Research Collection

Doctoral Thesis

Mass bias study of Cu and Zn using (LA)-MC-ICP-MS and isotoperatio determinations by atmospheric aerosol sampling

Author(s): Dorta, Ladina Corsina

Publication Date: 2013

Permanent Link: https://doi.org/10.3929/ethz-a-010030879

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DISS. ETH No. 21359

MASS BIAS STUDY OF CU AND ZN USING (LA)-MC-ICP-MS AND ISOTOPE RATIO

DETERMINATIONS BY ATMOSPHERIC AEROSOL SAMPLING

A dissertation submitted to

ETH Zurich

for the degree of

Doctor of Sciences

presented by

Ladina Corsina Dorta

M. Sc. ETH Zurich

born

August 28th, 1985

citizen of

Scuol (GR) - Switzerland

accepted on the recommendation of

Prof. Dr. Detlef Günther, examiner

Prof. Dr. Wendelin J. Stark, co-examiner

Dr. Bodo Hattendorf, co-examiner

2013

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Acknowledgements First I want to thank Detlef for the opportunity of receiving a master degree and then the PhD (which

I only did because I met the Günther group) in his group.

Thanks to Bodo for the help all along the PhD thesis and for always having an open ear when the

Beast was not working properly, or not at all (which was not that seldom)! Thank you for the fruitful

discussion about the happenings in the instrument! And for being my co-examiner!

Thanks to Prof. Wendelin J. Stark for accepting to be co-examiner at my thesis defence.

Thanks to Henry for the valuable corrections and comments!

Nicole, for the perfect management of the group and the hours spend together in the fitness centre,

sweating!

Katrin for the discussion about golf, the weather and so many more… Beni for his cheerful (mostly)

nature and Fandorin for letting me pet you (hoffe nur du verlürsch mal dini biisserlis)!

Joachim for your help with the femto, whenever it was not working correctly and for being afraid to

play against me at töggeli!

Gisela, you were the first contact I had in the group during my master thesis. Thank you for showing

me how to work with and take care of the Beast, even if sometimes nothing helped (not even the

largest hammer in the lab)!

Mattias and Bob for introducing me to the dark side and to the che cazzos! And for giving me the

best nickname ever, at least I feel a bit bigger with it…

Markus, Karin, Rokky, Giovanni, Mohamed, Tatiana, the “old” PhDs for making the group what it was

when I first had a glimpsed of it. There were discussions around coffee, beer, and orange juice and of

course the many töggeli games and tournaments (molten torch trophy)! And for giving me the

opportunity to live out my creativity when making your PhD hats!

Reto and DAF for the great time in the group but also during free time, for the many parties at your

homes and for liking my säucelis!

Tabasco for taking over Gisela’s place but not her outfits and for being che cazzo as well! And Luca

for being Luca.

Olga, Sabrina, Hao, Nataliya, Caro, Steffen, Abi, Kevin for funny discussions in the morning coffee,

during lunch time and the paper beers! Keep the habits of the group, they make it so special! Niko

for helping when the Beast was making strange noises!

Merci à mon français préféré, Julien, pour l’occasion de pouvoir parler un peu de français, question

de ne pas tout à fait l’oublier.

Lukas Bregy and Bianca Gusmini for being my master students and working hard! The many students

in the “pharma-praktikum”, to let me forget about the thesis for some time and for showing me that

I really learned something in the 4 years of study!

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You all made my time in the Günther Group special, thank you a lot!

Tinu and Thea for sharing the flat for 3.5 years, it was a fun time (even if I wasn’t at home much, gäll

Tinu ;-) )! I miss our grill evenings on the terrace!

Judith and Gian, thank you for the opportunity I had to leave the house and go studying in another

city, even if chemistry would also have been possible around the corner. Thank you for the support

during all the years of study and PhD! Thank you so much!

Natalia and Not for being the best siblings one can imagine, even if I was not always of this mind!

And thank you Fanny and Guy for enlarging the family and for the fun during our “family reunion”!

Thank you Andreas for cheering me up when I’m down and making me laugh, when I don’t feel like

laughing. Thank you for your love, support and endless positive thinking (chunt scho guet)!

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Contents Acknowledgements ...................................................................................................................... 3

Contents ...................................................................................................................................... 5

Abstract ....................................................................................................................................... 9

Zusammenfassung ...................................................................................................................... 11

Résumé ...................................................................................................................................... 15

Glossar ....................................................................................................................................... 17

1. Introduction........................................................................................................................ 19

1.1 Mass Discrimination in Isotope Ratio Measurements .......................................................... 19

1.1.1 Mass Discrimination ...................................................................................................... 20

1.1.2 Correct for Mass Fractionation ..................................................................................... 23

1.1.3 Mass Fractionation Correction for Cu and Zn ............................................................... 26

1.2 Instruments ........................................................................................................................... 28

1.2.1 Multi Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) ............. 28

1.2.2 Sample Introduction Systems ........................................................................................ 33

1.2.3 Measurement Routine .................................................................................................. 36

1.3 Aim of the Project .................................................................................................................. 39

2. Mass Bias of Cu and Zn ....................................................................................................... 41

2.1 Introduction to the Cu and Zn Isotope Systems .................................................................... 41

2.2 Samples ................................................................................................................................. 42

2.2.1 Introduction ................................................................................................................... 42

2.2.2 Experimental ................................................................................................................. 42

2.2.3 Results ........................................................................................................................... 44

2.2.4 Conclusion ..................................................................................................................... 46

2.3 Dependency of Mass Bias of Cu and Zn on ICP Operating Conditions with Three Different

Introduction Systems ........................................................................................................................ 47

2.3.1 Background .................................................................................................................... 47

2.3.2 Experimental ................................................................................................................. 47

2.3.3 Results and Discussion................................................................................................... 49

2.3.4 Summary and Discussion ............................................................................................... 60

2.4 Mixing Conventional Nebulisation and Dry Aerosol: Effect on Mass Bias Stability .............. 63

2.4.1 Introduction ................................................................................................................... 63

2.4.2 Experimental ................................................................................................................. 64

2.4.3 Results and Discussion................................................................................................... 64

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2.4.4 Summary ........................................................................................................................ 74

2.5 General Discussion and Conclusion ....................................................................................... 75

2.6 Outlook .................................................................................................................................. 76

3. LA-GED-MC-ICPMS .............................................................................................................. 77

3.1 Introduction to GED/air ablation ........................................................................................... 77

3.2 Experimental ......................................................................................................................... 78

3.2.1 Samples ......................................................................................................................... 78

3.2.2 Interferences ................................................................................................................. 79

3.2.3 Instruments ................................................................................................................... 79

3.2.4 Liquid Sample Measurements ....................................................................................... 80

3.2.5 Visualization of the Sampling Process ........................................................................... 80

3.3 Experiments ........................................................................................................................... 81

3.3.1 Isotopic Determination of Liquid Samples .................................................................... 81

3.3.2 Closed Cell and Atmospheric LA Sampling .................................................................... 82

3.4 Results and Discussion .......................................................................................................... 82

3.4.1 Solution Measurements ................................................................................................ 82

3.4.1 Laser Ablation Measurements ...................................................................................... 82

3.5 Conclusion ............................................................................................................................. 89

4. Application of LA-MC-ICPMS ............................................................................................... 91

4.1 Boron Isotope Ratio Determination in Boron Carbide Samples ............................................ 91

4.1.1 Introduction ................................................................................................................... 91

4.1.2 Samples ......................................................................................................................... 91

4.1.3 Tuning ............................................................................................................................ 91

4.1.4 Calibration ..................................................................................................................... 92

4.1.5 Results and Discussion................................................................................................... 92

4.1.1 Conclusion ..................................................................................................................... 95

4.2 Copper Isotope Ratio Determination in Andesines ............................................................... 97

4.2.1 Introduction ................................................................................................................... 97

4.2.2 Experimental ................................................................................................................. 97

4.2.3 Results and Discussion................................................................................................... 99

4.2.4 Conclusion ................................................................................................................... 103

4.3 Copper and Lead isotope Ratio Determination in Pb and Cu Minerals............................... 104

4.3.1 Introduction ................................................................................................................. 104

4.3.2 Experimental ............................................................................................................... 104

4.3.3 Results and discussion ................................................................................................. 104

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4.3.4 Conclusion ................................................................................................................... 115

4.4 Outlook ................................................................................................................................ 115

5. Overall Outlook ................................................................................................................. 117

References ................................................................................................................................ 119

Curriculum Vitae ....................................................................................................................... 125

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Abstract Mass Bias Study of Cu and Zn. Inductively coupled plasma mass spectrometry (ICPMS) is an efficient

ionisation source for almost every element in the periodic table. The coupling to a multi collection

(MC) detector array enables the precise measurement of isotope ratios. The direct coupling of this

method to solution nebulisation and laser ablation enables a high sample throughput and the

capability of in-situ measurements, respectively. The accuracy of the isotope ratio measurement is

hampered by the mass bias induced in the instruments due to diffusion, collision and space charge

effects. Different methods have been used for the correction of the mass bias; however they all

require a stable mass bias during the measurement session. The correction methods can either be

external or internal. For external methods, the stability of the mass bias is even more important as

similar mass bias have to be measured for sample and standard to ensure accurate correction. For

internal correction either isotopes of the same element or an element with similar mass, with known

isotope ratios, are used for the correction. This type of correction is often applied for Cu and Zn,

where the mass bias of Zn is used for the correction of the mass bias of Cu or vice versa.

In this study, the isotope ratio variation of Cu and Zn was investigated as a function of the gas flow

entering the ICP. The study was carried out on three different brass samples and two samples of

almost pure Zn and Cu. The study was carried out using three different sample introduction systems:

Solution nebulisation

Desolvating nebulisation (DSN)

Femtosecond laser ablation (fs-LA)

For the solution nebulisation measurements, the nebuliser pressure was varied, for the DSN

variations the membrane gas flow rate was changed and the additional Ar gas flow of the fs-LA setup

was changed for the isotope ratio measurements. For solution nebulisation, the variation curve

shows a plateau region for gas flows slightly higher than highest signal intensity, in which the mass

bias is stable, and therefore fluctuation in the plasma are less or minimally affecting the isotope

ratio. Measuring in this region gave the lowest relative standard deviation (RSD) for the isotope ratio.

For DSN and fs-LA (dry aerosols), the shape of the variation curve was slightly different. Instead of a

plateau region a broad maximum for the isotope ratios in the region of the maximum signal intensity

was found. The RSDs in this region are lowest but increase again for higher gas flow rates. By plotting

the logarithm of the 65Cu/63Cu ratio against the logarithm of the 68Zn/64Zn ratio a correlation over a

large range of the measured gas flows was observed for solution nebulisation. For the dry aerosols,

however, the correlation was divided into different branches. The correlation of the logarithm of the 66Zn/64Zn ratio and the logarithm of the 68Zn/64Zn ratio was good for solution nebulisation but was

worse for dry aerosols. With these findings, a setup where dry aerosols were mixed with solution

nebulisation was developed. The solution was desolvated in the DSN and mixed with the

conventional nebulisation of a 1% HNO3 solution before the ICP by the means of a T-type piece. For

the fs-LA generated aerosol, additional Ar was mixed through a laminar flow adapter to the He

carrier gas flow as in the conventional setup and before the ICP conventional nebulisation aerosol of

a 1% HNO3 solution was mixed using the T-type piece. With small amounts of moisture in the dry

aerosol, a stabilisation of the isotope ratio in the high gas flow region was observed. The correlation

in the Cu vs. Zn log plots is better than without conventional nebulisation mixing. The RSDs for DSN

measurements were better for the coupling with conventional nebulisation, for fs-LA aerosol, the

RSDs were not lowered for the high gas flow region. The intensity was however decreased by a factor

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of two, when comparing the conventional nebulisation and dry aerosol coupling to the dry only setup

with wet cones.

For fs-LA, different matching of the intensities were investigated to check for mass bias

dependencies. The Cu intensities, Zn intensities and total ion beam intensities were matched

between the samples for testing the mass bias dependency on the intensity and the mass load in the

plasma. No differences in the mass bias between the samples were found for the tests carried out.

The only conclusive statement is to ensure high enough intensities of all elements analysed.

LA-GED-MC-ICPMS. Isotope ratio determinations in solids sampled in air at atmospheric pressure

using laser ablation with direct introduction into an ICPMS was investigated. The results obtained on

the metal brass, and two minerals galena and zircon are reported. The samples were ablated in air

and the laser-generated aerosol was aspirated into a gas exchange device (GED), where the air was

replaced with Ar and transported into the MC-ICPMS. To demonstrate the capabilities of this

sampling system, the results were compared to results obtained using conventional laser ablation

(LA)-MC-ICPMS in helium in a sealed cell. Data show that comparable in-run (0.02-1.6%) and external

(0.005-0.254%) precisions can be obtained. The accuracy (0.001-0.115%) of both methods was also

comparable. However, the atmospheric sampling method gave lower intensities, by up to a factor of

5. Visualization of the aerosol extraction indicates that some material was lost prior to the gas

exchange. However, this method is suitable for isotopic determination of bulk materials for samples

which are too large to fit into an ablation cell or too valuable to be cut into smaller pieces.

Applications of LA-MC-ICPMS. Following three studies were carried out:

1. The B isotope ratio homogeneity of 10B enriched boron carbide sticks was measured. Mass

bias correction was carried out by sample standard bracketing. The enrichment of about

0.7% on 10B was confirmed and no trends in the isotope ratios were observed.

2. The Cu isotope ratio in andesines was determined and a relation between the isotope ratio

and the authenticity of the red colour of the andesines was investigated. The isotope ratio

alone was not able to differentiate true red andesines from treated ones.

3. The capability of Cu and Pb isotope ratio determination for provenance determination was

studied. Bronze and mineral samples from different regions in China were analysed. The

isotope ratios did not cluster by region, which did not allow provenancing.

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Zusammenfassung Massendiskriminierungsstudie von Cu und Zn. Die induktiv gekoppelte Plasmamassenspektrometrie

(ICPMS) ist eine effiziente Ionisationsquelle für die Ionisierung aller Elemente im Periodensystem.

Eine Kopplung mit einem Multi Kollektor (MC) Detektor ermöglicht die präzise Messung von

Isotopenverhältnissen. Die direkte Kopplung dieser Methode mit Lösungzerstäubung erlaubt einen

grossen Probendurchsatz oder mit Laserablation besteht die Möglichkeit in-situ Isotopenmessungen

durchzuführen. Die Genauigkeit der Messung von Isotopenverhältnissen ist beeinflusst durch die

Massendiskriminierung, welche durch Diffusion, Kollisionen und Raum-Ladungseffekte verursacht

wird. Verschiedene Methoden werden für die Korrektur der Massendiskriminierung verwendet, die

jedoch alle eine relativ stabile Massendiskriminierung über den gesamten Zeitraum eines Messzyklus

erfordern. Die Korrekturmethoden können von interner oder externer Natur sein. Bei externen

Korrekturmethoden ist die Stabilität der Massendiskriminierung noch wichtiger, da ähnliche

Massendiskriminierung für die Probe und den Standard gemessen werden sollten, um eine genaue

und präzise Korrektur zu gewährleisten. Bei der internen Korrekturmethode werden entweder

Isotope des selben Elementes, wobei ein bekanntes Isotopenverhältnis im Isotopensystem vorliegt,

oder ein Element mit ähnlicher Masse verwendet. Die interne Korrektur von Cu mit Zn wird häufig

verwendet, wobei die Massendiskriminierung von Zn für die Korrektur der Massendiskriminierung

von Cu benutzt wird.

In dieser Arbeit wurden die Veränderungen des Isotopenverhältnisses von Cu und Zn in Abhängigkeit

des Zerstäubergasflusses untersucht. Die Studie basiert auf drei unterschiedlichen Messingproben

und zwei Proben aus annähernd reinem Zn und reinem Cu. Die Studie wurde mit drei verschiedenen

Probeneinführungsystemen durchgeführt:

Lösungzerstäubung

Desolvatisierte Zerstäubung (DSN)

Femtosekunde-Laserablation (fs-LA)

Für die Lösungszerstäubungsmessungen wurde der Druck im Zerstäuber variiert und bei den DSN

Messungen wurde der Membrangasfluss variiert. Bei der fs-LA wurde der zusätzliche Ar Gasfluss

verändert und die Massendiskriminierung untersucht. Die Isotopenverhältnisse der

Lösungszerstäubung zeigen eine Plateauregion für die Gasflüsse, die oberhalb der optimalen

Gasflüsse für die maximale Intensität liegen. In dieser Region ist die Massendiskriminierung stabil

und Fluktuationen im Plasma beeinflussen das Isotopenverhältnis nicht oder nur geringfügig. Bei

diesen Parametern ist auch die relative Standardabweichung (RSD) der Isotopenverhältnisse am

kleinsten. Für die DSN und fs-LA (trockene Aerosole) wurden leicht verschiedene Variationskurven

gemessen. Statt eines Plateaus wurde ein breites Isotopenverhältnismaximum für die Region mit

maximaler Signalintensität beobachtet. Die RSDs bei diesen Gasflüssen sind am tiefsten, steigen

jedoch bei höheren Gasflüssen signifikant an. Wird das logarithmierte 65Cu/63Cu Verhältnis gegen das

logarithmierte 68Zn/64Zn Verhältnis aufgetragen, erkennt man eine Korrelation über einen Grossenteil

der Gasflüsse für die Lösungszerstäubung. Für die trockenen Aerosole ist die Korrelation in

verschiedene Bereiche aufgeteilt. Die Korrelation des Logarithmus des 66Zn/64Zn Verhältnisses gegen

den Logarithmus des 68Zn/64Zn Verhältnisses ist für die Lösungzerstäubung sehr gut, wird hingegen

kleiner für die trockenen Aerosole. Diese Beobachtung führte zur Entwicklung von einem Aufbau für

die Kopplung von trockenen und angefeuchteten Aerosolen. Die Lösung wurde mit dem DSN

getrocknet und dann in einem T-Stück mit 1% HNO3 befeuchtetem Aerosol gemischt. Für die fs-LA

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generierten Aerosole wurde ein zusätzlicher Ar Gasfluss mittels einem Laminarflussadapter zum

Aerosolträgergas hinzugefügt. Dieses Aerosol wurde kurz vor dem ICP mittels 1% HNO3 Lösung

angefeuchtet, wodurch feuchte Plasmabedindungen erreicht werden konnten. Mit dieser Studie

konnte gezeigt werden, dass bereits kleine Mengen an befeuchteten Aerosol ausreichen, um eine

Stabilisierung der Isotopenverhältnisse bei hohem Gasfluss zu beobachten. Die Korrelation von Cu

gegen Zn wird signifikant grösser als es für trockene Aerosol erreicht werden kann und die RSDs für

die DSN Messungen werden kleiner, wenn man angefeuchtete Aerosole ins Plasma einträgt.

Allerdings ist dies nicht der Fall für die fs-LA, bei der keine Verringerung der RSDs bei hohem Gasfluss

nachgewiesen werden konnte. Die Signalintensitäten verringern sich um einen Faktor zwei für

Messungen mit befeuchtetem Aerosol, wenn man sie mit den Messungen von nur trockenem

Aerosol vergleicht. Um den Einfluss von dem Probeneintrag auf die Massendiskriminierung zu

bestimmen, wurden für die fs-LA verschiedene Abgleichungstechniken der Intensität untersucht.

Dafür wurden die Cu Intensitäten, die Zn Intensitäten und der gesamte Ionenstrahl in den

Intensitäten zwischen den Proben angeglichen. Diese Tests zeigten keinen Unterschied in der

Massendiskriminierung zwischen den Proben.

LA-GED-MC-ICPMS. Neben den Massendiskriminierungsstudien wurde die Genauigkeit und Präzision

der Bestimmung von Isotopenverhältnissen fester Proben unter atmosphärischen Bedingungen

untersucht, wofür mittels Laserablation generierte Aerosole direkt in ein ICPMS eingeführt wurden.

Der Aufbau basiert auf einem Gasaustauschsystem, welches die vollständige Überführung von luft-

basierten Aerosolen in Argon erlaubt. Isotopenverhältnisse in Messing als Metall und zwei Minerale

(Bleierz und Zircon) wurden untersucht. Die Proben wurden unter Normalatmosphäre (Luft) ablatiert

und das laser-generierte Aerosol wurde in ein Gasaustauschgerät (GED, engl. Gas Exchange Device)

gesaugt, wo die Luft durch Ar ausgetauscht und anschliessend ins MC-ICPMS transportiert wurde.

Um die Leistungsparameter dieser Methode als neue Probenahmestrategie zu bestimmen, wurden

die resultierenden Messwerte mit den Resultaten aus konventionellen LA-MC-ICPMS Messungen in

Helium mit geschlossener Zelle verglichen. Die Messdaten zeigen vergleichbare interne (0.02-1.6%)

und externe (0.005-0.254%) Präzisionen. Die Genauigkeiten (0.001-0.115%) der beiden Methoden

sind ebenfalls vergleichbar. Allerdings wurden bei der Laserablation in der Luft bis zu einem Faktor 5

tiefere Intensitäten gemessen. Eine Visualisierung der Aerosolextraktion deutet darauf hin, dass ein

Teil des ablatierten Materials schon vor dem Austauschprozess verloren geht. Trotz der

Intensitätsverluste konnte nachgewiesen werden, dass sich diese Methode sehr gut für die

Bestimmung von Isotopenverhältnissen eignet. Ein besonderer Vorteil bietet sich für die Analyse von

grossen Proben, die nicht in einer Ablationszelle luftdicht eingeschlossen werden können, was zu

einer wesentlichen Verbesserung der Probenahme führt.

Anwendung von LA-MC-ICPMS. Anhand von drei anwendungsorientierten Studien wurde die

Leistungsfähigkeit der LA-MC-ICPMS demonstriert:

1. Es wurde die Homogenität vom B Isotopenverhältnisses in 10B angereicherten

Borcarbidstäbchen untersucht. Die Anreicherung der Proben von etwa 0.7% in 10B wurde

nachgewiesen. Eine lokale Variation der Isotopenverhältnisse innerhalb der Probe konnte

nicht festgestellt werden.

2. Es wurden die Cu Isotopenverhältnisse in Andesinen und dessen Zusammenhang mit der

Echtheit der roten Farbe untersucht. Anhand der Cu Isotopenverhältnisse konnten die

echten von den behandelten Andesinen nicht unterschieden werden.

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3. Die Möglichkeit der Herkunftsbestimmung von Bronze- und Mineralproben von

verschiedenen Regionen Chinas mit Hilfe von Cu und/oder Pb Isotopenverhältnissen wurde

untersucht. Anhand der erhalten Messwerte und deren statistischer Bearbeitung konnte

nachgewiesen werden, dass die Varation der Isotopenverhältnisse dieser Elemente nicht

ausreichen, um Aussagen über die regionale Herkunft zu treffen.

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Résumé Etude de biais de masse sur les éléments Cu et Zn. La spectrométrie de masse couplée à un plasma

par induction (ICPMS) est une source d’ionisation efficace capable d’ioniser tous les éléments du

tableau périodique. La possibilité de la coupler à un système de mesure de type multi-collection (MC-

ICPMS) permet de mesurer des rapports isotopique d’un même élément. Le couplage direct de cette

méthode à la nébulisation de solution et/ou à l’ablation laser permet un grand débit d’échantillon et

donne la possibilité d’analyse in situ à très petite échelle. Néanmoins l’exactitude de la mesure du

rapport d’isotopes est détériorée par le biais de masse instrumental engendré par la diffusion, des

collisions et des effets d’espace-charge. Différentes méthodes sont utilisées pour la correction de ce

biais de masse, considérant que ce dernier doit être plus ou moins constant pendant la durée de la

session de mesure pour une correction efficace. Les méthodes de correction peuvent être soit

externes ou internes. Pour les méthodes externes, la stabilité temporelle du biais de masse est

encore plus importante puisque le biais de masse doit être similaire entre l’échantillon et le standard

pour garantir une correction exacte. Pour la correction interne soit les isotopes d’un même élément,

avec un rapport d’isotope connu, ou d’un autre élément de masse similaire sont utilisés. Ce type de

correction est souvent utilisé pour la correction par exemple du biais de masse du Cu avec le biais de

masse du Zn.

Dans cette étude, la variation du rapport d’isotopes du Cu et du Zn est analysée en dépendance de la

quantité de gaz qui entre dans l’ICP. L’étude a été effectuée sur trois échantillons différents de laiton

et deux échantillons Zn ou Cu presque purs. Trois différents systèmes d’introduction d’échantillon

ont été appliqués:

la nébulisation de solution

la nébulisation désolvatisée (DSN)

l’ablation laser femtoseconde (fs-LA)

Pour les mesures avec la nébulisation de solution, la pression du nébuliser a fait l’objet de variation,

pour les mesures avec DSN le flux de gaz de la membrane a été changé et pour la configuration fs-LA

le flux additionnel d’Ar a été changé. La courbe de variation montre un plateau pour des flux de gaz

légèrement supérieurs au flux nécessaire pour une intensité de signal maximale, ce qui indique en

conséquence que les fluctuations dans le plasma affectent peu ou pas le rapport isotopique mesuré.

Une mesure dans la région du plateau produit les plus bas écart-type relatifs (RSD, angl. relative

standard deviation) pour le rapport d’isotope. Pour la DSN et la fs-LA (aérosols secs), la forme de la

courbe de variation est légèrement différente. Au lieu du plateau, un large maximum pour les

rapports isotopique est obtenu dans la région d’intensité maximale. Les RSDs dans cette région sont

au plus bas, mais augmentent à nouveau pour des flux de gaz supérieurs. Un graphique du

logarithme du rapport 65Cu/63Cu par rapport au logarithme du rapport 68Zn/64Zn montre une

corrélation claire pour la majorité des flux de gaz mesurés avec la nébulisation de solution. Pour les

aérosols secs, par contre, les corrélations sont divisées en différentes branches. La corrélation du

logarithme du rapport 66Zn/64Zn avec le logarithme du rapport 68Zn/64Zn est bonne pour la

nébulisation de solution mais se détériore pour les aérosols secs. Avec ces résultats, une

configuration analytique a été développée pour coupler les aérosols secs avec la nébulisation de

solution. La solution est désolvatisée dans le DSN et est mixée avec l’aérosol d’une solution de 1% de

HNO3 devant l’ICP par les moyens d’une pièce en T. Pour l’aérosol généré avec fs-LA, le flux d’Ar

additionnel est ajouté au gaz transporteur d’aérosol par un adaptateur de flux laminaire avant d’être

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mixé avec l’aérosol de 1% de HNO3 par une pièce de T. Des petites quantités d’aérosol mouillé

ajoutées à l’aérosol sec sont suffisantes pour obtenir une stabilisation du rapport isotopique dans la

région de haut flux de gaz. Les corrélations dans les graphiques logarithmiques montrant Cu contre

Zn sont meilleures que sans l’addition. Les RSDs pour les mesures avec DSN sont meilleures pour le

couplage avec aérosol mouillé, et ne sont pas diminuées pour la région de haut flux de gaz pour les

mesures avec la fs-LA. L’intensité du signal est réduite d’un facteur deux, si l’on compare le couplage

d’aérosol mouillé et sec avec seulement l’aérosol sec.

Pour la fs-LA, différents ajustages d’intensité sont étudiés pour contrôler la dépendance du biais de

masse. Les intensités du Cu, du Zn et l’addition des intensités du Cu et Zn ont été ajustées entre les

échantillons pour tester la dépendance de l’intensité mesurée sur biais de masse. Le biais de masse

ne montre pas de différence entre les échantillons pour les tests effectués. La seule conclusion à tirer

est de mesurer avec des hautes intensités sur tous les éléments analysés.

LA-GED-MC-ICPMS. La détermination de rapport isotopique dans des solides analysés dans l’air (en

l’absence de cellule d’ablation conventionnelle) par l’utilisation de l’ablation laser couplée à l’ICPMS

a été étudiée. Les résultats obtenus sur du laiton (métal) et deux minéraux (galène et zircon) sont

rapportés. Les échantillons sont ablatés dans l’air et l’aérosol généré par l’ablation laser est aspiré

dans un appareil à échange de gaz (GED, angl. gas exchange device), où l’air est ensuite échangé par

de l’Ar puis transporté dans le MC-ICPMS. Pour démontrer la capacité de la méthode, les résultats

obtenus sont comparés aux résultats obtenus en utilisant l’ablation laser conventionnelle dans une

cellule d’ablation hermétique et dans de l’hélium. Les valeurs mesurées montre des précisions

internes (0.02-1.6%) et externes (0.005-0.254%) comparables. L’exactitude (0.001-0.115%) des deux

méthodes est aussi comparable. Par contre, l’intensité de signal pour le prélèvement d’échantillon

dans l’air est jusqu’à un facteur de 5 plus bas. Une visualisation de l’extraction de l’aérosol indique

qu’une partie de l’aérosol est perdue avant l’échange de gaz. La méthode est appropriée pour la

détermination du rapport isotopique d’échantillons trop grand pour être placés dans une cellule

d’ablation ou trop précieux pour être coupés en plus petits morceaux adaptés aux cellules d’ablation.

Application du LA-MC-ICPMS. Trois études ont été effectuées :

1. L’homogénéité des rapports isotopique du B dans des bâtonnets de carbide de bore enrichis

en 10B a été examinée. L’enrichissement en 10B de 0.7% a été détecté. Par contre, aucune

tendance pour la variation des rapports isotopique ne fut constatée.

2. Le rapport isotopique du Cu pour des andésines et sa relation avec l’authenticité de la

couleur rouge de ce minéral a été examiné. Le rapport isotopique seul ne permet pas de

différencier les vraies andésines des andésines traitées.

3. La capacité de la détermination de provenance de certains matériaux par mesure de leurs

rapports isotopique de Cu et de Pb fut examinée. Des échantillons de Bronze et de minéraux

de différentes régions de Chine ont été analysés. Les rapports d’isotopes ne se groupent pas

en fonction de la région d’origine de l’échantillon ce qui empêche une détermination claire et

définitive de provenance des échantillons étudiés.

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Glossar Ar Argon

DSN Desolvating Nebulisation

ESA Electrostatic Analyser

FC Faraday Cup

fs femtosecond

GD Glow Discharge

GED Gas Exchange Device

He Helium

IC Ion Counters

ICP Inductively Coupled Plasma

LA Laser Ablation

MB Mass Bias

MC Multi Collector

MS Mass Spectrometry/Spectrometer

ns nanosecond

ps picosecond

RF Radio Frequency

RSD Relative Standard Deviation

SD Standard Deviation

SE Standard Error

SF Sector Field

SIMS Secondary Ion Mass Spectrometry

SSB Sample Standard Bracketing

TIMS Thermal Ionisation Mass Spectrometry

TOF Time Of Flight

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1. Introduction

1.1 Mass Discrimination in Isotope Ratio Measurements In analytical chemistry, one of the aims is to detect and determine the concentration of different

elements and their isotopic composition present in a sample. The concentration of different

elements can give information about the sample origin, its toxicity, and its authenticity. The isotope

ratios of the elements can provide more information about the sample. When an ancient artefact is

found, elemental analysis can tell what it is made of and which ores were used to manufacture it.

Element fingerprints can give information on the region where the ore came from. Isotopic analysis

however, can sometimes be used to determine from which mine the ore was collected or if mixing of

different ores occurred. This in turn, can shed light on trade routes at the time the artefact was

produced.1 Another application of isotopic analysis is isotope ratios determination in plants which

can give information about the uptake and the transport mechanism within the plant.2

Physical processes (such as diffusion, evaporation, absorption, precipitation, freezing, and boiling)

and chemical reactions (redox and acid-base) can lead to fractionation or changes in the isotopic

composition of an element. This fractionation is usually mass dependent.3 Some mass independent

fractionation can occur (natural or induced),4-6 but is rather rare, and often occurs in conjunction

with complex solution chemistry. The study of isotopic fractionation gives information on different

biological, geological, and environmental processes occurring in nature. For radioactive elements

additional isotope ratio changes occur.

One of the very important fields for isotope ratio determination is geological dating (geochronology)

of rocks, speleothems, and archaeological artefacts, using the radioactive decay of different

elements: U-Th-Pb, K-Ar, Rb-Sr, Sm-Nd, Lu-Hf, La-Ba and 14C. As an example, one of the oldest dating

method is the U-Pb method, where the decay of 238U to 206Pb (t1/2 = 4.47 x 109)7 and 235U to 207Pb (t1/2

= 7.04 x 109)7 is measured. This dating method was used for age determination of the oldest zircons

found in the earth crust, which have an age of 4.404 ± 8 Ga.8

The isotopic composition of Pb is also an important pollutant marker, used to distinguish

anthropologic and natural provenance9 or for provenance determination of archaeological artefacts

or ores. The variation in Pb isotope composition in natural samples is caused by the U to Pb decay

mentioned previously as well as the decay of 232Th to 208Pb.

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Other fields are also interested in the analysis of isotope ratios, especially in the newer non-

traditional isotope systems, which have developed because of the increased research and

development of ICP-MS instruments. In radiological studies, quality control in the production of

nuclear containment, control rod, which are enriched with a neutron absorbing isotope (10B) or the

identification of depleted, or more potentially dangerous, enriched U are determined. In provenance

studies, not only Pb, as mentioned, is analysed but also provenance determination of food based on

Sr isotopes has been performed. Authenticity of gem stones with Cu or Ar isotopes can be

monitored. Nutritional studies (Fe, Cu, Zn) are performed to shed light on biochemical mechanisms in

the human body and in plants. Isotope ratio determination can also be used for the high quality

determination of concentration in a sample by applying isotope dilution methods, methods which

must be used when using TIMS instruments.10

Isotope ratios can be measured by all mass spectrometers. When choosing an appropriate

instruments and method for an isotopic analysis one of the main consideration is the precision with

which the isotope ratio needs to be determined. Further the required sample preparation which is

needed for the different instruments and samples should be considered, i.e., if the sample has to be

dissolved and separated from interferences or if direct sample analysis can be performed. The

sample throughput of an instrument can also be an issue for sample measurements as this has a

direct relationship to costs. Thermal ionisation mass spectrometry (TIMS) is one of the highly precise

methods for isotope ratio determination. Some limitations, such as not being able to analyses

elements with ionisation potentials higher than 7.5 eV 3 and a usually tedious and time consuming

sample preparation are needed, were reason enough for the development of techniques without

these drawbacks. In 1990, VG Elemental introduced the first prototype of a sector field ICPMS with

multi collection capabilities.11 Now TIMS and ICPMS are most often used for isotope ratio

determination, yielding precision in the hundreds of ppm region and lower.12,13 Their advantages and

drawback will be discussed in the following sections. However, isotope ratios do not always need to

be acquired using highly precise methods. Single detector quadrupole or sector-field-ICPMS,

secondary ion mass spectrometry (SIMS), glow discharge-time of flight-MS (GD-TOFMS) are all

techniques with isotope ratio determination capabilities in the low per mil range,3,14 which are useful

for many applications.

1.1.1 Mass Discrimination When determining an isotope ratio using a mass spectrometer, the ratio measured in the sample

almost always is not equal to the true isotope ratio. This systematic deviation of a measured value

from the true value is caused by several phenomena. One is the mass fractionation occurring in TIMS

measurements. This mass fractionation is time dependent (over the short time of the introduction of

the total sample, which is not to be confused with drift) and occurs during the evaporation of the

sample from the filament. Light isotopes are slightly more volatile compared to heavier isotopes and

thus initially the vapour phase contains an excess of the low mass isotopes. Thus as evaporation

proceeds, the material remaining on the filament acquires a “heavier” isotope ratio. As evaporation

proceeds, more and more heavy isotope will be evaporated, ionised and measured. The isotope ratio

measured thus shifts eventually to a heavier one than in the original sample.

A second phenomenon is called mass discrimination or mass bias and can occur in all mass

spectrometric techniques. The discrimination usually reduces light isotope’s intensities. Different

effects are responsible for the mass bias and the contribution of each effect changes for different

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21

operating parameters and conditions of the MS. The magnitude of these contributions of the

different effects is not predictable from fundamental principles. In the MC-ICPMS, these effects are:15

Diffusion in the plasma. The ions diffuse in the plasma. The mass discrimination for lighter elements

in the plasma by diffusion was shown by Dziewatkoski et al.16 The diffusion is dependent on the ICP

conditions (sampling depth, gas flow rates, RF power, spray chamber temperature, and sample

uptake rate), the gas types, particle sizes when using LA, and also on the properties of the elements

(ionisation potential and vaporisation energy).17 Diffusion not only affects elements but also the

isotope of an element. Light isotopes are more affected by diffusion than heavy isotopes of the same

element.18 Thus there is usually a loss of light isotopes in the region sampled into the high vacuum of

the MS compared to heavier isotopes, giving a measured ratio which is biased to the heavier isotope.

Collisions in the supersonic expansion. Collisions with Ar neutrals during the supersonic expansion

after the sampler and skimmer cone occur.19 Because all ions gain the same velocity from the

supersonic expansion, these collisions also affect the light isotopes more than heavy ones.20 A

collision with a neutral atom leads to an energy loss and scattering, leading to less efficient collection

of the ions to the entrance slit of the MS.

Space charge effects in the high current ion beam. After the skimmer cone, electrons from the

expanding plasma are preferentially lost leaving a positively charged ion beam with high ion currents.

This high charge density leads to Coulomb-repulsion of the ions. The dispersion is mass21 and

energy21,22 dependent leading to a larger loss from the beam axis for the lighter and the less

energetic ions. As the space charge effect is dependent on the ion current, a variable matrix

composition of the samples analysed can result in variation of the magnitude of the space charge

effects,9 especially for a sample matrix of heavy elements. Space charge effects are also dependent

on the operating conditions of the ICPMS.23 Space charge effects are observed for element as well as

for the isotopes of elements.12

Energy selective ion transmission. The expansion in the interface is dominated by the expansion of

the hot argon gas from the ICP source. The terminal velocity is thus determined by the energy of the

argon atoms. The isotopes’ kinetic energies are therefore lower for light ions than for heavy ones.

The light ions are thus pushed to the perimeter of the beam and are lost at apertures where beam

clipping occurs.24-26 The high acceleration voltages used for sector field instruments (4-10 kV) should

reduce the energy spread.12

For all those mass discriminating effects mentioned above the lighter isotopes are more strongly

suppressed. Therefore the observed isotope ratio measured by sector field instruments is almost

always heavier than the true ratio. Mass discrimination is dependent on matrix composition, analyte

concentration, and on the operation conditions of the ICP. Total mass discrimination can reach 10%

per mass unit for element with masses less than 10, 5% for masses between 20 and 120, and less

than 1% per mass units for masses heavier than 120.12

Mass fractionation can however also occur before the ICP. Sample preparation can introduce mass

fractionation. When the analyte has been separated from the sample matrix using chromatography,

the elution of the different isotopes occurs at different times; it is therefore important to achieve a

100% yield.24 The isotopic composition of the drain of the spray chamber was found to be the same

as found in the sample.18 The aerosol reaching the ICP is therefore likely to be representative for the

sample solution.

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Laser ablation (LA), used as introduction system for solid samples, may also induce fractionation

occurring during the sampling process. Elemental and isotopic fractionation was shown to occur

when laser aerosols were introduced into the ICP. Elemental fractionation can be induced by many

factors:27

Element dependent fractionation. The chemical and physical properties of an element can influence

the way it is ablated. The (oxide) melting28,29 and boiling points are factors which influence the

fractionation. However, also ionic radii, speciation, and vapour pressure dependencies of the

fractionation have been reported.27

Laser properties dependent fractionation. The properties of the laser, including wavelength, pulse

duration, laser energy, and the crater aspect ratio also affect fractionation.

As the element property dependent fractionation cannot be totally removed, fractionation can only

be reduced by optimisation of the LA process.

The wavelengths of the laser used in LA range from infrared (IR) through visible (Vis) and ultraviolet

(UV) to vacuum-ultraviolet ((V)-UV), i.e. 1064 nm (fundamental Nd:YAG), 795 nm (fundamental

Ti:Sapphire), 532 nm (doubled Nd:YAG), 266 nm (quadrupled Nd:YAG), 265nm (tripled Ti:Sapphire),

213 nm (quintupled Nd:YAG), 193 nm (ArF) and 157 nm (F2). In case of non-metals, the particle size is

significantly a function of the wavelength, the longer the wavelength, the larger the aerosol particles

produced.30 The generation of small aerosol particles is an important factor for reducing elemental

fractionation. The smaller the particle the more completely they are vaporised in the ICP. If however

large and small particle with heterogeneous composition are transported to the ICP, fractionation in

the ICP occurs due to less complete vaporisation and transport of larger particles.30 Therefore,

elemental fractionation has been observed to be reduced when using shorter wavelengths.28,31 This

wavelength dependency has not been observed for metals.32

The laser energy and the crater depth to diameter ratio (aspect ratio) have also been found to

influence fractionation. Liu et al.29 observed representative Pb/U measurements when laser energies

above 0.6 GW/cm2 were used. The fractionation was found to increase when the aspect ratio

increases. The volatile element signal was larger the deeper the crater.33

The dependence of elemental fractionation on the melting point of the corresponding oxides in case

of nanosecond LA of glasses showed that ablation is a thermal process. The energy of the

nanosecond (ns) laser used to remove material from the sample can be converted to heat, as heat

transfer occurs in the picosecond range. When lowering the pulse duration to a few hundreds of

femtoseconds (fs), heat transfer between adjacent atoms does not occur. The morphology of ns- and

fs-craters showed different features. While in ns-craters the rims were melted, the fs-craters showed

well defined rims with no apparent melting.34 Different studies have shown that fs-LA is also more

useful for non-matrix matched quantification on various materials (metals, glass, and minerals

(dielectrics)).27,35-39 However, fluence dependencies of the fractionation for fs-LA were also

observed.40 And recently, studies of non-uniform aerosols composition of fs-LA generated aerosol

were reported.41,42 Nevertheless, when using fs-LA generated aerosols fractionation is usually

reduced and a lower matrix dependency of the aerosol generation compared to ns-LA aerosols has

been observed.43,44

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Isotopic fractionation was found to occur when performing laser ablation of single holes in metallic

copper.45 The smaller particles (< 200nm) were found to be enriched in lighter isotopes. The most

significant fractionation was observed for ablation times between 30 and 60 seconds. The particles

undergo partial vaporisation in the laser plume when using these ablation times, which favours the

lighter isotopes. The vapour, from which the smaller particles are formed, is therefore enriched in

lighter isotopes. As ablation proceeds, the longer residence time in the laser plume leads to more

complete vaporisation and to an equilibration of the isotope ratio in the vapour phase. Low laser

energy also results in fractionation, due to the preferential vaporisation of the lighter isotopes.46 The

preferential accumulation of light isotopes in small particles (< 180nm) for fs-LA was recently shown

for Fe isotopes.47 An additional isotopic fractionation source occurring in the ICP was observed,

namely the preferential vaporisation of the lighter isotope from large particles. This fractionation can

be reduced by filtering the aerosol to obtain only particles smaller than 0.5 µm. Using fs-LA for

isotopic studies, the fractionation was observed to be essentially eliminated for Fe isotopes.48 As

observed for elemental fractionation, the fs-LA is still fractionated, even if to a much lower extent

than for ns-LA. A complete absence of isotopic fractionation is therefore not possible and an

accumulation of lighter isotopes in the smaller particles was observed.47

The carrier gas has also been shown to influence the particle size distribution of the laser generated

aerosol and the aerosol transport to the ICP. Smaller particles are obtained when using He as the

carrier gas.31,46 It was suggested that this was due to the higher thermal conductivity of He in the

laser plume and therefore to the faster removal of particles from the ablation site, reducing the

condensation of particles. The sample re-deposition around the crater was reduced when using He as

the carrier gas compared to the use of Ar.27 The plasma temperature is also raised if it contains He,

leading the better ionisation efficiencies.

1.1.2 Correct for Mass Fractionation For accurate measurement of isotopic ratios, the mass fractionation must be corrected. Different

mathematical approaches have been proposed for this correction. Three possible correction

strategies have been proposed. In the external standardisation paradigm, an external standard with

known isotope ratio is used to correct for mass fractionation. The internal normalisation used an

isotope pair of the analyte element with known ratio to determine the degree of mass fractionation

and uses the same mass fractionation factor for other analyte ratios. For this correction the elements

of interest must have at least three isotopes, two of which must be “stable”. Another disadvantage

of the internal normalisation is that natural (stable isotope) fractionation cannot be determined.49

However for high precision isotope ratio analysis using TIMS, it is the only paradigm which can be

used. When using ICP ion sources, for isotopes, a second type of inter-element normalisation can be

used, when an element with similar mass is added to the sample or can be naturally present. A

similar correction technique is then used as for internal normalisation. When using this correction,

natural (stable isotope) fractionation can be measured.

1.1.2.1 Intra- or inter-element normalisation for mass fractionation correction

Three relationships were empirically suggested by Russell et al. for TIMS analyses.50 The use of these

so-called “laws” is essentially to provide a means to extrapolate the mass fractionation factor from

one isotope to another. The equations shown in the following are taken from Albarède et al.51 The

transmission T(Mk) of an isotope with mass Mk is defined by ⁄ , where nk is the number of ions

detected and Nk the total number of ions entering the ion source of the mass spectrometer. The first

law is the linear law, in which T(M) is expanded in a Taylor series at mass Mk:

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( ) ( ) ( )

( ) [( )

] (1)

where O stands for the higher order term. If equation (1) is calculated for and

divide it by T(Mk), equation (2) follows:

( )

( )

( )

( ) [( )

] (2)

If only considering the first order term:

( )

( ) ⁄

( ) (3)

where ri is the measured isotope ratio and Ri the true value of the isotope ratio. And for:

( )

(4)

where δ is the linear mass bias factor. This law is not consistent, as the ratio of two isotope ratios

fractionating according to the linear law, does not follow the linear fractionation law.

The second law is the so-called “power law”, where the logarithm of the transmission is expanded as

a function of the mass difference and reference mass Mk:

( ) ( ) ( )

( ) [( ) ] (5)

The same simplifications are done as for the linear law giving:

( ) ( ) ⁄

⁄

( ) (6)

The mass fractionation power law is defined as:

( )

( ) (7)

where g is the mass bias constant.

The exponential law of Russell et al.50 is defined by the expansion of the transmission as a function of

ln M:

( ) ( ) ( )

( ) [( )

] (8)

Keeping only the first order term:

( ) ( ) ⁄

⁄

( )

(

) (9)

And the mass bias factor is defined as:

( )

(10)

The measured isotope ratio is then:

( )

(11)

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A more general form of the power and exponential law is the generalized power law, where the

logarithm of the transmission is expanding as a function of Mq, q being an adjustable arbitrary

constant.

( ) ( ) ( )

(

) [(

) ] (12)

The measured isotope ratio is therefore defined as:

(

) (13)

With the fractionation coefficient:

( )

(14)

The generalized power law is transformed into the power law for and into the exponential law

for .

In 2003, Yang and Sturgeon compared the different correction laws for Hg isotope ratio

determination. The exponential law and the power law resulted in very similar results and the linear

law showed slightly less accurate results. The exponential law is however most commonly used for

mass bias correction in ICPMS.23

1.1.2.2 External standardisation for mass fractionation correction

1.1.2.2.1 Sample standard bracketing (SSB) method

The method of external standardisation is used in the sample-standard bracketing correction

paradigm. This method uses a standard which is measured before and often after the sample, or

using several samples between the standards to give a higher efficiency, with only a slight loss in

precision. From the standard, with known isotope ratio(s), a mass fractionation factor is derived; a

mass fractionation factor for the sample can then be interpolated from the standards bracketing it.

( ) ( ) ( )

( ) ( )

(15)

If the samples are measured alternatingly with the standards, θ will be 0.5. However, if more than

one sample is measured between two standard runs, a time factor can be introduced to the θ value.

The assumption is that the mass bias for standards and sample is the same i.e. not matrix dependent.

For obtaining precise and accurate isotope ratio determination, it is thus important to matrix match

the standard to the unknown sample. The correction factor is calculated for each isotope ratio of the

standard and applied to the sample. It needs to be noted however that this approach also corrects

for instrument drift in the mass bias correction.

1.1.2.2.2 Delta values

The corrected isotope ratios of a sample can either be reported as absolute value, which is possible

when the standard used has known isotope ratios or as relative values. Relative values are often used

in geology, where variations in the reported isotope ratios from a reference material are reported. By

reporting the relative variation from a standard, no “true” value is used, therefore the results always

remain correct even if the accepted isotope ratio of the standard changes. This relative value is

indicated as the delta value (δ-value):52

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(( ) ( )

) (( )

( )

)

(( ) ( )

)

(16)

where K is the mass bias factor. For , the last term of the equation is unity and can

easily be calculated. For mass bias factors to be equal, the standard and the sample should have the

same matrix and similar concentration. Therefore either perfect matrix separation must be achieved

for the sample or the standards can be spiked with the same amount of matrix contained in the

sample. The δ in this equation is not the linear mass fractionation term used in equation (4). The

need of matrix match of standard and sample is also important for this correction method, to yield

accurate results.

Further correction possibilities used are double or triple spike procedures, which can produce very

precise concentration determinations of ultra-trace elements.52 For this technique, however, more

than three, e.g., four isotopes must be available.

The correction technique chosen depends upon the sample, the elements measured, the standard(s)

available, and the instrumentation available, and the precision and accuracy requirements for the

application.

1.1.3 Mass Fractionation Correction for Cu and Zn Maréchal et al.24 were the first to determine Cu and Zn isotope ratios using MC-ICPMS. Their study

aimed at identifying natural variation in the Cu and Zn isotope ratios, in order to better understand

natural and anthropogenic processes of Cu and Zn. These variations are in a range of 1% or less. Prior

to this study, only TIMS and single collector SF-ICPMS measurements had been reported. For the

correction in TIMS, internal normalisation was previously used for Zn which has a multiple number of

isotopes, which was then not able to detect natural (stable isotope) variations. For other methods,

the precision was not sufficient.

Making the measurements on a MC-ICPMS with an inter-element normalisation procedure (use of Zn

to correct measured Cu or vice versa), the precision of the correction and thus the resulting isotope

ratio which was determined for Cu and Zn could be improved to a level where natural variations

were quantified.

The expression of the external correction used was derived from the exponential law (11). The

exponential law for the Cu isotope ratio, 65Cu/63Cu, is:

(

)

(17)

A similar expression is found for the Zn isotope ratio, 66Zn/64Zn:

(

)

(18)

By taking the logarithm and dividing (17) by (18), the following expression was derived:

( ⁄ )

( ⁄ )

( ⁄ )

( ⁄ ) (19)

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This expression describes also the slope of the data in a graph where ( ⁄ ) is plotted against

( ⁄ ). Such an inter-element normalisation was first proposed for application to ICPMS by

Longerich et al.53 They suggested that if f was the same for all elements, i.e. it would

suggest that the mass fractionation line in the above mentioned plot is only depending on the

respective masses of the isotopes analysed. They observed that the bias could be reduced by half of

the bias observed in the data set. Later in a study by Ketterer et al.54 the mass bias correction with Tl

for Pb was shown to be successful, as the biases obtained were less than 0.5% for the ratios with

respect to 204Pb isotope, which made the technique suitable for routine analysis of environmental

samples.

Maréchal et al.24 in this high precision study found, as reported by Longerich et al., that equivalence

of the fractionation factors was not true, but as long as ⁄ stays constant over the

measurement time, the correction is not affected.

In a plot where ( ⁄ ) is plotted against ( ⁄ ), the slope is:

( ⁄ )

( ⁄ ) (20)

And the intercept is:

(

)

(

)

(21)

If standards and samples are measured alternatively, the standards should fall on a line in the

( ⁄ ) vs. ( ⁄ ) plot. A linear regression through the points of the standard provides

the slope and intercept. The adjustment required to the intercept value to pass through the points of

the samples in the same plot resembles the difference of the Cu isotope ratio between samples and

standards.

A drawback of this method is the need of a mass bias spread for achieving a reasonable correlation in

the regression. If variation of mass bias is small during the measurements, the data will form a cluster

instead of a line. The slope of the regression line will therefore have a lower confidence level and the

accuracy of the correction will not be sufficient for the application.55 Several methods to increase the

mass bias spread can be used. The addition of matrix elements at different concentrations into the

standard solution will spread the mass bias.49,56-58 Archer et al.49 reported that care should be taken

when choosing the dopant matrix. Fe and Sr were tested for Cu and Zn. While Sr extended the spread

of the mass fractionation, while staying on a same regression line, Fe did not spread the mass

fractionation, additionally; the points did not fall on the line, which resulted in a degradation of the

correlation.

An alternative method was proposed by Mason et al.55 where a combination of the SSB and an inter-

element correction is used. In this approach, the mass bias for Cu was monitored internally by

comparison to a Zn spike, which was added to both the sample and the standard solution. Delta

values for both elements were calculated relative to the standard using equation (16). The delta

value of Zn was then subtracted from the delta value of Cu. This method was found to deliver more

accurate results, compared to a method which did not use this additional correction. Especially when

considering the time dependency of the mass bias, which can change within a period of 5 minutes,55

it is important to make use of internal standardisation which gives simultaneous monitoring of the

changes in mass bias.

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Additionally, they tested SSB and inter-element correction used for both dry and conventional

nebulisation. They found that the stability was increased for dry aerosols compared to conventional

nebulisation, which is favourable for SSB method, however less favourable for the inter-element

correction. They therefore suggested the use of SSB for dry aerosols and inter-element correction for

conventional nebulisation.

1.2 Instruments

1.2.1 Multi Collector Inductively Coupled Plasma Mass Spectrometry (MC-

ICPMS) In 1953 Alfred O. Nier and Edgar Johnson published their theory about “Angular Aberrations in Sector

Shaped Electromagnetic Lenses for Focusing Beams of Charged Particles”.59 With this publication, a

new sector field (SF) geometry was discovered and the fundamental theory for the multi collector-

IPCMS geometry used in this thesis was established. The instrument consists of a 90° electric sector

followed by a magnetic sector with a 60° angle with the same direction of curvature.60 The double

focusing property of this arrangement permits focusing of the energy and the mass to charge (m/z)

ratio and focusing several m/z ratios on one focal plane.

The ICP was first developed by Greenfield in 1964.60,61 While initially applied for optical spectrometry,

it was used as ion source for elemental mass spectrometry starting in the 1980s.19 The first

commercially available ICPMS with Nier-Johnson geometry was introduced in 1988 by VG Elemental

(UK) and was called Plasmatrace. Several companies subsequently developed their own ICP-SFMS

instruments. All those instruments had only one detector, which allowed single element analysis at

higher mass resolution capability than traditional quadrupole based systems. In 1992, the first results

for isotope ratio measurements with a multi collector (MC)-ICPMS were published by Walder et al.62

In that study, a Plasma 54, also developed by VG Elemental, was employed. With this first

commercial instrument several studies on a variety of elements (Hf, Mo, Te, Sn, W, Pb, Th, U)63-67

were performed. With this instrument, the MC-ICPMS technology was born. Eighteen years after the

introduction of this instrument, around 250 MC-ICPMS from 4 manufacturers were sold.60

Until the introduction of MC-ICPMS in the early 1990s, thermal ionisation mass spectrometry (TIMS)

was the method of choice for isotope ratio determination of heavier elements, meaning different

from the “stable isotope”; H, C, N, O and S, which are traditionally measured on gas source

instruments.68 However, for refractive elements and elements with high ionisation potentials,69

precise isotope ratio measurements were challenging because of low signal intensities, which in turn

resulted in large uncertainties. The high ionisation efficiency of ICPMS permits ionisation for these

elements resulting in a better precision.70

Comparing TIMS to MC-ICPMS, advantages and disadvantages are noteworthy. As mentioned

previously, the plasma is a more efficient ion source compared to the filament in the TIMS

instruments. This efficient ionisation has also drawbacks, as not only the first, but also the second

ionisation potential for some elements is reached, and therefore, additional interferences can be

present. TIMS needs a tedious and time-consuming matrix separation of a sample; which is not

mandatory for ICPMS. However, the best achievable precision and accuracy for ICPMS are reached

when matrix separation is carried out on the sample.71 The ease of coupling an ICPMS to an

atmospheric direct aerosol introduction from a solution or from a laser enables a larger sample

throughput, and in case of LA sampling, in situ analysis at high spatial resolution is important.5,70

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The isotope fractionation is not time dependent for ICPMS, but it is for TIMS. However the

fractionation is by about a factor of 10 lower for TIMS than for ICPMS and much better understood.71

The best precision for both instruments is achieved using isotope dilution.52 Precision can also be

improved if internal correction for the fractionation is used. The technique is less precise if no such

correction was used and only an external correction must be used instead, which means measuring

all samples under identical conditions.70,72 In ICPMS, the fractionation is mass dependent and

fluctuates with instrumental settings which in turn change slightly with time.71 The correction can be

done internally and externally as for TIMS and a third option is possible for ICPMS.70,72,73 The inter-

element normalisation permits to correct for mass bias with the help of another element in the same

solution or solid sample, as explained in chapter 1.24,53

In other words, both techniques are complementary; the method of choice should be selected for

each sample type.71 Criteria such as sample preparation, sample throughput, robustness, ease of

operation, precision and accuracy are relevant for the consideration of the method choice.74

The multi collector instrument used in these studies was a NuPlasma HR (Nu Instruments Ldt,

Wrexham, U.K.) and was developed between 1996 and 1998,60 when the first results and the setup

were published (Figure 1).73 This instrument has the Nier-Johnson geometry consisting of an electro

static analyser (ESA) with a 70° angle followed by a magnet with a 70° angle. The interface is at high

potential (6kV) and the analyser is grounded. A set of lenses is place after the interface and before

the ESA to extract the ions from the plasma and to shape the circular beam to the defining slit.73 This

slit is used for the selection of resolution and three different slits can be chosen. The alpha slits can

further be narrowed to enhance the resolution. After the magnet, the ions beams are dispersed or

compressed and focussed with a zoom lens into the detector array. The different components of MC-

ICPMS will be discussed in the following subchapters.

1.2.1.1 Inductively Coupled Plasma (ICP)

The inductively coupled plasma was developed by Greenfield et al. in 196461 and the first coupling of

an ICP with a quadrupole mass spectrometer was achieved by Houk et al.19 The ICP is used to

generate the ions, which will be detected in the MS. The Ar plasma is generated by a high voltage

discharge and sustained by an electromagnetic field generated by radio frequency (RF) power

applied to a load coil. The temperature in the plasma is about between 6000 and 10’000K.75 The

aerosol entering the plasma is first desolvated (if liquid), vaporised, atomised, and finally ionised. The

ionisation is achieved by collisions of the analyte atoms with positive Ar ions and free electrons. The

ions are then sampled by the sampler and skimmer cone through the vacuum interface and further

through the ion optic into the mass spectrometer. The interface (approximately 1 mbar) is the first

transfer stage between atmospheric pressure and high vacuum (5x10-9 mbar) in the mass analyser.

Three further chambers with pressures between 5x10-4 and 9x10-8 mbar and in which the ion optic is

located complete the vacuum system. The first extraction voltage is 6kV and is applied to the sampler

and the skimmer cone.

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Figure 1 An overview of the NuPlasma is shown. The main parts of the instrument are listed. The ICP is followed by the ESA, the magnet, the zoom lens and finally the detector block, where the ions are detected (adapted from instrument manual).

1.2.1.2 Electrostatic Analyser (ESA)

After ion formation and charge separation, the highly positive beam is directed into the mass

analyser. To ensure that the dispersion in the magnet is only a function of mass, the energy of the

ions must be the same. The ESA is used for the energy focussing. The force F of a charged particle in

an electric filed is given by the charge q (=ze) and the electric field E:

(22)

combining the centripetal force and the equation (22) gives:

(23)

where m is the mass of the ion, v its velocity and rE the radius of the trajectory. This corresponds to

the motion of a charged particle in an electric field, as in the ESA. The energy of the ions gained in

the extraction optic is defined by:

(24)

where Uoptic is the potential of the ion optics. The velocity is therefore:

√

(25)

combining equation (23) and (25):

(26)

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resolving equation (26) for rE:

(27)

Equation (27) shows that the ESA does not separate ions by mass, but rather by energy. A slit placed

behind the ESA ensures that only ions with nearly the same energies enter the magnetic field. This

ensures a mass-only separation in the magnet.60

Figure 2 The working principle of the ESA. The blue ions all have the same energy, while the red ions have a different energy. The beams with different energies are focused at different points after the ESA. With the slit, a energy filter is achieved and only the ions with the same energy are able to reach the magnet.

1.2.1.3 Magnet

After the ESA, the beam is separated according to the mass to charge (m/z) ratio in the magnetic

field. The behaviour of a charged particle in the magnetic field is described by the Lorentz force:

(28)

Where q is the charge (=ze), v the velocity, m the mass and rB the radius. The particle follows a

curved path perpendicular to the magnetic field with radius rB:

(29)

The radius is a function of the mass and the energy of the ions. The ions are separated by mass and

energy on different curved paths in the magnetic field. The velocity is again described by equation

(25), the radius can consequently be described by:

√

(30)

The radius of an ion at constant magnetic field is dependent on the mass to charge ratio and the

energy (contained in Uoptic). The importance of the ESA is also shown in equation (30). The ESA is

responsible for maintaining a constant energy and thus a constant Uoptic of a same ion mass. Figure 3

shows the dispersion achieved by the magnet.60

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Figure 3 The working principle of the magnet is shown. Dispersion is achieved separating mass to charge ratio and energies. In this case, the energy of the ions is assumed to be the same. The blue and red lines show different masses of ions. The blue ions (lighter) are all collected in one detector, while the red ions (heavier) are collected in another detector.

1.2.1.4 Zoom lens

The collector block of the NuPlasma is fixed; and the ions are directed into the separate detectors by

means of a zoom lens. The zoom lens consists of two lens systems. Voltages applied on these 13 to

15 lens elements create two individual electrostatic fields across the ion path. These fields can

magnify or de-magnify the beam width to match the respective isotope foci with the detectors.

Figure 4 shows the working principle of the zoom lenses, which can be compared to optical lenses. In

the upper row, no voltages are applied on the lenses; the beams are not altered, while in the lower

row, the voltages applied to the zoom lens 1 and 2 de-magnify the beam. The mass separation is the

mass difference between two detectors, and it is defined by the separation of the detectors in the

array, the dispersion of the instrument and the magnification of the zoom lens. The maximum mass

difference between the lightest and heaviest isotopes to be measured in this detector array is 15% of

the lower mass. The two Li isotopes can therefore be measured on the outermost detectors, while

for Pb, 204Pb to 208Pb can be measured in 5 adjacent detectors.

1.2.1.5 Faraday cup (FC)

The detector block of the NuPlasma consists of 12 Faraday cup and 3 ion counting detectors. The

Faraday detectors are small cups with an inner wall of graphite. When ions enter the cup and hit the

wall, the net charge of the detector is changed. To neutralise the cup, the electric charge has to be

equilibrated by electrons. This current causes a current to pass through a resistor, generating a

voltage drop over this resistor. The potential over this resistor is measured and thus the number of

ions can be calculated.76 Faraday cups are robust and have high efficiencies. The saturation limit is at

10 V, on this system, corresponding to about 625 Million ion counts per second. At the lower end of

the dynamic range, however, 1 mV corresponds to 62.5 kcps, which is an issue for low abundant

isotopes. For such cases ion counters can be used.

1.2.1.6 Ion counter (IC)

Three ion counters are available in this instrument and they are placed at the low mass end of the

detector block. When an ion impacts on a surface several secondary electrons are generated. With a

voltage applied between this and an adjacent surface, the electrons are accelerated and will impact

and release several electrons from the second surface, which in turn will be accelerated on a third

one, and so on.77 By the impact of one ion, an avalanche of electrons is produced, which are then

detected. Low noise levels and the high sensitivity make it the detector of choice, when low

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abundant isotopes are measured. The efficiency is lower and they are less robust in comparison with

FC.

The entire detector block is shown in Figure 5.

Figure 4 Working principle of the zoom lens. The upper row shows the ion beams leaving the magnet and entering the detectors without zoom lens. Two beams hit the wall of the collector block. In the lower row, the zoom lenses direct the beams into adjacent cups. The zoom lenses could direct the beams into the outer two cups, depending on the mass separation needed (adapted from instrument manual).

Figure 5 Detector block in the NuPlasma HR MC-ICPMS. Twelve faraday cups and the three ion counters are shown (adapted from instrument manual).

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1.2.2 Sample Introduction Systems The aim of a sample introduction system is to convert the sample into a fine and homogenous

aerosol. Different systems are available for this purpose. The aerosol can be generated starting from

a liquid or from a solid sample. The different sample introduction systems used in the following

chapters will be described.

1.2.2.1 Liquid introduction

To generate a fine aerosol from a liquid, two main components are needed. The nebuliser is

responsible for the aerosol generation, while the spray chamber is responsible for the droplet

selection.78 Different designs of nebuliser and spray chambers are available. The aerosol can then

either be introduced to the plasma directly or it can first be dried from its solvent in a desolvation

unit.

1.2.2.1.1 Wet nebulization

For the direct, wet aerosol introduction, a micro-concentric nebuliser was used (Glass Expansion,

Melbourne, Australia) in this study. In Figure 6, a picture of a micro-concentric nebuliser is shown.

The nebuliser consists of an inner capillary and a concentric outer tube. Through the outer tube, Ar

with a flow rate of about 1 L/min is supplied. At the capillary tip, reduced pressure is created, which

can aspirate the solution. The gas speed is high enough the break the solution in a multitude of tiny

droplets. The sample uptake rate is around 100 µl per minute. The spray chamber used was the

cyclonic spray chamber (Glass Expansion, Melbourne, Australia). The aerosol enters the chamber

perpendicular to the drain-output axis; large droplets collide with the wall, some condense and are

collected at the drain. Only the small droplets (5-10 µm) exit the spray chamber and enter the tube

leading to the ICP. In the system used, the spray chamber was Pelletier-cooled. Both, nebuliser and

spray chamber, were made of borosilicate glass.

Figure 6 A micro-concentric nebuliser77

and a cyclonic spray chamber

78 are shown.

1.2.2.1.2 Desolvating nebulization (DSN)

The aerosols can also be desolvated before entering the ICP. Enhanced sensitivity and lower oxide,

hydroxide and hydride formation are the benefits of a dry aerosol. The desolvation system used in

the following chapters was a membrane desolvator (DSN-100, Nu Instruments Ltd, Wrexham, U.K.).

The DSN is equipped with a micro-concentric nebuliser and a Scott-type spray chamber. The working

principle of the Scott spray chamber is the same as that of the cyclonic spray chamber. The larger

droplets impact the wall, while the small droplets are evaporated by heating the spray chamber to

105°C. The aerosol is then introduced into a heated (110°C) membrane d