Microchim Acta 150, 55–58 (2005)

DOI 10.1007/s00604-005-0332-9

Original Paper

Systematic Errors in the Determination of Trace Metals,Part II. Memory Effects of Quartz Vessels Used for SamplePreparation in the Determination of Ultra-Trace Levels of Platinum

Joanna Kowalska1;�, Monika Asztemborska1, Beata Godlewska-Zylkiewicz2,

and Jerzy Golimowski1

1 Chemistry Department, Warsaw University, Pasteura 1, PL-02-093, Poland2 Institute of Chemistry, University of Bialystok, Hurtowa 1, PL-15-399, Poland

Received August 19, 2004; accepted January 19, 2005; published online March 21, 2005

# Springer-Verlag 2005

Abstract. The influence of different cleaning proce-

dures applied to quartz vessels used for sample pre-

paration on the Pt contents in blank solutions was

studied. The platinum concentration was determined

by inductively coupled plasma mass spectrometry.

The strong dependence of the memory effect of the

quartz crucible on the previously stored Pt content of

the sample was observed. It was shown that cleaning a

quartz crucible previously used for Pt determinations

with hot nitric acid was insufficient. When heating the

vessels with a mixture of nitric, perchloric and hydro-

chloric acid, considerable amounts of Pt are released.

In the case of new crucibles, the platinum contents in

blanks were very low after cleaning with hot nitric acid.

Key words: Systematic errors; platinum; quartz vessels; ICP-MS.

Platinum determination in various geological, indus-

trial and environmental samples is still a challenge for

many analytical laboratories. Among different analyt-

ical techniques, inductively coupled plasma mass

spectrometry (ICP-MS) [1, 2] and adsorptive stripping

voltammetry (AdSV) [3, 4] are usually used to deter-

mine pg levels of Pt.

Most analytical problems occurring during Pt deter-

mination arise from the complex nature of its solution

chemistry and the numerous interactions between the

analyte and the constituents of the sample matrix,

which can significantly influence the limit of detection

(LOD) and the accuracy of determinations [5]. The

high procedural blank and=or losses of analyte during

pretreatment steps may be a source of systematic

errors during Pt determinations [6]. The high tendency

of Pt towards hydrolysis resulting in the formation of

several mixed aquo=chloro complexes of Pt(II) and

Pt(IV) [7, 8] and sorption on the walls of the storage

vessels [9–11] as well as the reduction of its oxida-

tion state may cause analyte losses and transforma-

tions during storage and sample pretreatment steps. It

is well-known that Pt forms very stable complexes

with chloride ions. Therefore, all solutions containing

traces of platinum should contain sufficient amounts

of chloride ions to prevent platinum losses on the

walls of quartz and polycarbonate containers [11].

The necessity of applying high chloride concentra-

tions in sample=standard solutions of Pt was shown

in the Wickbold combustion process performed in

quartz vessels [12]. However, even in the presence

of substantial amounts of chlorides, platinum loss

was found to occur during storage of its standard� Author for correspondence. E-mail: askow@chem.uw.edu.pl

solutions in vessels made of polyethylene of different

densities and PTFE [9, 10]. Hidalgo et al. [9] reported

that only polystyrene containers provided good stabil-

ity of the Pt chlorocomplexes in aqueous solutions.

A digestion step usually precedes Pt determination

in real samples. The kind of digestion procedure

depends on the type of matrix and the method of detec-

tion. When samples are dissolved in aqua regia, a pre-

treatment procedure must include elimination of the

excess acids from the solution, e.g. by evaporation

[2, 5], as an acid concentration that is too high may

cause corrosion of some parts of the apparatus during

Pt determination by ICP-MS. Elimination of nitrates is

also necessary before Pt determination by AdSV, since

the presence of nitrites in supporting electrolyte can be

a source of strong interference [3, 4, 11]. High purity

quartz is the common material used for pressure vessels

of high pressure digestion systems as well as for cru-

cibles used for evaporation of excessive amounts of

acids before analyte determination. Alt et al. [10] found

that the use of quartz vessels can prevent the loss of Pt

during the decomposition process; however, when sam-

ples with different Pt concentrations were subsequently

decomposed in such vessels, significant memory

effects were observed [3, 10]. A special three-step

cleaning procedure to eliminate Pt contamination from

the surface of the vessel was proposed by Alt et al.

[10]. During these operations, the blank was reduced

to below the LOD of graphite furnace atomic absorp-

tion spectrometry (40 ng mL�1). However, this proce-

dure was not effective when ultra-traces of Pt were

determined by AdSV. A very unpredictable Pt blank,

varying over a wide range (18–360 pg), was detected

by Hoppstock et al. [3], even when the vessels under-

went a repeated, drastic cleaning procedure, such as

cleaning decomposition. Therefore, for the determina-

tion of extremely low Pt content, only vessels made of

synthetic high purity quartz (‘‘Suprasil’’) were applied.

With these quartz vessels, metal ion diffusion into the

material is slower compared to normal quartz glass,

and vapor-cleaning with nitric acid can be performed

successfully.

The aim of this study was to examine the occurrence

of systematic errors during acid evaporation from the

digested samples. The influence of quartz crucibles,

previously used for Pt determination or completely

new ones, on platinum content in blanks, obtained after

evaporation of nitric, perchloric and hydrochloric acid

and estimation of the potential risk of contamination

during this analysis step was studied.

Experimental

Apparatus

An Inductively Coupled Plasma Mass Spectrometer SCIEX

‘‘Elan 6100 DRC’’ Perkin Elmer (USA) was used for platinum

determination.

The following ICP MS operating parameters were used: R.F.

generator 1100 W, lenses voltage 7 V, nebulizer gas flow

0.98 L min�1, plasma gas flow 15 L min�1, dwell time 0.1 s.

Reagents and Solutions

HNO3 (d¼ 1.40 g mL�1), HCl (d¼ 1.15 g mL�1), H2SO4

(d¼ 1.84 g mL�1), HClO4 (d¼ 1.67 g mL�1), Suprapur (Merck).

Standard solutions of Pt(II) containing 1 mg mL�1 were prepared

from ampoules of AAS standard (Merck). All solutions were

prepared using de-ionized water from Milli-Q-Water-System

(Millipore, USA).

Procedures

For the studies three sets of quartz crucibles were selected. Series I

(n¼ 12): crucibles which had been used for microwave digestion of

hydroponically cultivated Indian mustard or Anawa maize plants

containing high amounts of Pt (5–15 mg kg�1); series II (n¼ 12):

completely new quartz crucibles, not used before; series III

(n¼ 8): crucibles used for recovery studies of Pt using standard

solutions.

Cleaning procedure: the quartz crucibles were initially cleaned

according to the standard procedure, i.e. in hot nitric acid (1:5, v=v)

for 2 hours and then rinsed with MQ water and dried. Then a

mixture of 1 mL HNO3, 1 mL HClO4 and 0.5 mL HCl was added

into each crucible. The crucibles were covered with quartz lids and

heated on the electric heater for 0.5 h, then the lids were removed

and heating was continued to evaporate the acids. The heating

procedure was stopped when the acid residue volumes were about

0.05 mL. Then MQ water was added to each crucible to make up a

volume of 5 mL and the Pt content was determined by ICP-MS.

To control the efficiency of elimination of Pt contamination, the

evaporation of HNO3, HClO4 and HCl was repeated three times in

some crucibles of series I.

Results and Discussion

Platinum occurs in biotic and environmental materials

in the range of a few ng kg�1, so its concentration in

digested sample solutions is in the range of a few

pg mL�1. In order to eliminate excess acids and to

preconcentrate the analyte, evaporation of the digested

solutions from quartz crucibles is often performed.

This step of the pretreatment procedure is often ne-

glected when considering the sources of systematic

errors in analytical procedure. However, at such low

analyte levels, it seems extremely important to control

Pt losses or contamination of the samples during this

analysis stage.

In our experiments we determined the Pt content

that had leached from completely new quartz

56 J. Kowalska et al.

crucibles during evaporation of acids and from quartz

crucibles formerly used for evaporation of acids from

the aliquot of the digested samples containing high

analyte concentrations. It was found that during acid

evaporation from crucibles previously used for diges-

tion of plants (series I), considerable amounts of Pt

(230–1630 pg) were released (Table 1). After acid

evaporation from new crucibles (series II), the plati-

num content never exceeded 50 pg, which is accep-

table in most cases. In the third group of crucibles

(series III), which was used for the experiments with

Pt standard solution for 6 crucibles, the obtained

results were comparable with those obtained for new

crucibles, while in 2 cases the Pt content was higher.

These experiments demonstrated that quartz crucibles

used for evaporation of acids from the aliquot of the

digested sample solution could be a significant source

of systematic error during Pt determination. Such high

blank values are not acceptable when determining Pt

at the pg mL�1 level.

In the next experiment, successive evaporation of

the acids from the same crucible (of series I) was

repeated twice to check if Pt would still be released.

The obtained results (given in Fig. 1) show that only

in one case does the Pt content in the solution achieve

the level of the blank (approx. 20–30 pg) after the

second step of acid evaporation. In the case of the

other crucibles (3 and 4), the platinum content in

the obtained solution was not acceptable even after

the third evaporation. These results are in agreement

with the results of other authors [3, 10] concerning the

removal of contamination from quartz vessels during

a three-steps cleaning procedure (blanks in the range

of 30–350 pg).

It is known that quartz surfaces contain varying

amounts of silanol groups which exhibit ion-exchange

properties. These groups are usually saturated with pro-

tons by the usual treatment of quartz vessels after

digestion with nitric acid vapors (or a mixture of nitric

and hydrochloric acids). This treatment is usually very

advantageous, since most of the trace elements occur in

digested solutions in the form of cations. Yet it is ex-

tremely undesirable in the case of Pt, as it is present in

the form of anions (PtCl62�, PtCl4

2�). The changes in

surface morphology are a function of the number of di-

gestions carried out in the quartz vessel, and adsorption

as well as leaching phenomena increase with rising

surface roughness of a material [13]. Despite this fact

such vessels are usually applied hundreds of times in

decomposition procedures which can promote the pro-

cess of analyte adsorption on the surface of the vessels.

It is clear that extreme care must be taken when

using old quartz crucibles for sample pretreatment

before the determination of pg amounts of Pt.

Conclusions

Only new quartz crucibles cleaned with hot nitric acid

can be used for Pt determination when acid evaporation

from the aliquot of the sample is required. Otherwise

this step of analysis can be a significant source of sys-

tematic error during Pt determination resulting in exces-

sively high platinum content in the analyzed materials.

Table 1. Platinum content in samples obtained after acid

evaporation from quartz crucibles (series I–III)

Platinum content [pg]

Sample Crucibles I

previously used

for plant

digestion

Crucibles II

new ones

Crucibles III

previously used

for experiments

with Pt standard

solutions

1 341 � 3 40.0 � 6.3 46.3 � 4.4

2 232 � 11 32.9 � 3.5 38.6 � 5.3

3 652 � 30 27.1 � 3.4 76.3 � 11.1

4 483 � 23 25.5 � 4.4 22.9 � 4.9

5 1633 � 26 10.0 � 3.1 39.7 � 5.6

6 312 � 9 40.0 � 5.2 123.8 � 7.3

7 383 � 10 15.0 � 5.7 31.8 � 5.8

8 404 � 11 45.0 � 5.9 29.2 � 3.3

Mean 524 29.4 51 Fig. 1. Platinum content released during subsequent evaporation

of acids from quartz crucibles previously used for digestion of

plants with high platinum content (different bars represent the

results obtained after subsequent cleaning cycles)

Systematic Errors in the Determination of Trace Metals 57

When applying quartz crucibles, which were pre-

viously used for Pt analysis, in the determination of

pg mL�1 Pt, special attention must be paid to the

cleaning of the analytical vessels prior to their utiliza-

tion in analysis.

Acknowledgements. This study was supported by grant No. 120-

501=68-BW-1602=4=2003.

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58 Systematic Errors in the Determination of Trace Metals