calibration considerations using atomic spectroscopy

Calibration Considerations Using Atomic Spectroscopy

“We’re not exactly rocket scientists”

And, luckily we don’t have to be.

Why do we Calibrate? Direct vs. Indirect Methods

Direct Measurement Method:• The Measurement of a Physical Property.

• Instruments rarely need to be calibrated.

• Examples: Weight and Volume Measurements.

Relative Measurement Method:• Using an instrument which requires calibration prior

to the measurement.

• Many Dynamic System Variables

• Examples: AAS, Quantitative ICP-OES, ICP-MS.

Accuracy and Precision

Accuracy: Usually expressed as error. The difference between a measurement and the True Value is its absolute error (mg/L).• Accuracy can also be expressed as Percent Relative Error.• How much error is in a typical ICP determination?

Precision: Simply the degree of reproducibility of a set of replicate measurements.

• Precision can expressed by Standard Deviation (SD) or Percent Relative Standard Deviation (%RSD).

• What is typical precision for a set of GFAAS replicates?

Types of Errors

Determinate Errors:• Have specific, identifiable, and correctable causes.

• Examples: Contaminated Method Blank, Incorrect Standard Concentration.

• Usually main source(s) of most error, can be large.

Indeterminate Errors:• Random

• Frequently from Multiple Sources

• Examples: Flicker (Nebulizer) Noise in an ICP, Mechanical Vibrations, Electronic Noise.

• Hopefully small in magnitude

• Usually determines detection limits

Sources of Calibration Error

1. Improper Blanks

2. Improperly Prepared Calibration Standards

3. Calibration Curve Algorithm Type

27

Blank Control: Do you have a Contamination Problem?

If you are reporting negative answers,

you could have a contamination problem!

Run lots of different blanks and compare results

• Different sources of water

• Different sources of acids

• Different Flasks

• Different Analysts

Run blanks overnight and check stability

• Checks cleanliness of the instrument

Sources of Contamination (Post-sampling)

Analytical Containers (Volumetric flasks, pipettes,..) Storage Containers (bottles) Lab Reagents (including lab pure water) Lab Environment (dust) Analyst (yes, you!) Instrumentation (carry-over)

Volumetric Flasks … What are they good for?

Very Accurate, but do you really need them? They are NOT for storage! They are NOT for digestions!

Clean with 10% HNO3 4 hours

Rinse with lots of lab pure water Store filled with lab water Rinse out prior to use

Do you use graduated Cylinders? Why?

You can use 50mL Autosampler Vials for accurate volumetric measurements. Advantage is less potential sample contamination.

You can do Volumetric measurements with a Balance

1 mL of distilled water weighs 1 gram

Minimize the number of container surfaces the sample touches.

Pipettes

Glass Pipettes: Very Accurate, but do you really need them? Use only larger volumes (>10mL). Clean, check delivery. Re-clean Store dry, away from dust

Use Pipettors with disposable tips whenever possible Don’t contaminate the tips!

How clean are your pipettes?

2% Nitric acid run through 5mL pipets and scanned on ICPMS

How clean are your pipettes?

2% Nitric acid run through 5mL pipets and scanned on ICPMS

0.49Zn0.0162.56Mg

0.00751.53Tl0.751.62Fe

0.0030.56Ti0.280.91Cr

0.00030.24Th0.0042.02Co

0.00330.55Sn2.918.8Ca

0.135.4Pb0.00061.07Bi

0.180.96Ni0.0072.62Be

0.619.1Na0.136.43Al

0.0121.72Mn0.00882.33Ag

Detection limit

Conc. PPBElementDetection limit

Conc. ppbElement

Si,B,Na49714Borosilicate Glass

Ca,Zn,Si65422High Density PE-HDPE

Ca,Mg,Zn17814Polymethyl Pentene-PMP

Cl,Mg,Ca51921Polypropylene-PP

Ca,Cl,K2318Low Density PE-LDPE

Cl,Br,Al8510Polycarbonate-PC

K,Ca,Mg24125Teflon-FEP*

Ca,Pb,Fe,Cu1924Teflon-TFE*

Na,Ti, Al48Polystyrene-PS

Major

Impurities

Total

PPM

Total No. of Elements

Material

*TFE-Tetrafluoroethylene *FEP=FluorinatedEthylenePropylene

Impurities in Container Materials

Every Standard needs a Container, but Be Careful

Plastic Packaging Container Purity

FEP (FLUORINATEDETHYLENEPROPYLENE)PFA (PERFLUOROALKOXY)FLEP (FLUORINATED HIGH-DENSITY POLYETHLYENE)PMP (POLYMETHYLPENTENE)PP (POLYPROPYLENE)HDPE (HIGH-DENSITY POLYETHYLENE)LDPE (LOW-DENSITY POLYETHYLENE)

Laboratory Pure Water

100/ml10/ml0/ml0/mlBacteria Count

high10ug/lNDNDSoluble Silica

10106060Min. color retention time of KMnO4 mins

5-86.2-7.5NANApH

0.2>1.0115-18Specific Resist.

(megohm-cm)

210.1<0.1Total matter

(mg/L max.)

IVIIIIIIASTM Type

Laboratory Pure Water

The Direct-Q ultrapure water system produces 18.2 Megohm-cm reagent water containing less than 30 ppb Total Organic Carbon directly from potable tap water. The system is ideal for scientists needing 5 to 15 L/day of ultrapure water for the preparation of culture media, buffers, blanks and standard solutions.

www.elgalabwater.com/

http://www.elgalabwater.com/

$203/500mlNA3Optima

$213/500ml1003Ultrex

$51.4/500mlNA32Trace Metal

$55.18/2.5L500NAACS NF

$51.14/2.5L804000ACS Reagent

Fisher

$66.4/500ml 801000Trace Metal

$57.40/2.5L1001500ACS NF

$56.90/2.5L1002100ACS Reagent

Baker

PriceChlorides MetalsSupplier

Contaminates in Nitric Acid from Major Suppliers (ppb)

• Environment of class 100 (less than 100 particles of 0.3microns per m3)• Walls, ceilings and floors sealed and dust free• HEPA filters mounted in the ceiling• No fuming Acids• All work performed under clean hood

Clean Laboratory

Airborne Contaminants (ug/g)

Clean Techniques

• No jewelry, cosmetics or lotions

• Wear gloves, Powder-Free

• Cover hair and mouth

• Beware of dust, airborne fallout, cover samples

How do you determine if you have a clean lab?

By running blanks!

http://terrauniversal.com/

http://www.aircleansystems.com/

http://terrauniversal.com/

Clean Instruments

AA Instruments Graphite Components

• Modified Contact Cylinders: Exhibit less carry-over and cross contamination for samples with high dissolved solids content.

• UltraClean Graphite Tubes: Deliver exceptionally low levels of residual contamination due to extra high-temperature gas-phase cleaning procedure. Extremely low traces of Na, Ca, Fe, Al, Si, Ti, Cr, Ni.

Check parts of the instrument that contact the sample.

Clean Instruments

ICP-OES Glass Spray Chambers Quartz Nebulizers Ryton Spray Chambers Teflon(s) Polyethylene Sample Tubes PEEK Alumina Injectors

ICP-MS Platinum Cones, Injectors Quartz Spray Chambers Sapphire Injectors

Primary Calibration Standards – Match to your Task

AA Grade• Single Element Accuracy

• Stability

• Traceability

ICP Grade• Accuracy

• Purity

• Stability

• Chemical Compatibility

• Traceability

• Often You can Choose Acid Matrix

Multi Element • Reliable if you need lots of elements

• More Expensive

ppb Standard Stability Study

A blend of 65 elements from Inorganic Ventures / IV Labs' CMS-SET was prepared at the 0, 2, 10, and 100 ppb concentration level in 1 % (v/v) HNO3 at the start of the study.

The set consists of the following;

CMS-1 - 10 µg/mL Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Sc, Tb, Th, Tm, U, Yb, Y in 3.5 % HNO3

CMS-2 - 10 µg/mL Au, Ir, Pd, Pt, Re, Rh, Ru, and Te in 3.5 % HCl

CMS-3 - 10 µg/mL Ge, Hf, Mo, Nb, Ta, Sn, Ti, W, and Zr in 3.5 % HNO3 tr. HF

CMS-4 - 10 µg/mL Sb, As, Ba, Be, Bi, B, Cd, Ga, In, Pb, Se, Tl, and V in 3.5 % HNO3

CMS-5 - 10 µg/mL Ag, Al, Ca, Cs, Cr+3, Co, Cu, Fe, Li, Mg, Mn, Ni, K, Rb, Na, Sr, and Zn in 3.5 % HNO3

The LDPE bottles were acid leached with 1% nitric acid for 59 hours at 60 °C. New blends prepared in the same way were compared to the original preparation at 1, 3, 25, 75, 137, 300, and 375 days.

http://www.ivstandards.com/tech/reliability/part07.asp

http://www.ivstandards.com/products/go.asp?product=CMS-5





Experiment Results -

Hg was not stable long enough to measure (minutes).

Au was the next most unstable element, showing instability at the 2, 20, and 100 ppb levels at 3 days.

Pd showed instability only at the 2 and 10 ppb levels at 3 days.

Pt and Ta showed instability only at the 2 and 10 ppb levels at 137 days.

Ag showed instability only at the 10 and 100 ppb levels at 137 days.

Mo, Sn, and Hf showed instability only at the 2 ppb level at 375 days.

Ir showed instability only at the 2 ppb level at 300 days.

All other elements showed no instability at 2-100 ppb for 375 days, including: Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Sc, Tb, Th, Tm, U, Yb, Y, Re, Rh, Ru, Te, Ge, Nb, Ti, W, Zr, Sb, As, Ba, Be, Bi, B, Cd, Ga, In, Pb, Se, Tl, V, Al, Ca, Cs, Cr+3, Co, Cu, Fe, Li, Mg, Mn, Ni, K, Rb, Na, Sr, and Zn.

Paul Gaines, Ph.D.Author of Reliable Measurements and other guides

http://www.ivstandards.com/tech/reliability/default.asp

How do I know if my Primary Standard is Good?

Check against a Second Source or SRM• NIST, NRC Canada, Brammer…

Check Characteristic Concentration Flame AAS.• Also can use Sensitivity Check

Check Characteristic Mass Graphite Furnace AAS (M0).

For ICP and ICP-MS, you can check (count/sec) Intensity History.

Example Proper Calibration Scheme

Find Linear working Range. Find the range of your samples.

Flame Cu Calibration for Samples in the 25-50 ppb range

Calibration not quite good enough. Let’s try something… anything. Must meet 0.995 Law


Much better, don’t you think? 0.995 condition satisfied What is a little curve fitting among friends?


Oh yes, this is the answer: Linear Fit with much higher standards 0.995 Law more than satisfied, cc=0.997 Problem solved! Or is it?


What is wrong with this picture?


Look at the change one remade standard can make

Calibration Mental Mistakes - Review

1. Choosing a curve algorithm to fit data which you know should be linear.

2. Being a “slave” to arbitrary rules like “c.c. must be > 0.995”.

3. Using standard concentrations which are way too high, way beyond your expected sample range, just to get better c.c. statistics.

4. Being lazy, re- make the standards and /or run a second source standard.

Example: Lead in Calcium Nutritional Supplements

Abstract: ICP-MS

Intercalibrated measurements of lead in calcium supplements indicate the importance of rigorous analytical techniques to accurately quantify contaminant exposures in complex matrices. Without such techniques, measurements of lead concentrations in calcium supplements may be either erroneously low, by as much as 50%, or below the detection limit needed for new public health criteria. In this study, we determined the lead content of 136 brands of supplements that were purchased in 1996. The calcium in the products was derived from natural sources (bonemeal, dolomite, or oyster shell) or was synthesized and/or refined (chelated and nonchelated calcium) . The dried products were acid digested and analyzed for lead by high resolution-inductively coupled plasma-mass spectrometry. The method's limit of quantitation averaged 0.06 µg/g, with a coefficient of variation of 1.7% and a 90-100% lead recovery of a bonemeal standard reference material. Two-thirds of those calcium supplements failed to meet the 1999 California criteria for acceptable lead levels (1.5 µg/daily dose of calcium) in consumer products.

Environ Health Perspect 108:309-313 (2000) .

Example: Lead in Calcium Supplements (<1 ug/g Pb)

Sample Prep; 0.5g sample to 500 mL with acid dissolution.• Sample prep may contaminate samples low level Pb• We will need to accurately measure below 1 ug/L for Pb• Check Acid Reagent Blanks• Check Method Blanks – acids plus containers

Is my instrument clean enough for sub ppb work?• Replace or clean any contaminated parts, like cones, injector, …• Check blanks

What is the best primary standard to use?

What is Best Calibration Range and Curve Type to use?

Is a similar matrix SRM available?

ICP-MS Calibration for Pb

•Simple Linear Calibration up to 1.25 ug/L Pb

•Second Source QC at 1ug/L; +/- 10%

NIST 1486 Bone Meal SRM = 1.335 +/- 0.014 ugPb/g

Results for Reference Materials

NIST 1400 - Bone AshNIST 1486 - Bone Meal

Sample ID

Measured Conc (g/g)

NIST Certified Value (g/g)

10 g/g pre-dissolution spike recovery

0.1 g/g Post-dissolution spike recovery

SRM 1400 9.10 0.11 9.07 0.12 106% 109% SRM 1486 1.207 0.008 1.335 0.014 101% 99%

Matrix Type

Nitric Acid IDL =

(3)(1% nitric acid)

Calcium Phosphate MDL =(3)(matrix)

Solution 0.001 g/L 0.005 g/L

Solid 0.001 g/g 0.005 g/g

Detection Limits for Pb in Calcium Matrix

5% relative Error from Certified Value

Results for Calcium Tablets

Sample ID Mean (ug/g) %RSD (n=3) 0.05ug/g

%Spike Recov

Tricalcium

Phosphate -A

0.105 0.88 99

Tricalcium

Phosphate -B

0.108 0.60 92

Calcium

Carbonate

0.315 1.03 90

Antacid -A 0.114 2.84 93

Antacid -B 0.259 1.28 106

Data Reporting

No analysis is complete until the final results have been correctly calculated and properly reported. The report should give the best values obtained and also indicate the probable accuracy or reliability of the results.

A single result can express the degree of uncertainty by the number of

Significant Figures.• For Example; A weight given as 0.5 g implies that a rough type of balance was used

and that the actual weight is between 0.45 and 0.55 g.• Furthermore, any subsequent computation using the 0.5 g weight in the calculation of

a final value cannot contain any more than 1 significant figure. Obviously, a calculator or computer cannot improve the precision of the original data!

Expression # of Sig. Figs.

5.063 4

3600 2

3.600x103 4

0.00123 3

More Data Reporting

Standard Deviation and % Relative Standard Deviation can indicate the reliability of the method of measurement. Example:

MEAN (n=3) SD %RSD

27.6 ug/L 0.35ug/L 1.27%

QC or SRM Measurement Accuracy is commonly expressed as Percent Recovery rather than Percent Relative Error. Example:

MEAN (SD) Known QC % Recovery

19.3 (0.22) ug/L 20.0 ug/L 96.5%

%Recovery = 100 – (Known-Measured)/Known *100

Useful Sources and Links

http://ts.nist.gov/measurementservices/referencematerials/index.cfm Nation Research Council Canada http://www.nrc-cnrc.gc.ca/ BRAMMER http://www.brammerstandard.com/

http://www.standardmethods.org/ American Water Works Asso. http://www.awwa.org/

http://www.astm.org/ http://www.astm.org/cgi-bin/SoftCart.exe/SNEWS/MA_2008/

index.html?L+mystore+eswo6699

http://www.inorganicventures.com/tech/reliability/ http://www.spexcsp.com/ http://www.highpuritystandards.com/

http://ts.nist.gov/measurementservices/referencematerials/index.cfm

http://www.nrc-cnrc.gc.ca/

http://www.standardmethods.org/

http://www.inorganicventures.com/tech/reliability/

http://www.spexcsp.com/

http://www.highpuritystandards.com/

http://www.standardmethods.org/

Thanks for Your Time !

calibration considerations using atomic spectroscopy

Documents

instrument slide

sources of calibration

lab water

detection limits slide

atomic spectroscopy

different blanks

multiple sources examples

relative measurement