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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.

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We re not exactly rocket scientists
“We’re not exactly rocket scientists”

And, luckily we don’t have to be.

Page 2


Why do we calibrate direct vs indirect methods
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.

Page 3


Accuracy and precision
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?

Page 4


Types of errors
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

Page 5


Sources of calibration error
Sources of Calibration Error

  • Improper Blanks

  • Improperly Prepared Calibration Standards

  • Calibration Curve Algorithm Type

27

Page 6


Blank control do you have a contamination problem
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

Page 7


Sources of contamination post sampling
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)

Page 8


Volumetric flasks what are they good for
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.

Page 9


You can do volumetric measurements with a balance
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.

Page 10


Pipettes
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!

Page 11


Element

Conc. ppb

Detection limit

Element

Conc. PPB

Detection limit

Ag

2.33

0.0088

Mn

1.72

0.012

Al

6.43

0.13

Na

19.1

0.6

Be

2.62

0.007

Ni

0.96

0.18

Bi

1.07

0.0006

Pb

5.4

0.13

Ca

18.8

2.9

Sn

0.55

0.0033

Co

2.02

0.004

Th

0.24

0.0003

Cr

0.91

0.28

Ti

0.56

0.003

Fe

1.62

0.75

Tl

1.53

0.0075

Mg

2.56

0.016

Zn

9

0.4

How clean are your pipettes?

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

Page 12


Impurities in container materials

Material

Total No. of Elements

Total

PPM

Major

Impurities

Polystyrene-PS

8

4

Na,Ti, Al

Teflon-TFE*

24

19

Ca,Pb,Fe,Cu

Teflon-FEP*

25

241

K,Ca,Mg

Polycarbonate-PC

10

85

Cl,Br,Al

Low Density PE-LDPE

18

23

Ca,Cl,K

Polypropylene-PP

21

519

Cl,Mg,Ca

Polymethyl Pentene-PMP

14

178

Ca,Mg,Zn

High Density PE-HDPE

22

654

Ca,Zn,Si

Borosilicate Glass

14

497

Si,B,Na

Impurities in Container Materials

Every Standard needs a Container, but Be Careful

*TFE-Tetrafluoroethylene *FEP=FluorinatedEthylenePropylene

Page 13


Plastic packaging container purity
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)

Page 14


Laboratory pure water

ASTM Type

I

II

III

IV

Total matter

(mg/L max.)

<0.1

0.1

1

2

Specific Resist.

(megohm-cm)

15-18

1

>1.0

0.2

pH

NA

NA

6.2-7.5

5-8

Min. color retention time of KMnO4 mins

60

60

10

10

Soluble Silica

ND

ND

10ug/l

high

Bacteria Count

0/ml

0/ml

10/ml

100/ml

Laboratory Pure Water

ASTM Water Specifications

Page 15


Laboratory pure water1
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/

Page 16


Contaminates in nitric acid from major suppliers ppb

Supplier

Metals

Chlorides

Price

Baker

ACS Reagent

2100

100

$56.90/2.5L

ACS NF

1500

100

$57.40/2.5L

Trace Metal

1000

80

$66.4/500ml

Ultrex

3

100

$213/500ml

Fisher

ACS Reagent

4000

80

$51.14/2.5L

ACS NF

NA

500

$55.18/2.5L

Trace Metal

32

NA

$51.4/500ml

Optima

3

NA

$203/500ml

Contaminates in Nitric Acid from Major Suppliers (ppb)

Page 17


Clean laboratory
Clean Laboratory

  • 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

Page 18



Clean techniques
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/

Page 20


Clean instruments
Clean Instruments

Check parts of the instrument that contact the sample.

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.

Page 21


Clean instruments1
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

Page 22


Primary calibration standards match to your task
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

Page 23



Ppb standard stability study
ppb Standard Stability Study

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

  • 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.

Page 25


Experiment results
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

Page 26


How do i know if my primary standard is good
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.

Page 27


Example proper calibration scheme
Example Proper Calibration Scheme

  • Find Linear working Range.

  • Find the range of your samples.

Page 28


Flame cu calibration for samples in the 25 50 ppb range
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

Page 29


Flame cu calibration for samples in the 25 50 ppb range1
Flame Cu Calibration for Samples in the 25-50 ppb range

  • Much better, don’t you think?

  • 0.995 condition satisfied

  • What is a little curve fitting among friends?

Page 30


Flame cu calibration for samples in the 25 50 ppb range2
Flame Cu Calibration for Samples in the 25-50 ppb range

  • 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?

Page 31


Flame cu calibration for samples in the 25 50 ppb range3
Flame Cu Calibration for Samples in the 25-50 ppb range

  • What is wrong with this picture?

Page 32


Flame cu calibration for samples in the 25 50 ppb range4
Flame Cu Calibration for Samples in the 25-50 ppb range

  • What is wrong with this picture?

Page 33


Flame cu calibration for samples in the 25 50 ppb range5
Flame Cu Calibration for Samples in the 25-50 ppb range

  • Look at the change one remade standard can make

Page 34


Calibration mental mistakes review
Calibration Mental Mistakes - Review

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

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

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

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

Page 35


Example lead in calcium nutritional supplements
Example: Lead in Calcium Nutritional Supplements

  • Abstract: ICP-MSIntercalibrated 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 erroneouslylow, 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 standardreference 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) .

Page 36


Example lead in calcium supplements 1 ug g pb
Example: Lead in Calcium Supplements (<1 ug/g Pb)

Possible Pitfalls

  • 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?

Page 37


Icp ms calibration for pb
ICP-MS Calibration for Pb

  • Simple Linear Calibration up to 1.25 ug/L Pb

  • Second Source QC at 1ug/L; +/- 10%

Page 38


Nist 1486 bone meal srm 1 335 0 014 ugpb g
NIST 1486 Bone Meal SRM = 1.335 +/- 0.014 ugPb/g

Page 39


Results for reference materials nist 1400 bone ash nist 1486 bone meal
Results for Reference MaterialsNIST 1400 - Bone AshNIST 1486 - Bone Meal

5% relative Error from Certified Value

Detection Limits for Pb in Calcium Matrix

Page 40



Data reporting
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!

Page 42


More data reporting
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

Page 43


Useful sources and links
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/

Page 44



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