Measuring of samples with low concentrations ... · Measuring of samples with low concentrations: contamination problem and clean working conditions Anna Kowalska, Zuzanna Cieśla

Measuring of samples with low concentrations: contamination problem

and clean working conditions

Anna Kowalska,Zuzanna Cieśla

Meeting of the Heads of the Laboratories – Pallanza, 7-8 September 2017

Low concentrations


What are low concentrations? - Below 100 ppm?- Below 0,01 ppm?- Close to LOQ?

Are samples with low concentrations important? Are we really interested in low concentration samples? What is the % of the samples of low concentration in forest monitoring? Meaningful?

Why it is important to deliver precise and accurate values for low concentration?

- Because small errors sum up and cause problems e.g. in canopy bugdet models:

TFWA = TF∑Kat - TF∑An and BDWA = BD∑Kat - BD∑An

and in quality checks

Low concentrations in 8th WRT


Sample Cond pH Cl N-NO3 P-PO4 S-SO4N-NH4 Ca Mg Na K Alk. DOC Ntot

Deposition M M M M O M M M M M MM

pH>5M M

Soilsolution

M M M M - M M M M M M M M M

S 1 Stemflow H L H L H H H H L H H L x H

S 2 Throughfall H L L L x L L H L H H L H H

S 3 Bulk Depo L L L L x L H H L L L L L H

S 4 Soil Sol. H H H L x H x H H H L x H H

S 5 Soil Sol. H H H H H H L H H H H x H H

S 6 Syntetic H

H - concentration range highL - concentration range lowx - sample/parameter excluded

0,3-0,8 mg/l

12 µeq/l

0,5 mg/l

pH5,9 0,1

mg/l

0,8-3,9 mg/l

The lower concentration of a parameter in sample the more labs tends to fail in WRT for this sample

Factors that affect the content of trace elements in water


Factors that can affect concentrationlevels of trace components in a liquid sample:1 - contact of sample with laboratory air; 2 - residues of components of dishwashing mixtures; 3 - distilled water; 4 - reagents and solvents employed; 5 - contact with analyst;6 - vaporization of the most volatile components; 7-8 - processes of adsorption-desorption (wall memory effect); 9 - adsorption of analytes on suspension; 10 - precipitation of sediment; 11 - leaching of components from vessel; 12-13 - permeation of solution components through vessel walls; 14 - reaction of analyte with vessel material; 15 - chemical reactions among solution components

Namieśnik J., 2002 Critical Reviews in Analytical Chemistry, 32(4):271–300.

Sources of contamination and error in low concentration analysis


Pipetting –Changing the tips (single use) after each sample → cross-contamination of samplesStorage in vertical position → contamination of the inside of the pipetteTricky two-step button… → wrong volume sampled

Rinsing instruments between samples – with what solute, what volume and how long/howmany times – optimizing analytical method

Operations



Opening, reopening bottles many times → PO4?

Ambient conditions during analysis – dusts in the air, gases, temperature, etc.

As much particulates as 200 µg/m3 have been reported in the air of an analytical laboratory, they contained large amounts of Ca, Si, Fe, Al, Na, Mg, K, Cu, Mn and lesser amounts of several other elements (T. Murphy, National Bureau of Standards Special Publication 422, "Accuracy in Trace Analysis:

Sampling, Sample Handling, and Analysis", Proceedings of the 7th IMR Symposium).

See also Carmen I. presentation from 1st Meeting of the HoL in 2008, Hamburg – runningacid mineralisation of soil or foliar material back to back with analysis of precipitationsamples; vapours of acids as a potential contaminant of water samples

Operations



Storage → Time of storage, temperature, light, preservation

freezing → concentration of phosphate steadily decreased in samples stored for longer than 4 months in -20degC, no effect on nitrate conc. (Clementson L.A.,. Wayte S.E., 1992, Wat. Res. 26:1171-1176).

See also presentation by Nils K. and others from 2013, Meeting of the HoL in Zadar, Effects of storage in field and in laboratory and influence of temperature and light on the chemistry of forest water samples.

Dissolving of metals after acidification of water sample

Operations

Sources of contamination


Filters –Paper filters – NH4+, Ca, Mg releasingFilter material may be adsorbing some metal from the solution phase or may be releasing

metals to the solution (Hedberg et al 2011).

Blank values of 10 elements in 19 different filter types from Gelman, Millipore, Nucleporeand Whatman have been compared by extraction with dilute nitric acid:

Glass fiber filters - high blank values for a number of elements,PTFE filters - the cleanest ones,Most filter - high blank values of one or more elements of environmental interest (Cr, Zn,

B, Cr, Mg, Mo, Na, Zn) (Berg. T, Royset. O. 1993. Atm. Env., 27:12435-2439).

Whatman 1.2 µm pore size glass microfibre –

Materials, consumables

(Matoug, I.O.M., 2013, IOSR-JESTFT, 3(5):59-61.)



Labware: bottles, caps, flasks… -Cd from red caps on plastic bottles, Hg, Zn from plastic bottles, Fe, Na from glass bottles.

Al, Pb, Ca are also common contaminants (source: Hill Laboratories, New Zealand)

N from bottle caps – see Daniel Ž. presentation from 1st Meeting of the HoL in 2008 in Hamburg and 2nd in 2009 in Warsaw - N determination by peroxodisulfate digestion in autoclave followed by spectrometer determination of nitrate at 220 nm in SCHOTT DURAN autoclavable bottles 50 mL

Ca, Mg realeasing from walls of the PE bottles that were used for samples containing high concentration, see Carmen I. presentation from 1st Meeting of the HoL in 2008, Hamburg

Ageing of the materials of which the bottles are made




Clothes –Gloves not containing talc powder → Mg contaminationGloves → contact of samples/instruments (pipette, plastic and glass labware,..) with skin

has to be avoided


Human sweat - Na 863 µg/mL; K 222 µg/mL, Ca 16 µg/mL, Mg 1,3 µg/mL, Zn 355-655 µg/mL, Cu 80 ng/mL.Montain SJ, Cheuvront SN, Lukaski HC. Int J Sport Nutr Exerc Metab. 2007;17(6):574-82

Concentrations of major (Na, K, Ca, and Mg) and trace elements (Zn, Cu, Fe, Ni, Mn, and Cr) in sweat tends to be lower in summer than in winter. Hoshi A., Watanabe H., Chiba M.,Inaba Y., Kobayashi M., Kimura N., Ito T., 2002. Environ Health Prev Med, 7:60-63

https://www.ncbi.nlm.nih.gov/pubmed/18156662



Sources of contaminations


Pipes, tubes –→ algae growth in tubes→ sorbed and not removed/rinsed residues from past samples in tubing, detector cells etc.Replace when a bends or crincks are visible in the tubing

Torch in ICP-OES - clean or change when the torch begins to build up residue

Parts of the instruments



Reagent grade/purity

Reagent manufacturer

Reagent LOT

Reagents after the expiration date?

Reagents



Water

(source: Dr Paul Whitehead, ELGA LabWater materials)

←splashing

← collecting waterby flowing it along walls, no splashing

When ultra-pure water is dispensed it is important that there is as little air entrainment as possible.

The contamination from the atmosphere was compared by anion analysis by ion chromatography:

Measures to improve analysis of low concentration samples


Washing and cleaning- Use of tested dishwashing agents, develop procedures for cleaning, washing, drying of

labware- Test the vessels before use and test occasionally later

Methods- Proper validation and optimization,- Spectral interferences – background correction,- Internal standards,- …

Calibration –See the presentation of Gabriele T. and Aldo M. from Meeting of the HoL in 2011, Arcachon: Measurements of low values in IC, Examples for SO4=, NH4+ and Na+.



Calibration cont. - calibration range low/high; example: NH4+ in WRT samples, Ion chromatography, Dionex

system, MSA eluent,

No. Sample

Calibrationlow range (0,01-1,4

mg/L)

Calibrationhigh range

(1-10 mg/L)

high/lowin %

NH4+ NH4+1 C17-ICR 024 (0,2M) 0,2058 0,0438 21,29%2 C17-ICR 024 (0,2M) 0,2056 0,0434 21,09%3 CW14-2 LRM 0,2552 0,0880 34,49%4 CW16-1 LRM 1,2060 1,2002 99,52%5 Sample 1/ bottl. 97 1,1381 1,1223 98,61%6 Sample 1/ bottl. 104 1,1243 1,1063 98,41%7 Sample 2/ bottl. 195 0,3519 -0,1959 -55,68%8 Sample 2/ bottl. 199 0,3467 0,1772 51,10%9 Sample 3/ bottl. 291 0,6364 0,5434 85,40%

10 Sample 3/ bottl. 293 0,6354 0,5424 85,36%11 Sample 4/ bottl. 389 0,0326 -0,1581 -485,19%12 Sample 4/ bottl. 391 0,0244 -0,1827 -750,11%13 Sample 5/ bottl. 481 0,1871 0,0153 8,15%14 Sample 5/ bottl. 487 0,1949 0,0325 16,67%



Calibration cont. - fitting of the calibration curve: example of anion analysis, Dionex system, eluent NaHCO3-

Na2CO3:

No. Ret.Time Peak Name Cal.Type Points Offset Slope Curve Coeff.Det.

ECD_1 ECD_1 ECD_1 ECD_1 ECD_1 ECD_1 ECD_1 ECD_1 ECD_1

min (C0) (C1) (C2) %

1 5,28 Cl Y0LOff 7 -0,0048 0,3924 0,0000 99,7005

No. Ret.Time Peak Name Cal.Type Points Offset Slope Curve Coeff.Det.

ECD_1 ECD_1 ECD_1 ECD_1 ECD_1 ECD_1 ECD_1 ECD_1 ECD_1

min (C0) (C1) (C2) %

1 5,27 Cl Y0QOff 7 0,0016 0,3409 0,0069 99,9926

Bad choice: Linear with 0 point for curve fitting

Better choice: Quadratic with 0 point for curve fitting



The lower the required detection limit for the parameterthe more important it is to analyse blanks.

Bad habits:- Not using blanks- Not enough frequently using blanks- Badlly prepared blanks, omitted sources of possible contamination (reagents, matrix,

solvents, bottles, etc.)

Blanks

Control charts for reference material

Bad habits:- Not using control charts- Not enough frequently using control charts- Too high concentration in reference material used for control charts in relation to the

sample content- Not reacting to exceedances of limits



Thank you for your attention

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Measuring of samples with low concentrations ... · Measuring of samples with low concentrations: contamination problem and clean working conditions Anna Kowalska, Zuzanna Cieśla