Topic 2 Contamination and Interferences

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Topic 2 Contamination and Interferences
A contaminant something that is inadvertently
added to or taken from the sample during the
sampling and/or the analytical process.

• In the field
• Sample collection (containers, equipment, cross
  contamination,
• Sample handling (filtration, sieving,
• Preservation (acids, poisons,
• Sample storage and transportation
  (adsorption/desorption, evaporation, leakage,
  precipitation, reactions??

• In the Laboratory
• Sample pretreatment filtration, sieving,
  grinding, digestion (reagents / volatilization /
  ) transferals, drying, dust, possible from each
  operation.
• Analysis equipment, reagents and solvents,
  ambient conditions, sample carry-over/cross-contam
  ination,

Control of contamination Field blanks (matrix
matched reference materials, rinsings,
containers, ), in-field spiking, co-located
samples, control site samples, Lab RMs,
blanks, spiking, duplicates, cleanliness,
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Interferences
Interferant A component of a sample that
prevents direct measurement of the analyte
concentration.
Additive enhance () the analyte
signal by a fixed amount A contaminant, or
sample component other than the analyte, that the
detector responds to.
Multiplicative increase or decrease the analyte
signal by a factor but do not generate a
signal of their own.
y m x c
Can be removed or allowed for by standard
additions
Must be removed or quantified
Standard methods generally list known
interferences, part of the validation process
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Topic 2.1 Standard Addition techniques

• Two ways to use Standard Additions
• a) Sampling and analytical method quality
  control - (many possible points for the addition)
• (amount analyte in spiked sample) (amount
  analyte in sample) (amount analyte added).
• Allow for matrix effects for multiplicative
  interferences.
• a) only meaningful if b) does not exist.

• Standard additions in the absence of
  interferences.
• Add a known amount of analyte to a sample and
  determine analyte in sample and in the spiked
  sample. If the analysis has gone successfully
  then the difference between the two
  concentrations should be due to the amount of
  analyte added.
• As analyte concentration in the sample
• Ass analyte concentration in the spiked
  sample
• Sa amount (moles or grams) of analyte added
  (the spike).

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Standard Addition techniques cont.
As analyte concentration in sample Ass
analyte concentration in spiked sample Sa
amount of analyte added. Sf amount spike
found. Then (Sf/Sa x 100) - recovery of the
spike. eg if the sample is a solution Moles
analyte in the sample (As x Vs)/1000 Moles
analyte in the spiked sample Ass(Vs
Vss)/1000 Moles spike found Ass/1000x(VsVss
) As/1000xVs Sf eg Fes 45 ppm,
Fess 64 ppm, Vs 10 mL, Vsp 200 µL,
Fesp 1000 ppm Then recovery
(64/1000x10.2 45/1000x10)/(1000/1000x0.2)x10
0 101.4. If solid sample mass sample
5.0 g, spike with 100 µL of 1000 ppm Fe, find
Fess 64 ppm and Fes 45 ppm (in the
SOLIDS). (ppm mg/kg) (64/1000x5
45/1000x5)/(1000/1000x0.1) x 100 95
recovery. Add sufficient spike to increase the
analyte by a factor of 1.5-2. BUT how much is
that?? Monitor spike recoveries over time using
Quality Control Charts.
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Standard Addition techniques cont.

• To correct for matrix effects for
  multiplicative interferences.

• Single spike method
• Add a spike to the analytical sample and analyse
  the analytical sample and the spiked analytical
  sample
• As, Ass, Asp, Vs, Vsp, Rs, Rss. Responses
  (R) corrected for reagent blank (R mA c
  force the line through 0 i.e. c 0).
• Rs mAs and ? m Rs/As Rss
  mAss. and ? Rss (Rs/As)Ass
• Ass (AsVs/1000 AssVsp/1000)/(Vs
  Vsp)x1000 AsVs AspVsp/Vs Vsp
• Rss (Rs/As)(AsVs AspVsp/Vs Vsp)
• Rearrange to isolate As Rss (As)Vs
  Vsp RsAsVs RsAspVsp
• RssAsVsVsp RsAsVs RsAspVsp
• As (RssVs Vsp) RsVs) RsAspVsp
• As Rs AspVsp / (RssVs Vspike) RsVs)

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Standard Addition techniques b) Matrix effects
for multiplicative interferences. cont.
1. Single spike method
Rss
R
R
Rs
Ass
As
Ass
Asss
As
Assss
Asssss
concentration
concentration
2. Multiple spike method While you dont know
As the spikes are known
Work in the linear response region of the
instrument. If slope of standard additions line
slope of the normal calibration curve then
there are no multiplicative interferences. Additiv
e interferences must be absent.
set As to zero and plot responses against
spike concentrations, determine least squares
best fit line. Calculate As from concentration
intercept at R 0. Higher precision than for the
single spike process
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Topic 2.2 Removal of Interferences
Selective precipitation eg in determining the
OH- content of Bayer (bauxite to alumina) process
liquors. Titrate the diluted liquor with standard
acid. Remove CO32- by precipitation with BaCl2
(KspBaCO3 5 x 10-9) and then titrate OH- to pH
7 equivalence point (see C 10 lab manual) after
masking Al(OH)4-.
Masking Use a reagent to speciate the
interferant so that it will no longer participate
in the analytical reaction. eg in determining
the base content (OH- and CO32- OH-) of Bayer
process liquors. Al(OH)3(s) OH- ? Al(OH)4-
reaction that used to dissolve the alumina
minerals (iron minerals Fe2O3, FeO(OH) are
insouble in base). Al(OH)4- tartrate
(-O2CCH(OH)CH(OH)CO2-) ? A l(OH)3Tn2n-
OH-. Titrate total base with standard acid, OH-
coordinated to Al no longer reactive.
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Removal of Interferences cont.
Solvent extraction (Vogel, Chapter 6) where the
analyte or a complex of it is separated from
interferences by its preferential solubility in
some solvent. Generally an aqueous solute is
extracted into an organic solvent eg pesticide
for GC in C 30J lab) inorganic solutes often
need to be complexed to enhance solubility in the
organic solvent. Partition Coefficient P
Aorg/Aaq. if extracting Vaq mL containing
ao millimoles of A with Vorg mL solvent, and a1
millimoles remain in the aqueous phase
then a1/ao Vaq/(PVorg Vaq) (derive this
equation) and an/ao Vaq/(PVorg Vaq)n.
To separate two components (ai and aj) must
consider both distribution ratios. eg if Pi 10
and Pj 0.1 and Vaq Vorg 100mL (Vogel
p163) for n 1 a1/ai 0.091 and a1/aj 0.91
(or 90.9 and 9.09 extracted) For n 2 a2/ai
0.0083, a2/aj 0.83 (or 99.2 and 17
extracted) If ai aj (11) initially then
increasing the number of extractions improves
extraction efficiency but also leads to increased
interferant concentrations (after 1 extraction
10ai1aj while after 2 extractions 5.8ai1aj)
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Removal of Interferences cont.
Complexing metals with ligands HL ? H L-
Ka HL-/HL ions in the aqueous
(polar) phase neutrals in the organic (nonpolar
phase Complexation equilibrium Mn nL- ?
MLn and Kd MLn/MnL-n Partitioning
P(MLn) MLnorg/MLnaq and P(HL)
HLorg/HLaq. the distribution of the metal
D total metalorg/total metalaq
MLnorg/Mnaq. but MLnorg P(MLn)MLnaq
and MLnaq KdMnL-n. then D
P(MLn)MLnaq/Mnaq P(MLn)KdMnaqL-naq/
Mnaq. D P(MLn)KdL-naq P(MLn) Kd
HLnaq Kan /Hn Or DHn P(MLn)
KdKanHLaqn P(MLn)KdKanHLorgn/P(HL)n If
excess reagent is used then HLorg will be
constant DHn K or
logD nlogH logK i.e. logD - npH
logK or logD logK
npH when D 1 (50 extracted) pH (1/n)logK
Dithiozone Vogel p171
logD logK npH
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Removal of Interferences cont.
Control separations by controlling pH
eg Fe(SCN)63- method for Fe. (Vogel p690)
extract the aqueous solution with
pentan-1-ol/Et2O or Et2O. Charged species and
therefore a polar solvent.