Topic 3 contamination anf interferences

1
Contamination
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,
2
Interferences
Interferant A component of a sample that
prevents direct measurement of the analyte
concentration.
Additive enhance the analyte signal
by a fixed amount (eg contamination, sample
component that responds to the detector)
Multiplicative increases or decreases 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
3
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.
4
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 profenofos
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 extract Vaq mL containing ao
millimoles of A with Vorg solvent and a1
millimoles remain in the aqueous phase
then a1/ao Vaq/(PVorg Vaq) 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 a1/ai
0.0083, a1/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)
5
Removal of Interferences cont.
HL ? H L- Ka HL-/HL ions in
the aqueous phase neutrals in the organic
phase Mn nL- ? MLn Kd MLn/MnL-n P(
MLn) MLnorg/MLnaq and P(HL)
HLorg/HLaq. and 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 logD - npH logK
or logD logK npH when D 1 (50 extracted)
pH (1/n)logK
Dithiozone Vogel p171
logD logK npH