environmental
Applications
note
RestekCorporation • (800)356-1688 • (814)353-1300 •
environmental
#59175
GC/ECDAnalysisof HaloaceticAcids inWater SamplesUsingRtx
®
-CLPesticides
andRtx
®
-CLPesticides2Columns
SampleExtraction
Sample preparation requires amicroextractionof a 40mL
sample,methylationof the acids, and final neutralizationof
extract. The initial extraction of haloacetic acid compounds
involves transferring40mLof sample to a 60mLvial or
separatory funnel, and adding the surrogate 2,3-dibromo-
propionic acid.
The following steps are done in quick succession so that the heat
generated from adding the acid to the sample helps dissolve the
salts into the liquid phase: 1) Lower the sample pH to<0.5 using
concentrated sulfuric acid. 2)Add two grams of copper II sulfate
pentahydrate to the sample to color thewater,making it is easy
to distinguish thewater phase from the organic phase. 3)Add
16gmof pre-cleaned sodium sulfate to the sample to increase the
ionic strengthof the aqueous phase. 4)Add4mLofmethyl
tert
-
butyl ether (MTBE) to the sample and shake for twominutes.
CompoundMethylation
To begin haloacetic acidsmethylation, transfer approximately
3mLof theMTBE extract to a 15mLgraduated, conical
centrifuge tube.Add1mLof 10% sulfuric acid inmethanol to
the centrifuge tube. Cap the tube and heat at 50°C for two hours.
After cooling, neutralize the extractwith saturated sodium
bicarbonate solution, adding it in1mL increments. Continually
vent the centrifuge tube becauseCO
2
will be generated during
the neutralization process.
Transfer 1mLof theMTBE extract to an autosampler vial and
spike 10uLof internal standard (25ppm1,2,3-trichloropropane).
Archive the remaining extract portion for later use if necessary.
PeakGausianFactor:
ColumnChoice
The analysis of haloacetic acid compounds canbe performedon
a variety ofGC column phases.An important criterion for
column selection is the qualityof resolutionbetween the
methylatedhaloacetic acid compounds andknown interference
compounds like bromoform. Bromoformmaybe present
because of the partial decarboxylationof tribromoacetic acid in
themethylation stepusing acidicmethanol.
TheRtx
®
-CLPesticide andRtx
®
-CLPesticide2 columns provide
the necessary resolution for this analysis usingGC/electron
capture detection (ECD) (see chromatograms in
Figure 1
).
These columns have historically been used for the analysis of
chlorinatedpesticides (USEPAMethod508), and chlorinated
acids (Method515), using the same analytical configuration.
InstrumentCalibration
Analyze the laboratoryperformance check (LPC) solution to
verify instrument performance. TheLPC verifies three criteria:
instrument sensitivity, chromatographic performance, and
columnperformance. See
Table 1
, other side, for the results on
theRtx
®
-CLPesticide andRtx
®
-CLPesticide2 columns.
Monochloroacetic acid (MCAA) is used toverify instrument
sensitivity.At a concentration of 6µg/L,MCAAmust have a
signal-to-noise ratiogreater than three. Chromatographic
performance is verifiedusing ameasure of peak symmetry
called thePeakGaussianFactor (PGF). The calculatedPGF
must be between0.80 to1.15 for optimumperformance. The
compoundused for this is bromochloro- acetic acid (BCAA) at a
concentrationof 4µg/L (see below for calculation).
Column performance is verified using the peak resolution between chlorodi-
bromoacetic acid (CDBAA) and the surrogate (1,2,3-trichloropropane).
The criteria for resolution is greater than 0.5 using the following equation:
R= t/W
ave
where: t = the difference in elution time of the two peaks (inminutes)
W
ave
= the average peakwidth of the two peaksmeasured at baseline
(inminutes)
Haloacetic acids areabyproduct of chlorinateddisinfectionof drinkingwater.Recently, therehas been some concern that
theseanalytesmay represent a chronic risk tohumanhealth, and toxicological evidence suggests that someof themare
possiblehuman carcinogens. Elevated levelsof haloaceticacids indrinkingwater couldposeacutehuman riskbecause
of their corrosivenatures.With theproper samplepreparation techniqueanduseof theRtx
®
-CLPesticides and the
Rtx
®
-CLPesticides2 columns, environmental chemists canachieveaccurateanalysisof these compounds. (Refer toUS
Environmental ProtectionAgency [EPA]Method552).
PGF= (1.83 xW
1/2
)/W
1/10
where:W
1/2
= the peakwidth at half-height (in seconds)
W
1/10
= the peakwidth at one-tenth height (in seconds)
PeakResolution:
