Determination of Chloroacetic Acids in Natural and Tap Water by Gas
Chromatography and Mass Spectrometry (GC/MS)
Introduction
The concentrations of haloacetic acids such as dichloroacetic acid in
natural and drinking water is of concern because these compounds are believed
to be responsible for adverse human health effects. Dichloroacetic acid (DCAA),
in particular, is a by-product of the water disinfection process when compounds
such as chlorine or chloramine are used to make water potable. The
determination of concentrations of haloacetic acids in water is carried out by
liquid-liquid extractions, methyl esterification, and GC/MS. The method
described below is adopted in part from EPA method 552.3 for the analysis of
haloacetic acids in water.
Experimental
Please read the entire procedure before you start. You will work in
groups of two. Your instructor will give you water samples (40 mL) that are
known to have measurable quantities of three (mono-, di-, and tri-) chloroacetic
acids in them. You will extract the water with 4 mL of methyl tert-butyl ether
(MTBE) in a 125 mL separatory funnel. You will need to add several reagents to
the water sample before the extraction, as described in the following paragraph.
After adding the water to the separatory funnel, add 0.5 grams of copper
(II) sulfate and shake to dissolve well. Then add 12 grams of Na2SO4 (or 22
grams of NaCl) and shake vigorously for 3 to 5 minutes until almost all is
dissolved. The reasons for adding these reagents are discussed below.
Copper (II) sulfate is added to color the aqueous layer blue so that you
can distinguish it from the organic MTBE layer, making the extraction easier.
The Na2SO4 is added to increase the ionic strength of the aqueous layer, which
helps drive the undissociated haloacetic acids into the MTBE layer. Why is it
important to do this? Think about the structure of these chloroacetic acids.
Then add 400 µL of an internal standard, 2,2-dichloroproprionic acid, that
is 0.1 mg/mL in concentration in MeOH. Shake the funnel with the cap on to be
sure that the internal standard has mixed well with the sample. Then adjust the
pH of the water to approximately 1.0 by adding 2 mL of concentrated sulfuric
acid. This insures that almost all the acid is protonated so that it can be more
easily extracted.
Then add 4 mL of MTBE to the separatory funnel, and shake well for two
minutes to extract the DCAA and the DPAA internal standard into the MTBE.
The aqueous layer will be on the bottom. Transfer about 3 mL of the MTBE layer
1
to a 10 mL conical tube using a pasteur pipette. The esterification reaction takes
place in the conical tube.
Once the sample has been transferred, add 1.0 mL of 10% H2SO4 in
methanol (in the hood) to the conical vial. Then cap the conical vial and heat it in
a water bath for 2 hr at approximately 50 °C.
H3O+
CHCl2COOH + CH3OH
DCAA
CHCl2COOCH3 + H2O
DCAA methyl ester
Once the reaction is completed, add 4 mL of saturated NaHCO3 solution in
1.0 mL increments until the reaction mixture is neutralized. Shake the solution
gently during the neutralization. As the solution is being neutralized,
carefully vent the CO2 as it is being formed. As the neutralization becomes
complete, transfer 1 mL of the MTBE layer to duplicate autosampler vials for
analysis by GC/MS. You will be given your results in lab and your instructor will
go over them with you. Why are carboxylic acids often converted to methyl
esters prior to GC/MS analyses?
2