Page 220 - Tandem Techniques

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A sample of water 1 1 in volume was extracted three times with 100 ml portions of dichloromethane for
5 minutes. The combined extracts were concentrated on a water bath at 70-80°C and then divided into
two equal parts. Each part was evaporated to dryness. in a stream of nitrogen. One portion which was
used to determine six types of carbamate, 3-(sec-butyl)phenyl N-methylcarbamate, 2-isopropoxyphenyl
N-methylcarbamate, 3,5-xylyl N-methyl-carbamate, 3-methylphenyl N-methylcarbamate, 2-isopropyl-
phenyl N-methyl-carbamate and I-naphthyl N-methyl-carbamate. One part was dissolved in 0.2 ml ethyl
acetate and reacted with 0.2 ml of trifluoroacetic anhydride for 2 hours at 50°C. 5 ml of a n-hexane-
ether mixture (94 + 6), and 0.2 ml of an external standard solution of anthracene (1.0 ppm in n-hexane)
was then added. The mixture was washed three times with 5 ml of water, and then concentrated to a
volume of 2 ml. 3 µl of the solution was used for analysis by GC/MS employing the SIM mode. The
other portion was dissolved in 0.5 ml of methanol and added to 0.5 ml of diazomethane in diethyl ether
and allowed to stand for 1 hour at room temperature. The excess of diazomethane was decomposed
with 0.1 ml of acetic acid and 0.3 ml of the anthracene standard added and shaken vigorously. 3 µl
samples were used for analysis. The estimated detection limit was 14 to 180 pg per ml for a 1 liter water
sample. The recover from the water was estimated to range from 83 to 127%, and the relative standard
deviation for 11 analyses was between 2.6 and 22.6%

The identification of specific polychlorinated biphenyls is extremely difficult from mass spectra alone.
In contrast vibrational spectroscopy is and extremely sensitive technique for identifying molecular
structure and Hembree et al. [27] developed a triple system, involving a gas chromatograph, a mass
spectrometer and an IR spectrometer. The use of a gold plated light pipe as an interface, can produce
high sensitivities as previously discussed in chapter 4. However, the low temperature trapping
techniques coupled with the high sensitive HgCdTe IR detector can permit excellent spectra to be
obtained at the low picrogram level. Furthermore, the condensed spectra that are produced are very
similar to those produced by more conventional procedures at room temperature. The system was, in
fact, operated as two tandem instruments in parallel.