Water

The apparatus used for VOC analysis in water by high speed GC is essentially the same as that used for the air analysis work. This is because, as sample preparation is conducted by a static headspace equi-

Figure 2 High speed chromatograms using gasbag samples and the reverse-flow collection instrument: (A) six-component mixture; (B) nine-component mixture. Peaks in (A) A, n-pentane; B, n-hexane; C, benzene; D, n-heptane; E, toluene; F, n-octane. Peaks in (B) A, n-hexane; B and C, isomers of 2-hexene; D, 1-heptene; E and F, isomers of 2-heptene; G, 1-octene; H and I, isomers of 2-octene. (Reproduced with permisssion from Akard and Sacks, 1994.)

Figure 2 High speed chromatograms using gasbag samples and the reverse-flow collection instrument: (A) six-component mixture; (B) nine-component mixture. Peaks in (A) A, n-pentane; B, n-hexane; C, benzene; D, n-heptane; E, toluene; F, n-octane. Peaks in (B) A, n-hexane; B and C, isomers of 2-hexene; D, 1-heptene; E and F, isomers of 2-heptene; G, 1-octene; H and I, isomers of 2-octene. (Reproduced with permisssion from Akard and Sacks, 1994.)

Table 1 Limits of detection and quantitation measured for 13 common organic compounds

Table 2 Statistical data for air analysis using reverse-flow sample collection with two different sampling periods

Compound

Table 1 Limits of detection and quantitation measured for 13 common organic compounds

Compound

Pentane

2

7

Hexane

3

10

Heptane

4

13

Octane

5

17

Benzene

8

27

Toluene

2

7

o-Xylene

50

170

m-Xylene

1

3

Dichloromethane

< 1

<3

Chloroform

< 1

<3

Tetrachloroethylene

<1

<3

1,1,2,2 Tetrachloroethane

2

7

1,2,3 Trichloropropane

<1

<3

Table 2 Statistical data for air analysis using reverse-flow sample collection with two different sampling periods

Component

Correlation coefficient

Log-log slope

LOD (p.p.b.) 20 110

Benzene

0.993

0.99

26

4.6

n-Heptane

0.997

1.04

37

6.9

Toluene

0.999

1.07

25

4.0

Octane

0.995

1.07

47

8.4

p-Xylene

0.999

1.09

29

4.4

All values are expressed in parts per billion by volume and are based on a sample volume of 1 mL. Values reported as <1 required extrapolation below sample volumes that were actually tested.

LOD, sample mass producing a peak height equal to a blank pus three standard deviations of the noise; LOQ, the sample mass producing a peak height equal to a blank plus 10 standard deviations of the noise. Reproduced with permission from Mouradian etal. (1991).

librium method, the actual sample analysed is an air sample.

Detection limits using an FID were found to be <10 ig for benzene, toluene, ethylbenzene and the xylenes (BTEX compounds). Again, by increasing the injection loop volume, these detection limits could be reduced further. Real samples from ground water obtained near a leaking underground storage tank have been analysed and preliminary comparisons to more established methods made. Full validation of the high speed method has not been carried out.

Detection limits (LOD) for a signal-to-noise ratio of 3.0 for both the 20 s and the 110s sampling times. Reproduced with permission from Akard and Sacks (1994).

An alternative method for aqueous sample analysis combines solid-phase microextraction (SPME) with fast GC. The SPME extraction is conducted on the headspace above a water sample in a sealed vial. The analytes are then rapidly desorbed in a specially made injection port into a commercially available portable gas chromatograph. Separation of the BTEX compounds is achieved in less than 30 s. The photoioniz-ation detector for this work was not optimized for fast separations. Modification of the detector internal volume is required to reduce band broadening and provide performance suitable for use with high speed GC without the use of make-up gas. As the headspace above a sample is extracted with the SPME fibre, the method is easily adapted to soil sample analysis for VOCs.

One reason for the lack of published applications dealing with VOC analysis in water may be the large disparity between chromatographic analysis time and sample preparation time. Having a chromatographic time scale of 20 s (Figure 3), as is the case with this

Figure 3 High speed chromatogram of the static headspace above a sample of gasoline-contaminated ground water. Peak identification. 1, methyl t-butyl ether (2370 ig L~1); 2, benzene (97 ig L~1); 3, toluene (109 ig L~1); 4, ethylbenzene (2 ig L~1); 5, co-eluting m- and p-xylenes (12 ig L~1); 6, o-xylene ( < 0.3 ig L~1). (Reproduced with permission from Wang etal., 1991.)

Figure 3 High speed chromatogram of the static headspace above a sample of gasoline-contaminated ground water. Peak identification. 1, methyl t-butyl ether (2370 ig L~1); 2, benzene (97 ig L~1); 3, toluene (109 ig L~1); 4, ethylbenzene (2 ig L~1); 5, co-eluting m- and p-xylenes (12 ig L~1); 6, o-xylene ( < 0.3 ig L~1). (Reproduced with permission from Wang etal., 1991.)

BTEX analysis, has little advantage when the sample preparation time is between 20 and 60 min.

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