Vapor concentrations of signature chemicals in soil headspace samples

Figure 10. Mass of 2,6-DNT vapor sorbed on a polyacrylate SPME fiber at 23C.

Table 8. Mean analyte concentrations (pg/mL) in headspace vapor above sand containing buried U.S. military-grade TNT, at three temperatures and three moisture levels.

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13

Table 7. Effective sampling rates

cates for each of the three moisture conditions for

Ottawa sand at 23, 4, and -12C are presented in

(mL/min) for 2,4,6-TNT, 2,4-DNT,

2,6-DNT, and 1,3-DNB as a func-

tion of temperature.

space vapors for the silty and clayey soils at the

three temperatures are presented in Tables 9 and

23C

12C

Compound

10, respectively. Because of the numbers of indi-

vidual treatment conditions in this experiment,

we were unable to analyze individual replicates

with the frequency needed to accurately establish

1,3-DNB

2.16*

†

the breakthrough times for the various vapors to

* Value obtained at 22C.

penetrate the soil barrier and be detectable in the

† No estimate for vapor pressure available.

headspace. Nevertheless, we did detect the pres-

ence of 2,4-DNT and 2,4,6-TNT in the headspace

Vapor concentrations of signature chemicals

above the sand and silt soils maintained at 23C

in soil headspace samples

Peak heights for the target analytes detected in

at day 6 for all three moisture levels, and at day 15

the soil headspace SPME samples were converted

above the clay for the two higher moisture levels.

At 4C, 2,4-DNT and 2,4,6-TNT were detected in

to mass (ng) using SARM-based standards pre-

pared in acetonitrile. Peak heights for 2,5-DNT

the vapor above the sand and silt for all three

and 3-5-DNT were converted to mass of analyte

moisture levels at 12 days. For the clay, 2,4-DNT

was detected at 19 days above the 4C samples,

recovered using the response factors for 2,4-DNT.

After locating a source for these analytes, we

but 2,4,6-TNT was not detected until day 67, and

made the standards and compared the detector

then only at the two higher moisture levels. For

the 12C samples, 2,4-DNT was detectable in the

response to the response for the 2,4-DNT stan-

dard. A response conversion factor was applied

headspace at day 13 above the sand and silt soils,

to 2,5-DNT and 3,5-DNT so that these analytes

but 2,4,6-TNT was not detected until day 173. For

the clay at 12C, only at day 173 was either 2,4-

could be expressed as their own concentrations.

Individual values for the vapor concentration of

DNT or 2,4,6-TNT detectable.

each target analyte in the two replicates for each

Figure 12 is a plot of the vapor concentrations

combination of soil type, moisture level, and tem-

of 2,4-DNT, 1,3-DNB, and 2,4,6-TNT in the head-

space above the silty soil at 23C (low moisture

perature for each day when samples were collect-

ed are presented in Appendix A, Tables A1A9.

added), as a function of time after the start of the

The mass of each target analyte detected in

experiment (4112 days); 2,4-DNT and 1,3-DNB

each headspace sample was converted to vapor

have the highest concentration of the target ana-

concentration (pg/mL) using the effective sam-

lytes throughout the study. For example, at 112

pling rates (mL/min), which were obtained as

days, 2,4-DNT was present in the headspace at

described in the Calibration of SPME section, and

28.9 ng/L, and 1,3-DNB was present at 15.5 ng/L.

the sampling times (minutes) for each tempera-

In comparison, the vapor concentration of 2,4,6-

ture. The effective sampling rate for 2,4,6-TNT

TNT was only 2.65 ng/L after 112 days (Table 9).

(2.3 mL/min) was used to compute vapor con-

In the headspace above the Ottawa sand held at

23C, vapor concentrations of the target analytes

centrations for 2,4,6-TNT and other compounds

containing three nitro functions. The effective

exceeded those found above the silty soil by

sampling rate obtained for 2,4-DNT (4.5 mL/

about an order of magnitude (Table 8). For exam-

min) was used to estimate vapor concentrations

ple, the concentrations of 2,4-DNT, 1,3-DNB, and

of 2,4-DNT. Vapor concentrations for the other

2,4,6-TNT at 112 days in the low-moisture case

dinitro-containing compounds (DNTs and DNBs)

were 604, 401, and 135 ng/L, respectively. Con-

versely, for the clayey soil (low moisture, 23C),

were estimated from the mean effective sampling

rates measured for 2,4-DNT and 2,6-DNT (4.3

headspace concentrations were about an order of

mL/min). The effective sampling rate estimate

magnitude lower than for the silt. At 112 days, the

for 1,3-DNB was not used because we judged the

vapor concentrations of 2,4-DNT, 1,3-DNB, and

vapor pressure data that served as the basis for

2,4,6-TNT were 8.98, 1.68, and 0.24 ng/L, respec-

this value to be unreliable. These concentrations

tively (Table 10). These three analytes were gener-

for each headspace sample analyzed are presented

ally the target analytes present at highest concen-

in Tables A10A18.

tration in the headspace vapor above all three

soils maintained at 23C, although occasionally

Mean vapor concentrations for the two repli-

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