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Anal. Chem. 1004, 56,2602-2604
initial temperature, as well as the Qendat 20 “C which will approximate the required &, values when the column reaches the initial analytical temperature of 50 “C. Average velocities at both the initial and final temperatures for the three flow rates quoted are also included. While in this case the difference between Q,, and Qendis not substantial, for longer columns or higher flow rates where higher inlet pressure is employed, the difference can easily reach a factor of 2 or more, making it important that the type of flow rate is fully defined. The quoting of flow rates to three decimal places enables the average velocity and “dead time” to be calculated precisely where these are not given in the text, but it is unlikely that the actual column volume is known with this precision. A variation of only 1 Fm between the quoted diameter of 0.25 mm and the actual effective diameter will make a difference of almost 1% in the average flow rate. The conclusions reached by Jones et al. of flow rates of about 1 mL/min of helium (Qena of 1.2 mL/min) and 2.5 deg/min program rate as being a general optimum are in agreement with our observations. It is somewhat misleading to refer to these as “high flow rates”, however, particularly
when they represent initial values that fall by 28% during the run. For the test column the initial average velocity corresponding to a 1 mL/min average flow rate is 34 cm/s, falling to 25 cm/s at 250 “C, which is within the range that might be expected from van Deemter plots a t various isothermal temperatures. LITERATURE C I T E D (1) Jones, L. A.; Kirby, S. L.; Garganta, C. L.; Gerig, T. M.; Mulik. J. D.; Anal. Chem. 1983, 55, 1354-1360. (2) Jones, L. A., personal communication. (3) Zlatkis, A.; Fenimore, D. C.; Ettre, L. S.; Purcell, J. E. J . Gas Chromatogr. 1965 March, 75-81. (4) Nygren, S.; Mattsson, P. E. J . Chromatogr. 1976, 123, 101-108. (5) Grob, K., Jr.: Grob, G.; Grob, K. J . Chromatogr. 1978, 156, 1-20.
Noel W. Davies Central Science Laboratory University of Tasmania P.O. Box 252 C Hobart, Tasmania 7001, Australia RECEIVEDfor review January 30, 1984. Accepted July 2,1984.
Analysis of Groundwater Contamination by a New Surface Static Trapping/Mass Spectrometry Technique Sir: The common method of analyzing for organic groundwater contamination is to drill a well into the aquifer, remove a water sample and then use a procedure such as purge and trap followed by GC/MS analysis ( I ) . This approach, when used as a reconnaissance mode, is very expensive. I t is widely recognized that organic compounds in subsurface water can migrate into bed rock as well as undergo vertical migration to the earth’s surface (2). The quantities of these compounds at the surface are often too low for direct detection; however, a trapping procedure provides a viable approach. Recently, we developed a technique using a static collection device for surface geochemical exploration for petroleum (3). The static collection device consisted of a ferromagnetic wire to which charcoal had been glued with an inorganic cement. The wire was placed in a support in a 25-35 cm deep hole in the soil and covered by an aluminum can and the dirt backfilled. After equilibrating with the volatile5 from petroleum occurrences €or about 1week, the wires were removed and transported to the laboratory for analysis by Curie-point pyrolysis/mass spectrometry in conjunction with pattern recognition. The ferromagnetic wire served as a heat source to desorb the adsorbed compounds when placed in the high frequency field of the Curie-point pyrolyzer. In this paper we described how the static trapping/MS technique has also been successfully applied to the surface detection of low levels of tetrachloroethylene and other contaminates in groundwater. EXPERIMENTAL SECTION Sampling Site. A site of known tetrachloroethylene (PCE) contamination at an industrial site near Denver, CO, was used for the study. The PCE and other chemicals had been introduced into the groundwater (averaging13.4 m below surface) by seepage from a waste storage reservoir which had been used in the 1960s. A number of shallow wells had been drilled in the area to assess groundwater movement and PCE contamination. Sample Collector. A 358 OC Curie-point wire was used as the support for the static collector. The static trap was prepared by
applying sodium silicate solution to about 1cm of the roughened end of the wire followed by coating with fine activated charcoal (4). The wires were cleaned by heating to the Curie point under vacuum. A sealed culture tube was used to store and transport the wire to and from the field. In the field, the wires were placed at predetermined sites. Figure 1 illustrates the trapping device. Integration time for this study was 3 days, at which time the samples were removed and transported back to the laboratory for analysis. The loading on the wires was followed by removing duplicate samples at 1-day intervals. Three unexposed wires transported to and from the field were used as blanks. Mass Spectrometry Analysis. The adsorbed compounds were desorbed with a Fisher Curie-point pyrolyzer (1.5 kW, 1.1 MHz) in series with an Extranuclear Laboratories SpectrEL quadrupole mass spectrometer. Low-energy electron ionization (15 eV) was used to minimize fragmentation. A scan rate of 1200 amu/s and a scan range of 10-240 m u were used in all analyses. Data were collected on a DEC 1123 computer and stored on disk. Identification of individual species was made based on molecular weight and isotope distribution. Since a major portion of the compounds in the study contained chlorine, high confidence is placed on the identifications. The process of mapping fluxes of single or groups of compounds was completely computerized. The sample sites on a map were first digitized as X-Y coordinates. The X-Y data were then merged with selected mass spectral peak ion counts. A file was constructed which was subsequentlyrun through a plotting routine to produce the flux maps for each compound. The merging procedures were done on the DEC 1123 computer while digitizing and map construction were done on a DEC 1091 mainframe computer. All data were transferred between two computers on a hard line utilizing the NIH transfer program CLINK. R E S U L T S AND DISCUSSION The contamination of the groundwater occurred from leakage of a waste storage pond. Figure 2 illustrates an idealized cross section of the pond and sampling locations. The depth of the water table averaged 13.4 m. Movement of the groundwater is to the northeast. Soil is predominantly clay. The sampling scheme was chosen to traverse the sus-
0 1984 American Chemical Society 0003-2700/84/0358-2602$01.50/0
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
2603
a North
-
0
500'
Flgure 1. Schematic of trapping device. 4
k.-- 600'-+--1000'____.-
r4,
plant
well X23179
+
.
.
.
pond
.
f
t
- --
-- - t
I
Water table at 44'
'
t
t7--.pI-/L
L?r--
from pond ____ __t - _t _6eepage -__ I _
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Figure 2. Idealized cross section of pond and sampling slte; 0 , monltoring wells. 83
I
mRACHlOROETHYLENE
Figure 3. Typical mass spectrum of collected gases.
Table I. Compounds Identified by Trapping/MS Analysis for a Denver Industrial Site benzene toluene xylene phenol trichloroethylene tetrachloroethylene
dichlorobenzene chloroform trimethylbenzene naphthalene carbon tetrachloride I
pected east to west edge as well as the north edge of the contaminated groundwater plume. A total of 25 samples were taken on spacings ranging from 30.6 to 61 m. Figure 3 shows a typical mass spectrum of the trapped vapors from site 3. The major compound in this spectrum is PCE. The molecular ions plus major fragment ions are present, clearly defining the PCE. Table I summarizes the compounds which were observed in the 25 samples. Figure 4 shows the flux data for PCE using the m / z 164 peak. The fluxes for PCE in the area studied ranged from 0 to 12 300 ion counts. Although absolute concentrations are not given, ion counts for each site are proportional to the surface fluxes. The decrease in ion counts was sharp on the plume edges. For example, site 28 had 380 counts while the adjacent sample 27, had 10 171 ion counts. A qualitative comparison of the data obtained from the static trapping technique above well 23179 to the GC/MS purge and trap analysis of water from well 23179 is shown in
SURVEY DESl6l Figure 4. Tetrachloroethylene surface flux data: X, sample locations; hached area, PCE plume; 0, monitoring wells. Table 11. Compounds Identified by Static Trapping/MS and in Well Water by GC/MS trapping above well 23179 ion counts
I
toluene
(mlz 92)
xylene ( m / z 106) trichloroethylene ( m l z 130) tetrachloroethylene (mlz 164) chloroform" carbon tetrachloride (mlz 152)
well 23179 GC/MS concn, PPb
352
dichloroethylene toluene xylene benzene trichloroethylene
12
