Anal. Chem. 1995, 67,3659-3667
Determination of Nanogram per Liter Concentrations of Volatile Organic Compounds in Water by Capillary Gas Chromatography and Selected Ion Monitoring Mass Spectrometry and Its Use To Define Groundwater Flow Directions in Edwards Aquifer, Texas Paul M. Buszka*lt
U.S.Geological Survey, 801 1 Cameron Road, Austin, Texas 78754 Donna L. Rose*
U.S.Geological Survey, National Water Qualify Laboratoty, 5293 Ward Road, Arvada, Colorado 80002 George B. Ozuna and George E. Groschen*
U.S.Geological Survey, 435 lsom Road, Suite 234, San Antonio, Texas 78216 A method has been developed to measure nanogram per liter amounts of selected volatile organic compounds (VOCs) including dichlorodifluoromethane, trichlorofluoromethane, cis-1,2-dichloroethene,trichloroethene, tetrachloroethene,and the isomers of dichlorobenzenein water. The method uses purge-and-traptechniques on a 100 mL sample, gas chromatography with a megabore capillary column, and electron impact, selected ion monitoring mass spectrometry. Minimum detection levels for these compounds ranged from 1 to 4 ng/L in water. Recoveries from organic-free distilled water and natural groundwater ranged from 70.5% for dichlorodifluoromethane to 107.8%for 1,4-dichlorobenzene. Precision was generallybest for cis-1,2-dichloroethene,tetrachloroethene, and the dichlorobenzene isomers and worst for dichlorodinuoromethane and trichlorofluoromethane. Blank data indicated persistent, trace-level introduction of dichlorodifluoromethane, 1,4-dichlorobenzene, and tetrachloroetheneto samples during storage and shipment at concentrations less than the method reporting limits. The largest concentrations of the selected VOCs in 27 water samples from the Edwards aquifer near San Antonio, 'lX, were from confined-zonewells near an abandoned landfill. The results defined a zone of water with no detectable VOCs in nearly all of the aquifer west of San Antonio and from part of the confined zone beneath San AntOniO.
Studies in Europe and the United States have found an association between detections of volatile organic compounds (VOCs) in groundwater from unconfined aquifers and urban land Present address: US. Geological Survey, 5957 Lakeside Blvd., Indianapolis, IN 46278. Resent address: US.Geological Survey, 102 E. Main St., 4th Floor. Urbana, IL 61801. +
This article not subject to U.S. Copyright. Published 1995 Am. Chem. SOC.
uses.1-4 Potentially important sources of VOCs to groundwater include septic tanks and cesspools, seepage from leaking underground storage tanks, leakage of leachate from landiills, contaminants in urban runoff, and other point sources. Application of sensitive analytical techniques to detect VOCs in groundwater can define their migration and identify sources and processes affecting their concentrations. Previous investigations of the effects of wastewater on groundwater quality have emphasized determinations of nitrogen and phosphorus species, detergents,fecal coliform bacteria, and VOCs. The nutrient, detergent, and bacterial constituents may be useful only as indicators of pollution after contamination is widespread. In addition, VOCs may be sporadically disposed, not detected by random sampling, or present in concentrations less than the limits of detection of commonly used analyticalmethods. For example, VOCs used as septic tank cleaners include 1,l-dichloroethane, dichloroethene, l,l,l-trichloroethane, trichloroethene, and tetrachl~roethene.~,~ 0th-er organic compounds such as the 1,2 and 1,4 isomers of dichlorobenzene and 2,6-di-tert-butyl-~-benzoquinone, have been useful in tracing sewagecontaminated groundwater.6 These compounds originate from household cleaners, detergents, and disinfectants. They were detected during several of these investigation^^^^^^ at concentrationsless than the standard reporting limits of 0.2 pglL by Method 524.2 of the US. (1) Rivett, M. 0.; Lerner, D. N.; Lloyd, J. W.; Clark, L J Hydrol. (Amsterdam) 1990,113, 307-323. (2) Fusillo, T. V.: Hochreiter, J. J.; Lord, D. G. Ground Water 1985,23, 354360. (3) Eckhardt, D. A; Flipse, W. J.; Oaksford, E. T.US. Geological Survey, WaterResources Investigations Report 86-4142; U S . Government Printing Office: Washington, DC, 1986; 35 pp. (4) Buszka, P. M. US. Geological Survey, Water-Resources Investigations Report 87-4116; U S . Government Printing Office: Washington, DC. 1987; 100 pp. (5) Canter, L. W.; Knox, R C. Septic tank system effectson ground-water quality; Lewis Publishers: Chelsea, MI, 1985; 336 pp. (6) Barber, L.B., 11; Thurman, E. M.; Schroeder, M. P.; Leblanc, D. R Enuiron. Sci. Technol. 1988.22,205-211.
Analytical Chemistry, Vol. 67, No. 20, October 15, 1995 3659
Figun I. Location of study area, the Edwards
aquifer, and wells sampled for this Study.
Environmental Protection Agency (USEPA)? A reliable method to detect VOCs at smaller, nanogram per liter detection limits could give a useful early warning of water quality degradation to scientists and managers who work with environmental data. This paper describes a method for determiningconcentrations of VOCs in water rangingfrom about 2 to 250 ng/L ?his method exbacts VOCs from a 100 mL water sample using purgeand-trap techniques. The compounds are separated and identified by gas cbromatography/mass spectrometty (GC/MS) in the selected ion monitoring (SIM) mode. The instrumentaltime of 55 min/sample allows the automated analysis of several samples per day. This procedure, identified as the SIM method, was applied to analyses for nine target VOCs. The target compounds included cisl,2dichloroethene(c-DCE), dichloroditluoromethane (DCDFM), 1,Zdichlorobenzene (1,2-DCB), l,?dichlorobenzene (1,3-DCB), 1,4 dichlorobenzene (1,4DCB), 2,6diJertbutyl.gbenzcquinone(DBQ), tetrachloroethene (PCE), trichloroethene (ICE), and trichlore fluoromethane 0 . Several methods have been reported to determine VOCs in natural water. These include purgeand-trap extraction of 25 mL samples with conventional GC/MS analysis 0 , S adsorptionthermal desorption (All)) onto a sorbent trap with G U M S analysis (ATD)? pentane extraction followed by GC analysiswith electron capture detection @CD),’O and extraction by closed-loop stripping with GC/MS analysis (CIS).ll The SIM method should have lower reporting limits than PT because of the greater sensitivity of SIM and the larger mass of potentially extractable VOCs in a 100 mL sample. The SIM method uses a more commonly available extraction apparatus than the ATD method. 0 Guidelines establishing test procedures for the a n d p i s of pollutants under the Clean Water Act. C d e ofFedeml Rcgulotim, Part 136, Oct 26, 1984. US.Govemment Printing Ofice: Washington. DC. 1984; Fed Refit. 1984. 40, 198-199. (8)Wenhaw, R L,Fishman, M. 1.. Grabbe, R R. Lowe, L E., Eds. US. Geological Survey,Techniques of Water-Resources Investigations,Book 5; U S Govemment Printing Office: Washington. DC, 1987: Chapter A3, 80
PP.
(9) Pankow. 1. F.; Isabelle, L M.; Hewetson. I. P.; Cherry, J. A h m d Wafer 1985.23,775-782. (10) E k h d , G.;Jaseffson, B.; Raos. C.I. H k h Rerolut. Chmnatogr., chmnrotogr. Commun 1978. 7,34-40. (11) Barber, L B.. n. Gwchemistq of organic and inorganic c o m p m d s in a sewage contaminated aquifer, Cape Cod, Massachussets. M.S. Thesis. University of Colorado, Boulder. CO. 1985.
3660 Analytical Chemisiry, Vol. 67, No. 20,October 15, 1995
Recoveries of target compounds by purgeand-trap extraction are gene& superior to those reported for CIS.” Reporting limits of the SIM method should be comparable to those of the ECD method because of the enhanced sensitivity of the MS detector when run in the SIM modeJ2 The SIM method, unlike ECD, limits the chance for mistaken identification of coeluting compounds as target VOCs by identifying the target VOCs from their retention times and from the masses of selected ion fragments. These characteristics indicate the potential of the SIM method to detect nanogram per liter concentrations of VOCs in water. These very small concentrations may be d c i e n t l y small to warn of groundwater contamination in its early stages. The SIM method was applied to determine potential VOC tracers of groundwater flow and potential contamination in water from 27 domestic and public supply wells in the Edwards aquifer in Bexar, Medina, and Uvalde Counties near San Antonio, TX Fwre 1). The Edwards aquifer is a very permeable, dissolutionmodified, and faulted limestone and is the sole source of water for about 1.5 million people in the city of San Antonio and most of the adjacent counties to the west and northeast. Outcrop and hydraulidy unconfined parts of the aquifer under urbanized areas have been previously demonstrated to contain VOCs in concentrationsthat are gene& less than 2 pg/ L4 No VOCs were observed in water from deeper, hydraulically confined parts of the aquifer below central San Antonio.’ Hydraulically confined zones of the aquifer provide the best productivity for water supply. Vertical displacements of 50%or more along faults throngh the aquifer have been hypothesized as barriers to groundwater flow. These barriers would redirect groundwater flow containing VOCs toward the northeast away from the confined zone under San Antonio. The aim of this application was to determine whether SIM method analyses were useful in defining regionally important sources of recharge and fanlt barriers to contaminant flow. EXPERIMENTAL SECTION
Note: Use of finn or trade names in this article is for identification purposes only and does not constitute endorsement (12) Millington, D. S.; Nonuwd, D. L In Ogonic Cam’%o@mis Drinkiq Water: Detection, Teatment and Risk AuwnunG Ram. N. M., Calabrese, E. I.. Chrisman, R F.. Eds.; John Wiley and Sons: New York. 1986, pp 131152.
I- ?:[
w"EN' a ' ' '
10
\. . ' '
-.e-
l
-.-a ' ' '
15 20 WRGETIME, IN MlNCmS
'
'
'
l
25
Figure 2. Variation of compound abundance with purge time from 50 nglL spiked samples.
by the US. Geological Survey. Apparatus. The equipment used for the SIM procedure included the following items: (1) a purge-and-trap unit, Tekmar Models LSC 2000 and ALS 2032; (2) purge vessels with a 100 mL capacity with fritted glass at the sparge inlet; (3) a 0.2667 cm i.d. trap that was packed to a total length of 25 cm with one-third each of Tenax, silica gel, and charcoal, followed by a 1cm length of OV-1 column packing; (4) a gas chromatograph/mass spectrometer (Hewlett Packard 5996) equipped with a jet separator; and (5) a DB-624 megabore capillary column, 30 m x 0.53 mm id., with a 3 pm film thickness. To determine the best purge time, four 50 ng/L standards containing the eight target compounds were purged with helium at a flow rate of 40 mL/min for 11,14,17,and 20 min. The amount of each compound purged from the standards was determined by the abundance of the quantitation ion for each compound. The greatest amounts of cDCE, 1,2-DCB,1,3-DCB,1,4DCB, PCE, and TCE were purged from the standards using a 17 min purge time (Figure 2). The greatest amounts of DCDFM and TCFM were removed from the sample using a purge time of 11 min (Figure 2). A purge time of 15 min was selected for the SIM method as a compromise. The VOCs were collected on the trap at room temperature and desorbed onto the GC column at 180 "C for 4 min. The transfer line extending from the purge-and-trap unit to the megabore column was composed of silica (0.32 mm i.d.), externally coated with aluminum. The transfer line was inserted directly into the column, eliminating the injection port. The transfer line and valve temperatures were set at 100 "C. The gas chromatograph was operated using the following program: isothermal operation at 10 "C for 5 min, heated using a linear temperature increase to 190 "C (6 OC/min), and then isothermal operation at 190 "C for 2 min. The trap was baked at 225 "C for 8 min after desorption to prepare the trap for the next analysis. The oven was then allowed to cool for 10 min before
the next sample injection. The total time required for each analysis was 55 min. The carrier gas was helium at a flow rate of 15 mL/min. The instrument was tuned to pass bromofluorobenzene criteria7in the full-scan mode. The SIM mode was chosen for this analysis to get the greatest analytical sensitivity from the GC/MS equipment. Three ions were chosen for each compound. These ions were scanned for 333 ms each, for a total scan time of 1 s. The ion chosen for quantitation was usually the base peak. When a target compound interfered with the base peak scan, a secondary ion was used for quantitation. The quantitation ion, the secondary ions, and the retention time for each of the target VOCs are listed in Table 1. The source and analyzer temperatures were both 200 T,and the GUMS transfer line temperature was 170 "C. Reagents. All stock standards described in this method were made using commercially available, high-purity, purge-and-trap grade methanol. Five microliter aliquots of a 1pg/mL solution containing the surrogate and internal standards (SURRIS) were added to every standard and sample to check the performance of the operating system. The internal standards were 12-dichloroethaned4 and 1,2dichlorobenzene-d~.The surrogate standards were fluorobenzene, toluene$*, and 1,4bromofluorobenzene.A quality control check standard containing 5 pg/mL each of the VOCs was prepared from standards obtained from the USEPA Organic-free distilled water for field and equipment blanks was prepared by boiling distilled water for 1h and chilling the water with ice. The water was analyzed and determined to be free of contaminants. The water was bottled in cleaned and burned 1 L glass bottles with Teflon-lined septum plastic lids. All instrument blanks were prepared using 100 mL of organic-freedistilled water spiked with SURRIS. Laboratory Procedures. To ensure the integrity of the analytical procedure, several steps were taken to lessen background interferences. Each vessel was purged with helium for 25 min before the analysis. After all the purge vessels had been cleaned, the trap was baked for 30 min at 225 "C. An instrument blank was analyzed before sample analysis to determine if instrument and laboratorybackground concentrations of the target VOCs were acceptable. Separate 250 mL syringes were used for loading samples and standards. The syringes were cleaned before and after each use with sequential rinses of methanol, organicfree water, and sample. To prepare the sample at the laboratory for analysis, 100 mL was introduced into a clean 250 mL syringe equipped with a h e r Lock fitting. The plunger was inserted, and residual air was vented through the syringe valve. Five microliters of the SURRIS standard was injected through the syringe valve. The sample was transferred into a clean 100 mL purge vessel and analyzed using the protocol described in the Apparatus section. The groundwater samples were analyzed in duplicate or triplicate within 12 days of collection. A five-point calibration curve was generated before analysis for each of the chlorinatedVOCs using a concentrationrange from 10 to 250 ng/L. The calibration curve for DBQ ranged from 200 to 2000 ng/L. Working standards were prepared by spiking 100 mL portions of organic-free distilled water in a 250 mL syringe with the appropriate amounts of SURFUS and stock solutions.The standard was transferred to a clean 100 mL purge vessel and analyzed according to the protocol in the Apparatus section. A response factor (RF) was determined for each target compound Analytical Chemistry, Vol. 67, No. 20, October 15, 1995
3661
~~
~
~~
~~
Table I.Retention Times of the Target, Internal Standard, and Surrogate Standard Compounds and Their Characteristic Quantitation and Secondary Ions
compound name cis-1,2-dichloroethene 1,Zdichlorobenzene 1,Sdichlorobenzene 1,4dichlorobenzene dichlorodifluoromethane
absolute retention time (min) 9.72 23.11
tetrachloroethene trichloroethene trichlorofluoromethane 1,2-dichlorobenzene-d4 1,2-dichloroethane-d4
23.85 11.34
1,4bromofluorobenzene fluorobenzene toluene-ds
23.85
An asterisk
96 146* 146* 146*
23.90 22.89 2.30 34.49 16.35 12.76 4.19
2,&di-tert-butyl-fi-benzoquinone
mlP
quantitation ion Target Compounds
85*
60,61* 148 111, 148 111, 148 50,87
177 166* 132 101*
164,166 130*, 132 66, 103
internal standard referenceb
135,220
Internal Standard Compounds 152
65* Surrogate Standard Compounds
12.01 15.97
95* 96* 98*
150* 67,102 174,176 50,70 70, 100
2 1
na na na
indicates the ion is the base peak for that compound. na, not applicable.
at each concentration using the following calculation:
RE?=- A (c>C(i> A (9C(c> where A(c) is the GC peak area of the quantitation ion for the compound or surrogate standard, A(i) is the GC peak area of the quantitation ion for the internal standard, C(i) is the concentration of the internal standard (in ng/L), and C(c) is the concentration of the compound or surrogate standard (in ng/L). The quantitation ion and internal standard assignment for each VOC are listed in Table 1. If the relative standard deviation (RSD) of the response factors was 2M, a first or second degree curve was used for quantitation. A daily standard, prepared at a concentration of 50 ng/L, was analyzed to determine if the instrument was operating withii specifications. A response that was within 30% of the average response factor in the calibration curve was achieved by this method. A response within 40%of the concentration of a daily quality control standard was also achieved by the method. These responses meet the guidelines set by the USEPA for Method 524.2.7 Method detection levels (MDLs) were determined using standard procedure^.^ Method precision was determined by analyzing seven spiked replicates at two concentrations, 10 and 200 ng/L, in organic-free distilled water and a natural groundwater from the project area. The natural groundwater was collected from well TD-68-33-202. It had, at the time of collection, a pH of 7.1, a temperature of 22.5 “C,a specific conductance of 456 p S / cm, and a dissolved oxygen concentration of 4.4 mg/L. To identify a VOC in a sample, the retention time of the VOC had to agree within 0.1 min of the daily standard, and the ion abundances in the mass spectrum had to agree with their abundances in the daily standard. If a VOC was detected at or near the MDL, a duplicate, and in some cases a triplicate, was analyzed. All samples were also analyzed by USEPA Method 524.2.7 In this manner, the identity of each VOC was confirmed by a full-scan spectrum if the amount detected was > 10-20 ng/ 3662
secondary ions
Analytical Chemistry, Vol. 67, No. 20, October 75, 1995
L. Once an identification was made, the amount present was quantified using the following formula:
C = C O A( 4 RF (CIA(9 where C is the concentration of the VOC in the sample in ng/L, C (i) is the concentration of the internal standard in ng/L, A(i) is the area of the quantitation ion of internal standard, RF(c) is the average response factor for the VOC detected, and A(c) is the area of the quantitation ion of the VOC. If any VOCs were detected in the daily blank, the amount detected was considered when reviewing data at the MDL. For example, if TCE was detected at 2 ng/L in the daily blank, and it was detected in the sample at 4 ng/L, the concentration of TCE would be reported as (4 ng/L. Field Procedures. Groundwater samples were collected from largecapacity public supply wells and from domestic wells. Samples were collected after the water temperature, specific conductance, and pH were observed to stabilize and after at least three casing volumes had been purged. Samples were collected from faucets located at the wellhead using cleaned and burned, washerless brass hose bibs and copper tubing. The sample bottles were slowly filled from the bottom and allowed to overflow before being sealed. Five bottles were filled consecutively from each site and are called “replicate samples”. All samples were chilled to 4 “Cimmediately after collection and stored at that temperature until analysis. Chemical preservatives were not added to the samples collected for this study. The potential intrusion of the target organic compounds into the sample during storage and shipment was tested using a set of three, triplicate tripblanks. Organic-free water was sequentially prepared at the laboratory and shipped to the USGS office in San Antonio. From the office, it was transported to well 6 and poured into nine 250 mL sample vials, The sample vials consisted of precleaned 250 mL amber glass bottles that were sealed with a Teflon-lined septum lid cap. One set of three tripblanks was immediately chilled and shipped for next-day delivery in iced, sealed coolers to the USGS National Water Quality Laboratory in
Table 2. Method Statirtlcr for the Targot Compounds
compound name and concn in water m a w
organic-free distilled water matrix % recvrd RSD
&-1,2dichloroethene 10 200
1,2dichlorobenzene 10 200
1,3dichlorobenzene 10 200
1,4dichlorobenzene 10 200 dichlorodifluoromethane 10 200 2 ,&di-tert-butyl-p-benzoquinone 500 2000
tetrachloroethene 10 200
trichloroethene 10 200
trichlorofluoromethane 10 200 0
natural moundwater matrix % recvrd RSD
minimum
detectable concn“
104.4 100.6
8.2 3.7
103.8 103.8
8.3
2
100.5
7.3 7.5
98.4 107.3
7.5 3.2
1
106.2 94.3
6.0
95.3 102.9
5.9 2.3
1
5.3 6.3 5.5
98.4
7.5 2.2
1
107.8
81.6
24.2
18.9
96.8
91.4
11.0 7.6
4
70.5 93.5 88.2
16.9 8.8
61.8 87.3
14.4
180
106.6 90.8
6.4
7.2
98.5 93.1
3.1
2
100.4 90.8 106.
2.2
11.3 4.4
99.5
20.1 16.8
99.8 101.7
8.9 2.4
4
112.1
82.9
16.9 10.1
97.1 91.6
4.7 6.6
3
84.4
Concentrations are in nanograms per liter.
Arvada, CO, for analysis. The other two sets of three samples were held for 1 and 2 days, respectively, before shipment in a refrigerator at the San Antonio office to determine sample storage interferences. All tripblanks were analyzed immediately upon arrival at the laboratory. Data from these analyses were compared with those from an analysis of organic-free water that had been stored in the San Antonio office refrigerator for 34 days. The “34 day” water was stored without headspace in a 1L precleaned glass bottle with a standard Teflon-lined cap. RESULTS AND DISCUSSION Laboratory Study. The MDLs determined for the SIM method ranged from 1ng/L for 1,2-DCBand 1,CDCB to 4 ng/L for TCE and DCDFM (Table 2). These values represent the MDL with a 99% confidence interval. The typical reporting limit for standard PT analyses of these VOCs by USEPA Method 524.2 is 200 ng/L.7 The lower MDLs for SIM analyses are from 50-200 times lower than the MDLs from standard €Tanalyses. The primary reason for the lower detection limits is the use of SIM as compared to the scanning mass spectrometry that is used in standard PT analyses. The occasional presence of smaller concentrations of these target compound in the laboratory environment prevents the achievement of an additional decrease in MDLs that is possible using the SIM method as compared to the full-scan mode.’* Recoveries of the seven spiked replicates in two matrices ranged from 82 to 112%for all the chlorinated VOCs except DCDFM Vable 2). The recoveries of DCDFM at 200 and 10 ng/L concentrations in organic-free distilled water were 71 and 82%, respectively. The recoveries of DCDFM from the natural matrix at the same concentrationswere 97 and 91%,respectively. Larger
recoveries of TCFM also were obtained from the natural matrix than from the organic-free distilled water (Table 2). The lower recoveries of DCDFM and TCFM may reflect their enhanced solubility in a matrix with fewer dissolved solids. These recoveries are all within recommended tolerances for EPA Method 524.2 for these same compound^.^ The MDL for DBQ was 180 ng/L Vable 2). DBQ is more polar and less volatile than the chlorinated VOCs, enhancing its retention in the aqueous phase and increasing the M c u l t y of its desorption from the trap. Consequently, DBQ was not purged or desorbed as efficiently as chlorinated VOCs, resulting in a higher MDL The calibration curve had a RSD rangingfrom about 20 to 40%. A second-degree equation was used for quantitation. The recoveries of the seven spike replicates ranged from 62 to 94%for DBQ. The recovery of DBQ at a concentration of 500 ng/L from the natural groundwater matrix was 62%. The recovery of DBQ at 2000 ng/L from the natural matrix was 87%. The recoveries were 94 and 88%,respectively, for the same concentrations from the organic-free distilled water matrix. This method is suitable for samples containing e1 pg/L of the selected chlorinated VOCs but has not been evaluated for greater concentrations. EPA Method 524.2 is more suited to detect concentrations of VOCs that are > 1pg/L. All samples in this groundwater study were first analyzed by EPA Method 524.2 to determine if concentrations of VOCs greater than 1pg/L were present7 Concentrations of VOCs greater than 10 pg/L could easily saturate the mass spectrometer using the SIM method. The amount of water vapor that is transferred during the desorb cycle also can interfere with the SIM analysis. This was shown by a shift of the baseline between 10 to 14 min and a signi6cant increase in the amount of water seen in the ion source Analytical Chemistry, Vol. 67,No. 20, October 15, 1995
3663
Table 3. Summary Statistics for Concentrations of the Target Compounds in Laboratory-Prepared Blank and Field-PreparedTrip-Blank Samples
laboratory-preparedblank sample compound name cis-1,2-dichloroethene
1,2-dichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene dichlorodifluoromethane
no. of samples
min
field-prepared tripblank sample
concn (ng/L)a med max
37 37 37 37 37
