Anal. Chem. lQ87, 59, 1987-1990
types of asymmetric peaks not limited to the EMG model. Nonchromatographic Peak Profiles. The EMG model, and hence, the empirical equations de.veloped from this model [reported here and elsewhere (8,IO)],may also be applicable to nonchromatographic peaks. For example, many peaks reported in flow injection analysis and in graphite furnace atomic spectrometry bear a strong resemblance to EMG peaks (for example, see ref 21-26). In one preliminary study, application of the area test described in this report showed that nearly half of the graphite furnace atomic fluorescence peaks are examined were adequately modeled by the EMG function (26). Thus, although not yet justified on theoretical grounds, the EMG model and empirical equations should be of great benefit for characterizing many types of nonchromatographic peaks. Future Studies. We have reported empirical equations based on peak height, width, and asymmetry for the accurate calculation of peak area for Gaussian and EMG peaks. These equations are useful in quantitative analysis and in the modeling of symmetric and tailed chromatographic peaks. They can easily be incorporated into the software of computing integrators and data acquisition systems. In the future, we plan to examine the applicability of the empirical area equations and empirical statistical moment equations for the detection and characterization of coeluting and partially resolved peaks.
LITERATURE CITED (1) Dezaro, D. A,; Floyd, T. R.; Raglione. T. V.; Hartwick, R. A. Chromatogr. Forum 1986, 1 , 34-37.
lQ87
(2) Grushka, E.; Myers, M. N.; Schettler, P. D.; Glddings, J. C. Anal. Chem. 1969, 41, 889-892. (3) Petltclerc, T.; Guiochon, G. J . Chromatogr. Sci. 1976, 14, 531-535. (4) Chesler, S. N.; Cram, S. P. Anal. Chem. 1971, 4 3 , 1922-1933. (5) Yau, W. W. Anal. Chem. 1977, 49, 395-398. (6) Kirkland, J. J.; Yau, W. W.; Stoklosa. H. J.; Dilks, C. H. J . ChromatOgr. Sei. 1977, 15, 303-316. (7) Rony, P. R.; Funk, J. E. J . Chromatogr. Sci. 1971, 9 , 215-219. (8) Anderson, D. P.; Walters. R. R. J . Chromatogr. Sci. 1984, 22, 353-359. (9) Barber, W. E.; Carr, P. W. Anal. Chem. 1981, 53, 1939-1942. (IO) Foley, J. P.; Dorsey, J. G. Anal. Chem. 1983, 55, 730-737. (11) Foley, J. P.; Dorsey, J. G. J . Chromatogr. Sci. 1984, 22, 40-46. (12) Hanggi, D.; Carr, P. W. Anal. Chem. 1985, 5 7 , 2394-2395. (13) Delley, R. Chromatographia 1984, 18, 374-382. (14) Delley, R. Anal. Chem. 1985, 5 7 , 388. (15) Ball, D. L.; Harris, W. E.; Habgood, H. W. Sep. Sci. 1967, 2 , 61-99. (16) Foley, J. P. J . Chromatogr. 1987, 384, 301-313. (17) Mendenhall, W.; Scheaffer, R. L. Mathematical Statistics with Applications; Duxbury: North Scituate, MA, 1973; pp 128-131. (18) Hayes, J. M.; Schwartz, H. E.; Vouros, P.; Karger, 8.L.; Thruston, A. D.; McGuire, J. M. Anal. Chem. 1984, 56, 1229-1236. (19) Bldlingmeyer, 6 . A.; Warren, F. V., Jr. Anal. Chem. 1984, 56, 1583A- 1598A. (20) Nlessen, W. M. A.; van Vliet, H. P. M.;Poppe, H. Chromatographia 1985, 19, 357-363. (21) Vanderslice, J. T.; Rosenfeld, A. G.; Beecher, G. R. Anal. Chim. Acta 1986, 179. 119-129. (22) Tyson, J. F. Anal. Chim. Acta 1986, 179, 131-148. (23) Leclerc, D. F.; Bloxham, P. A.; Toren, E. C.. Jr. Anal. Chim. Acta 1986, 184, 173-185. (24) Sturgeon, R. E.; Berman, S. S. Anal. Chem. 1985, 5 7 , 1268-1275. (25) Cedergren, A.; Frech, W.; Lundberg, E. Anal. Chem. 1984, 56, 1382-1387. (26) Winefordner, J. D., University of Florida, personal communication, 1986.
RECEIVED for review December 1, 1986.
Accepted April 17,
1987.
Determination of Parts-per-Billion Concentrations of Dioxane in Water and Soil by Purge and Trap Gas Chromatography/Mass Spectrometry or Charcoal Tube Enrichment Gas Chromatography Paul S. Epstein,* Theresa Mauer, Michael Wagner, Susan Chase, and Betsy Giles Clayton Environmental Consultants, Inc., Novi, Michigan 48050
Two methods for the determination of 1,Cdioxane in water have been studied. The first method Is a heated purge and trap gas chromatography/rnass spectrometry system following salting out with sodium sulfate. The second method is an adsorption on coconut-shell charcoal and solvent desorptlon with carbon disuHide/methanoi followed by analysis of the desorbate by gas chromatography with flame ionization detection. The first method Is also successful for the determination of 1,440xane in sollds and sediments. The second method is shown to be successful for 2-butanone, 4-methyl2-pentanone, and butoxyethanoiin water. The two methods are compared by analyzing 15 samples by both methods and achieving slmliar results.
The determination of 1,4-dioxane at parts-per-billion concentrations in water and soil samples presents difficulties because of the high water solubility of the compound. Friant and Suffert (1) have published a study that uses 1,4-dioxane as a model compound in the comparison of methods to im-
prove the headspace analysis of organics in water. They reported a theoretical limit of detection of 740 ppb for 1,4dioxane by a heated headspace method. Higher levels of 1,bdioxane in water (1 ppm) are amenable to analysis by direct injection onto a Porapak Q column and detection by flame-ionization gas chromatography (GC-FID) (2). Initial attempts to use the gas chromatography/mass spectrometry (GC/MS) purge and trap method (USEPA Method 624) based on the work of Bellar and Lichtenburg (3,4)were moderately successful. A limit of detection of 20 ppb was achieved. We undertook a study to see if we could either improve on the purge and trap method for this difficult to determine compound or develop an alternate method that would provide low limits of detection. Recent articles on the use of graphitized carbon black for the enrichment of trace organics from water (5,6) indicated that we might be able to develop a similar analysis for the l,4-dioxane. There is an established method (7) for the analysis of 1,Cdioxane in air. This method involves the adsorption of 1,4-dioxane onto coconut-shell charcoal and desorption with carbon disulfide and analysis by GC-FID. Two
0003-2700/S7/0359-1987$01.50/00 1987 American Chemical Soclety
1988
* ANALYTICAL CHEMISTRY, VOL. 59, NO. 15. AUGUST 1. 1987
Table I. Recovery of 1,4-Dioxane Spiked into Water and Analyzed by the Charcoal Tnhe Enrichment Method spike level, ppb
amt recovered, 70
sample volume, mL
2 2 2 2
98 76 103 129
4000 4000
6
75
in
86 66 59
in
100
ion
100
ion
325
64
67 66
90
88 85 65 63 65
4000 4000
innn ~~~~
inno inno ion0 ion0
1000 1000 2000 2000 2000
500 500
inon
total spiked, Pg
8
8 8 8 6
5 IO 100 1M)
100
100 650 650 650 516 516 2100
samples of water were spiked with lP-dioxane at 100 pph and analyzed by passing 1 L of each sample over 3000 mg of coconut shell charcoal contained in three glass tubes. Desorption with carbon disulfide/methanol (95/5 (v/v)) and analysis of the desorbate by GC-FID achieved recoveries of 66% and 67% for the two samples. The initial results were followed by analyzing a series of spiked samples ranging from 2 to 2100 pph (see Table I). There seems to be a trend toward higher recoveries a t the lower levels. This may be due to the effect of the large volumes of water on the adsorption properties of the charcoal (8). Another possible cause of the losses a t higher spike levels (suggested hy one of the reviewers) may be evaporation of the dioxane from the open vessels. Although dioxane boils slightly higher than water (101.1 "C), it does form an azeotrope that boils at 87.8 "C. With a 4-h exposure (4 L at 1L/h) this loss may be significant. Analysis of soil and sediment samples is not amenable to the adsorption-desorption method and has been carried out successfully by a modified version of the Bellar and Lichtenburg method. Five grams of soil is mixed with 5 mL of a 1.6 M sodium sulfate solution and purged at 50 " C for 11 min in a vessel modified for soils and sediments (see Experimental Section). Water samples can also he analyzed by this salted out purge and trap analysis hy dissolving enough sodium sulfate in the sample to bring the molarity of the sample to 1.6 M. The molarity level was chosen only for convenience and ease of reaching a complete solution as Friant and Suffet (I) reported that the optimum molarity for the analysis of 1,4-dioxane was 3.5 M sodium sulfate.
EXPERIMENTAL S E C T I O N Charcoal Tube Adsorption Method for Water Samples. The charcoal tube enrichment apparatus included (a) 1000-mg coconut shell charcoal tubes (SKC 226-16 or equivalent), (b) a fitted with an integral chromatography column (500 cm X 22 cm), 500-mL sample bulb and a Teflon stopcock, and (c) 10-mL glass bottles with Teflon-lined caps for sample desorption. The gas chromatographused for the analysis was a HewlettPackard Model 5880 with a splitless injector and a flame ionization detector. The column used for most of the analytical work was a J & W DB-5 narrow bore 60-m fused silica capillary column. A packed column (6 ft by 'I4 in. glass 1%SP-1000 on Carbopack B from Supelco, Inc., Bellefonte, PA) also was used in the early development work and successfullyseparated the 1A-dioxane peak from the solvent peaks and any interferences present. Standards used were prepared from reagent grade chemicals: 1,4-dioxane (Alfa), tetrahydrofuran (Aldrich), acetone (Mallincrodt), acrylonitrile (Aldrich), 2-butanone (Aldrich), 1-butanol
Flgure 1.
Apparatus for charcoal tube enrichment analysis.
(Aldrich), ethoxyethanol (Aldrich), 4-methyl-2-pentanone (Aldrich), butoxyethanol (Aldrich), methanol (Fisher), and carbon disulfide (Baker). The desorbing solution was prepared as a 5% (v/v) solution of reagent grade methanol in reagent grade carbon disullide. The apparatus was assembed as s h o w in Figure 1. The water sample was placed in the sample hulh and drawn through the charcoal tubes at a rate of 1 L/h. The charcoal was removed from the tubes and placed in the 10-mL vials. The charcoal was then covered with 5 mL of the carbon disulfide/methanol solution and allowed to stand for a t least 4 h with occasional agitation. Linearity of the GC-FID was established by injecting a series of standards at concentrations of 518, 5.18, 2.59, 0.52, and 0.26 pg/mL of dioxane in carbon disulfide. The GC oven was programmed from 50 to 100 'C a t 5 deg/min. The injection was 1pL with a splitless time of 45 8. Amounts of dioxane were calculated by using an external standard method against the response established from the linearity plot. Concentrations of the 1,4-dioxanein samples and spiked samples were calculated by using the following formula: area (extract) X pg/(area) X elution vol (pL) pg/L = pL inj x sample vol (L) where pg/L is the sample concentration, area (extract) is the GC-FID peak area for the sample, pg/area is the external standard response, elution vol is the amount of CS,/methanol used to desorb the sample, sample vol is the total sample amount passed over the charcoal in liters, and pL inj is the desorbate volume injected onto the gas chromatograph. Gas Chromatograph/Mass Spectrometer Purge and Trap Methods. The apparatus used for this consisted of a commercially available Tekmar LSC-2 purge and trap sample concentrator fitted with a 10-position automatic laboratory sampler (ALS) module and connected by a 0.028 i.d. stainless steel heated transfer line to the head of the gas chromatograph column of a HewlettPackard 5985B gas chromatograph/mass spectrometer (GC/MS). The trap was a Tekmar 12-0084-003(12 in.X in. stainless steel packed with Tenax-silica gel). The column was 6 ft X 'I8 in. glass column packed with 1% SP-loo0 on Carhopack B (60180 mesh). The GC/MS interface consisted of an all-glassjet separator. The mass spectrometer was tuned to meet the hromofluorobenzene tuning requirements of the U S . Environmental Protection Agency Contract Laboratory Program (9). The mass spectrometer was scanned from 35 to 260 amu at 3 s/scan, emission current was 300 FA, electron energy was IO eV, and multiplier voltage was 2100 V. Data acquisition was under the control of a HewlettPackard 1000 (HP-1000) minicomputer with a RTE 6/VM ouerating system Linearity of the instrument was established by analyzing four calibration standards at 2,10, 100, and 200 ppb 1,4-dioxane vs. an internal standard of 50 ppb 1,4-difluorobenzene. Response factors were calculated for the extracted ion current profile of m / z 88 for the 1,kdioxane vs. m / z 114 for the internal standard. Surrogate standards as described in the USEPA method were also analyzed along with the 1C-dioxane to monitor the instrument reliability. Water GC/MS Methods. Water samples were analyzed by adding enough sodium sulfate to 5 mL of the sample to bring the concentration to 1.6 M. (Higher molarity solutions were too difficult to achieve a t room temperature.) The solutions were
ANALYTICAL CHEMISTRY, VOL. 59, NO. 15, AUGUST 1, 1987
Table 11. Recovery of Other Analytes Spiked into Water and Analyzed by the Charcoal Tube Enrichment Method
analyte acetone acetone acetone
acetone acrylonitrile acrylonitrile acrylonitrile acrylonitrile 2-butanone 2-butanone 2-butanone 2-butanone 1-butanol 1-butanol 1-butanol 1-butanol ethoxyethanol ethoxyethanol ethoxyethanol ethoxyethanol 4-methyl-2-pentanone 4-methyl-2-pentanone 4-methyl-2-pentanone 4-methyl-2-pentanone butoxyethanol butoxyethanol butoxyethanol butoxvethanol
no. of tests 4 4 2 2 4 4 2 2 4 4 2
spike level, ppb 2 5
50 500 2
5 50 500 2
5 50
2 4 4 2 2
500
4 4
2
2 2 4
50
4 2 2 3 4 2 2
amt recovered,
2
5 500 500 5
90
standard level, ppb
1,4-dioxane response factor
tetrahydrofuran response factor 1.1 1.2 1.1 1.2 1.3 1.3 1.4
2
0.041
10 50 200 50
26
200
0.065 0.051 0.045 0.041 0.022 0.029
100
sodium sulfate Yes Yes Yes
yes no no no
27
202 151 56 49 0 0 0 1
0 0 8 8
2
71
5 50
72
5 50 500
Table 111. GC/MS Response Factors for 1,4-Dioxane and Tetrahydrofuran
0 0 0 0 0 0
500
500 2
1989
77 91 34 40 43 54
prepared in the barrel of a 10-mL glass syringe just prior to injection of the sample into the purge vessel of the Tekmar. The internal standard and surrogate solution were added to the sample and the sample was placed in the purge vessel. The vessel was surrounded with a 50 "C water bath and the sample was purged for 11 min with helium at a flow rate of 30 mL/min. Soil and Sediment GC/MS Methods. Five grams of soil sample is mixed with 5 mL of a 1.6 M sodium sulfate solution and placed in a sediment purge vessel on the Tekmar. The analysis is identical with the water method aa stated above except for the addition of the sodium sulfate as a solution. All GC/MS calculations were performed by the Aquarius software package on the HP-1000 minicomputer.
RESULTS AND DISCUSSION The charcoal tube enrichment (CTE) method was tested on a series of spiked samples at levels ranging from 2 to 2100 ppb (see Table I). The recoveries ranged from 63 to 129% with an average recovery of 77.5% and a standard deviation of 19. The instrumental limit of detection (based on a 2.5 X signal to noise in an interval around the expected retention time of the dioxane) yields an experimental limit of detection of less than 1ppb in a 4-L sample. The h i t of quantitation is higher and around 2 ppb. The CTE method was tested on other water-soluble solvents with partially successful results (see Table 11). It appears that this method can be used to analyze water samples for 4-methyl-2-pentanone and butoxyethanol. There were some problems getting blank water free of 2-butanone, although it appears that the CTE method would be successful for this parameter as well. There was some recovery of acrylonitrile but not enough for this to be a viable method of analysis. Butanol and ethoxyethanol were not successful a t all and are probably not being desorbed by the carbon disulfide/methanol solution. The charcoal tubes are inexpensive and readily available and are used once and then discarded. This provides an analysis with little prospect for carryover and contamination from sample to sample. This is one major advantage of this method over the GC/MS purge
Table IV. Recovery of 1,4-Dioxane and Tetrahydrofuran Spiked into Water and Soil and Analyzed by the GC/MS Heated Purge and Trap Method spike analyte 1,4-dioxane 1,4-dioxane 1,Cdioxane 1,4-dioxane l,4-dioxane 1,4-dioxane 1,4-dioxane 1,Cdioxane tetrahydrofuran tetrahydrofuran tetrahydrofuran tetrahydrofuran tetrahvdrofuran
level,
amt recovered,
PPb
%
10
66
water
10
82 83 79 87 102 91 88 50
water water
10
10 20 20
50 50 10 10 10 50
50
47
106 99 98
matrix type
soil soil soil water
sodium sulfate added no no Yes
Yes
water water water
Yes Yes Yes Yes no no
water
yes
water water
Yes
ves
and trap methodology to be discussed below. The GC/MS purge and trap method, when modified by the salting-out technique described above, provides another method to analyze water and soil samples for 1,4-dioxane at parts-per-billion levels. Linearity was established down to the 2 ppb level (see Table 111) as opposed to the standard purge and trap method where linearity was established down to 50 ppb with a lower average response factor (0.0505 vs. 0.0306) and a larger coefficient of variation (0.20 for salting our vs. 0.29 for the standard method). The differences in relative response to the internal standard were on the borderline of statistical significance because of the small number of measurements. However, the trend seems to indicate a higher relative response for the 1,4-dioxane. The major difference operationally between salting out and not salting out was the ability to establish linearity down to the 2 ppb level with a small sample volume ( 5 mL purged). Spike recoveries for samples spiked at several levels with 1,4-dioxane averaged 85% with a standard deviation of 9.7 (see Table IV). Recoveries for samples spiked with tetrahydrofuran averaged 99% for the salted-out heated purge and trap method compared with 48.5% for two samples spiked at 10 ppb and analyzed by the standard US EPA Method 624 (Table IV). The success of increasing the salt concentration for the analysis of the water-soluble compounds 1,4-dioxane and tetrahydrofuran indicates that this technique may be generally applicable to the large number of water-soluble solvents that are appearing as environmental contaminants. Compounds such as acetone, 2-butanone, and butoxyethanol are difficult to analyze at part-per-billion levels because of their water solubility. When 1,4dioxane is analyzed for by GC/MS, it is important that the analyst scan for and monitor the ion at 58 amu. An interference was observed with a similar retention time and a parent ion at amu 88 (identical with the 1,4-dioxane). The interference exhibited a small ion abundance at amu 58 and
Anal. Chem. 1987, 59, 1990-1995
1990
Table V. Comparison of the Charcoal Tube Enrichment Method and GC/MS Purge and Trap Analysis
sample 1 2 3 4
5 6 7 8 9
10 11 12 13 14 15
results by
results by
GC/MS,
CTE GC-FID, PglL
PdL
28
68
31 85
57 130
98
