1494
Anal. Chem. 1985, 57, 1494-1496
alyzing the extract by GC/MS by on-column methylation. A 4:l ratio of a methanolic solution of the sample extract and 10% TMAH in methanol in tandem in a syringe was injected onto the GC column under standard GC/MS conditions. The results were similar to those obtained from eluants which were further cleaned up in phase transfer alkylation described above. The advent of these disposable cartridges presages a wider use in analytical toxicology because of a number of inherent advantages. They provide a rapid extraction of a relatively large volume of urine with a small volume of solvent. The recovery of a polar cannabinoid compound is virtually quantitative and the results show excellent cleanup and reproducibility. We feel that further adaptions of these cartridges to the extraction of other drugs-of-abuse and organic poisons in biological materials are in the offing. ACKNOWLEDGMENT The authors are indebted to John S. Polles, Commander of this laboratory, for reading the manuscript and suggesting improvements. Registry No. g-THC-COOH, 56354-06-4.
LITERATURE C I T E D Williams, P. E.; Moffat, A. C. J . fharm. Pharmacol. 1980, 32, 445. McBay, A. J.; Dubowski, K. M.; Finkle, B. S. J . Am. Med. Assoc. 1983, 249, 881. Whiting, J. D.; Manders, W. W. J . Anal. Toxicol. 1982, 6 , 49. Foltz, R. L.; Fentiman, A. F.: Foltz, R. B. "GCIMS Assays for Abused Drugs in Body Fluids"; National Institute of Drug Abuse: Rockville, MD, 1980; NIDA Res. Monogr. 32, p 62. Whiting, J. D.: Manders. W. W. Aerosp. Med. 1983, 54, 1031. Nakamura, G. R.; Stail, W. J.; Folen, V. A.; Masters, R. G. J . Chromatogr. 1983, 264, 336. Elsohly, M. A.; Elsohly, H. N.; Jones, A. B.; Dimson, P. A,; Wells, K. E. J . Anal. Toxicol. 1983, 7 , 262. Kogan, M. J.; Newman, E.; Wilson, N. J. J . Chromatogr. 1984, 306, 441. Kaistha, K. D.; Tadrus, R. J . Chromatogr. 1982, 237, 528. Narasemhachari, N. J . Chromatogr. 1981, 225, 189. Mutaz, M.; Narasimhachari, N.; Friedel, R. 0. Anal. Biochem. 1982, 126, 365. Suzuki, S.;Inoue, T.; Niwaguchi, T. J . Chromatogr. 1983, 267, 381. Shackleton, C. H. L.; Whitney, J. 0. Clln. Chim. Acta 1980, 107, 231. O'Conner, J. E.; Rejent, T. A. J . Anal. Toxicol. 1981, 5 , 168.
RECEIVED for review November 5, :984. Accepted February 15,1985. The article represents the opinions of the authors and does not necessarily reflect the views of the Department of the Army or the Department of Defense.
Recovery of Dichlorobenzenes, Nitrobenzene, and Naphthalene during Evaporative Concentration E d w a r d W. M a t t h e w s
U.S. Geological Survey, 6481 -HPeachtree Industrial Blvd., Doraville, Georgia 30340 Extraction of several semivolatile compounds from a water sample, notably dichlorobenzenes, nitrobenzene, and naphthalene, can be achieved with methylene chloride, as indicated by the analytical methods proposed in the Federal Register ( I ) and implemented by the Environmental Protection Agency (2). Concentration of the extract is carried out with a Kuderna-Danish concentrator (K-D) equipped with a Snyder column and a graduated receiver. In this laboratory, the extract is concentrated further in a warm water bath by passing nitrogen over the extract surface. The total process combines the best macro and micro concentration techniques that were evaluated by Erickson et al. (3) for a variety of organic compounds. The procedure is neither labor-intensive nor time-consuming. However, it can lead to both low and nonreproducible recoveries of these compounds, typically from about 40 t o 90%, as reported by the EPA ( 2 ) and the U.S. Geological Survey ( 4 ) . Clearly, meaningful determinations cannot be performed with these compounds until losses are significantly reduced. The work of Higgins and Guerin (5) showed that losses of naphthalene from spiked solvent systems could be minimized by the use of a Tenax-GC trapping system during evaporative concentration and suggested the possibility that the same trap might be employed for the benzene derivatives as well. Although the Tenax trapping system proved to be technically feasible, the entire procedure was complicated and required far too much time for a production laboratory. Therefore, a simpler and faster technique was sought. T o establish whether conditions in the last stage of evaporative analyte concentration could be standardized to improve the recovery of the benzene derivatives and naphthalene, a precision pressure regulator and a toggle cutoff valve were
added to the nitrogen line of the evaporative concentrator. Subsequent testing established that these minor alterations significantly improved recoveries of all compounds. EXPERIMENTAL S E C T I O N Materials. Naphthalene, nitrobenzene, and 0- and p-dichlorobenzene standards were obtained from EPA, Research Triangle Park, NC. Solvents (hexane, isooctane, methanol, methylene chloride, and toluene) were from Burdick and Jackson, Muskegon, MI. Apparatus. The following were employed: a 250-mL K-D with a 10-mL graduated concentrator tube and a one-ball Snyder column; an evaporative concentrator (N-Evap, Organomation Associates, Inc.) with a 1to 10 psi pressure regulator (Model 8601, Brooks Instruments) and an in-line togggle cutoff valve. Analyses were performed with a Hewlett-Packard5880A gas chromatograph equipped with a flame ionization detector (FID). Separations were accomplished with a 0.2 mm i.d. by 12 m fused silica capillary column with a thin film coating of OV-101. Conditions of analyses were as follows: column flow, 1.7 mL/min nitrogen, splitless injection with injector at 200 "C; initial oven temperature 45 "C for 4 min and then programmed at 5 "C/min for 7 min; FID at 275 "C, hydrogen 30 mL/min, air 410 mL/min, nitrogen makeup 28 mL/min. Procedure. Analyte standards were prepared in toluene. About 7 5 mL of solution (50 mL of hexane and 25 mL of methylene chloride) was placed in the K-D. (In this laboratory, water samples are extracted with methylene chloride. Sufficient hexane is then added to bring the methylene chloride t o the surface of the water. This technique has been found to be helpful to eliminate emulsion difficulties which are frequently encountered with environmental samples.) Standard solutions were transferred to this mixture with volumetric micropipets. A boiling chip and about 2 mL of isooctane "keeper" (to prevent total solvent and analyte loss) were added to the solution. A Snyder column was
Thls article not subject to US. Copyright. Published 1985 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985 e 1495
-
_ I
Table IV. Recovery from Spiked Solvent System with 2 mL of Isooctane"
Table I. Recovery from 500-mL Spiked Water Samples" amt added, recovery, mean recovery, re1 std ng (70 7c dev, 7~
compound p-dichlorobenzene
21
40-93
68
25
naphthalene
20
57-88
74
15
mL of isooctane added before K-D concentration. N-Evap controls not standardized. 3' = 9. O5
Table 11. Recovery from Spiked SolventSystem with 5 mL of Isooctane" amt added, recovery, mean recovery, re1 std ng % % dev, %
compound p-dichlorobenzene nitrobenzene naphthalene a
244
31-81
59
28
236 257
36-85 42-89
62 71
25 21
Same conditions as in Table I. N
ng
= 10.
p-dichlorobenzene o-dichlorobenzene nitrobenzene naphthalene
21 12 32 20
in hexane (2 mL)
in methanol ( 1 mL)
81 74 79
94 97 97 100
100
-
p-dichlorobenzene o-dichlorobenzene nitrobenzene naphthalene
11
-
attached and the apparatus was placed on an 85 "C water bath. The apparatus was removed from the bath when the column ball stopped bouncing, the contents were cooled, and the bottom joint was dried with a towel. Between 2 and 5 mL of solvent remained in the receiver. The receiver was removed, and the ground glass joints were rinsed into the receiver with small amounts of isooctane (total rinse volume, about 1 mL). The N-Evap water bath temperature was set at 53 i= 1OC. The nitrogen flow rate on the apparatus was set by opening all needle valves fully and then the in-line pressure regulator was adjusted until the rate at each needle was 60 i= 5 mL/min as measured with a bubble flowmeter. Once the flow was set, valves were not changed; nitrogen was turned off at all needles with a single in-line toggle valve. Receivers were placed on the N-Evap rack and were immersed in the water to the 0.5 mL mark. Needles were positioned so that each was 4 mm below the lip of the receiver. When the analyte was reduced to approximately 1mL, the receivers were removed and capped with a ground glass stopper. Subsequently, the analyte was brought to volume with isooctane and stirred with a vortex mixer before analysis.
RESULTS AND DISCUSSION Initial work assessed the recovery of p-dichlorobenzene and naphthalene from spiked 500-mL water samples. In the final concentration step, the N-Evap water temperature was 55 "C, and all other conditions were estimated. Table I shows that recoveries ranged from 40 to 93%. When spiked solvent systems were similarly concentrated, nearly the same results were obtained, as seen in Table 11. Evaluation of the Tenax trapping system confirmed that when hexane was used as an eluent, naphthalene recovery was quantitative but recovery of the other compounds of interest was not. When methanol was substituted as eluent, more than 93% of each compound was recovered, as indicated in Table 111. Trap preparation, solvent evaporation, and trap elution
%
dev, %
86-99
94
10
31
88-103
94
6
46 32
81-98 92-102
93 98
3
70
6
N-Evap conditions controlled as specified in text. N = 7.
-
Table V. Recovery from Spiked Solvent System with 5 m c of Isooctanen
o-dichlorobenzene nitrobenzene naphthalene
% recoverv
amt added,
ng
compound
Table 111. Recovery from 0.5-mL Tenax Trap after Solvent Evaporation to Dryness, N = 2
compound
amt added, recovery, mean recovery, re1 std
compound
amt added, recovery, mean recovery, re1 std 70 % dev, % ng 31
75-97
80
5
46 32
75-95 88-102
82 95
6 4
"Same conditions as for Table IV. N = 8. required about 4 h more than standard evaporation techniques. Further work on evaporative conditions with spiked solvent systems established that the wide variations of recoveries (usually between 40 and 90%) of semivolatile, solvent-extracted compounds were primarily because standardized conditions were not employed in the final reduction step. Specific problems included: (1)nitrogen flow rates increased at other sample ports as each sample was removed and its valve was turned off; (2) the needle above the solvent was repositioned as the volume of the analyte decreased; (3) the depth that each receiver was immersed in the water bath depended upon the amount of analyte present, which affected the rate of heat transfer t o the sample; and (4) higher than optimum temperatures were employed with the water bath to speed the evaporation process. After a precision pressure regulator and a toggle cutoff valve were added to the N-Evap, the nitrogen flow rate over each analyte could be accurately controlled. This minor change, together with standardization of all other conditions during the final analyte concentration phase resulted in the improved recoveries shown in Table IV. Average recoveries of all compounds exceeded 90%. The relative standard deviation of recoveries was less than or equal to 6%, with the exception of p-dichlorobenzene, which was 10%. Although recovery studies were not carried out for rn-dichlorobenzene, this procedure should apply to it as well. During initial controlled evaporation studies, 5 mL of isooctane keeper was used during the K-D concentration phase. This amount resulted in about 80% recovery for most of the compounds, as given in Table V. The additional 3 mL of isooctane caused the N-Evap concentration time to be extended considerably, which probably contributed to additional losses. Therefore, the volume of the isooctane was reduced to 2 mL, which led to signficantly improved recoveries, in the order of 95%. In conclusion, for a small number of samples either the procedure of Higgins and Guerin (5) using methanol to elute the Tenax trap, or stringent controls on the evaporative conditions of analytes can yield between 90 and 100% recoveries for these compounds. This is an improvement of nearly 30% over nonstandard evaporative conditions. The standardized procedure presented here requires about 3 I I 2 h less for sample preparation than the Tenax trap, without
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Anal. Chem. 1985, 57, 1496-1498
signficantly affecting the quality of determinations. For a production facility with a large sample throughput, when both labor and time are important factors, the latter procedure will be more cost-effective. Registry No. p-Dichlorobenzene,106-46-7;o-dichlorobenzene, 95-50-1; nitrobenzene, 98-95-3; naphthalene, 91-20-3; water, 7732-18-5; Tenax-GC, 24938-68-9.
LITERATURE CITED
Cincinnati, OH, 1982; EPA 600/4-82-057. (3) Erickson, M. D.; Giguere, M. T.; Whitaker, D. A. Anal. Lett. 1981, 74 ( A l l ) , 841-857. (4) U S . Geological Survey "Methods for the Determination of Organic Substances in Water and Fluvial Sediments"; USGS Techniques of Water Resources Investigations, Book 5, Laboratory Analysis, Chapter A3; U S . Geological Survey: Reston, VA, 1983; USGS Open-File Report 82-1004, 165-173. (5) Higgins, C. E.; Guerin, M. R. Anal. Chem. 1980, 52, 1984-1987.
RECEIVED for review September, 17,1984. Accepted January
(1) Fed. Reglst 1979, 44 (No. 233), 69540.
(2) U S . Environmental Protection Agency "Test Methods for Organic Chemical Analysis of Municipal and Industrial Wastewater"; Test Method 625; USEPA Environmental Monitoring and Support Laboratory:
14,1985. The use of brand names in this report is for identification Purposes only and does not imply endorsement by the U.S. Geological Survey.
Stabilized Gravimetric Standard for Sulfites and Sulfur Dioxide Knut Irgum Department o f Analytical Chemistry, University of Umei, S-901 87 Umei, Sweden Preparation of standards in the analysis of sulfites and sulfur dioxide can be a problem, especially if higher accuracy is needed, as the sulfite salts commonly used to prepare such standards are not available with guaranteed assays due to oxidation (1). They must therefore be assayed by titrimetry immediately before use. Sulfate originating from oxidation of the sulfite salt can also be troublesome if a mixed sulfite and sulfate standard is to be prepared for, Le., ion chromatography. Another compound that has been used as standard for tetravalent sulfur is sodium hydroxymethanesulfonate (HMS) (2, 3 ) . Aldehyde addition compounds are also used for stabilization of sulfur dioxide after sampling ( 4 , 5).
HSO3- + CHZ(0H)Z +== CHZ(0H)SOs-+ HzO
(1)
The reaction is fast and strongly shifted toward the addition compound a t pH 4-5. Stability of HMS toward oxidation is good, but commercially available preparations are practical grade (6) and must be purified before use. HMS is furthermore hygroscopic and decomposes on heating (7). These are drawbacks which make it far from ideal as a standard substance. This paper describes in situ preparation of this compound by purging a buffered formaldehyde solution with S02(g). Stabilities of both stock solutions and dilutions are checked.
EXPERIMENTAL SECTION Reagents. Sulfur dioxide (99.97%)was purchased in a tin can from Fluka (Buchs,Switzerland) and fitted with the recommended valve. All other reagents were reagent grade. Preparation of Stock Standard. The aldehyde and carboxylate salt were dissolved in 300 mL of water in a tall 400-mL polypropylene beaker and placed on an electronic balance, whose capacity was 400 g with a three-decimal window of f 4 0 g around the tared weight. The beaker was fitted with a sheet of plastic filmhaving a 8-mm hole provided for the glass capillary, through which the gas was applied. The capillary was drawn from a dropping pipet and silylated with dimethyldichlorosilane. Its orifice size was determined empirically to give a stream of tiny S02(g)bubbles that were absorbed by the solution before reaching the surface. The balance was tared and the SOz(g)flow turned on. When the tube connecting the valve and the capillary was filled with S02(g),the capillary was lowered through the hole in the plastic cover and 3-4 cm into the solution. After approximately 5 min, when nearly 0.1 mol of sulfur dioxide had been added to the solution, the SOz(g) flow was turned down and the capillary slowly
lifted out of the solution with a low flow still being continued. This is the most critical part of the preparation, as no liquid must stick to the capillary on retraction. It is also important that the capillary is lifted out of the solution before the SOz(g)flow comes to a complete stop. If, despite these precautions, some of the solution sticks to the capillary, it can be transferred to the plastic cover, weighed together with the rest of the solution and later washed into it. The solution was allowed to stand for a short while to assure that all sulfur dioxide in the space above the solution was absorbed. It was then weighed, transferred to a 1-Lvolumetric flask, and diluted with water. Ion Chromatographic Determination of Sulfate. The prepared standards were diluted to 1 mM with 3.5 mM NaHC03. Their sulfate content was determined with an ion chromatographic system consisting of an LDC Constametric I11 pump, a Rheodyne 70-10 injection valve with 100-yL sample loop, two Dionex 4 X 50 mm Anion Guard columns (no. 30825) in series, and a 5.7 X 300 mm suppressor column packed with Bio-Rad AG 5OW-X16 (200-400 mesh). An LDC Conduct0 Monitor equipped with a Model 7011 cell was used to monitor effluent conductivity. Eluent was 3.5 mM Na2C03/2.6mM NaOH ( 3 ) . Diluted standards were analyzed for S(1V) by a pararosaniline colorimetric method ( 5 ) .
RESULTS AND DISCUSSION Several factors affect the accuracy obtainable in the preparation of a standard in the way described here. Some of these are related to the way the gas is applied to the solution, while others are connected with the ability of the aldehyde to prevent sulfur dioxide absorbed in the solution from being oxidized by molecular oxygen. Errors due to Inaccuracy in Weighing the S02(g). I t is obvious that the larger the amount of sulfur dioxide used to prepare the standard, the higher the accuracy will be, as the relative contribution from constant errors will decrease. The manufacturer specifies an accuracy of jz0.002 g for the balance used. In the preparation of a 0.1 M standard, the uncertainty due to the balance alone will be jz0.03%. Repeated experiments with dipping the capillary into the solution and raising it quickly to purposely make drops stick to its surface showed that this would introduce an error of 0.05% a t most. Attempts to use a balance of higher accuracy will not be fruitful, as the errors due to factors like drops sticking to the capillary and vaporization of liquid from the absorption solution are already limiting the accuracy. Vaporization of Water during Purging with S02(g). Absorption of sulfur dioxide by the buffered aldehyde absorber
0 1985 American Chemical Society 0003-2700/85/0357-1496$01.50/0
