Anal. Chem. W Q O , 62, 1355-1356
and deexcitation. Therefore, phase-specific ion isolation is simplified by eliminating the phase delay. These unique waveforms can be stored as single excitation pulses for future selected ion studies. (1) (2) (3)
LITERATURE CITED Alllson, J.; Stepnowskl, R. M. Anal. Chem. 1987, 59, 313. Comisarow, M. B.: Marshall, A. G. Chem. Pnys. Left. 1974, 26. 489. Anders, L. R.: Beauchamp. J. L.; Dunbar, R. C.; Baldeschwieler,J. D.
J . Chem. phys. 1966, 45. 1062. (4) Kerley, E. L.; Russell, D. H. Anal. Chem. 1989, 67, 53. (5) Castro, M. E.; Russell, D. H. Anal. Chem. 1985, 5 7 , 2290. (6) Wang. T. L.: Rlcca. T. L.; Marshall, A. G. Anal. Chem. 1988, 58, 2935. (7) Marshall, A. G.; Roe. D. C. J . Chem. phvs. 1980, 73, 1581. (8) Castro. M. E.; Russell, D. H. Anal. Chem. 1985, 5 7 , 2290-2293.
(9)
1355
Marshall, A. G.; Wang, T. C.; Rlcca, T. L. J . Am. Chem. Soc.1985, 107, 7893. Mullen, S. L.; Marshall, A. 0. J . Am. Chem. Soc. 1988, 170, 1766.
(10) (11) Hanm, C. D.; Kerley, E. L.; Russell, D. H. Anal. Chem. 1989, 67, 83. (12) Ghaderl. S.; Littlejohn, D. Roceedhgs of ihe 3W, Annual Contenmce on Mass Spectrometry and A M , Sen Dlego ; Amerlcan Sock ety for Mass Spectrometry: East Lansing, MI. 1985; p 727.
' r
RECEIVEDfor review August 7, 1989. Revised manuscript received January 22,1989. Accepted February 5,1990. This work was supported by the National Science Foundation (CHE-8418457). We gratefully acknowledge the Texas A&M University Office of University Research Services and the College of Science for providing a portion of the funds for purchase of 'the Nicolet FTMS 1000 mass spectrometer.
Salting-Out Solvent Extraction for Preconcentrationof Neutral Polar Organic Solutes from Water Daniel C. Leggett,* Thomas F. Jenkins, and Paul H. Miyares Research Division, US.Army Cold Regions Research & Engineering Laboratory, Hanover, New Hampshire 03755
It appears there has been very little exploitation of salting out with water-miscible solvents for extraction of organic solutes from water. Although this technique is known to many (particularly older) chemists, we found no specific literature references to salting-out of organic compounds as a prelude to their determination in water, save one recent abstract (I). This technique has, however, been used for a number of years for extraction of metal-chelates into organic solvents prior to atomic absorption, high-performance liquid chromatography, polarographic, or colorimetric analysis (2-5).So, although we freely acknowledge that the technique itself is not new, we do feel that its potential applications in organic trace analysis of water have not been properly appreciated or utilized. We describe here just one of many possible examples, which has found considerable utility in our laboratory.
Table I. Recoveries of Nitroaromatics, Nitramines, and Nitrate Esters by Solvent Extraction
EXPERIMENTAL SECTION Extraction. Solutes dissolved in acetonitrile were spiked into 400 mL of either Milli-Q water or well water in a 500-mL separating funnel. To this was added 130 g of sodium chloride. After dissolution of the NaC1,lOO mL of acetonitrile was added. The funnel was stoppered and shaken vigorously for 5 min. The phases were allowed to separate and both the acetonitrile and aqueous phases analyzed by reversed-phase high-performance liquid chromatography (RP-HPLC) (6). For comparative purposes, extractions were also done with methylene chloride, with and without 130 g of added salt. In either case 400 mL of water was extracted with 20 mL of methylene chloride. Instrumentation. Liquid chromatographic analysis was performed on a modular system composed of an SP8810 pump, an SP8490 variable wavelength detector (254 nm), and a Dynatech LC-241 autoinjector equipped with a 100-pL loop. A 25 cm x 4.6 mm LC-18 (Supelco) column was used for separation with 1:l (v/v) methanol/H20 eluting solvent (1.5 mL/min). Reagents. 2,6-Dinitroluene, 2,4-dinitrotoluene (DNT), 2,4,6trinitrotoluene (TNT), 1,3,5-trinitrobenzene (TNB), 1,3-dinitrobenzene (DNB), glyceryl trinitrate (NG), pentaerythritol trinitrate (PETN), hexahydro-1,3,5-trinitro-l,3,5-triazine (RDX), and octahydro-l,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX) were standard analytical reference materials (SARMs) obtained from U.S. Army Toxic and Hazardous Materials Agency, Aberdeen Proving Ground, MD; 2-amino-4,6-dinitrotoluene(2ADNT) was furnished by Dr. David Kaplan, U.S. Army Natick Research Development and Engineering Center, Natick, MA; nitrobenzene
NG 99.7 PETN >99.9 ONote: The pooled standard deviation for all analytes except HMX was 0.9%. For HMX it was 4%.
(%)O
analyte
CHzC12
HMX RDX TNB DNB NB TNT 2,4DNT 2,6DNT 2ADNT 2NT 3NT 4NT
23.6 59.6 88.1 89.8 89.7
extractant CHZCl2+ NaCl CH&N
94.0 94.4
73.6 83.1 92.7
94.5 95.7 96.3 96.5
+ NaCl
95.6 93.9 96.5 95.6 94.2 98.9 98.0
97.9 96.5 96.9 96.9 97.1
(NB), 2-nitrotoluene (2NT), 3-nitrotoluene (3NT), and 4-nitrotoluene (4NT) were obtained from Baker or Eastman Kodak. All were used without further purification. Acetonitrile, methanol, and methylene chloride were Baker HPLC grade; house-distilled water was further purified with a Millipore Milli-Q system.
RESULTS AND DISCUSSION A preliminary experiment was conducted to compare acetonitrile with methylene chloride both with and without added salt for extraction of these compounds. Methylene chloride was selected for this comparison as it is one of the most polar solvents of those generally used for extraction of analytes from water. Much poorer extraction efficiencies would be obtained with a nonpolar solvent such as hexane. Since the recoveries with acetonitrile were remarkably good, we did a second experiment in order to test its efficiency on a larger number of nitroaromatics and nitrate esters. These combined results are summarized in Table I. Acetonitrile is clearly superior to methylene chloride in extracting these compounds, especially RDX and HMX. Although significant improvement was also observed for methylene chloride when salt was added, the ease
This article not subject to U S . Copyright. Published 1990 by the American Chemical Society
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Anal. Chem. 19Q0,62,1356-1360
of phase separation was reduced. The final measured volumes of aqueous and acetonitrile phases were 518 and 22.0 mL, respectively. Thus in one extraction a volume reduction factor of 18.2 (400/22) was achieved with individual recoveries ranging from 93% to 100% for thew compounds, giving an overall preconcentration factor of >16. Near quantitative recoveries could be obtained with two successive extractions but would sacrifice overall convenience and sensitivity. However, the acetonitrile can be evaporated to give a greater preconcentration factor if desired (7). It has also been confirmed that recoveries are not dependent on analyte concentration over at least a 200-fold range in concentration (7), nor does the presence of chloride in the acetonitrile phase have any effect on analyte retention times. There are several reasons to consider salting out as a sample preparation technique. With the advent of reversed-phase HPLC as the method of choice for an increasing number of analytes, it is often desirable to have the analytes in a water-miscible solvent prior to HPLC analysis. Several HPLC solvents including acetonitrile, 2-propanol, and tetrahydrofuran can readily be salted from water. Salting out can thus be used as a preconcentration technique either with or without solvent evaporation. In addition, better recoveries of analytes are attained by salting out relative to normal solvent extraction, and matrix effects related to differences in ionic strength are minimized. We observed slightly higher recoveries of HMX and RDX from well water than from Milli-Q extracted with methylene chloride when no salt was added. Use of environmentally less acceptable chlorinated solvents is also avoided, and perhaps most importantly of all, polar solvents, in general, extract polar solvents more efficiently than do nonpolar solvents. Furthermore, as subsequent work has shown, greater advantage can be taken of specific solventsolute interactions due to the greater range of solvent func-
tionalities available in salting-out systems (8, 9).
ACKNOWLEDGMENT This work was funded by U.S. Army Toxic and Hazardous Materials Agency, Analytical Technology and Development Program, Martin H. Stutz, project monitor. The contents of this paper were also presented at the Poster Session of the 103rd Annual AOAC Conference, St. Louis, MO, Sept 25-28, 1989. W s t m NO. HMX, 2691-41-0; RDX, 121-82-4;TNB, 99-35-4; DNB, 99-65-0; NB, 98-95-3; TNT, 118-96-7; 2,4DNT, 121-14-2; 2,6DNT, 606-20-2; SADNT, 35572-78-2; 2NT, 88-72-2; 3NT, 9908-1; 4NT, 99-99-0; NG, 55-63-0; PETN, 1607-17-6;CH2C12,7509-2; CH,CN, 75-05-8; water, 7732-18-5. LITERATURE CITED Hertz, C. D.; Schnable, J. G.; Suffet, I. H. Preprint, American Chemical Society Conference, Dallas, TX, 1989. Morrison, G. H. Anal. Chem. 19S0, 22, 1388-1393. Matkovlch, C. E.; Christian, G. D. Anal. Chem. 1979, 45, 1915-1921. N ~ @ o sY. ~ ,Anal. Chkn. Acte 1980, 120, 279-287. Mueiler, 8. J.; Lovett. R. J. Anal. Chem. 1987, 59, 1405-1409. Jenkins, T. F.: Mlyares, P. H.; Walsh, M. E. An Improved RP-HPLC Method for Determining Nitroaromatics and Nltramines in Water; CRREL SR 88-23; U.S. Army Cold R w n s Research and Englneerlng Laboratory: Hanover, NH, 1988. Mlyares, P. H.: Jenkins, T. F. Saltingat Solvent Extraction Method for Lowlevel Determinatlon of Nitroaromatics and Nitramlneg in Water; LeboCRREL SR; US. Army Cold Regions Research and Engineratorv: Hanover. NH. in Dress. (8) Leggin, D. c . R~CWU& A ~ chsmlcei Y ~eseerdc.oevenopment and EnghkWng Center Sc&ntlflc Conlkenee on chsmlcal Defense Resaarch, A k d a e n Roving &wnd. w , November 1989, U.S. Army Chemical Research,Development and Engineering Center, Aberdeen Proving Ground, MD, in press. (9) Leggett, D. C. Sdvent/Water Partltbnhg and Extradon of Dimethylm e m n a t e : Importance of H-bondhg; CRRa SR; U.S. Army Cold Regions Research and Engineering Laboratory: Hanover, NH, in press.
us.
RECEIVED for review August 1,1989. Accepted March 2,1990.
Differential Densometric Analysis of Equllibrla in Hlghly Concentrated Media: Determfnatkn of the Aqueous Second Acid Dissociation Constant of H,S Stuart Licht,* Fardad Forouzan, and Kevin Longo Department of Chemistry, Clark University, Worcester, Massachusetts 01610 INTRODUCTION Conventional analytical techniques are often not suitable for in situ measurements of highly concentrated electrolytes. Yet there are research and industrial environments in which high concentrations are typical. In high concentration electrolytes, an initial dilution step can alter equilibria and may be inappropriate prior to solution analysis. Therefore, we sought alternative analytical techniques that would be highly appropriate to concentrated media. We wish to report a liquid phase differential densometric technique and demonstrate its utility to determine the problematic aqueous thermodynamics of free sulfide. Unlike conventional spectroscopic or electroanalytical techniques, this technique for analysis becomes increasingly useful with increasing concentration. A recent substantial revision of aqueous metal sulfide thermochemistry (1)was based on three (2-4) studies indicating AGOse (aqueous) = 111 *2 kJ/mol (compared to the commonly used IUPAC value which is 25 kJ/mol smaller (5)). As will be shown, this large free energy of formation for suKde
* Author to whom correspondence should be addressed. 0003-2700/90/0362-1356$02.50/0
suggests that significant free sulfide species will exist only in extremely high ionic strength solutions. Hence, each of the sulfide studies was carried out in highly concentrated (up to 18 M) aqueous solutions. Of these three studies, the spectroscopic studies (2, 3) assume concentration-independent absorptivities (2) or Raman coefficients (3)and can be strongly affected by the spectra of related species and contaminants, while the potentiometric study assumes concentration-independent liquid junction potentials (4). Validation of these assumptions is difficult in the highly concentrated medium necessary to achieve significant activities of free S2-. Aqueous sulfide thermochemistry is the basis for the processes in the food, petroleum, pulp, and paper industries, for fundamental studies in geochemistry, semiconductor physics, environmental and health sciences, physical, analytical inorganic chemistry, and new concepts in energy technology (6-11). A cornerstone of the thermodynamics of aqueous metal sulfide chemistry is the fundamental equilibria of hydrogen sulfide. There have been over 50 studies of the second aqueous acid dissociation of H2S
HS-zH++ S2@ 1990 American Chemical Soclety
