base. The compressed inverted file structure can be readily adapted to storage and retrieval of this information. The essential step is to affect a simple transformation on standard molecular formulas. This reduces to “dividing” each molecular formula by “-CH2-” (the homologous component) which removes all the -CH2- contributions to the molecular formula. If X is used to denote the basic unit -CH2-, molecular formulas can be written as homologous formulas in which the “class” component appears separately. A set of examples is given in Figure 3. The homologous series log-compressed inverted file is constructed by subtracting the homologous contribution from the accurate molecular mass, e.g. C18H2402
C12H24
+
X12c602
+
+
272.178
168.188 103.990
+
:. C 6 0 2-+
-
The residual accurate mass for CsOzcan be converted to an integer in the range 0 2 OOOOOO. The procedure for encoding the binary representations of the residuals then parallels the procedure for molecular formulas as does the retrieval operation. The inverted homologous series file can be coupled with a separate compressed inverted file that represents the number of -CH2- components for each molecular formula. By searching the respective files for C602and X12(referring to
the previous example) and then doing a Boolean AND to combine the two sets of results, the formula XlzCsOzand hence CI8Hz4O2is retrieved. This file structure tallows for ranges of homologous series (e.g., XI7 Xzo)to be searched and i t is also convenient for standard molecular formula searches. To accommodate molecular weights up to 2000 amu (e.g., XI x256 = 2 9 , an extra 9-array bit vectors are needed on top of the 22 needed for the pseudo-class component of the molecular formula.
-
-
ACKNOWLEDGMENT The author thanks, J. K. MacLeod and P. Keogh for their comments. The data base of molecular formulas was supplied by G. Milne and S. Heller. The author is grateful to J. Reinfelds for pointing out an error in the molecular weight retrieval algorithm. The author thanks Mrs. Greta Pribyl and Glenda Gregor for typing the manuscript. LITERATURE CITED (1) J. E. Ash and E. Hyde, Ed., “Chemical Information Systems”, Horwood,
Chichester, 1975. (2) D. R. Eakin and E. Hyde. in “Computer Representation and Manipulation of Chemical Information”, W. I. Wipke, S. R. Heller, E. Hyde, and R. Feldmann, Ed., Wiley, New York, N.Y., 1973. (3) S. R. Heller, Anal. Chern.. 44, 1951 (1972). (4) D. Lefkovitz, J . Chem. Inf. Cornput. Sci., 15, 14 (1975). (5) R. G. Dromey, J . Chern. Inf. Comput. Sci., submitted for publication.
RECEIVED for review May 23, 1977. Accepted July 21, 1977.
Trace Determination of Vinyl Chloride in Water by Direct Aqueous Injection Gas Chromatography-Mass Spectrometry Toshihiro Fujii The Wivision of Chemistry and Physics, National Institute for Environmental Studies, Yatabe, Tsukuba, Ibaraki 300-2 I, Japan
Described is a rapid, precise, and specific method for the analysis of vinyl chloride (VC) at the sub-ppb level in water samples. The method involves the use of mass fragmentography of the gas chromatography-mass spectrometry by simultaneously recording m / e 62 and 64 after direct aqueous injection of a large sample (1000 ML) on the precolumn (digiyceroi as a liquid phase) with no concentration or extraction required. Several tap waters in Tokyo areas were tested, and VC was not found to be present.
Vinyl chloride (VC) has been identified as a carcinogen ( I ) that is likely to be related to human cancer. Initially, the intense search for VC in the environment has centered upon foods (because poly(viny1 chloride) is a common food packing material and VC migrates into foods), and the occupational atmosphere of poly(viny1 chloride) plants. Therefore, there have appeared many reports on sampling (2-4) and analytical techniques ( 5 )capable of detecting VC at less than ppm levels, in air (6) or in food samples ( 7 ) . Recently, VC at the 0.1-ppb level was discovered in the drinking water of U.S. cities (8). This incident has generated considerable concern also on the search for VC in water samples. There have been many reports for the determination of volatile organics in water by vmious methods, such as the head space technique, the gas stripping technique, and solvent
extraction. However, very few methods have been applied to the trace analysis of VC in water. T o my knowledge, the only used method appears to be the gas stripping technique (9). VC is stripped off a water sample with helium or nitrogen from which it is separated by adsorption on adsorbents. The adsorbents then are transferred into gas chromatograph or gas chromatograph-mass spectrometers for analysis. This method is not entirely suitable with respect to simplicity of operation. In addition, it requires special equipment. This paper describes a simple, new method which allows precise quantitative determinations of the sub-ppb level VC in water by a direct aqueous injection gas chromatography-mass spectrometry (IO) without any special preparations. This method is based upon mass fragmentography (11)which provides the highest sensitivity of the detector with high specificity. The large sample injection (1000 pL) affords low detection limits.
EXPERIMENTAL Apparatus. All analyses were performed on a Finnigan 3300F’ gas chromatographquadrupole mass spectrometer equipped with a multiple ior. detector, by which mass fragmentography can be
carried out. The interface between the gas chromatograph and the mass spectrometer was an all-glass jet-type enrichment device. The mass spectrometer was set to unit resolution (10% valley between adjacent nominal masses). The resulting ion currents were recorded on a multichannel strip chart recorder. The instrument was operated in the electron impact mode. Other conditions held constant throughout the analysis were: helium ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
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carrier gas at a flow rate of 34 mL/min; temperature of the gas chromotograph injection port at 200 "C; pressure in the mass spectrometer of 1 X 10" Torr; ionization voltage of 70 eV; emission current of 490 pA. Column. A 600 cm X 2 mm i.d. metal coiled main column (5% SE-30 on 60/80 mesh Chromosorb W AW DMCS) in simple conjuction with a 40 cm X 6 mm i.d. metal straight precolumn (10% diglycerol on 60/80 mesh Chromosorb G NAW) was used. The diglycerol precolumn was used to strip water from the samples. This situation is made possible by the very long elution time of water through the diglycerol precolumn in comparison to the elution time of VC. The diglycerol has the further property of repetitive use whereas a calcium sulfate precolumn used for the stripping of water should be replaced each time. The large volume injection made the big precolumn necessary. The long main column was chosen to meet the required separation. Other columns capable of performing the required separation could be used for the main column, which will depend upon the interferences present in the practical sample. The column temperature was maintained isothermally at 55 "C. A vacuum diverter (12) for venting was installed to vent high volume effluent water in the sample (eluting after VC) from entering the mass spectrometer. Venting of the water allows continuous mass spectrometer operation without the possibility of damage to the filament or electron multiplier. Reagents and Standardization. A 125-mL hypo-vial (Pierce) was filled with ethyl alcohol (Wako Chemicals, special grade) and sealed with a silicone septum by means of an aluminum crimp seal. Using a 1-mL Pressure Lok syringe (Precision Sampling Corp.), 500 pL (the vapor density of VC at 21 "C is 2.56 mg/mL) at atmospheric pressure was injected into this hypo-vial to give the VC standard. Working solution over the range of 0.1-10 ppb (w/w) was prepared by adding the appropriate quantity of this standard to the organic-free water in a volumetric flask and diluting. This solution was immediately transferred to fill several 20-mL, glass-stoppered bottles. The bottles were over-filled, and part of the solution was displaced with the glass stopper so that no head space was in the bottle. The working solution bottles, when stored at 5 "C, were maintained stably for at least 1 week. One bottle was available only for one-time use. Water used as the diluent was prepared with a water purification (Milli-Q, Millipore). Standard VC was purchased from Takachiho Shoji Co. Procedure. VC analysis was performed as follows. The water sample was injected directly with a 100-pLor a 1OOO-pL Hamilton syringe. Positive identification of VC in the water samples is supported not only from known retention times of the standards but also from the selectivity afforded by selected ion monitoring. The ions chosen to monitor VC are CHC1=CH2+. Thus, from the chlorine isotope clusters, a ratio of 3:l would be expected at masses 62 and 64. This check was made, substantiating the absence of interferences. Quantitative information was obtained using peak areas. It should be noted that after the elution of VC, this analysis normally requires at least 20 min, as the column temperature was increased up to 120 "C so that water and less volatile materials would be removed before the next analysis. Although not essential, enough conditioning of the column is helpful in quickly establishing a steady baseline after each run.
RESULTS AND DISCUSSION Separation. Figure 1illustrates mass fragmentograms of specific ions of VC spike tap water samples, indicating the VC peak shapes of a 1000-pL sample of water containing 0.1 ppb VC and a 100-pL sample of water containing 1ppb were found to be virtually identical. VC was spiked to the tap water in which no VC was confirmed. During the course of these studies, it became apparent that some components in water responded to m / e 62 and 64 other than VC. As may be seen from Figure 1, the initial peak becomes higher as the injection volume is increased. It was identified as nitrogen and oxygen by interpretation of mass spectra. As the quantity of these gases in a 1000-pL sample of water is considered to be considerably large, their abnormal response would be understandable if such a large quantity of 1988
ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
L -)LL VI:
fib
1
~
.
[
r
rL
-L
/
d
+
6
qL-
Figure 1. Mass fragmentograms of VC spiked tap water samples. (1) a 1000-pL water sample containing 0.1 ppb VC, (2) a 100-pL water sample containing 1 ppb VC
Table I. Reproducibility of V C Analysis Amount Std Concn, injected, Mean peak dev, Re1 std bL area," mm2 mmz dev, % PPb 188 23 12 0.1 1000 1 100 196 12 6 10 10 208 12 6 a Represents the mean of five consecutive injections of each standard. these gases raised the pressure in the mass spectrometer sufficiently t o produce a general overall background in the mass spectrometer due t o collisions occurring during analysis of the ions. A peak, which had the retention time of 6.3 min, was also visible on the m / e 64 (but not on the m / e 62) recording and was not identified in the present study. In spite of these components, there is no problem in identification and quantitation because of the sufficient difference in retention time between them and VC. Detection Limit. As 1000 p L of standard solution was injected, it was possible t o obtain a clean m l e 62 mass fragmentogram (S/N = 15 or greater) from 0.1 ppb VC (refer t o Figure 1). The extension of the detection limit was possible, because of the ability of the big diglycerol precolumn to handle water samples as large as lo00 pL with no detrimental effecb to the separation performance, and because of the property of the present column system t o elute VC before the overload water peak (trace components are not easily determined when appearing on the tail of an overload water peak). Although the injection volume more than lo00 p L may be made to extend the detection limit before the columns have lost their efficiency, it could not provide the baseline separation of the VC peak and the above-mentioned peak because of the large quantity of nitrogen and oxygen in water samples. Linearity and Reproducibility. Detection response (peak area) was linear over the chosen range of 0.1-10 ppb VC standard solutions for the method with a 1000-pLinjection. The reproducibility of the method was determined from replicate analyses (five times) of t h e standard sample. It is presented in Table I, using the standard deviations and relative standard deviations from 0.1, 1, and 10 ppb VC. Although a little decrease in precision was observed at the large volume injections, this method, without the concentration and extraction steps, enjoys sufficient reproducibility. The favorable result is that the direct aqueous injection of considerably large quantities has no significant effect on the
reproducibility studies as well as the calibration works. Application to Water Samples. As the method described has proved to be sensitive and precise for the analysis of VC in the water samples, it was applied to the analysis of VC in the drinking tap water of five locations near Tokyo. VC has not been found to be present in all tested t a p waters, even at the ultra trace levels. A few river waters were also measured. One of them, the Itachi River water in Toyama prefecture, was found to contain VC a t the level less than 0.1 PPb. ACKNOWLEDGMENT T h e author thanks Y. Ambe for his helpful suggestions. LITERATURE CITED (1) Manufacturing Chemists Association Report, Chem. Eng. News, 52 (21), 16 (1974).
(2) M. R. B. Burnett, Am. Ind. Hyg. Assoc., J., 37, 37 (1976). (3) A. E. Gabany, Jr., and H. Senman, Am. Lab., July, 5 0 (1976). (4) R. E. Hill, Jr., C. S. McCammon, A. T. Saaiwaechter, A. W. Teass, and W. J. Woodfln, Anal. Chem., 48, 1395 (1975). (5) J. E. Purcell, Am. Lab., May, 99 (1975). (6) M. Ravey and J. Klopstock, J. Chromafogr. Sci., 13, 552 (1975). (7) J. D. Rosen, J. R. Morano, and S. R. Pareles, J. Assoc. Off. Anal. Chem., 58, 700 (1975). (8) W. E. Coleman, R. D. Lingg, R. G. Mon, and F. C. Kopfler, “Identification and Analysis of Organic Pollutants in Water", Ann Arbor Science, Publishers, Ann Arbor, Mlch., 1976, Chap. 21. (9) T. A. Beilar, J. J. Lichtenberg, and J. W. Eichelberger, Environ. Sci. Techno/., 10, 926 (1976). (IO) L. E. Haarrls, W. L. Budde, and J. W. Eicheiberger, Anal. Clem., 46, 1972 (1974). (1 1) C. G. Hammer, B. Holmstedt, and R. Ryhage, Anal. Biochem., 25, 532 (1968). (12) R. L. Wolen and H. E. Pierson, Anal. Chem., 47, 2068 (1975).
RECEIVED for review May 2,1977. Accepted August 19,1977.
Determination of Sulfur in Organic Molecules by Spectrophotometric Titration A. J. Burgasser,* K. F. Slngiey, and J. F. Coiaruotoio" Hooker Chemicals and Plastics Corporation, Research Center, Grand Island Complex, M.P.O. Box 8, Niagara Falls, New York 14302
A spectrophotometrlc titration of sulfate has been developed for determinatlon of the sulfur content of organic molecules afler oxygen flask decomposition. The sulfate was titrated wlth barium perchlorate using thorln as the Indicator. The color change (yellow to plnk) is monltored with a modlfled Brlnkmann PC/lOOO Probe Colorimeter. The llmlt of detectlon of the method is 5 pg sulfur. The absolute accuracy of the method is f0.19% with a standard devlatlon of 0.24%.
The method of Fritz and Yamamura ( 1 ) for the determination of sulfate by titration with barium using thorin indicator has undergone many modifications over the years to improve its accuracy and precision. Menis, Manning, and Ball (2) have reported a spectrophotometric titration of sulfate in reactor fuels using barium and thorin. The method suffers from the disadvantages of having to modify a spectrophotometer with an automatic titrating assembly and construction of a special titration cell. We have developed, using a Brinkmann probe colorimeter, an improved spectrophotometric titration of sulfate with barium using thorin as indicator for the determination of sulfur in organic molecules after oxygen flask decomposition. T h e convenience of carrying out a spectrophotometric titration has been increased dramatically with the developments in fiber optics and the commercial availability of compact, versatile colorimeters equipped with flexible probes enabling measurements to be taken directly in titration beakers. This becomes most advantageous for titrations which have color end points which are not sharp and require an analyst to practice in order to become proficient a t seeing the proper end point. The titration of sulfate with barium using thorin is a difficult end point and the probe colorimeter is ideally suited to this determination. Several organosulfur compounds of known composition were analyzed. The optimum analytical conditions and detection limit were established, and the effects of p H and various
anions and cations on the accuracy of the method were assessed. EXPERIMENTAL Apparatus. All titrations were carried out with a Metrohm Herisau Potentiograph Model E536 and Dosimat E535 equipped with a 5-mL automatic buret. Absorbance measurements were made with a modified Brinkmann PC/lOOO Probe Colorimeter using a 520- or 450-nm filter and a 1-cm probe tip light path. The PC/1000 colorimeter was modified to produce a 1000-mV output by installing an optional signal amplification attachment into the colorimeter output circuit. The amplifier and the instructions for its installation can be obtained from Brinkmann Instruments. The 1000-mV output was not essential for this determination but was necessary for other applications. The colorimeter output lead was plugged directly into the indicator electrode socket in the rear of the potentiograph. Samples were weighed with a Mettler ME-22 micro-balance and decomposed in a 500- or 1000-mL Schoniger combustion flask. Reagents. Distilled water and reagent isopropyl alcohol were used in the preparation of all solutions. Barium perchlorate solution, 0.01 and 0.05 M, was prepared by dissolving 1.95 and 9.76 g of Ba(C104)2.3H20,respectively, in 200 mL of water and diluting to 1000 mL with isopropyl alcohol. The solutions were standardized against standard reference material 143C, cystine, obtained from the National Bureau of Standards. Saccharin and sulfanilamide were obtained from Aldrich Chemical Company and purified by four recrystallizations from ethanol. Stock thorin indicator solution, 0.2%, was prepared by dissolving 200 mg of thorin, l-(o-arsonophenylazo)-2-naphthol3,6-disulfonicacid, (Aldrich Chemical Corporation, Catalogue No. 10456-6) in 100 mL of water. Stock methylene blue solution, 0.01%, was prepared by dissolving 10 mg of methylene blue indicator (Eastman Organic Chemicals, Catalogue No. C573) in 20 mL water and diluting to 100 mL with isopropyl alcohol. Five mL of the 0.2% thorin solution and 1mL of the 0.01% methylene blue solution were added to 500 mL of isopropyl alcohol and this solution was used for transferring the absorber solution in the oxygen flask to the titration beaker. All indicator solutions should be protected from light. ANALYTICAL
CHEMISTRY,
VOL. 49, NO. 13. NOVEMBER 1977
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