In order to confirm the separation, triose and tetrose MesSi oximes in a GC-MS analysis mass spectrometric fragmentation of compounds of this class were studied. The spectra of all compounds under investigation gave molecular ion and [M - 15]+ peaks (Figure 3), and peaks at mle 73 and 147, characteristic of MesSi derivatives (6, 7). Characteristic of the fragmentation of per-0-MeaSi dihydroxyacetoxime is the formation of [M - MesSi-OH]+ ions, not present in the spectra of D,L-glyceraldehyde. The spectra of the two isomeric MesSi glyceraldoximes (Table I) contain peaks characteristic of the cleavage of the C(2)-C(3) bond (Figure 3). The two isomers show qualitatively identical spectra, except for the presence of the [M - 15]+ions present in the spectrum of the first eluted isomer. The mass spectra of the MesSi oximes of L-glycero-tetrulose differ from those of the corresponding aldoses by the presence of [M - MesSi-OH]+ ion peaks. The spectra of the two pairs of isomeric MeaSi oximes of D-threose and D-erythrose (Figure 3) are qualitatively identical. Trioses from tetroses and aldoses from ketoses can thus be distinguished by means of their mass spectra alone, although the spectra are not informative as far as the stereochemistry of the substances is concerned. Compared with existing methods, the selectivity and specificity of the determination of trioses and tetroses by the method involving gas chromatography described herein is noteworthy. The polarographic method is selective only with respect to individual groups of aldoses, ketoses, and their dehydration products. Two substances of a class in mixture cannot be determined in this way. I t may be used as an analytical tool when only one substance of each class is present in a mixture. The appearance of polarographic maxima or catalytic (observed, e.g., when molybdic acid is present in the
reaction medium in high concentration (12) interferes with the determination of aldoses and ketoses. In such instances, or when products of transformation of tetroses (erythrose and threose) are to be monitored a method is required capable of determining the substances in question, present either individually or in a mixture. The new method is specific with respect to trioses and tetroses and can be successtklly applied in cases when polarography or other methods fail because of various interfering effects. It fills efficiently the gap in the existing analytical methods of determining trioses and tetroses. When 2- to 10-mg samples were analyzed, the mean relative error of determination of trioses is less than f2.3%.
LITERATURE CITED (1)D. C. Gutsche, D. Radmore, R. S. Buriks, and K. Nowotny, J. Am. Chem. Soc.. 89. 1235 (1967). (2) J. Konigstein and M. Fedorohko, "Proceedings of the 3rd Analytical Con-
ference", Vol. 2,Budapest, 1970,p I 13. (3)C. C. Sweeley, R. Bentley, M. Makita, and W. W. Wells, J. Am. Chem. Soc., 85, 2497 (1963). (4)H. Yamaguchi, T. Ikenaka, and Y. Matsushima, J. Biochem. (Tokyo), 68, _753 _ _ ( i w n-,.\ (5) B. S. Mason and H. T. Slover, J. Agric. FoodChem.. 19, 551 (1971). (6) G. Petersson, Carbohydr. Res., 33, 47 (1974). (7)A. Laine and C. C. Sweeley, Carbohydr. Res., 27, 199 (1973). (8)J. KrupiEka and J. J. K. Novak, Collect. Czech. Chem. Commun., 25, 1275 (1960). (9)R. L. Whistler and M. L. Wolfrom, "Methods in Carbohydrate Chemistry", Vol. 1, Academic Press, New York, London, 1962,p 64. (IO)H. Muller, C. Montegai, and T. Reichstein, Helv. Chim. Acta, 20, 1468 (1937). (11) V. Moses and R. I. Ferrier, Biochem. J., 83, 8 (1962). (12)J. Konigstein, Collect. Czech. Chem. Commun., in press. 1
RECEIVEDfor review July 21, 1976. Accepted October 6, 1976.
In Situ Generation of Standards for Gas Chromatographic Analysis D. J.
Freed* and A. M. Mujsce
Bell Laboratories, Murray Hill, N.J. 07974
Techniques for the direct generation of acrolein, acrylonitrile, and vinyl chloride for gas chromatographic analysis are described. Precolumns containing conversion reagents generate the desired compound from suitable precursors via direct injection. At the 5-ng level, a precision of better than 5 % is achieved, with high and reproducible yields being obtained over a wide dynamic range. The method obviates the necessity for manipulationand storage of hazardous or toxic materials and facilitates the preparation of precise standards.
Because of growing awareness of the hazards of industrial pollutants, methods for the determination of trace amounts of these materials are being introduced a t an ever-increasing rate. There are, however, relatively few standards or standardization methods for toxic or hazardous compounds. Exponential dilution ( I ) , permeation tubes ( 2 ) ,diffusion ( 3 , 4 ) , and standard mixtures (5, 6) have all been used for standardization and calibration. The above techniques are not without attendant difficulties, perhaps the greatest one being the necessity for storage and handling of relatively large quantities of potentially dangerous materials. In addition,
many of the most toxic materials are gases or low boiling liquids, thus requiring elaborate flow and temperature control systems for successful standard preparation. Also, since many of these compounds are highly reactive vinyl compounds, such as acrolein, the reliability and stability of dilute standard mixtures may be suspect. For the above reasons we have been prompted to explore alternative methods for preparing standards for trace gas analysis. This report describes techniques for the generation of vinyl chloride, acrolein, and acrylonitrile directly in the injection port of a gas chromatograph. Precursor compounds are used, which are not nearly as volatile or hazardous as the desired compounds. The yields of the desired compounds are uniformly high and the decreased volatility of the precursor facilitates the preparation of reliable standard solutions. Finally, the introduction of the standard into the chromatograph as a plug, rather than as a continuous flow, more nearly mimics the conditions used in most analytical procedures.
EXPERIMENTAL Instrumentation.A V a r i a n MAT 112 Gas Chromatograph-Mass Spectrometer was used f o r a l l investigations. Chromatograms were ANALYTICAL CHEMISTRY, VOL. 4 9 , NO. 1, JANUARY 1977
139
Table I. Yields and Conditions for Generation of Standards Precursor moles taken
Desired standard
(x 109)
Vinyl chloride CH*=CHCl
1.28
0.25 0.129 0.05
Acrolein CHz=CHCHO
1.47 0.74
0.29 0.15 Acrylonitrile CHz-CHCN
0.32
0.16 0.064 0.032
Moles of standard recovered a (x 109)
Absolute yield, % b
Optimum temperature, "C
1.24 f 0.02 10.24 & 0.01 0.125 f 0.005 0.049 f 0.002
97 f 2 96 f 4 97 f 4 97 f 4
325 325 325 325
1.41 f 0.02 0.71 f 0.02 0.275 f 0.01 0.14 f 0.006
96 & 1 96 f 3 95 & 4 95 f 4
200 200 200 200
0.31 f 0.01 0.155 f 0.006 0.063 f 0.003 0.031 & 0.001
98 f 1 97 f 4 98 f 5 97 f 3
225 225 225 225
a Moles recovered by comparison with standard prepared as described in text. The measured yields are the results of triplicate determinations for each point.
30t
PEAK
Figure 2.
O t
AREA
Calibration curve for vinyl chloride
(0)Gravimetrically prepared standard. ( A ) Generated standard (from 1,2-di-
chloroethane) 100
200
TEMPERATURE,
300
rool
O C
Absolute yields vs. temperature. Flow rate is 20 rnl/min (a)Vinyl chloride from 25-ng injections of 1,2dichloroethane. (b) Acrolein from 25-ng injections of allyl alcohol. (c)Acrylonitrile from 25-ng injections of cyanoethyl trimethylammonium iodide Figure 1.
* recorded by use of a second ionizer operated at 20 eV with detection by means of a Faraday cup and dc amplifier combination. For higher sensitivity, chromatograms were recorded directly from the electron multiplier output with the mass setting at the specific ion of interest. Columns were either 2 m by 3.2 mm of Tenax GC (used for vinyl chloride) or 3 m by 3.2 mm of 10%AN-600 on Anakrom ABS (used for acrolein and acrylonitrile) with helium used as the carrier gas. Injections were made using Hamilton 701N syringes which had been gravimetrically calibrated with mercury. These were found to be reproducible to f1.4%for a 1-111 injection. Materials. Cyanoethyl trimethylammonium iodide was synthesized according to standard literature procedures (7) and the purity determined by titration with silver ion. Vinyl chloride, acrolein, and acrylonitrile were twice distilled before use. Standards were prepared gravimetrically from the latter compounds by a sealed ampoule technique (8). Precolumns. The precolumns were 10 cm X 6 mm Pyrex tubing. They were packed with 2 g of the required reagent (100to 200 mesh) and plugged with glass wool at each end. All precolumns were preconditioned at the maximum temperature of operation for 12 h prior to being used.
RESULTS A N D DISCUSSION We have investigated three methods of generation of standards, namely dehydrohalogenation (generation of vinyl chloride from 1,2-dichloroethane), oxidation (generation of 140
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
/
*Ot
L
/I
I
I
I
IO 20 F L O W , rnl/min
I
I
30
vs. flow rates (0)Acrylonitrile. ( A ) Vinyl chloride.(0) Acrolein Figure 3. Yields
acrolein from allyl alcohol), and thermal decomposition of a quaternary ammonium salt (generation of acrylonitrile from cyanoethyl trimethylammonium iodide). Basically the method involves utilization of a precolumn containing a conversion reagent which will react with the precursor compound so as to give the desired standard in high yield with a minimum of secondary products. Precolumn reagents were selected so as to be stable at the required operating temperatures and to give reasonably long lifetimes. In all cases studied, precolumns gave stable yields for a minimum of 50 injections. Dehydrohalogenation. For the generation of vinyl chloride we selected anhydrous potassium carbonate for the precolumn reagent, both for its high melting point and because the commonly available granular material requires no further treatment to render it suitable for preparation of efficient precolumns. 1,2-Dichloroethane solutions were prepared in
hexane and injected directly onto the precolumn placed in the injection port. The extent of conversion to vinyl chloride as a function of injection port temperature is shown in Figure 1. For temperatures in excess of 300 "C, the extent of conversion (obtained by plotting integrated peak areas of vinyl chloride and dichloroethane with monitoring at m/e 62) is greater than 96%. The absolute yields for generation of vinyl chloride were determined both as a function of temperature and concentration by comparison with freshly prepared vinyl chloride standards in hexane. These are indicated in Table I together with the optimum conditions for generation. In order to ascertain whether the hexane solvent decreased the yields, vapor injections of dichloroethane standards (prepared by cracking a sealed, pre-weighed ampoule in a large volume flask) were also introduced. No differences could be seen. To ensure that vinyl chloride did not react with potassium carbonate, amounts from 1to 100 ng were introduced onto both empty and packed precolumns. Again, no differences could be seen. Linear calibration plots were obtained for both generated and prepared standards of vinyl chloride in the range 1 to 125 ng (corresponding to 1 cm3 gas injections from 1to 125 ppm by weight) (Figure 2). No dependence of yield on concentration was observed in this range (Table I). The dependence on flow rate was studied in the range 5 to 30 ml/min; however there was no change greater than 1%in this range (Figure 3). Oxidation. The oxidation of allyl alcohol to acrolein by potassium dichromate was investigated in a similar manner as for vinyl chloride generation. The absolute yields and efficiencies are greater than 95%as before, but in contrast to vinyl chloride generation, are markedly dependent on the temperature and flow rates. This is to be expected since further oxidation to acrylic acid could occur. The optimum temperature for generation of acrolein together with yields are given in Table I, and the extent of conversion as a function of temperature is shown in Figure 1. It can be seen that at temperatures in excess of 250 "C, the yield falls off sharply. Coincident with this we observed the mass spectrum of acrylic acid. The yields of acrolein fell from a maximum of 95% a t a flow rate of 30 ml/min (and above) to a minimum of 47% at 5 ml/min. Under the conditions chosen for generation, however, the yields were highly reproducible (95 f 4% at 30 ml/min and 200 "C for 10 replicate injections) (Figure 3). Absolute yields were determined by comparison with freshly prepared standards of acrolein. In this connection, it should be noted that acrolein standards so prepared remained stable for only a few hours (in concentrations ranging from 1to 125 ppm by weight the concentration decreased by as much as 50% in less than 3 h) regardless of the manner of storage. We believe that traces of ionic impurities on either the glass flasks or stainless steel cylinders catalyze polymerization, either by an aldol mechanism or through the vinylic double bond. As before, no changes in yields were observed as a result of introducing the precursor in hexane, and linear calibration plots were obtained in the range 1-125 ng. Thermolysis of Quaternary Ammonium Salts. The well
known thermal degradation of quaternary ammonium salts to vinyl compounds was investigated for the generation of acrylonitrile. Cyanoethyl trimethylammonium iodide was prepared by quaternization of 3-dimethylaminopropionitrile with methyl iodide. The purity of the product was ascertained by titration with silver nitrate and found to be greater than 9990. The salt is a stable, white crystalline solid, readily soluble in polar solvents. The optimum temperature for decomposition was determined both by thermogravimetric analysis and by gas chromatography (Figure 1). For generation of acrylonitrile, the salt was dissolved in N,N-dimethylformamide and aliquots were introduced into the injection port. A t 225 "C essentially quantitative decomposition to acrylonitrile occurs, the other product being trimethylammonium hydroiodide. This latter product further decomposed to trimethylamine and hydrogen iodide, neither of which interfered with the generation of acrylonitrile. The absolute yield of acrylonitrile generation was determined by comparison with freshly prepared standards of acrylonitrile and is given in Table I. No significant effects were seen as a result of varying the flow rate from 5 to 30 ml/min and, as before, linear calibration plots for both generated and prepared standards were obtained in the range 1to 125 ng. CONCLUSION The in situ generation of chromatographic standards offers an attractive alternative to the usual methods of calibration. Personal hazards are minimized, and the preparation of accurate and stable standards is facilitated because of the great volatility difference between precursors and the desired compounds (for example 98 "C in the case of vinyl chloride). Of even greater interest, however, is the generality of the reactions, since many members of a homologous series may be generated by selection of the proper precursor. We have only touched the surface of this method; however, preliminary experiments have already demonstrated the feasibility of generation of acetaldehyde, formaldehyde, methyl vinyl ketone, and many nitrosamines. The latter are of particular importance since their extreme carcinogenity renders the preparation of standards extraordinarily hazardous. LITERATURE CITED (1) J. E. Lovelock, "Gas Chromatography", R. P. W. Scott, Ed., Butterworths, Washington, D.C., 1960, p 26ff. (2) F. Bruner, C. Canulli, and M. Possanzini, Anal. Chem., 45, 1970 (1973). (3) G. A.Lugg, Anal. Chem., 41, 1911 (1969). (4) A. P. Altshuller, and I. R. Cohen, Anal. Chem., 32, 802 (1960). (5) H. Hachenberg, "Industrial Gas Chromatographic Trace Analysis", Heyden and Sons, New York, 1973, Chap. 3. (6) E. E. Hughes, and J. D. Taylor, Spec. Environ. Rept. 3, WMO No. 368, 1974, p 616ff. (7) B. Hazzard, "Organicum", Pergamon Press, New York. 1973, Chap. 7 and references contained therein. (8) S. Siggia, "Quantitative Organic Analysis via FunctionalGroups", John Wiley and Sons, New York, 1963, Chap. 26.
RECEIVEDfor review August 2, 1976. Accepted September 23, 1976.
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