from 1.1,2-trichloroethane, furfural, butyl mercaptan, and pyridine. The separation of water from the several classes of organic compounds represented by the terpenes together with the compounds mentioned above illustrate the utility of the method described for the analysis of a wide variety of organic compounds. If the compounds of interest in a sample cannot be separated from water using sorbitol, then another preparative
column can be used such as purified apiezon L on which water elutes prior t o most organic compounds. In general, any preparative column that would provide a significant difference in retention characteristics for a diluent and those compounds of interest may be used with the system. Received for review October 11, 1973. Accepted November 19, 1973.
Pyrolysis Esterification of Phosphorous-Containing Acids for Gas-Liquid Chromatographic Analysis William L. Clapp,' Robert J. Valis, Stanley R. Kramer, and John W. Mercer Process Technology Branch. Chemical and Plants Division, Manufacturing Technology Directorate. Edgewood Arsenal, Md. 21010
Conventional methods of analysis of dilute aqueous solutions of phosphorous acids have centered chiefly around their determination by means of the molybdovanadophosphoric acid complex ( 2 , 2). The problems associated with analysis of organophosphorous acids are even more complicated in that they necessitate the conversion of such compounds into inorganic phosphates prior to analysis ( 3 ) . In recent years, a technique has been described whereby carboxylic acids are converted into their methyl esters by injection port pyrolysis of their tetramethylammonium salts. Robb and Westbrook ( 4 ) found that the methyl esters of saturated fatty acids could be formed by direct injection of methanolic solutions of their tetramethylammonium salts into a heated injection port of a gas chromatograph. However, the yields were variable with different acids and poor with small sample sizes. Downing (5, 6) modified the injection procedure and obtained more reproducible results, not only with saturated fatty acids but with mixtures of fatty acids containing unsaturated components as well. This procedure, with some additional modifications, has been applied to the pyrolysis esterification of organophosphorous acids.
Reagents. Methylphosphonic acid and methylfluorophosphinic acid were prepared and purified in this laboratory. Structures and purity were confirmed by infrared, nuclear magnetic resonance, and mass spectral analysis. Procedure. A 0.500-gram sample of each acid was dissolved in 4 ml of distilled water in a 10-ml volumetric flask which was immersed in an ice bath. Tetramethylammonium hydroxide (Eastman, 25% in water) was added dropwise to a phenolphthalein end point, followed by dilution to the mark with distilled water. A 2-pl aliquot of the sample was placed on the probe tip and allowed to evaporate to dryness at room temperature. (Additional sample could be added as necessary.) The probe was then attached to the injection port of the chromatograph and pyrolyzed for 15 seconds. A typical chromatogram for the methyl esters of methylphosphonic acid and methylfluorophosphinic acid prepared by this technique is shown in Figure 1. The principal pyrolysis degradation product was determined to be trimethylamine, by combined gas chromatography-mass spectral analysis. Reference samples of the methyl esters were prepared and purified by conventional techniques. Standard solutions of the esters in methanol were chromatographed and plots of peak area cs. concentration were obtained. Area measurements were made with an Infotronics Model CRS-108 digital integrator. The quantity of methyl ester formed through pyrolysis was then determined from standard curves.
EXPERIMENTAL
RESULTS AND DISCUSSION
Apparatus. An F&M Scientific Corporation Model 720 gas chromatograph with a thermal conductivity detector was used with a 6-ft X ?/s-in. 0.d. stainless steel column packed with 5% Carbowax 20 M on 60/80 mesh Chromasorb W. The carrier gas was helium at a flow rate of 25 cm3/min. The injection port temperature was 190 "C while the detector temperature was 260 "C. The column was programmed from 125 "C to 200 "C a t 7.5 "C/ min. An F&M Scientific Corporation Model 80 pyrolysis unit equipped with a standard M-type probe was used without modification for pyrolysis o f samples. This unit and probe have been adequately described by Brodasky ( 7 ) . A temperature calibration curve for each probe was supplied by the manufacturer.
Several parameters which would be expected to have an effect on the yield of the pyrolysis methylation procedure were studied. Effect of Pyrolysis Temperature. To determine the effect of this variable, aliquots of a standard solution of the tetramethylammonium salt of methylphosphonic acid were pyrolyzed a t various temperatures. A mesa-like curve resulted (Figure 2). Incomplete conversion below 325 "C appears likely, with sample decomposition beyond 925 "C. The broad optimum range of pyrolysis temperatures permitted excellent control and reproduction of the pyrolysis phase; the standard deviation of the technique was typically *1.5%. All subsequent sample pyrolyses were made a t 500 "C . Effect of Sample Size. The relationship between the yield of the pyrolysis step and the sample size was also evaluated for samples containing 10-100 pg of acid. Excellent results were obtained with methylfluorophosphinic acid (Table I) over the entire range studied. However, the ester yield with methylphosphonic acid (Table 11) decreased significantly when the quantity of acid pyrolyzed
Present address, R. J. Reynolds Tobacco Company, Research Department, 115 Chestnut Street, S.E., Winston-Salem, N.C. 27102. (1) E. J Griffith. Ana/. Chem.. 2 8 . 5 2 5 (1956). ( 2 ) R. E Kitson and M . G . Mellon. Anal. Chem.. 16, 379 (1944). (3) S . G. Jankovic, D. T Mitchell, and J . C. Buzzell. J r . , Watef Sewage W o r k s . 1 1 4 , 471 (1967). (4'1 E W . RobbandJ. J. Westbrook I I l , A n a i . C h e m . , 35, 1644 (19631. (5) D. T. Downing, Anal. Chern.. 39,218 (1967). ( 6 \ D. T Downing and R. S. Greene, Anal. Chem.. 40, 827 (1968). (71 T . F Brodasky, J . Gas Chfornatogf., 5 , 31 1 (1967).
A N A L Y T I C A L CHEMISTRY, V O L . 46, NO. 4 , A P R I L 1974
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sults were obtained with the use of this apparatus in the esterification of fatty acids (8). The application of this type of unit for pyrolysis esterification offers several advantages over those cited earlier. First, the unit is commercially available and easily adapted to many types of chromatographs. Second, the configuration of the probe is such that, if necessary, repetitive additions to the probe tip of sample aliquots are easily accomplished prior to pyrolysis with a delay only for the solvent to evaporate. Since no degradation of the dry salts of these acids was observed on the probe, as many aliquots as were necessary could have been added, thus reducing the need to concentrate samples. Tme
(Minutes)
Figure 1. Chromatogram of a mixture of (1) pyrolysis degradation products, (2) methyl methylfluorophosphinate, and (3) di-
methyl methylphosphonate
0.8 l.Ol 1
0.6-
CONCLUSIONS While most workers have focused their work with this technique on the determination of carboxylic acids, the data suggest that the technique can be applied to phosphorous-containing acids. The method appears ideally suited to the analysis of moderately dilute aqueous solutions of these acids where the approach is to conduct the analysis through gas chromatography of their methyl esters.
P
Received for review August 22, 1973. Accepted October 26, 1973.
e
0.4 .c
-
t 0.2 0
(8) P. H . Latimer and W. L. Clapp, R . J. Reynolds Tobacco Company, unwblished work, 1968. 200 Probe
400
600
Temperoture
000
1000
(" C )
Figure 2. Yield of dimethyl methylphosphonate as a function of pyrolysis probe temperature
Table I. Effect of Sample Size on Yield of Methyl Methylfluorophosphinate Sample size, pg
100 75 37.5 11.3 a
Yield: %
*
99 1 99 Zk 1 98 2 97 3
+
*
Each analysis is the average of three Chromatograms.
Table 11. Effect of Sample S i z e o n Yield of Dimethyl Methylphosphonate Sample size, pg
100 50 30 10 a
Yield: %
99 Zk 1 99 i 1 52 Zk 2 10 Zk 5
Each analysis is the average of three chromatograms.
was less than 50 Fg. When the tetramethylammonium salts of both methylfluorophosphinic acid and methylphosphonic acid were pyrolyzed simultaneously, esterification was quantitative for both acids over the entire concentration range. An attempt was made to esterify phosphoric acid under these same conditions but low, erratic yields were obtained, probably because of an inability to neutralize the thud acid group. Pyrolysis Probe. The nature of the pyrolysis unit was the key to the study. By producing a rapid, yet well-defined, probe temperature, reproducible injection port esterification was possible. In an earlier study, similar re614
ANALYTICAL CHEMISTRY, VOL. 46, NO. 4, A P R I L 1974
CORRECT1ON Resolution by Gas-Liquid Chromatography of Diastereomers of Five Nonprotein Amino Acids Known to Occur in the Murchison Meteorite In this article by G. E. Pollock, Anal. Chem., 44, 2368 (1972), an error in nomenclature was made. The ( + ) - 2 alkanols have an S ( + ) configuration in the Cahn-Ingold system, not R(+) as written in this paper. The following changes should be made: p 2368, column 2, lines 11 and 12 the N-trifluoroacetyl-(SR)-aminoacid (S)-2-butyl esters p 2369, column 1 under Reagents, line 4, , optically active S - (+)-2-alkanols p 2371, Tables I11 and IV, bottom of table Peak 1-RS for N-methylalanine, p-amino-n-butyric, Peak 2-SS and pipecolic acids. Peak 1-SS for isovaline and P-aminoisobutyric acid Peak 2-RS p 2372, column 1, lines 9-15, l . e . , the RS peak elutes first and the S S peak second. Isovaline and /3-aminoisobutyric acid, however, show a reversal of elution pattern, the SS peak elutes first and the RS peak second. Up to this time, only one case of reversal of elution order has been reported. Charles-Sigler and coworkers ( I O , 13) report that N-TFA(RS)-phenylglycine-(S)(+)-%octanol ester has a reversed elution order while the S ( +)-2-butanol ester is normal. I wish to thank Dr. E. Gil-Av for calling to my attention this error in nomenclature.
