These results show that the apparatus reported is feasible for monitoring SO*,H& and CH3SHwithin the limits shown. Sample injections can be made every 15 min; however, if only SOzis monitored, as n o other sulfur pollutants are usually present in the air, injections can be made every 2 minutes. The reduction of the sampling times between one injection and the next makes the total SO2 monitoring with the gas chromatographic method, which is by definition a discontinuous one, approaching the value of the integral method obtained with a coulometric analyzer. Contemporaneous monitoring of the same ambient air has been carried out with the gas chromatographic apparatus described here, with the automatic coulometer available in our laboratory (6), and with the classical semimanual colorimetric system (7). Figure 5a shows a comparison of the results obtained with these three systems. The sampling rate of the chromatograph was 15 minutes and the same time elapsed between one titration and the next. The results of the West and Gaeke method are also reported and the integration is made in this case over a three-hour period. Figure 5b reports the same comparison, the only difference being the sampling rate of the FPD which was in this case 3 minutes. From a comparison of Figure 5a and 56, it can be seen that with the sampling rate of 3 minutes, the two curves, coulometric and gas chromatographic, are closer than in the 15-minute sampling, as it could be easily predicted by the fact that the latter is discontinuous while the former gives a n integral response. (6) A. Liberti, M. Possanzini, and R. Ricci, I/zqui/zamento, 9, 28 (1971). (7) P. W. West and G. C. Gaeke, ANAL. CHEM., 28, 1916 (1956).
The data of Figure 56 show, however, that the two procedures, gas chromatographic and coulometric, are comparable, and a discrepancy of measurements of about 7 % only is observed. As the analytical techniques vary and the detecting principles are completely different, it can be said that the agreement is very good. Concerning the West and Gaeke method, it can be said that, even though the total under the curve corresponds to that of the other two, a very rough picture of the variations of the SO2 concentration is observed. In addition, the standardization of the coulometric apparatus has been carried out with a sulfur dioxide permeation tube; the agreement of the graphs in Figure 5 confirms also that consistent results are obtained with the two calibration systems. Concluding, the EDF calibration system appears to be simpler than that of permeation tubes, which requires a very long procedure in order to know the permeation rate. Moreover, the permeation tubes system needs a very accurate mixing device which must be frequently operated t o obtain the calibration curve. With the EDF, one has only to inject the sample into the flask and wait about half an hour to get complete data for the calibration curve. ACKNOWLEDGMENT
The authors wish to thank Antonio D i Corcia for discussions, Roberto Rastelli, Giorgio Sirilli, and Claudio Canulli for technical assistance. RECEIVEDfor review March 6, 1972. Accepted June 12, 1972.
NOTES
Use of the Microwave-Excited Emissive Detector for Gas Chromatography for Quantitative Measurement of Inter-Element Ratios R. M. Dagnall, T. S. West, and Paul Whitehead Department of Chemistry, Imperial College of Science and Technology, London, S W7 2AY. U.K. A WIDE RANGE of selective detectors for gas chromatography has been described in recent years. In general, these detectors have been used to provide better detection for particular types of compounds, to enable the determination of such compounds in the presence of a high background, and to obtain qualitative information on heteroatoms present in the chromatographic eluates. Except for the use of mass spectrometers, only very limited attempts have been made to determine the quantitative ratios between elements present in an eluate. Many detectors highly suited to sensitive selective detection are unsuitable for this purpose. Some have an unpredictable response or enhance the response of more than one element (e.g., electron-capture detectors) and others, such as coulometric detectors, would require a second de2074
tector to determine the mass of compound present. In principle, emissive detectors are best suited to determine several elements ; ideally, the response should be structure independent and the emission at each selected wavelength characteristic of the element in question. The use of metal sensitized flames utilizing the emission from metal salts has been reported. Na2S04-sensitizedflame detectors for chlorine, bromine, and iodine compounds have been described by Nowak and Malmstadt (I), and Bowman, Beroza, and Hill (2), but the methods d o not distinguish (1) A. V. Nowak and H. V. Malmstadt, ANAL. CHEM., 40, 1108
(1968). (2) M. C. Bowman, M. Beroza, and K. M. Hill, J . Chromatogr. Sci., 9, 162 (1971).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
between the various elements. The indium-sensitized flame photometric detector for halogens of Bowman and Beroza (3) also made no distinction. On the other hand, Overfield and Winefordner ( 4 ) distinguished between the emission from InCl (359.9 nm), InBr (372.7 nm), and In1 (409.8 nm). They found that for chlorine and bromine the response per halogen atom was the same within a variation of *lOZ for a range of compounds. Bowman and Beroza (5) monitored the emission at 526 nm from a copper-sensitized flame and were also able to distinguish between different halogens by varying the hydrogen flow rate to the detector. Bowman and Beroza (6) used a flame photometric detector to view the chemiluminescent emission from the HPO and S2 species (7) above a hydrogen-rich hydrogen/air flame and measured the ratio of P:S emission. The ratios were found to be in the range 5.2 to 6 for PS compounds, 2.8 to 3.3 for PS2 compounds, and from 1.7 to 2.3 for PS3 compounds. Sulfur was found to have an interference effect on the phosphorus emission and the ratio showed some dependence on compound structure. The dc emissive system (8) also showed some variations in emission with structure, but no attempt was made to determine elemental ratios. An emission system potentially well suited to this type of investigation is the atmospheric pressure microwave excited detector first described by McCormack, Cooke, and Tong (9) which gives both atomic and diatomic emission from most types of compounds. However, no studies on atomic ratio determinations have been reported to date with this type of detector. Earlier work in our laboratory (10) showed that the atomic carbon emission at 247.9 nm was approximately independent of structure and heteroatoms present for a wide range of compounds. This communication describes attempts to measure the ratio of chlorine, iodine, bromine, sulfur, and phosphorus emission to carbon emission for a range of simple compounds, to measure the factors which affect these ratios, and to determine the potential of this technique for the determination of inter-element ratios. EXPERIMENTAL
Apparatus. The apparatus used was similar t o that described previously (10). The detector system was modified by the use of a narrow, water-cooled, three-quarter wave microwave cavity. Two apertures on opposite sides of the cavity were used for viewing the discharge in conjunction with a Beckman DU monochromator fitted with a 960IB photomultiplier (EM1 Electronics Ltd.) and a low resolution Hilger and Watts monochromator (type D292) fitted with a R166 solar blind photomultiplier (Hamamatsu TV Co. Ltd., Japan). Both photomultiplier tubes were operated from the same high voltage power supply at a potential of 1 kV. During later work on S : C and Br:C ratios, the D292 monochromator was replaced by a Hilger and Watts type D330 monochromator fitted with a 62568 photomultiplier which was operated with a Brandenburg 472R high voltage power supply at a potential of 1.25 kV. The responses of the two photomultipliers were recorded simultaneously on strip chart recorders using two independent electronic read-out systems. (3) M. C . Bowman and M. Beroza,J. Clzromafogr. Sci., 7, 484 (1969). (4) C . V. Overfield and J. D. Winefordner, ibid., 8, 233 (1970). ( 5 ) M. C . Bowman and M. Beroza, ibid., 9,44 (1971). ( 6 ) M. C . Bowman and M. Beroza, ANAL.CHEM., 40, 1448 (1968). (7) Anoosh I. Mizany, J . Chromatogr. Sci., 8, 251 (1970). (8) S. Braman and A. Dynako, ANAL.CHEM., 40, 95 (1968). (9) A. J. McCormack, W. D. Cooke, and S. S. C . Tong, ibid., 37, 1470 (1965). (10) R. M. Dagnall, T. S. West, and P. Whitehead, Anal. Chim. Acta, 60, 25 (1972).
i
2.0
I
I
a, h @J P
c c
c Gl
.-
c w 1 9
g 0 +
8N
1.0
ti;
EGl .-
c 0 1
m
-E 0
0 E 4
0
0
1 so
75
Peak height
at 247.9 nm
Figure 1. Variation of the ratio of peak heights Pcl/Pc with PC
13
+ Trichloroethylene
A
x Dichloroethane V Ethyl chloride
Carbon tetrachloride Chloroform @ Tetrachloroethylene 0 Dichlorornethane
A 0.7-m column of ’/,& stainless steel tubing packed with chromosorb 101 was used throughout this work. Injections were made directly onto the column and argon was used as the carrier gas. Procedure. In the investigation of each group of compounds, one of the monochromators was set to the emission wavelength of atomic carbon (247.9 nm) and adjusted for maximum signal using repeated injections of butane. The other monochromator was set to a suitable wavelength to record the emission characteristic of the particular heteroatom (e.g., 206.2 nm for iodine) and optimized using a compound with a relatively high proportion of that atom (e.g., methyl iodide). The two detection systems responded simultaneously to an eluate passing through the discharge, and the ratio of the responses was taken as the ratio (R)of the peak heights (P) of the two chart recorders (e.g., PI at 206.2 nm and Pc at 247.9 nm). Peak heights are in arbitrary units as it is only the variation of R with different compounds which is studied. The effect of microwave power on R for each group of compounds was investigated and the optimum power selected. The variation of R with concentration was measured by injecting a series of small quantities (from ca. 10-8 to 1 0 - ~ gram) of each compound. The exact quantity injected was not important because previous studies had shown that the atomic carbon emission was linear with respect to concentration for all the compounds investigated over the concentration range used; hence, Pc was taken as a measure of the concentration of each compound present.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
2075
0.1
no
/
I
nAaU
0.01
"
" " I
0 ',2
1.0
10
Theoretical chlorine t o carbon ratio
0
Figure 2. Plot of log (Pcl/Pc - R,) us. log r Captions as for Figure 1
RESULTS AND DISCUSSION Chlorine/Carbon Ratio. The D292 monochromator (slit width 0.5 mm) was set to 247.9 nm and the Beckman monochromator (slit width 0.1 mm) to 256 nm. The emission band at this wavelength, due to Clz (11) was the most sensitive for chlorine compounds. An oven temperature of 150 "C and a carrier gas flow rate of 2.07 L/hr were used. The effect of microwave power on R(Pcl/Pc) was studied using chloroform and trichloroethylene. The ratio of the ratios RCHcla/Rc2Hcla was constant above ca. 50 watts, and a microwave power of 70 watts was used for further studies. The following compounds were studied (retention times in minutes are shown after each compound): carbon tetrachloride (2.21), chloroform (1.77), dichloromethane (0.92), trichloroethylene (2.87), tetrachloroethylene (6.42), ethyl chloride (0.50), chlorobenzene (10.17), and dichloroethane (2.64). All the compounds showed a linear variation when R was plotted against Pc and all exhibited approximately the same intercept (R,) on the R axis, equivalent to R, for a compound containing no chlorine (Figure 1). It was also found that the plot of log (dR/dPc) (Le. log of the gradient) against log r , where r is the empirical formula ratio of chlorine to carbon for each compound (e.g., r = 3 for CHC13) is linear with a gradient of 2.08. Alternatively, a plot of log R-R, for all compounds (at any one value of Pc) against log r gives the same gradient (Figure 2). The above results can be explained from the fact that the chlorine emission is a Clz band emission. Let P, be the peak (11) A. G. Gaydon and R. W. B. Pearse, "The Identification of
Molecular Spectra," Third ed., Chapman and Hall Ltd., London, 1965. 2076
0
8
4 Theoretical carbon t o iodine ratio
Figure 3. Variation of the ratio of peak heights Pc/PI with the stoichiometric carbon-iodine ratio
+
0 Methyl iodide
x Ethyl iodide A n-Propyl iodide
+Amyl iodide Phenyl iodide
height due to emission species x and [XI be the instantaneous concentration of x in the discharge. If P, is proportional to [x]then:
Pc
=
KC[Cl
(1)
and PCl?
= KCI[C121
Now if: [Cl,]
=
Kl[cl]z
then PCl =
KClKI[C112
+Po
(4)
Where Pcl is the emission at 256 nm and Po is the emission due to nonchlorine containing compounds and
Powas found experimentally to be proportional to [C] and hence,
For a compound of the type CCl,, r is equal to the ratio total chlorine/total carbon, PC a [C] 0: [total carbon] and P c l a [C1]2 a [total c h l ~ r i n e ] ~ .
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
Figure 4. Variation of the ratio of peak heights Pp/Pc with the stoichiometric phosphorus-carbon ratio V Trimethylphosphite A Triethylphosphite
n Trimethylphosphate 0 Triethylphosphate x Tributylphosphate + Triphenylphosphate
0.4
0.2
0 Theoretical
phorphorur to carbon
ratio
Hence, (7) and
Pc 1 - = K3 Pc
+ K5r2[C]
Therefore, for any compound of constant r (9) And for all compounds
and therefore,
Hence, a plot of PCIIPCBS. PC for any compound should be linear and a plot of log
us. log r should be
linear with a gradient of 2. IodineiCarbon Ratio. The Beckman monochromator was set to monitoi the iodine atomic line at 206.2 nm. A temperature of 160 "C and a carrier gas Bow rate of 3.8 l./hr were used. The ratios of iodine to carbon emission for ethyl iodide and methyl iodide were constant at microwave powers greater than 60 watts, and a power of 70 watts was again used. The following compounds were studied: methyl iodide (0.38), ethyl iodide (0.72), n-propyl iodide (1.35), amyl iodide (6.20), and iodobenzene (15.0). For all these compounds, the ratio Pc/PI was independent of
concentration and carrier gas flow rate. The ratio of peak heights Pc/PI plotted against the theoretical carbon to iodine ratio is shown in Figure 3. The plot is linear as expected with a maximum deviation from linearity of 4 z (if PI E [ I ] , 111 then PI - a r for a compound of the type C L ) . Pc [CI Phosphorus/Carbon Ratio. The major phosphorus emission was the atomic line at 253.5 nm and, hence, the Beckman monochromator was set t o monitor this wavelength. As before, 70 watts microwave power was found to be optimum. Unlike in the previous studies, one set of conditions could not be established to obtain convenient retention times for all the compounds studied. The following conditions were used: temperature 150 "C and flow rate 3.3 L/hr for trimethylphosphite (0.66), triethylphosphite (0.90), and trimethylphosphate (0.95); temperature 230 "C and flow rate 2.7 L/hr for triethylphosphate (0.80) and tributylphosphate (1.70); and temperature 270 "C and flow rate 2.4 L/hr for triphenylphosphate (6.0). For each of these compounds, the ratio of the peak heights for phosphorus and carbon emissions was independent of concentration and carrier gas flow rate. The graph of Pp/Pc against the theoretical ratio is shown in Figure 4. The maximum deviation from linearity is 8 %. Sulfur/Carbon Ratio. The D330 monochromator was set to the atomic sulfur line at 182.04 nm and was purged with argon. The Beckman monochromator was set to 247.9 nm. In this instance, 50 watts microwave power was found to be optimum because the sensitivity of the atomic sulfur emission falls off rapidly at higher microwave powers. A flow rate of 3.3 l.,'hr and an oven temperature of 200 "C were used. The compounds studied were carbon disulfide (0.48), dimethylsulfoxide (2.92), thiophen (0.65), dibutylsulfide (1.33), and dibutyldisulfide (4.50). Again, for each of these compounds R was found to be independent over a wide range of concentration and of flow rate. However, at high concentrations (ca. lo-& gram) the value of R tended to decrease, particularly for compounds with a high carbon content. This was probably due to carbon depositing on the walls of
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
2077
Table I. Some Limits on Detection and Selectivity Ratios Selectivity Wavelength, Limit of ratio Element nm detection, (g/sec) (w.r.t. carbon) C 247.9 1 . 9 x 10-10 I 206.2 1.0 x 10-10 1,m S 182.0 4.0 x 460 P 3.0 x 150 253.5 I
c1
a
Br Band.
256a 292“
4.5 2.5
x x
.
.
10-9 10-9
the detector tube which would particularly reduce the sulfur emission. The graph of Ps/Pcus. the theoretical ratio showed a maximum deviation from linearity of 5 %. Bromine/Carbon Ratio. The D330 monochromator was set to monitor a Br2 band emission at 292 nm. A microwave power of 70 watts was found to be optimum and a column temperature of 170 “C and a carrier gas flow rate of 3.3 L/hr were used. The compounds studied were bromoform (5.00), bromobenzene (6.45), ethyl-2-bromopropionate (6.75), dibromoethane (2.57), n-amyl bromide (2.30), ethyl bromide (0.55),and propyl bromide (1.04). The results were similar to those obtained for chlorine. All the compounds showed a linear variation when R was plotted against PC with approximately the same intercept on the R axis. When the log of the gradients of these slopes were plotted against the log of the theoretical bromine/carbon ratio, a straight line with a gradient of 1.96 was obtained. CONCLUSIONS
The present study has shown that for the limited range of sulfur, iodine, and phosphorus compounds investigated, the use of the simple expedient of simultaneously monitoring emission from two atomic lines can be used to determine the quantitative relationship between the heteroatoms and the number of carbon atoms in the compound. In each
instance the measured ratio was found to be independent of carrier gas flow rate and concentration. Hence, when these atomic emissions are used, it is unnecessary to know the exact concentration of a compound under examination and it is not essential to obtain a good peak shape in order to determine the inter-element ratios. The results obtained for chlorine- and bromine-containing compounds can be satisfactorily explained on the basis that the emitting species are diatomic (C12and Br2). The ratios of chlorine or bromine emission to atomic carbon emission were both carrier gas flow rate and concentration dependent. A series of peak ratios for a range of concentrations of the sample under investigation would be required together with a standard sample in a similar range for analytical purposes. The analytical utility of this technique is clearly limited by the sensitivities of the detectors to the elemental emissions monitored (shown in Table I) and by the relative interference of other elements at these wavelengths. Interferences with this detector may be of two types. The first occurs when a large excess of another compound gram/sec) overloads the plasma; this will affect all types of emissions and must be avoided. The second effect arises from spectral emissions from other species at the wavelength being monitored. The principal interfering element is carbon which gives rise to a large number of emitting species (e.g., Cn,CN, CH, C). The relative selectivity of each element at the atomic wavelength used is also shown in Table I. These are given as the ratio of response per gram-atom of the element to the response per gram-atom of carbon. The use of microwave-excited atomic emissions for the determination of inter-element ratios has been shown to be feasible and, providing it proves applicable to a wider range of compounds, it is considered that this will offer a valuable aid in the identification of unknown eluates in gas chromatography. RECEIVED for review February 7, 1972. Accepted June 15, 1972.
Determination of Amide, Urea, and Nitrile Compounds Using Alkali-Fusion Reaction Gas Chromatography Stanley P. Frankoski and Sidney Siggia Department of Chemistry, Uniuersity of Massachusetts, Amherst, Mass. 01002
CARBOXYLIC AMIDES have been determined by alkaline hydrolysis (1-4). While reaction is sluggish, primary amides are more reactive than secondary and tertiary. Major limitations of procedures using alkaline hydrolysis have been the dependence on concentration of caustic and attainment of high enough temperatures to force the reaction to completion. (1) S . Siggia, “Quantitative Organic Analysis via Functional Groups,” John Wiley and Sons, New York, N.Y., 1963, Chap. 3. (2) S.Olsen, Die Chernie, 56, 202 (1943). (3) F. E. Critchfield, “Organic Functional Group Analysis,” Pergamon Press, New York, N.Y., 1963, p 52. (4) R. D. Tiwari and J. P. Sharma, “The Determination of Car-
boxylic Functional Groups,” Pergamon Press, New York, N.Y., 1970, Chap. 5. 2078
Other chemical and instrumental methods for amide determination have been reviewed (1-5). Some of these methods include determination by titration in nonaqueous solvents, spectrophotometric procedures, reduction with lithium aluminum hydride, gas chromatography, and reaction gas chromatography. Polymeric amides with functional groups on the polymer backbone resist alkaline hydrolysis. Polymeric nitriles are extremely difficult to determine hydrolytically. Infrared analysis can be applied to polymeric nitriles; however, diffi(5) S . Siggia, “Instrumental Methods of Organic Functional Group Analysis,” Wiley-Interscience, New York, N.Y., 1972, Chap. 3.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
