In general, then, the compounds which undergo extensive acylation under the conditions specified here for ionizing voltage and pressure have either 0 or N contained in functional groups. Compounds containing halogen react only to a slight extent. From the double resonance signals, the acetyl ion itself (CH3CO+,mass 43), although present in every spectrum, is not the actual acylating agent (except possibly in one relatively ambiguous case). Rather, the butanedione molecular ion (mass 86) and the mje 129 ion formed by reaction 10 are the precursors to the acylated products. Loss of a neutral from the collision complex remains a requirement for formation of a product ion at these pressures. Finally, for the most part, if an ion other than these two acts as a precursor to the acylated product, it comes from a molecule of relatively low ionization potential. There is an obvious implication for analysis from these studies. With butanedione as a reagent, one can detect functional groups in a mixture with hydrocarbons, particularly the N- and 0-containing functional groups we have examined here. Table I would suggest that this sort of reaction can be used to distinguish oxygen-containing compounds from hydrocarbons of similar molecular weight, for example, under low-resolution conditions. While the commercial ICR instruments available today have a limited mass range, chemical ionization with groups larger than a proton should eventually be extended to type analysis of nitrogenous compounds in hydrocarbons. One may be able to estimate fractions of functionalized molecules in a given mixture under a routine set of conditions by comparison of low-voltage mass and ICR spectra. The observation of borderline reactivity for the alkyl halides may lead to an additional analysis by reactivity unlike mass spectrometry. By using other acylating agents, it should
be possible to find reactions whose observation is borderline for other functional groups, and so to be able to set up a series of microanalyses for functional groups within a unit complementing a mass spectrometer. More ideally, one can envision reagents which because of their bulkiness could react to greatly different extents with different stereoisomers and thus provide a rationalizable, or even predictable, substitute for stereochemical analysis of small quantities by mass spectrometry. Part of the immediate problem of ICR spectrometry is the fact that molecules of interest to persons concerned with stereochemistry are just barely within range of present instrumentation. For full development of the technique for analysis, a wider mass range will have to be introduced. In the meantime, there is a wide-ranging survey of reactivities of functional groups toward reagent ions, and of reagent ions from different sources, to be explored. We have only scratched the surface here, and the possibilities for further exploitation of this technique are impressive indeed.
RECEIVED for review May 18, 1970. Accepted July 27, 1970. Support of this work by the National Institute of General Medical Sciences (GM 15,994), the National Science Foundation (GP 8096, G U 2059), and the Advanced Research Projects Agency (Contract SD-100 with the University of North Carolina Materials Research Center) is gratefully acknowledged. The ICR-9 ion cyclotron resonance spectrometer was purchased through support by Hercules, Inc., the Shell Companies Foundation, the National Science Foundation (GU 2059), and the North Carolina Board of Science and Technology (159). M. M. Bursey is a Fellow of the Alfred P. Sloan Foundation, 1969-1971,
Determination of the Presence and Extent of Labeling in Hydrocarbons by Selective Monitoring of Optical Emissions from Gas Discharges David A. Luippold and J. L. Beauchamp The Arthur Amos Noyes Laboratory of Chemical Physics, California institute of Technology, Pasadena, Calif. 91109 The AzA -,X 2 r emissions of CD* and CH* generated in a low pressure microwave discharge were selectively monitored to determine the presence and extent of labeling in hydrocarbons. Differences in the emission intensities of CH* and CD*from the decomposition of partially deuterated methanes and ethanes are presented, and it is shown that the CD* and CH* fractional emission intensities can be related to the,extent of labeling. The feasibility of applying the microwave discharge as a selective detector for labeled species in the effluent from a vapor phase chromatograph is explored. An increase in ionic emissions and a decrease in atomic emission intensities which was observed in helium and argon upon addition of hydrocarbon may yield a simple method to distinguish between ionic and atomic lines.
DETECTION OF ORGANIC COMPOUNDS in a gas chromatographic effluent by the selective monitoring of emissions produced in a 1374
microwave discharge was first reported by McCormack, Tong, and Cooke (1). Spectral emissions have been correlated with fragments of functional groups and other specific structural features for a variety of compounds in gas discharges (2-6). Sensitivities of up to 2 X 10-16 g sec-' in a microwave disg sec-l in a dc discharge ( 4 ) have been charge (I) and to obtained. The success of the applications of gas discharges as selective detectors for gas chromatography has led us to in(1) A. J. McCormack, s. C. Tong,and W. D. Cooke, ANAL.CHEM., 37, 1470 (1965). (2) C. A. Bache and D. J. Lisk,. ibid.,. D_ 1477. ( 3 ) Ibid., 39, 786 (1967). (4) R. S. Braman, ibid., 38, 734 (1966). (5) R. S . Invet and R. Durbin, J. Gus Chromutogr., 1, No. 12, 14 (1963). ( 6 ) R. S. Braman and A. Dynako, ANAL. CHEM., 40, 95 (1968).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
Microwave Generator
To
Bubble
Gas
Meter
/Antenna
/I
II
Reservoir
T h ermocou ple Vacuum
Gauge
I
I
To
Pump
Figure 1. Discharge apparatus
vestigate the possibility of detecting the presence and extent of labeling in hydrocarbons by selectively monitoring CD* and CH* emissions from these species. The ability of radio gas chromatography to detect tritiated hydrocarbons makes it uniquely applicable to a range of experiments which involve the study of tritium incorporation into organic molecules. These include hot atom chemistry, tracer studies, radiation chemistry, and ionic reactions (7-14). Many studies of a similar nature, in which deuterium is incorporated in organic molecules, would benefit from the availability of a gas chromatograph detector capable of selectively identifying deuterated or partially deuterated compounds. It is shown in the section on emission spectra in a microwave discharge that with the limited resolution of 1.O A it is possible to obtain quantitative information on the relative CH* and CD* emission intensities. Procedures for separating CH* and CD* fractional emission intensities from overlap are presented along with neat CH* and CD* A2A + X*T emission spectra in that section. Differences in observed CH* and CD* emission intensities from the decomposition of partially labeled methanes and ethanes are discussed later on. I n the last section, the technique is applied to the detection of labeled species in the effluent from a gas chromatograph.
(7) D. Seewald and R. Wolfgang, J. Chem. Phys., 47, 143 (1967). (8) J. W. Root and F. S . Rowland, J . Amer. Chem. SOC.,85, 1021 (1963). (9) J. W. Root and F. S . Rowland, J. Chem. Phys., 46,4299 (1967). (10) S. Lukac, Jad. Energ., 12, 130 (1966). (11) F. Cacace and S . Caronna, J. Amer. Chem. Soc., 89, 6848 (1967). (12) F. Cacace, M. Caroselli, R. Cipollini, and G. Ciranni, [bid.,90, 2222 (1968). (13) F. Cacace and G. Perez, ANAL. CHEM., 39, 1863 (1967). (14) F. Cacace, Nudeonics, 19, No. 5,45 (May 1961).
EXPERIMENTAL
Samples were injected with a 0-10 pl syringe in typical amounts of less than 0.1 pl into a Varian Aerograph 90-P gas chromatograph. The column used was 10-ft X 1/4-inch 3 % SE-30 on 120/100 Diatom W operated at 60 “C with a flow rate of 50 ml/min, using helium as a carrier gas. After flowing through the thermal conductivity detector in the chromatograph, the effluent entered the discharge apparatus (Figure l), located 5 cm from the 90-P collector output. The plasma was produced in a borosilicate glass discharge tube, initiated by a Tesla coil and sustained with a 2450 MHz Diathermy microwave generator (85 Watts maximum) through a loosely coupled antenna. The light emission traversed ,a 1.7-mm silica window (passing wavelengths 1600 to 8000 A) (15) and a lens system before entering the Jarrell-Ash Model 82-000 0.5-meter Ebert spectrometer located at a distance of 6 feet. The separation was necessary to avoid noise in the photoelectric detection system, consisting of an uncooled 1P28 photomultiplier tube operated at 600 volts. The feciprocal linear dispersion of the grating employed was 16 A/mm. The spectrometer slit dimensions were 3 mm Y 64 p, corresponding to a resolution of approximately 1.0 A. Emissions at selected wavelengths could be registered simultaneously with thermal conductivity detector response on a dual-channel Hewlett-Packard 7100 B Strip Chart Recorder. Only one particular spectrometer resolution was used after initial investigations. Increased resolution requires a smaller wavelength bandwidth, thus reducing sensitivity. On the other hand, too little resolution, wit! great sensitivity,*reduced selectivity by including both 43 10-A CD* and 4315-A CH* emissions in the same intensity measurement. Our choice of slit dimensions provided the highest attainable sensitivity without sacrificing appreciable selectivity. (15) James A. R. Samson, “Techniques of Vacuum Ultraviolet
Spectroscopy,” 1st ed., John Wiley and Sons, Inc., New York, N. Y., 1967, pp 181, 252.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1375
CH emission
i
The sample inlet nozzle port B was used to establish a constant flow of additive into the gas discharge through a Granville-Phillips variable leak valve. A Hastings DV-23 thermocouple vacuum gauge recorded the pressure in the discharge tube. A mechanical pump drawing 25 1. min-1 was used, giving flow rates through the plasma tube between 2 and 50 ml-atmos-min-l for pressures between 250 and 2 torr. This corresponds t o a velocity of -104 cm sec-’ through inlet port A , and to sample residence time in the 35 cm3 discharge of -0.1 sec. Helium flow rates through the discharge apparatus and gas chromatograph were measured utilizing a standard bubble meter. After extended periods of operation, or following injection of a sample in excess of 8 X 10-5 gram per component with Cg and C, hydrocarbon, deposits formed on the inside of the discharge tube. The extent of formation of carbonaceous deposits was somewhat more severe with higher molecular weight species. Operating the microwave discharge with O2 rapidly removed the visible films. Deuterated hydrocarbons C6DI2, CH3D, CH2D2, CHD3, and CD, with a stated isotopic purity of 99.5 atom per cent deuterium were obtained from Stohler Isotope Chemicals and were used as supplied, as were the ethane samples CD3CH3, CH2D-CH2D, and CSDjH with a purity of 99 atom per cent deuterium from Merck Sharp and Dohme. Helium from the Linde division of Union Carbide, with a stated purity of 99.99 was used as supplied unless stated otherwise. EMISSION SPECTRA OF CH* AND CD* IN A MICROWAVE DISCHARGE In a gas discharge, low molecular weight hydrocarbons are fragmented into products consisting principally of diatomic, 1376
CD
emission
A~A-X*II
A*A-X*II
atomic, and ionic species (1, 2, 6). These fragments can be generated in, or excited to, high lying electronic states from which photon emission may occur. The intense CH* emission at 4314 A ( A 2 A--f P a ) characterizes the presence of hydrocarbons in gas discharges (16, 17). Emission spectra in the region of the A 2 A-,X ~ transition T obtained by admitting CH, and CDa into the gas discharge through port B are displayed in Figure 2. (The molar concentration of hydrocarbon in helium remained between and Introduction of higher molar concentrations causes the characteristic color of the discharge to change and the plasma volume t o shrink.) The spectral region in which this band is observed is free of significant helium background. The relative A 2 A + X2a emission intensities of CH* and CD* would thus provide relations for the numbers of emitting CH* and CD* present, except that both CH* and C D * (0””) vibrational heads at 4315.3 A (CH*) and 4310.1 A (CD*) (16-18) degrade t o the violet in rotational fine structure (19, 20) which overlaps and (16) L. Gero, 7. Physik, 117,709 (1941). (17) Zbid., 118, 27 (1941). (18) Gerhard Herzberg, “Molecular Spectra and Molecular
Structure: Spectra of Diatomic Molecules,” Vol. I., 1st ed., D. Van Nostrand Company, New York, N. Y., 1950. (19), C. E. Moore and H. P. Broida, J. Res. Natl. Bur. Standards, 63A, 19 (1959). (20) Arnold M. Bass and H. P. Brodia, “A Spectropbotometric Atlas of the Spectrum of CH from 3000 A to 5000 A,” United States Department of Commerce, National Bureau of Standards, Washington, D. C., 1961. \ -
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
prevents easy separation into CH* and C D * components. Emission maxima, however, of the C H * (4313 A) and C D * (4308 A) in this transition are observed t o be shifted 4.65 A (Figure 2). Relationships between the maxima and the total integrated A U -+ X2ir band intensities for C H * and CD* can thus be established, and after correcting for CH*-CD* overlap a t 4313 A and 4308 A, the relative emission intensities can be determined. Relating the maximum to the total (O’, 0”) band intensity was complicated by the presence of residual nitrogen in the system, as evidenced by C N and N2+ emissions (4216.0, 4197.2, 4181.0; 4278.1, 4236.5, 4199.1, and 4166.8 A, respectively) ( 2 1 ) in the region of interest. The amount of nitrogen impurity could be minimized by employing a cold trap containing silica gel at liquid nitrogen temperatures to purify the helium carrier and by maintaining a soap solution in the bubble meter to prevent back diffusion of air into port A . (However, a significant decline in sensitivity was observed (approximately a factor of 3) in going t o purified helium. This suggests that trace impurities such as NSor Os contribute t o the decomposition of hydrocarbons either directly or indirectly through changing conditions in the helium plasma.) Despite these precautions a detectable C N emission at 4216 A was always present (Figure 2), and it was eliminated from the integrated 4314 A band intensity only by recording spectra from 4315 A to 4223 A. A second difficulty in obtaining a n accurate ratio of the emission maximum t o the integrated band intensity involves the overlap of different vibrational branches, preventing the isolation of a single complete transition in the 4315-4220 A The main branches of interest in the A 2 A -+ X 2iT regio;. 4314 A system are listed in Table I for C H * (19, 20). The ratio of the maximum to the total intensity depends on the populations of the various rotational levels i a the excited states which give rise to transitions near 4313 A in CH or 4308 A in CD*. The vibrational and rotational temperature is therefore important, requiring care in reproducing the pressure and microwave power input. The ratios of maximum intensity over the total integrated band intensity for C H * and CD*, taken from the separate neat spectra of C H I and C D 4 at 1 torr, are given in Table 11. Possibly because of a reported disparity in C D * and C H * vibrational and rotational temperatures in similar decomposition conditions ( 2 2 ) , the intensity maximum divided by the total band intensity is not equal for C H * and CD*. To correct for this, a value R has been defined in Equation 1, where C H * and CD* refer to :he intensities of the emission maxima monitored at 4313 A and 4308 A, respectively.
-
Table I. Principal Vibration@Branches in the A2A X*TCH* 4314-A Transition Vibrational branch Spectral region,a A Q (O‘,O’‘) 42654314 R (1’”’’) 41854299 R (0’”‘’) 4 194-426 1 Q (1‘”‘‘) 4297-4313 R (2‘”’’) 4238-4314 Data from References 19 and 20. ____
Table 11. R Values and (Intensity Maxima)/(Total Integrated Band Intensity) for CH* and CD* Emissions at 1.0 Torr Pressure in a Microwave Discharge I,,,
.f4315 42231(X)dX
Standard deviation
2 938 3 284
0 157 0 043
CH~Q CD4
Rb = 0.895 & 0.057
I,,,
.f
Standard :::%) dh deviation
CH4 3.175 0.163 Rc = 0.893 & 0.050 CD4 3.555 0,037 a The molar concentration of CH4 and CD4 remained between and * R value for band intensity integrated over 92 A. R value for band intensity integrated over 73 A.
which in the next section are related t o the extent of labeling in partially deuterated methanes and ethanes. RELATIVE INTENSITIES OF CH* AND CD* EMISSIONS FROM PARTIALLY LABELED METHANES AND ETHANES Emission intensities of CH* and C D * taken from spectra of CH3D, CH2D2,CHDs, CH2D-CH2D, CH3-CDs, and CIDSH introduced through port B into the discharge tube with a helium pressure of 1.O torr are compared to the extent of labeling in Table 111. The maximum emission intensities are expressed in terms of the quantitiesfca and fCD defined in Equations 2 and 3, where CH* and CD* are the emission maxima used in Equation 1, and are corrected for the overlap apparent in Figures 2a and 2b.
+ CD*) = CD*/(CH* + CD*)
~ C = H
CH*/(CH*
~ C D
R,
=
CH*/.f2z’5ZCH(X) dX CD*/ .f23151CD(X)dX
The subscript o( in expression 1 refers to the two different integration lengths of band spectra given in Table 11, where a represents 4223 A or 4242 A. The respective R values a t 1.0 torr are 0.895 and 0.893, and are thus independent of the integration length. In this work, the intermediate R value of 0.894 is used. Thus, by fpplying overlap and R value corrections to the C H * (4313 A) and C D * (4308 A) intensity maxima, the C H * and C D * fractional emission intensities can be obtained, ~
(21) R. W. B. Pearse and A. G. Gaydon, “The Identification of Molecular Spectra,” 3rd ed, Chapman and Hall Ltd., London, England, 1963. (22) W. Brennen and T. Carrington, J . Chem. Phys., 46,7 (1967).
(2) (3)
The fractional emission intensities of C H * (Sea) and C D * (SCD) can then be calculated as indicated in Equations 4, 5, and 6. SCD = CD*
R/(CD*
*
R
+ CH*)
(4) (5)
The quantities and SCD are those which should most nearly represent the fractional hydrogen and deuterium content of the species admitted into the gas discharge ( 5 , 6,23). It (23) R. G. Brewer and F. L. Kester, J . Chem. Phys., 40, 812 (1964).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1377
Table 111. Fractional Emission Intensities (SCH and S c ~for ) Partially Deuterated Methanes and Ethanes at 1.0 Torr Pressure in a Microwave Discharge Fractional Emission intensities deuterium Sample content fCH fCD SCHa SCD" Std Dev R2 XC 0.008 0.653 1.25 0.313 0.338 0.687 CHID 0.25 0.662 0.443 0.557 0.001 0.714 1.172 0.50 0.416 0.584 CHzDz 0.002 0.774 0.175 0.795 0.225 0.775 0.75 0.205 CHD3 0.640 0.360 0 I002 0.785 0.33 0.611 0.389 CHzD-CHID 0.547 0.003 0.743 0.574 0.453 0.50 0.426 CHa-CDs 0.001 0.774 0.852 0.866 0.148 0.83 0.134 C2DaH 0 Fractional emission intensities calculated employing R = 0.894. b R value needed to equate fractional emission intensity with stoichiometric fractional deuterium content. 0 Individual isotope effect for R = 0,894 calculated as the probability of losing a hydrogen atom us. a deuterium atom at each step in a sequential decomposition.
1.0
I
iI
I
/ P
SCDMethanes v S c D Ethanes
A
0.8 h ._ f
In E
c
0.6 c
._
v)
.E
W
=
0.4
.-c6 0
tt 0.2
0
I
I
I
25
50
75
Deuterium
Content
100
(%)
Figure 3. Fractional emission intensity dependence on extent of labeling in methanes and ethanes
+
I .o
c .-
0,8
it still appears from the available electron impact data (26) that atomic hydrogen loss will dominate in the sequential decomposition process which leads eventually to CH*. Under these circumstances it is possible to calculate a n isotope effect for the probability (x)of losing a hydrogen atom over that of losing a deuterium atom in each step of the decomposition. The results of such a calculation are presented in Table 111, and are in the same range as values reported in the literature (27-31).
ln
0) c
e
-c .-s lI n n .-
0.6
E
W
-
is apparent in Table I11 that CD* emission is favored over that of CH*, as is graphically illustrated in Figure 3. The R values in Table 111which are required to equate the fractional emissions to the extent of labeling are significantly less than the R value of 0.894 given in Table 11, suggesting a n isotope effect. The mechanism of decomposition of alkanes in a rare gas microwave discharge is not yet understood in sufficient depth to permit a detailed interpretation of the above results. Fayard (24) has suggested that in the decomposition of methanes in a plasma, a primary ionization-decomposition sequence is initiated by collisions with energetic electrons, excited atoms and ions which leads partially t o the production of CH+. Recombination with a n electron into a n excited state then yields electronically excited CH* which may give rise to the 4313 A emission band (25). If such is the case, it seems reasonable to assume that most ionization-decomposition proceeds in steps where one hydrogen or deuterium atom is lost. It is in addition highly probable that an appreciable extent of decomposition leading to the production of excited CH* also proceeds through neutral intermediates. While it has been established in the nonionizing photodecomposition of methanes that molecular hydrogen elimination (Equation 8) is more important than atomic hydrogen elimination (Equation 7), CHI * CHI H (7)
0.4
0 0 c
.L
0
0.2
0
(24) (25) (26) (27) 0
50
25 CD,
75
100
Content (%)
Figure 4. Fractional emission intensity SCDus. CD4 in total CD4-CH4 mixture, from decomposition in a microwave discharge at 1.0-torr pressure 1378
F. Fayard, J . Chim. Phys., 60,651 (1963). L. M. Yeddanapalli, J. Chem. Phys., 10, 249 (1942). C. E. Melton and P. S . Rudolf, ibid., 47, 1771 (1967). V. H. Dibeler and F. L. Mohler, J. Res. Natl. Bur. Standards, ' 45, 441 (1950). (28) J. Turkevich, L. Friedman, E. Solomon, and F. M. Wrightson, J . Amer. Chem. SOC.,70, 2638 (1948). (29) M. W. Evans, N. Bauer, and J. Y. Beach, J. Chem. Phys., 14, 701 (1946). (30) D. P. Stevenson and C. D. Wagner, ibid., 19, 11 (1951). (31) D. 0. Schissler, S . 0. Thompson, and J. Turkevich, Trans. Faraday SOC.,10, 46 (1951).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
Table IV. Fractional Emission Intensities (SCH and ScD) for CH4-CD4Mixtures at 1.0-Torr Pressure in a Microwave Discharge Mixture composition,” CDd CH4 24 45 47 74 76 a
Emission intensities
76 55 53 26 24
fCH
fCD
SCH
SCD
Std dev
0.738 0.481 0.464 0.231 0.217
0.262 0.519 0.536 0.769 0.783
0.759 0.509 0.491 0.251 0.236
0.241 0.491 0.509 0.749 0.764
0.002 0.002 0.002 0.002 0.004
Mixtures were prepared volumetrically and checked by mass spectrometric methods on an ion cyclotron resonance mass spectrometer.
o 4338 ‘
10.0 -
+
~ e * Increase
3889 j He
8
i
Decrease
0
l
l
1
-E
0 al
t
7.5-
a
d %
c .-
In
al
c 5.0 c
CH* and CD* emissions from CHrCD4 mixtures were recorded, with no appreciable isotope effect being observed, as shown in Figure 4 and Table IV. Thus, the deviations of ScD emission intensity from the fractional deuterium content are probably due to isotope effects of hydrogen atom LIS. deuterium atom elimination at each step in a sequential decomposition. “Probability integral isotope effects” (8), where the extent of labeling affects the overall rate of decomposition of the molecule, are avoided by selecting discharge conditions such that all the hydrocarbon species are totally decomposed. It is of interest to note that CHICDI exhibits a preferential CD* emission (Table 111). This suggests that C-C bond cleavage is not the initial step in the decomposition of ethane in the gas discharge; equal CH* and CD* emission intensities would otherwise be observed. Braman and Dynako (6) note that the emission intensities of
z
.In ._ E
W
2.5
-
0
6
3 Thermal
Conduciivity
9
Detector Response (mv)
” u”
Emission
Figure 5. Intensity changes of 4338-A He+ and 3889-A He* emissions with introduction of cyclohexane
d
2
c Y
0 .G
F
n
f
z
Y
4308 A
Thermal
Emission
Conductivity Detectw (GC) Response
G C Detectw Response
I
I
I
3
4
5
I
I
I
6 Time (mid
7
8
Figure 6. Variation of 4308-A emission intensity with gas chromatograph thermal conductivity detector response for C6D12 and C6HiiCH3
I 3
I
1
I
1
4
5
6
7
I 8
Time (min)
Figure 7. Variation of 4313-A emission intensity with gas chromatograph thermal conductivity detector response for CBDlland CeHnCH3 ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1379
~~
~
~~
Table V. Ratio of Emission to Thermal Conductivity Detector Response in C.SH~Z-CBD~Z Mixtures Sample, Z 4313-A Emission 4308-A Emission CsDiz Ratio Std dev Ratio Std dev. CeHiz 0 0,3343 0.05 0.0868 0.002 100 0.2516 0.02 0.2336 0.007 75 25 0.1764 0.004 0.3259 0.01 50 50 0.1305 0.003 0.3970 0.03 25 75 0.0535 0.004 0.4216 0.02 0 100
transitions between excited states of helium decrease with the introduction of a trace impurity, and suggested that metastable states are important in the chemistry of hydrocarbon decomposition. We investigated this phenomenon with similar results, but in addition we found that the intensities of ionic helium increased with the addition of hydrocarbon. The linearity of the atomicoHe I (3889 A) intensity decrease and the ionic He I1 (4338 A) intensity increase with introduction of cyclohexane is shown in Figure 5. Because this indicates a n altering of conditions in the discharge, studies were performed using argon as a carrier gas. Corresponding behavior was observed in the intensities of excited atomic and ionic argon emissions, and a general method to distinguish between emissions from excited atomic and ionic transitions may be suggested. DETECTION OF LABELED SPECIES IN A GAS CHROMATOGRAPHIC EFFLUENT Deuterated and nondeuterated hydrocarbons may be teadily identified by monitoring the 4308-A CH* or the 4313-A CD* emission maximum simultaneously with the gas chromatograph thermal conductivity detector response, as is illustrated with cyclohexane-dlz and methylcyclohexane in Figurts 6 and 7. With cyclohexane-dlz, the corresponding 4308-A CD*
emission (Figure 6) is substantial in comparison to the unlabeled methylcyclohexane, which is $istinguished from the deuterated species by its strong 4313-A CH* emission (Figure 7). If the sample of interest is only partially deuterated, or if species simultaneously elute (such as cyclohexane and cyclohexane-dl*), then the extent of labeling (or the amounts of labeled and unlabeled materials) may be ascertained by the following methods. The simplest method would involve the rapid wavelength scanning of the emission spectra as the species elute from the gas chromatograph. This was not possible with our present instrumentation, An alternative but somewhat m y e tedious protedure is to determine the ratio of the 4308-A CD* or 4313-A CH* emissions to the thermal conductivity detector response as presented in Table V and Figure 8 for C6D12-C& mixtures. These ratios can be obtained fairly accurately from the slopes of the emission intensity plotted us. thermal conductivity detector response for multiple sample injections of varying size as is demonstrated in Figures 9 and 10. The small standard deviations given in Table V should make the determination of CBHlP inoC6H12 accurate to within 1 % by a monitoring of the 4313-A CH* emission for moderately and heavily deuterated samples and a
12.5
I
I
A 100% C6 Dl, 0
I0.C
50% C ~ D Q 50 % C6 HI,
75% C6H12
2 5 % C6Dl,
b
100% C6 H,,
'j,
E
-621
.=e
-
7.5
C
._
.-In
E
5.0
OQ
ae
(0
2.5
0
0
0
25 50 Deuterium Content
75
Figure 8. Variation of ratios from 4313-Aand 4308-A emission graphs in Figures 6 and 7 with extent of labeling in CeHlZ-C6Dl2 mixtures 1380
2
0 Thermal
(%)
4
6
Conductivity Detector Response (mv)
Figure 9. Determination of extent of labeling in slightly deuterated C&&Z-C~DIZ mixtures : 4308-A emission response
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
monitoring of the 4308-A CD* emission for slightly deuterated species. Following the method of Johnson and Stross (32) the limit of detection for C6HI2in helium plasma at 1.0 torr was 7 X 10-9 gram sec-I. In the 0.25-2.0 torr range studied, increasing the pressure in the discharge increased the sensitivity, which is inversely proportional to the limit of detection. Also, no attempt was made in the present study to cool the phototube or otherwise reduce the noise level. Since the limit of detection is directly proportional to the noise area (32), a higher sensitivity would be expected if this were done. (A referee has suggested that the analytical technique described would be most suitable for use with capillary columns. Higher concentrations of sample in the effuent with lower He flow rates would allow for the entire output to be delivered into the low pressure discharge tube with a significantly improved sensitivity. This intriguing possibility was not examined in the present studies.)
o 100 %
C,H,
5 0 % %HI, v 25%
? I
A
C6H12
50%
75 %
100% C6DI2
E
: I
.-
Y)
E
5 0
n 10
*
2.5
CONCLUSION
It is evident from the present study that the fractional emission intensities of excited CH* and CD* fragments from labeled hydrocarbon species admitted to a low pressure microwave discharge in helium can be quantitatively related to the fractional deuterium content in the sample. From the analysis of the emission spectra of methanes and ethanes, it is apparent that isotope effects in fragmentation are observed which indicate that in the decomposition process, loss of H is favored over loss of D. The increase in ionic emissions and the decrease in atomic emission intensities of helium and argon upon addition of a trace hydrocarbon suggests a simple method to distinguish between ionic and atomic lines from rare gas discharges.
RECEIVED for review April 30, 1970. Accepted July 24, 1970. D. A. L. is a National Science Foundation Undergraduate (32) H. W. Johnson, Jr., and F.H. Stross, ANAL.CHEM., 31, 1206
(1959).
0
0 Thermal
2 4 Conductivity Detector Response
6 (mv)
Figure 10. Determination of extent of labeling in heavily deuterated C6HI2-C& mixtures: 4313-A emission response Research Fellow, 1969-1970, and J. L. B. is an Alfred P. Sloan Fellow, 1968-1970. This research has been supported in part by funds made available by the United States Atomic Energy Commission (Contract AT(04-3) 767-81, the Alfred P. Sloan Foundation, the National Science Foundation (Grant No. GY-5881 and GY-7351), and American Cyanamid. Contribution No. 4044 from the Arthur Amos Noyes Laboratory of Chemical Physics.
Field Ionization Mass Spectra of Dialkyl Phthalates James C. Tou Chemical Physics Research Laboratory, The Dow Chemical Company, Midland, Mich. 48640
The field ionization mass spectrum of each of nine dialkyl phthalates shows an intense molecular ion peak, an intense metastable ion corresponding to the transition M f (M - R 2) .(R - 2) or M t 4 (M - OR) + .OR in cases of diallyl and diphenyl phthalates, and two characteristic peaks at m / e = 148, 149. An example is given of the qualitative analysis of a mixture to demonstrate the great usefulness of metastable ions in field ionization mass spectrometry. Beckey’s rules in the comparison of field ionization mass spectra with electron impact mass spectra are generally applicable to the phthalates studied. -f
+ + +
+
A HIGHELECTRIC FIELD has been considered as a much milder means for ionizing gaseous molecules ( I ) and radicals (2) than (1) For review see H. D. Beckey in “Mass Spectrometry,” R. I. Reed Ed., Academic Press, London and New York, 1965 pp
93-127.
(2) H. Butzert and H. D. Beckey, 2.Phys. Chem. (Frankfurt am Main), 62, 83 (1968).
electron impact. Field ionization (FI) mass spectrometry has the advantages of giving relatively stronger molecular ion peak as well as a simplified mass spectrum in the elucidation of organic molecular structure (3). The technique complements electron impact (EI) mass spectrometry. The E1 mass spectra of dialkyl phthalates have been studied extensively by many investigators (4-6). The intensities of the molecular ion peaks in the spectra show a rapid decrease with increasing size of the alkyl group. N o detectable molecular ion has been reported for a number of phthalates with unsaturated alkyl groups and alkyl groups whose carbon number (3) G. G. Wanless and G. A. Glock, Jr., ANAL. CHeM., 39, 2
(1967). (4) F.W. McLafferty and R. S . Gohlke, ibid., 31, 2076 (1959). (5) E. M. Emery, ibid., 32, 1495 (1960). (6) C. Djerassi and C. Fenselau, J. Amer. Chern. Soc., 87, 5756 (1965).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
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