eventually a gel forms on the walls of the container. Other elements behave differently. Iron is almost unaffected by time, whereas high barium or zirconium (>1%) shows marked decreases in concentration after several weeks or even days. A good rule to follow is to prepare several standards a t the same time as the samples to be analyzed, and to run the samples within a day or so of preparation. Use of the same sample for emission spectrometry, flame photometry, and other determinative techniques lends considerable flexibility to the proposed approach t o rapid silicate analysis. An obvious extension of the method is the use of flame absorption spectrometry for minor and trace elements. The solution technique adds the possibility of preparing standards simply by adding measured amounts of metal nitrate solutions to the dissolving acid. However, unnecessarily large additions of foreign anions may give rise to matrix effects-e.g., standards prepared by adding sulfuric acid solutions
of titanium to a rock solution are not satisfactory. The proposed method gives results generally as good as or superior in precision and accuracy to rapid colorimetric methods, and the time per analysis is in general much less. Twenty to 40 samples per day can be examined for 14 or more elements. The IL flame photometer with digital readout offers almost instant NazO and KzO determination. ACKNOWLEDGMENT
Lithium metaborate (LiB02) was first suggested to us by M. L. Keith of The Pennsylvania State University, who has made an extensive study of the fluxing properties of the alkali borates. LITERATURE CITED
( 1 ) Baer, W. K., Hodge, E. S., A p p l . Spectry. 14, 141 (1960). (2) Baksay, I., Anderson, C., Pittsburgh
Conference on Analytical Chemistry and Applied Spectroscopy, 1965. ( 3 ) Ellestad, R. B., Horstman, E. L., ANAL.CHEM.27, 1229 (1955).
Table VI.
Effect of Solution Aging on Si02 Concentration Test sample. Diabase, W-1 Age,of Age of
solution, solution, days % SiOz days 3 4 84
52.3 52.7 50.5
98 363 440
% Si02 48.3 44.8 44.5
(4) Heyes, M. R., Metcalfe, J., U,K.
Atomic Energy Authority Production Group Rept. 251 (S)(1963). (5) Ingamells, C. O., Talanta 9, 507
(1962). (6) Zbid., p. 781. ( 7 ) Sen Gupta, J. G., ANAL.CHEW35, 1971 (1963). ( 8 ) ShaGro, La, Brannock, W. W., U. S. Geol. Survey Bull. 1036C (1956), 11 14A (1962). (9)Wilkinson, L. P., A p p l . Spectry. 16, 185 (1962).
RECEIVED for review January 7, 1966. Accepted March 15, 1966. Financial support for this study came from NSF Grant GP3853.
Flame Emission and Dual Flame Emission-Flame Ionization Detectors for Gas Chromatography ROBERT S. BRAMAN IIT Research Insfifufe, Chicago, I / / . b A hydrogen-air flame emission detector was constructed employing interference filters and standard gas chromatography instrumentation. Instrumentation variables were studied. Detection sensitivity was in the microgram range, wavelength dependent and generally greatest for heteroatom containing compounds. The study of emission response a t 589, 515, and 415 mp indicates that the emission intensity attributed to CZ or CH molecules in the flame plasma are dependent upon the structure of the chromatographed compounds. The design and operation of a dual flame emission-flame ionization (FE/FI) detector for gas chromatography i s also described. The influence of structure on response ratios was studied on a chlorinated methane series of compounds, an aromatic series of compounds, and a three-carbon series of compounds. The influence of structure on response ratios was demonstrated thus establishing the potential use of the dual detector in qualitative identification of peaks.
-
of gas chromatognumerous techniques for
IKCE THE ADVENT
734
ANALYTICAL CHEMISTRY
detecting the vapors in the column effluents have been described. Selectivity of response has ranged from highly unselective for thermal conductivity and flame ionization detectors t o partially selective for electron capture, beta-ray absorption, and infrared absorption detectors. Insufficient selectivity of response of current detectors and the use of increasingly smaller sample sizes have combined to make the identification of eluted compounds difficult. Emission-type detectors, because of their dependence on wavelength and high inherent sensitivity, show considerable promise for component identification. Nevertheless, despite their potentially high selectivity of detection, until recently, emission methods have been given scant attention. Microwave-stimulated plasmas (8), high-frequency discharges (9) , and flame plasmas have been reported and are being investigated. A flame emission detector reported by Grant (3) was based upon the increase in the total emission of a hydrocarbon air flame, but no wavelength discrimination was attempted. Sensitivity of the device was restricted by the high emission of the
flame background. Response was linear with sample weight, and aromatic compounds exhibited a higher emission intensity per weight than aliphatic compounds. Juvet and Durbin reported the detection of metal chelates and organic compounds (4, 6) in a hydrogen-air flame. The potential advantages of a hydrogen-air flame emission detector were first realized by this author during the development of a portable flame emission instrument for monitoring pentaborane in air (1). Comparatively high sensitivity and wavelength dependency were observed when comparing the response of various volatile compounds. Detection sensitivity for heteroatomcontaining compounds was in the part per million range. These observations led to the conclusion that a flame emission detector would be suitable for gas chromatographic instrumentation and, in addition, would have a high degree of selectivity of response if selected band or line emission wavelengths were found for each different functional group or heteroatom. This eventually led to the construction of a hydrogen-air flame emission detector and the study of its operation.
n
EMISSION PROCESSES
Emission detectors are based upon t'he energy emitted from products of the various processes that occur when materials are introduced into plasma. The potedial selectivity of these detectors lies in the wavelength dependence of emission processes; the wavelength of emission depends on the eniitt'ing atom, molecule, ion, or group of atoms. When compounds are suitably injected into a hydrogen-air flame, the resulting processes produce emission that can be categorized into two general types: thermal emission and chemiluminescence. Organic compounds in general exhibit narrow-band emission, which is attributed mainly to CH and C2 (Swan bands). Compounds containing heteroatoms give characteristic narrow bands ( 2 ) , but they also can exhibit broader bands of thermal emission, or cheiniluminescence. For example, phosphorous-containing compounds appear to exhibit chemiluminescence. In experiments to further characterize the emission of different compounds, the emission spectra of pentane, triethylamine, benzene, carbon tetrachloride, carbon disulfide, and several other compounds were obtained experimentally. il hydrogen-air burner and a recording flame photometer were employed. The emission spectra obtained for pentane and triethylamine are shown in Figure 1. The major bands are the C4 and CH bands, observed for bot,h compounds, and an unresolved group of a KH2 band, observed for triethylamine. Some emission attributed to CN bands is observed near 388 mp, but the intensity is well below that of the CH and C2 bands. The observation of higher sensitivity at the CH and C4 wavelengths over that a t the 388-mp CN region is not in accord with the findings of McCormick, Tong, and Cooke ( 8 ) , who used a microwave-stimulated plasma. Apparently, a hydrogen-air flame suppresses CN molecule formation or the subsequent excitation process. Suppression of CX emission is advantageous in work with metal chelates of elements having line emission in the 380-mp region. As shown in Figure 1, both pentane and triethylamine exhibit a general emission above the hydrogen background, more pronounced in the case of triethylamine. This may be attributed to a broad continuum for C 0 2 or CO produced in the flame by combustion in the case of pentane, or it may be due to a series of unresolved KH2 bands extending through the spectral region recorded in the case of triethylamine. One furtJhervariable that can be considered in flame emission detection is the difference in the type or portion of the flame observed. Different chemical
700
Figure 1 .
553
509 468 WAVELENGTH,
CH 431.5
415
389
rnp
Hydrogen-air flame emission spectrum of pentane and triethylamine
processes occur in the fuel-rich and the oxygen- or air-rich portions of a flame. In the present study no attempt was made to observe any selected portion of the flame. DESIGN OF THE FLAME EMISSION DETECTOR
Initial experiments were carried out on the detector device (suitably modified) described by Braman and Gordon ( I ) . A separate detector based upon this design was constructed and used throughout this study. Interference filters were selected for wavelength discriminat ion. As can be seen from Figure 1, interference filters are suitable for work with organic compounds because the bandpass of the filters is equal to or narrower than the emission bandwidths of C2, CH, NH2 and other bands observed in hydrogen-air flames containing organic compounds. Inorganic chelates, which can also be separated by gas chromatography, would be better suited for a detector incorporating a medium-resolution monochromator. The emission lines of the metal elements are much narrower than the bandwidth of interference filters, and some advantage in detection selectivity and sensitivity could be gained by employing a prism or grating monochromator. Since the development of a simple interference filter detector was contemplated, it was necessary to select filters having specific wavelengths of greatest interest. The emission spectra of the compounds examined showed that several wavelengths would be suitable as flame-emitting detection wavelengths. The interference filters selected were: 589 mp, near the maximum emission for nitrogen-containing compounds: 515 mp, near the 516.5-mp C1 bandhead: 415 mp, near the 431.5-mp CH bandhead: and 380 mp, a general ultraviolet emission detection point near CN bands but separate from the OH bands near 306 mp. The 415-mp interference filter was selected on the ultraviolet side of the 431 .5-mp CH bandhead to avoid the 538-mp CBband on the red side. Since
the structure of compounds introduced into the hydrogen-air flame influences the population of C2 and CH molecules formed, the emission intensity of these prominent bands would be of greatest use in identifying the structure of chromatographed fractions. No interference was observed from flashes of sodium emission at 589 mp. The air flow through the burner was apparently sufficient to block this interference from atmospheric dust. The apparatus arrangement used for the flame emission detector is shown in Figure 2 . The flame ionization detector instrumentation also shown in Figure 2 were added to the same apparatus after completion of the initial study. A Dumont 6291 tube, S-11 response, was used. d DC power supply designed originally for use with gas chromatography detectors was used for the PM tube high voltage. The recorder output of the electrometer was attached through a zero suppression and voltage divider network to a variable-range strip-chart recorder. The chromatographic columns used were: (1) a 6 foot long and l/s-inch diameter copper tube packed with 60to 80-mesh 5% silicon gum rubber on Anakrom-S and ( 2 ) a 135-cm. X l/s-inch diameter copper column packed with 0.75 gram of 20% UCON polar substrate on Chromasorb-W. Nitrogen was used as the carrier gas, and an oil bath heater was used. The chromatographic column effluent was passed directly into the hydrogen stream instead of into the air stream. The chromatograph was operated a t a fixed flow rate to maintain a constant hydrogen-flame size. The hydrogen flow rate was regulated by pressure across a packed column. The air supply was maintained a t 2 cubic feet/hour and had little effect on the stability of the hydrogen flame. Thus a flame emission detector of comparatively simple design and general application can easily be constructed from conventional gas chromatographic components. The detector does not require high wavelength resolution beVOL. 38, NO. 6, MAY 1966
e
735
r
- - - --
+
cause interference filters can be used instead of an expensive monochrometer. It should be noted, however, that if many different wavelengths are of interest, or if high wavelengths resolution is required, a monochrometer will prove to be more convenient. OPERATIONAL CHARACTERISTICS OF THE FLAME EMISSION DETECTOR
The recorded output is primarily a function of the concentration of the emitting species in the hydrogen-air flame plasma. The recorded emission response, however, is a function of the wavelength, the bandpass of the interference filter used, the flame size and background, the Pbl tube wavelength response, the PM tube voltage, the electrometer sensitivity setting, and the recorder range. Because of the difficulty of relating all these variables to concentration, the output is conveniently reported in terms of PM tube current and time (amp.-sec.). Major limitations of the detection system are the bandpass of the interference filters and the spectral response of the PM tube in the bandpass region of interest. Glass filters were combined with interference filters having more than one transmission range in order to eliminate the undesired wavelength region. Bandwidths a t half peak transmission of most interference filters used ranged from 10-15 mp. As indicated previously, the S-11 response Dumont 6291 PRI tube was used. The background radiation of the hydrogen flame, the PM operating voltage, and the interference filter employed are important in defining optimum operating conditions. The flame background and the dark current were determined as a function of interference filter wavelength and PM voltage (Figure 3). The dark current for the PM tube was obtained at room temperature, 22' C. 736
ANALYTICAL CHEMISTRY
high light intensity levels can saturate or cut off the PM tube. Operating a t higher PM tube voltages also is a contributing cause. This is usually not a problem when PM tubes are operating with optical devices having narrow slits. The FE detector described here was operated without slits and was observed to cut off at higher flame background currents. The input impedance of the electrometer (usually lo6 to 1Olo ohms, depending on the range setting) is a contributing factor. For example, at an input impedance of 107 ohms (see Figure 3), the PM tube was observed to cut off at currents above 3 x 10-6 ampere. Optimum operating conditions for the detector were sought in order to obtain the most favorable signal-to-noise ratio. Because previous data were obtained on flame background only, it was decided to obtain data on actual samples. The area responses of 1-p1. cyclohexane samples were determined from chromatographed samples, using the nonpolar gas chromatography column. Area responses (amp-sec.) were determined as a function of hydrogen flow rate and PM tube voltage. As expected, the responses were approximately an exponential function of PM voltage. Greater responses were observed for lower hydrogen flow rates than for higher flow rates. The greater re-
FI RECORDER
ELECTROMETER
A general close grouping of the flame background currents was observed with the 320- to 579-mp interference filters, indicating that the flame emission response of the detector would be linear with wavelength for this series of filters. The flame background intensity increased with increased hydrogen flow rate. The flame background current increased exponentially as a function of the PM tube voltage. Excessive phototube currents caused by exposure of the photocathode to lo-'
lo-'
P
a
lo-' Dark Current
I0-
I
I
I
I
I
I
200
400
600
800
1000
1200
v.
DC
Figure 3. Dark current and flame background as function of wavelength and PM voltage
sponses in the case of low hydrogen flow rates were tentatively attributed either to a longer residence time of the molecules in the flame or to a decreased dilution of the eluted peaks. Background noise was determined as a function of pressure and P M tube voltage. The noise here represents both electronic noise and fluctuations due t o the instability of the hydrogenair flame. The background noise increased with PM tube voltage and with the size of the hydrogen flame. The signal-to-noise ratio for the 1-111. cyclohexane chromatographed samples is shown in Figure 4. These data are the ratio of the area responses for 1-pl. samples at several hydrogen flow rates employed and the background noise figures for the same flame. The ratio improves with decreased flame size and with decreased PM tube voltage. The greatest influence on sensitivity is probably flame flicker (IO) which increases with flame size since the entire flame is observed in this detector. illthough the signal-to-noise ratios do not reach a maximum with the conditions studied, the ratio probably decreases below 400 volts as the dark current is approached. The operating point selected for further experimentation in the present study was 120 ml./minute hydrogen and 600 volts. The higher hydrogen flow rate was selected to avoid extinguishing the flame by sudden changes in carrier gas flow rate on sample injection. INFLUENCE OF FUNCTIONAL GROUP RESPONSE
ON
The potential advantage of the flame emission detector is the identification of functional groups and the detection of differences in the structure of similar compounds. In order to study this application, different types of organic compounds were chromatographed. The flame emission detector was used with the UCON polar substrate column heated to 102' C, a 40-ml./minute flow of nitrogen carrier gas, and a 120-ml./ minute flow of hydrogen. Four different interference filters, corresponding to the band emission wavelengths or the wavelength regions of interest, were used. The sample size used depended upon the response of the compound under study. Liquid samples were 0.5 to 2 pl., and gas samples were 50 to 1000 p1. In each case, two to five samples of different size were injected. The response for each chromatographed sample was calculated on the basis of amp.-sec./pmoles. Peak areas were determined by means of a planimeter. Table I shows molar responses as a function of wavelength. The reproducibility of these data averaged *8%. The noise level is shown so that the limit of detection can be estimated. Peak widths ranged from 10 to 30 seconds for
10000 I 8000
69 ml/mi
6000 4000
2000
42
1000
800 600
400
\
2oo 100
t I
L-4
I
I
1
I
400
600
800
1000
PM Voltage Figure 4.
Signal-to-noise ratio as function of PM voltage and hydrogen Row rate
the compounds indicated, depending upon the amount of sample injected. The limit of detection for most compounds not containing heteroatoms was 1 to 2 pg. The limit of detection for the others was in the fractional microgram range. The limit of detection (S/N=2) of the flame emission method for acetonitrile, nitromethane, nitrobenzene, diethylsulfide, and isobutyronitrite was estimated from earlier work to be 0.02 to 0.2 pg. in the 540-mp wavelength region. The detector was especially sensitive to carbon disulfide. The greatest emission response per mole was observed when heteroatoms were present in the chromatographed compounds. It was also apparent that ratios of responses obtained a t different wavelengths should be useful in qualitative identification.
Table 1.
Compound
In addition to identification of heteroatom-containing components, it was considered desirable to determine the linearity of response with sample size, the typical response observed with homologous series of compounds, and the influence of structure on the response of similar compounds a t various wavelengths. These three effects were studied on two series of compounds, the CI to Ch aliphatic alcohols and an aromatic hydrocarbon series. The aromatic compound series consisted of benzene, cyclohexane, chlorobenzene, toluene, and xylene. Samples from 0.5 to 3 pl. in size delivered from a syringe were chromatographed on the silicone gum rubber column a t 105' C. Nitrogen carrier gas a t approximately 40 ml./minute was used; the hydrogen flow rate was 120 ml/minute. Two
Flame Emission Responses for Selected Compounds
589 mp 113 57 92
..
73 50 20
7
2280
4800 Noise level, amp. X 1Olo 1 . 2
Response (amp.-sec./pmole) X 1010 515 mp 415 mfi 563 214 316 400 208 308 71 31 31,600 572 high 2.2
562 148 184 162 136 282 99 130
...
482 38,000 4
380 mu 95 116 35 48 65 49 19 14 106 35 5200 1.1
VOL 30, NO. 6, MAY 1966
0 .
737
120
80
I
-
70
o Benzene o Xylene 0
60
100
-
A
50
Tolnene
0
o Chlorbenzene Cyclohexane
0
415 Millimicrons
.
*
5 40
80
30
-
20
10
20
10
30
Micromoles
Figure 5.
Analytical curves for selected aromatic compounds
wavelength regions were selected, the 515-mp Cz emission band and the 415mp CH emission band. Figures 5 and 6 show the analytical curves on a molar basis. Figures 7 and 8 show the analytical curves on a weight basis. The best linear response was obtained for larger sample sizes. Curvilinear response for small sample sizes may have been due to errors in sample injection. As shown by Figures 5 through 8, the compounds selected have markedly different response curves. The addition of methyl groups to the benzene markedly influences the CZ emission intensity on a molar basis, while the CH emission intensity remains approximately the same. This may be attributable to the manner in which the compound is decomposed in the flame and the resulting population of Cz and CH fragments. Figure 9 illustrates a possible relationship between response and structure.
Table 11.
Figure 6. Analytical curves for selected aromatic compounds
If it is assumed that the double bond is attacked in benzene, toluene, and xylene, then the number of Czfragments available per molecule is different for each, while the number of CH bonds remains the same. The carbon number appears to have little influence on the emission intensities, since the molar Cz band intensity decreases with increasing carbon number. Chlorobenzene does not duplicate the results obtained with toluene. The
Flame Emission Responses
ResDonse (amD.-sec./umolel X 10'' Wavelength, mp Ratio
A
B
CHaCHzCHzOH CHaCHCHa
3 3
3 3
589 0.037 0.029
515 0.22 0.24
415 0.14 0.19
515/415 1.6 1.3
OH CHa-C-CHa
3
2
0.047
0.11
0.20
0.6
CHa-CH-CHa
3
3
0.25
0.37
0.27
1.4
Br CHa-CH-CHs
4
3
0.25
0.60
0.26
2.3
I
/I
0
I
I
CN A Carbon number B Number of CH fragments per molecule
738
ANALYTICAL CHEMISTRY
chloro group apparently decreases the decomposition of the molecule into CZ fragments. Note that the reduction of CH groups from 6 to 5 for chlorobenzene decreases the emission intensity proportionately. The aliphatic alcohols were run under experimental conditions identical to those used for the aromatic compounds. Response curves were obtained a t three wavelengths: 589 mp, 515 mp (Cz), and 415 mp (CH). The typical molar responses a t 415 mp are given in Figure 10. Responses a t 589 mp and 515 mp were essentially the same in character as the responses shown in Figure 10. Response curves on a weight basis are not shown. Weight responses a t 589 mp and 515 mp for all the alcohols are nearly the same-Le., similar to those in Figure 7 for the aromatic compounds. The weight responses for C z C 4 alcohols a t 415 mp are similarly grouped, but that for methanol was definitely lower. In Table I1 the molar response of some 3-carbon compounds is compared. The Cz emission (515 mp) is generally proportional to the carbon number: the CH (415 mp) emission is generally proportional to the CH bands available in the molecules. Acetone is an interesting case. The CH emission is similar to that of is0 propanol but the CZemission per mole is markedly reduced from that of the other compounds.
90
1
415 Millimicrons
Mg
Figure 7. pounds
Analytical curves for selected aromatic com-
5
mg
The reduced Cz emission may be attributed to the decreased availability of the carbonyl carbon atom. The comparison of n-propanol to iso-propanol indicates the probable influence of small structure changes on emission. The increase in CH emission for isopropanol may indicate a more general fragmentation of this molecule into C-H groups than is observed for npropanol. The addition of heteroatoms to the three carbon series gives a general increase in background luminosity as seen from the emission response at 589 mp and also carried over into the other wavelengths. Response ratios of the 515-and 415-mp emission were calculated. The difference in ratios shown, although not very large, suggest the possibility of qualitative identification based upon emission ratios. The ratio of emission a t 589 mp to that a t 515 or 415 mp appears to be better suited to indication of heteroatoms in molecules.
Figure 8. Analytical curves for selected aromatic compounds
Since this was an initial study, considerable additional work in improving and expanding the usefulness of the technique is suggested. The limit of detection of the present device could probably be improved through a decrease in hydrogen flame background by removing low concentrations of impurities in the hydrogen or by redesigning the burner. There is probably a practical lower limit to the improvement imposed by leakage of liquid substrate from chromatographic columns. THE DUAL FLAME EMISSION-FLAME IONIZATION DETECTOR
During work with the flame emission detector it was considered likely that certain advantages could be realized if Structure
-C
flame emission and flame ionization detectors were combined into a single dual detector. First, heteroatom-containing components of a separated mixture could readily be identified as such by observing the enhanced FE emission or by determining the FE/FI response ratio and comparing it with standard response ratios. Second, components of mixtures that are detected poorly or not a t all by FI and that have good emission sensitivity (such as CO, Con, NzOa, SOZ, NzF4, HF, and CS2) could be sensitively detected by the F E mode, thus extending the applicability of the dual detector. Third, by comparing the FE/FI ratio of the component a t different wavelengths or the F E / F I ratio Micromolar Resuonse. an2
Fragments
C-H Grouus
DISCUSSION
The flame emission detector is well suited for identifying heteroatom-containing compounds. In practical applications the peaks can be identified by obtaining ratios of response a t different selected wavelengths and comparing them to those of known compounds at the same wavelengths. The flame emission method also has potential use for the study of the structure of organic compounds, even those containing more than C, H, and 0 atoms. This arises from the apparent dependency of CZ and C H emission intensity per mole on the structure of the organic molecule.
2
6
3122
2823
CH3
Figure
9. Effect of structure on flame emission response VOL. 38, NO. 6, MAY 1966
739
40
30
a
Y
j 20
Weight 10
Figure 1 1 . responses
-
Relationship of FE and FI
RESPONSE RATIO STUDIES 50
of a mixture component with that of a known compound at the same wavelength, the structure or functional group could possibly be qualitatively identified. The designs of the FE and FI detectors easily permit construction of a single detector in which the two techniques are combined. Thus the emission a t one wavelength over a narrow wavelength band and the total ionization process for a chromatographed component could be monitored simultaneously. Because hydrogen fuel is used, the problems of high ionization background are avoided. For example, the F E detector of Grant (S), which uses a hydrocarbon flame, cannot be successfully combined with a FI detector because of relatively large concentrations of ions in the flame gases. In order to investigate the potential advantages, a combined FE/FI detector was constructed and suitable experiments were performed to test its operation. EXPERIMENTAL DESIGN OF APPARATUS
To provide for the FI detection mode, a platinum-wire electrode was added to
the flame burner housing of the F E detector. Since this wire is not heated to incandescence and is placed outside the optical path of the FE detector, it does not interfere in FE detection. A borosilicate-glass cylindrical tube was required to channel the burner gases and reaction products. After a few minutes of operation, the borosilicate-glass tube is sufficiently heated t o prevent moisture condensation. The schematic diagram of the dual detector is shown in Figure 740
ANALYTICAL CHEMISTRY
2. The FI circuit is a conventional arrangement for this type of detector. The spacing of the platinum-wire ion collector was made by experimentation. The electrometer used in the F I readout system is a Research Specialties, Inc., instrument designed specifically for FI detection. The FE detector instrumentation and flame supply system is essentially the same as used in the FE detector study. In later work a dualpen recorder instead of two strip-chart recorders was used to facilitate comparison of data and to provide more compact recording of data. The effect of changes in hydrogen flow rate and carrier gas flow rates on the FE/FI detector responses to 1-111. samples of ethanol a t 415 mp was studied. The response of the FI detector was more markedly influenced by flow rate changes than the response of the FE detector. The carrier-gas flow rate appeared to be more critical than the hydrogen flow rate. Since both responses were influenced by flow changes, one set of conditions was selected and used throughout all experimental work. A 135cm. X 0.3l-cm.-diameter column packed with 0.75 gram of 20% UCON polar substrate on Chromasorb and heated in an oil bath to 105' C. was used. The hydrogen flow rate was 170 ml./minute, and the nitrogen carrier-gas flow rate was approximately 100 ml/ minute. Liquid samples were 0.5 t o 5 pl. Three interference filters were selected for the emission wavelengths of interest: 589 mg, near the NH2 emission bands, 515 mp, a Cp emission band; and 420 mp, a CH emission band.
Figure 11 illustrates the relationship of FE response (at three different wavelengths) to linear FI response, which is independent of wavelength. Response constants are empirical and are related to the number of ions present or the number of emitting species in the flame by variables of the detection system. In both systems emission intensity or ionization current is presented ultimately in a recorded form. Emission readout is basically a function of the number of emitting species being detected at any particular time or over a set period of time when a chromatographed sample component is presented to the flame. Factors that influence the emission readout include bandpass and transmission of the interference filter, quantum efficiency and gain of the photomultiplier tube used, flame operating conditions, and burner housing and design. Because these factors vary somewhat for each apparatus, accurate intercomparison of data and intercomparison of response ratios with those of standards should be made only when obtained on the same instrument. FI response is more directly a function of ions present, but flame composition, flame temperature, electrode spacing, and impressed voltage also influence the readout. For most organic compounds, sensitivity of the FI method is 10 to 100 times greater than that of the FE method. Consequently, the gain of the FI detection system must be set lower by this factor to obtain chromatographic peaks that are comparable in size. If the experimental variables are controlled, the FE/E I ratio becomes a function of the manner in which the compound decomposes to emitting and ionized species in the flame. Thus the response ratio is indicative of the
structure and the type of heteroatom or functional group present. Work with the flame emission detector indicated that the C2 and CH emission band intensities are influenced by structure rather than the carbon number of compounds. At the same time, FI response is proportional to carbon number (6). For many compounds the FE/FI ratio can thus be considered the emission intensity per carbon atom. Nevertheless, because of the present empirical nature of the F E and FI responses in practical applications, the response ratios a t any selected wavelength should be compared with those of standard materials. The detection readout device can be set to give a F E / F I ratio of 1.0 for the standard, and the ratios of unknowns can be compared for purposes of qualitative identification. In order to investigate and demonstrate the relation of structure to response, experiments were carried out using compounds with similar structures. Because linearity of response with weight of both detectors was established in the flame emission study, this study was mainly concerned with the effect of functional groups, heteroatoms, and structure on the response ratio. The sensitivity settings of the electrometer circuits were set to give convenient and comparable-sized readout peaks for both F E and FI with the sample sizes of interest. The FE/FI ratio is independent of weight and can be treated as dimensionless, although it also can be considered to be in units of watts per ion. The first series consisted of methanol and the chlorinated methane compounds from chloroform t o carbon tetrachloride. Samples from 0.5 to 2 pl. in size were chromatographed, the peak areas obtained and normalized for the electrometer reference gains, and the response ratios calculated. Average data from a t least two samples of each compound a t each wavelength are presented in Table 111. The ratios vary an average of -f 10%. The FE/FI ratios a t 589 mp were lower than those a t 420 and 515 mp, because the quantum efficiency of the S-11 response tube is markedly lower a t this wavelength while the FI response remains the same. Responses at 515 and 420 mp were comparable. Note that the FE/FI ratio a t 515 mp and 589 mp increases by a factor of ten as the number of chlorine atoms in the molecule increases from zero to four. The CH band intensity also increases according to the ionization response but less markedly than the Cz response. Note that to obtain a CH molecule from CCL requires that all four chlorine atoms be removed and recombined with hydrogen. The Cz molecule requires combination of two carbon atoms after the chlorine atoms have been removed.
Table 111.
Response (FE/FI) Ratios for Chlorinated Methane Series
Filter Wavelength, mr Compound
589 0.72 0.65 0.28 0.06 0.07
cc1,
CHCL CHzClz CHsCl CHaOH
Table IV.
515 2.3 1.6 0.90 0.40 0.23
420 0.59 0.44 0.31 0.20 0.22
Table V.
589 0.084 0.11 0.082 0.025 0.027
515
420 0.48 0.48 0.37 0.11 0.085
...
0.68 1.12 0.21 0.11
FE ratio (CZ/C) ... 1.4 3.0 1.9 1.3
Response (FE/FI) Ratios for Three-Carbon Series
Filter Wavelength, mfi Compound Methanol n-Propanol Isopropanol Acetone 2-Bromopropane i-Butyronitrile Methyl nitrate Acetonitrile
4.6 4.1 2.9 2.0 1.0
Response (FE/FI) Ratios for Aromatic Series
Filter Wavelength, mfi Compound %-Pentane Cyclohexane Benzene Chlorobenzene Bromobenzene
FE ratio (CZ/CH)
589 0.070 0.037 0.029 0.047 0.25 0.25 0.58 0.56
Despite the apparently complicated nature of these processes, both must occur, since C2 emission and CH emission are observed, the former apparently to the greater extent. The ratio C2/CH is independent of the FI factor and simply compares emission a t 515 and 420 mp wavelengths. The C2/CH emission ratio increases in direct proportion to the increase in chlorine atoms in the molecule. The bond energies (7) of the various species and compounds involved may account for some of the observed relationships: CH (98.2 kcal./mole), C2 (90 kcal./mole), and CC1 (78 kcal./ mole). Despite the high energy available in the hydrogen-air flame, which is well above CH dissociation or bond energies, the increase in the number of hydrogen atoms bonded to carbon atoms suppresses the formation of G, which can occur only after complete removal of all surrounding bonded atoms. Since the CC1 bond is less stable, it is apparently more readily broken, . permitting the formation of Cz in a higher concentration than that of the less highly halogenated methane nuclei. The response ratios calculated for a second series of compounds are shown in Table IV. Samples from 0.5 to 2 pl. were chromatographed: a t least two samples of different size were used to determine the ratio. Variation in the individual ratio values shown was
515 0.23 0.22 0.24 0.11 0.37 0.60 1.05
...
420 0.22 0.14 0.19 0.20 0.27 0.26 0.33 0.38
FE ratio (Cz/CH) 1.0 1.6 1.3 0.6 1.4 2.3 3.2
...
= t l O % . The effect of substituted chlorine or bromine on the decrease of the Cz FE/FI ratio from that of benzene alone is quite marked. The decrease in Cz emission for methyl-substituted benzene was also observed in the aromatic series when a FE detector was used alone. Bromobenzene and chlorobenzene also exhibit a ratio a t 420 mp that was less than that observed for benzene, n-pentane, or cyclohexane. This is probably in part due to the fewer CH bonds originally present in bromo- or chlorobenzene. The third series of compounds consisted of the three-carbon series: n-propanol, isopropanol, acetone, 2bromopropane, and isobutyronitrile. Methanol was included for comparison. Data for methyl nitrate and acetonitrile are added to show the increase in the response ratios a t 515 and 590 mp for heteroatom-containing compounds. Data precision was f10% to *15% for most ratios. Ratios obtained a t the three selected wavelengths are shown in Table V. As previously, the ratio a t 589 mp is smaller than a t other wavelengths because of the lower photomultiplier (FE) sensitivity. Ratios for acetone and isopropanol are markedly different a t the 515-mp CZemission region, despite the similarity of these compounds. Isopropanol apparently decomposes in the flame to give a higher population of C2molecules per ion than acetone. This can be VOL 38, NO. 6, MAY 1966
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attributed to the higher energy of the carbonyl bond (173 kcal./mole) compared with that of carbon-oxygen bond (79 kcal./mole). The carbonyl carbon atom in acetone may not be so readily available for forming Cz molecules, thus reducing the effective carbon number of the molecule and reducing Cz emission. Because of the poor molar emission response of methanol observed previously, its FE/FI ratio value appears high in comparison with that of n-propanol. Nevertheless, this may be due to the smaller ionization response rather than to a higher emission response. The ionization response of methanol has been reported t o be lower than that of the other alcohols (6). The introduction of a bromo or cyano group into the three-carbon chain markedly increases the emission response a t the 589-mp region: consequently the ratio is larger than for the other compounds. This observation is similar to the increase in FE/FI ratio observed when halogens were added to the methane nucleus, but it is opposite to the effect observed when halogens are added to the benzene nucleus. The combustion processes are therefore distinctly different for each type of compound, emphasizing the need for comparison of ratios with those of standards. The increased CJCH ratio for isobutyronitrile is probably partially due to the extra carbon. The cyanogen nitrogen does not give the very great increase observed for amine-type compounds.
This is attributed to the high energy of the CN bond (209 kcal./mole), which thus reduces the effective carbon number of this compound to three and does not make the nitrogen atom available for NHz nor NH band emission. DISCUSSION
The FE/FI dual detector that is reported here for use in gas chromatography has also been found in unpublished work to be useful in following the course of vapor-phase reactions. In applications of this type the capability of sensitivity detecting FI-insensitive molecules not containing CH bandssuch as CO, CO,, SO-becomes of considerable advantage. The present sensitivity of the dual detector is limited by the sensitivity of the F E mode. The detector sensitivity can probably be increased by improving the design of the burner assembly and possibly by selecting other fuel-oxidizer combinations for the flame plasma. For high sensitivity, however, hydrogenoxygen and hydrogen-air flames are likely best. Other fuels will likely produce either a high F I background or a high F E background. The use of an integrating sphere with burner will increase the efficiency of light collection and may permit development of a direct relationship between emitting species concentration and readout signal. The dual FE/FI detector is somewhat similar to a dual electron capture-FI detector, since both electron capture
and FE detectors are more sensitive to heteroatoms. Nevertheless, because of the variety of wavelengths available and the emission a t different wavelengths by different molecules, the FE/FI detector is more selective. The FE detection mode could also be combined with other detection modes, for example, thermal conductivity. The thermal conductivity mode is a better comparison mode than FI because it is less dependent upon the elemental makeup and structure of molecules than the FI mode. LITERATURE CITE0
( I ) Braman, R. S., Gordon, E. S., ZEEE Trans. Instrumentation and Measurement, IM-14, 11-19 (1965). (2) Gaydon, A. G., “The Spectroscopy of Flames,” Riley, New York, 1957. (3) Grant, D. W., “Gas Chromatography 1958,” D. H. Desty, ed., p. 158, Aca-
demic Press, New York, 1958. (4) Juvet, R. S., Durbin, R. P., Gas Chromatog. 1 (12), 14, 1963. (5) Juvet, R. S., Durbin, R. P., ANAL. CHEK 38, 565 (1966). (6) Perkins, G., Jr., Rouayheb, G. M., Lively, L. D., Hamilton, W. C., “Gas Chromatography,” N. Brenner, J. E. Callen, hl. D. Weiss, eds., pp. 269-285, Academic Press, New York, 1962. (7) Pitzer, K. S., J. A m . Chem. SOC.70, 2140 (1948). (8) McCormack, A. J., Tong, S. C., Cooke, W. D., ANAL.CHEM.37, 1470 (1965). (9) Sternberg, J. C., Poulson, R. E., J. Chromatog. 3,406 (1960). (10) Winefordner, J. D., Vickers, T. J., ANAL.CHEM.36, 1939 (1964). for review December 20, 1965. RECEIVED Accepted March 7, 1966.
An Assay Method for Vinyl Grignard Reagents Using Gas Chromatography ANATOLE WOWK and SALVATORE DiGIOVANNI’ M&T Chemicals Inc., Rahway, N. .I. A method for the determination of vinyl Grignard reagents has been developed which assays the reactive vinyl magnesium moiety in the product. It distinguishes the vinyl magnesium from other titratable compounds resulting from the hydrolysis, oxidation, or decomposition of the vinyl Grignard. The method involves the reaction of the vinyl Grignard reagent with an excess of tributyltin chloride followed by analysis of the reaction product mixture by gas-liquid chromatography, The content of tributylvinyltin in the mixture indicates the concentration of the reactive vinyl group in the Grignard. The method has been used for the analysis of Grignard reagent made from vinyl chloride and magnesium in tetrahydrofuran.
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ANALYTICAL CHEMISTRY
S
the preparation of vinyl Grignard in high yields from vinyl chloride and magnesium in tetrahydrofuran (THF) was described in our laboratory in 1957, (5, 6) no specific method for assaying this reagent has been described in the literature. A general method of assaying Grignard compounds by an acid-base titration developed by H. Gilman ( I ) has been used for this purpose. This method, however, indicates the total content of C-Mg and 0-Mg bonds present in the material under test and not the content of the reactive CH2C=CHMg moiety which is desired. Thus, we have found in many instances that vinyl Grignard reagents which have deteriorated for one reason or another give, by titration, much higher assay figures than their INCE
true vinyl magnesium content as evidenced by reduced yields in coupling reactions. To utilize the vinyl Grignard agent as a synthetic tool, it became necessary to develop an accurate assay method. In our search for a more appropriate method of assay, the gas-volumetric method was investigated. The gases obtained on hydrolysis of a Grignard reagent with dilute acid were subjected to chromatographic and mass spectrometric analyses. In the case of a vinyl Grignard reagent stored for a period of time these analyses showed, besides the theoretically expected ethylene, the presence of considerable amounts of 1 Present address, Hewlett-Packard Co., Englemood, N. J.
