Rapid-Scanning Mass Spectrometry Continuous Analysis of Fractions from Capillary Gas Chromatography J. A. DORSEY, R. H. HUNT, and M. J. O'NEAL Houston Research laborafory, Shell Oil Co., Deer Park, rex.
b The wetted-wall capillary column has made possible at least an orderof-magnitude increase in the resolving power of gas chromatography. Because the sample capacity of such columns is very small, special techniques are required to detect the emerging components. The coupling of a rapidscanning mass spectrometer to a capillary gas chromatographhas made possible detection and identification of the eluting components in an integrated system which maintains the high resolving power of the capillary columns. The method is sensitive, giving usable mass spectral data on components 0.1% by weight or less, and is versatile, so that capillary or packed columns can be interchanged without modification of the mass spectrometer inlet system. The technique can also be used with short capillary columns for high-speed analyses.
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advent of capillary gas liquid chromatography, with its attendant increase in resolving power, has extended the analybis of a wide variety of materials both qualitatively and quantitatively. At present, however, the separating ability of such columns has exceeded the means available for identification of the eluted fractions. Comparative retention time methods utilizing known compounds have reached the stage where they are limited by calibration compound availability. The application of the methylene insertion reaction ( l a ) served to advance the retention time principle by a t least one c a r h i number, but even this is not enough to keep pace with improvements in capillary gas chromatographic separations. Although trapping techniqum have found nidespread use with packed Chromatographic columns, they are not successful with capillary columns because of the small sample size. Hence, some other analytical method must be employed for identifying the eluting fractions. One such method, used successfully by Holmes and Morrell (10) to identify the components of city gas, is the direct HE
coupling of a packed gas chromatographic column to a mass spectrometer. This technique provides separation and identification in one integrated system and circumvents the problems of trapping and transferring small samples. Subsequent investigators (6, '7, 9, If) have extended this method to the analysis of more complex petroleum fractions. Conventional mass spectrometers have been utilized with capillary columns by monitoring a single massto-charge ratio during the capillary separation (8). By judicious selection of mass-to-charge ratios on subsequent separations, a partial mass spectrum of each fraction can be assembled and the peaks identified. This is obviously a time-consuming analysis and requires extremely stable operation of both the capillary gas chromatographic column and the mass spectrometer. A more suitable technique is the use of high-speed mass spectrometry to record the entire mass spectrum of each component as it emerges from the capillary column. Recently BrunnBe, Jenckel, and Kronenberger ( 2 ) have discussed the physical basis, with several experimental designs, for the use of a high-speed mass spectrometer with capillary columns. The present report is to show the usefulness of such techniques for the analysis of complex mixtures. EXPERIMENTAL APPARATUS
Capillary Gas Liquid Chromatograph. The sample introduction sys-
tem is of a design described by Durrett, Simmons, and Dvoretzky (4, which permits small samples t o be introduced into capillary columns without fractionation. I t is typically operated a t 300" C., from 30 to 50 p.s.i.g., and a t split ratios of 50:l to 150: 1. The various capillary columns utilized are attached with a Swagelok fitting in order to permit interchangeability. Helium is used as a sweep gas in all work with the mass spectrometer, since it is easily pumped by the vacuum system and does not interfere with the mass spectra in the mass ranges normally scanned. A 300-foot, 0.01-inch i.d. capillary
using squalane as the liquid phase was employed for the majority of the work reported, although any suitable substrate may be used and the capillary column may be replaced with packed columns if desired. In analyses requiring temperature programming, the columns were encased in cylindrical heating mantles and temperatures regulated with a conventional controller. Inlet System. T o make use of existing retention time data and permit correlations between the mass spectrometer system and conventional detectors, an inlet system was desired which would operate with the gas chromatographic column exit a t atmospheric pressure. At the same time, backmixing of the eluents had to be avoided if the capillary gas chromatographic separation was to be maintained.
These requirements were met by inserting an uncoated section of 0.004inch i.d. stainless steel capillary between the gas chromatographic column exit and the mass spectrometer in a manner similar to that described by BrunnBe, Jenckel, and Kronenberger ( 2 ) . The capillary tubing has an outside diameter of 0.008 inch and can be inserted directly into the chromatographic column, 0.01-inch i.d. The other end of the inlet capillary passes through a rubber serum cap on a 12/30 standard taper and into the mass spectrometer ion source. The leak orifice normally employed in mass spectrometer inlets has been eliminated, to provide unobstructed sample flow through the instrument. When working with the higher-boiling fractions the inlet capillary is heated by passing a small electric current through it. This also serves, along with adjustment of the capillary length, to control the flow of sample from the capillary exit into the mass spectrometer. A section of 0.004-inch i.d. capillary 100 cm. long a t room temperature passes about 0.45 ml. per minute of the capillary effluent (total capillary flow about 1 ml. per minute) to the mass spectrometer, Mass Spectrometer. The mass spectrometer employed was a cyVOL. 35, NO. 4, APRIL 1963
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Figure 1. Schematic representation of capillary GLC rapid-scanning mass spectrometer system
cloidal focusing instrument manufactured by Consolidated Electrodynamics Corp. (6). The instrument has resolution of 1 in 200 and a usable range of approximately 250 a.m.u. Scanning rates from 0.1 to 1000 seconds per scan are available and cover essentially the full range of the instrument. The only exception is exclusion of the masses below mle 20. These can be obtained by limiting the upper masses to about 150. For most applications a 1second scan time from mass 20 to 200 is employed for capillary GLC studies. This scan rate is generally adequate, since the duration time of capillary GLC peaks normally is in the 5 to 15second range. Scan rates of 0.1 second over a limited mass range have been found useful in some cases. Detection of the ion beam is through an electron multiplier which maintains high sensitivity a t the scanning rates employed. The combined GLC-LIS is schematically illustrated in Figure 1. Output Systems. The output of the mass spectrometer is fed to three parallel devices; an oscilloscope (Techtronix Type 531.4) is used to provide a continuous visual display of the mass spectra, a strip chart recorder provides a trace similar t o
conventional chromatograms, and a high-speed oscillograph (chart speed 2 feet per second) records the mass spectral patterns. The high-speed oscillograph has been found more convenient and accurate than photographic recording of an oscilloscope trace, used previously (2). Of the three recording devices, only the use of the strip chart recorder is somewhat unconventional. The recorder has a response time of 0.5 second and hence does not respond directly to the mass spectrometer output, which (n-ith 1second scanning time) has an average frequency of 150 peaks per second. This difference in rate tends to produce an integral trace of total ion intensity and hence provides a signal of high sensitivity. In practice, deflections of the recorder are approximately equal to those obtained Kith conventional flame ionization detectors. Figure 2 compares the M S total ion intensity trace with that of a conventional hydrogen-flame detector operating on the same sample and capillary column (sample of CS paraffin cut, separated on a 250-foot X 0.01-inch i.d. column coated with squalane).
-4s each component of the sample traverses the system, its occurrence in the mass spectrometer source is indicated on the oscilloscope and by the strip chart recorder. A t maximum deflection of the recorder (peak top) the high-speed oscillograph is engaged and two or more repetitive spectra are recorded. These spectra are then classified by their major mass-charge ratios and compared with reference spectra ( I ) for identification. The mass spectral patterns of the cycloidal focusing instrument are similar to those obtained with the larger analytical instruments, as indicated in Figure 3. Capillary Columns. The identification of the C9 isomers in the saturate fraction of a commercial reformate is taken as an illustrative example of the methods application with conventional capillary columns. The mass spectra of all 35 of the isomers are included in the API 44 tables for reference and studies of these isomers through methylene insertion provide a means for substantiating the results (12).
=1 250-foot X 0.01-inch i.d. column, coated with squalane, was used to effect the separation. Column temperature was maintained a t 33" C. for the initial 27 minutes of the run and was then increaPed t o 100' C. for the remainder of the separation. The resulting chromatogram is shown in Figure 4. Thirteen of the 16 peaks shown were identified directly by comparison with the API 44 spectra. Of the other three, two did not correspond to any of the standards and one peak was similar to two of the -4PI 44 spectra. Further information was necessary to
4 Figure 2. Chromatographs from GLC-RSMS system and GLC-flame detector system showing relative resolution of each
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Figure 3. Comparison of spectra from GLC-RSMS system with API 44 spectra
parison of the normalized values for the m/e 57, m/e 70, and m/e 85 revealed the following changes as the fraction eluted : The m/e 57/43 ratio decreased about 10%. The m/e 70/43 ratio increased twofold. The m/e 85/43 ratio decreased to half its original value. These changes provided a picture of a two-component fraction, beginning with a compound having a large m/e 85 and ending with a compound having a large m/e 70. Both the compounds had approximately the same m/e 57/43 ratio. Of the nine isomers possible, only the 4-ethylheptane and 3,4-dimethylheptane had spectral patterns which fit these conditions. These two compounds form an ideal mixture for illustrating the calculation of relative concentrations. The 4ethylheptane has a large m/e 85 and small m/e 70, while the 3,4-dimethylheptane sensitivities for these two masses are just the reverse. This makes the solution of simultaneous equations very sensitive to small changes in the concentration of either compound. The absolute sensitivities for these compounds were not determined and the calculation is based on an assumed sensitivity for the 3,Cdimethylheptane m/e 70. The assumption of a sensitivity for this m/e is not critical, since
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Figure 4.
GLC-RSMS separation of
elucidate the structure of the peaks, and this can be derived from the chromatogram of Figure 4 and the boiling points of the identified components. The resulting curve of boiling point us. retention time (Figure 5) indicates that of the 11 known structures establishing the curve, none deviates from the line by more than 11’ F. Since all the components are of similar structure and boil over a narrow range, it is safe to conclude that the unknown components will follow this behavior. Hence, the possible structures are limited considerably and some further identifications can be made. For peak A on Figure 4 there are seven Cs paraffins which fall within the selected boiling point range. Of these, only one, 4,4-dimethylheptane, produces an ion intensity of m/e 85 which could account for the magnitude of this fragment found in the unknown. If the contribution of the 4,4-dimethyl isomer is removed from the mass spectrum, it is possible to assign the structure of 2,Bdimethylheptane from the remaining fragments.
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paraffins in reformate
Peak B was a case of two compounds having virtually the same cracking patterns and boiling points and, hence, cannot be resolved by the technique vith the particular column used. The boiling range of peak C included nine of the CS isomers. The presence of m/e 128 indicated that a t least one of these nine was present, but the spectrum of the fraction was not similar to any of the API 44 spectra. To obtain further information, another GLC separation was made and mass spectra mere obtained continuously during the emergence of this fraction. The m/e 43 was the major peak in each of the spectra and, therefore, each of the individual spectra, 28 in all, was normalized to the base m/e 43. Com1
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