Anal. Chem. 1907, 5 9 , 443-448
(1) (2)
LITERATURE CITED Vossen, J. L.; Cuomo, J. J. In Thin Fllm Processes; Vossen, J. L., Kern, W., Eds. Academic: New York, 1978; pp 12-73. Hecq, M.; Hecq, A.; Delrue, J. p.; Robert, T. J . Less-Common Met.
1079, 84, 25. (3) Chapman, B. olow Discharge Processes; Wiley: New York, 1980. (4) Bail, D. J. J . Appl. Phys. 1072. 42, 3047. (5) Greene, J. E.; Sequeda-Orsorio,F. J . Vac. Scl. Technoi. 1073, 10, 1144. (6) Harrison, W. W.; Hess, K. R.; Marcuss, R. K.; King, F. L. Anal. Chem. W86. 58. 341A. (7) Werner, H. W.; Garten, R. P. H. Rep. Prog. PhYs. 1964, 4 7 , 221. (8) Belglan Patent 01216896, 1986. (9) Statham, P. J. In Microbeam Analysis; Nemerg, E., Ed.; Sari Francisco Press: San Francisco, CA, 1979; p 247.
----. - - .
443
(IO) Berth. E. P. Princlpies and Practice of X-ray Spectrometric Analysis;
(12)
Plenum: New York, 1975. Vossen, J. L. In Physics of Thin Films Hass, G., Francombe, M. H., Hoffman, R. W., Eds. Academic: New York, 1977. Manifacier, J. C.; Spessy, L.; Bresse, J. L.; Perotln, M.; Stuck, R. Ma-
(13)
Roche, A.; Charbonnier, M.; Gaillard, F.; Romand,
(11)
fer.Res. Bull. 1070, 74, 163.
Surf. Sci. 1081, 9 , 227. (14) Lowry, R. K. Anal. Chem. 1086, 58, 23A.
M.; Bador, R. Appl.
RECEIWDfor review June 23, 1986. Accepted September 17, 1986. The authors thank Le Ministere de 1'Energie de la RBgion Wallonne for the support of this research.
Serially Interfaced Gas Chromatography/Fourier Transform Infrared Spectrometer/Ion Trap Mass Spectrometer System Edwin S. Olson* and John W. Diehl
University of North Dakota, Energy Research Center, Box 8213, University Station, Grand Forks, North Dakota 58202
A serial gas chromatography/Fourier transform infrared spectrometer/mass spectrometer (GC/FTIR/MS) system has been developed with an ion trap detector or mass analyzer that Is interfaced to the light pipe In the FTIR spectrometer. A modlficatlon of the manufacturer-supplied open-spilt interface to the ion trap was required to obtain chromatographic r e d s free of dlscrimlnation and activation effects. The flow rate of the helium make-up gas in the light pipe was used to control the amount of material that enters the ion trap. Hydrogen carrier gas was used for the chromatographic separatkns with no adverse effects on the mass spectra obtained. The analyses of three samples are described to demonstrate the capabilities of the system. These include the Grob test mixture, a steam c#sHlate from a gadfkation water treatment plant, and a sample of methyl esters of coal oxidation products.
The powerful analytical capabilities of a gas chromatography/Fourier transform infrared spectrometer/ mass spectrometer (GC/FTIR/MS) system have been reported in previous publications (1,2). These systems have employed either splitting of the GC column effluent between the FTIR and MS in a parallel configuration (1)or serial interfacing where the column effluent passes through the light pipe and then directly into a jet separator on the MS (2). A GC/ F"lI'R/MS system that differs from those reported previously has been assembled in our laboratory. An ion trap detector (ITD) with a modified open-split interface was employed for the MS, and this was serially coupled to the light pipe exit in such a way that the make-up gas to the light pipe controls the amount of material actually entering the ITD transfer line. EXPERIMENTAL SECTION Gas Chromatography. A Hewlett-Packard Model 5890A gas chromatograph with a J&W on-column capillary injector was connected t o the Nicolet GC/FTIR interface as shown in the system configuration diagram (Figure 1). A J&W 30 m X 0.32 mm i.d. 1.0 pm film DB1701 column was used for analyses reported in this paper. The carrier gas was ultra-high-purity hydrogen further purified by a drying tube and oxygen trap in line 0003-2700/87/0359-0443$0 1.50/0
and the carrier flow was set at a linear velocity of 42 cm/s (2 mL/min) at 300 "C. The GC oven temperature programming used an initial temperature of 40 "C and then a rapid increase (30 "C/min) to 50 "C, followed immediately by a rate of 5 OC/min from 50 to 300 "C.
Fourier Transform Infrared Spectrometry. A Nicolet POSXB FTIR spectrometer with the Nicolet GC/FTIR light pipe interface was used. The gold-coated light pipe dimensions were 15 cm X 1.5 mm i.d. and the light pipe temperature was 250 "C. Helium (ultra-high-puritygrade, scrubbed with a General Electric GO-GETTER oxygen and water removal system) make-up gas for the light pipe was adjusted to a flow rate of 0-10 mL/min, depending on the analysis. The data system was the Nicolet 1280 computer equipped with a fast Fourier transform coprocessor and an 86-Mbytehard disk. The number of scans per chemigram data point was varied between 4 and 16 to produce a data point every 1-3 s, respectively. Heated, 1/16 in. glass-lined tubing (GLT) connected both the light pipe entrance and exit to the inside of the GC oven (Figure 1). The GC column end was inserted through the GLT to the light pipe entrance. A short section of the same column which was used for the chromatography was inserted into the light pipe exit tube as far as bends in the GLT would allow, and the other end of this section was connected to the open-split interface inside the GC oven. Mass Spectrometry. The Finnigan Model 700 ion trap detector was used to obtain mass spectra of the GC eluents. This instrument has a mass range of 10-650 amu with unit resolution. A range of 50450 amu was used for the experiments in this paper. The interfacing of the mass spectrometer utilized a modification of the original Finnigan-supplied all metal open-split interface. The open-splitinterface was connected to the MS via a 4 ft section of heated metal tubing through which a flow restrictor was installed (Figure 2). This restrictor was a section of SGE 0.1 mm i.d., 0.21 mm o.d., 0.1 pm film BP-10 (OV 1701) column, with the restrictor extending 5 cm beyond the open split interface into the GC oven. The section of column coming from the light pipe exit was then inserted over this narrow piece of fused silica tubing and far enough into the open split interface to allow the column nut and ferrule to be tightened. A helium sweep of 2 mL/min in the open-split interface and a transfer line temperature of 250 "C were maintained. The data system for the ITD was an IBM PC/AT with a 30-Mbyte hard disk and standard GC/MS software for tuning, total ion chromatogram generation, selective ion monitoring, and library search routines. 0 1987 American Chemical Society
444
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987 ITD Transfer line
Figure 1. Schematic diagram of interfaced instruments. Column n and
exit lube
transfer line 10 MS
Flgure 2. Modified open-split interface to the ion trap transfer line.
DISCUSSION The ability of an instrument system to perform both vapor-phase infrared spectrometry and mass spectrometry during high-resolution gas chromatographic analysis on a fused silica open tubular capillary column allows the complementary data obtained for each peak to be analyzed for the identification of the components of complex mixtures. We have demonstrated that this system can be constructed with a relatively inexpensive mass spectrometer and that the interfacing can be advantageously accomplished with an opensplit interface in a serial arrangement of the instrumentation. Since use of hydrogen carrier gas is preferred for the chromatographic separations (3),it was necessary to demonstrate that both the infrared spectrometry and the ion trap mass spectrometry do not suffer adversely from the use of the hydrogen carrier in mixtures with helium as a make-up gas. Vapor-phase FTIR using a light pipe requires a relatively large amount of sample as compared with the more sensitive mass spectrometer. In the parallel configuration of an FTIR and FTMS described by Laude ( I ) , a splitter was employed at the end of the GC column which provided a 200 to 1 split ratio of the GC effluent to the FTIR and FTMS, respectively. Restriction of the flow to the MS was obtained by using a crimped stainless steel tube which allowed a pressure of 1.5 x lo-’ torr in the transfer line. The use of a tee at this point restricts the flexibility of the system and does not allow for easy adjustment of the flow rates to the instruments. The serial configuration of the instruments described here and in the earlier paper of Wilkins’ group (2) allows independent adjustment of the flow and make-up gas. The serial configuration reported by Wilkins (2) employed a jet separator interface to the MS, whereas we have used an open split interface modified from the Finnigan-supplied equipment. In our system, the infrared spectrometer was the Nicolet 20SXB equipped with a gold-plated light pipe. This interface normally requires the addition of helium make-up gas to the capillary effluent, in order to overcome the peak broadening caused when peak volumes are less than the light pipe volume (4). Although the hydrogen carrier is infrared inactive, there was some concern that leakage of the hydrogen carrier in the interface would present an explosion hazard. The reason for this concern is that in this interface there are two relays which control the heaters for the light pipe and transfer line whose intermittent opening and closing might ignite a hydrogen-air mixture, if present. However, no leakage of hydrogen in the interface could be detected with the J&W hydrogen-sensing detector (GC protector) which detects hydrogen at one-tenth
the explosion potential level and shuts off ac power to the light pipe assembly. Variation of the helium make-up flow rate to the light pipe did not significantly affect the sensitivity, peak shape and intensity, or quality of the spectra. Extension of the GC capillary column through the glass-lined tubing to the entrance of the light pipe avoided detrimental effects on the components eluting from the column which result from contact with active surfaces such as would be present in transfer lines or a postcolumn splitter which are not deactivated and phase-coated. Thus no loss of chromatographic resolution or peak intensity of polar components eluted from high-resolution capillary columns could be observed when using this direct coupling of the GC column to the light pipe. The interfacing method to the mass spectrometer which is described below is not unique to the ion trap mass spectrometer, and modification of the Finnigan-supplied equipment for improved operation may be generally applicable. However, some problems have been considered which are unique to the ion trap operation. The Finnigan Model 700 ion trap detector uses the ion trap principle to obtain mass spectra of GC effluents (5). Not only is the cost very low but the unit is small and little maintenance is required, which make it advantageousfor remote applications and low-budget programs. Extreme care must be exercised, however, in the analysis of some types of polar materials where the high source pressure and the ion confinement in the trap may result in extensive ion/molecule reactions (“self-CI”)(6). In the factory installation, the ion trap MS was connected to the GC via a 4 ft section of heated metal tubing through which a 1.2 m X 0.15 mm i.d., 0.25 pm f i i DB-5 flow restrictor was installed. This was designed to terminate in the all-metal open split interface inside the GC oven and the end of the GC column was inserted into the open split such that it would be a short distance (1-2 mm) from the restrictor entrance. The gap between the two was swept with helium gas. This design was troublesome since much discrimination occurred with higher boiling compounds and poor peak shape was exhibited due to active sites. To correct this problem, the Finnigan restrictor line was replaced with a section of nmow fused silica tubing, coated with the same phase material as was used in the GC separation. This restrictor section was extended 5 cm beyond the open split interface into the GC oven. The section of column from the light pipe exit was inserted over this narrow restrictor tube and far enough into the open split interface to allow the column nut and ferrule to be tightened. Helium gas was introduced to sweep the open split interface. This modification eliminated discrimination and active site problems and has been reported previously as the best design for open split interfacing (7). The amounts of the components that enter the flow restrictor, and consequently the ion trap, are related to the amount of helium used in the make-up flow in the light pipe; the greater the flow, the less material reaches the trap. This relationship is not linear. For example, peak intensities calculated for the trap when operated with a make-up flow of 6 mL/min were one-fifth of those with no make-up flow. Since adjustment of the make-up flow provides the ability to control the amount of material entering the ion trap, larger samples can be injected to obtain good spectra in the lesssensitive FTIR, without overloading the ion trap. Operation at high make-up flows does not seem to reduce light pipe sensitivity; spectra from 100 ng of most compounds can be obtained with a make-up flow of 6 mL/min, and maximum sensitivity can be obtained, of course, by simply turning off the valve. Operation of a GC/FTIR system with similar make-up flows has been previously reported ( 4 ) . Setting the make-up flow above 10 mL/min causes longer retention times, apparently due to pressurization of the light pipe.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
445
Table I. Spectral Data from GC/FTIR/MS Analysis of Grob Test Mixture
peak
component
1 2
2,3-butanediol decane
3658,3630, 1092,1059 2961, 2934, 2878, 2867
3 4
undecane 1-octanol
2961, 2934, 2867
5
nonanal 2,6-dimethylphenol 2-ethylhexanoic acid 2,6-dimethylaniline methyl decanoate dicyclohexylamine methyl undecanoate methyl dodecanoate
6
7 8 9 10 11 12 a
FTIR,cm-'
MS ( r n / z ) O 55, 57, 73, 75 b b
3667,1050 2710, 1742 3652,1199, 760 3575,1770
b b b b 106, 121 74, 87, 101, 115, 129, 143, 187 56,100,138, 181,182 74, 87, 101, 115, 129, 143, 157, 201 74, 87, 101, 115, 129, 143, 215
3497,1627, 1477, 1264,750
1759, 1169 2935, 2861, 1464, 1130 1759, 1169 1759, 1169
Major peaks above m / z 50. *MatchedEPA/NIH library spectrum. 128
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198
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Figure 3. Mass spectrum of naphthalene from a GC/FTIR/MS run using hydrogen carrier and no helium make-up gas in the light pipe. Other condklons are described in the Experimental Section.
Helium carrier is recommended for operation of the ion trap detector since it provides a damping effect important for stabilizing the ring of ions in the trap (5) without forming adduct (M 4)+ ions. It is important to know what happens to the mass spectra produced in a mixed hydrogen-helium system resulting from the use of hydrogen carrier in the chromatographic separation followed by mixing with helium in the light pipe interface and open split interface to the ion trap. Although hydrogen molecules or atoms might provide similar damping effects, they are more likely to result in chemical ionization with formation of (M + 1)+ions. Neither this protonation nor the self-CI is necessarily detrimental to analysis of the mass spectrum and in fact may be useful for determining molecular weights or locating molecular ions in the spectra of certain classes of compounds. The extent to which hydrogen carrier results in excessive chemical ionization which might lead to problems in the analysis of hydrocarbons was evaluated by examining the mass spectra obtained for hydrocarbons using different proportions of helium make-up gas in the light pipe interface. When no helium make-up gas was used,the hydrogen carrier was diluted only by the helium flow in the open split interface. Under these worst case conditions, only a small excess of the (M + 1)+ions was observed in the mass spectra The mass spectrum of naphthalene obtained at the GC peak maximum is shown in Figure 3. The ratio of (M + 1)+to M+ is 0.1284, slightly higher than the ratio expected on the basis of the isotopic abundances (0.1094). The mean for the ratio across the entire peak is 0.1254. Plots for the (M l)+, M+, and total ions through the naphthalene GC peak (Figure4) show a consistent and symmetrical distribution. As the make-up flow of helium was increased, thus diluting the hydrogen further, the (M + 1)+to M+ peak ratio decreased only slightly. It should be pointed out that at high concentrations of GC eluents, self-CI will result in high (M + 1)+ to M+ ratios using either carrier gas, and traces of water have the same effect. These factors are much more important in determining the extent of ion/molecule reactions than is the use of the hydrogen as the GC carrier gas.
I4 21 I4 U 15 91
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Flgure 4. Selected ion chromatogram profiles of the naphthalene peak for the M+ = 128, (M 4- 1)' = 129, and total ions. Conditions are the same as those in Figure 3. 1'
10
12 i
+
+
I , 2 34
6 14
-
- ~- __..I .__-__ T ' ~ .-. 9 95 13 7 5 1 7 5 5 2 1 3 4 2 5 1 4 RETENTION TIME (MIN)
28 94
32 74
Figure 5. Gram-Schmidt reconstructed(FTIR) chromatogram (lower) and reconstructed total ion chromatogram (upper) of Grob test mixture. See Table I for peak identification.
Analysis of Mixtures. The use of a standardized test mixture allows the comparison and evaluation of GC performance, whether it be that of a column phase, instrumental component, or system. The Grob test mixture contains a variety of polar and nonpolar components which challenge any methodology (8) and thus was used to evaluate this GC/FTIR/MS combination. The FTIR Gram-Schmidt reconstructed chromatogram and the ion trap total ion chromatogram can be superimposedas shown in Figure 5. Except for the 2-ethylhexanoic acid, which does not give good peak shape on any kind of polysiloxane phase column, all the components exhibited acceptable peak shape and intensity and, thus,have not been adsorbed or decomposed on an active
446
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
Table 11. Spectral Data from GC/FTIR/MS Analysis of Steam Stripping Overhead
component
peak
FTIR, cm-'
MS (m/z)" b b b 66, 93, 107, 121 b b b b 123, 138 (C,) 107, 122 (C3) 121, 136 b
pyridine picoline pyrrole
1 2 3 4 5 6 7 8 9 10
m- and p-cresol 4-methylguaiacol alkylphenols
11
BHT
alkylpyridines phenol guaiacol o-cresol
3074, 1585, 1448, 1439 3074, 1595, 1475, 1444, 755 3516, 3196, 1537, 1415, 994 3650, 3054, 1602, 1498, 1257, 1182 3588,1501, 1262, 1221, 1034,750 3645, 3044, 1604, 1497, 1319, 1258, 1212, 1158, 1108, 852 3650, 3035, 2936, 1607, 1513, 1261, 1177, 936 (m), 834 (p), 802 (p), 769 (m) 3591, 3052, 2935, 1514, 1271, 795 3671, 2967, 1441, 1231, 1163, 771
"Major peaks above m / t 50. *Matched EPA/NIH library spectrum.
57
+
I
81
LE
88
fragmentation of alkanes, a severe limitation in the analysis of alkane samples. The mass spectra of the 1-octanoland the nonanal exhibit clusters of fragment ions typical of long aliphatic compounds and the molecular ions are not seen in either the library or ion trap spectra; however, the patterns of ions clearly match for these two compounds in the library and ion trap spectra. The ion trap spectra of the phenol, aniline, alcohols, and nonanal did not show (M + 1)+ions due to CI effects; however the dicyclohexylamineand esters did. The initial spectra obtained during the elution of the dicyclohexylamine showed a small molecular ion at 181 and no (M 1)+ion at 182. As the concentration of dicyclohexylamine reached a maximum, self-CI effects resulted in the formation of the 182 ion; However, the rest of the spectrum did not change significantly. The (M + 1)' ions were also found in the spectra of the three methyl esters a t 187, 201, and 215, respectively. The (M + 1)+ions have been shown to be useful in the identification of unknown esters, since E1 spectra of esters usually do not have either M+ or (M 1)+ ions to directly determine the molecular weights. The GC/FTIR/MS system has been useful in the analysis of a number of mixtures of polar compounds which have resulted from coal processing and oxidation. Two of these analyses will be discussed in this paper. Samples were obtained of the overhead or distillate obtained during steam stripping of UNDERC gasifier condensate water which had previously been extracted with diisopropyl ether in a watertreatment process (9). This aqueous ammoniacal sample was extracted with diethyl ether, dried over magnesium sulfate, and concentrated with a Kuderna-Danish apparatus. The residue was dissolved in methylene chloride and injected by using the on-column injector. The FTIR and MS reconstructed chromatograms of the extracted organic material are shown in Figure 7. The isopropyl ether and isopropyl alcohol peaks, which elute just after the solvent peak, were omitted from these chromatograms. Although infrared spectra were obtained for these peaks, mass spectra were not. All of the significant peaks in the chromatograms were identified (Table 11) and the FTIR and ion trap spectra matched the corresponding library spectra. The early part of the chromatogram contains a large peak for pyridine (peak l),followed by smaller peaks for 2-picoline (peak 2), pyrrole (peak 3), and several methyl and Ct pyridines (peaks 4). Because of the wide range of concentration of the components, it was difficult to obtain adequate signal to noise ratios for the minor componentsin this sample; thus the FTIR spectra of the alkyl-substituted pyridine components (peaks 4) were too noisy for adequate identification. The mass spectra of these alkylpyridineswere too similar to distinguish between isomers. The sample contains a large quantity of phenol (peak 5) followed by guaiacol (peak 6), o-cresol (peak 7), m-and p-cresol
199
ita
Figure 6. FTIR and mass spectrum of P-ethylhexanoic acld from GClFTIRlMS run of Grob test mixture.
surface. The 2-ethylhexanoicacid was eluted from the DB1701 phase column and was observed in both the FTIR and MS chromatograms (Figure 6). The data from a single GC/FTIR/MS analysis of the Grob test mixture are presented in Table I to show that there is no deterioration of the information which one expects from the infrared or mass spectra obtained in this system using hydrogen carrier and that the system provides usable information for a variety of compound types. Infrared spectra of eight of the components of the Grob test mixture are in the Aldrich vapor-phase library and superimposable spectra for these components were obtained in the chromatographic analysis. The other four test mixture components, nonanal and the methyl esters, matched library spectra of similar compounds. The 2-ethylhexanoic acid peak was broad and overlapped the 2,6-dimethylaniline peak; hence the infrared spectrum of the 2,6-dimethylaniline was obtained by subtraction of the acid spectrum. Identification of the position isomers of the substituted phenol and aniline was possible using the C-H bending absorption. Of course the spectra of the decane and undecane cannot be distinguished from other n-alkanes. The only problem in functional group characterization using the FTIR spectra was that the N-H stretching absorption of dicyclohexylamine, which is very weak and broad in the reference spectrum, was not observed at all in the spectrum from the chromatographic analysis. The mass spectra of the Grob test components obtained in the same run as the FTIR data discussed above closely matched the EPA/NIH library spectra and along with the infrared data allow the unequivocal identification of all the components except the decane and undecane, whose spectra are similar to any n-alkane. The high energy of the ionizing electron beam (>lo0 eV) in the ion trap results in extensive
+
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
447
6
11
n.. .
9.
-r
1 36
5 84 1 0 32 1 4 81 1 9 2 9 RETEYTIOU TIME (MIN)
Flgure 7. Gram-Schmidt reconstructed (FTIR) chromatogram (lower) and reconstructed total ion chromatogram (upper) of steamdistillate sample.
4
4l
4 66
1 1
60
' 8 5 4 2 5 47 32 4 ' 39 3 5 RETFNTION TIME (MIN)
46 2 8
53 22
Flgure 9. Gram-Schmidt reconstructed (FTIR) chromatogram (lower) and reconstructed total ion chromatogram (upper) of GC/FTIR/MS run of methyl esters of lignite oxidation products.
I
187
,!I,
95
48
60
BE
1
188
128
'
1
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1
I
148
Flgure 8. FTIR and mass spectrum of peak 9 (4-methyiguaiacol) in GC/FTIR/MS run of steam distillate sample.
(peak 8), and small amounts of Cz and C3 substituted phenols and 2,6-di-tert-butyl-4-methylphenol (BHT) (peak 11). The latter phenol was introduced into the wastewater as the antioxidant present in the diisopropyl ether used in the extraction step prior to the steam stripping. The mass spectrum of peak 9 (ions at m l e 123 and 138) (Figure 8) indicated that it was a methylguaiacol, and the FTIR spectrum of peak 9 (Figure 8) resembled that of guaiacol (peak 6). On the basis of our previous experience with gasifier tars ( l o ) ,only two C,-guaiacol isomers are formed, 4-methylguaiacol and 6methylguaiacol. Although not in the Aldrich vapor phase FTIR library, the infrared spectrum from the Chromatogram identified this peak as the 4-methylguaiacol. Further confirmation was obtained by retention-time matching. Mass spectra of some very small peaks provided evidence for the presence of 4-ethyl- and the 4-propylguaiacol. To further demonstrate the capabilities of the this GC/ FTIR/MS system, a sample of the methyl esters of the products from the oxidation of Beulah, ND, lignite with ru-
thenium tetraoxide was analyzed. The reconstructed FTIR and total ion chromatograms for this sample are shown in Figure 9. The major peaks were originally identified by using GC/MS and GC retention-time matching and the results have been reported in a preliminary paper (12). The peaks eluting early in the chromatogram are straight-chain and branched aliphatic dicarboxylate esters, followed by aliphatic polycarboxylatesand then aromatic polycarboxylates. Esters with molecular weights up to 400 were analyzed in this system without discrimination or loss. Some differences in response factors are evident in comparing the two reconstructed chromatograms. The phthalate and benzenetricarboxylate peaks in the ion trap chromatograms are very small relative to their size in the FTIR reconstructed chromatogram (and also the FID chromatogram). This response decrease had no effect on the quantitative analysis of the esters since an isotope dilution method was used in the quantitation reported earlier (11). In this paper we focus on the FTIR and mass spectra of the peaks in the center portion of the chromatogram (Figure 9) in order to characterize the unknown components, at least to the point of choosing likely structures based on molecular weight and aliphatic or aromatic character. Peak 2 in Figure 9 was identified earlier as the methyl ester of 1,2,3-propanetricarboxylate, peak 3 as dimethyl phthalate, peak 4 as 1,2,4-butanetricarboxylate, and peak 7 as 1,2,4-benzenetricarboxylate and 1,2,3-benzenetricarboxylate.The vapor-phase FTIR spectra of the aromatic esters are distinctively different from those of the aliphatic esters (Table 111). The vapor-phase carbonyl stretching frequencies of the aliphatic esters were found to be close to 1759 cm-l and those of the aromatic esters close to 1750 cm-*. A C-0 stretching band was found at 1164 cm-I for the aliphatic esters and at 1247 cm-' for the aromatic esters. On this basis, peaks 1,5, and 6 appeared to be aliphatic
448
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3,FEBRUARY 1, 1987
Table 111. GC/FTIR/MS Spectral Data from Analysis of Esters of Coal Oxidation Products peak
MS ( m / z Y
FTIR, cm-'
1 2 3 4
55,59,69,115,127, 145,159,187 55,59, 101,113, 115,126,127, 145,159, 173,187,219 59, 69,81,101,104, 108, 113,115,127,137,163,167 55, 59, 71, 101,113, 114,115,127,141,159,168,169,201, 233 55, 59, 100, 113, 114, 127,141,155,215 55,59, 101,113, 114, 139,153,167,199,259
2960,1758, 1440,1225,1165 2961,1759, 1440,1167 2960, 1753, 1441,1247,1075 2901, 1760, 1440,1165
propanetricarboxylate
2963, 1760, 1440,1168 2960, 1759, 1439,1166
pentanetricarboxylate butanetetracarboxylate
5 6
component methyl ester 1,2,3-propanetricarboxylate
phthalate and phthalate-d, (internal std) 1,2,4-butanetricarboxylate
"Major peaks above mlz = 50. a molecular ion of mass 290, which corresponds to a butanetetracarboxylate isomer. Mass spectra of the components of this mixture were also obtained on a quadrupole GC/MS instrument (HP 5985B) and are very similar to the ion trap spectra. Further characterization of this sample is in progress. Mass spectra of the minor components have been obtained; however the FTIR spectra of the corresponding peaks have low signal to noise ratios and thus the analysis of larger amounts of samples is required. The preliminary GC/FTIR/MS data described above have been useful in choosing likely structures for the unknown ester components, which are now being synthesized for spectral comparisons.
113
59
1, 1
I1
48
SPEC)
68
88
188
141
111
128
,
I
148
168
189
298
228
Flgure 10. FTIR and mass spectrum of peak 5 of GCIFTIRIMS run of methyl esters.
esters, with absorptions at 1757 and 1165,1760 and 1165, and 1760 and 1165, respectively (Figure 10). Spectral matching did not provide identification with the library spectra. The fragment ions observed in the ion trap spectrum of peak 1 ( m / e 55,59,74,69,115,127,145,159,187)are similar to those found in the methyl esters of aliphatic tricarboxylic acids such as 1,2,3-propanetricarboxylicacid and 1,2,4-butanetricarboxylic acid. The 187 ion is believed to be the (M - 31) fragment ion (loss of methoxy), resulting from expected fragmentation in these methyl ester mass spectra. The molecular ion was not normally observed in this type of ester. The unknown peak 1precedes the 1,2,3-propanetricarboxylate peak, and the unknown ester is thus suspected to be a propanetricarboxylate isomer. The ion trap spectrum of peak 5 (Figure 10) exhibits ions at m / e 55,59,100,113,114,127,141,155, and 215. This series of ions is similar to that observed in the butanetricarboxylate spectrum, but with a presumed (M - 31) fragment ion at 215. This gives a molecular weight of 246, and consequently the unknown peak 5 is believed to be a pentanetricarboxylate isomer. The fragment ions of peak 6 ( m / e 55,59,101,113,114,139, 153, 167, 199, 259) are again similar to those of the tricarboxylates. The 259 ion represents loss of methoxy (31)from
CONCLUSIONS The relatively inexpensive GC/FTIR/MS system assembled and interfaced in a serial configuration has provided useful infrared and mass spectra. Modification of the open split interface to the mass spectrometer resulted in excellent performance with regard to peak shape and intensity (no discrimination), in contrast with the performance of the manufacturer supplied interface. Hydrogen carrier was used in the gas chromatographic separation without significant effect on the ion trap mass spectra. The split ratio which determines the amount of material entering the ion trap was easily adjusted by varying the make-up flow rate. LITERATURE CITED Laude, D. A.; Brissey, G.M.; Ijames, C. F.; Brown, R. S.; Wilkins, C. L. Anal. Chem. 1984, 56, 1163-1168. Wilkins, C. L.; Glss, G. N.; White, R. L.; Brissey, G.M.; Onyirluka, E. C. Anal. Chem. 1982, 5 4 , 2260-2264. Jennings, W. Gas Chromatography wlth Glass Capillary Columns, 2nd ed.; Academic: New York, 1980. Hurrell, R. A. Instrum. Res 1986, (March), 28-39. Stafford,G. C.; Kelly, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J . Mass Spectrom. Ion Phys, 1984, 6 0 , 85-90. Ghaderi, S.;Kuikarni, P. S.: Ledford, E. B., Jr.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1981, 53, 428-437. Arrendale, R. F.; Severson, R. F.; Chortyk, 0. T. Anal. Chem. 1984, 56, 1533-1537. Grob, K.; Grob, G. HRC C C , J . High Resolut. Chromatogr. Chromatogr. Commun. 1983, 6 ,
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RECEIVED for review August 4, 1986. Accepted October 2, 1986. This research was supported by Contract No. DOEFC21-83FE60181 and DOE-FC21-86MC10637 from the U.S. Department of Energy Reference herein to any specific commercial product by trade name, mark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.
