1290
Anal. Chem. 1985,57, 1290-1295
knowledge of the general structure of the polymer, an average middle group length of 14.8 (average chain length of 16.8) is indicated. This result is substantiated by the quantitative proton spectrum of DC-710 in Figure 4b which indicates the average chain length is 16.6. Although static system quantitative proton spectra are readily obtained, the technique is not generally selective for the silicone middle and end groups; the separation of peaks in the proton aliphatic region was fortuitous.
LITERATURE CITED (1) Shoolery, J. N. Prog. NMR Spectrosc. 1977, 1 1 , 79-93.
(2) Forsyth, D. A.; Hedigev, M.; Mahmond, S. S.;Glessen, 8. C. Anal. Chem. 1982, 5 4 , 1896-1898. (3) Cooperus, P. A.; Clague, A. D. H.; van Dargen J. P. C. M. Org. Magn. Reson.1976, 8 , 426-431. (4) Shoolery, J. N.; Smithson, L. H. J . Am. Oil Chem. SOC. 1970, 4 7 , 153.
(5) Martin, M. L.; Delpuech, J. J.; Martin, G. J. “Practical NMR Spectroscopy”; Heyden: London, 1980; p 350. (6) Shaw D. “Fourier Transform NMR Spectroscopy”; Elsevier: Amsterdam, 1976; p 237. (7) Levy, G. C.; Edlund, U. J . Am. Chem. SOC. 1975, 9 7 , 4482-4485. (8) Cookson, D. J.; Smith, 8. E. J . Magn. Reson. 1984, 5 7 , 355-368. (9) Haw, J. F.; Glass, T. E.; Dorn, H. C. J . Magn. Reson. 1982, 4 9 , 22-3 1. (IO) Laude, D. A., Jr.; Lee, R. W.-K.; Wilkins, C. L. Anal. Chem. preceding paper in this issue.
RECEIVED for review October 16, 1984. Accepted February 11,1985. Support of the National Science Foundation through Grant CHE-82-08073 and a department research instrument grant, CHE-82-03497, is gratefully acknowledged. Partial support was also providing by the donors of the Petroleum Research Fund, administered by the American Chemical Society, for which we are also grateful.
Gas Chromatography/Multiphoton Ionization Fourier Transform Mass Spectrometry Thomas M. Sack, David A. McCrery, and Michael L. Gross* Department of Chemistry, University of Nebraska-Lincoln,
A capillary gas chromatograph/Fourler transform mass spectrometer (GCIFTMS) has been upgraded so that a pulsed laser operatlng at 266 nm can be used for multiphoton ionization (MPI). M P I is demonstrated to be a selective ionizatlon method for a variety of polycyclic aromatic hydrocarbons (PAHs) In both a test mlxture and a sample of gasoline, and the selectlvity Is conslstent with previous research. However, ions can now be detected at mass resolving powers of greater than 20000 (fwhm) up to m / z 200 by using the FT mass spectrometer rather than the often-used time-of-flight mass spectrometer. Detection llmits for several of the PAHs are reported to be in the low picogram range (Le., 7 pg for naphthalene) at both high (>20 000) and low (C5000) resolving powers wlth a linear dynamlc range of 2.5 orders of magnitude. Mass measurement accuracy is also evaluated, and the typical errors are found to be approxlmately 1-10 ppm (5 ppm) at moderate resolvlng powers wlth low ion densities.
Gas chromatography/mass spectrometry (GC/MS) combines the powerful separation abilities of GC and the exceptional quantitative and qualitative capabilities of mass spectrometry. Because of these features, GC/MS has been applied successfully to a myriad of problems involving the analysis of complex mixtures. However, there is a limit to the ability of GC/MS to separate completely all of the components in a mixture, because of the limited “peak capacity” of any given system ( 1 ) . This problem is primarily caused by the finite scan rate of the mass spectrometer along with the requirement that a large number of components be separated in a finite amount of time. Thus, there is a significant probability that there will be coeluting components in most separations of complex mixtures. One way to overcome this limitation is to develop methods to increase the specificity of the mass spectrometer. High-
Lincoln, Nebraska 68588
resolution mass spectrometry (HRMS) can greatly increase the specificity of a GC/MS system by enabling the separation of isobaric ion signals, thereby permitting accurate mass measurements (2) and reducing chemical noise in multiple ion detection schemes (3). Tandem mass spectrometry (MS/MS) can also increase the specificity of GC/MS by offering another dimension of mass analysis to the detection of the GC effluents (3, 4 ) . Another way to enhance the specificity of GC/MS is to use selective ionization techniques to form the ions for mass analysis. Thus, two coeluting components can be more readily separated by the mass spectrometer if one is preferentially ionized. Chemical ionization (CI) (5) is one technique for this purpose, because, with a judicious choice of reagent gas, one can specify the compound types which are to be ionized based on their reactivity (6). Multiphoton ionization (MPI) (7) is an alternative selective ionization method which is emerging as a potentially powerful companion to mass spectrometry. In MPI, a molecule irradiated by intense UV or visible light (- 190-600 nm) must absorb two or more photons to ionize, so that one need not employ a cumbersome vacuum-UV source to generate high-energy photons. Both physical (8-13) and analytical (14-1 7 ) studies of MPI-MS have been presented in the recent literature. The attractiveness of MPI as an ion source for a mass spectrometer stems from three basic features. Because MPI depends on absorption of photons by a molecule, there is a wavelength dependence of the ionization efficiency, yielding a moderate degree of ionization selectivity. MPI can also be very efficient when the photon energy coincides with a real electronic state in the molecule (e.g., resonance-enhanced MPI or REMPI), because there can be orders of magnitude enhancement in ion yield (14), which can lead to nearly 100% ionization efficiency (15). Finally, the fragmentation induced by MPI can be controlled. MPI can produce an abundance of fragmentation similar to or even exceeding that produced by E1 or very little fragmentation reminiscent of classical “soft” ionization techniques (12). This feature can be very important
0 1985 American Chemical Society 0003-2700/85/0357-1290$01.50/0
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
in trace analysis where it is desirable to obtain “sparse” masa spectra which have most of the ionization concentrated in one characteristic ion species. Reilly and co-workers (18)have demonstrated recently that these features of MPI-MS, when combined with a gas chromatograph, provide the basis for a powerful analytical technique. For their experiments, a time-of-flight (TOF) mass spectrometer using laser MPI was employed as a detector for a capillary gas chromatograph. Because of the efficiency of MPI and the excellent transmission characterktics of the TOF, they were able to obtain subpicogram detection limits for several polycyclic aromatic hydrocarbons (PAHs). Furthermore, the selectivity of MPI was demonstrated in the analysis of a mixture of chrysene and triphenylene, in which only the chrysene was ionized at 308 nm. However, the limitation of the GC/MPI-TOF arrangement is that the TOF has a relatively low mass resolving power (20&4000) (19). which can severely restrict the mass spectral specificity. In order to increase the mass resolution and hence the specificity of GC/MPI-MS, one needs to turn to another type of mass spectrometer that also enjoys the multichannel advantage required by pulsed laser MPI. The Fourier transform (FT)mass spectrometer can combine both the features of high resolving power and multichannel advantage (20,21). The MPI-FTMS combination was demonstrated first by McIver e t al. (22) and later by Carlin and Freiser (23). Like TOF, FTMS can he used to obtain an entire mass spectrum with each laser pulse. However, FTMS offers m r a l other features not available with TOF. For example, FTMS can be performed in the heterodyne mode (20,21)to acquire very high resolution spectra over narrow maaS ranges with no loss in sensitivity (24).because the ions are formed and detected in the same region. In fact, Carlin and Freiser (23) reported measuring the molecular ion of azulene formed by MPI with a resolution of over 2OooO. Furthermore, the FTMS can be used to perform accurate mass measurements (25-28) and GC/MS experiments (2!+33), which are not routinely done with a typical TOF mass spectrometer. Thus, it appears that the W masa spectrometer may be a better match for the detection of MPI-formed ions than the often-used TOF mass spectrometer. In this report, the performance characteristics of a capillary GC/FT mass spectrometer using MPI at a fixed wavelength will be described. Application of this instrumentation to the analysis of model and real mixtures will also be demonstrated. We will focus on those new features that the FTMS lends to the experiment, such as high resolving powers and accurate mass measurements, and also discuss the sensitivity and selectivity available with this configuration. Finally, the problems and the future applicability of GC/MPI-FTMS will be addressed based on the results of these experiments. EXPERIMENTAL S E C T I O N GC/MS. The capillary GC/FTmass spectrometerused in this work has been dwrihed previously (32). The gas chromatograph is fitted with a variable split injector which was maintained a t 275 OC with a split ratio of 251. A 30 m X 0.25 mm i.d. SE-54 column (Supelm, Bellefonte, PA) was used throughout this study. The gas chromatograph was coupled to the FT mass spectrometer by using the recently developed pulse valve interface (32). Since the initial experiments involving the pulse valve interface, several improvements have been made. Firstly, it was found that the pulse valve could operate with no loss of performance at temperatures well above the specified maximum of 100-150 “C. For this work, the temperatures of the pulse valve and the transfer line were maintained a t 250 OC. Secondly, the flow restrictor, which had originally been inserted to drop the pressure between the column and the pulse valve, was removed because the severe chromatographic band broadening observed in the initial experiments was occurring at this junction. However, because the valve vent port is continuously evacuated by a me-
*
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OUENCH VALVE LASER EXCITE DETECT
Flgure 1.
t-----w
Sequence of events In the GCIMPI-FTMS experiment.
chanicnl pump, the pressure drop oocurred along the column which decreases its chromatographic resolution. A simple calculation (34)reveals that approximately 10 m of a 0.25 mm i.d. column is required to effect a pressure drop from 760 to 1 mtorr. Therefore, the 30-m column that we employed has an effective length of ca. 20 m, although the 10-m length acting as the flow restrictor probably contributes somewhat to the plate number. The FTmass spectrometer was operated with a magnetic field of 1.2 T and was equipped with either a 2.54 or a 5.08 cm cubic cell (35),which was modified for laser work in an electromagnet-based system (36). The trap voltage was typically 1-2 V, except for the very high resolution experiments, where it was set to 0.5 V. Electron ionization (EI) spectra were acquired at an ionizing energy of 15 eV with an emission current of 3.5 MA and a beam duration of 5 ms. Typical low-resolution mass spectra were acquired in the direct mode from m/z W 5 0 0 by using 16K data sets. High-resolution spectra were acquired in the heterodyne mode over narrow mass ranges, usually 10 amu or less with as many data pints as were n e e w r y to acquire the entire transient Laser. The source of ultraviolet laser radiation used in these experiments was a Quanta-Ray (Mountain View, CA) DCR-2 NdYAG laser equipped with a Model H G 2 harmonic generator. The 266-nm fourth harmonic was used in all experiments described herein. The laser produces a pulse of 5 4 ns duration at this wavelength. The beam diameter was reduced to 5 mm by an iris after which it entered first the vacuum chamber ria a fused silica window and then the cell through a 6.3-mm prt In all case, the beam was unfocused, because more volume could be irradiated than with the focused beam. This resulted in better sensitivity and less fragmentation. Pulse energies at 266 nm were measured by using a disk calorimeter (Scientech, Boulder, CO). Typical pulse energies were 4 mJ/pulse for low sensitivity and up to 15 mJ/pulse for highest sensitivity work. Pulse Sequence. The event sequence for GC/MPI-FTMS is shown in F i e 1. The valve was triggered by the quench pulse and remained open for 10 ms. The laser was triggered by the FTMS beam pulse 100 m8 after the quench. Detection occurred from 100 to 250 ms after the laser was fired, depending on the mass resolution required. For low-resolution spectra (R < lOooO), the delay was 100 ms, and 3 scans were averaged every second. The highest resolution was obtained with the longer delay times, where the time per scan was approximately 2 s, due to the long observation time of the ion transient signals. Reagents. The PAHs (Aldrich, Milwaukee, WI) were used as received without further purification. Solutions of the PAHs were prepared in reagent grade chloroform in concentrations of 10,1, and 0.01 mg/mL. The gasoline was a commercial sample of regular, leaded gasoline obtained locally. The toluene (Mallinkrodt, Paris. KY) and 4-bromostyrene (Aldrich)employed as mans standards were used as received and subjected to two freeze pump-thaw cycles prior to admitting them to the FT mass spectrometer.
RESULTS AND DISCUSSION General Performance. The test mixture chosen for the initial evaluation of the GC/MPI-FTMS system was a series of PAHs because they are known to ionize at 266 nm (15,18)
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TIME (rnin)
Reconstructed chromatograms of the PAH mixture under (a)E1 and (b) MPI conditions. Approximately 40 ng of each component was detected. The column temperature was programmed from 150 to 225 O C at 10 'C/min. Inset: chromatogram resulting from the injection of the anthracene stock solution, showing the impurity at 8.6 min. The minor differences in the retention tlmes in (a)and (b) are due to small variations in the chromatographic conditions employed. Flgure 2.
, , , , , , , , , , , , , , , , , , / , ~ ,, , ,, , ,~, , , , , ,
100
140
140 160 MASS I N A . M . U .
180
200
Flgure 3. Representative mass spectra of acenaphthene from the chromatograms shown in Figure 2. Each spectrum is the average of three 16K data sets obtained in direct mode.
and, along with their analogues, are of current analytical interest (37-39) due to their toxic and carcinogenic properties. The chromatographic performance of the GC/MPI-FTMS using the pulse valve interface is illustrated in Figure 2 for the PAH test mixture. Peak widths are on the order of 6 s as compared to 20 s prior to the improvements in the interface described above. With data acquisition at the rate of 1file/s (3 scans/file), several spectra can be accumulated for each peak. The analogous chromatogram obtained by using electron ionization is also shown in Figure 2 for comparison. I t is evident that the chromatographic performance is not affected by the method of ionization. The mass spectra for acenaphthene from the reconstructed chromatograms in Figure 2 are shown in Figure 3. One notes the similarity of the 15-eV E1 and MPI spectra using the unfocused 266 nm light, which, in fact, was found to be a fairly general phenomenon in this work. The operation of the FT mass spectrometer using E1 or MPI was found to be very nearly the same except that the excitation amplitude required for MPI operation was typically lower than for E1 by nearly 2 d B for most experiments. Furthermore, the optimum conditions were not as clearly defined for the MPI experiments (i.e., the tuning was flatter). These differences can be attributed to the different spatial ion distributions in the cell for E1 and MPI. In EI, all the ions are created in a thin volume along the z axis (parallel to the
magnetic field) of the cell and are approximately all the same distance from the receiver and transmitter plates. Therefore, the ions all require approximately the same energy to reach optimum cyclotron radii. In MPI, however, the geometry of our vacuum system and cell dictates that the ions be formed in a cylindrical volume perpendicular to the z axis and on a diagonal through the cell. Therefore, ions are distributed with all possible distances from the transmitter plates so that it takes less energy to excite some fraction of ions into optimum detectable orbits. Accordingly, if the excitation energy is increased, those ions closest to the cell plates will be ejected, while those closer to the center will be more effectively excited. This distribution of ions along the x-y cell line causes the tuning as a function of excitation amplitude to be flat. Selectivity. Because most aromatic molecules show distinctive MPI spectra, due to their unique electronic structures, one would expect that the test compounds should exhibit different ionization efficiencies by MPI than by EI. This is clearly shown in Figure 2 where the relative chromatographic peak heights in the MPI trace are markedly different than those in the E1 trace. No ions were observed for the solvent in the MPI mode simply because chloroform does not ionize a t 266 nm. Likewise, the carrier gas, He, and any common contaminants such as O2or H 2 0 do not ionize at 266 nm. This ensures that any ions formed by MPI are due only to the primary ionization of the sample molecules and not to chemical ionization or charge-exchange reactions involving ions such as H30+or He+, respectively. The inability of MPI at 266 nm to ionize the carrier gas has an added benefit in GC/FTMS because the FTMS analyzer cell has a limited capacity for ions. If an ionization method produces He+ (e.g., 70-eV EI), from the excess neutral helium, the cell will be populated by a large number of unwanted ions. The excessive amount of He ions can reduce the cell capacity for analyte ions and degrade the performance because of coulombic effects. Therefore, most GC/FTMS experiments using E1 are carried out in our laboratory at an ionizing energy lower than the ionization potential of He (24.6 eV). A significant difference in the reconstructed chromatograms shown in Figure 2 is that the anthracene peak at 8.80 min in the MPI trace is relatively small and accompanied by another peak at 8.63 min (see insert, Figure 2). The unidentified peak was found to be an impurity in the anthracene, and its MPI-MS spectrum is nearly identical with that of anthracene. The only compound that would be expected to fit these data is an isomer, phenanthrene. This was confirmed by injecting authentic phenanthrene which exhibited the same retention time as the impurity. Since the phenanthrene was not observed in the E1 trace, its appearance in the MPI trace along with the reduction of the anthracene peak is attributed to the greater 266-nm MPI efficiency of phenanthrene compared to anthracene. In order to quantify the phenanthrene, the standard addition method was chosen. The complex areas (40) of the m / z 178 ion intensities for phenanthrene were plotted as a function of the concentration of added phenanthrene, yielding a line with a correlation coefficient of 0.997. From these results, the commercial anthracene sample was found to be contaminated with approximately 2.4% w/w phenanthrene. This result was corroborated by using conventional UV spectrophotometry. On the basis of the relative chromatographic peak heights of the phenanthrene and anthracene, it was calculated that the ionization efficiency of phenanthrene was approximately 18 times that of anthracene at 266 nm. This relative efficiency is due solely to the different electronic structures of these isomers and can be reversed if a wavelength which is more selective for anthracene than the one that had been used. For example, at 310 nm, anthracene can be preferentially ionized
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
~
Ill
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TIME (minl
Flgure 4. Reconstructed chromatograms for a neat injection of 0.1 pL of commercial leaded gasoline under (a) E1 and (b) MPI conditions. The column temperature was programmed from 50 to 250 'C at 5 'C/min. Files were stored at the rate of 1 file/s.
in the presence of phenanthrene (41). Thus, the use of a tunable UV laser source may permit selective detection of a single compound or class of compounds. Although the evaluation of instrument performance by using model systems is mandatory for new techniques, it is of considerably more interest to examine the analytical possibilities with real, complex samples. Gasoline was chosen because it contains hundreds of components, some of which are aromatic hydrocarbons that should ionize a t 266 nm. When the E1 and MPI reconstructed chromatograms of neat gasoline injections are compared, radical differences are evident (see Figure 4). Nearly all of the components eluting in less than 4 min in the E1 trace are absent in the MPI chromatogram. These retention times correspond to the light aliphatic hydrocarbon fraction of the gasoline. The only exception is the peak at 3.6 min which was found to be due to toluene. The saturated hydrocarbons do not ionize because they are optically transparent over most of the near-UV range and they have ionization potentials of over 9.6 eV. Therefore, they require absorption of three photons of 266-nm light to ionize (Ephoton = 4.66 eV at 266 nm). In addition to the exclusion of hydrocarbon peaks, MPI accentuates those compounds with large ionization cross sections a t 266 nm. As a result, the relative peak heights in the MPI trace differ from those in the E1 trace. Furthermore, some of the later eluting components (RT >20 min) which do not appear in the E1 trace are quite evident in the MPI chromatogram. These components (dimethyl- or ethylnaphthalenes) apparently are ionized more efficiently by using MPI a t 266 nm than by 15-eV electrons, much like the phenanthrene. In Figure 5, representative MPI mass spectra of both a minor and a major component from the gasoline chromatogram are shown in order to demonstrate the type of mass spectral data provided by ionization of these molecules at 266 nm. In general, with a direct mode acquisition of 8K data points over a mass range of m / z 50-200, it was possible to produce quality spectra with a mass resolution of lo00 (fwhm) at m / z 200 for most of the components. However, the disadvantage of the present system is that it is not possible to store an entire GC run in conventional IO-Mbyte disk storage. This problem should be overcome in the future by incorporating higher density disk drives and rapid data reduction hardware such as array processors. Mass Resolution. Mass resolution can be considered as one element of specificity, which can be improved by using high resolution. When operated in the direct mode under GC/MPI-MS conditions, resolution from 1000-3000 (fwhm) up to m / z 200 was obtained routinely by using 16K data sets. In all cases, the time domain response was longer than the
100 MASS
50
IN
200
150 A . M . U.
Figure 5. MPI mass spectra of selected components from the chromatogram shown in Figure 4b. Each spectrum is the average of three 8K scans. (a) C9H,, at 8.7 min. (b) Cj2HI2 at 21.6 min. ( 0 1 DIRECT MODE: R=1,500
d
-~ ( b ) HETERODYNE MODE
R=33,500
1 162
1$3
a
Flgure 6. Comparison of the molecular ion region of fluorene under GC/MPI-FTMS conditions in (a) direct mode and (b) heterodyne mode.
data acquisition period, but larger data sets (Le., longer acquisition times) were not used because of the limited disk capacity. The data acquisition time and hence the resolution could be increased by switching the FT mass spectrometer to the heterodyne mode and consequently sacrificing mass range. As an example, the m / z 162-168 regions of fluorene obtained from the GC/MPI-FTMS in both the direct and heterodyne modes are shown in Figure 6. All parameters were the same for the two experiments except that each direct mode spectrum is the average of 3 scans, while in heterodyne mode, each spectrum was from 1 scan so that a scan rate of 1spectrum/s was preserved. The resolution enhancement is evident in the heterodyne spectrum where R = 33 500 a t m / z 166, whereas the resolution is a moderate 1500 in the direct mode. Mass resolution of 30000-40000 was routinely obtainable a t m / z 128, and further increases were possible with minor changes in the experimental conditions. First, the delay after the valve pulse was increased to 250 ms to allow a longer time for excess neutral molecules to be pumped away. Second, the trap voltage was reduced to 0.5 V or less. Higher trap voltages, although leading to more intense signals (due to increased trapping efficiency), caused frequency broadening, which we believe is due to space charge effects. With these changes, the maximum resolution achieved under GC/MPI-FTMS conditions was 87 000 for the m / z 128 ion from naphthalene. However, the time required per scan for R = 87 000 was 1.96 s which is becoming too long for the narrowest peaks produced
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985 2.5
Table I. Limits of Detection for PAHs by GC/MPI-FTMS-at 266 nm
compound naphthalene
conditions
LOD, Pg"
b
7.7 (4.4) 10.4 (1.6) 50 76.2 (23.4) 85.5 (13.4) 78.9 (23.9) 50.4 (25.8) 34.3 (6.8)
C
fluorene biphenyl acenaphthene
d b
b d b d
10
"Limit of detection (standard deviation) calculated for a SIN = 2. bMPI, direct mode (R 20 000), which becomes very important in the analysis of trace amounts of targeted substances in complex matrices. The LOD's obtained by using GC/EI-FTMS are nearly the same for all the compounds studied, whereas the LOD's from the MPI experiments are somewhat different, especially in the case of naphthalene. Presumably, naphthalene has a larger ionization cross section at 266 nm than the other compounds under these conditions. If the laser wavelength could have been varied continuously, these detection limits could have been reduced significantly by simply working at the wavelength of maximum ionization for each analyte. It is feasible that detection limits in the femtogram range could be achieved
103
102
104
106
AMOUNT INJECTED (pg)
Figure 7. Calibration curve MPI-FTMS at 266 nm.
for naphthalene obtained by using GC/
by incorporating a tunable laser. The LOD's in the E1 and MPI modes are not markedly different, but there are difficulties which can complicate the spectra when electron ionization is used. For example, more ions can be produced by E1 by simply increasing the duration of the beam event. However, prolonged electron bombardment can lead to further fragmentation of the stored ions (42) which could actually reduce the S I N of the ion of interest. Moreover, electron bombardment also ionizes column bleed, carrier gas, and residual gases, so that the spectra of picogram amounts of PAHs are obtained under conditions of higher space charge. Nonetheless, these sources of chemical noise can be practically eliminated by using high resolution. To determine the linear dynamic range of GC/MPI-FTMS, several different amounts of naphthalene were injected, and the complex areas of the m / z 128 ions were computed and summed over the GC elution profile (see Figure 7). The linear portion of the curve extends upward from 20 pg for approximately 2.5 orders of magnitude until space charge effects begin to cause deviation from linearity. If a calibration at higher sample levels is required, the curve could be shifted to the right by simply lowering the laser power, which would extend the upper end of the calibration curve with a corresponding sacrifice of the LOD. Accurate Mass Measurements. One feature of FTMS is the ability to perform accurate mass measurements. However, there have been no reports of the use of FTMS to determine accurate masses of GC eluants, despite the fact that most of the calibration laws developed are relatively simple and only require as few as two lock masses. To perform accurate mass measurements in the GC/ MPI-FTMS experiment, a mass standard was needed which could be admitted at background levels into the FTMS and would form relatively abundant ions that encompass the mass range of interest. For the experiments reported here, the mass range was limited to 100-200 amu, which spanned the constituent molecular ion masses of the PAH mixture. It was determined that a 1:2 mixture of toluene and 4-bromostyrene best fit the criteria, because the components yield abundant ions at m l z 91.054 77, 92.06260, 103.054 77, 181.973 10, and
Table 11. Mass Measurement Accuracy for PAH"
compound
theoret mass
naphthalene acenaphthene
128.062 60 154.078 25 154.078 25 166.07825 178.078 25
biphenyl fluorene
phenanthrene
b -4.96 -5.15 -3.59 -4.07 -6.53
(6.11) (5.78) (4.24) (5.34) (3.60)
calibration masses used C d -5.19 nd -4.41 -1.34 -5.78
(9.32) (2.30) (4.88) (3.46)
-5.43 nd -4.11 -6.26 -4.35
(6.78) (2.26) (6.95) (5.15)
e
-5.73 nd -4.22 -3.79 -5.17
(7.34) (1.90) (8.89) (2.92)
"Errors are in parts per million (standard deviation). *Calibrated with m / z 92, 103, and 182. 'Calibrated with m / z 92 and 182. dCalibrated with m / z 91, 103, and 184. "Calibrated with m / z 91, 92, 103, 182, and 184.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
183.971 07 when present at a total pressure of 1 X torr. Using the Nicolet mass calibration software based on the mass-frequency relationship derived by Ledford et al. (28), several combinations of two or more of these ions were used to calibrate over a maximum mass range of 91-184 amu. To evaluate the quality of the accurate mass measurements, triplicate injections of each sample were made at fairly low levels (i.e., 2 ng) in order to minimize space charge. The three most intense spectra for each GC peak were then calibrated, and the masses of the sample molecular ions were determined along with the parts per million mass errors with respect to the theoretical masses. The spectra were acquired in the direct mode from mlz 90 to mlz 250, with 16K data points and zero-filling (43-44) to 64K to provide enough data points across each peak to permit accurate centroid assignments. Under these conditions, the mass resolution at mlz 128 was nearly 3000 fwhm. Although the resolution is lower than one would normally use to minimize interference from isobaric ions, it is necessarily low when a wide mass range is desired. The use of a higher field magnet would easily permit direct mode acquisition of spectra at higher resolving powers. The mass measurement errors are typically in the range of -2 to -6 ppm, which in many cases are probably systematic, because the 0 ppm error line does not lie within one standard deviation of the mean error (see Table 11). Furthermore, all of the average errors are negative, which is additional evidence for a systematic bias. On the basis of the work by Ledford et al. (28), these errors can be attributed to changes in the number of ions, which cause the Coulombic effects from scan to scan to be different. Radical changes in the number of ions is unavoidable in GC/MS experiments, and since the frequency shifts due to these effects are dependent on mass (28), the measured masses can vary. Choosing different reference masses for the calibration table does seem to have an effect on the accuracy and precision of the mass measurements for a given mass, but the changes are not systematic with respect to any particular variable. Therefore, there is no real advantage in using one set of calibrant peaks over another for this application, as they all lead to similar accuracy in the mass measurements. These results do show, however, that reasonably good mass measurements (1-10 ppm) are possible in the GC/FTMS experiment if the number of ions is kept relatively low and the number data points across each mass peak is large enough for good centroid measurement. Perhaps the most important factor influencing the mass measurement accuracy is the ion density in the cell (28). This could conceivably be a problem in GCIFTMS, where the ion population cannot be controlled as well as for sample introduction from a direct probe or leak valve. Nonetheless, it is useful to know that these types of measurements can be made with a moderate degree of accuracy, even a t the medium resolving powers which were used in this study. In conclusion, the GCIMPI-FT mass spectrometer, although potentially a powerful analytical instrument, is by no means complete at the present time. Addition of a tunable dye laser with UV capabilities would greatly enhance the selectivity of the technique by enabling the wavelength corresponding to the maximum ionization cross section of any given analyte to be chosen. Maximum sensitivity would be expected in this case and would probably be in the subpicogram range. Furthermore, progress in the understanding of the ion dynamics in the FTMS cell should improve accurate mass measurements under the varying space charge conditions of the GC/MS experiment. Registry No. Naphthalene, 91-20-3;acenaphthene, 83-32-9;
1295
biphenyl, 92-52-4;fluorene, 86-73-7;phenanthrene, 85-01-8;anthracene, 120-12-7.
LITERATURE CITED ( 1 ) Rosenthal, D. Anal. Cbem. 1982, 54, 63-66 (2) Kimble, B. J. I n "High Performance Mass Spectrometry"; Gross, M. L., Ed.; American Chemical Society: Washington, DC, 1978; pp 120-1 49. (3) Yost, R. A.; Fetterolf, D. D.; Hass, J. R.; Harvan, D. J.; Weston, A. F.; Skotnicki, P. A.; Simon, N. M. Anal. Cbem. 1984, 56, 2223-2228. (4) Gaskell, S. J.; Millington, D. S. Biomed. Mass Spectrom. 1978, 5 , 557-556. (5) Harrison, A. G. "Chemical Ionization Mass Spectrometry"; CRC Press: Boca Raton, FL, 1983. (6) Hilpert, L. R.; Byrd. G. D.; Vogt, C. R. Anal. Chem. 1984, 56, 1842- 1846. (7) Johnson, P. M.; Otis, C. E. Annu. Rev. Phys. Chem. 1981, 32, 139-157. (8) Zandee, L.; Bernstein, R. B. J . Chem. Phys. 1979, 71, 1359-1371. (9) Reiliy, J. P.; Kompa, K. L. J . Cbem. Pbys. 1980, 73, 5468-5476. (IO) Rettner, C. T.; Brophy, J. H. Chem. Pbys. 1981, 56, 53-61. ( 1 1 ) Rebentrost, F.; Ben-Shaui, A. J . Chem. Pbys. 1981, 74,3255-3264. (12) Bernstein, R. 6 . J . Phys. Chem. 1982, 86, 1178-1184. I
Ltchtin, D. A.; Sauire, D. W.; Winnik, M. A.; Bernstein, R. B. J . Am. Cbem. SOC.1983, 105,2109-2117. Seaver, M.; Hudgens, J. W.; DeCorpo, J. J. I n t . J . Mass Spectrom. Ion Phys. 1980, 34, 159-173. Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982, 54,660-665. Lubman, D. M.; Zare, R . N. Anal. Chem. 1982, 54, 2117-2120. Trembreull, R.; Lubman, D. M. Anal. Chem. 1984, 56, 1962-1967. Rhodes, G.; Opsal, R. B.; Meek, J. T.; Reilly, J. P. Anal. Chem. 1983,
55, 280-286. Neusser, H. J.; Boesl, U.; Weinkauf, R.; Schlag, E. W. Int. J . Mass Spectrom. Ion Proc. 1984, 60, 147-156. Comisarow, M. B.; Marshall, A. G. J . Cbem. Pbys. 1975, 62,
293-295. Comisarow, M. B. Adv. Mass Spectrom. 1980, 88,1698-1706. Irion, M. P.; Bowers, W. D.; Hunter, R. L.; Rowland, F. S.; McIver, R. T., Jr. Cbem. Phys. Len. 1982, 93, 375-379. Carlin, T. J.; Freiser, B. S. Anal. Cbem. 1983, 55,955-958. Whle, R. L.; Ledford, E. B., Jr.; Ghaderi, S.; Wilkins, C. L.; Gross, M. L. Anal. Cbem. 1980, 52, 1525-1527. Ledford, E. B., Jr.; Ghaderl, S.; White, R. L.; Spencer, R. 8.; Kulkarni, P. S.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1980, 52,463-468. White, R. L.; Onyiriuka, E. C.; Wilkins, C. L. Anal. Chem. 1983, 55,
339-343. Francl, T.; Sherman, M. G.; Hunter, R. L.; Locke, M. J.; Bowers, W. D.; McIver, R. T., Jr. Int. J . Mass Spectrom. Ion Proc. 1983, 54,
189-199. Ledford, E. B., Jr.; Rernpel. D. L.; Gross, M. L. Anal. Chem. 1984, 56,
2744-2748. Ledford, E. B., Jr.; White, R. L.; Ghaderi, S.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1980, 52, 2450-2451. Wilklns, C. L.; Giss, G. N.; White, R. L.; Brissey, G. M.; Onyiriuka, E. C. Anal. Chem. 1982, 54, 2260-2264. White, R. L.; Wilkins, C. L. Anal. Cbem. 1982, 54,2443-2447. Sack, T. M.; Gross, M. L. Anal. Chem. 1983, 55, 2419-2421. Laude, D. A., Jr.; Brissey, G. M.; Ijames, C. F.; Brown, R. S.; Wilkins, C. L. Anal. Chem. 1984. 56. 1163-1168. Dushman, S. "Scientific 'Foundations of Vacuum Technique"; Wlley: New York. 1963: D 82. Comisarow, M. B Int. J . Mass Spectrom. Ion Phys. 1981, 37,
251-257. McCrery, D. A.; Ledford, E. B., Jr.; Gross, M. L. Anal. Cbem. 1982,
54, 1435-1437. Bjorseth, A., Dennis, A. J., Eds. "Polynuclear Aromatic Hydrocarbons: Chemistry and Biological Effects"; Battelle Press: Columbus, OH,
1980. Yu, W. C.; Fine, D. H.; Chiu, K. S.; Biemann, K. Anal. Chem. 1984,
56, 1158-1162. Stray, H.; Mano, S.; Mikalsen, A.; Oehme, M. HRC CC, J . High Resolot. Chromatogr. Cbromatogr. Common. 1984, 7 ,74-82. Huang, S.K.; Rempel, D. L.; Gross, M. L. Paper presented at the Thirty-second Conference on Mass Spectrometry and Allied Topics, San Antonio, TX, May 27-June 1 , 1984, pp 596-597. Klimack, C. M.; Wessel, J. E. Anal. Cbem. 1980, 52, 1233-1239. Cody, R. B.; Freiser, B. S. Anal. Cbem. 1979, 51, 547-5511, Comisarow, M. B.; Melka, J. Anal. Chem. 1979, 51, 2196-2203. Francl, T. J.; Hunter, R. L.; McIver, R. T., Jr. Anal. Cbem. 1983, 55,
2094-2096.
RECEIVED for review November 5, 1984. Accepted February 11, 1985. This work was supported by the National Science Foundation, Grant CHE-8018245, and the National Institutes of Health, Grant GM 30604. Presented in part a t the 32nd Annual Conference on Mass Spectrometry and Allied Topics, San Antonio, TX, May 27-June 1, 1984.
