221 1
Anal. Chem. 1982,5 4 , 2211-2215
Low-Pressure Collision-Induced Dissociation Analysis of Complex Mixtures by Fourier Transform Mass Spectrometry Robert L. White and Charles L. Wilklns” Department of Chemistry, University of California-Riverside, Riverside, California 9252 1
High mass resolution (10 000 full width at half height (fwhhp) of daughter Ions resuitlng from collision-induced dissociation of a complex mixture le demonstrated In Fourier transform mass spectrometry. Theoretical aspects of the experiment are discussed and applications of the technique to mixture analysis are presented for both synthetic and unknown Samples. The performance is evaluated by comparing results with previously publislied data obtained by using triple quadrupole MSIMS.
Collision-induced dissociation (CID) is a mass spectrometric technique that cam rival GC/MS in sensitivity and selectivity for complex mixture analysis (1-3). In fact, enhanced selectivity by CID has recently been applied to GC analysis (GC/MS/MS) (4)even though the technique is generally used without prior component separation. The)advantages of CID derive from the dimination of background “chemical noise”. An example of chemical noise in GC/MS is column bleed. Also, a reduction in sample analysis time compared with GC/MS can be achieved since prior component separation is not needed. ‘Whereas conventional mass spectrometric methods must be combined in tandem t o perform CID experiments, only minor computer software changes are necessary to operate a Fourier transform mass spectrometer (FTMS) as a CII) instrument. Freiser and co-workers have reported a series of studies demonstrating this fact (5-7). With further software additions, the potential for multiple CID experiments (MS/MS/MS ...) exists (8)and MS/MS/MS also has been demonstrated recently by Freiser and co-workers using a prototype Nicolet FT/MS-1000 iron magnet system (9).
In MS/MS, collision-induced dissociatiion is accomplished in three phases: ion selection, ion fragmentation, and mass analysis of the fragments. Thus, mass spectra derived from single selected ions are obtained. It has been suggested that the resulting patLerns can be a valuable source of structural information (2). In conventional MS/MS, a single ionic species with a specific m / z value is chosen for CID analysis by some means of mass filtering (e.g., magnetic sector, quadrupole filter, double focusing MS). The analogous step is performed in FTMS by ejecting all ions except those with the selected m / z value from the analyzer cell. This is done with radio frequency sweeps of sufficient amplitude and appropriate bandwidth to accelerate the unwanted ions into the cell plates where they are neutralized. Ion fragmentation of selected ions is accomplished in conventional MS/MS by accelerating the ions into a collision chamber filled with collision gas such as nitrogen or argon where they undergo collisions and subsequently dissociate. FTMS mass-selected ions are accelerated by irradiating them with a low intensity radio frequency electric field oscillating at the characteristic cyclotron resonance frequency of the ion. Once “heated”, the ions can collide with inert gas atoms and fragment. Mass analysis of the collision-induced fragments can be achieved by normal FTMEI detection which involves an rf excitation
sweep followed by image current detection of the total ion signal, Although conventional MS/MS daughter ion detection schemes suffer from poor mass resolution, high resolving power is possible in FTMS-CID by operating at low analyzer cell pressures. Improved selectivity can be expected with high resolution single reaction monitoring FTMS-CID.
EXPERIMENTAL SECTION The Fourier transform mass spectrometer used in this study was a Nicolet FT/MS-1000 equipped with a 1.9-T superconducting magnet, 128K Nicolet 1280 computer, and a 0.0254-m cubic analyzer cell. Samples and reagent and collision gases were introduced to the vacuum system via a dual gas/liquid inlet system as well as a third inlet leak valve which was probe-mounted and inserted into a vacuum lock. The probe-mounted leak valve was constructed in our laboratory and consisted of a Varian Model 951-5106 variable leak valve mounted on a flange to which was welded a 9 in. X 0.5 in. 0.d. stainless steel tube. The third inlet was bakeable to 450 “C! and background pressures as low as 4 X lo4 torr were attained with this inlet inserted. All samples were reagent grade quality and were used without further purification. The operating principles of FTMS have previously been described in detail (IO). For CID studies, typically a 10-ms,15-eV electron beam was used for sample ionization. Following the beam pulse, two ejection sweeps of 12-V p-p amplitude and sweep rates of 100 Hz/ps were employed to remove the majority of ions (Le., those with m / z values below and above that of the ion selected for study) in the cell. A 1.0-V trapping potential was used. The ion accelerating pulse consisted of a radio frequency oscillation 2.0 V p-p in amplitude and 500 ps in duration centered on the cyclotron resonance frequency of the selected ion. A collision delay of 50 ms (vide infra) was used before ion detection and 64K transient signal data points were digitized and transformed without zero filling.
RESULTS AND DISCUSSION High mass resolution of CID daughter ions in Fourier transform mass spectrometry is illustrated in Figure 1 for cyclohexane molecular ion (mlz 84). The mass spectrum in Figure l a was obtained with low-energy (15 eV) electron impact ionization of cyclohexane at a partial pressure of 3 x 10P torr. A large molecular ion as well as significant fragmentation was observed under these conditions. The spectrum in Figure l b shows the effect of an ion ejection sweep (350 kHz-2 MHz at 100 HzIps and peak-to-peak voltage of 12 V) to remove the majority of ions from the cell. Mass selection of m / z 84 was not perfect and residual ion intensity was noted a t m / z 83 and m / z 85 (Figure lb). Figure ICshows the results of CID (3 X torr argon collision gas partial pressure) of the molecular ion initiated by a 500-ws rf accelerating pulse a t 346.472 kHz and 2.10 V peak-to-peak amplitude. The decrease in m / z 84 intensity resulting from ion acceleration (Figure lb-c) was 73% while the m / z 83 and m / z 85 intensities were essentially unchanged (the change in these ion intensities was less than 1% of the decrease in intensity of m / z 84). Thus, even though mass selection was poor (less than unit resolution), ion acceleration was specific so that the interfering ions (mlz 83 and m / z 85) did not appreciably affect the fragmentation pattern observed for the selected parent ion. Because a low FTMS operating pressure was used (3.3 X
0003-2700/82/0354-2211$01.25/0 0 1982 American Chemical Society
2212
ANALYTICAL CHEMISTRY, VOL.
54,NO. 13, NOVEMBER 1982
a lool
I
6?
8 t
I I
z
loo** 75
~i
W
01 0
I
40
I
80
COLLISION DELAY (ms) Figure 2. Change in parent ion ( m l z 114) relative intensity function of collision delay time for 3-heptanone molecular ion.
50
60 70 80 m/z
90
IO0
Figure 1. (a) Electron impact (15eV) spectrum of cyclohexane at a partial pressure of 3 X torr. (b) Result of ion ejection to remove all but the selected ion ( m l z 84). (c) Collision-induceddissociation of the parent ion using 3 X torr of argon collision gas and accelerating m l z 84 to 245 eV translational energy. For clarity, (b) and (c) are plotted on the same vertical scale.
lo-’
torr), high mass resolution (8500 fwhh at m/z 84) was obtained for each of the spectra by using a heterodyne data acquisition mode ( m / z 40 to m / z 100). In this instance, the resolution was limited by the available computer memory (64K) and the time domain signal was visibly truncated. Nevertheless, this example demonstrates a daughter ion mass resolution which easily surpasses that obtained previously using MIKES ( I ) , triple quadrupole MS/MS (2), and FTMS-CID a t higher analyzer cell pressures (5-7). Much higher mass resolution could have been obtained by using a heterodyne data acquisition over a narrower mass range. The collision efficiency of a parent ion in CID is determined by the probability of an accelerated ion colliding with a neutral gas atom or molecule and is therefore a function of the ion residence time in the collision chamber as well as the concentration of collision gas particles. In triple quadrupole MS/MS, the residence time of a typical ion in the center (collision) quadrupole is on the order of microseconds. As a result, collision gas pressures in the lo4 torr range are needed to maximize the number of effective ion-neutral collisions. In FTMS, by prolonging ion residence time in the collision chamber, it is possible to use low collision gas pressures to achieve collision efficiencies comparable to those obtained in the triple quadrupole experiment. For this technique, lower operating pressures result in less signal damping and higher mass resolution (IO). The trapped-ion FTMS cell design provides an ideal means of increasing the ion residence time in the analyzer (collision) cell. It has been demonstrated that ions can be trapped for many seconds a t low operating pressures (11). By inserting a “collision delay” period between ion acceleration and fragment detection, the number of ion collisions can be increased to the point that all ions collide with neutrals. The collision efficiency a t a given low pressure then becomes a function of the collision delay time. This behavior was observed experimentally and is illustrated in Figure 2. The plot of relative intensity vs. collision delay time was generated by using the
as
a
3-heptanone molecular ion ( m / z 114) at a partial pressure of 2x torr and argon collision gas a t a partial pressure of torr. A sharp decline in intensity with increasing delay 3X time was observed for short delay times but for delays longer than 15 ms, the intensity was relatively constant. The fact that the m / z 114 intensity did not approach zero with increasing collision delay times but instead leveled off indicated that a fraction of the parent ions did not dissociate upon collision and the ions were thermalized. Residual parent intensity was reduced when more intense accelerating pulses were used. Presumably, this resulted from a shifting of the energy distribution of the accelerated ions to higher energies and thus ejection, in addition to CID. In order to more fully understand the interaction of accelerated ions with neutrals, a simple model describing the collision can be developed by assuming spherical volumes for the colliding particles. If the accelerated ion and neutral particles possess collision cross-section diameters denoted ‘SI and uN, respectively, then the effective collision cross-section for the ion-neutral collision is given by Ueff
=
uI -k “N ~
2
The effective collision cross-section diameter ( ueff) is the distance that the centers of the spheres must approach each other before contact can occur. Each of the spheres are moving through space at different velocities. The ion is not at thermal energy but has been accelerated by a radio frequency pulse. The increase in velocity resulting from acceleration can be calculated
Vi,, = 2 ~ r f , where f, is the cyclotron resonance frequency of the ion and rc is the increase in the radius of ion motion. The value of rc is dependent on the accelerating conditions and has been derived from fundamental principles for accelerating frequencies at the ion cyclotron resonance frequency (12) EIftIf
rc = 2B
(3)
Erfis the intensity of the accelerating radio frequency oscillation, tIf is the duration of the pulse, and B is the applied magnetic field strength. In most instances, the velocity of the accelerated ion will be much greater than the thermal velocity
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
2213
m I-
50
0 0
20
40
COLLISION
60
80
DELAY (ms)
Comparison of experimental (points)and theoretical (curve) relative fragment ion intensity as a function of collision delay time for the m / z 57 daughter ion from 3-heptanone. Figure 3.
of the collision particle and, as a first approximation, the neutral collision particle can be considered at rest with respect to the ion. In this case, the average distance that an ion must travel before collision occurs becomes
where CN is the neutral concentration and is assumed to be much larger than the concentration of ions. It follows that the average time required for an ion to collide with a neutral is given by dcollisionl Vion or
substituting B = 27fcm/z yields
where m/z is the mass-to-charge ratio of the ion. The relation in eq 6 shows the expected result that the average collision time is inversely proportional to collision gas pressure, collisional cross-section, and rf irradiating power. If the effective collision cross-section diameter is known, Equation 6 provides a means of estimating the average collision time of an ion with a neutral. For the 3-heptanone example illustratied in Figure 2, the value for the collision cross-section diameter was estimated to be the same as that for n-heptane and was calculated by using the van der Waals constant b (13),which is a measure of the molecular volume. With this, an average collision time of 6.5 ms was calculated. Figure 3 shows a comparison of the relative change in ml2 57 daughter ion intensity from 3-heptanone CID as a function of collision delay time with a theoretical curve. The theoretical curve was generated by assuming that ion-neutral collision probability as a function of collision delay time could be represented by a Graussian distribution centered a t the average collision delay time calculated in eq 6. Furthermore, the area of the Gaussian wiu chosen to be the m/z 67 intensity at long collision delay times (complete fragmentaition of the parent ion). The curve in Figure 3 was created by integrating the Gaussian function with respect to collisioin delay time. The agreement between experiment and theory is rather good, indicating that the assumptions made in calculating the average collision time were valid for this example. On the basis of the results presented in Figure 3, a collision delay time of 50 ms was selected for all of the experiments described here in order to assure complete ion dissociation prior to daughter ion analysis. Low-pressure fragmentation efficiency of a parent ion was found to vary with collision gas pressure at a constant collision
V '
0
2
4
6
8
1
0
ARGON PRESSURE (16~torr) Variation of fragmentation efficiency with collision gas pressure for cyclohexane m / z 8 4 accelerated to 80 eV. Figure 4.
delay time. This is illustrated in Figure 4 for the fragmentation of m / z 84 from cyclohexane. The partial pressure of cyclohexane was 3 x torr. Percent efficiency was defined as follows daughter intensity % efficiency = x 100% ( 7 ) A(parent intensity) where A(parent intensity) was measured by subtracting the mlz 84 peak intensity after CID from the peak intensity before CID and daughter intensity was simply the sum of the daughter ion peak intensities in the CID spectrum. The % efficiency leveled off at about 22% at an argon collision gas partial pressure of 2 X torr; approximately 7 times the cyclohexane partial pressure. The translational energy imparted to mlz 84 was insufficient to eject the ion from the analyzer cell (0.006 m calculated ion cyclotron radius compared with a cell radius of 0.0125 m). It is possible that 100% efficiency was not measured due to scattering of the accelerated parent anid daughter ions within the cell. Such scattering could result in incoherent ion excitation during ion detection and would be expected to yield fragment ion peak heights that did not reflect the total number of ions which were actually in the cell. The technique of single reaction monitoring in CID is often used for increased selectivity in complex mixture analysis. In this method, a selected daughter ion i!s monitored in order to link a given parent ion to a specific daughter. This technique has been demonstrated to be effective at the 50-pg level for TCDD analysis (14). Single reaction monitoring can be especially useful for distinguishing isomeric parent ions when fragmentation patterns are different. In FTMS-CID, the specificity of single reaction monitoring can be enhanced by measuring daughter ions at high resolution. Cody and Freiser have recently reported FTMS-CID etudies of m/z 43 ions derived from acetone and 2-chloropropane molecular ions (7), demonstrating a resolution of about 3000 fwhh. Those spectra were obtained with a magnetic field of 0.9 T, corresponding to resolution of about 6300 at 1.9 T. As; an illustration of what appears to be the highest resolution CI[D mass spectrum thus far reported, the m/z 866 daughter ion from CID of the m/z 1166 ion of tris(perfluorohepty1)-s-triazine(CZ4Fd5N3, mol wt 1185) was measured in our laboratory with a mass resolution of 9200 (fwhh). A similar analysis of the mlz 43 CID daughter ions formed from a mixture of 2-pentanone, heptanal, and 3-heptanone was carried out under low pressure conditions. Figure 5 shows the mass spectra obtained by using the heterodyne mode by monitoring a mass range of 0.4 amu from mlz 42.8 to mlz 43.2 as each of the molecular ions were dissociated under identical conditions Reasonable S I N was obtained even though the collision energy of the parent ions
2214
54, NO. 13, NOVEMBER 1982
ANALYTICAL CHEMISTRY, VOL.
Table I. Synthetic Mixture CID Results parent 84 (mixture) m/z 41 42 55 56 69 [100]' [15] [75] [801 [451 84 (cyclohexane) 86 (mixture) m/z 43 58 71 86 (2-pentanone) [40] [85] [loo] 114 (mixture) m/z 41 42 43 55 56 57 [ ~ O O I [io1 ~ 5 1[421 114 (n-octane) [81 [51 114 (heptanal) ~ 5 1 ~301 114 (3-heptanone) [301 [351 a Numbers in brackets are the relative abundances of the daughter also observed in the CID spectrum of the pure substance.
CH3COt
daughters
69 70 71 72 84 85 87 99 ~ 0 1[51 ~ 3 2 1 [51 1381 [go1 [51 [721 ~301 [ ~ O O I [1001 BO1 [251 ions observed in the mixture CID experiment that were
43.01839
'I
'
11 I
loo:
~
C
I irl
id$
h
/'
i
lf#-
\
43
mh
40
60
80
m /z
100
120
Figure 5. Narrow band heterodyne mode spectra of m l z 43 for (a) 2-pentanone, (b) heptanal, and (c) 3-heptanone. Spectral resolution is about 10 000 fwhh.
Flgure 6. Spectra of CID daughter ions produced from (a) Sheptanone M H+ and (b) 3-heptanone molecular ion (M').
was not optimized to yield the maximum signal a t mlz 43. The resolution obtained in these spectra (10 000 fwhh) was sufficient to distinguish between CH3CO+(mlz 43.01839) and C3H,+ (mlz 43.05478). Most of the nominal mlz 43 intensity for 2-pentanone (Figure 5a) was due to CH3CO+fragments while virtually all of the signal obtained for heptanal (Figure 5b) was in the form of C3H7+ions. Dissociation of 3-heptanone molecular ion, however, was found to yield appreciable quantities of each type of ion (Figure 5c). Heptanal and 3-heptanone are isomeric, and, for this reason, low-resolution single reaction monitoring at m / z 43 would be incapable of distinguishing these compounds. However, with high-resolution FTMS-CID, the compounds were readily distinguished. For comparison of FTMS-CID with a more conve?tional MS/MS technique, mixture component analysis was performed for an equivolume mixture of cyclohexane (mlz 841, 2-pentanone (mlz 86), n-octane (mlz 114), heptanal (mlz 114), and 3-heptanone (mlz 114). This mixture was chosen because the components have been successfully identified by Yost and Enke using low-energy triple quadrupole MS/MS (2). The results of the component analysis are contained in Table I. Cyclohexane and 2-pentanone were easily identified in the mixture by comparing the CID results for the mixture with those of the pure substance. In fact, the fragmentation pattern observed for cyclohexane was in good agreement with the previously published results. The m / z 114 ion in the mixture was a composite of n-octane, heptanal, and 3-hep-
tanone molecular ions. Yost and Enke found that n-octane was distinguished by a characteristic peak at m/z 70, 3-heptanone produced a unique fragment a t mlz 99, and m / z 81 or mlz 96 could be used to identify heptanal. Although daughter ions at m/z 70 and 99 were obtained, peaks at m/z 81 and 96 were not observed in our measurements. This was probably due to the relative involatility of heptanal with respect to the other components (25 times less volatile than cyclohexane at room temperature) and the fact that the CID yield of these fragments was small in comparison with the other daughter ions obtained from pure heptanal CID. In fact, the mlz 81 peak was not observed in the pure heptanal CID spectrum. Additional verification of the presence of 3-heptanone in the mixture was obtained by inspection of the m / z 43 peak in the spectrum. The resolution of the direct mode experiment was sufficient (4000 fwhh) to distinguish the CH3CO+ and C3H7+ions. The CH3CO+fragment observed could only have come from 3-heptanone since neither pure heptanal nor n-octane produced it (see Figure 5 ) . A more sensitive method of ionization in FTMS is chemical ionization. This can be performed a t low pressures and is therefore compatible with CID. Low-pressure chemical ionization is accomplished by creating reagent ions in the analyzer cell and using a delay period to ensure complete reaction of the reagent ion with the sample (11). By use of 2 X lo-, torr torr argon, and 2 X torr methane in addition to 3 X 3-heptanone, the CI-FTMS-CID spectrum of the M H+ ion
+
+
2215
Anal. Chem. 1982, 5 4 , 2215-2219
for 3-heptanone was obtained. All operating conditions were the same as in E1 measurement with the exception that a 2.0-s delay period was inserted between the electron beam pulse and the ejection sweeps. Figure 6 shows a comparison of fragmentation patterns obtained from M + H+ and M' FTMS/CID of 3-heptanone. The fragmentation pattern was significantly altered by protonation of the sample. Another area of research in our laboratory is development of a linked GC/FTIR/MS analysis systern for complex mixture analysis. Recently, this system was used to analyze a commercial lacquer thinner sample containing more than 30 components (15). One of the components identified by the linked system was butyl acetate. Using chemical ionization FTMS-CID for direct mixture analysis of the lacquer thinner, supporting information was obtained which elucidated the following fragmentation pathways for this component
r-C4H:
CH3COC4Hg
m/z
57
mk
+ 4 H ;
COC4H9
nih
73
101
and further confirms the assignment.
LITERATURE CITED (1) Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 50, 81A-92A. (2) Yost, R . A.; Enke, C. G. Anal. Chem. 1979, 57, 1251A-1264A. (3) Burinsky, D. J.; Cooks, R . G.; Chess, E. K.; Gross, M. L., Abstracts of the 29th Annual Conference on Mass Spectrometry and Allied Topics, Minneapolis, MN, 1981; p 663. (4) Fetterolf, D. D.; Yost, R. A. "Abstracts of Papers," 183rd National Meeting of the American Chemical Society, Las Vegas, NV, 1982; American Chemical Society: Washington, DC, 1982; ANYL 93. (5) Cody, R. B.; Burnier, R. C.; Freiser, B. S. Anal. Chem. 1982, 54, 96-101. (6) Cody, R. 8.; Freiser, 8. S. I n t . J. Mass Spectrom. Ion Phys. 1982, 41, 199. (7) Cody, R. B.; Freiser, B. S. Anal. Chem. 1982, 54, 1431-1433. (8) McIver, R. T. Workshop on Newer Aspects of Ion Cyclotron Resonance and Fourier Transform Mass Spectrometry, 29th Annual Conference on Mass Spectrometry and Allied Topics, Minneapolis, MN, 1981; p 791. (9) Cody, R. 6.; Burnler, R. C.; Cassady, C. J.; Freiser, B. S. Anal. Chem. 1982, 54, 2225-2228. (10) White, R. L.; Ledford, E. B., Jr.; Ghaderi, S.;Wilkins, C. L.; Gross, M. L. Anal. Chem. 1980, 52, 1525-1527. (11) Ghaderi, S.; Kulkarni, P. S.;Ledford, E. B., Jr.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1981, 53, 428-437. (12) Comisarow, M. B. Int. J. Mass Spectrom. Ion Phys. 1978, 26, 369-378. (13) "Handbook of Chemistry and Physlcs", 52nd ed.; The Chemical Rubber Co.: Cleveland, OH, 1971; p D146. (14) Chess, E. K.; Gross, M. L. Anal. Chem. 1980, 52. 2057-2061. (15) Wilkins, C. L.; Giss, G. N.; White, R. L.; Brissey, G. M.; Onyiriuka, E. C. Anal. Chem. 1982, 54, 2260-2264.
RECEIVED for review May 3,1982. Accepted August 6,1982. The support of the National Science Foundation under Grants CHE-79-10263, CHE-81-13612, and CHE-80-18245 is gratefully acknowledged.
High and Low Energy Collision Mass Spectrometry/Mass Spectrometry of Aza and Amino Polynuclear Aromatic Compounds in Coal-Derived Liquids J. D. Ciupek, D,. Zakett,' and R. G. Cooks* Department of Chemistty, Purdue University, West Lafayette, Indiana 47907
K. V. Wood" Engine/Fuels Laboratory, Institute of Interdisc@linaryEngineering Studies, Chemistry Building, Purdue University, West Lafayette, Indiana 47907
Comparisons are made between colllsion-induced dissociation spectra obtained by mar& spectrometry/mass spectrometry (MS/MS) using triple quadrupole and sector instruments. Data are glven for methyl-substituted aza polynuclear aromatlcs (PNAs) arid the isomeric prlmary amino PNAs. The spectra of massselected (M H)' ions (formed by chemlcal Ionization ( C I ) ) show substantlal differences which depend upon the collision energy but they are independent of reagent gas (isobutane or ammonia). Isomer dlstlnctlon is possible for standard compounds In both high- and How-energy MS/MS spectra and Is based upon the same processes, losses of 15, 16, 17, and 18 mass unlts, In both energy reglmes. Ammonia reagent gas slmpllfles the C I mass spectrum of solvent reflned coal (SRC 11) and enhances the relative abundance OX bask constltuentr. The MS/MS methodology showed the m / z 144 component in the SRC I 1 to be a mlxture of at least three methylqulnallne isomers.
+
Present address: Dow 'Chemical Co., Midland, MI 48640. 0003-270O/82/0354-2215$01.25/0
Mass spectrometry/mass spectrometry (MS/MS) is becoming a useful tool in mixture analysis (1-3) and structure elucidation (3-5). Early work employed mass-analyzed ion kinetic energy spectrometry (MIKES), that is a reverse geometry mass spectrometer with a collision cell located between the magnetic and electrostatic sectors. Individual components are mass selected after ionization. Dissociation products arise by fragmentation in the region between the sectors, as a result of either unimolecular decomposition (metastable ions) or collision-induced dissociation (CID). Fragment ions are then identified by kinetic energy analysis. Analogous data can be obtained from a conventional double focusing mass spectrometer by simultaneously scanning the magnetic and electrostatic sectors (6, 7). Characteristic of the CID process in sector instruments is the high axial kinetic energy of the ions, typically 3-10 keV, entering the collision cell. Recently, tandem quadrupole (3, 8) and hybrid sector/ quadrupole devices (9) have enabled MS/MS to be performed using ions with translational energies on the order of 3-100 eV. In triple quadrupole instruments, the first quadrupole 0 1982 Amerlcan Chemical Society
