Anal. Chem. lQ86, 58, 1993-2601
the high reflectivity of Mo at the vaporization laser wavelength results in very little energy absorbed. The amount of energy deposited in Mo should be similar to that for polished Al. The difference in volatility accounts for the results in Figure 7 and Figure 5. The distribution for molybdenum is unusual compared to all other cases. Apparently, some material is released from the surface with high velocity initially to occupy the region about 0.5 cm above the surface. These disperse slowly due to loss of momentum through collisions and gravitational force. More material then builds up over a longer time period adjacent to the surface, resembling a slow boiling process. Tantalum (Figure 8) absorbs laser light very efficiently, perhaps comparable to Cu. But, because it is less volatile than copper (Figure 4), less material is vaporized and more uniform particles are released. The scattering map for Ta is comparable to that for nonpolished A1 (Figdre 6). The higher absorption for Ta offsets its lower volatility. In summary, we have demonstrated a unique spatial and temporal mapping scheme for particulates that are formed in transient atom sources such as the laser microprobe. Acoustooptic beam deflectors allow consecutive time-lapse maps to be obtained. Scan rates of 10 ps per one-dimensional map have been demonstrated previously (15), so that two acoustooptic deflectors at orthogonal orientations can extend the results here to two-dimensional maps. Since commercial deflectors with higher spatial resolution (N in eq 1)and higher modulation frequencies are already available, the limit seems to be digitization rate (100 MHz) of the wave form analyzer in providing a total of 10000 points in 100 ps, the duration needed to effedively “freeze”the particles in time. We showed that particulate formation is dependent on power density, laser wavelength, surface characteristics, and volatility of the material. Obviously, the spatial maps presented here are not general since we have not characterized in detail the laser mode structure, surface roughness (B.g., by microscopy), reflectivity and absorption at the laser wavelength, thermal conductivity, etc. Now that particulate distributions can be monitored for a single transient event, one can proceed with
1993
systematic studies of the laser-surface interaction. Analogous
to other atom sources for analytical measurements, one wants to eventually arrive at experimental conditions that produce uniform atom plumes (for better reproducibility) and also negligible amounts of particular matter (for efficient conversion to free atoms). Also, it should be noted that the present 90’ observation geometry is the simplest arrangement. More information about the particles can be obtained in a more sophisticated arrangement that allows one to study the dependence of the signal on polarization, wavelength, and observation angle, especially if the particles are much smaller than those observed here. Finally, the present system can be easily adapted for similar furnaces, sparks, combustion chambers, and propellants. Registry No. Cu, 7440-50-8 Al,7429-90-5;Mo, 7439-98-7;Ta, 7440-25-7.
LITERATURE CITED (1) Mossotti, V, G.; Laqua, K.; Hagenah, W. D. Spectrochim. Acta, Part 6
198f, 238,197-206. (2) Conzemius. R. J.; Svec, H. J. Anal. Chem. 1978, 50, 1854-1860. (3) HiHenkamp, F.; Unsold, E.; Kaufmann, R.; Nitsche, R. Appl. Phys. 1975, 8,341-348. (4) Steenhoek, L. E.; Yeung, E. S. Anal. Chem. 1981, 53,528-532. 6)Huie, C. W; Yeung. E. S. Saectrochim. Acta, Part 6 1985, 406, 1255-1258. Yeung, E. S.;Steenhoek, L. E.; Tong, W. G.; Bobbin, D. R. Anal. Chem. 1981. 53,1936-1938. Huie, C. W.; Yeung, E. S. Appl. Spectrosc. 1984, 38,660-663. Horvath. J. J.; Bradshaw, J. D.; Bower, J. N.; Epstein, M. S.; Winefordner. J. D. Anal. Chem. 1981, 5 3 , 6-9. Hartley, D. L. AIAA J . 1972, 7, 687-689. Piepmeier. E. H.; Osten, D. F. Appl. Spectrosc. 1971, 25,642-652. Chromiak, J. Combust. Flame 1972, 78,429-434. Manabe. R. M.; Piepmeier, E. H. Anal. Chem. 1979, 57,2066-2070. Jones, D. G.;Mackie. J. C. Combust. Flame 1978. 27,143-146. Penlat, M.; Bailly, R.; Taran, J. E. Optics Commun. 1977, 22,91-94. Huie, C. W.; Yeung, E. S. Appl. Spectrosc., in press.
RECEIVED for review January 24, 1986. Accepted April 14, 1986. The Ames Laboratory is operated for the US. Department of Energy by Iowa State University under Contract No. W-7405-eng-82. This work was supported by the Office of Basic Energy Sciences.
Atmospheric Pressure Ionization Mass Spectrometry with Laser-Produced Ions Leonidas Kolaitis and David M. Lubman* Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109
I n this work laser ionization at atmospheric pressure has bmn characterized as an lonlzath source for mass analyzed Ion mobliity measuremerits. Laser ionization has been explored in different atmospheric pressure gases ineluding N,, air, Ar, P-10, and CO, and the resuitkrg Ion moMiRies in the dmerent gases have been compared. Further, ionization via Ni @ source ionization in these gases in an IMS has been studled and the r d s were compared to the laser bnlzatlon. I n each case the lorn produced In the IMS are injected into a quadrupole mass spectrometer and mass analyzed. We have found that in ahnost every example studied the moiecYlar ion (M’) or MH’ Is formed wlth no resuiting fragnentatkn from the laser or Ni p source. I n addition, the K Ovalue of each species has been generally found to change as a tunction of a,the poiarizablity, except for CO, drift gas where large dusters are found to form around the organic knic core. 0003-2700/86/0358-1993$01.50/0
Atmospheric pressure ionization methods can provide a sensitive and rapid means of monitoring trace contaminants in the environment (1-11). The great sensitivity of this technique, which can reach the parts-per-trillion level ( 4 ) ,is based upon the fact that ionization occurs under ambient conditions, i.e., in situ, so that the actual concentration is examined during analysis. In conventional mass spectrometry injection, only a limited flux of molecules per unit time can be introduced through a pinhole leak into vacuum, thus the number of molecules examined may be decreased by 3 orders of magnitude or more. At atmosphefic pressure a partsper-billion concentration level mearis detection of nearly 3 x 1O’O molecules/cm3. This number represents a density comparable to or better than that normally probed in molecular beam experiments. The second aspect of atmospheric pressure detection that 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
1994
AIMS 14 I
760 torr
I
MOBILITY REGION
51
I
MS IO-^^^^^
1 SHIELD I
:I
I
LENS
APERTURE
I Ill
DRIFT
GAS
Figure 1. Schematic of the atmospheric pressure ionization mass spectrometer-ion mobility drift tube apparatus.
provides great sensitivity is the ionization efficiency. The two sources often used are the Ni ,i3 source and Corona discharge methods. Both techniques utilize kiloelectronvolt electrons to ionize components in the air which then transfer their charge through a series of ion/molecule reactions to the trace species of interest. In effect, this is atmospheric pressure chemical ionization and, because of the large number of collisions at atmospheric pressure, is nearly 100%efficient. The main drawback of this method is that background contaminants in the sample may result in a number of competing reactions and thus confusion may arise in the interpretation of the data. Further, if a strong base is present (NH,), then a weak base such as benzene may not be detected due to proton abstraction or charge transfer. An alternate means to achieve atmospheric pressure ionization is to use laser induced resonant two-photon ionization (RZPI) (12,13).In this process an ultraviolet or visible photon excites a molecule to a real intermediate state and a second photon ionizes the molecule. Thus, the sum of the two photons must be greater than the IP of the molecule for ionization to occur efficiently. The RZPI process has been shown to be an efficient means of soft ionization where only the molecular ion is produced, i.e., no fragmentation at modest laser energies, with efficiencies that may range up to several percent within the laser beam (14,15).One advantage of RZPI as an atmospheric pressure ionization source is that it is a direct ionization method and thus may be less prone to competing ion/molecule reactions. However, the most unique aspect of this method is that selectivity can be obtained based upon the wavelength dependence of ionization in the R2PI process (23). In our work, laser ionization has been explored as an API source for mass analyzed ion mobility spectrometry (MAIMS). Ion mobility measurements provide a means of separating ions produced at atmospheric pressure with high sensitivity in a rapid and continuous manner for monitoring of the environment. In essence, IMS is an atmospheric pressure drift tube in which ions are separated according to their ionic mobility under the influence of an electric field. A mobility spectrum is obtained at a collector as a function of time, where the typical drift time is in the millisecond regime. Although ion mobility measurements cannot provide high resolution since the peak widths are ultimately diffusion limited, in essence the resolution has been traded-off for the speed of analysis
and sensitivity of the technique. The ions are then focused into a quadrupole mass spectrometer for exact mass identification. The combination of ultrahigh sensitivity and speed provided by this technique have made it the method of choice for solving several very specialized process monitoring applications including the quality control analysis of volatile contaminants on semiconductorsand of the ultra-high-purity gases used in this industry (4,16)as well as the actual monitoring of clean room environments (16). In addition, it has found important use in environmental pollution monitoring and in detection of explosives and other harmful agents in the environment (17,18).The combination of laser ionization, ion mobility measurements, and mass spectrometry thus form a tandem combination of three methods for selective ionization and detection of molecules at atmospheric pressure. In this paper laser ionization at atmospheric pressure is explored in several different background gases. In one set of experiments, ion mobility measurements are evaluated as a function of drift gas and the reduced mobility (KO) obtained using laser ionization vs. the Ni /3 source is compared for a number of aromatic species of environmental interest. In particular, mass analysis is used for the first time to evaluate the ionization products formed by laser ionization at atmospheric pressure and compared to product ions formed by the Ni ,i3 source. Laser ionization at atmosphericpressure is shown to produce soft ionization of aromatic molecules and related species even under relatively high laser power densities. Further, ionization in different gases is evaluated in terms of background interferents and the different clusters and dimer products that may be formed.
EXPERIMENTAL SECTION The laser-IMS setup has been described previously (12)and is a modified commercial version of the PCP Phemtochem 160 LI spectrometer (West Palm Beach, FL) in which an IMS has been interfaced to an Extranuclear quadrupole mass spectrometer as shown in Figure 1. The present setup can be operated as a conventional plasma chromatograph with a Ni @ source or with the laser ionization by changing the bias circuit and introducing a laser beam through the cell window. In the former mode of operation ion control grid one is biased to allow reactant ions formed due to the @ source into the ion/molecule reactor region where ionization occurs and the ions are subsequently injected into the drift region by pulsing grid 2. The typical pulse width used in these experiments was 0.2 or 0.5 ms and the drift distance
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
to the detector was 8.0 cm. The shorter pulse width provides higher resolution at the expense of sensitivity since the total number of ions injected per pulse is less. Thus, for direct measurements of the products of Ni /3 source ionization by the atmospheric pressure mass spectrometer, the sensitivity may be increased by nearly 1000 times by keeping both grids open and allowing all the ions to pass through for mass analysis. In the case of the laser ionization source, grid one must be biased to block ionization from the /3 source and grid two is left open to allow ions produced by the laser source to pass through to the collector. There is always a small continuous leak of high energy electrons through grid one which results in a relatively low continuous background from the ion/molecule reactor; however, this continuous dc background does not significantly affect the large pulsed signal obtained from the laser ionization. This continuous j3 source background does interfere with analysis in the mass spectrometer so that we have developed a circuit to gate open grid 2 in front of the ions produced by the laser pulse at the correct delay so that the laser-produced ions are transmitted and the background is minimized. The gate width in this experimentmay be varied from 0.5 to 15 ms where a narrower gate width will improve the SIN against the background interference. The drift distance from the center line of the laser beam to the ion collector is 12.5 cm. Suprasil quartz windows were used to transmit laser UV radiation for ionization. The IMS device was operated at above 200 O C at all times in order to keep the apparatus walls free from contamination. Generally the 30-pm inlet into the mass spectrometer was operated at 10 "C above the IMS body in order to prevent clogging and the quartz inlet to the IMS was operated at 275 "C. A countercurrent flow of liquid Nz boiloff is constantly flowed through the drift tube at -600 cm3/min in order to prevent clogging of the orifice and to keep the drift region clean in order to prevent further ion/molecule reactions in the region. The Nz is filtered through a molecular sieve trap in order to remove water vapor and other contaminants. The mass spectrometer is an Extranuclear quadrupole device that can be scanned up to mass 1200 and has a resolution up to 1OOO. In our experiments we generally scan over a range of 1-500 amu with unit resolution either in the positive or negative mode. The present apparatus is set up in the pulse-counting mode with a high-gain channeltron electron multiplier. Since we presently do not have a digital signal averager the signal is converted to an analog mode and signal averaging is performed by slowing scanning the mass spectrometer at a rate of -0.5 m u / s and the signal recorded on a chart recorder. In this averaging mode the rapid analysis capability is lost, but for the purpose of evaluating the ions produced by these techniques, this method is sufficient. The laser ionization source was the fourth harmonic (A = 266 nm) of the Quanta-Ray NdYAG laser. Typically 1-3 mJ of energy was used to obtain the mass spectra shown in this work. The sample concentrations were prepared using the diffusion tube method (19). Diffusion tubes (Vici Metronics, Santa Clara, CA) with an aperture of 0.5 cm and 7.62 cm in length were used to create samples in concentrations on the order of 10 ppm by flowing N2 over the diffusion tube enclosed in a stainless steel housing and through l/g in. tubing into the inlet of the IMS. Provided the carrier gas was flowing slowly (,+l
6
30
,
1ffiY'
I,
36 ,
,
,
I?
,
,
101 MH*
,
,
,
I06 M'
50
e 6
li6
, O P M +
4652
Flgure 6. API spectra of o-xylene at 220 OC where the ions are produced by either 63Ni p source or laser ionization at 266 nm in different drift gases where (a) air, (b) Ar, (c) P-10, and (d) COP.
I1
I
I
6 B
0
I
II
I
I II
1, I
Flgws 7. API spectrum of aniline at 89 OC in C 0 2 produced by laser ionization at 266 nm. Note the presence of CO, clusters.
species appear as shown in Figures 5 and 6 for the case of methylnaphthalene and o-xylene. Thus, it should be expected that the change in KOas a function of drift gas should be related to the (Y of that gas. There are several exceptions to this rule such as benzene and naphthalene; however, the reason for this is not clear from our data, which shows the formation of molecular ions with no unexpected species present. A more interesting case where the KOdoes not change as a function of a for a drift gas occurs when COPis used. In each case a long drift time is obtained in COzand thus rather broad peaks are obtained a t the detector. The most logical explanation for the unexpected variation with CY is the formation of large clusters of the highly polarizable COParound the ionic core. However, at 210 "C we see no such clusters in our mass spectrometer when either ionization source is used. If the temperature is lowered to 160 "C, no clusters are detected; however, at 89 "C significant clustering of ions with COPis reported as shown in Figure 7. Clustering of COz had been shown by Ellis e t al. (25) in mass analyzed atmospheric pressure ionization a t room temperature. At elevated temperature the ion-COP clusters are probably weak enough so that, upon expansion into the mass spectrometer under vacuum, the clusters dissociate to give the molecular ion. This probably occurs due tu collisions of the jet expansion with the focusing structure of the quadrupole which causes sufficient interference to destroy the jet and produce shock waves that break up these weak clusters. Thus, C02 may not be useful in many cases for separation in the IMS, but laser ionization
10
I
I
I
I
I
1
30
50
70
90
110
130
MASS (AMU)
Figure 8. API spectra of benzene in N2 at 220 OC where ionization is produced by (a) Laser ionizatlon at 266 nm and (b) 63Nip source.
can still be efficiently produced in COP and identification achieved by the molecular ion with little background interference at elevated temperatures in the mass spectrometer. Another important point is that the ionization induced by the laser source is less prone to memory effects than the Ni p source. This is basically for the same reason that the laser source ionization is less susceptible to background ion/molecule reactions, i.e., the laser ionization can be produced in a small volume near the clean drift region where the ions can be quickly separated as opposed to the 8-cm ion/molecule reactor where there is extensive time for such reactions to occur and which inherently is much closer to the contaminated source region. One example of this effect is shown in Figure 8b where benzene M+ cannot be detected by Ni /3 source ionization due to pyridine contamination as evidenced by the strong peak a t molecular weight 80. However, in Figure 8a where the laser ionization source is used, a strong M+ benzene peak at mass 78 is observed and no pyridine background is evident. A key feature of the laser ionization process is that one can produce direct efficient soft ionization with no fragmentation. We have found that even under fairly extreme power conditions at 266 nm, it was difficult to produce any substantial fragmentation. Up to 30 mJ of 266-nm light was focused into the IMS with a 30-cm focal length positive lens and no fragmentation was produced in aniline a t a concentration of -15 ppb in N2. This corresponds to a power density of 30 MW/cm2 and under comparable conditions in vacuum severe fragmentation to mainly C+ would be observed in a mass spectrometer (26). The mechanism for such fragmentation has been shown by Schlag et al. (27) to be due to initial ionization by R2PI to produce the molecular ion followed by subsequent absorption of additional photons which excite the molecular ion to highly excited ionic states that rapidly dissociate to produce both ionic and neutral fragments. Subsequent absorption of light by the ionic fragments will cause further fragmentation to smaller products. In previous work (23),the experiments of Schlag were repeated a t atmospheric pressure where an ultraviolet laser was used to produce mo-
2000
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
ANlLiNE (mw 93)
Drift Time (msec)
LASER 532 nm
(0)
NH
2.2
20
7
94 MH'
60
100
15
IO % .
140 180 220 M/Z
260
20
25
"
I
m e 9. API spectrum of aniline at 220 OC In N, where fragmentation has been produced by laser ionization at 532 nm.
lecular ions followed by a visible laser to indtice fragmentation. Our results indicate that excited ionic states M*+ induced by the second laser are rapidly quenched by collisions to the ground ionic state by the drift g& before fragmentation can occur. This lack of fragmentation a t high laser energies was found to be true in Nz,air, COz,Ar,and P-10 gas, and the mass spectrometer confirms the fact that no ionic mass fragments are formed in the IMS. In order to induce fragmentation, the visible 532-nm radiation from the second harmonic of a Nd:YAG laser was focused into the IMS with a 30 cm focal length lens at an input of nearly 100 mJ or >lo0 MW/cm2. The result shown in Figure 9 is that fragmentation can be induced in aniline but the smallest fragment observed is C&I,+ at mass 79 m u and no small fragments such as C+ are induced even under these extreme power conditions. No breakdown products of atmospheric constituents were observed in these experiments. The green light is generally more effective than UV light in producing fragmentation since organic ions generally absorb strongly in the visible region. The key point is that laser ionization at atmospheric pressure may serve as a means of softly ionizing molecules that might undergo extensive fragmentation in the ionization process. One very important chemical system studied in this work is dimethyl methylphosphonate ( D M m ) [CH,P(O)(OCH,),] since it serves as an analogue of many pesticides and nerve inhibitors (28-31). DMMP is also an interesting system in that it is a nonaromatic molecule that has an absorption at 266 nm in a hot bulb, but it also has an ionization potential of 10.4 eV so that radiation at 266 nm should not efficiently produce ionization. Using ionization by the Ni j3 source at 220 "C in dry N2, we observe one peak in the IMS at KO = 1.90, which corresponds to the DMMP monomer (Figure loa). The concentration in this figure is 15 ppb, but as the concentration is raised above this level, rapid formation of dimer occurs at the expense of the monomer and the dimer peak appears in the IMS at KO = 1.38 (Figure lob). As the concentration is raised more, only the dimer peak appears in the IMS (Figure 1Oc). This is believed to occur by first ionization of the monomer followed by formation of the dimer ion as shown by the long tail between peaks in the ion mobility spectrum, which shows the interaction between these two species. After formation of the monomer ion, the dimer is formed along the drift length as a function of time. In the mass spectrometer at low concentration, M+ and MH? are observed in a ratio of 1:2. At the lowest concentration levels no dimer peak is visible in the mass spectrometer; however, as the concentration is raised, a peak at M2H+appears (Figure lla). If the laser at 266 nm is used as the ionization source, then one peak at KO= 1.90 is observed a t low concentrations of 15 ppb and 100 ppb (Figure 10d,e) and in the mass spectrometer M+ and MH+ are observed (Figure Ilc,d). A t the same concentration only the dimer in the IMS is observed by using Ni /3 source ionization (Figure 10). The laser-ionized species are less prone to dimerization since they are immediately biased into the clean drift region. whereas with the
-
-
Figure 10. Ion mobility spectrum for DMMP at 220 OC in N, at a concentration of (a) 15 ppb, (b) 50 ppb, and (c) 100 ppb for ions produced by the %' I 0source and (d) 15 ppb and (e) 100 ppb for ions produced by laser radiation at 286 nm. IMMP CH, PIO)LOCH3)2(mwi24) ANI ,cone 15ppb
6JNi C O ~ C100 PPb
, 'p
>.
t in W
c
824
z
W
249 M2"*
I
.ASER 266 nm,
? C
t
COW I2SH"
w
I5Ppb
,
LASER 266 nm, C O ~ CIOOppb
d
125MH.
124
N 124 .
Flgure 11. API spectra of DMMP at 220 O C in N, (a) 15 ppb and (b) 100 ppb where ionization is produced by the e3Ni 0source and for (c) 15 ppb and (d) 100 ppb where ionization is produced by laser radiation
at
266
nm.
Ni j3 source ionization there is the ion/molecule reactor region in which the dimerization process can occur. Nevertheless, DMMP is a system that is susceptible to rapid dimerization. In the case of laser ionization, no fragmentation of DMMP was observed in the mass spectrometer even at the highest laser energies ( 30 mJ a t 266 nm, unfocused beam) used in this work. Laser ionization has also been studied in COz and P-10 gas and in C02we observe M+/MH+ in a ratio of 1:3 and (M + 44)H+ and small dimer peaks at 247 and 249. For P-10 we observe (M + 16)+and (M + 16)Hf in a ratio of 1:3 due to clustering with CHI and also dimer peaks at 246,247,248, and 249 at a concentration of 15 ppb. DMMP should not ionize efficiently at 266 nm because of its high ionization potential; however, ionization is observed due to hot band population of low energy modes that can provide ionization at much lower effective IP's (32). DMMP cannot be ionized at comparable laser energy under vacuum in an effusive beam (33). Below 150 OC only the dimer peak appears in the IMS N
Anal. Cheni. 1986, 58, 2001-2009
and only (M2- H)+ and MzH+appear in the mass spectrometer for both the Ni /3 and laser ionization source even a t the lowest concentration levels studied. Below 150 "C the laser ionization signal becomes very weak due to lack of hot-band population. Although direct ionization of the neutral dimer is possible, at low concentration, formation of M+ followed by MH+ + M MzH+ is believed to be the mechanism of dimer formation.
-
ACKNOWLEDGMENT We thank Michael Tierney for technical assistance given during the course of this work. Registry No. P-10,39390-76-6;DMMP, 756-79-6;COP,12438-9; N2, 7727-37-9; Ar, 7440-37-1; 63Ni,13981-37-8; benzene, 71-43-2; phenol, 108-95-2;toluene, 108-88-3;p-xylene, 106-42-3; rn-xylene, 108-38-3;o-xylene, 95-47-6;p-cresol, 106-44-5;m-cresol, 108-39-4; o-cresol, 95-48-7; aniline, 62-53-3; azulene, 275-51-4; naphthalene, 91-20-3; 2-methylnaphthalene, 91-57-6; indene, 95-13-6;4-picoline, 108-89-4;indoline,496-15-1; pyridine, 110-86-1; pyrazine, 290-37-9; triazine, 290-87-9; pyridazine, 289-80-5; quinoline, 91-22-5;isoquinoline, 119-65-3;quinoxaline, 91-19-0; 1-methylnaphthalene,90-12-0.
LITERATURE CITED Siegel, M. W.; Fite, W. L. J. Phys. Chem. 1978, 80, 2871-2881. Spangbr, G. E.; Lawless, P. A. Anal. Chem. 1978, 5 0 , 884-892. HIII, H. H., Jr.; Baim, M. A. In P&sma Chromatography Carr, T. W. Ed.; Plenum Press: New York, 1984; Chapter 5, pp 143-176. Mlsui, Y.; Kambara, H.; KoJima, M.; Tomita, H.; Katoh, K.; Satoh, K. Anal. Chem. 1983, 5 5 , 477-481. Kambara, H.; Ogawa, Y.; Mlsui, Y.; Kanomata, I.Anal. Chem. 1980, 5 2 , 1500. Iribarne, J. V.; Dziedzic, P. J.; Thomson, 8 . A. Int. J. Mass. Spectrom. Ion Phys. 1983. 5 0 , 331-347. Karasek, F. W. Int. J. Envlron. Anal. Chem. 1972, 2 , 157-166. Karasek; F. W.; Hill, H. H.; Kim, S. H. J. Chromatcgr. 1978, 117, 327-336. Meier, R. W. Am. Ind. Hyg. Assoc. J . 1978, 3 9 , 233-239. Carroll, D. I.; Dzidic, R. N.; Stillwell, M. G.; Hornlng, M. G.; Horning, E. C. Anal. Chem. 1974, 46, 706-710.
2001
Griffin, 0. W.; RzMlc, I.; Carroll, D.; Stillwell, R. N.; Hornlng, E. C. Anal. Chem. 1973, 45, 1204-1209. Lubman, D. M.; Kronlw, M. N. Anal. Chem. 1982. 5 4 , 1546-1551. Lubman, D. M.; Kronick, M. N. Anal. C h m . 1982, 5 4 , 2289-2291. Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982, 54, 660-665. Dietz, T. 0.; Duncan, M. A.; Liverman, M. 0.; Smaiby. R. E. Chem. Phys. Len. 1980, 7 0 , 246. Carr, T. W. I n Pksma Chromatography; Can, T. W., Ed.; Plenum Press: New York, 1984; Chapter 7, pp 215-235. Dam. R. J. I n Plasma Chromatwaphy; Carr. T. W., Ed.; Plenum Press: New York, 1984; Chapter 6, pp 177-213. Karasek, F. W. Anal. Chem. 1974, 46, 710A. Altshuller, A. P.; Cohen. J. R. Anal. &em. 1980, 32, 802. Carr, T. W. Anal. Chem. 1979, 51, 705. Revercomb, H. E.; Mason, E. A. Anal. Chem. 1980, 32, 802. Kim, S. H. PhD. Dissertation, 1978, The Guelph-Waterloo Centre for Graduate Work in Chemistry, Unhrerslty of Waterloo, Waterloo, Ontario, Canada N2L 3G1. Lubman, D. M. Anal. Chem. 1984, 5 6 , 1298. Tembreuii, R.; Sin, C. H.; Pang, H. M.; Lubman, D. M. Anal. Chem. l985i 5 7 , 2911-2917. Ellis, H. W.; Pai, R. Y.; Gatiand, I.R.; McDaniel, E. W.; Werniund, R.; Cohen. M. J. J. Chem. Phys. 1978, 64, 3935-3941. Lubman, D. M.; Naaman. R.; Zare. R. N. J. Chem. Phys. 1980, 72, 3035. Boesl, U.; Neusser. H. J.; Schiag, E. W. J. Chem. Phys. 1980, 72. 4327. Kim, S. H.; Spangler, 0. E. Anal. Chem. 198S, 5 7 , 567. Spangler, 0. E.; Campbell, D. N.; Carrlco, J. P. 1983 Pittsburgh Conference, Paper 641. Spangler, 0. E.; Suh, S. W.; Carrico, J. P. 1980 Pittsburgh Conference, Paper 549. Preston, J. M.; Karasek, F. W.; Kim, S. H. Anal. Chem. 1977, 49, 1746. Lubmah, D. M.; Kronick, M. N. Anal. Chem. 1983. 5 5 , 867-873. Lubman, D. M., unpublished results, The University of Michigan, 1985.
RECEIVED for review January 13, 1986. Accepted April 14, 1986. We gratefully acknowledge support of this work from the Army Research Office under Grant DAAG 29-85-K-1005 and also the Department of Defense through the US.Army Research Office under Grant No. DAAG 29-85-G-0018 for purchase of the equipment used in this work. We also acknowledge partial support under NSF Grant CHE 83-19383.
Methane High Pressure Collisional Activation Mass Spectrometry of Aromatic Hydrocarbons Karl F. Blom and Burnaby Munson* Department of Chemistry, University of Delaware, Newark, Delaware 19716
The hlgh-pressure colllslonal actlvatlon mass spectra wlth methane as the reagent/colllslon gas are reporled for benzene and eight alkylbenrenes. Colllslon energy resolved spectra are used to deduce dlssoclatlon pathways and structures of fragment Ions. The structures of fragment Ions are indicated by thelr colilslonally actloated decomposltlon patterns and by their chemlcal reactlvlty wlth methane. Fragmentationsof the protonated afkylbenzenes appear to be controlled by both thermochemical and klnetlc factors. Hlghgressure col#slonal acttvatkm spectra are compared wlth conventional CID mass spectra and the C I spectra resulting from exothermic proton transfer reactlons.
Chemical ionization mass spectrometry (CIMS) often produces fragmentation that is diagnostic of the structural features of the sample substrate as well as ions that provide 0003-2700/86/0358-2001$01.50/0
molecular weight information (1-4). Much of the research in CIMS has been toward the development of low-energy reagent ions which give very simple mass spectra (3-6). These spectra are useful for the qualitative and quantitative analysis of mixtures and for the determination of the molecular weight of the sample compound (7, 8). But, since these spectra contain essentially no fragmentation they usually provide no structural information. A few special cases have been reported in which the CI reagent ions react selectively with a particular functional group or combination of functionalities (8-1 1). These spectra can provide information about specific structural features of the sample substrate, but interpretation requires some prior knowledge of the sample. It is often necessary to use additional techniques that produce fragmentation to obtain the desired structural characterization. Selective CI reagent ions have frequently been used with collisional activation techniques to obtain the molecular weight and structural information (12-1 7). 0 1986 American Chemical Soclety
