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Time-resolved laser desorption mass spectrometry. I. Desorption of preformed ions
International Journal of Mass Spectrometty
Elsevier
Scientific
Publishing
TIME-RESOLVED I. DESORPTION
RICHARD
Company,
and Ion Physics, 49 (1983)
Amsterdam
- Printed
LASER DESORPTION OF PREFORMED IONS
B. VAN BREEMEN,
MARK
SNOW
35
35-50
in The Netherlands
MASS
and ROBERT
SPECTROMETRY.
J. COTTER
Department of Pharmacology and Experimental Therapeutics, The Johns Hopkins School of Medicine, 725 North Wolfe Street, Baltimore 21205 (U.S.A.)
(First
received
7 June
1982;
University
in final form 28 June 1982)
ABSTRACT A laser desorption time-of-flight (LD-TOF) mass spectrometer has been used to study the time-resolved desorption of nonvolatile organic salts following a laser pulse. An electron beam, pulsed prior to the pulse which draws the ions from the source, can be used to examine the neutral species desorbed by the laser. Experiments on tetraalkylammonium halides indicate desorption rates for neutral decomposition products vastly different from those for intact ions and neutral clusters, so that the method can be used to resolve competing thermal processes.
INTRODUCTION
A recent extensive review by Conzemius and Capellen [l] reveals the breadth of interest in the use of laser ionization in the mass-spectrometric analysis of solid materials. The major efforts using this instrumental combination have generally been elemental analyses of solid surfaces [2-5] and pyrolysis of nonvolatile compounds to produce volatile fragments in order to obtain structural information [6]. However, an article by Kistemaker et al. [7] in 1978 generated a great deal of interest in the analysis of large, nonvolatile bio-organic molecules, using conditions which produce in tact molecular ions. Similar to the case for field desorption (FD), they found the laser desorption (LD) technique able to produce molecular ion species from several oligosaccharides, giycosides and oligopeptides by alkali ion attachment. Since that time, there have been a number of contributions to the literature on the LD of thermally labile and nonvolatile compounds_ Stoll and Rollgen [8] used a continuous-wave CO, laser and a quadrupole mass spectrometer to produce molecular ions of carboxylic acids and tetra-n0020-738
1/83/0000-0000/$03.00
0 1983 Elsevier
Scientific
Publishing
Company
36
butylammonium salts * . Heresch et al. [9] and Cotter [lo] combined pulsed LD and mass scanning on sector instruments_ The latter effort, in our own laboratory, also introduced the combination of LD and chemical ionization (CI) [lo] which permitted a secondary means of soft ionization for the work indicated that intact neutral desorbed neutral species. Preliminary species are emitted from the probe surface for time periods longer than for molecular ions [ 111. More recently, Kistemaker and co-workers [ 12,131 conducted a number of experiments which were intended to establish the thermal nature of the desorption process. Heinen 1141, using a laser with a much shorter wavelength (256 nm), confirmed the thermal nature of ion formation by alkali attachment or by desorption of preformed ions from salts, but suggested that electronic (multiphoton) mechanisms may be needed to explain the formation of M+ from nonpolar aromatic solids. Cooks and co-workers [15] reported ionization by Ag+ ion attachment when solid samples are deposited onto silver foil with ammonium chloride. Hercules and co-workers [ 161 used a laser-microprobe mass spectrometer for the analysis of polar molecules, observing alkali and halide ion attachment in the positive and negative ion mass spectra [16], and have recently reviewed applications of this type of instrument in the analysis of solids [17]. In our own laboratory, we have been interested in the mechanisms involved in a variety of desorption techniques and particularly the significance of heating (or heating rates) for the ions observed in direct-exposure CI [18-201, thermal desorption (TD) [21,22], FD [23] and fast-atom bombardment (FAB) [23-251. Specifically, as applied to this current work in LD, we have attempted to investigate further: (1) the question of ion and neutral species lifetimes, i.e., the production of and competition between desorption of elemental ions (such as the alkali metals), molecular ions, intact neutral molecules, clusters, and decomposition products, with the purpose of exploiting not only the direct desorption of ions, but combination ionization techniques; and (2) the thermal aspects of LD as they pertain to the observation of “preformed ions” in the desorption mass spectra of organic salts. For these investigations of the nature of the desorption process, an instrumental configuration has been assembled which allows observation of ions by direct LD, and of ions plus neutral species by generating an
* Strictly speaking, organic salts such as the quaternary molecular ions. Rather, they produce ions representative
ammonium halides do not produce of the intact organic cation portion
of the salt. Since the cation observed is the same cation which exists in the solid or solution phase, these are referred to in this paper as “preformed ions”.
37
electron-beam pulse just prior to sampling the current contents of the ion source (LEI); and which involves a real-time data acquisition system which records the timing interval between the laser and drawout pulses as well as the time of flight (TOF) of the ions (and ionized neutral species) which originate from the same laser pulse. This technique is referred to as time-resolved laser desorption (TLD) mass spectrometry. ION/NEUTRAL
SPECIES
LIFETIMES
Several investigators have noted that neutral species are also desorbed from a surface by a laser pulse [26,27], and may be produced in greater abundance than ions. Vastola and Pirone [26] have also shown that desorbed neutral species are present for much longer time periods than are ions, after the laser pulse is completed_ In particular, they employed an 800 ~LSlaser pulse; ions were observed during the peak power of the pulse, while neutral species, detected by electron-impact (EI) ionization, were present for several hundred ,US after the laser pulse. Their observations were made using an “open” source in a TOF mass spectrometer. In our own laboratory, we combined these observations with the fact that at higher source pressures, ions are retained in the source for longer time periods [28]. We then demonstrated both the feasibility and advantages of combining a short pulsed laser and a scanning magnetic instrument to produce LD/CI mass spectra of polar compounds [lo]. In these preliminary studies, we showed that a laser pulse of - 0.7 J and 40 nS in length .produces neutral species which are ionized in the source (0.5 torr of isobutane) and emitted for the next 4-10 ms [lo]. Figure 1 schematically summarizes the results of these investigations, and indicates the extension of analysis times (following a laser pulse) resulting from ionization of neutral species, or using high pressures in the ion source. Time-resolved mass spectrometry was reported by Lincoln [29-3 l] as early as 1964, using a TOF instrument, but differs from the current work here in several respects. First, the work of Lincoln involved the vaporization of graphite to study the vaporization times of such species as C,H, and C,, rather than molecular ions and neutral species from nonvolatile organic compounds. Secondly, the techniques for producing time resolution were quite different from those in the current work. For complete mass spectra, the TOF mass spectrometer was operated in a repetitive mode and produced successive spectra as displaced oscilloscope traces, from which qualitative differences along the time scale could be observed. For more-quantitative results, selected-ion monitoring using pre-dynode gating to follow several prominent ions was used [30].
3x
LASER I
DESORPTfON -+
I
+
DESORPTION FROM
THE
SURFACE ILASER
4076
+A+--
]lONS
IpS
-
!schematic)
METHODS
PULSE] DESORBEDI
INEUTRALS
II:
IONS
A
LEAVING LOW
THE
SOURCE
(1) ELECTRON
(2)
B
ELECTRON
HIGH
SOURCE
(11 ELECTRON
(2)
ELECTRON
Fig. 1. Schematic mass-analyzed
DESORBED]
SOURCE PRESSURE BEAM
BEAM
OFF
BEAM
gas)
ILASER
IONIZATION]
ON
PRESSURE BEAM
(no reagent
(ISOBUTANEI
OFF
ILASER
IONIZATION]
ON
representation
of ions and neutral species desorbed
by a laser pulse, and
as a function of time, source pressure and whether or not an electron beam is
present.
DESORPTION
OF PREFORMED
IONS
The ease with which preformed cations and anions can be vaporized and analyzed by desorption techniques is now well recognized [32]. Prior to the use of secondary-ion mass spectrometry (SIMS), plasma desorption mass spectrometry (PDMS), LD, TD and FAB on such compounds, the quaternary ammonium halides were considered to be one of the more difficult classes of compounds- to analyze by mass spectrometry. EI and CI required prior volatilization of the neutral salt (R,NX), which generally resulted in decomposition to the neutral tertiary amine (R,N) through loss of IIX or RX (R = organic radical, X = halide). Ionization of this more volatile species
39
then produced R,N+ (El) or R,NH+ (CI), from which the structure of the parent cation was deduced [33 1. Two separate experiments were performed which indicated two alternative pathways resulting from heating quaternary ammonium salts, both of which result in observation of the intact molecular ion R,N+. First, Daves and co-workers [34] used rapid heating in combination with EI to observe the quaternary ammonium ion. The failure to observe the molecular ion without the electron beam led to the conclusion that some other neutral species, e.g., was generated under fast heating R,N+Xor a cluster (R,N+X-),, conditions, and that rapid heating enhanced this process over the decomposition to neutral tertiary amines. Secondly, Rollgen et al. [35] were able to observe the R,N+ ion in an FD source at very low field, an experiment which suggested (1) that such preformed ions are more easily desorbed in FD than are ions produced by electron abstraction, and (2) that such preformed ions might be observed by heating alone. Following these experiments, several laboratories reported the observation of quaternary ammonium ions by simple thermal processes [ 12,2 1,22,36-381. Cotter and Yergey 1211 have suggested that this thermal desorption of organic ions is explainable on the basis of a ‘modified’ Langmuir-Saha equation which considers the preformed nature of these ions and, rather than ionization energies, would use ionic bond strengths or lattice energies. This interpretation would predict that the (CH,),N+ ions should be more easily formed than Nat or Kf from their respective halide salts, and Fig. 2 illustrates this point. In a purely thermal experiment reported earlier [21], the quaternary ammonium ion is desorbed at lower heating currents than are
500
HEATER
-
.
.
I
I.5
2
4
;
A
CURRENT
Fig. 2. Thermal desorption sample-probe wire [2 11.
of (CH3),,N+,
K+
and Na+
as a function
of current through
K+ or Nat. As the heating current is increased, Kf (because of its lower ionization or lattice energy) desorbs more readily than Nat, while the quaternary ammonium ion intensity continues to increase. At still higher temperatures the production of Na+ and Kf reaches rates which reflect their natural abundance as impurities, while the (CH3)4N+ ion decreases owing to both depletion and competing decomposition processes. The time-resolved LD and LEI experiments reported here were designed to examine the desorbed ions and neutral species in a way which resolves the competing processes of decomposition [33] into tertiary amines and alkyl halides, evaporation of neutral ‘salt molecules’ or clusters described by Daves [34] as resulting from fast heating, and direct desorption of intact molecular ions as observed in the TD experiments of Stoll and Rollgen [36,37] and in our own laboratory [21,22]. Electron-beam pulsing (to distinguish ions and neutral species) and time-resolved ion extraction are used to distinguish the processes. EXPERIMENTAL
Instrumenta
tton
A block diagram of the instrument is shown in Fig. 3. The mass spectrometer is a standard CVC (Rochester, NY) Model 2000 TOF instrument, except that the flight tube has been extended to 2m. Basically three modifications were made to the mass spectrometer itself. (1) An entry port for the laser beam was constructed by mounting a 1” diameter germanium window on the normally blind flange opposite the direct-insertion probe. This flange was then connected rigidly to an optical bench on which a silicon mirror set at 45” could be finely adjusted to direct the laser beam onto the sample probe. (2) A new sample probe was constructed from stainless steel. The end of the probe was designed to hold Vespel (polyimide) rods - l-in. long and 1/l&in. diameter, so that the sample on the flat end of the rod could be placed in the area between the backing plate and the first ion-drawout plate. (3) The 10 kHz oscillator used to generate repetitive spectra was disabled and the mass spectrometer was instead made to respond to single external pulses from which it would then begin its own timing sequences. The analog scanner supplied with the instrument for recording spectra was not used, as it does not monitor events in real time. Using the terminology developed by Conzemius and Capellen [I], the geometric configuration of the LD source is 90/O, i.e., the angle + between the laser beam and sample surface is 40”, while the angle 8 between the ion optic axis and the sample surface is 0’. In addition, the angle between the
41
SfLlCON MI RRDR
MULTI
PLI
ER
SCOPE ANODE FILAMENT
ELECTRONICS
WAVEFORM
8-SIT PARALLEL INTERFACE
APPLE
II+
MICROCOMPUTER
Fig. 3. Block diagram
48~ II
of laser desorption
mass spectrometer.
beam (when it is used) and both the laser beam and ion optic axis is axis to 90”. The position of the sample is - 3-4 mm off the electron-beam prevent electron bombardment of the surface. This position is determined by monitoring the trap current and withdrawing the probe until it ceases to interfere with the electron beam. The laser is a Tachisto (Needham, MA) Model 215A CO, laser, with a pulse width of 40 ns and a maximum repetition rate of 1 Hz. The wavelength of the radiation is 10.6 pm and the energy of the unfocused pulse in these experiments was 0.1-0.5 J, as measured using a Scientech (Boulder, CO) Model 362 energy power meter. Analog signals resulting from detection of ions at the multiplier are digitized and stored in the 2048 channels of a Biomation (Cupertino, CA) Model 8100 waveform recorder which can access data at a rate of 0.01 ,us to 10 s/channel_ For the data discussed in this paper, the rate of analog to digital conversion was 0.05 ,us/channel, so that the digitized trace covered a period of 102.4 ~LS. The input to the waveform recorder was taken directly from the scope anode output of the mass spectrometer without additional amplification. The 50 a termination of the scope anode lead is provided internally in the waveform recorder input. The current contents of the electron
42
waveform recorder are displayed continuously on a Tektronix 7603 oscilloscope. Digitized spectra were transferred to an Apple II+ (Cupertino, CA) 48K RAM microcomputer as 2048 &bit words via a parallel interface. When multiple real-time spectra from several laser pulses were recorded, the amplitude from each channel was added to the total amplitudes from previous spectra in double precision, and stored in semiconductor memory as 1&bit words. In this manner the microcomputer/waveform-recorder combination served as a multichannel analyzer. Data transfer, addition and storage for each entire spectrum were accomplished in machine code and took - 1 s. Cumulative spectra were stored on a disk and could be reread for smoothing, plotting peak centroids, peak detection, mass assignment, normalizing or replotting. Base-peak and total ion intensities were displayed on the video display unit of the Apple II+. The input sensitivity of the waveform recorder was adjusted so that single ion peaks were just able to set the least significant bit (LSB) of the appropriate channel. At this sensitivity, the g-bit dynamic range of any single channel was rarely exceeded. By the addition of 20 spectra in double precision the total dynamic range is therefore slightly more than 5000; for 100 spectra, slightly more than 25000. In a particular spectrum, a total ion current of 200-2000 counts may be registered following a 40 ns laser pulse. This corresponds to - 5 X 104-5 X lo5 ions/s. In reality, ion- production rates are much higher, since, as discussed below, the sampling time is much shorter than the ion production time. Software for data acquisition and processing was stored on disks, and spectra were plotted 3n an Integral Data Systems (Milford, NH) Model 460 printer/plotter, or a AHewlett-Packard (San Diego, CA) Model 7044B X-Y recorder, controlled through a Mountain Hardware (Santa Cruz, CA) interface which contains 16 (analog to digital) input and 16 (digital to analog) output ports. One of the output ports is used to send a pulse to the time-delay circuitry, which begins the series of timing events necessary to produce an LD mass spectrum. Time-resolved
mass spectra
The actual circuitry used in the time-delay unit is similar to that used in the scanning mass spectrometer [lo], and has been described in detail elsewhere [37]. Its main function is to provide a delay between the pulse which triggers the laser and the pulse which initiates the mass-spectrometer the timing. In addition, several trigger pulses are available for beginning waveform recording_ The timing sequence is shown in Fig. 4. The’ circuit accepts a 5 V pulse
43
CKJCK PULSE *I
I-
230 ps
\\ ((
(FIXED)
LASER TRIGGER PULSE -I-ous
I-
\, I I
730 us VARIABLE DELAY PULSE
ELECTROd GRID PULSE
1 TO 5 115 la )I II
I
C
TOTAL DELAY TiME
_;--+
::
ION DRAWOUT PULSE
C-l
1
TO
500 fis TOF
-
SPECTRUM)
I I---+-
I Fig. 4. Timing diagram
for laser desorption
DATA TRANSFER 1.5 s
1
mass spectrometer.
from the Apple II+ and after a delay of 230 ps, triggers the laser with a 35 V pulse. A 5 V pulse for triggering the waveform recorder is also available at this time. Coarse and fine controls on the time-delay unit can be used to adjust a second delay pulse which is used to initiate the electron-grid and drawout pulses. In the normal mode for recording mass spectra this is set for a time after the laser trigger pulse which produces the best resolved and most intense mass-spectral trace on the oscilloscope. A pulse for triggering the waveform recorder is available at this point as well. If the waveform recorder is triggered at this point, and the delay is set before the laser trigger pulse, it is possible to ‘see’ the laser flash on the scope trace, and to set the coarse adjust to align the variable delay with the laser itself, since there is some finite delay between laser triggering and firing. Upon receiving the delay pulse, the mass spectrometer’s in ternal timing-circuit pulses the electron grid for l-5 ps. If the filament current is on, this will produce an electron beam for this time period, Otherwise, it will simply produce an additional delay. When the electron-grid pulse has been turned off, an ion-drawout pulse removes the ions from the source:. An output pulse for triggering the waveform recorder at this point is also available. There are three modes for triggering the waveform recorder. (1) When the ion-drawout pulse is used then the mass spectra only are recorded. Changes in delay times, electron-grid pulsing times, etc., will be reflected only in changes in the intensity and quality of the spectra, but the mass scale will
44
remain fixed. (2) When the variable-delay pulse is used, the mass scale will shift by an amount which corresponds to the length of time for which the electron beam is on. Finally (3), when the laser trigger pulse is used, the 100 ps time-frame records both the total delay time and the TOF (mass spectrum) of the ions. The drawout pulse produces a one-channel-wide noise pulse on the trace to separate the delay and flight times. The software detects this pulse to determine the delay time and assign masses based upon displacement from this channel. The spectrum is ‘time-resolved’, since the single digitized trace records both the full mass spectrum and the time between the laser pulse and ion extraction on the same time axis. At any particular delay setting, the delay pulse is reproduced at the same channel in successive spectra, so that the time resolution of the delay circuitry is 50 ns. Procedure
Samples were dissolved in methanol, generally as saturated solutions, and 2-3 ~1 of solution were deposited on the Vespel tip. Generally 20 spectra from 20 laser shots were acquired, added and displayed by the data system for each delay-time setting. Spectra were acquired with the electron beam on base-peak intensities and mass (LEI) and off (LD). Total ion currents, spectra are then stored on diskettes. RESULTS
Figure 5 shows the LD mass spectrum of tetratiethylammonium chloride. The electron beam was off. The waveform recorder was triggered by the drawout pulse so that the entire trace represents the TOF (mass spectrum) of the ions, The laser pulse energy was 0.1 J. The base peak is the molecular ion is observed. In addition, Naf and K+ (CH&N+ 3 and no fragmentation ions from impurities in the sample and/or probe tip are observed_ At this laser power, a cluster ion is also observed. Figure 6, on the other hand, is a ‘time-resolved’ LEI mass spectrum_ The waveform recorder has been triggered by the laser pulse, so that the mass scale is displaced 486 channels, corresponding to the 24.3 ,XS between the time the laser is pulsed and the ions are drawn out of the source. In addition, there are a number of other peaks in the spectrum since the ions recorded include ions desorbed directly by the laser, plus neutral species desorbed and then ionized by the electron-beam pulse prior to ion drawout. The molecular t is observed at m/z 74. The base peak consists of unresolved ion (CH,),N (59). Neutral CH,Cl is observed at (CH,),N+=CH, (58) and (CH,)3Nf masses 50 and 52. The broad peak at m/z 43 may consist of several
45
unresolved species, including CH2=N+=CH2 (42), CH,-N *=CH, (43) and CH,-N+ H=CH, (44). K+ is observed as m/t 39. The peak group below that may include +CH=NH, (29), CH2=N+HH (30) and +CH,-NH, (32).
Y
51.2
JJ’S
?
Fig. 5. Laser desorption
(LD)
energy=O.l
beam off, drawout pulse triggered.
J. Electron
mass spectrum of tettamethylammonium
chloride.
Laser pulse
ELECTRON
BEAMON
1
43
DRbCUTFU_SE o(AMLM LASER PULSE
x KC2 8 ~
I*
2048 CHANYELS x ,m ~‘5
Fig. 6. Time-resolved
laser desorption
/cHpsJyEL =
102.4 I-‘S
,I electron impact (LEI) mass spectrum of tetramethylam-
monium chloride. Laser pulse energy=0.5 J. Electron .beam on, laser pulse triggered. (This figure originally appeared in Trends in Analytical Chemistry, 1 (1982) 307.)
46
Finally, CH: and NH: general the ions observed in the EI flash desorption Desorption
are observed at m/z 15 and 18, respectively. In are those described by Daves and co-workers [34] mass spectrum of the same compound.
times for ions and neutral species
Initial experiments, in which the drawout pulse followed the laser pulse very closely, produced poor ion currents, while delays of 5- 10 ps produced better ion currents. Furthermore, intensities were unchanged when the electron beam was pulsed. Longer delay times produced two results. First, the abundances of molecular ions for the quaternary salts improved, relative to alkali ions, and secondly, the electron beam enhanced the intensity of the ions, indicating desorption of neutral precursors to these ions. These results are summarized in Fig. 7, which compares the desorption times of Na+ , Kf and (CH,),N+ under LD and LEI conditions. The spectra change more dramatically when longer desorption times are examined, as shown in Fig. 8. Above 50 ps delay no ions are observed without the electron beam. The neutral products (CH,),N and CH,Cl of the pyrolysis of the quaternary amines produce the ions shown in this figure.
L.ASER
ESORPTI
CM
Fig. 7. Intensity of ions observed during 30 ps following laser pulse. (This figure originally appeared in Trends in Analytical Chemistry, 1 (1982) 307.)
47
Fig. 8. Intensity of ions observed during first 500 ~LSfollowing laser pulse. (This figure originallyappearedin Trends in Analytical Chemistry, 1 (1982) 307.)
Effects
of anion on LD
spectra
Production of the molecular ion (CH3)4N+ by direct LD appeared to be relatively insensitive to whether the anion was Cl-, Br- , or BF,. At laser pulse energies of 0.3 J or more, the ion (CH,),-N=CH, (58) was observed under laser-only conditions for the chloride and bromide, but not for the tetrafluoroborate. The most outstanding feature was the intensity of the CH,i ion in the LEI mode which followed the order Cl- > Br- > BFL, perhaps reflecting its origins from desorbed methyl halide. DISCUSSION
Recently
Kistemaker
and co-workers
[
131 described
a thermal
model
for
the desorption of organic species coated on a surface, based upon the thermal effects of high-powered lasers on surfaces as described by Ready [40]. In this model, it is assumed that for a substrate having high thermal
adsorptivity, the presence of an organic layer does not influence the temperature distribution on the substrate. The laser energy is absorbed mainly in the substrate and transferred to the organic layer, and while the heat flux in the organic layer is small compared to the flux in the substrate, it is sufficient to heat the sample to the momentary substrate temperature [ 131. Using Kistemaker’s model, a sub-microsecond laser pulse will produce maximum temperature in the substrate almost immediately, but the dissipa-
48
tion of heat will require of the order of several ,us when the substrate is stainless steel. In our experiments, the use of Vespel as a substrate may produce a slightly longer time-constant for the temperature drop, and it is during this time that the ‘time-dependent’ desorption of ions and neutral species is observed. In Fig. 7 the maximum Na + ion desorption occurs at 5 (us, while the Kt ion reaches a maximum at - 12 ps. Since the ionization energy is higher for sodium (5.1 eV) than for potassium (4.4 eV), it can be assumed that the substrate temperature is higher at 5 ps than at 12. The maximum substrate temperature is undoubtedly reached earlier (( 1 ps), so that some of the time delay can be rationalized on the basis of the heat flux from substrate to sample. In addition, during the period from 3 to 8 ,us, the desorption is entirely ionic, which, according to the Langmuir-Saha equation [41], would correspond to the highest temperatures of the sample. The time-resolved spectra of tetramethylammonium chloride -itself can be used to explain the results observed in TD [21,22,36-381, EI/flash vaporization [34], and normal probe heating [3] of the quaternary amines. A neutral precursor for the (CH3),N+ ion, which simply contains one more electron, is probably unrealistic, since this even-electron ion would have to originate from a neutral species having an additional electron in an antibonding orbital. In addition, like the (CH,),N+ ion, IQ+ and Na+ exist as preformed ions in the form of chloride salts. In a previous discussion of TD, Cotter and Yergey [21] suggested that the gas-phase heats of formation of these ions, or the lattice energies using the same anion, might be more appropriate predictors of the relative energies necessary for cation desorption. Using the lattice energies for NaCl (8.0 eV), KC1 (7.2 eV) and (CH,),NCl (C 6.6 eV) [21], desorption of the (CH3)4Nf ion should. occur at lower temperatures than for K + ions, which in turn should be desorbed at lower temperatures than Na + ions. Figure 7 verifies this order of desorption. experiments of Daves and co-workers [34] are The El/f1 us h vduatifization simulated by the appearance of these same ions using LD combined with an electron-beam pulse. As predicted in their experiments, these occur during a very short time, and as a result of rapid heating. Finally, the normal probe heating of quaternary amines is reflected in the longer time (Fig. 8) in which thermal decomposition results in the production of the volatile tertiary amine (CH,),N through the loss of CH,Cl. Ions formed from both of these neutral pyrolysis products are observed. CONCLUSIONS
Despite their tendency to decompose upon heating, the quaternary ammonium cations are easily desorbed intact under conditions which are milder
49
than those for the desorption of Na+ and K+ ions [21]. The ease with which ‘preformed’ ions can be desorbed has been observed for all of the desorption methods, so that the results reported here may have implications for the other methods. The amount of neutral species generated is considerable. At 20 ps delay (a point at which the molecular ion is optimal for both LD and LEI spectra of the tetraalkylammonium salts) the total ion currents with the electron beam on are from 6 to 20 times the currents from the laser desorbed ions alone. Since probably only a small fraction of the neutral species actually become ionized by the electron beam, it is clear that neutral species are the major product of the desorption event. More importantly, the kinds of neutral species desorbed depend a great deal upon the temperature and/or heating rate. TD techniques which employ heating of wires [21,22] may produce both pyrolysis products and sublimed clusters in much greater abundance than preformed ions. The neutral species are not seen with the electron beam off. On the other hand, the particle bombardment techniques (SIMS, PDMS and FAB) produce instantaneous localized collision events, perhaps followed by rapid increases in temperature in the areas surrounding the collision, as a secondary event [42]. The rapid transfer of heat from the collisionally damaged substrate to the sample might be expected to encourage the desorption of preformed ions and neutral clusters, but perhaps to minimize the formation of neutral pyrolysis products. For the LD work itself there appear to be two basic conclusions. The first is that the large number of intact neutral species which are desorbed suggest the future utility of using secondary means of ionization, such as EI or CI. There have in fact been several reports of laser-assisted FD studies [43,44]. The second conclusion is that a thermal model continues to work well as a description of LD since the time for desorption is not appropriate for an electric-field or spectroscopic explanation. Considering the time-resolved spectra, it is understandable that lasers used for so long to pyrolyze organic samples are now being used to desorb molecular ions intact! ACKNOWLEDGEMENT
This Science DHHS.
work was funded by Grants CHE So-16440 from the National Foundation, and CA 09243 from the National Cancer Institute,
REFERENCES 1 R.J. Conzemius and J.M. Capellen, Int. J. Mass Spectrom. Ion Phys., 34 (1980) 147. 2 N.C. Fenner and N.R. Daly, Rev. Sci. Instrum., 37 (1966) 1735.
50 3 4 5 6 7 8 9 10 11 12 13 14 1.5 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
R.A. Gingham and P.L. Salter, Anal. Chem., 48 (1976) 1735. R.J. Conzemius and H.J. Svec, Anal. Chem., 50 (1978) 1854. R.H. Scott, P.F.S. Jackson and A. Strasheim, Nature (London). 232 (1971) 623. M.A. Posthumus, N.M.M. Nibbering, A.J.H. Boerboom and H.-R. Schulten, Biomed. Mass Spectrom., 1 (1974) 352. M.A. Posthumus, P.G. Kistemaker, H.L.C. Meuzelaar and M.C. Ten Noever de Brauw, Anal. Chem., 50n (1978) 985. R. Stoll and F.W. Rollgen; Org. Mass Spectrom., 14 (1979) 642. F. Heresch, E.R. Schmid and J.F.K. Huber, Anal. Chem.. 52 (1980) 1803. R.J. Cotter, Anal. Chkm., 52 (1980) 1767. R.J. Cotter, Anal. Chem., 53 (1981) 719. G.J.Q. van der Peyl, K. Isa, J. Haverkamp and P.G. Kistemaker, Org. Mass Spectrom., 16 (1981) 416. G.J.Q. van der Peyl, J. Haverkamp and P.G. Kistemaker, Int. J. Mass Spectrom. Ion Phys., 42 (1982) 125. H.J. Heinen, Int. J. Mass Spectrom. Ion Phys., 38 (1981) 309. D. Zackett, A.E. Schoen, P.H. Hemberger and R.G. Cooks, J. Am. Chem. Sot., 103 (1981) 1295. S.W. Graham, P. Dowd and D.M. Hercules, Anal. Chem., 54 (1982) 649. D.M. Hercules, R.J. Day, K. Balasanmugan. T.Z. Dong and C.P. Li, Anal. Chem.. 54 ( 1982) 280A. R.J. Cotter, Anal. Chem., 5 1 (1979) 3 17. R.J. Cotter and C. Fenselau, Biomed. Mass Spectrom., 6 (1979) 287. R.J. Cotter, Anal. Chem., 52 (1980) 1589A. R.J. Cotter and A.L. Yergey, J. Am. Chem. Sot., 103 (1981) 1596. R.J. Cotter and A.L. Yergey, Anal. Chem., 53 (198 1) 1306. R.J. Cotter, G.E. Hansen and T.R. Jones, Anal. Chim. Acta, 136 (1982) 135. C. Fenselau and R.J. Cotter, in K.J. Laidler (Ed.), IUPAC: Frontiers in Chemistry, Pergamon, New York, 1982, pp_ 207-216. V.T. Vu, C. Fenselau and O.M. Colvin, J. Am. Chem. Sot., 103 (1981) 7362. F.J. Vastola and A.J. Pirone,.Adv. Mass Spectrom., 4 (1968) 107. R.O. Mumma and F.J. Vastola, Org. Mass Spectrom., 6 (1972) 1373. R.C. Dunbar and P. Ormentrout, Int. J. Mass Spectrom. Ion Phys., 24 (1977) 465. K.A. Lincoln, Rev. Sci. Instrum., 35 (1964) 1688. K.A. Lincoln, Int. J. Mass Spectrom. Ion Phys., 2 (1969) 75. K.A. Lincoln, Int. J. Mass Spectrom. Ion Phys., 13 (1974) 45. R.J. Cotter, Trends Anal. Chem., 1 (1982) 307. J_ Shabanowitz, P. Byrnes, A. Melicke, D.V. Bowen and F.H. Field, Biomed. Mass Spectrom., 2 (1975) 164. T.D. Lee, W.R. Anderson, Jr. and G.D. Daves, Jr., Anal. Chem., 53 (19Sl) 304. F.W. Rollgen, U. Giessmann, H.J. Heinen and S.J. Reddy, Int. J. Mass Spectrom. Ion Phys., 24 (1977) 235. R. Stoll and F.W. Rollgen, J. Chem. Sot., Chem. Commun., (1980) 789. R. St011 and F.W. Rollgen, Org. Mass Spectrom., 16 (1981) 72. M. Ohashi, R.P. Barron and W.R. Benson, J. Am. Chem. Sot., 103 (1981) 3943. R.J. Cotter, Chem. Biomed. Environ Instrum., 11 (1981) 57. J.F. Ready, Effects of High Power Laser Radiation, Academic Press, New York, 1981. I. Langmuir and K.H. Kingdon, Proc. R. Sac. (London), Ser. A, 6 1 ( 1925) 107. C.J. McNeal, Anal. Chem., 54 (1982) 43A. H.-R. Schulten, P.B. Monkhouse and R. Muller. Anal. Chem., 54 (1982) 654. H.-R. Schulten, W.D. Lehmann and D. Haaks, Org. Mass Spectrom., 13 (1978) 361.
Elsevier
Scientific
Publishing
TIME-RESOLVED I. DESORPTION
RICHARD
Company,
and Ion Physics, 49 (1983)
Amsterdam
- Printed
LASER DESORPTION OF PREFORMED IONS
B. VAN BREEMEN,
MARK
SNOW
35
35-50
in The Netherlands
MASS
and ROBERT
SPECTROMETRY.
J. COTTER
Department of Pharmacology and Experimental Therapeutics, The Johns Hopkins School of Medicine, 725 North Wolfe Street, Baltimore 21205 (U.S.A.)
(First
received
7 June
1982;
University
in final form 28 June 1982)
ABSTRACT A laser desorption time-of-flight (LD-TOF) mass spectrometer has been used to study the time-resolved desorption of nonvolatile organic salts following a laser pulse. An electron beam, pulsed prior to the pulse which draws the ions from the source, can be used to examine the neutral species desorbed by the laser. Experiments on tetraalkylammonium halides indicate desorption rates for neutral decomposition products vastly different from those for intact ions and neutral clusters, so that the method can be used to resolve competing thermal processes.
INTRODUCTION
A recent extensive review by Conzemius and Capellen [l] reveals the breadth of interest in the use of laser ionization in the mass-spectrometric analysis of solid materials. The major efforts using this instrumental combination have generally been elemental analyses of solid surfaces [2-5] and pyrolysis of nonvolatile compounds to produce volatile fragments in order to obtain structural information [6]. However, an article by Kistemaker et al. [7] in 1978 generated a great deal of interest in the analysis of large, nonvolatile bio-organic molecules, using conditions which produce in tact molecular ions. Similar to the case for field desorption (FD), they found the laser desorption (LD) technique able to produce molecular ion species from several oligosaccharides, giycosides and oligopeptides by alkali ion attachment. Since that time, there have been a number of contributions to the literature on the LD of thermally labile and nonvolatile compounds_ Stoll and Rollgen [8] used a continuous-wave CO, laser and a quadrupole mass spectrometer to produce molecular ions of carboxylic acids and tetra-n0020-738
1/83/0000-0000/$03.00
0 1983 Elsevier
Scientific
Publishing
Company
36
butylammonium salts * . Heresch et al. [9] and Cotter [lo] combined pulsed LD and mass scanning on sector instruments_ The latter effort, in our own laboratory, also introduced the combination of LD and chemical ionization (CI) [lo] which permitted a secondary means of soft ionization for the work indicated that intact neutral desorbed neutral species. Preliminary species are emitted from the probe surface for time periods longer than for molecular ions [ 111. More recently, Kistemaker and co-workers [ 12,131 conducted a number of experiments which were intended to establish the thermal nature of the desorption process. Heinen 1141, using a laser with a much shorter wavelength (256 nm), confirmed the thermal nature of ion formation by alkali attachment or by desorption of preformed ions from salts, but suggested that electronic (multiphoton) mechanisms may be needed to explain the formation of M+ from nonpolar aromatic solids. Cooks and co-workers [15] reported ionization by Ag+ ion attachment when solid samples are deposited onto silver foil with ammonium chloride. Hercules and co-workers [ 161 used a laser-microprobe mass spectrometer for the analysis of polar molecules, observing alkali and halide ion attachment in the positive and negative ion mass spectra [16], and have recently reviewed applications of this type of instrument in the analysis of solids [17]. In our own laboratory, we have been interested in the mechanisms involved in a variety of desorption techniques and particularly the significance of heating (or heating rates) for the ions observed in direct-exposure CI [18-201, thermal desorption (TD) [21,22], FD [23] and fast-atom bombardment (FAB) [23-251. Specifically, as applied to this current work in LD, we have attempted to investigate further: (1) the question of ion and neutral species lifetimes, i.e., the production of and competition between desorption of elemental ions (such as the alkali metals), molecular ions, intact neutral molecules, clusters, and decomposition products, with the purpose of exploiting not only the direct desorption of ions, but combination ionization techniques; and (2) the thermal aspects of LD as they pertain to the observation of “preformed ions” in the desorption mass spectra of organic salts. For these investigations of the nature of the desorption process, an instrumental configuration has been assembled which allows observation of ions by direct LD, and of ions plus neutral species by generating an
* Strictly speaking, organic salts such as the quaternary molecular ions. Rather, they produce ions representative
ammonium halides do not produce of the intact organic cation portion
of the salt. Since the cation observed is the same cation which exists in the solid or solution phase, these are referred to in this paper as “preformed ions”.
37
electron-beam pulse just prior to sampling the current contents of the ion source (LEI); and which involves a real-time data acquisition system which records the timing interval between the laser and drawout pulses as well as the time of flight (TOF) of the ions (and ionized neutral species) which originate from the same laser pulse. This technique is referred to as time-resolved laser desorption (TLD) mass spectrometry. ION/NEUTRAL
SPECIES
LIFETIMES
Several investigators have noted that neutral species are also desorbed from a surface by a laser pulse [26,27], and may be produced in greater abundance than ions. Vastola and Pirone [26] have also shown that desorbed neutral species are present for much longer time periods than are ions, after the laser pulse is completed_ In particular, they employed an 800 ~LSlaser pulse; ions were observed during the peak power of the pulse, while neutral species, detected by electron-impact (EI) ionization, were present for several hundred ,US after the laser pulse. Their observations were made using an “open” source in a TOF mass spectrometer. In our own laboratory, we combined these observations with the fact that at higher source pressures, ions are retained in the source for longer time periods [28]. We then demonstrated both the feasibility and advantages of combining a short pulsed laser and a scanning magnetic instrument to produce LD/CI mass spectra of polar compounds [lo]. In these preliminary studies, we showed that a laser pulse of - 0.7 J and 40 nS in length .produces neutral species which are ionized in the source (0.5 torr of isobutane) and emitted for the next 4-10 ms [lo]. Figure 1 schematically summarizes the results of these investigations, and indicates the extension of analysis times (following a laser pulse) resulting from ionization of neutral species, or using high pressures in the ion source. Time-resolved mass spectrometry was reported by Lincoln [29-3 l] as early as 1964, using a TOF instrument, but differs from the current work here in several respects. First, the work of Lincoln involved the vaporization of graphite to study the vaporization times of such species as C,H, and C,, rather than molecular ions and neutral species from nonvolatile organic compounds. Secondly, the techniques for producing time resolution were quite different from those in the current work. For complete mass spectra, the TOF mass spectrometer was operated in a repetitive mode and produced successive spectra as displaced oscilloscope traces, from which qualitative differences along the time scale could be observed. For more-quantitative results, selected-ion monitoring using pre-dynode gating to follow several prominent ions was used [30].
3x
LASER I
DESORPTfON -+
I
+
DESORPTION FROM
THE
SURFACE ILASER
4076
+A+--
]lONS
IpS
-
!schematic)
METHODS
PULSE] DESORBEDI
INEUTRALS
II:
IONS
A
LEAVING LOW
THE
SOURCE
(1) ELECTRON
(2)
B
ELECTRON
HIGH
SOURCE
(11 ELECTRON
(2)
ELECTRON
Fig. 1. Schematic mass-analyzed
DESORBED]
SOURCE PRESSURE BEAM
BEAM
OFF
BEAM
gas)
ILASER
IONIZATION]
ON
PRESSURE BEAM
(no reagent
(ISOBUTANEI
OFF
ILASER
IONIZATION]
ON
representation
of ions and neutral species desorbed
by a laser pulse, and
as a function of time, source pressure and whether or not an electron beam is
present.
DESORPTION
OF PREFORMED
IONS
The ease with which preformed cations and anions can be vaporized and analyzed by desorption techniques is now well recognized [32]. Prior to the use of secondary-ion mass spectrometry (SIMS), plasma desorption mass spectrometry (PDMS), LD, TD and FAB on such compounds, the quaternary ammonium halides were considered to be one of the more difficult classes of compounds- to analyze by mass spectrometry. EI and CI required prior volatilization of the neutral salt (R,NX), which generally resulted in decomposition to the neutral tertiary amine (R,N) through loss of IIX or RX (R = organic radical, X = halide). Ionization of this more volatile species
39
then produced R,N+ (El) or R,NH+ (CI), from which the structure of the parent cation was deduced [33 1. Two separate experiments were performed which indicated two alternative pathways resulting from heating quaternary ammonium salts, both of which result in observation of the intact molecular ion R,N+. First, Daves and co-workers [34] used rapid heating in combination with EI to observe the quaternary ammonium ion. The failure to observe the molecular ion without the electron beam led to the conclusion that some other neutral species, e.g., was generated under fast heating R,N+Xor a cluster (R,N+X-),, conditions, and that rapid heating enhanced this process over the decomposition to neutral tertiary amines. Secondly, Rollgen et al. [35] were able to observe the R,N+ ion in an FD source at very low field, an experiment which suggested (1) that such preformed ions are more easily desorbed in FD than are ions produced by electron abstraction, and (2) that such preformed ions might be observed by heating alone. Following these experiments, several laboratories reported the observation of quaternary ammonium ions by simple thermal processes [ 12,2 1,22,36-381. Cotter and Yergey 1211 have suggested that this thermal desorption of organic ions is explainable on the basis of a ‘modified’ Langmuir-Saha equation which considers the preformed nature of these ions and, rather than ionization energies, would use ionic bond strengths or lattice energies. This interpretation would predict that the (CH,),N+ ions should be more easily formed than Nat or Kf from their respective halide salts, and Fig. 2 illustrates this point. In a purely thermal experiment reported earlier [21], the quaternary ammonium ion is desorbed at lower heating currents than are
500
HEATER
-
.
.
I
I.5
2
4
;
A
CURRENT
Fig. 2. Thermal desorption sample-probe wire [2 11.
of (CH3),,N+,
K+
and Na+
as a function
of current through
K+ or Nat. As the heating current is increased, Kf (because of its lower ionization or lattice energy) desorbs more readily than Nat, while the quaternary ammonium ion intensity continues to increase. At still higher temperatures the production of Na+ and Kf reaches rates which reflect their natural abundance as impurities, while the (CH3)4N+ ion decreases owing to both depletion and competing decomposition processes. The time-resolved LD and LEI experiments reported here were designed to examine the desorbed ions and neutral species in a way which resolves the competing processes of decomposition [33] into tertiary amines and alkyl halides, evaporation of neutral ‘salt molecules’ or clusters described by Daves [34] as resulting from fast heating, and direct desorption of intact molecular ions as observed in the TD experiments of Stoll and Rollgen [36,37] and in our own laboratory [21,22]. Electron-beam pulsing (to distinguish ions and neutral species) and time-resolved ion extraction are used to distinguish the processes. EXPERIMENTAL
Instrumenta
tton
A block diagram of the instrument is shown in Fig. 3. The mass spectrometer is a standard CVC (Rochester, NY) Model 2000 TOF instrument, except that the flight tube has been extended to 2m. Basically three modifications were made to the mass spectrometer itself. (1) An entry port for the laser beam was constructed by mounting a 1” diameter germanium window on the normally blind flange opposite the direct-insertion probe. This flange was then connected rigidly to an optical bench on which a silicon mirror set at 45” could be finely adjusted to direct the laser beam onto the sample probe. (2) A new sample probe was constructed from stainless steel. The end of the probe was designed to hold Vespel (polyimide) rods - l-in. long and 1/l&in. diameter, so that the sample on the flat end of the rod could be placed in the area between the backing plate and the first ion-drawout plate. (3) The 10 kHz oscillator used to generate repetitive spectra was disabled and the mass spectrometer was instead made to respond to single external pulses from which it would then begin its own timing sequences. The analog scanner supplied with the instrument for recording spectra was not used, as it does not monitor events in real time. Using the terminology developed by Conzemius and Capellen [I], the geometric configuration of the LD source is 90/O, i.e., the angle + between the laser beam and sample surface is 40”, while the angle 8 between the ion optic axis and the sample surface is 0’. In addition, the angle between the
41
SfLlCON MI RRDR
MULTI
PLI
ER
SCOPE ANODE FILAMENT
ELECTRONICS
WAVEFORM
8-SIT PARALLEL INTERFACE
APPLE
II+
MICROCOMPUTER
Fig. 3. Block diagram
48~ II
of laser desorption
mass spectrometer.
beam (when it is used) and both the laser beam and ion optic axis is axis to 90”. The position of the sample is - 3-4 mm off the electron-beam prevent electron bombardment of the surface. This position is determined by monitoring the trap current and withdrawing the probe until it ceases to interfere with the electron beam. The laser is a Tachisto (Needham, MA) Model 215A CO, laser, with a pulse width of 40 ns and a maximum repetition rate of 1 Hz. The wavelength of the radiation is 10.6 pm and the energy of the unfocused pulse in these experiments was 0.1-0.5 J, as measured using a Scientech (Boulder, CO) Model 362 energy power meter. Analog signals resulting from detection of ions at the multiplier are digitized and stored in the 2048 channels of a Biomation (Cupertino, CA) Model 8100 waveform recorder which can access data at a rate of 0.01 ,us to 10 s/channel_ For the data discussed in this paper, the rate of analog to digital conversion was 0.05 ,us/channel, so that the digitized trace covered a period of 102.4 ~LS. The input to the waveform recorder was taken directly from the scope anode output of the mass spectrometer without additional amplification. The 50 a termination of the scope anode lead is provided internally in the waveform recorder input. The current contents of the electron
42
waveform recorder are displayed continuously on a Tektronix 7603 oscilloscope. Digitized spectra were transferred to an Apple II+ (Cupertino, CA) 48K RAM microcomputer as 2048 &bit words via a parallel interface. When multiple real-time spectra from several laser pulses were recorded, the amplitude from each channel was added to the total amplitudes from previous spectra in double precision, and stored in semiconductor memory as 1&bit words. In this manner the microcomputer/waveform-recorder combination served as a multichannel analyzer. Data transfer, addition and storage for each entire spectrum were accomplished in machine code and took - 1 s. Cumulative spectra were stored on a disk and could be reread for smoothing, plotting peak centroids, peak detection, mass assignment, normalizing or replotting. Base-peak and total ion intensities were displayed on the video display unit of the Apple II+. The input sensitivity of the waveform recorder was adjusted so that single ion peaks were just able to set the least significant bit (LSB) of the appropriate channel. At this sensitivity, the g-bit dynamic range of any single channel was rarely exceeded. By the addition of 20 spectra in double precision the total dynamic range is therefore slightly more than 5000; for 100 spectra, slightly more than 25000. In a particular spectrum, a total ion current of 200-2000 counts may be registered following a 40 ns laser pulse. This corresponds to - 5 X 104-5 X lo5 ions/s. In reality, ion- production rates are much higher, since, as discussed below, the sampling time is much shorter than the ion production time. Software for data acquisition and processing was stored on disks, and spectra were plotted 3n an Integral Data Systems (Milford, NH) Model 460 printer/plotter, or a AHewlett-Packard (San Diego, CA) Model 7044B X-Y recorder, controlled through a Mountain Hardware (Santa Cruz, CA) interface which contains 16 (analog to digital) input and 16 (digital to analog) output ports. One of the output ports is used to send a pulse to the time-delay circuitry, which begins the series of timing events necessary to produce an LD mass spectrum. Time-resolved
mass spectra
The actual circuitry used in the time-delay unit is similar to that used in the scanning mass spectrometer [lo], and has been described in detail elsewhere [37]. Its main function is to provide a delay between the pulse which triggers the laser and the pulse which initiates the mass-spectrometer the timing. In addition, several trigger pulses are available for beginning waveform recording_ The timing sequence is shown in Fig. 4. The’ circuit accepts a 5 V pulse
43
CKJCK PULSE *I
I-
230 ps
\\ ((
(FIXED)
LASER TRIGGER PULSE -I-ous
I-
\, I I
730 us VARIABLE DELAY PULSE
ELECTROd GRID PULSE
1 TO 5 115 la )I II
I
C
TOTAL DELAY TiME
_;--+
::
ION DRAWOUT PULSE
C-l
1
TO
500 fis TOF
-
SPECTRUM)
I I---+-
I Fig. 4. Timing diagram
for laser desorption
DATA TRANSFER 1.5 s
1
mass spectrometer.
from the Apple II+ and after a delay of 230 ps, triggers the laser with a 35 V pulse. A 5 V pulse for triggering the waveform recorder is also available at this time. Coarse and fine controls on the time-delay unit can be used to adjust a second delay pulse which is used to initiate the electron-grid and drawout pulses. In the normal mode for recording mass spectra this is set for a time after the laser trigger pulse which produces the best resolved and most intense mass-spectral trace on the oscilloscope. A pulse for triggering the waveform recorder is available at this point as well. If the waveform recorder is triggered at this point, and the delay is set before the laser trigger pulse, it is possible to ‘see’ the laser flash on the scope trace, and to set the coarse adjust to align the variable delay with the laser itself, since there is some finite delay between laser triggering and firing. Upon receiving the delay pulse, the mass spectrometer’s in ternal timing-circuit pulses the electron grid for l-5 ps. If the filament current is on, this will produce an electron beam for this time period, Otherwise, it will simply produce an additional delay. When the electron-grid pulse has been turned off, an ion-drawout pulse removes the ions from the source:. An output pulse for triggering the waveform recorder at this point is also available. There are three modes for triggering the waveform recorder. (1) When the ion-drawout pulse is used then the mass spectra only are recorded. Changes in delay times, electron-grid pulsing times, etc., will be reflected only in changes in the intensity and quality of the spectra, but the mass scale will
44
remain fixed. (2) When the variable-delay pulse is used, the mass scale will shift by an amount which corresponds to the length of time for which the electron beam is on. Finally (3), when the laser trigger pulse is used, the 100 ps time-frame records both the total delay time and the TOF (mass spectrum) of the ions. The drawout pulse produces a one-channel-wide noise pulse on the trace to separate the delay and flight times. The software detects this pulse to determine the delay time and assign masses based upon displacement from this channel. The spectrum is ‘time-resolved’, since the single digitized trace records both the full mass spectrum and the time between the laser pulse and ion extraction on the same time axis. At any particular delay setting, the delay pulse is reproduced at the same channel in successive spectra, so that the time resolution of the delay circuitry is 50 ns. Procedure
Samples were dissolved in methanol, generally as saturated solutions, and 2-3 ~1 of solution were deposited on the Vespel tip. Generally 20 spectra from 20 laser shots were acquired, added and displayed by the data system for each delay-time setting. Spectra were acquired with the electron beam on base-peak intensities and mass (LEI) and off (LD). Total ion currents, spectra are then stored on diskettes. RESULTS
Figure 5 shows the LD mass spectrum of tetratiethylammonium chloride. The electron beam was off. The waveform recorder was triggered by the drawout pulse so that the entire trace represents the TOF (mass spectrum) of the ions, The laser pulse energy was 0.1 J. The base peak is the molecular ion is observed. In addition, Naf and K+ (CH&N+ 3 and no fragmentation ions from impurities in the sample and/or probe tip are observed_ At this laser power, a cluster ion is also observed. Figure 6, on the other hand, is a ‘time-resolved’ LEI mass spectrum_ The waveform recorder has been triggered by the laser pulse, so that the mass scale is displaced 486 channels, corresponding to the 24.3 ,XS between the time the laser is pulsed and the ions are drawn out of the source. In addition, there are a number of other peaks in the spectrum since the ions recorded include ions desorbed directly by the laser, plus neutral species desorbed and then ionized by the electron-beam pulse prior to ion drawout. The molecular t is observed at m/z 74. The base peak consists of unresolved ion (CH,),N (59). Neutral CH,Cl is observed at (CH,),N+=CH, (58) and (CH,)3Nf masses 50 and 52. The broad peak at m/z 43 may consist of several
45
unresolved species, including CH2=N+=CH2 (42), CH,-N *=CH, (43) and CH,-N+ H=CH, (44). K+ is observed as m/t 39. The peak group below that may include +CH=NH, (29), CH2=N+HH (30) and +CH,-NH, (32).
Y
51.2
JJ’S
?
Fig. 5. Laser desorption
(LD)
energy=O.l
beam off, drawout pulse triggered.
J. Electron
mass spectrum of tettamethylammonium
chloride.
Laser pulse
ELECTRON
BEAMON
1
43
DRbCUTFU_SE o(AMLM LASER PULSE
x KC2 8 ~
I*
2048 CHANYELS x ,m ~‘5
Fig. 6. Time-resolved
laser desorption
/cHpsJyEL =
102.4 I-‘S
,I electron impact (LEI) mass spectrum of tetramethylam-
monium chloride. Laser pulse energy=0.5 J. Electron .beam on, laser pulse triggered. (This figure originally appeared in Trends in Analytical Chemistry, 1 (1982) 307.)
46
Finally, CH: and NH: general the ions observed in the EI flash desorption Desorption
are observed at m/z 15 and 18, respectively. In are those described by Daves and co-workers [34] mass spectrum of the same compound.
times for ions and neutral species
Initial experiments, in which the drawout pulse followed the laser pulse very closely, produced poor ion currents, while delays of 5- 10 ps produced better ion currents. Furthermore, intensities were unchanged when the electron beam was pulsed. Longer delay times produced two results. First, the abundances of molecular ions for the quaternary salts improved, relative to alkali ions, and secondly, the electron beam enhanced the intensity of the ions, indicating desorption of neutral precursors to these ions. These results are summarized in Fig. 7, which compares the desorption times of Na+ , Kf and (CH,),N+ under LD and LEI conditions. The spectra change more dramatically when longer desorption times are examined, as shown in Fig. 8. Above 50 ps delay no ions are observed without the electron beam. The neutral products (CH,),N and CH,Cl of the pyrolysis of the quaternary amines produce the ions shown in this figure.
L.ASER
ESORPTI
CM
Fig. 7. Intensity of ions observed during 30 ps following laser pulse. (This figure originally appeared in Trends in Analytical Chemistry, 1 (1982) 307.)
47
Fig. 8. Intensity of ions observed during first 500 ~LSfollowing laser pulse. (This figure originallyappearedin Trends in Analytical Chemistry, 1 (1982) 307.)
Effects
of anion on LD
spectra
Production of the molecular ion (CH3)4N+ by direct LD appeared to be relatively insensitive to whether the anion was Cl-, Br- , or BF,. At laser pulse energies of 0.3 J or more, the ion (CH,),-N=CH, (58) was observed under laser-only conditions for the chloride and bromide, but not for the tetrafluoroborate. The most outstanding feature was the intensity of the CH,i ion in the LEI mode which followed the order Cl- > Br- > BFL, perhaps reflecting its origins from desorbed methyl halide. DISCUSSION
Recently
Kistemaker
and co-workers
[
131 described
a thermal
model
for
the desorption of organic species coated on a surface, based upon the thermal effects of high-powered lasers on surfaces as described by Ready [40]. In this model, it is assumed that for a substrate having high thermal
adsorptivity, the presence of an organic layer does not influence the temperature distribution on the substrate. The laser energy is absorbed mainly in the substrate and transferred to the organic layer, and while the heat flux in the organic layer is small compared to the flux in the substrate, it is sufficient to heat the sample to the momentary substrate temperature [ 131. Using Kistemaker’s model, a sub-microsecond laser pulse will produce maximum temperature in the substrate almost immediately, but the dissipa-
48
tion of heat will require of the order of several ,us when the substrate is stainless steel. In our experiments, the use of Vespel as a substrate may produce a slightly longer time-constant for the temperature drop, and it is during this time that the ‘time-dependent’ desorption of ions and neutral species is observed. In Fig. 7 the maximum Na + ion desorption occurs at 5 (us, while the Kt ion reaches a maximum at - 12 ps. Since the ionization energy is higher for sodium (5.1 eV) than for potassium (4.4 eV), it can be assumed that the substrate temperature is higher at 5 ps than at 12. The maximum substrate temperature is undoubtedly reached earlier (( 1 ps), so that some of the time delay can be rationalized on the basis of the heat flux from substrate to sample. In addition, during the period from 3 to 8 ,us, the desorption is entirely ionic, which, according to the Langmuir-Saha equation [41], would correspond to the highest temperatures of the sample. The time-resolved spectra of tetramethylammonium chloride -itself can be used to explain the results observed in TD [21,22,36-381, EI/flash vaporization [34], and normal probe heating [3] of the quaternary amines. A neutral precursor for the (CH3),N+ ion, which simply contains one more electron, is probably unrealistic, since this even-electron ion would have to originate from a neutral species having an additional electron in an antibonding orbital. In addition, like the (CH,),N+ ion, IQ+ and Na+ exist as preformed ions in the form of chloride salts. In a previous discussion of TD, Cotter and Yergey [21] suggested that the gas-phase heats of formation of these ions, or the lattice energies using the same anion, might be more appropriate predictors of the relative energies necessary for cation desorption. Using the lattice energies for NaCl (8.0 eV), KC1 (7.2 eV) and (CH,),NCl (C 6.6 eV) [21], desorption of the (CH3)4Nf ion should. occur at lower temperatures than for K + ions, which in turn should be desorbed at lower temperatures than Na + ions. Figure 7 verifies this order of desorption. experiments of Daves and co-workers [34] are The El/f1 us h vduatifization simulated by the appearance of these same ions using LD combined with an electron-beam pulse. As predicted in their experiments, these occur during a very short time, and as a result of rapid heating. Finally, the normal probe heating of quaternary amines is reflected in the longer time (Fig. 8) in which thermal decomposition results in the production of the volatile tertiary amine (CH,),N through the loss of CH,Cl. Ions formed from both of these neutral pyrolysis products are observed. CONCLUSIONS
Despite their tendency to decompose upon heating, the quaternary ammonium cations are easily desorbed intact under conditions which are milder
49
than those for the desorption of Na+ and K+ ions [21]. The ease with which ‘preformed’ ions can be desorbed has been observed for all of the desorption methods, so that the results reported here may have implications for the other methods. The amount of neutral species generated is considerable. At 20 ps delay (a point at which the molecular ion is optimal for both LD and LEI spectra of the tetraalkylammonium salts) the total ion currents with the electron beam on are from 6 to 20 times the currents from the laser desorbed ions alone. Since probably only a small fraction of the neutral species actually become ionized by the electron beam, it is clear that neutral species are the major product of the desorption event. More importantly, the kinds of neutral species desorbed depend a great deal upon the temperature and/or heating rate. TD techniques which employ heating of wires [21,22] may produce both pyrolysis products and sublimed clusters in much greater abundance than preformed ions. The neutral species are not seen with the electron beam off. On the other hand, the particle bombardment techniques (SIMS, PDMS and FAB) produce instantaneous localized collision events, perhaps followed by rapid increases in temperature in the areas surrounding the collision, as a secondary event [42]. The rapid transfer of heat from the collisionally damaged substrate to the sample might be expected to encourage the desorption of preformed ions and neutral clusters, but perhaps to minimize the formation of neutral pyrolysis products. For the LD work itself there appear to be two basic conclusions. The first is that the large number of intact neutral species which are desorbed suggest the future utility of using secondary means of ionization, such as EI or CI. There have in fact been several reports of laser-assisted FD studies [43,44]. The second conclusion is that a thermal model continues to work well as a description of LD since the time for desorption is not appropriate for an electric-field or spectroscopic explanation. Considering the time-resolved spectra, it is understandable that lasers used for so long to pyrolyze organic samples are now being used to desorb molecular ions intact! ACKNOWLEDGEMENT
This Science DHHS.
work was funded by Grants CHE So-16440 from the National Foundation, and CA 09243 from the National Cancer Institute,
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