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Time-resolved laser desorption mass spectrometry. II. Measurement of the energy spread of laser desorbed ions
151
International Journal of Mass Spectromezry and Ion Processes, 54 (1983) 15 l- 158
Elsevier Science Publishers B.V., Amsterdam
TIME-RESOLVED II. MEASUREMENT DESORBED IONS
JEAN-CLAUDE
TABET
- Printed in The Netherlands
LASER DESORPTION MASS SPECTROMETRY. OF THE ENERGY SPREAD OF LASER
* and ROBERT
J. COTTER
Department of Pharmacology and Experimental Therapeutics, The Johns Hopkins School of Medicine, 725 North Wolfe Street, Baltimore, MD 21205 (U.S.A.)
University
(Received 5 May 1983)
ABSTRACT The energy spread of laser desorbed ions can be measured with a time-of-flight analyzer, using a drawout pulse with variable delay after the laser pulse to separate the desorption time profile from the energy spread information. The energy spread varies from 15 eV immediately following the laser pulse to 0.31 eV at longer times.
INTRODUCTION
In 1966 Fenner and Daly reported the use of a high powered laser for mass analysis using a time-of-flight (TOF) mass spectrometer [l]. In this early instrument, 30 ns pulses from a ruby laser with powers up to 10 mJ were focused onto an area of 2 X lo-’ cm2, giving a power density of the order of 10” W cme2. They employed an energy filter to achieve a mass resolution of 30, since the measured energy spread of the vaporized atomic ions was as much as 500 eV. More recently, the energy spreads of desorbed ions from a commercially available laser TOF microprobe (LAMMA) have been reported to be below 50 eV [2]. In this instrument, which uses a frequency-quadrupled neodymium laser (347 nm) focused to a very small spot size (0.1-1.0 pm), power densities in the range of 108-10” W cmm2 are used. Recently there has been much interest in the application of pulsed high power lasers for the desorption and mass analysis of polar, thermally labile organic solids [3], where the energy spread of the laser desorbed ions is of concern in order to produce minimally unit resolution for the higher masses * Visiting faculty, University of Orsay, Paris XI. 0 168- 1176,‘83/$03.00
0 1983 Elsevier Science Publishers B.V.
152
of such compounds. Using magnetic sector instruments, Kistemaker et al. [3], Heresch et al. [4] and Cotter [5,6] have generally employed somewhat lower power densities for their observation of molecular ions by alkali ion attachment (e.g., sucrose + Na+) and quatemary ammonium ions. This application is consistent with observations by Stoll and Riillgen [7] and Cotter and Yergey [8] that such ions may be produced by thermal methods which employ rather moderate temperatures. Using a hemispherical electrostatic analyzer, Kistemaker et al. have recently measured the energy spread of desorbed Na+ ions in their instrument [9]. Comparison of the energy spread of the laser desorbed alkali ion (FWHM = 0.26 eV) with thermionic emission results (FWHM = 0.23 eV) and correction for instrumental- effects resulted in an estimate of - 0.1 eV energy spread, consistent with the thermal model for laser desorption described earlier by these authors [ lo]. In addition, Kistemaker and coworkers have pointed out the difficulty of determining the ion energy spread by time-of-flight analysis, because of the convolution of the desorption time profile with the time spread which arises from the actual spread in initial energies [9]. Nevertheless, such time-of-flight measurements were used very cleverly by Hardin and Vestal [ 1 l] using a quadrupole instrument to study the metastable gas-phase decomposition of laser desorbed ions and to determine the precursors (protonated molecules and cluster ions) for fragment ions observed in the mass spectrum. They found energy spreads in the range of 6-25 eV. Work in our laboratory [5,6,12] indicates that desorption of ions following a CO, laser pulse may extend for tens of microseconds for desorbed ions and for even longer times for neutral species, making such time-of-flight estimates of energy spread difficult. The actual length of the desorption time depends most probably on the total energy (in this case up to 0.3 Jj pumped into the sample/substrate and the ability of the substrate to dissipate the instantaneous high temperatures. In this paper we use a laser/TOF configuration, described earlier [ 121, in which the ions are formed by the laser, but are mass analyzed following an ion drawout pulse. This system enables analysis of the time profile of ion desorption and permits partial deconvolution of the desorption profile from the initial energy spread information. METHOD
In the time-of-flight mass spectrometer, resolution depends upon the initial velocities (energy spread) and positions of the ions at the time of the drawout pulse. However, the mass resolution is also affected by the length of the ionization time, which in the electron impact mode (EI) is kept short ( < 1 ps). Because of the long desorption times following a laser pulse, some time broadening of individual mass peaks will occur even when a drawout
153
pulse is used. In the method which is described below, variation of the ionization time in an EI case is used to study this effect so that the time resolution of peaks in the laser desorption mass spectrum can be correlated with the initial energy spreads of desorbed ions. In 1955 Wiley and McLaren [13] described the time-of-flight of the ions as the sum T(U,, s) = r, + Td’+ T,,
0)
where T,, Td and Tn are the times in the ion source, ion accelerating region, and drift region, respectively. For a given set of voltage and distance conditions, the time-of-flight of a particular mass is also a function of initial energy, U,, and position, s, in the source. The major portion of the time spread in the mass spectrum resulting from initial energy spread occurs in the source region [ 131. T, =
1.02(2~-$‘2 [(Q + z,sE~)“~ -+ (U,)“‘] S
where ES is the electric field (V cm-‘) in the source when the drawout pulse is applied. For mass 28, at 373 K (0.049 ev), the time spread will be 0.0116 ~_ls.At 0.1 eV a variation of 0.0151 ps in the time-of-flight can be expected. It is well known that increasing the length of time that the electron beam is on increases the sensitivity but decreases the resolution. The increase in sensitivity is not linear, since many of the ions drift away from the ion formation region and eventually out of the source prior to the application of the drawout pulse. This same drift increases the range of values of s {distance from the drawout grid) and therefore the range of values of initial acceleration, zsES, received by the ions. Variation of the time that the electron beam is on can be used to determine when the steady-state condition between ion formation and ion loss is achieved, as well as a value for the peak width for a known initial energy spread. For the’electron impact ionization of a gas at constant source pressure the rate of formation of ions will be essentially constant (k,) during the time that the electron beam is on, while the rate of loss of ions through drift from the source will be first order ( k2[A]) at low source pressures. = k, - k,[A]
d[A]/dt
(3)
Integrating for the length of time, t, that the electron beam is on, the intensity .of ions observed in the detector will be proportional to [A]
=
$(I -
e-kzt)
2
from which k, and k, can be determined.
The half-life
of ions in the source
154
can be calculated from k,. A plot of eqn. (4) can be used to determine the point at which the peak width for a known energy spread has reached its maximum value. This value is then used to correlate peak widths with initial energy spreads in the laser desorption experiment. EXPERIMENTAL
The laser desorption mass spectrometer has been described in detail previously [ 121. The mass spectrometer is a standard CVC (Rochester, NY) Model 2000 TOF with a 2 m flight tube, modified for laser desorption. 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 energy of the unfocused pulse in these experiments was 0.1-0.5 J, as measured on a Scientech (Boulder, CO) Model 362 energy/power meter, corresponding to a power density of 2.5-12 MW cm- 2. Analog signals from the multiplier are digitized and stored in 2048 channels of a Biomation (Cupertino, CA) Model 8100 waveform recorder, with a time resolution of 0.01 ps channel-‘. Digitized spectra are transferred to an Apple II+ (Cupertino, CA) microcomputer via a parallel interface. Generally, twenty “real time” spectra were added. The intensity of a given mass peak was calculated by numerical integration of the intensity/channel information for that mass, and the FWHM was calculated from the standard deviation. The SENSITIVITY ADJUST on the CVC instrument selects five electron beam time widths, which were measured accurately with a Tektronix 7603 oscilloscope. For the laser desorption experiment, the delay between the laser pulse and drawout pulse was varied between 0 and 50 pus. The delay between the pulses was measured from their appearance on the waveform recorder. RESULTS
Figure 1 shows the intensity of the ion signals for N2+ (m/z 28) as a function of the time that the electron beam is on. Using a least-squares fit of eqn. (4), k, for N2+’ was determined as 0.6936 pus-‘, giving a half-life in the source of 1.0 ps for that ion. Table 1 shows the time widths as a function of the time the electron beam is on. Using eqn. (2), the value for the shortest time the electron beam is on (0.72 ps) corresponds to an energy spread of approximately 0.17 eV. At 7.2 ps the ion formation/drift has reached steady-state, so that the value of 0.0352 ps for the width represents the maximum width for this energy spread for long ionization times. Figures 2 shows the FWHM of the K’ ion peak (m/z 39) as a function of
15s 3600
t
Electron
beam pulse width (11s)
Fig. 1. Intensity of NC as a function of the time the electron beam is on.
the delay between the laser pulse and the ion drawout pulse. Using the fact that the time spread is proportional to initial energy and mass. T=
C&2(71/2
(5)
0
the corresponding
initial energy spreads can be calculated
U, = 0.17(28/39)(
T/0.0352)*
from (6)
so that energies range from 15.3 to 0.3 1 eV during the times examined. DISCUSSION
As reported in an earlier paper [ 121 the ions produced by laser desorption are observed for some time after the laser pulse is applied. Assuming a TABLE
1
Time widths as a function of the time the electron beam is on Electron beam on
width of peak
(PSI
(PSI
0.72 1.8 4.5 7.2
0.0194 0.0256 0.0324 0.0352
10.0
5.0
5
5
6 5
,g 0.2 3
; w
E” ‘F LO
0.1 -
t.0
I.5 12 3.1
01 0
’
’
10
’
’
’
20 Time
’ 30
after
laser
’
’ 40
1
I 50
3.0
pulseIps)
Fig. 2. Peak width of K+ ion peak as a function of the time delay between the laser pulse and the drawout pulse.
thermal model for the formation of gaseous ions, this time profile occurs because the substrate on which the sample is deposited continues to transmit heat to the sample after the conclusion of the laser pulse. The ions leaving the surface do so at a rate of d[A]/dt
= k,Ne-AE’kT
(7)
where N is the number of particles on the surface and AE is the desorption energy [lo]. If the maximum temperature of the substrate (which is a function of the laser intensity and the absorptivity of the substrate) is reached very quickly, the temperature will then decay exponentially with time, and the rate of ion emission will decrease as well. Normally, using time-of-flight measurements with a constant drawout
157
field, the intensity
bl=j
of ions observed will be proportional
to the integral
tmok,Ne-AE~kTdf
and the peaks will be broadened by the desorption time. The use of a drawout pulse provides a means of looking at the desorption profile, since many of the ions are cleared from the source prior to the pulse d[A]/dt
= k,Ne-AE’kT - k, [A]
(9)
The integral form, taken from t = 0 to the time of the drawout pulse, is somewhat more complex, since the temperature, T, is itself a function of time. However, for delay times which are much longer than the source residence time, the ions observed will for the most part have been formed just prior to the pulse, and the integral form can be approximated by
- rtAtk,Ne-AE1kTdf M-j
(10)
so that the use of the drawout pulse results in at least a partial deconvolution of the desorption profile, eqn. (8), and the peak widths reflect (mainly) the energy spread of ions formed at a particular time. CONCLUSIONS
The combination of pulsed laser and pulsed drawout field has some distinct advantages for laser TOF measurements. First, it permits examination of the time dependence of the rates of desorption and the energy spreads which correlate well with a thermionic emission model for desorption. Secondly, it suggests an interesting analytical method for improving resolution for laser desorptiori mass spectrometers. In this experiment, the majority of ions are observed from 13 to 20 ms after the laser pulse, in the energy range 0. l-l .O eV. At later times, the energy spread is even lower, so that a delayed drawout pulse may be used as an energy filter to improve mass resolution with, of course, some loss of sensitivity. ACKNOWLEDGMENT
work was funded by a grant, Science Foundation. This
CHE-80-16440,
from
the National
REFERENCES 1 N.C. Fenner and N.R. Daly, Rev. Sci. Instrum., 37 (1966) 1068. 2 R. Nitsche, R. Kaufmann, F. Hillenkamp, E. Unsold, H. Vogt and R. Wechsung, Israel J. Chem., 17 (1978) 181.
158 3 M.A. Posthumus, P.G. Kistemaker, H.L.C. Meuzelaar and M.C. Ten Noever de Brauw, Anal. Chem., 52 (1980) 1803. F. Heresch, E.R. S&mid and J.F.K. Huber, Anal. Chem., 52 (1980) 1803. R.J. Cotter, Anal. Chem., 52 (1980) 1767. R.J. Cotter, Anal. Chem., 53 (1981) 719. R. Stoll and F.W. Riillgen, J. Chem. Sot., Chem. Commun., (1980) 789. R.J. Cotter and A.L. Yergey, J. Am. Chem. Sot., 103 (1981) 1596. G.J.Q. Van der Peyi, W.J. Van der Zande, K. Bederski, A.J.H. Boerboom and P.G. Kistemaker, Int. J. Mass Spectrom. Ion Phys., 47 (1983) 7. 10 G.J.Q. Van der Peyl, J. Haverkamp and P.G. Kistemaker, Int. J. Mass Spectrom. Ion Phys., 42 (1982) 125. 11 E.D. Hardin and M.L. Vestal, Anal. Chem., 53 (1981) 1492. 12 R.B. Van Breemen, M. Snow and R.J. Cotter, Int. J. Mass Spectrom. Ion Phys., 49 (1983) 35. 13 WC. Wiley and 1-H. McLaren, Rev. Sci. Instrum., 26 ( 1955) 1150.
International Journal of Mass Spectromezry and Ion Processes, 54 (1983) 15 l- 158
Elsevier Science Publishers B.V., Amsterdam
TIME-RESOLVED II. MEASUREMENT DESORBED IONS
JEAN-CLAUDE
TABET
- Printed in The Netherlands
LASER DESORPTION MASS SPECTROMETRY. OF THE ENERGY SPREAD OF LASER
* and ROBERT
J. COTTER
Department of Pharmacology and Experimental Therapeutics, The Johns Hopkins School of Medicine, 725 North Wolfe Street, Baltimore, MD 21205 (U.S.A.)
University
(Received 5 May 1983)
ABSTRACT The energy spread of laser desorbed ions can be measured with a time-of-flight analyzer, using a drawout pulse with variable delay after the laser pulse to separate the desorption time profile from the energy spread information. The energy spread varies from 15 eV immediately following the laser pulse to 0.31 eV at longer times.
INTRODUCTION
In 1966 Fenner and Daly reported the use of a high powered laser for mass analysis using a time-of-flight (TOF) mass spectrometer [l]. In this early instrument, 30 ns pulses from a ruby laser with powers up to 10 mJ were focused onto an area of 2 X lo-’ cm2, giving a power density of the order of 10” W cme2. They employed an energy filter to achieve a mass resolution of 30, since the measured energy spread of the vaporized atomic ions was as much as 500 eV. More recently, the energy spreads of desorbed ions from a commercially available laser TOF microprobe (LAMMA) have been reported to be below 50 eV [2]. In this instrument, which uses a frequency-quadrupled neodymium laser (347 nm) focused to a very small spot size (0.1-1.0 pm), power densities in the range of 108-10” W cmm2 are used. Recently there has been much interest in the application of pulsed high power lasers for the desorption and mass analysis of polar, thermally labile organic solids [3], where the energy spread of the laser desorbed ions is of concern in order to produce minimally unit resolution for the higher masses * Visiting faculty, University of Orsay, Paris XI. 0 168- 1176,‘83/$03.00
0 1983 Elsevier Science Publishers B.V.
152
of such compounds. Using magnetic sector instruments, Kistemaker et al. [3], Heresch et al. [4] and Cotter [5,6] have generally employed somewhat lower power densities for their observation of molecular ions by alkali ion attachment (e.g., sucrose + Na+) and quatemary ammonium ions. This application is consistent with observations by Stoll and Riillgen [7] and Cotter and Yergey [8] that such ions may be produced by thermal methods which employ rather moderate temperatures. Using a hemispherical electrostatic analyzer, Kistemaker et al. have recently measured the energy spread of desorbed Na+ ions in their instrument [9]. Comparison of the energy spread of the laser desorbed alkali ion (FWHM = 0.26 eV) with thermionic emission results (FWHM = 0.23 eV) and correction for instrumental- effects resulted in an estimate of - 0.1 eV energy spread, consistent with the thermal model for laser desorption described earlier by these authors [ lo]. In addition, Kistemaker and coworkers have pointed out the difficulty of determining the ion energy spread by time-of-flight analysis, because of the convolution of the desorption time profile with the time spread which arises from the actual spread in initial energies [9]. Nevertheless, such time-of-flight measurements were used very cleverly by Hardin and Vestal [ 1 l] using a quadrupole instrument to study the metastable gas-phase decomposition of laser desorbed ions and to determine the precursors (protonated molecules and cluster ions) for fragment ions observed in the mass spectrum. They found energy spreads in the range of 6-25 eV. Work in our laboratory [5,6,12] indicates that desorption of ions following a CO, laser pulse may extend for tens of microseconds for desorbed ions and for even longer times for neutral species, making such time-of-flight estimates of energy spread difficult. The actual length of the desorption time depends most probably on the total energy (in this case up to 0.3 Jj pumped into the sample/substrate and the ability of the substrate to dissipate the instantaneous high temperatures. In this paper we use a laser/TOF configuration, described earlier [ 121, in which the ions are formed by the laser, but are mass analyzed following an ion drawout pulse. This system enables analysis of the time profile of ion desorption and permits partial deconvolution of the desorption profile from the initial energy spread information. METHOD
In the time-of-flight mass spectrometer, resolution depends upon the initial velocities (energy spread) and positions of the ions at the time of the drawout pulse. However, the mass resolution is also affected by the length of the ionization time, which in the electron impact mode (EI) is kept short ( < 1 ps). Because of the long desorption times following a laser pulse, some time broadening of individual mass peaks will occur even when a drawout
153
pulse is used. In the method which is described below, variation of the ionization time in an EI case is used to study this effect so that the time resolution of peaks in the laser desorption mass spectrum can be correlated with the initial energy spreads of desorbed ions. In 1955 Wiley and McLaren [13] described the time-of-flight of the ions as the sum T(U,, s) = r, + Td’+ T,,
0)
where T,, Td and Tn are the times in the ion source, ion accelerating region, and drift region, respectively. For a given set of voltage and distance conditions, the time-of-flight of a particular mass is also a function of initial energy, U,, and position, s, in the source. The major portion of the time spread in the mass spectrum resulting from initial energy spread occurs in the source region [ 131. T, =
1.02(2~-$‘2 [(Q + z,sE~)“~ -+ (U,)“‘] S
where ES is the electric field (V cm-‘) in the source when the drawout pulse is applied. For mass 28, at 373 K (0.049 ev), the time spread will be 0.0116 ~_ls.At 0.1 eV a variation of 0.0151 ps in the time-of-flight can be expected. It is well known that increasing the length of time that the electron beam is on increases the sensitivity but decreases the resolution. The increase in sensitivity is not linear, since many of the ions drift away from the ion formation region and eventually out of the source prior to the application of the drawout pulse. This same drift increases the range of values of s {distance from the drawout grid) and therefore the range of values of initial acceleration, zsES, received by the ions. Variation of the time that the electron beam is on can be used to determine when the steady-state condition between ion formation and ion loss is achieved, as well as a value for the peak width for a known initial energy spread. For the’electron impact ionization of a gas at constant source pressure the rate of formation of ions will be essentially constant (k,) during the time that the electron beam is on, while the rate of loss of ions through drift from the source will be first order ( k2[A]) at low source pressures. = k, - k,[A]
d[A]/dt
(3)
Integrating for the length of time, t, that the electron beam is on, the intensity .of ions observed in the detector will be proportional to [A]
=
$(I -
e-kzt)
2
from which k, and k, can be determined.
The half-life
of ions in the source
154
can be calculated from k,. A plot of eqn. (4) can be used to determine the point at which the peak width for a known energy spread has reached its maximum value. This value is then used to correlate peak widths with initial energy spreads in the laser desorption experiment. EXPERIMENTAL
The laser desorption mass spectrometer has been described in detail previously [ 121. The mass spectrometer is a standard CVC (Rochester, NY) Model 2000 TOF with a 2 m flight tube, modified for laser desorption. 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 energy of the unfocused pulse in these experiments was 0.1-0.5 J, as measured on a Scientech (Boulder, CO) Model 362 energy/power meter, corresponding to a power density of 2.5-12 MW cm- 2. Analog signals from the multiplier are digitized and stored in 2048 channels of a Biomation (Cupertino, CA) Model 8100 waveform recorder, with a time resolution of 0.01 ps channel-‘. Digitized spectra are transferred to an Apple II+ (Cupertino, CA) microcomputer via a parallel interface. Generally, twenty “real time” spectra were added. The intensity of a given mass peak was calculated by numerical integration of the intensity/channel information for that mass, and the FWHM was calculated from the standard deviation. The SENSITIVITY ADJUST on the CVC instrument selects five electron beam time widths, which were measured accurately with a Tektronix 7603 oscilloscope. For the laser desorption experiment, the delay between the laser pulse and drawout pulse was varied between 0 and 50 pus. The delay between the pulses was measured from their appearance on the waveform recorder. RESULTS
Figure 1 shows the intensity of the ion signals for N2+ (m/z 28) as a function of the time that the electron beam is on. Using a least-squares fit of eqn. (4), k, for N2+’ was determined as 0.6936 pus-‘, giving a half-life in the source of 1.0 ps for that ion. Table 1 shows the time widths as a function of the time the electron beam is on. Using eqn. (2), the value for the shortest time the electron beam is on (0.72 ps) corresponds to an energy spread of approximately 0.17 eV. At 7.2 ps the ion formation/drift has reached steady-state, so that the value of 0.0352 ps for the width represents the maximum width for this energy spread for long ionization times. Figures 2 shows the FWHM of the K’ ion peak (m/z 39) as a function of
15s 3600
t
Electron
beam pulse width (11s)
Fig. 1. Intensity of NC as a function of the time the electron beam is on.
the delay between the laser pulse and the ion drawout pulse. Using the fact that the time spread is proportional to initial energy and mass. T=
C&2(71/2
(5)
0
the corresponding
initial energy spreads can be calculated
U, = 0.17(28/39)(
T/0.0352)*
from (6)
so that energies range from 15.3 to 0.3 1 eV during the times examined. DISCUSSION
As reported in an earlier paper [ 121 the ions produced by laser desorption are observed for some time after the laser pulse is applied. Assuming a TABLE
1
Time widths as a function of the time the electron beam is on Electron beam on
width of peak
(PSI
(PSI
0.72 1.8 4.5 7.2
0.0194 0.0256 0.0324 0.0352
10.0
5.0
5
5
6 5
,g 0.2 3
; w
E” ‘F LO
0.1 -
t.0
I.5 12 3.1
01 0
’
’
10
’
’
’
20 Time
’ 30
after
laser
’
’ 40
1
I 50
3.0
pulseIps)
Fig. 2. Peak width of K+ ion peak as a function of the time delay between the laser pulse and the drawout pulse.
thermal model for the formation of gaseous ions, this time profile occurs because the substrate on which the sample is deposited continues to transmit heat to the sample after the conclusion of the laser pulse. The ions leaving the surface do so at a rate of d[A]/dt
= k,Ne-AE’kT
(7)
where N is the number of particles on the surface and AE is the desorption energy [lo]. If the maximum temperature of the substrate (which is a function of the laser intensity and the absorptivity of the substrate) is reached very quickly, the temperature will then decay exponentially with time, and the rate of ion emission will decrease as well. Normally, using time-of-flight measurements with a constant drawout
157
field, the intensity
bl=j
of ions observed will be proportional
to the integral
tmok,Ne-AE~kTdf
and the peaks will be broadened by the desorption time. The use of a drawout pulse provides a means of looking at the desorption profile, since many of the ions are cleared from the source prior to the pulse d[A]/dt
= k,Ne-AE’kT - k, [A]
(9)
The integral form, taken from t = 0 to the time of the drawout pulse, is somewhat more complex, since the temperature, T, is itself a function of time. However, for delay times which are much longer than the source residence time, the ions observed will for the most part have been formed just prior to the pulse, and the integral form can be approximated by
- rtAtk,Ne-AE1kTdf M-j
(10)
so that the use of the drawout pulse results in at least a partial deconvolution of the desorption profile, eqn. (8), and the peak widths reflect (mainly) the energy spread of ions formed at a particular time. CONCLUSIONS
The combination of pulsed laser and pulsed drawout field has some distinct advantages for laser TOF measurements. First, it permits examination of the time dependence of the rates of desorption and the energy spreads which correlate well with a thermionic emission model for desorption. Secondly, it suggests an interesting analytical method for improving resolution for laser desorptiori mass spectrometers. In this experiment, the majority of ions are observed from 13 to 20 ms after the laser pulse, in the energy range 0. l-l .O eV. At later times, the energy spread is even lower, so that a delayed drawout pulse may be used as an energy filter to improve mass resolution with, of course, some loss of sensitivity. ACKNOWLEDGMENT
work was funded by a grant, Science Foundation. This
CHE-80-16440,
from
the National
REFERENCES 1 N.C. Fenner and N.R. Daly, Rev. Sci. Instrum., 37 (1966) 1068. 2 R. Nitsche, R. Kaufmann, F. Hillenkamp, E. Unsold, H. Vogt and R. Wechsung, Israel J. Chem., 17 (1978) 181.
158 3 M.A. Posthumus, P.G. Kistemaker, H.L.C. Meuzelaar and M.C. Ten Noever de Brauw, Anal. Chem., 52 (1980) 1803. F. Heresch, E.R. S&mid and J.F.K. Huber, Anal. Chem., 52 (1980) 1803. R.J. Cotter, Anal. Chem., 52 (1980) 1767. R.J. Cotter, Anal. Chem., 53 (1981) 719. R. Stoll and F.W. Riillgen, J. Chem. Sot., Chem. Commun., (1980) 789. R.J. Cotter and A.L. Yergey, J. Am. Chem. Sot., 103 (1981) 1596. G.J.Q. Van der Peyi, W.J. Van der Zande, K. Bederski, A.J.H. Boerboom and P.G. Kistemaker, Int. J. Mass Spectrom. Ion Phys., 47 (1983) 7. 10 G.J.Q. Van der Peyl, J. Haverkamp and P.G. Kistemaker, Int. J. Mass Spectrom. Ion Phys., 42 (1982) 125. 11 E.D. Hardin and M.L. Vestal, Anal. Chem., 53 (1981) 1492. 12 R.B. Van Breemen, M. Snow and R.J. Cotter, Int. J. Mass Spectrom. Ion Phys., 49 (1983) 35. 13 WC. Wiley and 1-H. McLaren, Rev. Sci. Instrum., 26 ( 1955) 1150.