Photoemission electron impact ionization in time-of-flight mass spectrometry: an examination of experimental consequences

International Journal of Mass Spectrometry and Ion Processes 131 (1994) 125-138 016%1176/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved

125

Photoemission electron impact ionization in time-of-flight mass spectrometry: an examination of experimental consequences Steven M. Colby, James P. Reilly* Department of Chemistry, Indiana University, Bloomington, IN 47405, USA (Received 24 February 1993; accepted 24 June 1993) Abstract The use of photoemission electron impact ionization in time-of-flight mass spectrometry is examined. It is found to complement laser ionization by providing many of the advantages of electron impact ionization. Some ions are more easily produced by photoemission electron impact ionization. Under certain conditions, these ions can act as an impediment to laser ionization experiments. Methods for distinguishing between ions produced by laser ionization and photoemission electron impact ionization are presented. A new source, designed for simple conversion between these two ionization methods, is presented. This source is easily adapted to the many laser ionization time-of-flight instruments already in use. Key words: Electron impact ionization; Photoemission electron impact ionization; Laser ionization

1. Introduction Time-of-flight mass spectrometry (TOF-MS) is finding an increasingly wide variety of applications. Its sensitivity, simplicity, and unlimited mass range make it one of the most versatile mass spectrometric techniques. The popularity of the method has accelerated in the past decade with the development of high power, short pulse lasers. These provide TOF-MS with a sensitive and selective ionization source. Electron impact ionization is now rarely used in TOF-MS. It is, however, the most common ionization method in other types of mass spectrometers [1,2]. Electron impact ionization sources are usually continuous. For use with TOF-MS an electric field must be pulsed to establish the start of the time-of-flight. * Corresponding

author.

SSDI 0168-l 176(93)03878-P

Recently, however, Rohwer et al. introduced a new pulsed electron impact ionization method. Their approach, which we call photoemission electron impact ionization, uses a pulse of photogenerated electrons for inducing ionization [3]. The electrons are produced by directing a laser pulse at a wire within the source of a time-of-flight instrument. The resulting short pulse of photoelectrons is accelerated across the source to an energy great enough to cause ionization. This method provides the advantages of electron impact ionization to the large number of time-of-flight instruments that currently use laser ionization. These advantages include the abilities to ionize all types of compounds, to produce mass spectra whose characteristics do not strongly depend on instrumental parameters, and to use the large database of electron impact reference spectra currently available. Since its introduction, the photoemission

126

SM.

Colby and J.P. ReiNy/Int. J. Mass Spectrom. Ion Processes 131 (1994) 125-138

electron impact ionization method has been applied only sparingly [4,5]. This paper expands on the initial work by exploring advantages of the technique and pointing out how the phenomenon can affect conventional laser ionization experiments. We start by showing that photoemission electron impact ionization can sometimes be much more efficient than laser ionization. We then demonstrate that, because of this, it also can cause severe interference in laser ionization timeof-flight experiments. Methods are suggested for detecting this interference. Finally, we describe a new ion source designed for both photoemission electron impact ionization and laser ionization. This source is easily adaptable to the many laser ionization time-of-flight instruments already in use and has some distinct advantages over the sources proposed to date. The availability of a dual, easily interconvertible source will further expand the range of applications to which TOF-MS is suited. Laser ionization and electron impact ionization are complementary techniques. The advantages of laser ionization are that it provides good temporal and spatial resolution, controlled fragmentation, and high spectrometric selectivity. There are also several drawbacks to laser ionization, each of which can be overcome by electron impact ionization. The first is that the high selectivity of laser ionization makes it inappropriate for the analysis of unknowns. Some analytes may be difficult to laser ionize because they require the energy of many photons, do not absorb at convenient wavelengths, or fragment before ionizing. Ion yields and fragmentation patterns of different neutrals may vary widely under the same set of experimental conditions. The second drawback to laser ionization is that the exact conditions under which experiments are performed strongly affect the observed mass spectra. This makes it virtually impossible to form a large database of reference spectra. Laser induced photoemission has been used as a source of electrons in several applications other than TOF-MS [6-231. These include microwave generators [lo-121, particle accelerators [13,14],

and free electron lasers [14-l 81. All require very short electron pulses with narrow kinetic energy and angular distributions. Research in these areas involves examining the effects of various wavelengths, laser pulse widths, and types of irradiated targets. The latter include conductors, semiconductors, and organic materials. One particularly imaginative use of laser induced photoemission is the measurement of potentials in ultra-high-speed electronic circuits. This is difficult because any probe making contact with a wire would introduce a perturbation to the circuit. However, since the number of electrons produced in photoelectric emission depends on the field above a surface, a laser beam directed on a wire can be used as a non-intrusive probe of its potential. This has been demonstrated experimentally on a picosecond time scale [ 19,201. The mechanism of the laser induced photoemission process is primarily based on the photoelectric effect. However, thermal emission also may occur if the electron source is significantly heated by either absorption of energy from the laser light or through other means. The density of electrons emitted thermally depends on the temperature through the Richardson-Dushman equation: J = AT2 exp(-@/kT) where J is the current density in Acme2, A is a constant, and @ is the work function of the emitter [24]. Thermal emission is not desirable in applications requiring short pulses of electrons, because it may take place over a time longer than the temporal duration of the laser pulse [25]. Another physical phenomenon that can influence the photoemission process is the Schottky effect, the reduction of a work function due to the presence of an external field [24,26,27]. It lowers both the tunneling barrier controlling the rate of thermal emission and the energy needed for electron emission. The Schottky effect becomes important as the local field approaches tenths of a millivolt per Angstrom [lo]. For some ionization processes photoemission electron impact ionization can be much more effective than direct laser ionization. For example, through this process it is possible for a single

SM.

Colby and J.P. Reilly/Int.

J. Mass Spectrom. Ion Processes

near-UV photon to produce ions whose appearance potentials are tens of electron volts. This can occur because photoemitted electrons, which require only a few electron volts to produce, can easily be accelerated to hundreds of electron volts. Under some conditions, the conversion of photons into electrons may even have an efficiency greater than unity [lo, 281. The relative efficiency of the two ionization processes is a function of photon flux, electron production rate, ionization crosssections, and energetics. Photoionization and electron impact cross-sections tend to be similar (i.e. in the realm of lo-l6 to 1O-‘8 cm*) but laser fluence is generally much higher than electron fluence. Consequently, resonantly enhanced laser ionization can be significantly more sensitive. However, helium, argon, and other substances whose high lying excited states are beyond the range of powerful tunable lasers can be more effectively ionized by electron impact. This is particularly true for the production of multiply charged ions. An important difference between photoemission electron and conventional electron impact ionization is the magnitude of the electron currents involved. Conventional filament sources typically work in the 100 PA to 1 mA range [l]. However, if a 1OOd laser pulse were converted into electrons during lOns, with a conversion efficiency of 1%, then a 16 A electron pulse would be produced. Such high currents would exist only during the short period of the laser pulse. Although the time integrated number of electrons may be similar to that of a filament source, short bursts of high electron current may yield more extensive fragmentation or even multiple ionization because a molecule or fragment may be hit by more than one electron. In our experiments most of the laser light passes through the instrument and we produce only a fraction of the maximum possible electron current. We obtained the data presented in this paper with a combined gas chromatograph-time-offlight mass spectrometer (GC-TOF-MS). Our experiments were originally directed toward developing new techniques for the ultrasensitive

131 (1994) 125-138

12-l

detection of organometallics [29,30]. However, we found that photoemission electron impact ionization was the dominant ionization process under the conditions of some of our experiments. We have therefore, to an extent, focused on how photoemission electron impact ionization affects GC-TOF-MS. Our choice of reagents was also determined by our original goals. They served well, however, for illustrating the photoemission electron impact ionization process and its effects. 2. Experimental 2.1. Reagents Tetraethyltin was purchased from Aldrich Chemical Company. Samples were prepared by dilution in “spectranalyzed” methylene chloride from Fisher Scientific. Final concentration was lOOngmL_’ of tetraethyltin. Zero Grade helium and argon were purchased from Air Products. They were passed through a Varian filter before entering the gas chromatograph at a flow rate of z 0.6 mL min-’ n-butylbenzene, used to demonstrate the dual source, was purchased from Aldrich Chemical Company and introduced without dilution. 2.2. Instrumentation Figure 1 shows the three TOF-MS sources used in this experiment. Source A had two acceleration regions adjusted to Wiley-McLaren space focusing conditions [31]. Source B was shaped like a cup in an attempt to partially confine the chromatographic effluent and thereby increase instrument sensitivity. The cup had holes to allow the laser beam to pass through and the chromatography column to enter. It will be described further elsewhere [29]. Both of these sources were designed for laser ionization but photoemission electron impact ionization can occur in them when electrons are produced at the first (nickel) grid and then accelerated across the source to the bottom plate or cup.

SM.

128

Colby and J.P. ReillylInt. J. Mass Spectrom. Ion Processes 131 (1994) 125-138

hv

Fig. 1. The three instrument

sources: (A) standard;

(B) cup; (C)dual ionization. CI, column inlet; HB, heating G,,, solid plate or cup.

Source C was specifically designed so that it could be used for either laser or photoemission electron impact ionization. This allows the two complementary techniques to be used interchangeably without modifying the instrument. Source C was similar to A except that an additional grid, Gs, was added to enable the use of both modes. For gas phase laser ionization, G2 is grounded and the source functions just as it does in configuration A with ionization taking place between Go and Gt. For photoemission electron impact ionization the laser is directed at the plate Go of source C. The field between Go and G1 accelerates the resulting photoelectrons toward and through G1. Electron impact ionization then takes place just above Gt . The distance the electron can travel beyond Gt depends on the electric fields in the acceleration regions. The fields between G, and GZ, and G2 and G3 are adjusted to space focusing conditions. Figure 2 shows the trajectory of the photoemitted electron. The cup and the plate Go were manufactured from aluminum while the grid holders G, through Gs were stainless steel. The grids were 90% transmitting nickel mesh (Buckbee Meers). The work functions of aluminum and nickel are 4.28 and 5.15 eV respectively [32]. For some experiments, 8.4cm long deflection plates were mounted parallel to the flight tube axis just above the source (not shown in Fig. 1.) Table 1 lists the voltages and

block; G1_3, grid holders;

dimensions used for different instrument configurations. The remainder of this paper will refer to these as A, B, C, and C’ as defined here and in Fig. 1. For tetraethyltin analysis chromatography was performed using a Varian 3700 Gas Chromatograph with on-column injection and a 30m long, 250pm i.d. fused silica capillary column (Supelco SPB-1). The chromatographic temperature program ranged from 40 to 200°C. The interface between the gas chromatograph and the TOFMS, was held at 250°C and has been previously described [33]. When in use, the cup was also heated to approximately 270°C. The end of the capillary column was positioned within the first acceleration region. The mass spectrometer was evacuated through a liquid nitrogen trap by a Varian VHS-6 diffusion pump. The base pressures were approximately 1.0 x lop4 and 4.6 x 10e5 Torr with the helium or argon carrier gas flowing and approximately 2.0 x 10e6 Torr without carrier ION

G1-

Fig. 2. Schematic process.

of photoemission

electron

impact

ionization

SM.

Colby and J.P. ReillylInt.

Table 1 Instrument

J. Mass Spectrom.

Ion Processes

configurations Configuration Bb

Aa Distances

Go GI G2 G3

C’d

(mm)

Go-G, GI-Gz Gz-Gs G2-MCP Gs-MCP Voltages

CC

12.52 4.27 _

12.60 8.56 _

626.6 _

632.5 _

2096.82 1828.82 Ground

2092.60 1834.47 Ground

10.13 9.39 9.29 _

10.13 9.39 9.29 _

623.6

623.6

2119.38 1880.62 Ground Ground

1907.76 2007.76 1820.84 Ground

(V)

_

_

a Standard source. b Cup shaped source. ’ Dual source with photoionization. d Compare with Fig. 1.

gas flow. Ions were detected by a pair of tandem microchannel plates (Galileo). Their output was amplified (LeCroy WlOl ATB) and then digitized by a high-speed waveform recorder (Biomation 6500). The data were transferred to a microcomputer where software was used to record mass spectra and integrate the ion signal over the isotopic mass envelope of tin. The light source used for these experiments was a Lumonics TE-861 ArF excimer laser with a pulse duration of about 8ns and a repetition rate of 20Hz. It has previously been shown that this 193 nm wavelength can produce tin ions [34] and induce the photoemission of electrons [28]. The laser beam was apertured and then focused into the source of the mass spectrometer using a 1OOOmm focal length lens. The total distance from the laser to the instrument was approximately 2m. The rectangular focal spot produced by this lens was 4.5mm wide and 0.6mm high. This was measured by passing a sharp edged blade through the focal plane and monitoring the transmitted light intensity. The energy of the laser beam was often adjusted by either changing the laser discharge voltage or the size of the aperture. The laser energy also declined as the laser gas mixture matured. The lens was positioned so that

131 (1994)

129

125-138

the narrow edge of the focal spot intercepted the chromatographic effluent approximately 1.Omm from the tip of the chromatographic column. The laser energy was measured before the instrument with a Gentec ED-500 joulemeter or a Laser Precision Rm-660 Universal Radiometer. In this experiment the energy of the ionizing electron depended on the point of ionization. In sources A and B this point is not well delined because the electrons are produced at the first grid and then continuously accelerated across the source to the bottom plate or cup. Once the electron has sufficient energy it can induce ionization at any position. However, since the average energy of the electrons during their acceleration is equal to half of their final energy, we can assume that the typical electron/molecule collision occurs when the electron has half of its total possible energy. Under conditions A and B this corresponds to an energy of approximately 130 eV. Since this is above the energies used in conventional electron impact ionization, our instrument may produce more fragmentation. When using source C’ the electron energy is still dependent on the point of ionization. However, the range of possible ionization positions can now be controlled by varying the potential on Go. As shown in Fig. 2, electrons decelerate as they enter the region between Gi and GZ. For all of the Go potentials listed in Table 1 their energy is low enough for them to be turned back toward Go. Ions formed between G, and G2 are accelerated toward the detector while those that are produced between Go and Gi are accelerated toward Go and cannot be detected. It is therefore possible to limit the production of detectable ions to the lower part of the region between Gi and G2 by limiting the potential drop between Go and G, . 3. Results and discussion 3. I. Eficiency ionization

of pho toemission electron impact

Photoemission electron impact ionization is able

S.M. Colby and J.P. Reilly/Int. J. Mass Spectrom. Ion Processes 131 (1994) 125-138

130

Are+

Ar3+

A+

Ar+

\

xl 0 I

0

i!Lb I

I

10

30

20

I

40

1

50

Mass/Charge Fig. 3. Mass spectrum

showing multiply

charged

argon ions. (Other noteworthy peaks are m/z 14, 16,28, and 32, corresponding Of, Nz, and 0: respectively.)

to produce some ions more efficiently than laser ionization. We have done two experiments to illustrate this. They demonstrate the formation of multiply charged ions and the uncharacteristic production of ions at extremely low light levels. 3.1.1, Multicharged argon Photoemission electron impact ionization’s ability to produce ions with high appearance potentials is illustrated by the degree to which it can generate multiply charged argon. Figure 3 shows a mass spectrum recorded when argon was used as the GC carrier gas. The data represent the summation of 200 laser shots, with a typical intensity of 32 MW cmw2, taken with mass spectrometer configuration A. Argon ions with massto-charge ratios of 40 (Ar+), 20 (Ar2+), 13.33 (Ar3+), and 10 (Ar4+), along with some ions attributable to air, are seen. (The ion with a mass-tocharge ratio of eight could be 02+ and therefore cannot positively be identified as A?‘.) The data were obtained using the most sensitive range of the waveform recorder. The largest peaks are therefore off scale. Spectra taken on less sensitive ranges displayed these peaks in their entirety. Table 2 contains data relevant to the ionization of argon. This includes the energy required to produce multi-

to N+,

ply charged argon ions [35], the number of 193 nanometer photons that would be required if the ions were produced by photoionization, and the electron impact cross-sections for the production of the ions [2,36-391. Clearly, photoemission electron impact ionization is able to produce ions with very high appearance potentials. To produce these by laser multiphoton ionization would require an extremely intense light source. It has previously been shown that, even with = lOI W cme2 of 193 nm light, argon is not ionized beyond the 2 + state [40]. Table 3 lists the relative peak areas in our mass spectra. These are compared with electron impact cross-sections from Table 2, normalized to the cross-section for producing Ar3+. Their remarkable similarity is further evidence that most of these ions are produced via photoemission electron impact ionization. Conventional 70eV electron impact ionization produces only Arf and Ar2+ in a ratio of approximately 1 to 0.13 [41]. 3.1.2. Signal at ultra low light levels A second illustration of the efficiency of photoemission electron impact ionization is the very low amount of light needed to produce ions. We explored this by using very small amounts of

SM.

Colby and J.P. ReillylInt. J. Mass Spectrom. Ion Processes I31 (1994)

Table 2 Ionization,

energetic

and cross-section

Ion

Appearance

He+ He2’ Ar+ Ar2+ Ar’+ Ar4+ Ar2+ Ar’+ Ar4+

24.587 54.416

potential

131

125-138

data (eV)”

15.759 27.629 40.74 59.81

Photons

Mechanism

Cross-sectionb

4 9 3 5 7 10

He+e+Hef+2e He + e * He’+ + 3e Ar+e+Art+2e Ar + e + Ar2+ + 3e Ar + e + Ar’+ + 4e Ar + e + Ar4+ + 5e Ar+ + e + Ar2+ + 2e Ar2+ + e + Ar3+ + 2e Ar’+ + e + Ar4+ + 2e

3.7 1.4 2.5 1.6 8

x x x x x

(m2)

10-Z’ 10-24 lO-2o lO-2’ 10-24’

_ 9 x 10-2’ 3 x 10-Z’ 1.6 x IO-*’

a From ref. 35. b From ref. 2, for 1OOeV electrons. ‘4 x 1O-23 at 150eV.

incident light, Instead of sending the laser beam into the instrument, we directed it to a point on the exterior of the vacuum chamber approximately 8cm from the 19mm diameter input window. The only light that entered our mass spectrometer was a small amount scattered from the primary beam as it passed through the one meter lens 9Ocm away from the instrument. Figure 4 shows a mass spectrum of helium carrier gas taken under these conditions. It was recorded with 4000 laser shots and mass spectrometer configuration B. The spectrum shows a strong He+ ion signal. To produce this by multiphoton ionization would require the absorption of four ArF laser photons; however, the incident light was so weak that it was not possible to measure it without a calibrated

photomultiplier.

We conclude

that the

few 193nm photons that do enter the instrument generate electrons at an interior surface and the helium ions are thereby produced by photoTable 3 Ionization

data

Ion

Measured peak areaa

Relative cross-sectionb

Ar’ Ar2+ Ar3+ Ar4’

676.6 64.4 1 0.27

625 40 1 _

a From Fig. 3. b From Table 2.

emission electron impact ionization. Since this depends linearly on light intensity, it is possible to observe ions at even very low light levels. 3.2. Photoemission electron impact ionization as a cause of interference in laser ionization TOF-MS The additional ions produced by photoemission electron impact ionization can act as an impediment to the detection of ions produced by photoionization. This occurs through two processes: space charge effects and detector saturation. We performed experiments to observe both of these. 3.2.1. Detector saturation Following the arrival of a large current of ions, microchannel plate response becomes nonlinear. It is possible for ions produced by photoemission electron impact ionization to cause this effect. For example, Fig. 5 shows the relative He+ signal as a function of laser energy for instrument configuration A. Different sets of points represent data taken at different times or on different days. The He+ ion yield is not exactly reproducible because any variation in the laser beam profile changes the amount of light scattered off the tip of the chromatography column. Figure 5 shows that the He+ signal increases linearly with laser energy and then levels off as saturation begins. The very large He+ signal reduces the sensitivity and linearity of the microchannel plates for many

SM.

132

Colby and J.P. Reilly/M. J. Mass Spectrom. Ion Processes 131 (1994) 125-138

1

I

I

30

40

I

0

20

I

50

Mass/Charge

Fig. 4. Mass spectrum

obtained

at very low light levels. The principal

peak is He+.

some conditions the Sn+ signal actually increased as the laser light intensity and He+ peak decreased. An example of this is shown in Fig. 6(C) which was recorded with 9.1 mJ of light. Although this is about one-third of the fluence used to obtain the spectrum in Fig. 6(B), the signal intensity is clearly stronger. It is possible to reduce the detector saturation by deflecting the carrier gas ions out from their normal flight path. We did this by placing a +lOOOV potential on one of the deflection plates and then dropping the potential to OV after the carrier gas ions had passed and before the ions of interest entered the deflection region. Deflection He+ increases the chromatographic signal from a

microseconds. Figure 6 shows three mass spectra of Sn+ ions from tetraethyltin taken with instrument configuration A. These ions arrive approximately 10 ~LSafter the He+ signal. The He+ photoemission electron impact ionization signal (not shown) was much stronger in the second spectrum than in the first. The isotope ratio in Fig. 6(A) displays the expected distribution. This includes the stable isotopes at 112~ (l.Ol%), 116~ (14.8%), 117~ (7.75%), 118 u (24.3%), 120~ (32.4%), 122~ (4.56%), and 124~ (5.64%) [42]. In contrast, Fig. 6(B), recorded with 25mJ of light, shows a distorted isotope distribution. The microchannel plate saturation effect was so strong that under 2500 2250

0

F

0

iooo 1 UI

1500

t -

1250

-

1000

-

750

-

500

-

250

-

2

;

s 9 ‘Z 0 ru OL

0

0

9

0

1750

x0

0

X 0’0.

o8

A A

‘A” x

\AA

QX @

0

q

00

00 a

‘~‘~‘~~~~‘~~‘~‘~~‘1”~~~~‘~‘,~’ 5 10 Laser

Fig. 5. Relative He+ signal as a function

15 Energy

20

25

30

(mJ)

of laser energy for instrumental configuration different times.

A. Different

symbols correspond

to data taken at

SM.

Colby and J.P. ReillylInt.

J. Mass Spectrom.

Ion Processes

131 (1994)

125-138

133

A A

h

B

Mass/Charge

Fig. 6. Tetraethyltin mass spectra showing tin isotope distribution as indicator of nonlinearity: (A), the normal isotope distribution; (B), the isotope distribution when He+ ions are saturating the detector; (C), when the laser power is reduced by two-thirds. Only (B) and (C) are plotted on the same vertical scale.

100 pg sample of tetraethyltin by a factor of 20. The effect is even greater for smaller samples whose signal is unobservable without deflection. 3.2.2. Space Charge Efects Space charge effects are also exacerbated by photoemission electron impact ionization. Excess space charge reduces both the sensitivity and the resolution of the instrument. Sensitivity is reduced when analyte ions are pushed radially away from the path that would take them to the detector. This problem can be reduced by confining the ions to a narrow tube [29] or placing a wire with a negative potential down the length of the flight tube [43,44]. It is not possible to eliminate space charge effects with deflectors because they are strongest before the ions are separated. Resolution is impaired because space charge increases the width of the ion’s kinetic energy distribution; conditions that maximize carrier gas signal eliminate the structure in later parts of the mass spectrum. For example, there are usually several background peaks arising from pump oil. However, when the helium or argon signal is extremely high the pump oil signal remains strong but appears extremely broadened and unrecognizably distributed.

4. Methods for distinguishing photoemission electron impact ionization from photoionization

Photoemission electron impact ionization can clearly have serious consequences for experiments that depend on the wavelength selectivity of laser ionization. It is, therefore, important to be able to detect its presence. We have developed two methods for identifying the ions produced via this process. The first method relies on varying the electric field in the ionization region. As the potential of Go approaches that of Gr the maximum kinetic energy of the photoemitted electrons is lowered. When this energy drops below the appearance potential of an ion, the ion’s signal disappears. Figure 7 shows this for He’+ and He+. The He2+ ion signal is the first to disappear. As expected, it does so around the point where the maximum electron energy is near the ion’s 54.4 eV appearance potential. These data were taken with instrumental conditions B and 044mJ of light. The second method for identifying photoemission electron impact ionization is to exchange the potentials on Go and G,. Under normal conditions, positive ions are formed in the first acceleration region and accelerated toward the detector. Electrons are ejected from the grid Gr

134

S.M. Colby and J.P. ReillylInt.

80000

9

z ms 5 2+

,140o

r

70000 T .z 3 4

J. Mass Spectrom. Ion Processes 131 (1994) 125-138

O

60000

1200

t -

50000

.

40000

-

0 +$

+

-

20000

-

10000

-

0

0

-300

0

++OC? +++y$), I I -200 -100 Go-Q

Fig. 7. The effect of Go-G,

4

600

5 = & .G

oo+ tt I 100

0

-

1

5

- 400

+“+

+oO

0 -400

-

+o+o O+ - 600

+ 30000

z .= s

i 1000

ZJ

200

g

0

I 200

300

Voltoge Difference

acceleration

field on ion signals: (+),

and accelerated toward the plate Go. When the grid potentials are interchanged, positive ions formed in the first acceleration region are directed away from the detector. One would therefore not expect to see any ions produced by photoionization under these conditions. However, ions are still detected. These must be produced by photoemission electron impact ionization. This occurs because electrons formed at Go can now be accelerated through G,. After passing through the grid, into the second acceleration region, these electrons can collide with neutrals to form positive ions that are able to reach the detector. As referred to above, this

He2+; (O), He+.

process is shown in Fig. 2. Figure 8 presents spectra taken with instrumental conditions B. The signal arises from helium introduced as the carrier gas. The spectrum in Fig. 8(A) was recorded using normal grid potentials, while that in Fig. 8(B) was obtained with those potentials switched. The shift in ion flight times is approximately that expected for ions produced just above Gt. The reduction in signal is due to the fact that the electrons travel a much smaller distance in the second acceleration region than they do in the first and the amount of light striking each grid may be substantially different. Both spectra are plotted on

B h

0

I

10

1

I

20

30

40

I

50

Mass/Charge

Fig. 8. Effect of ion acceleration

field direction on observed mass spectrum of helium: (A), normal field direction; switched. The peak at m/z0 is due to the detection of scattered photons.

(B), grid potentials

S.M. Colby and J.P. ReillylInt.

J. Mass Spectrom.

Ion Processes

the same vertical scale. Switching the grid potentials is the easiest method for detecting photoemission electron impact ionization because the changes and measurement are simple. 5. Advanced suurce design The instrumental conditions C and C’ represent a new ionization source designed for simple conversion between photoionization and photoemission electron impact ionization. It has some distinct advantages over previous designs [3-51. These include better field uniformity, reduced thermal emission, better space focusing, and the ability to change modes without disturbing the instrumental setup. Previous designs involved the placement of a thin wire within the source region. This wire was used as the electron source and perturbed the uniformity of the electric field. Reducing the spatial distribution of the ionization by using the wire to form a potential well contributes even more to this problem [4]. The lack of field uniformity reduces the effectiveness of space focusing conditions since the field potential is no longer simply a function of distance between the grids. Thermal emission is undesirable because it may continue long after the end of the laser pulse. With the sources previously proposed it may occur when too much laser light is directed on the thin wire. In the new source thermal emission is less likely because the light is distributed over the larger surface area and volume of the plate Go. This should allow the use of greater photon fluxes and the generation of higher currents. Space focusing in this new source is as effective as that in traditional laser ionization sources. Under the conditions used with the new source, ionization by electrons, whose energy is at least 50eV, is only found within a region 2.5 mm above Gt. Calculated times-of-flight for He+ ions produced within this region vary by only 14ns. This is substantially below the time variation resulting from initial ion velocity distributions [45]. The greatest advantage of the new source is the ease of switching between ionization methods. No

131 (1994)

125-138

135

physical changes need to be made within the instrument. Only the three grid voltages and, if desired, the path of the laser beam need to be adjusted. We have found that for photoemission electron impact ionization, redirecting the laser beam, so that it hits the plate Go instead of passing directly between Go and Gi , is not absolutely necessary because there is sufficient scattered light. However, doing so substantially increases the signal intensity. To move the beam we either adjust the 1 m lens or insert a second 400mm focal length lens in front of the instrument. The second lens makes it possible to quickly return to the same laser ionization conditions. 5.1. Results with advanced source design

To demonstrate the new dual source we analyzed a sample of n-butylbenzene using both ionization modes. This molecule was chosen because there are clear differences between its electron impact and laser ionization mass spectra. The sample was allowed to diffuse into the instrument through a needle valve and the gas chromatograph was disconnected. Figure 9 shows the resulting mass spectra. Figure 9(A) was obtained with conventional laser ionization. It shows the relatively soft ionization possible with even this short wavelength of light. The principal peaks are at 134, 119, 105, 91, and 92 u. These masses correspond to the parent ion and fragments resulting from the loss of parts of the alkyl chain. The data were obtained using configuration C with 1000 laser shots, 45 PJ per pulse, and a 1 m focal length lens. The resolution is limited by the rather large laser focal spot. The second mass spectrum, (Fig. 9(B)), was obtained with photoemission electron impact ionization under instrumental conditions C’. It shows the greater fragmentation expected in this case. Significant signals are seen for all the lower mass fragments. The resolution is slightly worse than that of the laser ionization mass spectrum but much better than that of the spectrum shown in Fig. 8(B), which was obtained without space focusing conditions. Figure 9(C) displays a 70eV

136

SM.

Colby and J.P. Reilly/M

A

ML,

0

20

IL

40

LA.

I-

60 80 100 Mass/Charge

A

120

140

Fig. 9. Three mass spectra of n-butylbenzene with: (A), laser ionization; (B), photoemission electron impact ionization; (C), 70 eV electron impact ionization.

electron impact ionization spectrum taken from the literature [46]. As expected, there is greater fragmentation in Fig. 9(B) than in Fig. 9(C) because of the higher range of electron energies and currents. The only changes made between recording the spectra in Figs. 9(A) and 9(B) involved adjusting the lens and setting power supply voltages. The same light fluence was used. 6. Summary and conclusions A primary goal of our research has been to maximize the analytical sensitivity of laser ionization GC-TOF-MS [30]. We found, however, that photoemission electron impact ionization produced the major limitation to developing a more sensitive technique. For example, our attempts to increase tetraethyltin signal included increasing the laser power and confining the GC

J. Mass Spectrom. Ion Processes 131 (1994) 125-138

effluent with the cup shaped source. However, both of these steps led to more photoemission electron impact ionization, which actually reduced our sensitivity to tetraethyltin. As shown, this reduction resulted from photoemission electron impact ionization of the GC carrier gas that is present in much greater abundance than the analytes. Carrier gas ionization is not ordinarily a problem in laser ionization GC-TOF-MS because the carrier gas or laser wavelength can be selected to prevent it. We expect that problems may be found whenever an experiment depends on specific properties of laser ionization such as selectivity or controlled fragmentation. Photoemission electron impact ionization can occur whenever there is the possibility of light striking the source and high intensities, high fields, or short wavelengths are used. Short wavelengths are not necessarily a requirement because multiphoton absorption can also induce photoemission [47,48]. The degree to which photoemission electron impact ionization occurs is a function of instrument geometry. The more confined the ionization region, the more likely it is that scattered light will strike a surface. In our experiments this was exemplified by the cup source, which had only small holes (2.5 mmx 13.5 mm and 3 mm diameter) for the laser beam and capillary column to pass through. Photoemission electron impact ionization greatly expands the versatility of TOF-MS. It is a universal ionization method that combines electron impact ionization’s advantages of reproducibility and generality, with a short temporal duration and reasonable spatial definition. One of its most attractive characteristics is that it can be adapted to the large number of time-of-flight instruments already in use. Our new dual source should be particularly convenient. It requires only slight modification of standard laser ionization time-offlight instruments. Photoemission electron impact ionization is an excellent complement to laser ionization. It should be useful for the analysis of unknowns because the mass spectra obtained can be compared with those

SM.

Colby and J.P. Reilly/Int. J. Mass Spectrom. Ion Processes 131 (1994)

from conventional electron impact ionization. It also makes it possible to detect molecules that, because of low cross-sections, very short excited state lifetimes 1491,or high ionization thresholds, are difficult to laser ionize. With this method of photoemission electron impact ionization, the advantages of time-of-flight are retained. These include low cost, high throughput, low weight, simplicity, and the ability to obtain a complete mass spectrum with each laser shot. In the past, for many applications, these advantages have been outweighed by the lack of a convenient electron impact ionization source. With photoemission electron impact ionization this is no longer the case. It is important to be aware that photoemission electron impact ionization can also be a major source of background signal in laser ionization experiments. We have pointed out several ways of identifying signal from photoemission electron impact ionization. Once identified, steps can be taken to eliminate it by reducing scattered light or adjusting laser parameters.

7

8 9 10

11

12

13 14 15 16

17 18 19

Acknowledgment 20

This work has been supported Environmental Protection Agency.

by the U.S. 21 22

References F.A. White and G.M. Wood, Mass Spectrometry, Applications in Science and Engineering, Wiley, New York, 1986, p. 18. T.D. Mark and G.H. Dunn (Eds.), Electron Impact Ionization, Springer-Verlag, New York, 1985, pp. 176, 306. E.R. Rohwer, R.C. Beavis, C. Kiister, J. Lindner, J. Grotemeyer and E.W. Schlag, Z. Naturforsch. Teil A, 43 (1988) 1151. J.G. Boyle, L.D. Pfefferle, E.E. Gulcicek and S.D. Colson, Rev. Sci. Instrum., 62 (1991) 323. K. Zipperer, R.C. Beavis, E.R. Rohwer, J. Lindner, C. Kostner, J. Grotemeyer and E.W. Schlag, Proc. 37th ASMS Conference on Mass Spectrometry and Allied Topics, Miami Beach, FL, 21-26 May 1989, pp. 5688569. R.E. Honig and J.R. Woolston, Appl. Phys. Lett., 2 (1963) 138.

23 24

25 26 27 28 29 30 31 32

125-138

137

A.Sh. Airapetov, G.A. Gevorgyan, I.V. Levshin and B.N. Yablokov, Sov. Techn. Phys. Lett., 15 (1989) 428 (and references therein). 0. Prokoviev and P.G. Seiler, Nucl. Instrum. Methods Phys. Res. A, 278 (1989) 608. C.H. Lee, Appl. Phys. Lett., 44 (1984) 565. M. Boussoukaya, H. Bergeret, R. CheHav, B. LeBlond and J. Le Duff, Nucl. Instrum. Methods Phys. Res. A, 279 (1989) 405. M. Yoshioka, M. Mutou, Y. Fukushima, T. Kamei, H. Matsumoto, H. Mizuno, S. Noguchi, I. Sato, T. Shidara, T. Shintake, K. Takata, H. Kuroda, N. Nakano, H. Nishimura, K. Soda, M. Miyao, Y. Kato, T. Kanabe and S. Takeda, Proc. 1984 Linac Conference, Gesellschaft fur Schwerionenforschung, Darmstadt, Germany, Report GSI-84-11, 1984, p. 469. E.L. Garwin, W.B. Hermannsfeldt, C. Sinclair, J.N. Weaver, J.J. Welch and P.B. Wilson, IEEE Trans. Nucl. Sci., 32 (1985) 2906. W. Peter and M.E. Jones, Part. Accel., 24 (1989) 231. Y. Kawamura, K. Toyoda and M. Kawai, Appl. Phys. Lett., 45 (1984) 307. Y. Kawamura, K. Toyoda and M. Kawai, Appl. Phys. Lett., 45 (1984) 1287. C.H. Lee, P.E. Oettinger, E.R. Pugh, R. Klinskowstein, J.H. Jacob, J.S. Fraser and R.L. Sheffield, IEEE Trans. Nucl. Sci., 32 (1985) 3045. J.S. Fraser, R.L. Sheffield and F.R. Gray, Nucl. Instrum. Methods Phys. Res. A, 250 (1986) 71. J.S. Fraser and R.L. Sheffield, IEEE J. Quantum Electron., 23 (1987) 1489. J. Bokor, A.M. Johnson, R.H. Storz and W.M. Simpson, Springer Ser. Chem. Phys. (Ultrafast Phenom. S), 46 (1986) 123. R.B. Marcus, A.M. Weiner, J.H. Abeles and P.S.D. Lin, Appl. Phys. Lett., 49 (1986) 357. T. Anderson, I.V. Tomov and P.M. Rentzepis, J. Appl. Phys., 71 (1992) 5161. N.K. Sherman, P.B. Corkum and T. Srinivasan-Rao, Proc. SPIE-Int. Sot. Opt. Eng., 664 (1986) 74. L.H. Luthjens, M.L. Horn and M.J.W. Vermeulen, Rev. Sci. Instrum., 57 (1986) 2230. A. Modinos, Field, Thermionic, and Secondary Electron Emission Spectroscopy, Plenum, New York, 1984, pp. 20, 22. S.D. Moustaizis, M. Tatarakis, C. Kalpouzos and C. Fotakis, Appl. Phys. I&t., 60 (1992) 1939. W. Schottky, Z. Phys., 14 (1923) 63. J.G. Potter, Phys. Rev., 58 (1940) 623. P.E. Oettinger, Proc. SPIE-Int. Sot. Opt. Eng., 998 (1988) 24. S.M. Colby and J.P. Reilly, Anal. Chem., submitted. S.M. Colby, M. Stewart and J.P. Reilly, Anal. Chem., 62 (1990) 2400. WC. Wiley and I.H. McLaren, Rev. Sci. Instrum., 26 (1955) 1150. R.C. Weast (Ed.), Handbook of Chemistry and Physics, 64th edn., CRC Press, Boca Raton, FL, 1983, p. E-76.

138

S.M. Colby and J.P. ReillylInt. J. Mass Spectrom. Ion Processes 131 (1994) 12.5138

33 R.B. Opsal, Ph.D. Dissertation, Indiana University, (1985). 34 R. Larciprete, S. Fontana and E. Borsella, Chemtronics, 4 (1989) 184. 35 R.C. Weast (Ed.), Handbook of Chemistry and Physics, 64th edn., CRC Press, Boca Raton, FL, 1983, p. E-63. 36 K. Stephen, H. Helm and T.D. Mark, Measurement of Partial Electron Impact Ionization Cross-Sections of Rare Gas Atoms and Dimers, in A. Quayle (Ed.), Advances in Mass Spectrometry, Vol. 8A, Heyden, London, 1979, p. 122. 37 D.C. Gregory, P.F. Dittner and D.H. Crandall, Phys. Rev. A, 27 (1983) 724. 38 A. Miiller, E. Salzborn, R. Frodl, R. Becker, H. Klein and H. Winter, J. Phys. B, 13 (1980) 1877. 39 K. Stephen, H. Helm and T.D. Mark, J. Chem. Phys., 73 (1980) 3763. 40 T.S. Luk, H. Pummer, K. Boyer, M. Shahidi, H. Egger and C.K. Rhodes, Phys. Rev. Lett., 51 (1983) 110. 41 F.W. McLafferty and D.B. Stauffer (Eds.), The Important

42

43 44 45 46

47 48 49

Peak Index of the Registry of Mass Spectral Data, Vol. 1, Wiley, New York, 1991, p. 100. G. Friedlander, J.W. Kennedy, E.S. Macias and J.M. Miller, Nuclear and Radiochemistry, 3rd edn., Wiley, New York, 1981, p. 624. N.S. Oakey and R.D. Macfarlane, Nucl. Instrum. Methods, 49 (1967) 220. R.D. MacFarlane, D.F. Torgerson, Y. Fares and CA. Hassell, Nucl. Instrum. Methods, 116 (1974) 381. R.B. Opsal, K.G. Owens and J.P. Reilly, Anal. Chem., 57 (1985) 1884. E. Stenhagen, S. Abrahamsson and F.W. McLafferty (Eds.), Registry of Mass Spectral Data, Wiley, New York, 1974, pp. 179,263. P. Martin, R. Trainham, P. Agostini and G. Petite, Phys. Rev. B, 45 (1992) 69. E.M. Logothetis and P.L. Hartman, Phys. Rev., 187 (1969) 460. C.W. Wilkerson Jr., S.M. Colby and J.P. Reilly, Anal. Chem., 61 (1989) 2669.