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A d . Chem. 1991, 63, 990-993
Beam Voltage Manipulation for Time-of-Flight Mass Analysis of Continuous Ion Beams Kenneth A. Cowen, Christopher J. Frank, and James V. Coe* Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210
Beam voltage manlpulatlon has been used for tlme-of-fllght mass analyds of a contlnuous Ion beam wlthout separation of pulses from the continuous beam. The procedure defines a pulse of Ions In the continuous beam by tagging the pulse wlth a slightly different beam energy. Thls Is accompllshed, In a remarkably slmple fashion, by dlrectlng the Ion beam through a short tube to which a 1 0 4 square wave Is applled. Ions that have had thelr beam energy manlpulated leave a hole at the time they would have arrived and accumulate at thelr new arrlval t h e . These changes In the Ion current are readlly detected when observed on the time scale of lon f#ght through the beam line. I t Is hoped that this partlcular appllcatlon lndlcates a general utility for beam voltage manlpulation-spedflcaly the abWlty lo modulate a continuous Ion beam wlth great control over the wave form of the Ion beam’s resulting Intenshy. Thls attribute should prove useful In a variety of Ion optlcal appllcatlons.
INTRODUCTION The possibilities of beam voltage manipulation became apparent to us from experience gained in pushing ion beam velocity modulation techniques (1-3) to the highest frequencies possible. In these experiments, the signal falls dramatically as the period of modulation becomes comparable to the ion flight time in the ion-photon interaction tube. At high modulation frequencies (- 1 MHz), a significant fraction of the ion beam has had its beam energy altered. The same physics has been used by several investigators ( 4 , 5 )in pulsed decelerators for the injection of ions into a Fourier transform ion cyclotron resonance spectrometer. A new type of mass spectrometer is being developed for continuous ion beams based upon beam voltage manipulation. The technique works by changing the voltage on a tube through which ions are moving on a time scale faster than the ions can travel through the tube. Since the ions are inside a conductor, they do not feel the force normally associated with a changing potential. The ions may only experience the change upon emerging from the tube, whereby their beam energy has been changed by an amount equal to the tube’s potential change while they were inside. The following sequence may be more illustrative: (1)ions with a certain beam energy at ground potential enter a tube with no potential difference from ground; (2) the tube’s potential is changed rapidly compared to the ion’s transit time through the tube; (3) being inside a conductor, the ions feel no force due to the tube’s changing potential and remain at the same velocity although the tube’s potential is now different than ground; (4) emerging from the tube into a region at ground potential, they now experience the potential difference between the tube and ground; (5) the ions must accelerate (or decelerate) and assume a different beam energy at ground potential than they had initially. Beam voltage manipulation of this sort has been used to define pulses of ions within a continuous ion beam by tagging pulses with a slightly different beam energy. This procedure
results in some very simple techniques for recording timeof-flight mass spectra. Our earliest version (6) involved filtering off pulses by directing the ion beam toward an aperture maintained at a potential that stops unmanipulated ions but allows pulses defined with higher beam energy to pass. Upon separation of pulses, the device could be configured as a conventional time-of-flight mass spectrometer or alternatively as a mass filter. The latter arrangement was used to produce a 50000-kHz, mass-selected beam of protonated nitrogen with packets of about 5 x lo5ions arriving a t 2-ps intervals, only 16 cm from the ion source. Recently, we have realized that it is not necessary to separate the tagged pulses from the continuous ion beam in order to record a time-of-flight mass spectrum. When the total ion current from a continuous source is monitored in real time (triggered by the rising edge of the beam voltage manipulating wave form), ions that have had their beam energy manipulated leave a hole a t the time they would have arrived and accumulate at a new arrival time. Mass spectra are recorded by observing changes in the continuous ion current as opposed to separation of the pulses. This configuration should be of particular interest to those performing continuous ion beam experiments when mass spectrometry is an aspect but not the final goal of the investigations. I t represents a simple way to add mass analysis to a beam line or post mass analysis of a mass-selected beam. This technique is attractive because it is both mechanically and electronically simple. A short tube to which is applied a 10-V square wave and a means of looking at the ion current in real time are all that must be added to an existing beam line. In more general terms, beam voltage manipulation can be used to modulate a continuous ion beam with great control over the wave form of the ion beam’s resulting intensity. In the latter part of this paper, we explore the mechanism of beam voltage manipulation by tracing the trajectories and velocities of ions through the time-dependent electrostatic potential of the tube used to produce these effects. The technique is likely to be generally useful in ion optical applications. It could be employed, for instance, to bunch ions in order to achieve higher ion densities downstream in a ion beam. We have used it to produce sinusoidal variations in ion intensity, suggesting the possibility of a Fourier transform mass spectrometric technique.
EXPERIMENTAL SECTION A schematic diagram of our apparatus is shown in Figure 1.
The ion source used to demonstrate this technique was a cold cathode discharge source (CCDS).The CCDS consisted of two hexagonal iron electrodes 3.7 cm across separated by a 4-cm glass tube with a 9-mm inner diameter. Viton O-rings were used to form a vacuum seal between the glass and the electrodes. The front plate (6 mm thick) had a 2-mm hole drilled through the center and had a 22’ taper cut into both the front and the back. Gas was introduced into the source through a 6-mm hole in the side of the back plate (1.8 cm thick). Two small insulated magnets were placed across the two electrodes, increasing the total ion current extracted from the source. A three-apertureelectrostatic lens was used to extract and focus the ions from the source anode which was floated at +2000 V. The beam voltage manipulator (BVM) consisted of a 3-mm-long
0003-2700/91/0383-0990$02.50/0 0 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 10,MAY 15, 1991 ion
Source
Ion Lens
Faraday
BVM
Drift Region
CUP
l I I I L l I ,
h
I DR
I
0 5
0
Figure 1. Schematic diaqam of the experimental apparatus. The ions are extracted from a cold cathode discharge source that is floated at +2000 V. The electrostatic lens (Ll) is used to optimlze ion current co#ected on a faraday cup at a desired drift region (DR) potential. Ions that are inslde the beam voltage manipulator (BVM) when its potential is changed have their beam energy changed by an amount equal to the change applied to the BVM. The “tagged” ions travel at a different beam velocity and therefore have their arrival times altered. Manipulation of this type leads to characteristic variations in ion current when monitored in real time. These changes in ion current allow a timeof-flight mass spectrum to be recorded within continuous wave ion beams.
tube to which was applied a square wave (10 V, 50 kHz, 10-ns rise time). The tube was encased in a grounded shell to reduce through-space pickup of the rf waveform on the faraday cup. All apertures in the lens and BVM were 3.2 mm in diameter. The beam entered the drift region through a 3.2-mm aperture but was not constrained by small apertures thereafter. The ions were then decelerated, sent through a drift region (30 cm long, 2.2 cm i.d.), and collected on a faraday cup (1.1-cm diameter). This arrangement eliminated potentially detrimental focusing effects that might have been caused by weak lensing of the BVM wave form (10-eV changes in the 2000-eV beam energy). The ion current was converted to a voltage by using a transimpedance amplifier (Texas Optoelectronics Inc. TIEF-151), which could be used due to the nearly space-charge-limited currents available in our experiments. These currents would saturate electron multipliers or even high-current microchannel plates. The signal was viewed in real time on an oscilloscope triggered by the BVM wave form. The signal-to-noise ratios observed in this arrangement were small (3:l) and limited by the noise of the amplifier. This system was very convenient for tuning the apparatus and for optimizing the production of a specific ion. To record spectra, the output of the transimpedance amplifier was amplified with a gain of 100 (Princeton Applied Research Model 115) and sent to a boxcar averager (Princeton Applied Research Model 160). The boxcar was controlled and monitored by a minicomputer (CompuAdd 286, Metrabyte DASH 16F 1/0 board). A reasonable signal-to-noise ratio was obtained with 60-s scans. The setup is also amenable to signal-averaging techniques. Beam voltage manipulation occurs at the rising and the falling edges of the square wave. Ions caught in the beam voltage manipulator (BVM) during the rise of the wave form experience an increase in beam energy, while those caught in the BVM during the fall experience a decrease in beam energy. Ions that receive an increase in beam energy arrive sooner than ions that have not had their energy manipulated, and those that experience a decrease in beam energy arrive later than untagged ions. In Figure 2, the BVM wave form is superimposed upon an observed ion signal to demonstrate certain features of this technique. The upward peaks in the ion current correspond to accumulations of ions that have had their beam energy altered. The downward peaks correspond to holes in the ion current left by the tagged ions. The edges of the square wave define a zero in time at which pulses are generated. Two full time-of-flight spectra are defined in each period of the BVM wave form. In the first, the trapped ions have gained energy and accumulate at times sooner than their holes. In the second, the holes appear before the accumulations because the tagged ions experienced a decrease in beam energy. A 6:l mixture of H2/02was used to produce the mass spectrum in Figure 3. This mixture favored the production of H30+over
lime-of-Flight
25
20
15
10
(pa)
Figure 2. Illustration of the beam voltage manipulation technique. A trace of the BVM wave form is superimposed on the signal. The edges of the BVM wave form indicate when the pulses of tagged ions are defined. Two mass spectra are determined for each period of the BVM wave form. The different masses displayed correspond to hydrated hydronium cluster ions. H,O‘
V I , . , , , ,
2
3
,
,
,
4
Time-of-Flight
,
,
,,,,,,,,,,I 5
7
6
(microseconds)
Figure 3. Timeof-flight mass spectrum of a 6:l H2/02discharge mixture. Observed ion cwent as a function of the delay from the rising edge of a 50-kHz square wave applied to the BVM. The features observed are the changes in 2.0 pA of total ion current. The drift region was +1409 V, giving the ions 591 eV of beam energy.
I
I
Time-of -Flight
(microseconds)
Figure 4. Timeof-fligM mass spectrum of hydrated hydronium cluster ions. Neat water vapor was the source gas, and 1.6 pA of total Ion current was collected. The potential applied to the drift region was +1830 V, giving the ions a beam energy of 170 eV in this section.
02+.Neat water vapor was used as the source gas for the mass spectrum in Figure 4 of hydrated hydronium ions. Under both circumstances, the pressure in the source was held a t approximately 2 Torr. With the Hz/Oz mixture, the discharge voltage was 2000 V. When water was used,the discharge voltage was 1600 V. Discharge current was maintained at approximately 2 mA with 250 kQ of ballast resistance in series with the discharges.
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 10, MAY 15, 1991 6cm
1.0
1.1
15
2.0
3 .I
3 0 om
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4.0
7.0
4 5 om
7 .6
10.6
11.0
60
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14.21
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14.71
Time-of -Flight (microseconds)
Figure 5. Computer model of the evolution of the observed line shape as a function of drift region length. The ion current has been normalized such that 1.O represents the average continuous Ion current collected at the faraday cup. At 15 cm, bunching can be observed, causing the upward peak to be taller and narrower than the downward peak. At 60 cm, the upward peak has been spread out. The length of the drift region is perhaps not as critical to the evolution of the line shape as the nature of the BVM or the beam energy of the ions. The potential on the drift region was +1410 V for the spectrum shown in Figure 3 and +1830 V for the spectrum in Figure 4, resulting in beam energies of 590 and 170 eV, respectively. This arrangement maximized the total ion current collected and minimized detrimental focusing effects. The Hz/Oz mixture produced a total ion current of 1.6 PA, and neat water vapor produced 2.0 PA.
RESULTS AND DISCUSSION The observed line shapes have a derivative appearance that is the result of unresolved upward and downward peaks. The accumulations are not fully separated from the holes because the tagged ions have had their beam voltage manipulated by only a small amount. The point at which the observed line shape crosses the baseline is the average of the arrival time of the upward and downward peaks. This point is well estimated by the following expression
where the sum is over three different sections of the beamline. In this expression, diis the length of each section, m is the ion mass, q is the charge, Vi is the beam energy in each section, and 6 is the peak-to-peak amplitude of the wave form applied to the BVM. The lengths of the BVM field ( d l ) , the drift region ( d 2 ) ,and the distance at ground potential (d3)were 0.005,0.30,and 0.05 m, respectively. For a given set of conditions, all of the parameters in eq 1,except m, are constant, giving
t = km’/2
(2) A linear least-squares fit of the arrival times in Figure 4 versus the square root of mass gives the following expression
t = (1.690 f 0.004)m*/2+ (0.124 f 0.024)
(3) in which the times are in ps and the masses are in amu. The small offset is due to the detection electronics. The standard deviation of the time for this fit is 11 ns, which is approximately the interval between adjacent points in the scan. Propagation of errors from the fitted expression above gives a 6mlm of 0.4% in the limit of high mass. Defining resolution ( R ) is somewhat problematic. It is typically calculated from the following relationship (7-10) R = -I -t (4) 2 bt where t is the arrival time of an ion and 6t is the FWHM of a pulse of that ion. The H30+feature in each spectrum was fit by using a nonlinear least-squares procedure assuming Gaussian profiles for both the accumulated peaks and the holes. The widths of the fitted Gaussian line shapes were
found to be 96 ns in Figure 3 and 120 ns in Figure 4. The widths were used for 6 t in eq 4 above. Defiied in this manner, the resolution of the spectra in Figures 3 and 4 was found to be 20 and 25, respectively. Contributing factors to the resolution inclued (1)the geometry (in particular the length) of the BVM, (2) the finite rise time of the BVM wave form, (3) the energy spread of the ion source, and (4) the time resolution of the detection system. The potential in the BVM and the rise time of the square wave determine the basic line shape of the feature. The energy spread of the ion source and the time resolution of the detection scheme broaden the basic line shape. The energy spread (FWHM) of the CCDS employed in these experiments has been determined to be about 3 V (3),which for H30+translates into a contribution of 65 ns in the first spectrum and 10 ns in the second. The boxcar was found (afterwards) to have a time resolution of 100 ns. Scans in which the total ion current was varied showed no change in resolution, so space charge is thought to have a negligible effect. Finite difference grid calculations (11,12) were performed to characterize the potential introduced by applying the square wave to the BVM. This potential determines the velocity profile imposed on the ion beam by the BVM. Only ions that are in the field free region of the BVM during the entire rise time of the BVM wave form receive the full beam voltage manipulation. Ions in the fringe field of the BVM are tagged with less than the full voltage applied to the BVM. As a result of field penetration into the BVM, a somewhat Gaussian velocity profiie is produced from the BVM geometry employed in our experiments. The profile produced by the BVM was modeled by direct numerical integration of Newton’s equation for ions starting a t different locations throughout the BVM. This profile was then propagated through the drift region by using different beam energies and drift tube lengths. Plots of the evolution of the line shape at different distances in the drift region are shown in Figure 5. In this model, the ions have a beam energy of 170 eV, the rise time of the BVM wave form is 10 ns, the broadening width due to the boxcar is 100 ns, and the energy spread of the ion source is 3 V (FWHM). Ions that are tagged with the full BVM voltage can be bunched with ions on the leading edge of the packet of tagged ions due to the difference in velocity. At the right distance for a particular drift region energy, this results in an upward peak that is of greater amplitude and is narrower than the downward peak. At longer times, the velocity profile spreads the upward peak while the downward “hole” is left unaffected. Beam voltage manipulation has been exploited to produce a very simple time-of-flight mass spectrometer for continuous ion beams using inexpensive and readily available electronics. Model calculations have been performed which indicate that order of magnitude increases in resolution can be obtained
Anal. Chem. 1991, 63,993-1000
with longer beam lines. Mass spectrometrists may be interested in exploring the technique's potential for high resolution. By pushing the technique with faster rise times of the BVM wave form, higher beam voltages, smaller BVM tubes, and energy selection of the continuous ion beam, vastly higher resolutions could be achieved. The technique shows great promise as a method for high-frequency population modulation and appears to be amenable to Fourier transform mass spectrometric techniques.
ACKNOWLEDGMENT We are also indebted to A. G. Marshall for discussions about this instrument and our neighbors who have lent us equipment, including R. L. McCreery, C. W. Mathews, and T. A. Miller. LITERATURE CITED (1) Coe, J. V.; Saykaiiy. R. J. In Ion and Cluster Ion Spectroscopy and Stnrcfure; M e r , J. P., Ed.; Elsevier: Amsterdam, 1989; pp 131-154. (2) Coe. J. V.; Owrutsky, J. C.; Keim, E. R.; Agman. N. V.; Hovde, D. C.; Saykally, R. J. J. Chem. Phys. 1989, 90, 3893-3902.
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(3) Keim, E. R.; Polak, M. L.; Owrutsky, J. C.; Coe, J. V.; Saykally, R. J. J . Chem. Phys. 1990, 93, 3111-3119. (4) Alford, J. M.;Williams, P. E.; Trevor, D.J.; Smalley R. E. Int. J. Mess Spectrom. Ion Roc. 1988. 72, 33-51. (5) Hanson, C. D.; Keriey, E. L.; Russell, D. H. In Treatise on Analyticel Chemistry; Wlnefordner, J. D.. Ed.; Wlley Interscience: New York, 1989; Vol. 11, Part I, pp 128-133. (6) Cowen, K. A.; Coe. J. V. Rev. Sci. Instrum. WBO, 61. 2601-2604. (7) Cotter, R. J. B i d . Environ. Mass Spectrom. 1989, 18, 513-532. (8) Brunnee, C. Int. J. Mess Spectrom. Ion Roc. 1987, 76, 125-237. (9) Watson, J. T. Introduction to Mess Spectrometry;Raven, New York, 1985. (10) Pinkston, J. D.; Rabb. M.;Watson, J. T.; Allison, J. Rev. Sci. Instrum. 1985, 5 7 , 583. (11) Weber, C. In Focusing of Cherged Particles; Septier. A., Ed.; Academic Press: New York, 1967; Vol. 1, pp 45-99. (12) Computer program written by Charles Schmuttenmaer. University of California, Berkeley; modified for use on IBM PC clones by J. Coe.
RECE~VED for review November 26,1990. Accepted February 27,1991. We gratefully acknowledge the support of the Department of Chemistry at The Ohio State University as well as the Petroleum Research Fund, administered by the American Chemical Society.
Electrical Double-Layer Model for Sorption of Ions on Octadecylsilyl Bonded Phases Including the Role of Residual Silanol Groups Hanjiu Liu and Frederick F. Cantwell* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Sorption isotherms on a highly end-capped octadecyisiiyi (ODS) silica bonded phase packing have been measured at various ionic strengths, adlusted with NaCi, both for tetra-nbutyiammonium cation (TBA'), from a solution of its chloride salt, and for p-nitrobenzenesulfonate anion (NBS-), from a solution of its sodium salt, using the column equilibration technique. At constant ionic strengths the NBS- isotherms are strictly Langmuirian, while the TBA' isotherms are Langmulrian only after subtractlng the moles of TBA' that are strongly adsorbed on residual siianoi groups. The completeness of elution of TBA' in the experiment has been verified by neutron activation analysis. Sorption of each of the organic ions, NBS- and TBA', is quantitatively described by the Stern-Guoy-Chapman (SGC) model of the electrical double layer. The surface potential $, is Nernstian for the TBA' system but is markedly sub-Nernstian for the NBS- system. The latter is due to the presence of a small number of anionic siianoiate sites on the ODS packing.
INTRODUCTION The sorption of ions onto chemically bonded reversed-phase stationary phases in liquid chromatography is important both in the direct separation of ions (1)and in the separation of ions by the more popular technique of "ion-pair" chromatography (2). In both techniques the sample ions to be separated are sorbed onto the stationary phase, and in the latter technique this sorption occurs in the presence of "pairing ions", which also are sorbed. The sorption of ions onto nonionic interfaces has been the object of study for many years because of its importance in
surface and electrochemistry and in separation science and technology. On nonpolar sorbents the main chemicul attractive forces between the solute and the sorbent are dispersion forces but, because these are relatively weak, solvent-solute interactions, solvent-solvent interactions, and solvent structure (entropy) effects also account for a significant fraction of the chemical interaction energy ( 3 , 4 ) . For ionic solutes there exists an electrostatic interaction energy in addition to the chemical interaction energy. Nearly all of the well-establishedphysicochemical models for ionic sorption take into account the electrical potential and the electrical double layer that develop at the interface (5-7). This is true for ionic sorption a t air-liquid and liquid-liquid interfaces (8)as well as at solid-liquid interfaces involving both polar and nonpolar solids such as quartz, graphite, and polystyrene (3, 4 , 9). Since the introduction of classical electrical double-layer concepts to explain the sorption of ions on reversed-phase packings used in high-performance liquid chromatography (HPLC) (IO), they have increasingly been invoked to explain ionic sorption on both porous polymers and chemically bonded sorbents (11-27). A form of the Stern-Gouy-Chapman (SGC) theory of the electrical double layer was initially proposed (10). The SGC theory is derived by solving the Poisson-Boltzmann equation for a planar interfacial geometry under semiinfinite conditions (5). In this laboratory, experimental tests of the SGC model have been based on measuring the ionic strength dependence of the distribution coefficient for the ion. Recently, others have proposed modifications to this model, such as solution of the Poisson-Boltzmann equation for different geometries, in an effort to more realistically represent the electrical potential gradient within pores (21,25). The results of these efforts do not give significantly better
0003-2700/91/0363-0993$02.50/00 1991 American Chemical Society
