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Anal. Chem. 1983, 55, 1318-1323
hydroxy-l0,ll-dihydrocarbamazepine,35079-97-1; adenosine, 58-61-7;guanosine, 118-00-3;androsterone glucronide, 1852-43-3; tetramethylsilane, 993-07-7; tetramethylsilane-d,,, 18145-38-5.
LITERATURE CITED Carroll, D. I.; Nowlin, J. G.; Stillwell, R. N.; Hornlng, E. C. Anal. Chem. 1981, 5 3 , 2007-2013. Odiorne, T. J.; Harvey, D. J.; Vouros, P. J . Phys. Chem. 1972, 7 6 , 3217-3220. Odiorne, J. J.; Harvey, D. J.; Vouros, P. J . Org. Chem. 1973, 3 8 , 4274-4278. Hendewerk, M. L.; Well, D. A.; Stone, T. L.; Ellenberger, M. R.; Farneth, W. E.; Dixon, D. A. J. Am. Chem. SOC.1882, 104, 1794-1799. Ellenberger, M. R.; Hendewerk, M. L.; Well, D. A.; Farneth, W. E.; Dlxon, D. A. Anal. Chem. 1982, 5 4 , 1309-1313. Trenerry, V. C.; Blair, I. A.; Bowle, J. H. Aust. J . Chem. 1980, 3 3 , 1 143-1 146. Bowle, J. C. Acc. Chem. Res. 1980, 13, 76-82. Blair, I. A.; Philllpou, G.; Bowie, J. H. Aust. J. Chem. 1979, 3 2 , 59-64. Blair, I. A.; Bowie, J. H. Aust. J. Chem. 1979, 3 2 , 1389-1393. Trenerry, V. C.; Bowie, J. H.; Blair, I. A. J. Chem. SOC., Perkln Trans. 2 1979, 1640-1643. Blair, I . A.; Bowle, J. H.; Trenerry, V. C. J. Chem. Soc., Chem. Common. 1979, 230-231. Shea, K. J.; Gobeille, R.; Bramblett, J.; Thompson, E. J . Am. Chem. SOC. 1978, 100, 1611-1612. Morita, T.; Okamoto, Y.; Sakural, H. Tetrahedron Lett. 1980, 835-838. MacFarlane, R. D.; Torgerson, D. F. Science 1976, 191, 920-925. Barber, M.; Bordoli, R. S.;Sedgwick, R. D.; Tyler, A. N. J . Chem. Soc., Chem. Commun. 1981, 325.
(16) Barber, M.; Bordoll, R. S.;Sedgwlck, R. D.; Tyler, A. N. Nature (London)1981, 293, 270-275. (17) Barber, M.; Bordoli, R. S.;Sedgwlck, R. D.; Tyler, A. N. 29th Annual Conference on Mass SDectrometrv and Ailled Tonics. American Soclety for Mass Spectrometry, May 22-29, 1981, Abstracts pp 351-352. (18) Zackett, D.; Schoen, A. E.; Cooks, R. G.; Hemberger, P. H. 29th Annual Conference on Mass Spectrometry and Allied Topics, American Society for Mass Spectrometry, May 24-29, 1981, Abstracts pp 27-28. (19) Hardln, E. D.; Vestal, M. L. 29th Annual Conference on Mass Spectrometry and Allied Topics, Amerlcan Soclety for Mass Spectrometry, May 24-29, 1981, Abstracts p 29. (20) Krueger, F. R. 29th Annual Conference on Mass Spectrometry and Allied Topics, Amerlcan Soclety for Mass Spectrometry, May 24-29, 1981, Abstracts pp 25-26. (21) Busch, K. L.; Unger, S. E.; Cooks, R. G. 29th Annual Conference on Mass Spectrometry and Allied Topics, American Society for Mass Spectrometry, May 24-29, 1981, Abstracts pp 30-31. (22) Kambara, H.; Hlshlda, S. Anal. Chem. 1981, 5 3 , 2340-2344. (23) Bennlnghoven, A. Trends Anal. Chem. 1982, 1 , 311-314. (24) Zakett, D.; Schoen, A. E.; Cooks, R. G.; Hemberger, P. H. J. Am. Chem. SOC. 1081, 103, 1295-1297. (25) Cotter, R. J. Anal. Chem. 1081, 5 3 , 720-721.
RECEIVED for review April 8, 1982. Resubmitted January 19, 1983. Accepted February 23,1983. This work was supported by Grant GM-13901 from the National Institute of General Medical Sciences, Grant HL-17269 from the National Heart, Lung and Blood Institute, and Grant Q-125 from the Robert A. Welch Foundation.
Molecular Secondary Ion Mass Spectrometry with a Liquid Metal Ion Primary Source Douglas F. Barofsky,* Ulrich Giessmann, Anthony
E. Bell,
and Lynwood W. Swanson
Oregon Graduate Center, 19600 A/. W. Walker Road, Beaverton, Oregon 97006
The use of Ga, In, SIAu, and BI llquld metal ion (LMI) emmers as prlmary sources in matrix assisted molecular secondary Ion mass spectrometry (SIMS) was lnvestlgated. The prlnclpai observatlons were a very high molecular Ion abundance from the glycerol solution of every test compound (10-50 times that attainable wlth Ar fast atom bombardment), a strong increaslng dependence of molecular ion slgnal on prlmary beam current denslty up to a saturation level at lo4 A cm-*, a strong increasing dependence of molecular Ion abundance on atomic number of the bombardlng element, a secondary ion energy spread of -3 eV, and a statlstlcai fluctuation of -3-10% In the secondary ion slgnals. No essential, quailtative dlfferences were observed between L M I S I M S mass spectra and those of other soft ionizetion techniques. Applications In SIMS analyses, fundamental Investigations of secondary Ion processes, and, possibly, localized molecular analysis of blologlcai tlssue coupled wlth Ion microscopy are Indlcated.
-
Beginning with the introduction of field desorption mass spectrometry in 1969 (I),revolutionary advances have taken place in the mass spectrometric analysis of thermally labile, high molecular weight organic compounds. Relatively recently, secondary ion mass spectrometry (SIMS) employing either atomic ion (2-4) or neutral atom (5, 6) primary beams has contributed significantly to this progress. Some of the major developments in molecular SIMS have been brought about through the use of primary beams with
different physical properties. The initial molecular SIMS measurements (2)were, in fact, made after recognizing the significance of the static SIMS condition (7) whereby the primary particle flux is so low that individual surface molecules do not experience multiple impacts during measurement. Introduction of fast atom bombardment (FAB), in which neutral argon primary beams were used (5, 6), permitted widespread adaptation of molecular SIMS to magnetic sector instruments and, with the assistance of a glycerol sample matrix, realization of the inherent potential in the SIMS method for identification of high molecular weight compounds (8-11). Bombardment with heavier primary particles, such as cesium (12-14)and xenon (15-17), substantially increased secondary molecular ion abundances and permitted still higher molecular weight identifications to be made in both the static SIMS (18) and FAB/glycerol matrix (19)modes. In two recent studies, in which primary beams formed from mercury (20) and trimethylpentaphenyltrisiloxane(21)were used, secondary molecular ion abundances of compounds mixed in glycerol matrices were reported to be an order of magnitude or more greater than those obtained with an argon beam. Unfortunately, these experiments were quite limited in scope, and the reasons for the unusually high secondary ion abundances are, for the present, obscure. Moreover, the mercury and siloxane sources are apparently impracticable (22).The results are none the less significant because they demonstrate that conditions exist, relative to those commonly described for inert gas primary sources, whereby the abundance of secondary molecular ions can be substantially increased. Such increases in turn raise the prospects for ex-
0003-2700/83/0355-1316$01.50/0@ 1983 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
tending the range of applicability of molecular SIMS beyond its already considerable limits. This potential for advancing the capabilities of molecular SIMS by achieving higher secondary ion abundance8 was one of the major reasons for undertaking the present investigation of the use of the liquid metal ion (LMI) source in producing secondary ions from organic compounds dissolved in liquid matrices. This source is generally characterized as a very bright, virtually point, field emitter. Experimentally, it is known that emission takes place from a cone of liquid metal (23) which is formed and maintained by a relatively high electric field (24,W). The ionization and ejection processes occurring at the apex of the emission cone are not understood well enough at present to formulate a model consistent with the source’s emission properties. Scientific and engineering interests in the LMI source derive from potential applications in such diverse areas as microfabrication, ion beam lithography, maskless ion implantation, ion microprobe analysis, and ion propulsion (24-27). Focused beams which deliver 2(t120 pA into spots of about 100 nm diameter have been achieved recently in the development of LMI sources for ion microprobe analysis of solids (28, 29). This accomplishment is of interest within the present context because SIMS is an integral part of an ion microprobe a n a l p r and because there is potential for molecular mapping of biological tissue with an ion microprobe (30). The prospect of utilizing the point source characteristics of the LMI emitter to achieve localized, molecular analysis of biological specimens in combination with ion microscopy gave strong additional impetus to the present work. In the two most common LMI emitter configurations the cone of liquid metal is formed either at the end of a narrow caDillarv (24) or at the end of a verv sharn needle which is witted by.a film of the liquid (2.5).-The sources have been found to emit both inns and charged micropanicles from a variety of metals. The ionic component is comprised of singly w e d and multiply charged atomic and molecular ions. The relative ahundance of these different snecies and their individual energy distributions and angular divergences are strongly dependent on the total emission current ( 3 1 4 1 ) . The make-up of the charged microparticle component is not as well studied as that of the ionic part. Apparently, at emitter currents lesa than 100 PA the rate of loss of masa, dmldt, from an LMI emitter is proportional to the emission current, and charged microparticles account for about two-thirds of the ejected material (42). Both the size and size range of the charged microparticles (-5-10 nm at 40 pA, -1W500 nm at 140 PA) have been shown to increase measurably with increasing emission current (43). The possibility of exploiting this rich selection of ions and microparticles in the investigation of secondary ionization processes provided the final incentive for the present study. Preliminq accounts of the use of LMI emitters as p r i m q sources for molecular SIMS have been given by the present authors elsewhere (44,45).The present report fuUy describes and discusses the initial investigation of this application for LMI sources. ~
~
~
~.~ ~
~
~
~
~~~~~~
~~~~~~
~~~~~~
EXPERIMENTAL SECTION Mass analysis was performed with a conventional, single focusing instrument (Hitachi/Perkin-Elmer RMU-GE), which has a mass range of 400 amu at an accelerating voltage of 4 kV. Needle LMI sources of the type shown in Figure 1 were used with gallium, indium, silicon-gold eutectic alloy, and bismuth metals. Briefly, the needle source consists of a tungsten hairpin filament with a short length of small diameter (130pm) tungsten or Nichrome (nickel chromium) wire spot welded to it, The tip of the wire is electrochemically etched to a point with a radius of curvature at the apex of 2-5 pm. After the assembly is thermally cleaned under vacuum, the reservoir at the junction of the
1319
Flgure 1. Needle LMI emitter (top)before wening and (bottom] after Wening with silicon-gold eutectic alloy.
bend in the filament and the emitter wire is filled while still under vacuum by dipping it into a molten pool of the liquid metal. Wetting of the shaft and sharpened tip of the wire is facilitated by grooving and roughening the surfaceswith additional chemical etching. A more complete description of the fabrication and wetting of these needle sources is given elsewhere (34, 46). The LMI sources were operated in h t h unfocused and focused modes. The two configurations are illustrated schematically in Figure 2. The unfocused variation (Figure 2a) consisted of a simple emittercathode assembly mounted about 12 mm from the target with the liquid metal beam directed perpendicular to the optical axis of the mass spectrometer. The focused version (Figure 2b) utilized a three-element aperture lens to focus the beam into a spot of less than 1mm diameter at the sample target: the entire assembly was mounted on an X-Y table to facilitate mechanical alignment. Despite this latter provision alignment of the focused source as built was difficult to maintain. Operation of the LMI source is essentially the same as that of field ionization (FI) or field desorption (FD) sources. The LMI source filament is resistively heated to melt the metal film and promote ita flow to the emitter tip. Typically, the emitter or anode is positively biased 5 7 kV with respea to the LMI source cathode depending on the desired emission current. In the unfocused configuration the LMI source cathode is maintained at the Same potential as the secondary ion acceleration electrode. The LMI source anode current, without correction for secondary electron effects, is used to monitor source emission. FAB mass spectra were produced for comparison on the same instrument. For this mode of operation the LMI source was replaced with an Ion Tech FAB llNF saddle field gun. The source to target distance in this arrangement was about 40 mm. The gun was operated with argon at a positive acceleration potential
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ANALYTICAL CHEMISTRY, VOL. 55,NO. 8, JULY 1983
T -i
-I
(a) Unfocused AV 4 4 V
- Acceleration +7 @(a/node), electrode, I-3kV(VoI
Lens-
Vo+AV(ZO-45V)
(b) Unfocused AV=22V
(C)
Focused AV = 2 2 V
'
L M I E m i6-IOkV tter
/
L M I Source Cathode, I-3kV
-m/z
Flgure 3. Copper and gallium secondary ion mass spectra from a bare (a)
MS
t
MS entronce s l i t
Y
-I
Sample probe rod
4
Cu/Be sample target using unfocused and focused LMI sources and differenttarget bias voltages: (a) unfocused, 44 V; (E) unfocused, 22 V; (c) focused, 22 V.
RESULTS AND DISCUSSION LMI emitters characteristically started with an anode current of about 1-2 fiA and, depending on the purpose of the experiment, were operated at anode currents of 5-20 FA. The sources operated stably with all four metals despite the presence of relatively high vapor pressure organics ( Pa). The lifetime of a gallium source in the mass spectrometer was typically found to be 5-6 h. This operating time is probably shortened because of backsputtering from the extraction electrode, and it should be possible to significantly extend it through an improved electrode configuration. Spent sources are relatively easily rejuvenated by rewetting them with fresh metal. The other three metal sources have not yet been operated sufficiently long to determine their lifetimes. Bombardment of the bare copper/beryllium target with ions from a gallium source yields 63Cu+,66Cu+,69Ga+,and 71Ga+ as the principal secondary ions. The mass spectrum, as seen in Figure 3, actually exhibits a doublet for each of these four species. The taller peak in each doublet registers secondaries released from the target, while the shorter peak records ions coming off the electrode opposite the sample target. This was established by observing that the separation between the members of each pair is directly proportional to the electric potential between these two electrodes (Figure 3a,b) and further by noting that the shorter, lower energy peak vanished when using the focused primary beam which strikes only the target (Figure 3c). The copper present on the secondary ion acceleration electrode must be material sputtered from the target, whereas the gallium on this electrode is probably deposited both indirectly by sputtering from the target and directly from the unfocused primary gallium beam. The presence of solid arsenocholine chloride on the target does not noticeably alter the production of copper and gallium secondary ions. This suggests that the arsenocholine segregates into microparticles during some phase of application and that only a fraction of the metal substrate is actually covered by the sample, By contrast the signals from these two elements completely disapppear when glycerol, with or without sample, is spread over the target. The copper and gallium secondary ions reappear essentially simultaneously upon depletion of the liquid organic layer. Corresponding results are observed with the indium, silicon-gold, and bismuth LMI sources. Interestingly, no evidence for metal interaction has been observed with either glycerol or sample in the mass spectra of any of the compounds tested. In one extreme case a mixture of sucrose and NaI in glycerol was bombarded for 240 s with a 30 MAgallium beam. Taking into account the total mass N
~
u 2
t
Figure 2. Scondary ion source with a LMI prlmary gun in (a) an unfocused configuratlon and (b) a focused configuration. Typlcal operating ranges of electrode voltages are Indicated.
of 6-9 kV and an anode current of 20-25 CLA; the ionic component of the primary beam was not deflected away from the target. Primary beam current densities incident on the target were estimated from electrometer readings of the beam current collected by the sample target and independently from calculations which took into account the primary source anode currents and the geometry of the beam and target. The values obtained are upper limits only since the measured values of neither target nor anode currents were corrected for increases due to secondary electron or ion effects. A 2 mm square of copper/beryllium sheet, spot welded at a slight angle onto the end of an FD emitter holder (Figure 2), was used as the sample target. The sample probe was inserted through a vacuum lock assembly originally provided for FD. Generally, the target was positively biased 20-45 V relative to the acceleration electrode of the secondary ion source. A tripeptide (glutathione), a pentapeptide (H-Tyr-Ile-HisPro-Phe-OH), sucrose, stachyose, arsenocholine chloride, hemin, and cyanocobalamin (vitamin BIZ)were used as test compounds. The arsenocholine chloride was applied to the sample probe from an aqueous solution (M) and analyzed as a solid after evaporation of the water. All of the other test compounds were analyzed from liquid glycerol solutions. The sucrose and stachyose solutions contained NaI in mole ratios (sugar:salt) of either 4:l or 101. KOH was added to the hemin solution to raise its pH to 12. No other sample preparation was performed. To provide a basis for comparison, all experiments throughout the source of this work were conducted with 1 fig sample amounts.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
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Cyanocobalamin M W 1354
H e m i n I K O H (pH-12) MW 651
1
( a ) In Source
I I
I
I
1
I
1329 (MtH-CNI' I
1355 IMtHl' ~
( b ) 8 1Source
I-
-
m/z
Molecular Ion region of cyanocobalamin (vitamln BIZ): glycerol matrix, acceleratlon voltage 1.2 kV; (a) In LMI source (-4 PA), (b) Bi I-MI source (-4 MA). Figure 4.
&
Flgure 5. Molecular ion region of hemln: glycerol matrlx adjusted with KOH to pH 12; acceleration voltage 2.3 kV; Ga LMI source (-4 HA). (b) Ga Source StachyosdNoI ( I O : I mol)
(0)
loss rate from the LMI emitter, (drnG,/dt) N -2.5 X kg s-l(42), the nearly uniform angular intensity of the beam (34, 36), and the geometry of the system, the amount of gallium delivered to the glycerol during this experiment was calculated to be approximately 150 ng. If all of this metal was indeed retained by the glycerol, then near the end of the irradiation period the glycerol would have contained essentially the same amount of gallium (-2 nmol) as sucrose (-3 nmol). Whatever the actual circumstances in this exaggerated situation no gallium related mass peaks could be found, and the fate of the bombarding metal in the sample matrix remains to be ascertained. It was noticed while making measurements with both the argon FAB and gallium LMI primary sources that, when intense primary particle bombardment starts, the onset of measurable secondary ion emission (A) from a glycerol matrix and ita rise to a high, stable level have a relatively long delay perbod, 210 s. This cursory observation suggests that as a preliminary to abundant secondary ion emission from liquid matrices a macroscopic transformation of the ciurface region from its vacuum equilibrium state occurs. !Such a situation would apparently be distinct, at least physically, from that which exists under static SIMS conditions. Possible gains in understanding the secondary ion process in molecular SIMS warrant further investigation of this delay effect. Mass spectra were facilely produced from all of the test compounds. Not surprisingly, these spectra have the same, major, qualitative analytical features which are characteristic of soft ionization processes in general. The spectra of the two peptides and vitamin B12exhibit a (M H)+molecular ion peak. This peak is the base peak in the peptide spectra; however, the base peak in the molecular ion region of the vitamin B12spectrum, which is shown in Figure 4, corresponds to (M + H - CN)'. The two sugars, which were mixed with NaI in the glycerol, produce spectra exhibiting a (M -k Na)+ molecular ion peak that in both cases is also the base peak. In the arsenocholine chloride spectrum the base peak corresponds to the cation itself. The molecular ion region of hemin's mass spectrum, shown in Figure 5, exhibits no molecular ion peak, but peaks for (M - C1+ K)+ and (M - HCI -I- 2K)+ ions, the latter being the base peak, are quite pronounced. No major qualitative differences are evident between the fragment ions in tho LMI/SIMS mass spectra of the test substances and those observed in their mass spectra produced with other soft ionization techniques. A limited number of measurements were made of the increases in abundance of the molecular ion, (M Na)+, of sucrose, and of stachyose following increases in the anode current of a gallium source. The principal observation was
+
+
A r Source S t o c h y o s e / N a I (10:l mol)
-
m/z
Flgure 6. Molecular ion reglon of stachyose: NaI-glycerol matrlx; acceleratlon voltage 2.3 kV; (a) Ar FAB source (20-25 PA), current density 5 l O - ' A cm-', source pressure 7 X lo3 Pa; (b) Ga LMI sowce (-4 PA), current denslty 2 X 10" A cm-*, source pressure 5 X lo4 Pa.
a saturation in the secondary molecular ion beam intensity when the current density incident on the target was on the (44).This is about 4 orders of magnitude order of 10" A cmW2 greater than the current density A cm-2) given as the upper limit for static SIMS (7). Also significant was a cursory determination that the stachyose molecular ion abundance produced with indium, silicon gold, or bismuth LMI sources saturates at about the same current density as that found for the gallium source. This disposition for the secondary molecular ion emission from a glycerol matrix to level off is not understood a t present, but the rate of diffusion (47) and the rate of ion formation, however it occurs (48),have both been proposed, without prior knowledge of the present study, as possible rate-determining steps in the release of secondary molecular ions from liquid matrices. The LMI sources produce high secondary molecular ion abundances. As a typical example Figure 6 displays two mags spectra of the molecular ion region of stachyose which were produced with argon FAB and galium LMI sources. In this case as well as in all other comparison experiments the position of the sample target and the outputs of the primary sourct?s were set as best as could be determined to achieve the maximum secondary molecular ion emission. Referring to Figure 6, the peak height of the (M Na)+ ion produced by the gallium source is more than an order of magnitude greater than that of the same ion produced by the argon source, while the background signals in the two spectra are essentially the same. It should be noted, however, that there is too much uncertainty in the estimates of the primary beam current densities to interpret this peak height ratio in terms of relative secondary molecular ion yields.
+
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
Table I. Relative Secondary Molecular Ion Abundances
primary source element
atomic mass no.u
Ar Ga In Xe AuC
40 69 115 132 197
*
incident current density A cm-2)
--207 -7 -7
re1 stachyose (M t Na)+ abundance 1 8-12 35-40 -3 45-55
Most abundant stable isotope. Included for comparison; relative abundance estimated from results reported elsewhere ( 1 5 - 1 7). Most abundant ion produced by the SiAu source is Au' (39). (I
Relative abundances of the stachyose (M + Na)+ ion produced with argon, gallium, and silicon gold sources operating at comparable levels are tabulated in Table I. Although not included in Table I, results obtained in subsequent measurements with a bismuth source were comparable to those for the silicon-gold source. As mentioned in the preceding paragraph these values should not be taken as relative secondary molecular ion yields because of the questionable accuracies of the current densities given for the argon and LMI sources. In addition it should be noted that the current densities for the LMI sources are all near lo* A cm-2 and that the leveling off in relative abundances observed with the heavier metals may be due in part to the onset of the saturation effect described above. It is possible that the large disparity between the secondary ion emission produced by the argon and gallium sources simply reflects gross inaccuracies in the estimated current densities given in Table I. However, the observed changes in secondary ion abundance resulting from the use of different metals in the LMI source cannot be explained away so easily. The particles emitted by these sources have very similar angular and energy distributions at the same total emission currents (36),and thus, the probability for large discrepancies in the estimated particle fluxes is reduced. Moreover, the rise in secondary ion emission with increasing primary particle mass (Table I) is too large and too systematic to be accounted for solely by errors in the estimated current densities. It seems more likely that the observed changes in secondary ion emission with the different metals correspond to differences in secondary ion yields. If this is the case, dense collision cascades or "spikes", as they are called, may be involved; spikes are favored by bombardment with heavy ions and molecular ions and refer to a situation in which all of the molecules within the cascade volume of an impacted primary particle are set in motion at a level conducive to sputtering (49). Regardless of any possible explanations, the 50-fold increase in molecular ion abundance obtained by changing from an argon to a silicon-gold primary beam is impressive and, as in the cases of the recent measurements reported for mercury (20) and trimethylpentaphenyltrisiloxane(21) primary beams, demonstrates that alternate primary source conditions exist by which significant gains in the utility of molecular SIMS can be made. This is very strikingly demonstrated by the example of vitamin BIZ. No signal in the molecular range of this compound could be detected by using an argon source. With an indium source the (M H)+ ion (Figure 4a) was just barely detectable with a signal to noise ratio of about 3:l. The spectrum shown in Figure 4b was recorded by using a bismuth source and an ion source potential of 1.2 kV (mlAm 300); a similar spectrum was made by using a silicon-gold source. It is remarkable that an ion of this molecular weight can be so strongly detected despite the large loss in instrument
+
-
sensitivity at the low acceleration voltage. A reasonable measure of the dispersion in the energy distribution of the secondary ions is provided by the width of the mass peaks when very narrow mass analyzer slits are used. By this means the energy spread of the secondary ions produced with the gallium LMI source was determined to be about 3 eV. This value is less by about a factor of 2 than that observed with the same instrument using the argon FAB source. At LMI source current settings of less than 10 pA ( S O - 5 A cm-2 incident current density) a microgram sample of glutathione, sucrose, or stachyose yields molecular ions for periods well over 30 m. During the initial 30 m, the molecular ion beam intensity gradually decreases to a level which is 280% of the original. Relative statistical variations in the molecular ion currents range from about 10% for the gallium source to about 3% for the silicon-gold alloy source. The LMI emitter has several physical and operational attributes which lend themselves very well to its use as a primary source in SIMS. The source itself can be made quite small; it is compatible with high vacuum environments; and it does not require auxiliary vacuum or gas supply facilities for its operation. Thus, an unfocused version of the LMI source can be installed in most existing mass spectrometers with a minimum of modification to the source or source housing. A high voltage supply for the anode, a high voltage isolated power supply for heatng the emitter filament, and a high voltage isolated electrometer for reading the anode current are the only electronic requirements for an unfocused source. Mechanical and electrical equirements for focused sources depend, of course, on the characteristics of the ion optics. In operation an unfocused source establishes a steady state between deposition and removal of metal on the secondary ion acceleration electrode. This prevents buildup of organic matter on this electrode and, hence, essentially eliminates the need for cleaning it. Source insulators, however, must be shielded from the primary beam to prevent them from being metal coated. In conclusion this work strongly indicates that LMI sources can be used to significant advantage in practical analytical applications of matrix assisted SIMS. The pronounced variations between secondary ion abundances produced by sources with different metals and the strong dependence of secondary ion emissions on primary current density up to a saturation level at about lov5A cm-2 indicate that systematic use of LMI probes will indeed lead to significant information about secondary ion processes. Finally, the concept of coupling localized molecular SIMS of biological specimens with ion microscopy should be restated. Recent accomplishments in the development of finely focused ion beams from LMI sources (28,29) and a simple demonstration of direct analysis of plant material by SIMS (30) support the feasibility of this important idea. Registry No. Ga, 7440-55-3;In, 7440-74-6;SiAu, 12256-53-0; Bi, 7440-69-9. LITERATURE CITED (1) Beckey, H. D. Jnt. J . Mass Spectrom. Jon Pbys. 1989, 2 , 500-503. (2) Benninghoven, A.; Jaspers, D.; Slchtermann, W. Appl. Pbys. 1976, 7 7 , 35-39. (3) Benninghoven, A.; Slchtermann, W. Org. Mass Spectrom. 1977, 72, 595-597. (4) Grade, H.; Wlnograd, N.; Cooks, R. G. J. Am. Cbem. SoC. 1977, 99, 7725-7726. (5) Surman, D. J.; Vickerman, J. C. J . Cbem. Soc., Cbem. Commun. 1981, 324-325. (6) Barber, M.; Bordoll, R. S.; Sedgwick, R. D.; Tyler, A. N. J . Cbem. Soc., Cbem. Commun. 1981, 325-327. (7) Benninghoven, A. Surf. Scl. 1973, 35, 427-457. (8) Barber, M.;Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. Nature (London) 1981, 293, 270-275. (9) Rinehart, K. L., Jr.; Gaudioso, L. A,; Moore, M. L.; Pandey, R. C.; Cook, J. C., Jr. J . Am. Cbem. SOC. 1981, 703, 6517-6520.
Anal. Chem. 1983, 55, 1323-1330 Williams, D.H.; Bradley, C.; Bojesen. G.; Santikarn, S.; Taylor, L. C. E. J . Am. Chem. SOC. 1981, 703, 5700-5704. Barber, M.; Bordoll, R. S.; Sedgwlck, R. D.; Tyler, A. N. Blomsd. Mass Spectram. 1982,9 , 208-214. Standing, K. G.; Chalt, B. T.; Ens, W.; McIntosh, 0.; Beavls, IR. Nucl. Instrum. Methods 1982, 198, 33-38. Aberth, W.; Straub. K. M.; Burlingame, A. L. Anal. Chem. W82, 5 4 , 2029-2034. Rudat, M. A.; McEwen, C. N. Int. J . Mass Spectrom. Ion Phy$. 1983, 46, 351-354. Hunt, E). F.; Bone, W. M.; Shabanowitz, J.; Rhodes, J.; Ballavd, J. M. Anal. Chem. 1981,5 3 , 1704-1706. Martln, S. A.; Costello. C. E.; Biemann, K. Anal. Chem. 1882, 54, 2362-2368. Morrls, H. R.; Panico, M.; Haskine, N. J. Int. J . Mass Spectrom. Ion PhyS. 1963,46, 363-366. Ens, W.; Standing, K. C.; Westmore, J. 6.; Ogllvie, K. K.;Nemer, M. J. Anal. Chem. 1982,5 4 , 960-966. Dell. A.: Morris. H. R. Biochem. Bio~hvs.Res. Commun. 19f12, 106. 1456-1462. ’ Stoll. R.; Schade, U.; Rollgen, F. W.; Giessmann, U.; Barofsky, D. F. Int. J . Mass Spectrom. Ion Phys. 1982,4 3 , 227-229. Wong, S. S.; Stoll, R.; Rallgen, F. W. Z . Natufforsch., A 1982,37A, 718-7 19. Rollgen, F. W., Instltut fur Physlkallsche Chemie der Universltlt Bonn, personal communication, 1982. Taylor, G. I. Proc. R . SOC. London, Ser. A 1964, 280, 383-397. Mahoney, J. F.; Yahlku, A. Y.; Daley, H. L.; Moore, R. D.; Perel, J. J . Appl. t‘hys. 1969,4 0 , 5101-5106. ClamDitt, R.: Altken. K. L.: Jefferies. D. K. J . Vac. Scl. 7‘echnoi. 1975; 72,1208. Krohn, V. E.; Ringo, G. R. Appl. Phys. Left. 1975. 27. 479-461. Clampitt, R.; Jefferles, D. K. Nucl. Instrum. Methods 1978, 749, 739-742. . - - . .-. Seliger, R. L.; Ward, J. W.; Wang, V.; Kubena, R. L. Appl. Phys. Left. 1979,3 4 , 310-312. Ishitani, T.; Tamura, H.; Todokoro, H. J . Vac. Sci. Technol. 1982, 20.80-63. Day, R . J.; Unger, S.E.; Cooks, R. G. Anal. Chem. 1980,52, 557A572A. Sakurai. T.; Culbertson, R. J.; Robertson, G. H. Appl. Phy8. Lett. 1979,$14,11-13. Sudraud, P.; Colllex, C.; van de Walle, J. J . Phys. (Orsay, f r . ) 1979, 40, L207-L211, Mair, Ei. L. R.; von Engel, A. J . Appl. Phys. 1979, 5 0 , 5592-5595. Swanson, L. W.; Schwlnd, 0. A.; Bell, A. E.; Brady, J. E. J . VNC.Scl. Technol. 1979, 16, 1864-1867.
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Culbertson, R. J.; Robertson, 0. H.; Sakurai, T. J . Vsc. Sci. Technol. 1979, 76, 1868-1870. Swanson, L. W.; Bell, A. E., Schwlnd, G. A.; Orloff, J. “Proceedlngs”, Symposlum on Electron and Ion Beam Sclence and Technology, St. Louis, MO, May 1980; Electrochemical Society: New York, 1980; p 594. Swanson, L. W.; Schwlnd, 0. A.; Bell, A. E. J . Appl. Phys. 1980,51, 3453-3455. Gamo, K.; Ukegawa, T.; Namba, S. Jpn. J. Appl. Phys. lQ60, 19, L379-L362. Gamo, K.; Ukegawa, T.; Inomoto, Y.; Ka, K. K.; Namba, S. Jpn. J. Appl. PhyS. 1980, 19, L595-L598. Dlxon, A.; Colliex, C.; Sudraud, P.; van de Walle, J. Surf. Sci. 1981, 108. L424-L428. Bell, A. E.; Schwlnd, G. A.; Swanson, L. W. J . Appl. Phys. 1982,53, 4802-4605. Malr, G. L. R.; von Engel, A. J . Phys. D 1981, 74, 1721-1727. Thompson, S. P.;von Engel, A. J. Phys. D 1982, 75, 925-931. Barofsky, D. F.; Giessmann, U.; Swanson, L. W.; Bell, A. IE. “Proceedings”, 29th Internatlonal Field Emission Symposiurn, Goteborg, Sweden, Aug 1982; AndrBn, H-O., Norden, H., Ed%; Almqvist and Wlksell Int.: Stockholm, 1982; pp 425-432. Barofsky, D. F.; Glessmann, U.; Swanson, L. W.; Bell, A. E. Int. $1. Mass Spectrom. Ion Phys. 1983,4 6 , 495-497. Wagner, A.; Hail, T. M. J . Vac. Sci. Technol. 1979, 76, 1871-1874. Magee, W. C. “Abstracts”, 30th Annual Conference on Mass Spectrometry and Aliled Topics, Honolulu, HI, June 1982 American Society for Mass Spectrometry: 1982, p 577. Field, F. H. Presented In part at the 2nd International Meeting on Ion Formation from Organic Solids (IFOS 11), Miinster, FRG, Sept 1982. Slgmund, P. I n “Inelastic Ion-Surface Collisions”; Tolk, N. H., Tully, J. C., Heiland, W., Whlte, C. W., Eds.; Academic Press: New York, 1977; pp 121-152.
RECEIVED for review December 9, 1982.
Accepted March 3, 1983. This work was supported by grants from the National Institutes of Health (AM 20937) and the National Science Foundation (ECS-8206796). Preliminary aspects of this study were presented at The 29th International Field Emission Symposium, Goteborg, Sweden, Aug 9-13,1982, and the 9th International Mass Spectrometry Conference, Vienna, Austria, Aug 30-Sept 3,1982,,
Mass Spectrometry/Mass Spectrometry by ‘Time-Resolved Magnetic Dispersion John T. Stults and Christie G. Enke” Department of Chemlstty, Michigan State University, East Lanslng, Mlchigan 48824
John F. Holland Department of Biochemistty, Michigan State University, East Lansing, Michlgan 4 8 8 2 4
A new typo of multldlmenslonal mass spectrometer Is; presented. Utlllratlon of Ion beam pulsing and time-resolved detectlon techniques In a magnetlc sector mass spectrometer allows slmiiltaneous momentum and velocity analysis of the Ions. Thls oomblnatlon provldes energy-lndependent Ion mass asslgnments. Parent, daughter, and neutral loss spectra can be obtalnedl as In conventional MS/MS Instruments. Equatlons for mass determination are glven and various scannlng modes are described. Metastable Ion decomposltlons are used to experimentally conflrm the theory, and a comparlson Is made of the achieved and expected resolullon and sensltlvlty. The potentlal for the technlque In analytlcal appllcatlons I$ consldered, particularly In light of a proposed high-speed data acqulsltlon system.
The ability to identify relationships between parent and daughter ions that result from metastable ion decomposition
or from collison-induced dissociation (CID) has become increasingly important in recent years. Spectra showing all the daughters of a specific parent ion (daughter spectra) are proving invaluable for many applications in mixture analysis and structure elucidation (1-3). So also are spectra whiclh show all the parents of a particular daughter mass (parent spectra) or spectra of ions that undergo a particular neutral loss (neutral loss spectra). These types of spectra are normally obtained by a technique referred to as mass spectrometry/ mass spectrometry (MS/MS), that utilizes various tandem (sequential) arrangements of mass-selective devices. Two types of MS/MS instruments are commonly in use and are commercially available: tandem sector instruments of which the mass-analyzed ion kinetic energy spectrometer (MIKES) (4)is an example, and multiple quadrupole instruments such as the triple quadrupole mass spectrometer (TQMS) (5). Both types of instruments operate in a sequential manner: (a) ion formation (b) parent ion selection, (c) parent ion dissociation, (d) daughter ion selection, and (e) ion de-
0003-2700/83/0355-1323$01.50/0@ 1983 American Chemical doclety
