Secondary ion emission from solutions: time dependence and surface

Secondary Ion images of droplets of. TTAB solution showed that emission of both TTA+ and glycerol secondary Ions was homogeneous across the sample. Se...
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Anal. Chem. 1992, 64, 3052-3058

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Secondary Ion Emission from Solutions: Time Dependence and Surface Phenomena M. Scott Kriger and Kelsey D. Cook’ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600

R. T. Short and Peter J. Todd’ Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6365

The temporal behavlor of FAB maw spectra from glywrd sdutlonr of tetradecyltrlmethylammonlumbromlde (CI4H2)N ( C H ~ ) ~ ~ , T T A B ) a n d t ~ M t h y l a m ” l o d k l e ( T Ew Am I) Investigated. FAB spectra of the TTAB sdutlon displayed a contlnuorw decrease In TTA’ wlth t h e . Spectra obtained from the TEAI solution were Inltlalty Invariant for several mlnutes and then dkplayed a gradual Increase In the relatlve abundance of TEA’ to a maxl”, folkwed by a preclpnour drop In Ion intenstty. Secondary Ion images of droplets of TTAB sdutlon showed that emlsslonof both TTA’ and glycerol across the sample. Secsecondary Ions was ondary lon Images of droplets of TEAI solutlon showed heterogamous and segregated .mkrkn of both TEA’ and protonated glycerol. R e r u b from the FAB spectra and the secondary lon Imageswere corrdatd and ratlonakedon the bask of surface tenrlon-lnduced mass transport and matrht evaporatlon.

INTRODUCTION Secondary ion mass spectra (SIMS) and fast atom bombardment (FAB) mass spectra of solutions are time-dependent, i.e., the intensity of solute-characteristicsecondary ions relative to that of solvent-characteristic ions changes during the course of the experiment. The effect has been studied by a number of workers, and the time dependence appears to be a result of many factors, including primary particleinduced evaporation: sample size? surface phenomena,g10 etc. Generally, the time dependence of any particular solution is difficult to reproduce from instrument to instrument and even from run to run on a given instrument. One approach to compensating for this variability involves the comparison Addreas correspondencetoKeleeyD. Cook,Department ofchemistry, University of Tennessee, Knoxville, TN 37996-1600,or Peter J. Todd, P.O. Box 2008, Bldg. 5510,MS-6365,Oak Ridge National Laboratory, Oak Ridge, TN 37831-6365. (1) Field, F. H.J. Phys. Chem. 1982,86,5115-5123. (2)Cole, R. B.; Guenat, C.; Hass, J. R.; Linton, R. W. Anal. Chem. 1987,59,1930-1937. (3)Gale, P.J.; Bentz, B. L.; Chait, B. T.; Field, F. H.; Cotter, R. J. Anal. Chem. 1986,58,1070-1076. (4)Wong, S.S.;Rbllgen, F. W. Nucl. Zmt. Methods 1986,B14, 4 3 6 447. (5)Tuinman, A. A.; Cook, K. D. J. Am. SOC.Mass Spectrom. 1992,3, 318-325. (6)Barofsky,D. F.;Gieasmann,U., Barofsky,E.Znt. J.Mass Spectrom. Ion Phys. 1983,53,319-322. (7)Todd, P. J.; Groenewold, G. S. Anal. Chem. 1986,58,895-899. (8)Ligon, W.V.; Dom, S. B. Znt. J. Mass Spectrom. Ion Processes 1984,57,75-90. (9)Ligon, W. V., Dom, S. B. Znt. J. Mass Spectrom. Ion Processes 1986,78, 99-113. (10)Lacey, M. P.; Keough, T. Rapid Commun. Mass Spectrom. 1989, 3,46-50. 0003-2700/92/0364-3052$03.0010

of relative intensities of characteristic secondary ions from two or more solutes in the same sample. In pioneering work on liquid samples, Ligon and Dorn found that the relative intensity of solute-charaderistic secondary ions from glycerol solution correlates with relative surface a c t i v i t ~ .Lacey ~ and Keough later showed that the role of surfaceactivity appeared to vary with time;I0over the course of sample life,the intensity of secondary ions characteristic of compounds that are less surface active increases to a maximum and may dominate the SIMS spectrum toward the end of the experiment. From these studies an active role in mass transport has been assigned to surface p h e n ~ m e n a . ~The . ~ effect was best demonstrated in the study by Ligon and D ~ r n .When ~ the projection of the primary particle beam on the sample is smaller than the droplet of glycerol solution, differences in surface tension drive surface active solutes toward the spot where the beam strikes the droplet. Ligon referred to this phenomenon an “side-filling”. In the parlance of colloid chemistry, mass transport driven by surface forces is known as a Marangoni effect.” To understand the role of surface effects on sensitivity, it is important to recognize that the forces involved in surface phenomena extend only about 0.5-1 nm below the surface, so surfactants are not “pulled”to the surface;11J2rather, they must arrive at the surface by some mechanism, after which surface phenomena may dominate. Furthermore, the net force, i. e. surface tension, is always directed in the plane of the surface. That is, surface effects cannot be used to explain the mass transport of solute from the bulk to the surface. An alternative mechanism, diffusion, is a slow process in the time domain of a FAB experiment; in a classic test of Gibbs’s theory of enhanced surface concentration, McBain found that Surfactants required hours to reach equilibrium surface concentration in quiescent water.12 Diffusion coefficients in glycerol are about 104times smaller than in water so the time frame for diffusion should be even 10nger.l~Furthermore, in FAB and SIMS the surface is continually sputtered away, unlike the case in McBain’s experiments. Practically apeaking, there is simply insufficient time during the course of a FAB experiment for the Surfaceto be replenished by diffusion of solute to the ~ u r f a c e .LigonQJ4 ~ recognized this fact and further concluded that the mechanism of mass transport within the bulk of the droplet could not be clearly discerned from his experiments;he hypothesizedthat mechanical mixing and convectionwere necessary to replenish the surfacesupply of solute. (11)(a) Heimenz, P. C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker, Inc.: New York, 1986. (b) Adamson, A. W. Physical Chemistry of Surfaces, 2nd ed.; Wiley Interscience: New York, 1967. (12)McBain, J. W.; Swain,R. C. Proc. R. SOC.London 1936,A154, 608-618. (13)Evanoff, J. E.; Harris, W. E. Can. J. Chem. 1978,56,574-577. (14)Ligon, W. V. Znt. J. Mass Spectrom. Zon Phys. 1983,52, 189. @ 1992 American Chemical Society

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In contrast to results obtained from solutions containing involatile solutes, when volatile amines are introduced onto the surface of glycerol via the gas phase, the effects of surface phenomena are not apparent.15 Under such circumstances, amine characteristic secondary ion intensity is found to correlatewith the Henry’s law concentration of free base amine and is independent of the relative surface activity of the respective amine. This conclusion is consistent with observations that FAB and SIMS sampling depths ordinarily encompass more than the surface monolayers.16J7 In this context, the persistent and prominent role of surface phenomena and mass transport in FAB spectra of involatile compounds dissolved in glycerol may appear anomalous. This collection of disparate observations may be summarized simply. First, FAB and SIMS spectra of glycerol solutions are time-dependent, but the time dependence is difficult to reproduce and is by no means well understood or defined. Second, apparently definitive experiments have supported opposite conclusions concerning the role of surface activity in secondary ion emission. The most readily identifiable mechanisms for mass transport of solute from the sample bulk to the surface, mixing and convection, are by nature nonselective,yet lead (in combination with Marangoni effects) to highly selective emission of nonvolatile surface active analytes. In this paper, the time dependence of FAB spectra is compared with image data from a wide-angle secondary ion microprobe.18 This instrument has made it possible to determine the spatial distribution of secondary ion emission from sample droplets. We will demonstrate that much of the controversy about time dependence of FAB spectra arises because secondary emission from glycerol solutions is sometimes heterogeneous.

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with a mass resolution of at least lo00 (10% valley definition). Research grade Xe (MG Industries, North Branch, NJ) was used with an Ion Tech (Middlesex, UK) atom gun, operating with 1.0 mA emission current at 8.0 kV. Under these conditions, the ca. 3-mm-diameterbrass FAB target was fully and uniformly exposed to the primary atom beam. Spectra were obtained using a 10 s/decade magnet scan. Plots of intensity vs time were reconstructed from these scans, rather than by single ion monitoring. Reported ion intensities represent peak heights. The FAB gun was warmed up for 1h prior to collecting data. Samples. Glycerol (AFt Grade, Mallinckrodt),tetradecyltrimethylammoniumbromide (TTAB, C14Hd(CH&Br) (Reagent Grade, Sigma), tetraethylammonium iodide (TEAI) (Aldrich), and 3-nitrobenzyl alcohol (NBA) (Aldrich, 98%) were used as received. The 1 mM samples were vacuum degassed (ca. Torr) for a minimum of 8 h under low heat and constant stirring. The samples were stored under vacuum until loaded onto the sample probe. Procedures. For secondary ion imaging experiments, a thin film of sample was applied to approximately 1cm2of a 20-mm X 100-mmhorizontal copper plate sample stage. This assembly was inserted via a prevacuum chamber into the source housing. Data collection started immediately after inserting the probe into the source housing and continued for approximately 1.5 h. For the FAB experiments, approximately 2 mg of a glycerol solution were loaded onto the target by dipping. The FAB probe was then inserted into the source via a prevacuum stage. Data collection began immediately after inserting the probe into the source high vacuum housing. Each sample was run to dryness, requiring approximately 1 h.

RESULTS AND DISCUSSION

In addition to other obvious differences, the two mass spectrometers used in these experiments differ in sample orientation and exposure. Samples in the imaging SIMS source are oriented horizontally, which prevents “sagging” or “drooping” of the sample droplet under the influence of gravity.8 The primary ion dose experienced by the sample EXPERIMENTAL SECTION during the 2 min necessary to acquire data for an image is Instrumentation. Secondary ion (SIMS)spectraand images well below the static SIMS limit. Furthermore, exposure to were obtained using a wide angle secondary ion microprobe that the 100-”-diameter primary beam at any point on a sample has been described previously.18J9 Briefly, it consists of a Cs+ is intermittent because the beam is rastered across the sample. primary ion gun with a focusing objective lens and raster The SIMS source has a large field of view, by virtue of a capibility; a wide-angle secondary ion source; an Extrel (Pittsprocess known as dynamic emittance matching.20 Under burgh, PA) triple quadrupole mass filter (used here as a single MS, rather than in the tandem mode); and a postanalysis computer control, potentials applied to deflectors located acceleration detector (PAD). Control and data acquisition are within the secondary ion source cause the limited field of performed using an 80286-based computer, with software deview (ca. 100-pm diameter) of the secondary ion optics to veloped at Oak Ridge National Laboratory. Secondaryion images transmit ions from the exact location on the sample being reported here are based on the peak intensity of mass-resolved irradiated by the primary ion beam. This allows uniformly secondary ions. Dynamic emittance matchingl8S2Ois employed high transmission of secondary ions emitted from virtually to minimize effects of focus changes across the sample surface. any location on the SIMS target up to at least 1cm in diameter; The following instrumental conditions were employed: 6-keV i.e., the field of view is increased by a factor of 104. In contrast, Cs+primary ion beam; 3-nA primary ion current; 10-eVsecondary the ZAB employs a vertical sample orientation for FAB and ion kinetic energy;-6-kV PAD dynode voltage;-1.3-kvmultiplier heavy continuous primary particle flux over the entire sample. voltage. The data acquisition rate was 50 kHz, allowingaveraging of 128intensitymeasurementsat a selected mlz prior to deflection Gravity-induced flow and a high sputtering rate account in of the 100-rm primary beam spot to the next position on the part for the shorter persistance of FAB spectra. The sample. The time required to obtain data for a 200 X 200 pixel acceptance aperture2 of double-focusing instruments like the image over a 1-cm2area was approximateIy 2 min. Alternatively, ZAB is similar to that of the quadrupole SIMS instrument; a full spectrum could be obtained from a single point on the i.e., detected ions are limited to those originating from a rather sample in about 2 s. The primary ion gun was allowed to warm narrow zone (probably I 1mm2)of indeterminate position on up for a minimum of 1 h prior to collecting data. the sample. Despite these differences,FAB and SIMS spectra Fast atom bombardment (FAB)data were obtained with a VG obtained using the respectiveinstruments on the same samples Analytical (Manchester,UK) ZAB-EQ mass spectrometer using are qualitatively similar; that is, the same ions are evident in the standard VG FAB source and a VG 11-2505data system. The both spectra. However, as will be seen below, the quality of instrument was operated at an acceleration potential of 8 kV and the match in relative intensities depends on the time of (15) Todd, P. J. J. Am. Soc. Mass Spectrom. 1991,2, 33-44. comparison. (16) Standing, K. G.; Chait, B. T.; Ens, N.; McIntosh, G.; Beavis, R. Temporal Behavior of FAB Spectra. Figure l a shows Nucl. Imt.Methods 1982,198, 33. the temporal behavior of the intensity of ions a t mlz 256 (17) Ligon, W. V. Int. J. Mass Spectrom. Ion Rocesses 1983,52,189. (18) Grimm,C.C.;Short,R.T.;Todd,P.J.J.Am.Soc.MassSpectrom. (tetradecyltrimethylammonium cation ( C I ~ H ~ ~ N + ( C H ~ ) ~ , 1991,2, 362-371. TTA+) and m/z 93 (protonated glycerol, [G+ HI+) in FAB (19)Todd, P. J.; Short, R. T.;Grimm, C. C.; Markey, S. P. Anal. Chem. spectra of a TTAB/glycerol solution. The TTA+ intensity 1992,24, 1871-1878. initially dominates the spectrum, as shown in Figure lb; ions (20) Leibl, H. US. Patent 3,517,191.

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Flguro 1. (a) Intensitles of mlz 258 (llA+) and mlz 93 ([G HI+) versustlmefrom 1 mMTTAB In glycerolunder FAB. (b) Mass spectrum recorded from scan 1. (c) Mass spectrum recorded from scan 50.

characteristic of glycerol (e.g. m/z 93,185, ...I are present, but a t low intensity. After approximately 25 min of exposure, TTA+ is still prominent, but protonated glycerol becomes the base peak in the spectrum, as shown in Figure IC. As bombardment continues beyond 25 min, "A+ disappears from the spectra, and eventually glycerol ion intensities also fall to the level of background. Similar behavior has been reported elsewhere.4 Figure 2 shows data from an analogous experiment using a tetraethylammonium (TEA+)/glycerolsolution. Parts a and b of Figure 2 show that the intensity of TEA+is a t first low and fairly constant. TEA+ intensity then gradually rises until the spectrum is characterized by a short-lived burst of high intensity. Even at the peak intensity for TEA+, the spectrum still contains ions derived from glycerol, as shown in Figure 2c. Emission of TEA+ from a TEAI/glycerol sample could be restored after the precipitous decrease demonstrated in Figure 2a by the simple application of pure glycerol to the probe tip. Furthermore, the temporal behavior (including persistence) of the rejuvenated sample was similar to that shown in Figure 2, although the intensity of TEA+ ions (relative to solvent ions) was typically half that for the original sample. For TTAB samples, no such rejuvenation was observed; FAB of

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madcharge Flguro 2. (a) Inteneltks of mlz 130 (TEA+) and mlz 93 ([Q HI+) versus time from 1 mMTEAI In glycerol under FAB. (b) Mass spectrum recorded from scan 20. (c) Mass specbum record4 from scan 82.

+

a ''rewetted" sample provided a t best a weak, Short-lived,

TTAB spectrum. For both solutions, if a sample was allowed to dry in vacuum without primary particle bombardment, the temporal behavior following rewetting waa observed to be similar to that shown in Figures 1and 2. The fact that TEA+ intensity can be rejuvenated indicates that TEAI is removed at a rate less than the combined sputtering and evaporation rate of glycerol. Conversely,decay of the TTA+ signal and its failure to be rejuvenated indicates that TTAB is removed a t a rate greater than that of glycerol. The reason for these differences in sampling rate and their relation to relative analyta surface activities is not entirely clear, but the dominating maas transport processes can be identified from imaging data. Spatial Distributions of Solutes. Using the imaging capabilities of the SIMS microprobe to explain the temporal behavior of FAB spectra is subject to severe restrictions. The substantial differences in primary ion flux density between the FAB and SIMS experiments must be considered. This will be discussed in detail below. However, as noted above, differences in the respective spectra caused by these differences between primary particle parameters are minor; that is, the same ions are present in both spectra, even though their relative abundance8 differ. This appears to be the case for

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Figure 4. (a) Secondary ion image of mlz 130 (TEA+)from 1 mM TEAI in glycerol, using white as most intense, black as least intense. (b) Line analysis of mlz 130 along cursor mark in a. (c) Mass spectrum acquired at the center of the sample. (d) Mass spectrum acquired at the edge of the sample.

liquids2' in general. To reiterate this point, secondary ion mass spectra are included with the images for comparison with the FAB spectra. Figure 3a shows a map of the intensity a t m/z 256 (=A+) from a glycerol solution of TTAB. The intensity of TTA+ is fairly uniformly distributed across the surface of the sample, as can be seen from Figure 3b, which is a Cartesian plot of TTA+ intensity across the line indicated in Figure 3a. Individual spectra obtained near the middle and the edge of the sample, as shown in Figure 3c,d, respectively, are very similar to each other and to the FAB spectrum of Figure lb. In contrast with the FAB data, the spectra shown in Figure 3c,d persisted for the duration of the experiment, with no observable change in relative abundance. At no point in the SIMS experiment was a spectrum similar to that shown in (21)Aberth, W.;Straub, K.M.;Burlingame, A. L.Anal. Chem. 1982, 54,2029-2034.

Figure lcobtained, even when the primary beam was 'parked" in one position and repetitive spectra scanned. At the low doses employed for this static SIMS experiment, the TTAB could not be appreciably depleted by sputtering. The secondary ion image of TEA+ obtained from a glycerol solution of TEAI is shown in Figure 4a. The image is dramatically and revealingly different from that shown in Figure 3a. TEA+is preferentially emitted from the periphery of the sample droplet. The magnitude of TEA+intensification near the rim is evident from the intensity profiles of Figure 4b. Near the center of the sample, the spectrum roughly parallels the "normal" FAB spectrum; i.e., Figures 4c and 2b are quantitatively similar. Significantly, spectra recorded from the periphery of the droplet, like that shown in Figure 4d, closely resemble that shown in Figure 2c, the FAB spectrum near the maximum in TEA+ emission.

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a

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Flgure 5. Secondary ion image of (a),(c) and (e) m/z 130 (TEA+) and (b), (d), and (f) mlz 93 ( [ G HI+)15,32. and 66 min after introduction, respectively. Each image is internally normalizedto the most intense point in the image (indicated by white): except for the posilion of the respective maxima, no correlation is implied between the maximum intensities of mlz 130 and 93 in the respective images.

+

A series of SIMS images of the sample used for Figure 4 was taken over a period of time. Over the course of the experiment, the images of Figure 5a,c,e, show the distribution of TEA+ emission, while parts b, d, and f of Figure 5 show the distribution of m/z 93 emission characteristic of glycerol, mainly for reference. As the droplet shrinks, the original "doughnut" shape of intense TEA+ emission gradually fills. The filling correspondsto the shrinking of the region of intense emission of m/z 93. The relative intensity of TEA+ increases correspondingly. Figure 5 can be used to explain, in part, why the FAB spectrum of TEA+ is time-dependent. In both FAB and SIMS, emission from the center of the droplet initially consists of characteristic glycerol ions. If the center of the FAB target is the object point of the mass spectrometric ion optics, then only late in the FAB experiment will the "doughnut" of intense TEA+ emission reach this focal region. Thus, as the hole fills in, the intensity of TEA+ increases, as in Figure 2a. Such time dependence would also be observed using the SIMS spectrometer if the (100-pm diameter) focal region had been "parked" at the droplet center via the computer controlled deflectors within the secondary ion source. With FAB, where primary particle flux is higher than for static SIMS,22 characteristic secondary ion emission from the sample virtually ceases when the sample dries. This was confirmed by results from the rejuvenation experiments wherein addition of glycerolrestored characteristic emission. In contrast, static SIMS permits spectra to be obtained even from dry samples.= That is why the hole in the "doughnut" gets smaller in Figure 5, but the diameter of the doughnut hardly changes a t all. (22) Benninghoven, A.; Rudenaur, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry; Wiley: New York, 1987; pp 726-727.

The center of the doughnut may not coincide with the ion axis on some instruments or samples so that the spike of high intensity is seen a t a different time during the course of the FAB experiment. Thus, for the same solution, a different time dependence may be observed. For studies in which the mass spectrometer focus is readjusted between measurements, it may be difficult to see differences between results from surfactants and nonsurfactants. Given the wide range of variables involved, it is not surprising that time dependence is difficult to reproduce from instrument to instrument. By contrast, we found that the computer controlled ion optical properties of the SIMS instrument made images like those of Figures 3-5 remarkably reproducible. While heterogeneous emission is characteristic for TEN solutions, emission from TTAB solutions is reproducibly homogeneous. Furthermore, temporal behavior of spectra from these solutions appears to arise mainly from changes in concentration due to preferential sputtering of TTAB, rather than ion optical effects. This contention is supported by the continuous decrease in intensity a t m/z 256 (Figure la); by the results of rejuvenation experiments, which fail to yield a significant FAB spectrum after redeposition of glycerol; and by the homogeneous emission of TTA+ from the droplet (Figure 3a). Effects of Surface Activity. A simple and justifiable assumption is that the surface activity of TTAB is greater than that of TEAI in glycerol; this is certainly the case in water, another protic solvent. By surface activity, we mean that when a surface active molecule is on the surface, it has the effect of loweringthe surface tension of the liquid. Surface free energy is lowered by a surface excess concentration of such a solute. As noted earlier, diffusion from the bulk is not a sufficiently rapid mechanism to replenish the surface during the course of a FAB experiment.14 By the same token, diffusion is not sufficiently rapid to reduce any excess concentration of solute that may be present on the surface of the glycerol solution due to evaporation of the glycerol itself. As reported earlier: a primary beam current density of about 10 pA/cm2 will provide a rate of sputtering equal to the rate of glycerol loss from evaporation a t normal room temperature. The flux density that corresponds to this, 6 X 1013atoms/cm2-s, is a fairly typical value for a FAB experiment. The drying behavior of TEAI solutions is much like that of brine solutions in that as the solution evaporates, salt precipitates a t the periphery, leaving a residue behind the shrinking droplet. For both brine and TEAI/glycerolsolution, precipitation occurs a t the periphery. I t is well-known that hydrophilic salts tend to raise the surface tension of water. Excess salt on the surface (remaining after evaporation of solvent) is transported rapidly across the surface and to the periphery by a Marangoni effect. Consistent with this phenomenon, secondary emission of TEA+ is more intense from the periphery of the shrinking droplet than from the center until the very end of the experiment, when the sample has dried. The reproducible images shown in Figure 5 rule out any significant role for convection and mechanical mixing as mechanisms of mass transport of solute to the surface; such processes would tend to destroy the surface heterogeneity. Figure 6a-c confirms the absence of mixing and convection (at least for the SIMS experiment) even from a mixture containing TTAB,TEAI, and glycerol. By excluding important roles for diffusion,mechanicalmixing, and convection, only sputtering, evaporation, and Marangoni effects remain as mass transport mechanisms. Presumably, a glycerol solution of TTAB dries in the same manner as a solution of TEAI. Due to the surface activity

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Scan Number Flgure 7. Intensities of mlz 130 (TEA+) and mlz 154 ([NBA 4- HI+) versus time from 1 mM TEAI in mnitrobenzyl alcohol sample under FAB.

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by Flgure 6. Successive (a) T T A + , (b) 'EA', and (c) ITA+ images from the same droplet of TEAI and TTAB in glycerol solution, taken about 4 min between images.

of "TAB,the Marangoni effect will tend to cause "AB to be uniformly spread across the glycerol surface as it is enriched a t the surface by evaporation. This surface spreading can account, a t least in part, for the relatively rapid sputtering of TTAB compared with TEAI, as evident from the data from rejuvenation experiments. Surface replenishment via matrix evaporation, in conjunction with the Marangoni effect, reasonably accounts for the behavior demonstrated in Figure 1. The steady decrease in TTA+ intensity is consistent with an initially high surface concentration of TTAB, erosion of this surface by sputtering, and replenishment by evaporation of the matrix. With such a model, a very long time may be required for the system to reach a steady state even in the FAB experiment. In part, this may result from heating of the surface due to high primary particle flux densities. While sputtering increases linearly with primary particle flux, evaporation of glycerol increases exponentially with any temperature increase caused by elevated primary particle flux d e n ~ i t yFrom .~ Figure 1,the steady state of TTA+ emission and presumably of surface concentration is never reached in the FAB experiments. Because the sputtering rate is much lower in SIMS, the steady state of TTAB surface concentration is maintained throughout the experiment. Consistent with this fact, the SIMS spectrum of TTAB solution is essentially independent of time. Sputtering and evaporation are well-known processes of sample depletion. Whether heterogeneous secondary ion emission from liquid samples is a common phenomenon is less obvious. Figure 7 shows the temporal behavior of TEA+ and protonated m-nitrobenzylalcohol (NBA)23from a solution (23) Barber, M.; Bell, D.; Eckersley, M.; Morris, M.; Tetler, L. Rapid Commun. Mass Spectrom. 1988,2, 18-21.

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Flgure 8. Secondary ion image of (a) mlz 130 (TEA') and (b) mlz 154 (NBA H+) from 1 mM TEAI in mnitrobenzyl alcohol using white as the most intense, black as the least intense.

+

of TEAI in NBA under FAB conditions. The peak in TEA+ intensity (attributed to bringing the high-concentrationregion into focus) is similar to that in Figure 2 but less steep. The difference in slope may be a result of any of a number of factors, including different rates of evaporation and sputtering. Parts a and b of Figure 8 show the distribution of emission of TEA+and [NBA + HI+,respectively. While NBA has chemical and physical properties very different from glycer01,2~the behavior of emission of TEA+ from the NBA solution is very much like that from glycerol, and the images of samples from both solutions show similar distributions. By presumption that matrix evaporation is the main method of mass transport to the surface, it becomes possible to explain why the effects of surface activity are absent when solutes are admitted via the gas phase. With solutes that are a t equilibrium with the vapor, both the surface distribution and bulk concentration of solute are controlled by gas-phase mass transport. Even though Marangoni effects may be rapid in the context of a FAB or SIMS experiment, gas-phase mass transport is far more rapid. (24) Cook, K. D.; Todd, P. J.; Fryar, D. Biomed. Enuiron. Mass Spectrom. 1989,18, 492-497.

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CONCLUSIONS The most important finding of this work is that secondary ion emission from samples of liquid solutions can be heterogeneous across the surface. The time dependence of FAB spectra can be explained on the basis of this heterogeneity, plus variation in analyte mass transport (mainly from Marangoni effects) and sampling efficiencies. Practically speaking, the images make it possible to reconcile diverse sets of data and diverse conclusions reportad in the literature. At the same time, it is clear that results obtained from solution samples without accounting for time dependence may well be ambiguous. As notad earlier, diffusion coefficients in glycerol are generally so small that diffusion through the bulk solution cannot be a viable mass transport mechanism to establish surfacelbulk equilibrium during the courseof a SIMSor FAB experiment.la By the same token,the slow diffusion of solutes in glycerol, in conjunction with glycerol evaporation, necessarily results in enrichment of involatile solute near the surface. When the presence of excess solute on the surface increases the surface tension, the solute is driven to the periphery by a Marangoni effect. For surfactants, the solute is dispersed across the surface by the same forces, as has been recognized by Ligon.8 He coined the term "side fiiing" to mean replacement of solute a t the site of primary ion bombardment driven by surface forces. If the history of inorganic secondary ion imaging can be used as a guide, there is a tendency to overinterpret image

d a t a The conclusionsof this paper are baaed on a very small set of compounds, and it is presumptuous to make grand generalizations about all FAB data based on our resulta. Within these limits, however, the images and temporal behavior reported here seem to be consistent with data reported elsewhere,althoughthe conclusionsmay be different.

ACKNOWLEDGMENT

William Holland's development of computer programs for printing grayscale images is gratefully acknowledged. This work was supported in part by a grant from the National Science Foundation (CHES822787). The UT Chemistry Maw Spectrometry Center is funded by the Science Alliance, a Stateof TennesseeCenter of Excellence. Organic ion imagea were obtained using instrumentation on loan from the Biomedical and Instrumentation Engineering Program, National Center for Research Resources, NIH, and developed under Grant R01-GM41617 from the National Institutes of Health. Further funding was provided by the U.S.Department of Energy, Office of Basic Energy Sciences, under ContractDE-AC05-840R21400with Martin MariettaEnergy systems,Inc. RECEIVED for review March 26, 1992. Accepted September 8, 1992. Registry NO. TEAI, 68-05-3; TEA+,66-40-0;"A+, 1018292-0; CI&,N(CHs)aBr,1119-97-7; glycerol, 56-81-6.