Anal. Chem. 1986,58,895-899
895
Temporal Behavior of Secondary Ion Emission from Analyte/Liquid Matrix Samples Peter J. Todd* Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Gary S . Groenewold Bristol-Myers Company, Pharmaceutical Research & Development Division, Syracuse, New York 13221
Secondary Ion mass spectra of glycerol and of sulfuric and polyphosphorlc acids are studied as a function of tlme and primary Ion dose. The lntenslty of the protonated molecular ion characteristic of each matrix Is found to decrease exponentlally with time, and respective flrst-order decay constants correlate wlth sample vapor pressure. Sample depletlon as determined by welght loss measurements is a ilnear functlon of time If the sample is dlspersed over the target surface. When sample depletlon is circumvented by continuous replenishment or suppressed by the presence of involatile solutes, intensity of characteristic secondary lons ls Independent of tlme for perlods of at least 30 min.
Secondary ion mass spectrometry (SIMS) and fast atom bombardment mass spectrometry (FABMS) have been successfully used for the analysis of a wide range of involatile organic analytes (I). Often samples are prepared by dissolving the analyte in a relatively involatile liquid such as glycerol, referred to as a matrix, a term that derives from the unique matrix effect (2) of such solutions, namely, enhanced secondary emission of ions characteristic of the analyte. The mass spectra observed by use of these techniques frequently change over the course of an analysis (3-5). This variation presumably arises from several sources: radiation damage and sputtering of the sample, caused by the primary beam ( 3 , 4 ) ;evaporation of the matrix; and evaporation of the analyte (3-5). In addition to these effects, sample flow and convection can have a dramatic effect on measured secondary ion intensity. The goal of this study is to investigate these factors, which bear on the temporal variation observed in SIMS and FABMS analyses of liquid samples. The temporal behavior of a given SIMS and FABMS sample is difficult to predict. Recent studies of the effects of time and primary particle dose on neat samples indicate that the secondary ion mass spectrum of glycerol changes significantly with primary dose, and the intensity of secondary protonated ions decreases within minutes after bombardment is initiated ( 4 ) . For many analyte/glycerol solutions, a predominant analyte (M + H)+ion can be observed only over the course of a few scans. These observations contrast with reported behavior of other analyte/matrix samples, where secondary ion yields and relative intensities of analyte-characteristic ions are independent of time and dose ( 1 , 5 , 6 ) . Examples of this latter behavior include FABMS analyses of mixtures of surfactants in glycerol ( 5 ) and SIMS analyses of gaseous phosphonates using a polyphosphoric acid (PPA) matrix (6): constant analyte (M + H)+abundance without apparent radiation damage was observed in the SIMS/FAB spectra of these samples. Apparently, subtle differences between physical or chemical properties of neat matrices and their solutions have a pronounced effect on the temporal behavior of SIMS or FABMS spectra. For the above examples, it has been demonstrated that surfactants suppress evaporation of glycerol, and PPA is far less volatile than glycerol. This led
us to hypothesize that differences in the temporal behavior of SIMS/FAB spectra obtained from liquid samples largely reflect differences in the rate of sample depletion. This depletion apparently arises from both sputtering and normal evaporation. Temporal variations in secondary ion emission may also arise from primary particle induced damage to the sample. Experimental distinction between damage and depletion is difficult for a number of reasons. Both effects are primary dose dependent, and the most common primary particles employed are fast atoms, for which accurate measurement of primary atom flux is impossible. To overcome this problem, a primary ion beam is employed here; its current density may be measured by using a Faraday cup of known aperture. Secondly, sample flow and convection cause fluctuations in measured secondary ion intensity that obscure effects due to depletion and damage. To mitigate this effect, a fine mesh was attached to the sample probe tip. The mesh precludes macroscopic flow and aids in reducing sample surface charge buildup. We have calculated the rate of glycerol evaporation, the matrix cooling caused by evaporation, and the matrix heating caused by the primary beam. These calculations were performed in order t~ determine if evaporation would be expected to be a major contributor to matrix loss. The rate of (M + H)+intensity decay under primary ion bombardment was then compared for three matrices that have different rates of evaporation: glycerol, concentrated sulfuric acid, and PPA. If rate of sample evaporation is an important factor, then we expect that the (M + H)+ intensity from the sample having the greatest rate of evaporation will have the greatest decay rate. Sample weight loss as a function of time in vacuum and primary dose was also measured for comparison. The rate (M + H)+intensity decay for glycerol was also measured by using a reservoir probe tip, in which glycerol was replenished as it was depleted from the probe tip by evaporation and sputtering. Changes in the SIMS spectrum of glycerol with time can be ascribed solely to ion damage when such a device is employed. Finally, the rates of analyte (M + H)+intensity decay for a basic analyte (amylamine) sputtered from the three above-mentioned matrices were determined by use of a standard (nonreplenishing) probe tip. If evaporationof the amine significantly affects sample depletion and if this is the main cause of amine (M + H)+ intensity decay, then the rate of such decay from a glycerol solution can be expected to be much greater than the rate from solutions of the two more strongly acidic matrices. This expectation derives from the fact that amylamine dissolved in glycerol is certainly more volatile than its dissolved salt, which would be the expected product when dissolved in H,SO, or PPA.
EXPERIMENTAL SECTION Mass Spectrometer. The primary (7) and secondary ion sources (8) and the triple-sector (EBE) mass spectrometer have been described elsewhere (9). Primary ion current density was
0003-2700/86/0358-0895$01.50/00 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
a
b
C -
Figure 1. Schematic diagram of targets a-c
measured before each matrix run with a Faraday cup that was fitted into the ion source in place of the target. In addition, NaI was loaded onto a target and irradiated by the primary beam. Visual observation of the emitted light from the target indicated the entire sample surface was subjected to primary ion bombardment. Total surface irradiation limits the effects of lateral surface diffusion. Typical measured current density was 5 f 2 kA/cm2 of 5-keV Ar ions that strike the target 70" from target normal. Secondary ion source pressure in the absence of samples torr as measured by a Bayard-Alpert gauge located was 5 X 10 cm from the target surface. Targets. Three different target designs were employed. Diagrams of these targets are shown in Figure 1. Target a consisted of a stainless-steel plug, 6.35 X 9.5 mm diameter, to which was spot-welded a fine stainless-steelmesh (wire diameter 35 pm; 50% transmission). The mesh permits a uniform distribution of matrix on the target surface and serves to prevent sample charging. Use of the mesh results in a substantial reduction in secondary ion intensity fluctuations and high voltage arcing, compared to that observed when a target without mesh is employed. Target b, as illustrated in Figure 1, was made by drilling two holes of diameter 2.2 mm and 0.6 mm into the plug. As with target a, a fine stainless-steelmesh was spot-welded over the face of the target. Target c was made by boring a 7.9-mmdiameter recess to a depth of 0.08 mm in the face of the plug. Stainless-steel mesh was spot-welded onto the resultant raised perimeter of the face of the target, as shown in Figure 1. Sample Preparation. Samples consisted of pure matrices or 5% (w/w) solutions of amylamine/matrix. Targets a and c were washed with concentrated HCl and rinsed with deionized water and acetone and dried. A drop of sample was placed onto the mesh of the target, and then a glass tube of approximately 6 mm diameter was rolled across the face to force the matrix into the space between the mesh and the target plug. The face of the target was then held vertical, and excess matrix material collecting at the bottom of the face was removed with the tip of a glass transfer pipet. Target b was washed with concentrated HC1, rinsed with deionized water followed by a rinse with CC14,and dried. A small amount of vacuum grease was deposited around the perimeter of the target face to prevent matrix from flowing off the target while it was in the mass spectrometer source. Glycerol was the only matrix tested with this target and was introduced into a hole parallel to the target face via a glass transfer pipet. The target was repeatedly tapped to drive out air bubbles. When a darkened spot appeared on the mesh, indicating that it was wetted by the glycerol, the target was inserted into the mass spectrometer ion source via a vacuum lock. During operation in the mass spectrometer vacuum system, glycerol flow to the mesh surface is induced by gravity and the capillary action of the mesh. Target preparation affects the temporal behavior of the secondary ion intensity; we have found that unless the stainless surface is cleaned with HC1, glycerol tends to "bead on the surface and is not dispersed over the entire target surface. Sector mass spectrometers transmit only those ions formed from a small (approximately1-10 mm2)region generally near the target center, which we define as the sampling region. Factors such as matrix flow can cause this central samplingregion to be nonrepresentative thereby causing unpredictable mass spectral behavior. Weight Loss Measurements. Weight loss measurementswere conducted by using target a. Cleaned targets were weighed to within 0.1 mg, loaded with glycerol, reweighed, and inserted into the mass spectrometer. After insertion, samples were irradiated
by the primary Art beam as specified above and removed from vacuum. Samples were then allowed to equilibrate with ambient air for 5 min and reweighed. Samples were then reinserted into the mass spectrometer and the process repeated. Average flux density to which the samples were exposed was determined as the product of measured primary flux density and bombardment duration divided by the time under vacuum. Results of these measurements were compared to those made without primary radiation. Reagents. Glycerol and sulfuric acid were ACS reagent grade. Ninety nine percent amylamine and commercially available PPA were used. No impurities were evident from the SIMS spectra of these compounds. RESULTS AND DISCUSSION Sample Depletion. The rate of evaporation can be calculated for any pure liquid in equilibrium with its vapor (10). The rate of evaporation is defined as the number of molecules (n)leaving the surface of the liquid per unit area of surface, per unit time ( t ) ,and can be calculated by using eq 1 where
dn/dt = P
O
/
~
~
T
(1)
Po is the vapor pressure of the liquid (Pa), m is the mass of the molecules (kg/molecule), k is the Boltzmann constant (J/K), and T i s the temperature (K). Equation 1 describes a system where the flux of molecules from a bulk liquid surface equals the flux toward the sample surface and implies that the rate of evaporation is a linear function of surface area. If the surface area of a liquid sample decreases, the rate of sample depletion should decrease as well. Equation 1 is not applicable to desorption of otherwise volatile compounds that have adsorbed onto a surface. For a relatively involatile liquid under vacuum, the mean free path of molecules in the gas phase is sufficiently large that the flux of particles leaving the liquid surface is unaffected by the flux of molecules returning to the surface. Given the constraint that the vapor pressure of the liquid is independent of ambient pressure (a constraint met by glycerol) (11), evaporation of a liquid would proceed a t the same rate per unit area in vacuum as it would in equilibrium with its vapor. A flux (rate of evaporation) of 2.7 X 1OI6 molecules/cm2.s can be calculated using eq 1 and the equilibrium vapor pressure glycerol (23 "C) of 1.287 X lo-* torr (11). This is a rapid rate: a 1-mg sample with a surface area of 1 cm2would completely evaporate in about 4 min if the surface area remained constant. This rapid evaporation is not observed when a sample of glycerol is subjected to atmospheric pressure outside the mass spectrometer vacuum system because the mean free path of evaporated glycerol molecules is so short. The evaporation rate is a function of the temperature of the matrix, principally because the vapor pressure of a pure liquid is temperature dependent. Factors that affect the temperature of the matrix and, hence the evaporation rate, include evaporative cooling and heating by primary ion/atom bombardment. The magnitude of both factors may be extracted by calculation if we make the assumption that the energy lost by direct sputtering is negligible. In the case of evaporative cooling, the temperature drop across a glycerol film caused by evaporation into a vacuum (13) is given by AT = (heat loss rate) (thickness)/ (heat flow rate coefficient) (2) Here, the metal target is assumed to be a heat reservoir. The heat loss rate can be computed as the product of the evaporation rate (vide supra) and the molar heat of vaporization of glycerol, 20.7 kcal/mol (12). For the thickness of the glycerol film we generously estimate 1 mm, and the heat flow rate coefficient (14) (thermal conductivity) of glycerol is reported as 7.03 x 10" (cal/s.cm2)/(K/cm). Using these values
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
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c
I1
LOG INTENSITY
/
7L
'I Y 5
VI
TIME(MIN)
/
i
2 I
i
IO I 5 20 25
io
time (min)
i 5 40 45 50
'
Flgure 2. Weight loss of glycerol samples vs. time under vacuum
obtained by using target a. Soild data polnts refer to data taken In the absence of primary Ion bombardment; open data points refer to measurements taken where the timeaveraged current density was 3.5 pA/cm2.
and eq 2 yields a temperature drop of 0.13 "C. This small temperature drop is unlikely to significantly alter the evaporation rate of the glycerol sample. An upper limit to the amount of heating resulting from the incident primary beam may also be estimated by using 2 and substituting the heating rate of the primary beam for the heat loss rate. The maximum heating rate caused by the primary particle beam employed in SIMS/FABMS experiments may be estimated for a practical upper limit current density of lo4 A/cm2 (6.24 X 1014particles/cm2.s) at 5 keV kinetic energy. If this kinetic energy is quantitatively converted into thermal energy, then the heating rate is 0.12 cal/s.cm2. Again, assuming a glycerol thickness of 0.1 cm, the temperature change across the glycerol would be 17 "C, causing the temperature at the surface to be 40 "C. This temperature will cause the rate of glycerol evaporation to be double that at 23 "C. Since the current density employed here is 5 X low6A/cm2, we estimate a maximum temperature change due to beam heating of 0.85 "C for the present work. We conclude that the temperature or rate of evaporation of the glycerol sample is unlikely to change significantly as a result of evaporative cooling or primary ion bombardment heating. Therefore, eq 1should yield a reasonable estimate of the glycerol evaporation rate for comparison with sample weight loss vs. time measurements. The results of weight loss experiments are shown in Figure 2. The linearity of the plots indicates that the surface area of the glycerol exposed to vacuum is constant. This is the expected result as visual inspection of glycerol loaded onto the target shows that it is dispersed over the entire surface by a clean mesh, regardless of sample load. In fact, from the slope of mass vs. time, evaluation of eq 1indicates a surface area of 0.81 cm2. The actual surface area of the target without mesh is 0.71 cm', an area that is somewhat increased by the mesh. Given the uncertainty in weight loss measurements, which are in large part due to the hygroscopic nature of glycerol, agreement between the data and prediction of eq 1is gratifying. It appears from Figure 2 that an average primary current density of 3.5 x A/cm2 (2 x 1013particles/cm2-s) has a modest effect on the rate of sample depletion. This contrasts the results of FABMS experiments by Field (3) and Ligon and Dorn (5). Field estimated that a flux density of 2 x 1014ern+ s-' resulted in a primary ion induced loss 75% of total. Ligon and Dorn estimated 50% primary ion induced losses with a flux density of 1.9 X 1OI2 cm-2 s-l. We cannot explain the
1
E
,
0 PPA
[
1 t
H2s04
0 GLYCEROL
i
1 30 36 60 6
12
18
24
4 2 40 54
66
TIME, MIN Flgure 3. Intensity of secondary protonated molecular ions (M
+ H)'
characteristic of each matrix as a function of time, using target a.
disparity among the results. We do point out, however, that accurate measurement of fast atom flux is impossible, and that for results presented here, sample depletion rates measured in the absence of primary beam irradiation are consistent with rates predicted by simple evaporation. Furthermore, results obtained here are in good agreement with recent results obtained elsewhere ( 4 ) . According to Figure 2, an average particle flux of 2 X 1013 Arf/cm-2.s induces a 40% increase in the depletion rate of glycerol. Such an increase in depletion indicates that approximately 540 glycerol molecules are sputtered per primary 5-keV Ar+. Under equilibrium conditions, 5-keV energy would account for evaporation of about 8500 glycerol molecules. Given the experimental uncertainty incumbent with weight loss measurements, we are reluctant to derive any conclusions from this apparent 6% efficiency. However, the result is consistent with the thermodynamic argument that the liquid-gas transition under nonequilibrium conditions requires more energy than evaporation under equilibrium conditions. Time-Depen-dent Mass Spectral Behavior of Neat Liquid Samples. The secondary protonated molecular ion (It4 + H)' abundances of three matrices having different rates of evaporation were measured as a function of time. In each case, (M + H)+ abundance decreased exponentially (Figure 3) with time. Primary ion beam conditions were identical for each experiment. The ratio of the decay rate constants of (M + H)+intensity decay from glycerol, concentrated H2S04,and a polyphosphoric acid sample was 1.41.0:0.1, respectively. This ratio was determined by comparing the slopes of the lines plotted in Figure 3. Significantly, the ratio of the (M + H)+ intensity decay constants is roughly the same as the ratio of the evaporation rates for the three substances. The three matrices compared were chosen because they have different rates of evaporation. Under identical experimental conditions, the respective rates of evaporation are approximately in the same order as the vapor pressures of the pure compound ( P o )(see eq 1): glycerol (11)Po = 1.3 x torr > sulfuric acid (14) (Po = 1.0 X torr) > PPA (Po unknown). The rate of evaporation of polyphosphoric acid is probably an order of magnitude less than that of sulfuric acid. In the present apparatus, introduction of polyphosphoric acid into the source causes a pressure increase of 2 x lo+ torr,
898
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
R.A
Ob'
,001'
"
10
20
"
30
40
'
50
1
60
'
70
'
80
1
90
100
'
110
1
I
TIMEIMIN)
Figure 4. Relative intensities of various Ions from a pure glycerol sample when loaded onto target b. Intensity of m l z 93 is relative to Its maximum observed intensity (-40 min after introduction). Intensities of other ions are relative to the simultaneous intensity of m l z
93.
which is 10% that of the pressure increase observed (2 X torr) when either glycerol or sulfuric acid is introduced. The results for glycerol are consistent with the results of weight loss measurements, except that sample weight decreases linearly with time and secondary ion intensity decreases exponentially with time. This functional difference indicates that either the sampling region changes physically as sample is depleted or subtle chemical changes occur within the sample, i.e., radiation damage as suggested by Field, which affect secondary ion emission. The functional difference between sample depletion and secondary ion emission decay may have some bearing on the inconsistency between the results of Field and those of Ligon and Dorn. In the former case, the benchmark of secondary ion intensity was employed. In the latter, weight measurements were employed to establish sample depletion. Mass spectra of glycerol and sulfuric acid samples also varied with time and primary particle bombardment, but the interpretation of these spectra does not permit an unambiguous distinction between damage and depletion effects. From glycerol samples, behavior similar to that noted by Field (3) was observed for the ions m / z 57,61,75,113,123,129, and 183; their relative intensity increased with time and primary ion dose. These data suggest ion damage has an effect on emission. However, from sulfuric acid samples, we noted that the intensity of ions such as Ni+, Cr+, and Fe+ increased as the intensity of m / z 99 decreased. No changes were observed in the mass spectrum of PPA during 1h of primary ion exposure ( 6 ) . These data suggest that sample depletion is the cause of temporal variation in SIMS spectra in contrast to the results obtained from neat glycerol samples. When sample depletion and damage occur simultaneously,the effects of the two processes cannot be clearly distinguishedfrom each other. A simple modification of target a can be used to replenish glycerol in the sampling region as the glycerol is depleted, and thus maintain constant sampling conditions. As noted earlier, a clean mesh dispersed the matrix over the entire target surface regardless of sample load. This effect arises because the surface free energy of the stainless steel is greater than that of the matrix. By wetting the stainless-steel surface of the mesh, ita surface free energy is reduced by matrix coverage. Target b shown in Figure l b contains a reservoir for the glycerol and makes use of its wetting properties to resupply the glycerol to the sampling region as it is depleted. Any changes in secondary ion emission Or mass spectra obtained when using such a device could then be attributable to ion damage. When target b was loaded with matrix material, and irradiated as before, the characteristic secondary protonated molecular ions from glycerol showed nearly constant intensity for 2 h. This is demonstrated in Figure 4, where log ([93+]/[93+],,,) is plotted vs. time since introduction into
vacuum. An initial rise is observed principally because the instrument requires tuning following sample introduction. Secondly, short-term fluctuations in (939 intensity occur because air trapped within the reservoir eventually reaches the target face causing macroscopic changes in the surface. By use of target c, the air bubble problem can be overcome, although the duration of constant (M + H)+ is reduced. Use of a finer (400 mesh) stainless-steel grid can also be used to enhance emission stability. In any case it is clear that when depletion of sample is corrected by replenishment, long-term maintenance of a stable secondary (M + H)+ intensity from the neat sample is possible. Figure 4 also shows the relative intensity of various secondary ions emitted from the glycerol sample as a function of time. For each of the ions indicated, the intensity is measured relative to the simultaneous intensity of m / z 93. It is clear that as irradiation proceeds, no systematic increase is observed in the relative intensity of these ions, contrary to the report by Field and our own earlier observations using a simple probe tip. Fields interpretation (3) of these ions attributes them to protonated polyols and polyhydroxy aldehydes. However, we have been unable to observe secondary ions characteristic of either alcohols or aldehydes dissolved in glycerol. These species may well be products of ion damage to the matrix. However, unless glycerol is depleted, their presence in the matrix is not indicated by increasing intensity of their protonated molecular ions as the radiation dose increases. This has been observed by others, and a surface self-cleaning mechanism has been proposed to account for it (4). We conclude that the increased abundance of polyhydroxy1 secondary ion indicates a reduction in the thickness of glycerol from which secondary ions are emitted. Perhaps the relative intensity of these protonated polyol ions observed in the earlier experiments increases because they are formed from glycerol molecules in direct contact with the target surface. These secondary ions may be characteristic of metal-catalyzed reactions of the matrix and they may reflect changes in the internal energy distribution of the sputtered ions as well (vide infra). Significantly, we find the onset of increased relative intensity of these species occurs when indicated source pressure has diminished to 1 0 4 0 % of the value indicated at sample introduction. A second possible explanation for the origin of the "damaged glycerol" ions may be that ions which originate from areas where the thickness of the glycerol is only one or two monolayers possess a different internal energy distribution than do ions which originate from the surface of a bulk drop of glycerol. Consequently, ions sputtered from one or two monolayers would yield a different cracking pattern. Time-Dependent Mass Spectral Behavior of Amylamine Solutions. The behavior of (M + H)+ of a simple, volatile amine sputtered from the three matrices was studied to evaluate the effect that analyte volatility has on the temporal behavior of the SIMS spectrum. The volatility of an amine free base in glycerol is most certainly much greater than the volatility of the conjugate acid in concentrated H2S04. Even if (at low concentrations) the amine exists principally in its conjugate acid form in glycerol, deprotonation is sufficiently rapid that it should not inhibit evaporation. In sulfuric acid, deprotonation of the amine conjugate acid at any appreciable rate is unlikely. Temporal behavior of the secondary mlz 88 (protonated amylamine) intensity is shown in Figure 5 for three 5% (w/w) solutions of amylamine/matrix samples deposited on target A. Comparison with Figure 3 shows that from glycerol and PPA, the temporal behavior of the mlz 88 intensity parallels that of the corresponding protonated molecular ion from the
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
l-----T LOG INTENSITY vs T I M E ( M I N )
A H2S04
t
M/Z 88
0 PPA 0 GLYCEROL
6
12
10
24 30 36 42 4 8 5 4 60 6 6 TIME, MIN
+
Flgure 5. Intensity of secondary m l r 88 (amylamine H+)+ as a function of time from various liquid solutions. Samples were loaded onto target a.
neat matrix. The m / z 88 intensity from a sulfuric acid mixture, however, is independent of time and significantly different from the behavior of m / z 99 (H3S04+)from pure sulfuric acid, which decays rapidly. From all three amylamine matrix solutions, the mass spectrum of the amylamine consists mainly of the m / z 88 ion, and there is no evidence of cumulative radiation damage even after radiation for 50 min. The time-independent behavior of the (amylamine + H)+ intensity sputtered from H2S04probably arises from one of two causes. First, the volatility of the amylamine conjugate acid is much lower than the volatility of the free base. Second, the presence of protonated amine ions on the sample surface retards matrix evaporation. It is likely that the salt concentration of the surface increases as the matrix evaporates, and matrix molecules must diffuse through the salt layer in order to escape the condensed phase. Similar reduction in glycerol evaporation by organic surfactants has been reported by Ligon (5),who has attributed enhanced surfactant surface concentration to the reduction in surface free energy that it causes. The ratio of m / z 88 and m / z 93 intensities was constant in SIMS spectra of the amylamine/glycerol solution regardless of time, even though the amine is far more volatile than glycerol. This observation may shed light on the role of condensed-phase analyte diffusion in SIMS and FABMS experiments. These data indicate that the amylamine surface concentration is constant during the course of the experiment and that the amine analyte is not being depleted at a rate greater than the rate of depletion of the entire sample. Secondly, because the intensities of (M + H)+ secondary ions characteristic of both glycerol and amylamine decay a t the same rate as the (M + H)+ intensity from a pure glycerol sample, the presence of a volatile component upon a matrix surface does not apear to significantly alter the rate of sample depletion. This observation is more than mundane. For diffusioncontrolled evaporation, the surface concentration of the more volatile species is related to its bulk concentration by known functional behavior (15). Solutions of various volatile amines dissolved in glycerol might thus be used in quantitative studies relating concentrationto secondary ion emision intensity. This work is presently under way.
899
The effect that amylamine has on depletion of sulfuric acid matrix has some implications for quantitative measurements on involatile analyte/volatile matrices. Ligon has demonstrated that anal* surface activity affects secondary emission intensity and, by inference, surface concentration. The present results indicate that matrix volatility and the extent that an analyte suppresses sample depletion may also be important factors. For example; the surface concentration of any involatile analyte should always be greater than its bulk concentration, even given the small diffusion coefficients (16,17) of most compounds in glycerol because glycerol can evaporate from the surface whereas an involatile analyte cannot.
CONCLUSION Unlike techniques such as electron impact ionization and chemical ionization, where gaseous analytes are continuously supplied to the ionization source, secondary ionization involves a system where physical and/or chemical properties of the sample may change with time. Our results indicate that a major factor causing the sampled region to change with time is sample depletion. When a neat liquid sample is employed, evaporation of the sample results in secondary ion emission that arises not only from the surface of a bulk sample but also from locations where the metal surface of the target is involved. Any addition to the system that inhibits or prevents loss of matrix from the sampled region, such as an involatile solute, mitigates these effects. Conversely, any factor that enhances departure of the matrix from the sampled region, such as liquid flow, exacerbates the effect. Aside from these effects, we see no evidence in the SIMS spectra of the matrices and analytes to suggest radiation damage, until the majority of the matrix has evaporated. ACKNOWLEDGMENT We are grateful to Franz W. Rollgen for helpful discussions and for communicating results prior to their publication. Registry No. Glycerol, 56-81-5;sulfuric,7664-93-9;amylamine, 110-58-7. LITERATURE CITED Barber, M.; Bordoli, R. S.; Elliot, G. J.; Sedgwick, R. D.; Tyler, A. N. Anal. Chem. 1982,54,645A-657A. "Compendlum of Analytlcal Nomenclature (Definitive Rules 1977)"; Irving, H. M. N. H., Freiser, H., West, T. S., Eds.; Pergamon: Oxford, 1978; p 101. Field, F. H. J . Phys. Chem. 1982,86, 5115-5123. Wong, S. S.; Rollgen, F. W.; Manz, I.; Przybylsky, M. Biomed. Mass Spectrom. 1985, 12, 43-46; Wong, S. S.; Rollgen, F. W., in press. Ligon, W.; Dorn, S. B. Int. J . Mass Spectrom. Ion Processes 1984, 5 7 , 75-90. Groenewold, G. S.; Todd, P. J. Anal. Chem. 1985,57, 886-890. Menzinger, M.;Wahlin, L. Rev. Sci. Instrum. 1989, 40, 102-105. Todd, P. J.; Glish, G. L.; Christie, W. H. Int. J . Mass Spectrom. Ion Processes lg84,61, 215-230. Russell, D. H.; Smlth, D. H.; Warmack, R. J.; Bertram, L. K. Int. J . Mass Spectrom. Ion Phys. 1980,381-391. Hill, T. H. "Lectures on Matter and Equilibrium";W. A. Benjamin: New York, 1966; p 116. Cammenga, H. K.; Schulze, F. W.; Theuerl, W. J . Chem. Eng. Data 1977,22, 131-134. Boyce, W. E.; DiPrima, R. C. "Elementary Differential Equations and Boundary Value Problems"; Wiley: New York, 1967; p 391. Weast, R. C., Ed.; "Handbook of Physics and Chemistry", 64th ed.; CRC Press: Boca Raton, FL, 1983; p E4. Fasullo, 0. T. "Sulfuric Acid"; McGraw-Hill: New York, 1965; p 286. Crank, J. "The Mathematics of Diffuslon", 2nd ed., Clarendon: Oxford, 1979; p 60-61. Evanoff, J. E.; Harris, W E. Can. J . Chem. 1978. 56, 574-577. SJostedt,E. Trans. Faraday SOC. 1938, 3 4 , 1158-1163. Lamm, 0.;
RECEIVED for review July 12,1985. Resubmitted November 11, 1985. Accepted November 11, 1985. Research was sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. G. S. Groenewold acknowleges support from the U.S. Department of Energy Postgraduate Training Program administered by Oak Ridge Associated Universities.
