Anal. Chem. 1987, 59, 1378-1383
1370
Dominance of Gas-Phase Basicities over Solution Basicities in the Competition for Protons in Fast Atom Bombardment Mass Spectrometry Jan Sunner, Angelina Morales, and Paul Kebarle*
Chemistry Department, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Fast atom bombardment (FAB) mass spectra were obtalned for pals of basic anaiytes, An1 and An2, dissolved In glycerol. For thwe anaiyte pairs where An1 had the higher solutionphase baslclty but the lower gas-phase baslclty (GB), It was found that the peaks due to protonated An1 were suppressed by the presence of An2. Thus, the GB and not the soiutionphase basicity determines which protonated compound is preferentiaw expressed. SuppreSSh of An1 peaks Increases as the concentratlon In the matrlx of An2 increases and as the OB difference between An2 and An1 increases from 0 to ca. 10 kcai mol-’. The domlnance of gas-phase energetics over liquld phase energetics implies that extensive proton An2 = An2H’ Anl, occurs during the transfer, AnlH’ FAB event In an envhonment that, with regard to the sdvaUon of the ions, Is more characteristic of a gas than of a liquld.
+
+
The mechanism of ion formation in several related desorption ionization techniques has received considerable attention in recent years (1-3) and is still a subject of considerable controversy. In fast atom bombardment (FAB) and in molecular secondary ion mass spectrometry (SIMS) there are frequent experimental observationsthat the charge state of the emitted ion corresponds to the charge state of the precursor in the liquid matrix (3). Thus, addition of an acid frequently increases the MH+ peak in positive ion FAB spectra and, conversly, addition of a base enhances the (M - H)- peak in negative ion FAB spectra (4-6). Such observationshave often been cited in support of the “precursor model” (3). In this model, it is assumed that the mass spectrum is caused mainly by desorption from the matrix of “preformed” ions, i.e. of ions already present in the liquid matrix, for example MH’. This desorption may result from momentum transfer from recoiling atoms in the primary collision cascade (7,8). The precursor ions are assumed to lead to desorbed ions with the same atomic composition. It has also been considered ( 1 ) that subsequent to the actual desorption some gas-phasereactions, notably cationization, take place in the so-called ”selvedge” (9). Finally, the role of unimolecular dissociations in the vacuum in desorption ionization and the fact that the dissociation reactions mainly parallel those observed with other mass spectrometric techniques seem well established ( 1 , 10, 11). In addition to the results that can be interpreted to support the precursor model, there are also results that indicate that gas-phase-like ion/molecule reactions can play an important role in the desorption ionization techniques. For example, ion/molecule reactions have been implicated in laser desorption (12). Michl ( 1 3 ) has given an extensive discussion of gas-phase reactions occurring in SIMS on low-temperature matrices. Recently, we published results that showed that the gas-phase basicity (GB) plays an important role in FABMS ( 1 4 ) . Thus, if the GB of the analyte An was lower than the 0003-2700/87/0359-1376$01.50/0
GB of the matrix, the AnH+ peak had a low intensity. Conversely, when the GB of the analyte was higher than the GB of the matrix, the AnH+ peak was intense and the matrix ions were suppressed. It was also shown that the intensities of cluster ions, in particular, dimers of the matrix, show a positive correlation with the respective gas-phase stabilities of the clusters (14). In a second paper, we demonstrated the importance of gas-phase energetics for the extent of cationization (15)in FAB. It was also shown (15)how gas-phase ion pair formation can be invoked to explain both why the total ion current in FAB does not increase with the concentration of precursor ions in the matrix and also how cationization of analyte molecules may occur via ion/molecule reactions involving the ion pairs. We interpreted our results in terms of a “gas collision model” (GCM). This model is discussed in ref 15 and only a short description is given here. The fast bombarding atom causes some ions to be formed out of neutral molecules in the collision cascade. As a result of the energy released in the track and near surroundings by the bombarding atom, a “cavity” is formed that is filled with hot gaslike molecules and fragments. The cavity contains the ions formed by the collision cascade, as well as preformed ions if such were present in the liquid prior to the bombardment. A good portion of the “gas” in the cavity, ca. 1000 glycerol molecules (16),is expelled into the vacuum. During this process, the positive and negative ions in the “gas”undergo extensive recombination, which leads to the formation of ion pairs or neutral molecules and f r a p ments. Proton transfer and other ion/molecule reactions, on the other hand, preserve the charge, but change the nature of the ions. Cationization occurs as a result of ions reacting with ion pairs formed in the recombination (15). The reactions leading to the protonated matrix ions may be represented by the following simplified reaction sequence:
xe’+ Gly = PI++ 2 + e- (or C-) PI++ Gly = GlyH+ + (P, - H)
(1) (2)
where + Xe is the bombarding atom, Gly is the glycerol matrix,
P,+ is a primary ion and the negative charge is carried either by an electron, e-, or by a negative ion, C-. Reactions 1and 2 are followed by protonation of the analyte: GlyH+
+ An = AnH+ + Gly
(3)
The observation that the gas-phase basicity (GB) of the analyte must be higher than the GB of the matrix for a significant intensity of AnH+ to be observed can be understood since otherwise reaction 3 is endothermic and its rate constant will be very low. Protonated analyte molecule ions can presumably also be formed directly from primary ions:
P,+ + An = AnH+ + (PI- H)
(4)
Protonated analyte molecules formed by reaction 4 may transfer the proton to matrix molecules by the reverse of 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. I O , MAY 15, 1987
reaction 3, and this can be expected when GB(An) < GB(G1y). Though eqs 1-4 are written as if the ions were bare, it is likely, that in a high-density, high-temperature "gas", the ions at any instant are solvated by a few matrix molecules. Since a compound with a high GB also tends to have a high solvent basicity, it can be argued that the competition for the protons, reaction 3, occurs already in the liquid matrix. In order to clarify this point, we decided to study the competion for protons between pairs of analyte compounds 90 chosen that one of the compounds in the pair is more basic in the gas phase but the other is more basic in the liquid matrix. It is easy to find such pairs since there is only a rough correlation between gas-phase basicities (GB) and solvent basicities. The present paper thus represents a continuation in our studies of the importance of gas-phase energetics and gas-phase-like ion/molecule reactions to FAB spectra.
EXPERIMENTAL SECTION The FAE! measurements were performed on an MS9 equipped with a saddle field atom gun (17). The gun was operated with Xe gas, a discharge current of 1.0 mA, and a voltage of 8 kV. The atom gun is mounted at a 90° angle to the MS9 ion path and the incidence angle to the probe surface is 20°. The chemicals were obtained commercially except for 4cyanopyridine hydrochloride and (N&-dimethy1amino)pyridine hydrochloride for which salts were prepared from their neutral compounds. Gas-phase basicities are generally known only for small molecules, which are relatively volatile. Evaporation from the FAB probe surface occurs for such compounds. For example, aniline and pyridine were not detected in the FAB spectrum. Such compounds thus had to be added as hydrochloride salts. Even for 3-hydroxyanilineand 4-hydroxypyridineit was found that the FAB signals were low compared to what is generally observed for compounds with similar basicities. Diethanolamine (DEA) on the other hand has a similar volatility to glycerol, and FAE! spectra of DEA and of DEAHCl, respectively, in glycerol were very similar. When analytes are added to the matrix, attention has to be paid to both volatility and aqueous basicities as the following examples illustrate. When NH&l and DEA were present together as analytes, there were no NH4+peaks in the spectra. Because of the small difference in pKBH+between the two analytes, proton transfer may occur in the liquid from NH4+to DEA followed by evaporative loss of ammonia. As a second example, 4-cyanopyridine hydorchloride gave only very weak peaks when dissolved in glycerol. Again, this can be explained as a consequence of the small PKBH+difference between 4-cyanopyridineand glycerol and evaporative loss of cyanopyridine. The pK, measurements in glycerol were carried out with a Fisher Accumet Model 520 pH/ion meter with glass/calomel electrodes, Fisher 13-639-1and 13-639-52. Aqueous buffer solutions of pH 10 (borax), pH 7 (phosphate),and pH 4 (hydrogen phthalate) were used to calibrate the pK meter. The pK, values were obtained from the pH values at half neutralization volume. RESULTS AND DISCUSSION Suppression of Analyte Peaks in the FAB Spectrum in the Presence of a Second Analyte with a Higher Gas-Phase Basicity. Here, we describe the results of the study of the competition for protons between two basic analyks, An1 and An2, dissolved in the glycerol matrix. Crucial to the investigation is the choice of special pairs of analytes for which the basicity order in the gas phase is reversed relative to that in glycerol solution. Such pairs can be found in spite of the fact that on the whole there is a rough correlation between the basicity orders in the gas phase and in solution (18, 19).
The compounds that were used as analytes are listed in Table I. Gas-phase basicities (GB) (18) and aqueous pKBH+ values (19) are given in the table, together with relative pKBH+ values in glycerol determined by us. An almost linear relationship was found between the basicities in water and in glycerol. The basicity order was identical in the two liquids
1379
Table I. Basicities of Analyte Bases GB" (300 K), pKnqgHtb pKBHt(An) kcal mol-' ( 300 K) pKBH+( NHJC
-
compound, An 3"
NzH4
aniline 4-cyanopyridine 3-hydroxyaniline 4-hydroxyanaline pyridine DEA TEA (Na-dimethylamino)pyridine
195.6 196.7 202.5 202.5 206.4 213e 213.1 220' 225e
228.4
9.2d
0 -1.0 -4.5 -7.9
8.2d 4.6
1.9
7.8
-4.9 -6.1 -4.0 -0.4 -1.3
9.7
+0.4
4.4
3.2 5.2 8.9
"From ref 18, unless otherwise noted. bFromref 19, unless otherwise noted. cpK measured in glycerol at 300 K, this work. dFrom ref 24. eEstimated. 'Reference 14. #Estimate (14). NN' dimethylaminopyridine 0 DEA 0 TEA 0
0 N2H4
L
C Aniline.
190
200
3 OH aniline 0 Pyridine
210
220
230
GB/kcal mol-' Flgure 1. Aqueous basicity vs. gas-phase basicity for the compounds used in the present study; see Table I (DEA = diethanolamine, TEA
= triethanolamine).
with the single exception of (N,N-dimethylamino)pyridine, which was found to be less basic in glycerol than diethanolamine and ammonia. Because the aqueous values are well established, they are used throughout this paper. Figure 1 shows the aqueous basicities and the GB for the compounds used in the present study. I t is seen from the figure that, for example, for the ammonia/aniline pair, aniline is more basic in the gas phase whereas ammonia is more basic in the aqueous phase. Examples of mass spectra obtained with the ammonia/aniline analyte pair are shown in Figure 2. (As explained in the Experimental Section, it was necessary to add most of the analyks as hydrochloride salts to the glycerol matrix). It is seen in the figure that when both compounds are present as hydrochloride salts in the glycerol matrix a t a concentration of 1mol % (Figure 2c), the analyte peaks in the mass spectrum are close to what is found when the two analytes are present separately with a concentration of 1 mol % (Figure 2, parts a and b). Thus, the presence of one analyte does not significantly influence the spectrum of the other analyte; i.e. their contributions to the FAB spectrum are additive. Reactions 5-8 describe the suggested processes leading to the formation of protonated analyte ions, analyte cluster ions, and fragments according to the gas collision model (14, 15).
GlyH+ + An1 = AnlH+ + Gly
(5)
AnlH+ = fragment ions, cluster ions
(6)
GlyH+
+ An2 = An2H+ + Gly
An2H+ = fragment ions, cluster ions
(7) (8)
In contrast to the situation at low concentration, the presence of 10 mol % of aniline hydrochloride in the matrix
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ANALYTICAL CHEMISTRY, VOL. 59. NO. 10, MAY 15, 1987
Table 11. Results of Experiments with Pairs of Analytes in Glycerol analyte 1 (Anl) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
analyte 2 (An2)
AGB,b kcal mol-'
APKBH+'
R(An2/Anl)d
1.1 6.9 10.8 17.4 17.5 24.4 32.8 5.8 10.5 6.6 13.6 7 5 6.9 17.5 10.6
-1.0 -4.6 -4.8 -6.0 -4.0 -0.3 +0.5 -3.6 -1.4 -1.2 +4.5 +5.7 -1.1 +3.7 +4.3 +0.6
1.2 3.7 1.7e 4.2 4.8 3.0 13.3 3.7 9.lf 3.6 2.0 2.0 1.8 1.6 1.4 0.25
NZH4" aniline" 3-hydroxyaniline 4-hydroxyaniline pyridine' DEA" (N,N-dimethy1amino)pyridine" aniline" 4-hydroxypyridine 4-hydroxypyridine DEA DEA TEA DEA DEA pyridine"
"3'
"3" NH," NH," "3' "3"
"3" NZH*' aniline" 3-hydroxyaniline 3-hydroxyaniline 4-hydroxypyridine DEA pyridine" aniline" aniline"
" Analyte was added to glycerol as the hydrochloride salt. GB(An2) - GB(An1). p K a q ~ ~ + ( A -n 2pK'&+(Anl). ) Intensity ratio, eq 12 (see text), used to measure the suppression of An1 peaks in the presence of An2. eEvaporation of hydroxyaniline causes this value to be too low; R = 3.5 is best estimate. fEvaporation of analine due to proton transfer in glycerol liquid from aniline to hydroxypyridine causes this value to be too high: not dotted in Figure 4. liquid, but they are effectively deprotonated as a result of the recombination reactions 10a and lob, which occur in the hot
A+ + B- = neutrals
(104
+
AnH+Gly+ X-GIy= (AnH+X-) = Angas HX,,
103
L 2 . 1
I
,t t
MASS
I*
(1)
MASS
Flgure 2. FAB spectra of the following solutions in glycerol: (a) 1 mol YO NH,CI; (b) 1 mol YO aniline hydrochloride: (c) 1 mol % NH,CI and 1 mol YO aniline hydrochloride; (d) 10 mol % NH,CI; (e) 10 mol % aniline hydrochloride; (f) 10 mol YO NH,CI and 10 mol YO aniline hydrochlorlde.
results in a suppression of the ammonia peaks in the FAB spectrum compared to when no aniline hydrochloride is present (Figure 2, parts d and f). This can be understood as a result of proton transfer between the two analytes, reaction 9. AnlH+ + An2 = An2H+
+ An1
(9)
In order for reaction 9 to occur a t all, the analytes, or at least An2, must be present in the "gas" as neutrals. Prior t o the bombardment the analytes are protonated in the glycerol
(lob)
"cavity" created by the projectile. Evidence for extensive positive-negative ion recombination was presented in a previous paper (15). Because of ion pair formation and subsequent dissociation, it should not matter whether the neutral base or the hydrochloride salt is added to the matrix. For example, ammonium chloride will dissociate into ammonia and hydrochloride. This contention is supported by the observation that diethanolaminedissolved in glycerol gave nearly identical mass spectra whether the neutral base or the hydrochloride salt was used (15). The intensities of the ammonia and aniline peaks, respectively, with and without the presence of the other analyte, are given in Figure 3a for the whole concentration range from 1 to 10 mol %. Curve a shows the s u m of the intensities of the ammonia peaks, NH,+(Gly),, n = 0-4,and NH,+(NH,)(Gly),, rn = 1 or 2, expressed in percent of the total ion current, as a function of the concentration of NH4C1with no other analytes present. Similarly, curve b shows the total intensity of the aniline peaks, which include AH+(Gly),, n = 0-3, fragments 77' and 65+, and A2H+,where A = aniline. The aniline peaks are more intense than the ammonia peaks at the same concentration, which is consistent with a higher "preference factor" (pf) (eq 11) (14, 15), for aniline, pf = 37, pf =
I(analyte ions) I(matrix ions)
X
matrix mole fractions analyte mole fraction (11)
than for ammonia, pf = 20. Curve c in Figure 3a shows the ihtensity of the major ammonia peaks for an equimolar solution of ammonia chloride and aniline hydrochloride, and curve d shows the intensity of the aniline peaks for the same solution. Comparing the intensity of the ammonia peaks with no aniline hydrochloride present, curve a, with the intensity when aniline hydrochloride is present, curve c, we see clearly the gradually increasing suppression of the ammonia peaks by aniline. The ratio of curve a to curve c is plotted in Figure 3b. Results from FAB spectra of several other analyte ion pairs are given in Table 11. Of the 16 analyte pairs in the table,
ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987
'*
.w
c
4
1381
10
g! L
5
I
p '
$-
c
0 11
-m
: c
.I-
0
-1
la
0
16
10
20
30
GB(An21- GB(An1)
0
5
-
10
J1
*I2
*C
9
Mole (O/O) Figure 3. (a) Intensities in percent of total ion current of the sum of all major aniline and ammonia derived peaks, respectively, in FAB spectra of glycerol solutions as a function of analyte concentrations: (a) 0, ammonia ions from ammonium chloride solutions; (b) 0 , aniline Ions from aniline hydrochloride solutions; (c) A,ammonia ions (d) 0, aniline ions from equimolar solutions of ammonium chloride and aniline hydrochloride, respectively. (b) Ratio of curve a to curve c in part a.
-2
815
14@
o--;
-
50
1
*lo
-e
-1
-
3 .
3 =. -- .5 -
013
-
- 6
z
*8
0
16
0
2.03
04
-
5 7
.1
:b 10 fulfill the condition that An1 has the higher aqueous basicity and An2 has the higher GB. For the other pairs, An2 has the higher basicity both in the gas phase and in solution. As a measure of the degree of suppression of An1 by An2 the following expression can be used: R(AnB/Anl) =
I(An2,Anl) I(An1) XI(Anl,An2) I(An2)
I
1
1
I
I
(12)
I(An1) is the intensity of An1 derived ions in a FAB spectrum of 10 mol % of An1 in glycerol. I(Anl,An2) is the total intensity of the sane ions when, in addition to 10 mol % of Anl, 10 mol % of the other analyte, An2, is present in the same solution. I(An2) and I(An2,Anl) similarly refer to the intensities of An2 derived ions for glycerol solutions of 10 mol % of An1 alone and of 10 mol % each of An1 and An2 together. The ratio given by eq 12 is expected to be equal to 1 for two analytes that do not compete. Thus, R(An2/Anl) > 1, would show that An1 is depressed by An2. The ratio, R(An2/Anl), is given in Table I1 for all the analyte pairs in the table. It is seen that in all cases, except for the anilinepyridine pair, R(An21Anl) > 1. Thus, with a sole exception, it is the gas-phase basicity and not the solvent basicity that determines the direction of the proton transfer, reaction 9. The intensity ratios R(An2/Anl) are plotted vs. the differences in GB's in Figure 4a. Figure 4b shows the ratios vs. the aqueous-basicity difference. The results in Figure 4a show graphically that when An2 is more basic in the gas phase, R(An2IAnl) > 1 for all cases, except no. 16. Furthermore, although the data are quite scattered, there is an increase of R(AnS/Anl) with increasing gas-phase-basicity difference between An1 and An2. In contrast to Figure 4a, most of the pairs in Figure 4b have ratios R < 1. Had the aqueous basicities been important, the ratios would have been expected to be >1. The figure illustrates that there is no clear corre-
lation between the competition for protons between analytes in FAB and the aqueous basicities of the analytes. We feel that the most likely explanation for the scatter in Figure 4a is that the concentrations of the analytes in the surface layer are different from the bulk concentrations. Analytes containing hydrophobic groups would tend to be enriched in the surface. Such a pattern can possibly be discerned in Figure 4a. Thus, protonated aniline with the hydrophobic phenyl group could be expected to be enriched relative to analytes like NH4+,N2H5+,and pyridineH+. Pairs 2 and 8 have aniline as the high-GB analyte and the R ratios are somewhat higher than expected (Figure 4a). Pairs 15 and 16 have aniline as the low-GB analyte and the R ratios are depressed, which would be expected if there is aniline enrichment on the surface. The role of surfactants in FAB in known and in particular Ligon (20) has used such compounds to enhance the sensitivity of FAB. The compounds used in the present work are obviously far less surface active than the special surfactants in Ligons work. Furthermore, independent information on the surface enrichment of the present compounds does not seem to be available. With these uncertainties in mind, it still may be meaningful to speculate that the present FAB spectra are determined by the "gas"-phase reactions and particularly reaction 9, which is governed by the gas-phase basicities, but that the concentrations of the participating analytes are not proportional to the ligand bulk concentrationbut are modified by surface enrichment. It might be possible to select a larger
1382
ANALYTICAL CHEMISTRY, VOL. 59,
NO. 10, MAY 15, 1987
set of analytes for which the gas-phase basicities and surface enrichment follow distinct trends. Plots like Figure 4a for such a group of compounds might then allow one to assess the influence of surface activity with greater certainty. Obviously, it would also be very valuable to be able to measure surface concentrations with a totally independent method. It is well-known that the relative basicities in water and probably also glycerol are temperature dependent. For example, at 25 “C the difference between the pKBH+values for ammonia and aniline is 4.6, whereas at 50 “C this has decreased to 4.4 (19). For other analyte pairs like ammonia and pyridine this temperature dependence is stronger. In both of these cases, the direction of change, when solution temperature is increased, is toward the relative gas-phase basicities. However, this does not seem to be always the case (21). We have used room-temperature PKBH+’s. It could be argued that we should have used pKBH+ for higher temperatures. However, even if it would be the case that the proton transfer, reaction 9, occurs in what still might be called a liquid, the point made in the present paper is that the energetics of the proton transfer reaction is still characteristic of the gas phase. The argument in favor of the gas-phase model can also be put another way. The basicity reversals from gas to solution are examples of “specific” solvation effects, which are due to the specific binding of solvent molecules to a given solute. It has been shown that the specific solvent effects are revealed already with the attachment of a relatively small number of solvent molecules to the charged solute (22,23).For example, the basicity reversal between ammonia and aniline to the basicity order found in water occurs in the gas phase already when the anilinium ion, at equilibrium,is solvated by ca. three water molecules. Thus, the solvation of the ions in the cavity should be less extensive than this in order for the gasphase-basicity order to be observed in FAB spectra. The present experimental findings appear incompatible with the notion that the FAB mass spectral intensities reflect solution acidities and basicities in such a way that even solution pK, values can be deduced from FAB spectra (25). The present results clearly show that the presence of AlH+ in the matrix will not lead to the appearance of these ions in the FAB spectrum when a coanalyte of lower solution basicity but higher gas-phase basicity is present. This fact, taken together with the previous findings (15),which showed that the total FAB secondary ion current does not increase with addition of electrolytes (preformed ions) to the matrix, indicates that the presence of preformed analyte ions, like AH+, in the matrix and their direct ejection may not be the direct cause (3)for the increased intensity of quasi-molecular analyte, ions, like AH+, in the spectrum, relative to the spectra when only neutral A is present. Indirect causes like an increase of the solubility and surface concentration of the analyte or a decrease of the extent of fragmentation of the quasi-molecular ions (15) may be more important consequences of the preparation of ionized analyte in the matrix. Some of the features of the “gas” collision theory used by the present authors to rationalize the present and earlier observations ( 1 4 , 1 5 ) appear not to be too far removed from some more recent versions of the ”selvedge”theory. Thus, a recent review ( 1 )states: “The picture of the selvedge that emerges is that of a very hot, high pressure region in which multiple collisions are possible.” It is well-known that large ionized clusters are emitted in FAB and SIMS. A typical example is the H30+(H20),cluster ions emitted from ice at 70 K, observed by Rabelais et al. (26). Also, in pure glycerol a small fraction of the total ionization is due to large protonated glycerol clusters, GlyH+(Gly),. The exact mechanism leading to large clusters is not well established. Rabalais (26, 27) assumed that they are formed by
aggregation in the selvedge region. More recently, Michl et al. (13,281 proposed a somewhat different mechanism. The formation of the larger clusters was assumed to occur “...in a dense flowing gas in the form of preferred solvation shells around charged particles”. Rollgen et al. (29) has formulated a mechanism according to which quasi-molecular ions like AnH+ and AnH+ clustered with matrix molecules are due to the rapid desolvation in vacuum of “charged clusters sputtered from the liquid solution” by the rapidly expanding gas produced by the fast particle impact. According to this view (30), the expression of gas-phase ion stabilities rather than solution stabilities is brought about by the cluster desolvation process. As the clusters shed matrix (solvent) molecules, the conditions for the ions contained in the cluster become more gaslike. According to the cluster desolvation model it would appear that the preferred expression of the coanalyte An2H+ with the higher gas-phase basicity requires the initial presence of An1 and An2 in the desolvating cluster. In the present experiments, it was observed that the high-gas-phase-basicity analyte, An2, typically accounts for ca. 50% of the total ionization when its concentration is 2 mol % . An estimate of the average cluster size is obtained from this result since at least 50% of the clusters must contain An2. A cluster must contain a total of ca. 34 molecules (1- 0.9834= 0.5) in order to have a 50/50 chance to contain at least one An2. On the basis of the large intensity observed for bare An2H+, about 90% of the clusters would have to dissociate all the way to the bare ion. The declustering process is presumably adiabatic. Thus, removal of the energy required for the desolvation will cool the remaining ionic cluster. The desolvation of such large clusters, even if initially very hot, all the way down to the bare An2H+ ion appears unlikely, although some declustering is expected to occur. ACKNOWLEDGMENT We thank Allan Hogg for the use of and help with the FAB-MS9 instrument and Raweendra Kulatunga for help with preliminary experiments. LITERATURE CITED Pachuta, S. J.; Cooks, R. G. I n Desorptlon Mass Spectrometry: Are SIMS and FAB the Same?; American Chemical Society: Washlngton, DC, 1985; p 1. MacFarlane, R. D. Acc. Chem. Res. 1082, 15, 268. Benninghoven, A. I n Secondary Ion Mass Spectrometry; Benninghoven, A., Okano, J., Shimizu, R., Werner, H. W., Eds; SDringer-Veriaa: . Berlin, 1984 p 342. Martln, S. A , ; Costello, C. E.: Biemann, K. Anal. Chem. 1982, 54, 2362. Busch, K. L.; Unger, S. E.; Vincre, A.; Cooks, R. G.; Keough, T. J . Am. Chem. SOC. 1082, 104, 1507. Inchaouh, J.; Blais, J. C.; Bolbach, G.; Brunot, A. Int. J . Mass Spectrom. Ion Processes W84, 61, 153. Magee, C. H. Int. J . Mass Spectrom. Ion Phys. 1083, 49, 211. Garrison, B. J. In Desorption Mass Spectrometry: Are SIMS and FAB the Same?; American Chemical Society: Washington, DC, 1985; p
.-.
A3
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RECEIVED for review October 16, 1986. Accepted February 2*1987. This work was by a grant from the Canadian Natural Sciences and Engineering Research Council.
Increased Sensitivity in Laser Microprobe Mass Analysis by Using Resonant Two-Photon Ionization Processes F. R. Verdun,’ G. Krier, and J. F. Muller* Laboratoire de Spectrometrie de Masse et de Chimie Laser, UniversitC de Metz, B.P. 794, 57012 Metz Cedex 1 , France
The coupling of a tunable dye laser to a laser microprobe mass analyzer (LAMMA), used to both atomize and Ionize matter, allows slgnlficant increases in sensitivity by using resonance lonizatlon properties (if we compare the results wlth those of a standard LAMMA 500 system). We present first the results obtained from studying metal-doped polymer thin sections and then discuss the matrix effects observed. Finally we will present the analysis of a steel alloy as an applkatlon of our coupling to more current samples.
Resonant ionization mass spectrometry (RIMS) is a relatively new technique that exploits both resonance ionization spectrometry (RIS) and mass spectrometry (MS) properties. RIS is a photoionization method in which atoms in the gas phase are ionized by the absorption of photons (produced by dye lasers) that energetically match transitions between quantum states of these atoms (1,Z). Hurst et al. (I) have proposed five basic photoionization schemes, according to the relative energy position of the intermediate quantum level(s) to the continuum, to obtain a very selective and sensitive ionization of a majority of the atoms. However, to be very selective, RIS needs the use of wavelengths in the visible range which requires in many cases, more than one order. The idea ww then developed to use lower wavelengths (UV range) to ionize atoms in the gas phase by a two-photon ionization process: (A ( w l , w l e) A+) (1)where two photons from the same pulse (1)are successively absorbed by the atoms which are then exited into their own ionization continuum. However, even though the advantage of increased sensitivity is obtained with this procedure, the selectivity of the method is strongly reduced because only one resonant step is involved. To counteract that, a mass spectrometer is generally used as the ion detector (3-6). Fassett et al. (6) have already published very convincing results on noticeable increases in sensitivity for about 50 elements photoionized by a well-tuned laser in the range of 260-355 nm, after a thermal vaporization step. Present address: Chemistry Department,Ohio State University, Columbus, OH 43210.
It is clear that the main advantage of using the RIS properties is to obtain an increase in sensitivity which is particularly important in searching for trace elements. In this range of application, it is often very important to know the precise location in a sample where the trace element has been detected, so the idea has been to associate the RIMS and the “microprobe” techniques. Williams et al. (7) have shown some exciting results with a system in which the vaporization of a small area mm2) of the sample is obtained by a well-focused Nd:YAG laser (1064 nm) and in which an excimer laser is involved in the resonant ionization step. In this system, the resulting ions are analyzed by a time-of-flight spectrometer. Finally, Donohue et al. in a recent paper (8)described the use of an ion microprobe (2 pm of spatial resolution) to produce gas-phase atoms which were subsequently ionized by resonant processes with a tunable dye laser. However, with this system, the use of only one ionizing tunable dye laser in the visible range does not allow one to exploit RIS advantages with a great number of atoms. Our purpose is to examine the possibility of using the RIS advantages with only one UV-tunable laser used for both the vaporization and the resonant photoionization of a spot area in the range of a few square micrometers. That has been done by the coupling of a commercial laser microprobe LAMMA 500 with a dye laser (9-11). EXPERIMENTAL SECTION The laser mass analyses were performed on a LAMMA 500 (laser microprobe mass analyzer, Leybold Heraeus, Germany) which analyzes thin samples (d 5 1 pm) in a transmission arrangement. In its commercialized version the ionization of the sample is induced by a Q-switched quadrupled NdYAG laser (266 nm, 15 ns), which is power controlled by a set of filters, and focused by a microscope on an area of 2-5 pm2. A visible He-Ne pilot laser beam, collinear with the ionizing laser, locates the sample area to be analyzed. Positive or negative ions are extracted into a time-of-flightmass spectrometer though an “einzel”lens. The common resolution (M/dM) of this instrument is about 800. The signals are stored in a 100-MHz transient recorder (2K memory, Biomation 8100) (12-14). In our modified system the NdYAG laser second harmonic (532 nm) pumps a TDL I11 dye laser (Quantel, France) and the dye laser output is frequency doubled (or frequency mixed with the NdYAG residual IR)before being focused onto the sample (Figure 1) (15). The dyes used in these experiments were the Rhodamines
0003-2700/87/0359-1383$01.50/00 1987 American Chemical Society
