J . Phys. Chem. 1991, 95, 7824-7830
7824
The calculation in the Z direction, after application of the boundary conditions to eq A9, is of the form
where each of the terms are defined in Table 11. The values for the vector (d*]l,kSNZ-l for each column are calculated from the corresponding c , , k values evaluated at the previous half-time step. Values of cl*j,k are then calculated along each column, starting a t the R = 0 boundary and working across to the radial edge of the glass sheath. The calculation starts with species R followed by 0 for all k at each value of j . It is apparent from Table I1 that the calculation of CR**j,k(for each column in the range 0 Ij I NE) requires a knowledge of CO**j,Nz-I,which remains to be calculated. This problem was circumvented by adopting an iterative procedure in which, for a particular column, CO+*j,Nz-l was initially approximated by CO*j,Nz-lfollowed by successive calculations of cR**j,kand co**,,k, using the newly determined value for CO**j,Nz-I at each iteration, until the values of CR**,,Nz-I and co**j,Nz-I were unchanged (to within 0.01%) upon further iteration. The validity of this procedure was checked by
calculating the current-time characteristics for feedback from a conductor in a similar fashion, and comparing the results to those obtained by invoking the concept of mass conservation (since Do = DR),and solving the equivalent "one species" case (which avoids the above-mentioned problem). At the end of the second half-time step, the current at the disk electrode was evaluated by the summation of the local fluxes in the normal direction. A five-point finite-difference approximation for the flux was used, as recommended by other workers.s9 The calculation then proceeds to the next time step, with the values for (d]O,,j320 (Fe+) (5000) (9000) (0) (0) retinol IO 13 200 (Fe+) >320 (Fe') (7000) 19700) (0)
Leu-Phe Tyr-Val
14 (150) 20 (1200)
220 (Fe+) (0)
NS' (0)
100 (Fe+) NS
NS (0)
NS (0) (0) (0) 'All values given in this table are observation thresholds which are dependent on the sample absorption and ionization characteristics as well as the instrumental configuration. bNumbers in parentheses are solution-phase absorptivities in L/(mol.cm). Pentacene values were measured in benzene. All other values were measured in methanol. 'Underlined desorption thresholds were measured by using focused LD conditions. "(Fe+) indicates that appearance of the molecular ion signal was observed to coincide with the observation of postionized substrate-specific Fe+ ions. e NS indicates no signal from sample-specific postionized neutrals was observed to the limits of the desorbing laser power density. thinner at the edges. The edges of the probe tip were left exposed. We estimate, from the depth of the machined depression, that the thickness of the sample film at the center of the probe tip was about 100 pm. Ala-Trp (alanyltryptophan), Leu-Phe (leucylphenylalanine), Tyr-Val (tyrosylvaline), Leu-Trp (leucyltryptophan), Met-Trp (methionyltryptophan), Arg-Trp (arginyltryptophan), and L-WM-R-F(leucyltryptophylmethionylarginylphenylalanine)were purchased from Serva Chemicals, Heidelberg. Retinol, &estradiol, and gram. D (gramicidin D) were purchased from Sigma Chemical Co., Munich. Pentacene was obtained from Aldrich Chemicals, Steinheim. Coumarine (coumarine 307)was obtained from Radiant Dyes, Wermelskirchen. All chemicals were used as received without further purification.
Results Table 1 shows a list of test compounds that were investigated by desorbing at the four harmonic wavelengths of the Nd:YAG laser, 1064,532, 355, and 266 nm, and the measured thresholds for observation of postionized parent neutral molecules. The thresholds for parent ion detection were measured by first determining the appropriate delay between desorbing and ionizing laser pulses which gave maximum parent ion signal (typically 275-325 ps) at somewhat higher desorbing laser intensities and then reducing the desorbing laser intensity until a parent ion signal could no longer be observed in the analog oscilloscope trace at highest sensitivities. This procedure was repeated several times on fresh sample material to arrive at a representative value for the threshold desorbing laser power. The uncertainty in these values arose principally as a result of the shot to shot fluctuations in the desorbing laser output which were ca. f200 pJ (e.g., ca. 1.5 MW/cm* for the unfocused 266 nm desorbing radiation). It should be emphasized that the reported values are observation thresholds which are strongly dependent on the instrument design and individual sample molecule absorption and ionization characteristics. Whenever possible, desorption of the sample was performed with an unfocused desorbing laser to avoid exposure of the substrate material and to minimize changes in the sample morphology during repeated LD pulse exposures. The desire was to achieve a relatively constant neutral desorption rate for a brief number of LD events so that observations regarding neutral distributions
Kinsel et al. in the jet expansion and relative neutral intensities of the compound of interest could be made. This approach was very effective when resonant wavelengths were used for desorbing, and, depending on the desorbing laser intensity and specific sample probed, constant postionized neutral signals could be obtained for the compounds listed in Table I for several hundred LD shots or more. When a sample was desorbed by using a nonresonant desorption wavelength, however, this approach was often ineffective as no signals could be observed up to the energy limit of the unfocused desorbing laser. Therefore, the desorbing laser was softly focused to increase the power density in the desorption volume. It has been suggested in the literature that the desorption mechanism may change as a function of desorbing laser focus.23 In particular, it has been noted that the threshold desorption energy for direct LD of ions does not appear to scale as a function of the area of the laser focus. This behavior was attributed to a nonlinear photon absorption/desorption process. We also observed a similar nonlinear relation between threshold desorbing power and laser focal area for those samples which could be desorbed under either focused or nonfocused conditions, i.e., resonantly absorbing samples. However, no significant changes in resulting sample mass spectra or jet profiles were noted under the conditions used in these experiments. This observation suggests that changing the desorbing laser focus does not significantly alter the actual desorption process but that care must be excerised when focused and unfocused desorption thresholds are compared. Spectra obtained under focused desorption conditions are noted in Table I. Another important point to be discussed when focused desorption laser conditions are used relates to the appearance of Fe+ ions in the mass spectra. These ions arose from neutral iron species being desorbed from the substrate surface and transported into the ion source region. When focused nonresonant LD was employed, detectable ion signals from the sample of interest could only be observed concurrent with large signals of Fe+ ions. Typically, when a fresh sample surface was exposed, tens or hundreds of LD shots at high power occurred initially where no observable postionized neutral signal of any kind could be detected. After a period of time, a signal began to appear of postionized Fe and, at the same time, signals from postionized neutral sample molecules were also detected in the mass spectrum. Inspection of the sample probe showed that a hole had been ablated through the sample, exposing the steel substrate material below. We emphasize that it was not possible, under the experimental conditions described, to obtain a signal from the test compounds under nonresonant desorption conditions without also observing an intense Fe+ signal in the mass spectra. Often the Fe+ signal was accompanied by signals from Na+, K+, and FeO+. The precursors for these ions must enter the ion source region as neutral species since positive ions are prevented from entering the RETOF-MS source region under the current instrumental design. It is not presently known what these species are or how ionization occurs. It is likely that other ionization mechanisms besides atomic MUPI, such as ionization and dissociation of metal clusters, play a role. Desorbing conditions under which the formation of postionized Fe was observed to coincide with observation of the postionized sample neutral molecules are also indicated in Table I. At desorbing wavelengths where no parent ion signal could be observed under any conditions, the delay time between desorption and ionization was scanned at a variety of desorbing laser powers to ensure that the signal was not being overlooked at some significantly different arrival time. Table I also lists the measured solution-phase absorption coefficients for the various test compounds at the four desorbing wavelengths. These values are meant only as a general guideline to indicate gross absorption characteristics and are not intended as accurate measures of solid-phase absorptivities. The cutoffs of the absorption bands as well as the absorption strengths may change for the solid-state and solution-phase species. It has been shown that solid-phase absorption characteristics can also change as a function of solid sample thickness and/or crystal morphol(23) Hillenkamp. F. Adu. Mass Spectrom. 1989, I I A , 354.
Laser Desorption of Neutral Organic Molecules
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7827
I M*'
A
X
I B
I
FeO*
I
11
M+' ,
X
C
l M *'
I
X
D
D
IM*' 70
0
140
210
280
INa' i"* 3%
Figure 1. Mass spectra of laserdesorbed pentaccne neutrals (MW = 278 amu) postionized by 255-nm MUPI. Neutral molecules are desorbed at (A) 266, (B) 355, (C) 532, and (D) 1064 nm. Spectra are individually normalized to the most intense signal.
l A
I
B
I I
L
_.
Fe'
No*
70
140
210
280
%"
Figure 2. Mass spectra of laser-desorbed coumarine neutrals (MW =
271 amu) postionized by 255-nm MUPI. Neutral molecules are desorbed at (A) 266, (B) 355, (C) 532, and (D) 1064 nm. Peaks marked with an X are identified as background signals. Spectra are individually normalized to the most intense signal. ~ g y The . ~ measured ~ values, however, are adequate, to give a general indication at which desorption wavelengths a compound may be expected to absorb strongly, weakly, or not at all. Examples of typical spectra obtained at the four desorbing wavelengths for pentacene, coumarine, and Leu-Phe are shown in Figures I , 2, and 3, respectively. The pentacene mass spectra (Figure 1 ) represent the behavior of a compound which gives parent ion containing mass spectra without Fe+ ions at all four desorbing wavelengths. The coumarine mass spectra (Figure 2) are representative for the compounds which yield parent ion (24) Hillenkamp, F.; Karas, hi.; Holtkamp, D.; Kluesener, P. Inr. J . Mass
Spccfrom. Ion Proc. 1986, 69, 265.
350 amu
Figure 3. Mass spectra of laserdesorbed Leu-Phe neutrals (MW = 278 amu) postionized by 255-nm MUPI. Neutral molecules are desorbed at (A) 266, (B) 355, (C) 532, and (D) 1064 nm. No molecular ion signal could be detected when 532- and 1064-nm wavelengths were used for desorption. Peaks marked with an X are identified as background signals. Spectra are individually normalized to the most intense signal.
"All values given in this table are observation thresholds which are dependent on the sample absorption and ionization characteristics as well as the instrumental configuration. Underlined desorption thresholds were measured by using focused LD conditions. '(Fe') indicates that appearance of the molecular ion signal was observed to coincide with the observation of postionized substrate-specificFe+ ions. N S indicates no signal from sample-specific postionized neutrals was observed to the limits of the desorbing laser power density.
1 1 0
280
a6 a
X
1
210
wavelength of desorbing laser irradiance comcd 266nm 355nm 532nm 1064nm Ala-Trp 7 (Fe+)' >250 (Fe+) >320 (Fe+) Leu-Trp 7 (Fe+) >250 (Fe+) NSd Met-Trp 7 150 (Fe') >250 (Fe+) NS 14 150 (Fe+) NSd NS Arg-Trp NS 160 (Fet) NS L-W-M-R-F gram. D 82 180 (Fe+) NS NS
IFe* 1.
140
TABLE II: Measured Tbresbolds" (MW/cm2) for the Observation of Postionized Molecular Ion Signals of Laser-Desorbed Neutral TrpContaininn Peptides
I M+'
I
70
M*'
I,.
X
0
X
containing mass spectra a t both resonant and nonresonant desorbing wavelengths. The mass spectra and characteristic LD behaviors of retinol, estradiol, and Ala-Trp are similar to that of coumarine. The Leu-Phe mass spectra (Figure 3) are representative for compounds which do not give parent ion containing mass spectra at longer wavelength nonresonant LD conditions. The Tyr-Val mass spectra and LD behavior are analogous to that of Leu-Phe. Table I1 lists the results for a series of Trpcontaining dipeptides as well as two larger Trp-containing polypeptides. This test compound group was chosen to investigate the effect of increasing molecule size, and presumably decreasing thermal stability, in LD of neutral molecules. For the compounds in Table I1 the Trp residue is expected to determine the general absorption characteristics of the samples at the four Nd:YAG desorbing wavelengths since it provides the strongest absorbing chromophore. Therefore, the absorption characteristics for all the samples in Table I1 are expected to be very similar to those obtained for the Ala-Trp dipeptide shown in Table I. Thresholds for the compounds listed in Table I 1 were determined in the same manner as previously described for the samples
7828
The Journal of Physical Chemistry, Vol. 95, No. 20, 15)91
listed in Table I. It should be noted that it was not possible to desorb the two larger polypeptides, even at the resonant 266-nm desorbing wavelength, without focusing the desorbing laser. However, the results obtained for resonant LD of the polypeptides under focused conditions were significantly different from those obtained for the focused nonresonant LD of the smaller test compounds given in Table I. Specifically, once the threshold for sample desorption was reached, a signal from the postionized polypeptide neutrals was observed immediately, even when a fresh sample surface was exposed. It was possible to obtain mass spectra of postionized neutrals of the polypeptides without creating the abundant Fe+ ions which were always observed under nonresonant LD conditions. This observation indicates that it was not necessary to expose the substrate surface for neutral molecule desorption of the resonantly absorbing polypeptides. It appears that simply an increase in desorption power is required to desorb sufficient numbers of larger molecules to exceed the observation threshold. The increased power necessary for the desorption of the polypeptides only allowed a limited number of LD shots to be performed on any given sample spot before the substrate material was exposed, which was evidenced by the appearance of Fe+ ions in the mass spectra. To eliminate this effect, the desorbing laser focal point was scanned across the sample surface to expose fresh sample material throughout data acquisition and spectra without Fet signals were thus obtained. The spectra shown and the compounds listed in Tables I and I1 are just a small sample of a larger set of compounds that have been investigated in the manner described in this paper. These examples were chosen to illustrate the various characteristics of resonant and nonresonant LD. In general, the conclusions that are derived from analysis of the LD behavior of these compounds have been found to be applicable to all compounds thus far invest iga ted.
Discussion The most striking and reproducible feature of the data given in Table 1 is the consistent decrease in the LD threshold power when the desorbing laser wavelength is resonantly absorbed by the sample compound. This observation is in agreement with previously reported results for resonant direct LD of ions.12 All seven compounds in Table I show appreciable solution-phase absorption cross sections at 266 nm ranging from 150 L/(mol-cm) for Leu-Phe to 21 000 L/(mol.cm) for pentacene. It was possible to desorb all seven test compounds by using unfocused radiation in the range 3-20 MW/cm2. An abundant molecular ion signal is produced in all cases with very few fragment ions (Figure I A , 2A, and 3A). It was not always clear whether the fragment ions that were observed were produced during LD and subsequently postionized or were a result of MUPI at 255 nm. Both processes may contribute to the fragment ions observed. Power- and wavelength-dependence studies have shown that many of the fragment ions observed are created as neutral species during LD together with the desorbed intact molecular sample.25 There is no general correlation between the measured threshold powers for the various compounds and their solution-phase absorptivities. This is not unexpected since the threshold powers are effectively observation thresholds which are dependent on all factors related to eventual ion detection. In particular, the ionization efficiencies for these compounds at 255 nm may vary widely. When desorption is done at 355 nm, Table I again shows lower thresholds for the desorption of the resonantly absorbing compounds pentacene, coumarine, and retinol. Abundant molecular ion signals with minimal fragmentation for these compounds are again observed (Figures IB and 2B). However, estradiol and Ala-Trp, which do not have appreciable solution-phase absorptivities at 355 nm, also give relatively low thresholds. This may be due to the thermal stability of these compounds or weak solid-phase absorptions which are not observed in the solutionphase absorption spectrum. In the case of Ala-Trp, it seems likely (25) A more detailed discussion of fragment ion formation will be published in a future paper.
Kinsel et al. that the solid material may have a weak absorption at 355 nm. The published solid-phase absorption spectra for pure tryptophan show a weak absorption tail which extends out beyond 355 nm and which appears to increase as a function of sample thickness." The situation is less clear for estradiol, where no published solid-phase absorption spectrum exists for comparison, but the low threshold value and lack of observation of postionized Fe suggest a similar weak absorption at 355 nm. Sample-specific signals from Leu-Phe and Tyr-Val could only be obtained under focused high-power desorption conditions with simultaneous observation of postionized Fe (Figure 3B). Postionized parent ion signals could be observed for both compounds, with some fragment ion formation. When desorption of the nonabsorbing compounds is done at 532 and 1064 nm, signal from postionized intact neutral molecules could only be obtained under high-power (200 MW/cm2 or more) focused LD conditions and with concurrent observation of postionized Fe (Figure 2C,D). In all cases, the absolute postionized molecular ion signals were more than an order of magnitude less intense than that observed under resonant desorption conditions. In many cases these signals were extremely unstable and reproducible spectra from shot to shot could not be produced. In the cases of Leu-Phe and Tyr-Val, no signal from postionized sample-specific neutrals could be observed at any desorbing power, even under conditions where abundant Fe+ ion signals were produced (Figure 3C,D). A limiting desorption power of 1000 MW/cmZ was determined by the maximum output of the desorbing laser system under the focal conditions used. The only compound which could be easily desorbed by using unfocused light at 532 and 1064 nm and without the observation of postionized Fe was pentacene (Figure 1C,D). The solutionphase absorption spectra of pentacene indicates this compound absorbs at 532 nm but not at 1064 nm. The anomolous behavior may be due to a very low sublimation temperature for pentacene which allows this material to be desorbed at temperatures below that which also desorbs the Fe-containing substrate material. Or, this behavior at 1064 nm may be due to a solid-state absorption band resulting from an extended delocalized aromatic system in the solid material which is disrupted and not observed in the solution-phase spectrum. It cannot presently be determined which of these effects allow the pentacene material to be efficiently nonresonantly desorbed at the 1064-nm desorbing wavelength. It seems clear from the majority of these results that, when a pure sample does not resonantly absorb at the desorbing wavelength, less efficient desorption and collection of neutral intact molecules is the result. In many cases it is impossible to observe postionized intact sample-specific neutrals under nonresonant desorbing conditions with the instrumental configuration described. For those compounds that can be observed under focused nonresonant desorption conditions, detectable desorption of neutral sample species occurs only after the metal substrate material is exposed. Apparently, once the metal substrate is exposed, the desorbing energy is coupled into the substrate material, producing a rapid temperature rise which, if the sample material is thermally stable enough to withstand the increase in temperature, results in desorption of the sample and substrate material simultaneously. At least some fraction of this desorbed material is neutral intact molecular sample species. The compounds Leu-Phe and Tyr-Val appear to have insufficient thermal stability to withstand the temperature rise of the substrate material at these desorbing powers. An inspection of the structures of the compounds investigated in Table I suggests that these two compounds might indeed be the least thermally stable. The nonpeptide compounds can all be mass spectrometrically analyzed by using conventional desorption probe techniques. For the dipeptides, it is known that alkyl amino acids become increasingly thermally labile as the alkyl chain increases in size. The aromatic amino acids, on the other hand, tend to be relatively stable due to the high degree of conjugation within the side chain. A further support of this interpretation is given by the results shown in Table 11. These results show that as the size of the alkyl side chain increases, nonresonant desorption of the
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 1829
Laser Desorption of Neutral Organic Molecules
M
A
I
L
lB
L
..
X
i
.I.
0
jYf
ID 0
+-
200
400
600
800
1000
amu
I 80
160
240
320 4%1
Figure 4. Mass spectra of resonant 266 nm laser desorbed Trp-containing dipeptide neutrals postionized by 255-nm MUPI. Spectra shown are (A) Ala-Trp (MW = 275 amu), (B) Leu-Trp (MW = 317 amu), (C) MetTrp (MW = 335 amu), and (D) Arg-Trp (MW = 360 amu). Spectra are individually normalized to the most intense signal.
intact neutral dipeptide molecule becomes less likely, to the extent that only Ala-Trp can be observed when desorption is performed at 1064 nm. In contrast, all Trp-containing peptides up to the mass of gramicidin D (1 88 1 Da) can be efficiently desorbed at the resonantly absorbed 266-nm desorption wavelength. In all cases, an abundant molecular ion signal from postionized intact neutrals with minor fragmentation may be observed (Figures 4 and 5). It is also interesting to note that all Trp-containing peptides, including gramicidin D, can be desorbed with focused 355-nm radiation as well. One interpretation of this behavior may be the onset of a competition between a weak resonant bulk sample desorption process and a substrate-mediated thermal desorption process. If the desorption process were purely a result of substrate heating, then the efficiency of peptide desorption at 355 nm should be the same as for the longer desorbing wavelengths of 532 and 1064 nm. Clearly this is not the case for the larger peptides. As stated previously, the solid-phase absorption spectrum of Trp shows a weak absorption at 355 nm. Comparison of the results at 355 nm with those at 532 and 1064 nm supports the existence of this weak solid-phase absorption and suggests it is a necessary factor for the successful desorption of intact neutral molecules from the larger peptides. It cannot be stated with certainty that no intact neutral molecules of the larger compounds are desorbed under high laser power nonresonant conditions. The neutral collection limitations imposed by the jet entrainment and transfer process may partially filter neutrals desorbed with high kinetic energies. If nonresonant desorption conditions produce high translational energy distributions in the desorbed neutrals, then a lower fraction of the desorbed species will be collected and introduced to the RETOF-MS source region for postionization. However, nonresonant desorption does produce detectable numbers of the smaller Ala-Trp molecule, indicating that the kinetic energy distribution of this molecule when desorbed under nonresonant conditions is within the collection limitations of the jet entrainment process. Further experiments in which LD and postionization are performed directly within the RETOF-MS source region may clarify this uncertainty. A further limitation of high-power nonresonant desorption may be imposed by the apparent increase in desorption threshold observed as a function of molecule size. When resonant desorption
Figure 5. Mass spectra of resonant 266 nm laser desorbed Trp-containing polypeptide neutrals postionized by 255-nm MUPI. Spectra shown are (A) L-W-M-R-F (MW = 751 amu) and (B) gramicidin D (MW = 1881 amu). Peaks marked with an X are identified as background signals. Spectra are individually normalized to the most intense signal.
is performed at 266 nm, the data in Table I1 show that the threshold for detectable desorption of intact neutrals increases as the molecular weight of the compound increases. If a similar trend exists for nonresonant desorption, where the observed thresholds are already very high for smaller molecules, then extremely large powers may be necessary for the desorption of larger intact neutral molecules. Such high-power LD conditions do not seem appropriate for desorption of intact neutral molecular species. On the other hand, the successful desorption of gramicidin D at 266 nm indicates that resonant coupling of moderately low power LD energy directly into the sample bulk is an effective means for the desorption of even relatively large intact neutral molecules. The resonant desorption mechanism for the desorbing wavelengths investigated appears to be a bulk sample absorption/desorption process. It was never necessary to observe substratespecific Fe+ ions to resonantly desorb the investigated compounds. Under resonant desorption conditions, Fe+ ions were not observed until almost all sample material had been ablated from the probed area, even when desorbing powers well above those measured to produce Fe+ ions from the bare probe were used. Similar results have been reported in direct LD of ions, where it has been shown that even submonolayer coverages of absorbing species can effectively shield the substrate material from the desorbing laser power.24 It has been proposed that this behavior is due to a nonlinear resonant absorption by the sample material. Changes in both the substrate support material (quartz and graphite substrates) and the sample exposure conditions (thin and thick sample exposures) produced no substantial variations in the results presented. Models of the LD process which depend upon substrate heating, therefore, seem inappropriate as descriptions of the resonant desorption process. It is not presently known how the results described in this paper may be extended to infrared LD of neutrals at the C02wavelength of 10.6 r m . Previous investigations at this desorbing wavelength by this group indicated that resonant absorption of the desorbing wavelength was detrimental to LD of intact molecules, although, strictly speaking, these results related to absorption by a mixed sample/matrix combination rather than to absorption by a pure c o m p o ~ n d . ~Unknown ~J~ chemical interactions between the matrix and sample and the longer pulse duration of the C02 desorbing laser complicate comparisons with the current results. However, desorption of transparent molecules at the 1064-nm wavelength, which may be considered a near infrared excitation wavelength, is clearly not an efficient process. Whether the apparent change
7830
J . Phys. Chem. 1991, 95, 7830-7838 ditions substrate absorption with subsequent rapid laser heating plays a minor role in the desorption process. The best description seems to be one where the LD energy is nonlinearly coupled directly into the sample bulk with subsequent efficient neutral desorption. Nonresonant desorption, on the other hand, requires the participation of the substrate material as the energy uptake medium for the desorption process to occur. However, the higher threshold powers, subsequent substrate temperature rise, and abundant substrate ion formation processes severely limit the overall production of intact neutral species. As a result, the most efficient method for desorbing and detecting neutral intact molecules of organic compounds and in particular of larger, thermally labile compounds is using resonant LD conditions.
in LD behavior at 10.6 pm is due to a fundamental change in the desorption mechanism as a function of desorbing wavelength, a result of the longer duration desorbing pulse, or specifically related to the chemical nature of the matrices chosen will require further investigation.
Conclusion The experimental results show that the wavelength used for desorption of neutral species from a pure sample is clearly an important parameter. In a manner very similar to direct LD of ions, resonant desorption of neutrals can be achieved at lower thresholds with stronger yields of postionized molecular ion signals over nonresonant desorption. Depending on the choice of conditions, abundant postionized molecular ion signals with minimal fragment ion formation can be achieved for a wide variety of resonantly absorbing samples over a relatively large mass range. Certain characteristics of the resonant and nonresonant neutral desorption mechanisms can be determined from the data presented in this study. It seems clear that under resonant desorbing con-
Acknowledgment. This work was supported by grants from Deutsche Forschungsgemeinschaft (GR917/ 1-2) and the Bundesministerium fur Forschung und Technologie (1 3NS307-2). G.R.K. gratefully acknowledges the fellowship support of the Alexander von Humboldt Stiftung, Bonn, Germany.
ESR and Electron Spin-Echo Studles of MnAIPO5 Z. Levi, A. M. Raitsimring: and D. Goldfarb* Department of Chemical Physics, The Weizmann Institute of Science, 76 100 Rehouot, Israel (Receiued: February 14, 1991)
The immediate environment of Mn(I1) in MnALPOS after synthesis, calcination, hydration, and dehydration has been investigated by both conventional E S R spectroscopy and by several electron spin-echo (ESE) methods. Several samples with Mn(I1) contents between Mn/P = 0.05 and O S atom Iwere synthesized, and samples of impregnated Mn-AIPOS and exchanged Mn-SAPOS were used as references. The X-band and Q-band E S R spectra of all as-synthesized and calcined MnAlPOS samples and of impregnated Mn-AIPOS are characteristic of Mn(I1) in a slightly distorted octahedral symmetry with an hyperfine coupling constant of 90 G and a zero-field splitting parameter of 140 G. The spatial distribution of the Mn(I1) cations was investigated by the "2 + 1" E S E experiment. A random homogeneous distribution of isolated Mn(I1) cations was found only in the sample with the lowest Mn(I1) content. In all other samples regions with enriched Mn(I1) concentrations, where most of the Mn(I1) are located, were found to exist. Dehydration causes the migration of the Mn(I1) cations toward each other, and consequently the spin exchange interaction increases, resulting in the averaging of the hyperfine interaction. Electron spin-echo envelope modulation (ESEEM) experiments, which detect only the isolated Mn( 11) species, showed that nuclei at a distance of 5 A. the isolated Mn(I1) cations interact through weak dipolar interactions with an average of 6 This does not agree with a model of Mn(I1) substituting for framework AI. Significant changes in the AI modulation after calcination and hydration indicated that this treatment results in a change in the Mn(I1) location. Mn(I1) in calcined and hydrated MnAIPOS and in impregnated Mn-AIPOS is coordinated to three or four framework oxygens, and the remaining ligands are water molecules and/or hydroxyl groups. All the above observations lead to the conclusion that the majority of the Mn(ll) in MnAIPOS does not occupy framework sites but is probably coordinated to the external surface.
Introduction The aluminophosphate molecular sieves have drawn considerable attention due to their ability to form structures with relatively large channels;'.2 for instance, the microporous solid with the largest channel known today, VPI-5, is a member of this family.3 The drawback of these materials in terms of their application as catalysts is the absence of BrBnsted acid sites, which is a consequence of the neutrality of their framework. This disadvantage can be circumvented by framework substitution, which introduces negative charges into the framework, thus requiring the presence of exchangeable cations to balance the charge. Flanigen et aL4 have reported the incorporation of several transition-metal cations, such as Ti(I), Mn(II), Fe(II)/(III), and Co(II), into tetrahedral sites in the framework of a number of aluminophosphate (AIPO) and silim-aluminophosphate (SAPO) molecular sieves during synthesis. Several characterization studies on CoAIPOS reported that the Co(11) is tetrahedrally coordinated 'On leave from the Institute of Chemical Kinetics and Combustion, SBAS USSR, Novosibirsk, USSR.
0022-3654191/2095-7830%02.50/0
in framework sites both after synthesis and following c a l ~ i n a t i o n . ~ ~ It was also found that the amount of Co(I1) that can be incorporated into the framework of AIPOS is limited, Le., CoJP < 1 atom %.7 Evidences for Mn(I1) incorporation into tetrahedral framework sites in AIPOS or any other aluminophosphate mo( I ) Wilson, S. T.;Lok, B. M.; Flanigen, E. M. U S Pat. 4310440. (2) Wilson. S.T.; Lok, B. M.; Messina, C. A.; Cannan, T.R.; Flanigen, E. M. J . Am. Chem. Soc. 1982, 104, 1146.
(3) Davis, M. E.; Saldasiaga, C.; Montes, C.; Garces, Y.; Crowder, C. Nature 1988, 3317, 698. (4) Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S.T. New Developments in Zeolites Science Technologv; Proceedings of the 7th International Zeolite Conference; Murakami, Y., Ijima, A., Ward, J . W., Eds.; Kodansha: Tokyo, 1986; p 103. ( 5 ) Iton, L. E.; Choi, 1.; Desjardins, J. A.; Maroni, V. A. Zeolites 1989, 9, 457. ( 6 ) Shiralkar, V. P.; Saldariaga, C. H.; Perez, J. 0.;Clearfield, A.; Chen, M.; Antony, R . G.; Donohue, J. A. Zeolites 1989, 9, 474. (7) Scoonheydt, R. A.; de Vos, R.; Pelgrims, J.; Leeman, H. Zeolites: Facts, Figures & Future; Jacobs, P. A,, van Santen, R. A., Eds.; Elsevier: Amsterdam, 1989; p 559.
0 1991 American Chemical Society
