International Journal of Mass Spectrometry and Zon Processes, 112 (1992) 205-214 Elsevier Science Publishers B.V.. Amsterdam

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The laser desorption of organic molecules in ion mobility spectrometry

Julie Phillips and John Gormally’ Department of Chemistry and Applied Chemistry, University of Saljord, Salford h45 4 WT (UK)

(First received 10 July 1991; in final form 4 September 1991)

ABSTRACT

An application of pulsed ultraviolet laser desorption as a source of ions in ion mobility spectrometry is described. Examples of ion mobility spectra obtained for polyaromatic hydrocarbons and indole derivatives are given. A feature of laser desorption under atmospheric pressure conditions is found to be an apparent absence of ion fragmentation for most molecules studied.

Keywords: laser desorption; polyaromatic hydrocarbons; indole derivatives; ion mobility spectrometry.

INTRODUCTION

Ion mobility spectometry (IMS) has been developed as a simple but sensi- tive technique for the detection of molecules at trace levels in gases at atmos- pheric pressure [l]. In the standard technique, primary ions are produced in a carrier gas by a 63Ni beta emitter. These primary ions then give rise to a series of ion/molecule reactions which eventually yield molecular ions of the trace species. In some more recent versions of the technique, pulsed lasers have been used to directly ionize the molecules of interest [2-51. The use of tunable ultraviolet lasers to selectively ionize molecules by resonant multi- photon ionization can enhance the ability of the technique to discriminate between different molecular species. In addition, the spectra obtained with laser ionization are generally simpler and easier to interpret than those obtained with the more usual Ni beta source.

A further potential application of lasers to this technique is that the same laser that is used for ionization can also be used, with small moditications to the ion mobility spectrometer, for the laser desorption and ionization of substances that are not particularly volatile at the temperature of the experi- ment. The application of laser desorption in the field of mass spectrometry continues to be studied extensively but we are not aware of any similar studies

‘Author to whom correspondence should be addressed.

0168-I 176/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

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b)

Fig. 1. (a) The ion mobility spectrometer drift tube assembly. The drift tube is fitted with two tubular PTFE sidearms through which the laser beam is admitted so as to fall on the sample plug at an angle of 20”. (b) The anode end of the drift tube shown rotated through 90” with respect to (a). The voltage applied between the anode and cathode was 2500 V.

relating to IMS. However, some related work in which neutral molecules have been desorbed using a laser and subsequently ionized using a Ni beta source has been reported by Kolaitis and Lubman [6]. Here, we describe experiments that we have carried out on the combined laser desorption and ionization of substances introduced in the solid form into an ion mobility spectrometer.

EXPERIMENTAL

The equipment used in this work was very similar to that already described in a recent report on the use of laser ionization in IMS [5]. The essential difference was in the design of the ion mobility spectrometer ionization region, which has been modified so as to allow the laser beam to fall obliquely onto an aluminium plug that is fitted into the anode plate (Fig. 1). The distance between this plug and the detector disc was 143mm and the voltage applied across the drift region was 2500 V. In all experiments, the temperature of the drift tube was between 22 and 25°C. A current of dry nitrogen (100 ml min-‘) entered the drift tube through eight holes in the cathode plate.

Usually, the sample to be studied was dissolved in an appropriate solvent

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and a drop of the solution was put onto the face of the plug and the solvent allowed to evaporate. This procedure generally gave uneven polycrystalline deposits. The plug was then inserted into the anode plate and irradiated with the focussed beam from the laser. The laser wavelength used was in the region of 280 nm and the pulse energy was typically below 300 ,uJ in 5 ns pulses at a repetition rate of 10 Hz. The area of sample irradiated could not be measured accurately but it was approximately 2 mm*. Prior to use, plugs were cleaned either by abrading the surface on silicon carbide paper or by polishing with a tine grade diamond paste followed by extensive ultrasonication in isopro- panol. Before applying the sample, the cleaned plug was inserted into the ion mobility spectrometer and irradiated with the laser beam in an attempt to detect ions resulting from surface contamination. The term contamination is used loosely here. It denotes the presence of a weak but persistent ion peak that was observed when a cleaned plug was irradiated at relatively high laser energies. The drift time of this peak was always in the region of 40 ms and this time did not change significantly when the aluminium plug was changed for one made of brass or for another aluminium plug that had been coated with gold. Whatever its origin, this peak did not give rise to problems when a sample was present because, at the energies generally used, it would be very small or unobservable. It was found that plugs with a rough surface were much more likely to give rise to the contamination peak and, for this reason, they were not used. In the experiments described below, the plugs had a smooth mirror-like surface. For this type of plug, large numbers of positive ions were not detected from the cleaned surface until the irradiation energy density was in the region of 20 mJ cm-* (400 PJ pulse energy). The smoothness of the substrate did not appear to have any significant effect upon the ease with which ions were produced.

The substances that have been studied are anthracene, 2,3-benzanthracene, pentacene, indole-3-acetic acid, and tryptamine (3-(2-aminoethyl)indole). These were used as supplied without further purification. The first three compounds are polyaromatic hydrocarbons which together with naphthalene that was studied earlier [5], constitute successive members of a linear poly- aromatic ring system. The intention here was to investigate how the ion mobilities correlate with the dimensions of the ions concerned. Indole acetic acid and tryptamine were chosen because they have been studied using laser ionization time-of-flight mass spectrometry [7,8] and are known to undergo resonant two photon ionization at wavelengths in the region of 280nm.

Anthracene was sublimed in air onto the surface of the sample plug. Toluene was used as a solvent for benzanthracene, tryptamine was dissolved in methanol or dichloromethane and indole acetic acid in acetone.

Following pre-amplification at lO*VA-‘, all traces were recorded on a digitizing oscilloscope which was interfaced to a personal computer. Drift

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0 ms 100 ms

Fig. 2. (a) The ion mobility spectrum of anthracene desorbed at a laser pulse energy of 3Ofl at 280 nm. (b) The spectrum of anthracene desorbed at a pulse energy of 150 fl.

times could be measured to a resolution of & 0.2 ms (a 1OOms trace was digitized at 500 points) which represents about f 0.4% for the spectra dis- cussed here. The spread in values of the reduced mobility for the same ion peak observed under similar conditions on different days indicated a measurement accuracy for this quantity of within f 2%.

RESULTS

The ion mobility spectra obtained are shown in Fig. 2-5. The scaling factors written by the side of each trace indicate the extent of vertical mag- nification provided by the computer program. A factor of x 1 corresponds to a vertical sensitivity of 5 mV per division on the oscilloscope and a factor of x 10 would represent an effective sensitivity of OSmV per division. Ion mobility spectra of anthracene desorbed at different laser energies are given in Fig. 2. The low energy result shows a single peak with the suggestion of a shoulder on the higher mobility side. A peak with a similar shape has been observed in the low energy laser ionization spectrum of naphthalene [5]. The value of the reduced mobility derived from the anthracene peak is 1.69 cm2 V-’ s-’ . This compares with a value of 1.87 cm2 V-’ s-’ obtained for naphthalene [5]. The trace obtained at higher laser energy again shows the main peak in the same position but there is also evidence for the desorption of some lower mobility species and, significantly, there is no indication of increased fragmentation. This has been found to be a feature of all our results. The effect of increasing the laser energy is to enhance the production of larger, less mobile ions rather than to fragment the parent molecule. In no instance

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0 ma 100 ms

Fig. 3. (a) The spectrum obtained when a cleaned aluminium plug was irradiated at an energy of 17OpJ. (b) The spectrum obtained when a plug coated with benzanthracene was irradiated at an energy of 190 pJ. (c) The ion mobility spectrum from a benzanthracene sample irradiated at a pulse energy of 35OpJ.

have we observed the emergence of higher mobility peaks, which would indicate fragmentation, as the laser pulse energy was increased. It has been noted in accounts of laser ionization studies of molecules in the gas phase by Lubman and co-workers [9,10] that ions arising from fragmentation of an aromatic ring appear to be much less common at atmospheric pressure than under vacuum conditions. The lower mobility species observed could arise from the desorption of ionized clusters or they might arise from the presence of some higher mass impurity in the sample. An increase in the tendency for cluster ions to form as the irradiance increases has been noted by Hillenkamp [l l] as one of the general features of laser desorption under vacuum con- ditions. In our experiments, the evidence for cluster formation is more con- vincing in the results for indole acetic acid and tryptamine described below than in the case discussed here.

Ion mobility spectra of benzanthracene are given in Fig. 3. Higher laser energies were required to desorb and ionize benzanthracene than were necess- ary in the case of anthracene. The lower energy spectrum (Fig. 3b) consists of two definite peaks with shoulders on the left sides. The left peak corresponds to a mobility higher than that found for anthracene and, on this basis, we can be confident that it does not represent benzanthracene. Furthermore, this peak coincides with the peak that is observed when a cleaned aluminium plug is irradiated at relatively high energies (lower trace in Fig. 3). As the energy is increased, the peak to the right grows in relative size and the shoulder disappears. We consider that the peak on the right does represent benzan- thracene ions and the corresponding reduced mobility is 1.47 cm2 V-’ s-’ .

Pentacene was obtained in the form of a fine powder which could be

0 ms 100 ms

Fig. 4. (a) The ion mobility spectrum of indole acetic acid at an irradiation pulse energy of 20 fl at 280nm. (b) and (c) Spectra of the same sample as in (a) but irradiated at 5OpJ and 160~5 respectively.

suspended in benzene and applied to the surface of the sample plug. We do not give mobility spectra for this compound as they are essentially similar to those obtained for anthracene and benzanthracene except that the main peak appeared at longer drift times, as expected, and there was evidence of some lower mass contamination of the sample. For a given laser energy, the ion yield from pentacene was considerably higher than from benzanthracene. The value of the mobility derived from the main peak was 1.32 cm* V-’ SC’.

The evidence for cluster ion desorption is strongest in the results obtained for indole acetic acid shown in Fig. 4. At low laser energies, a single ion peak was observed (reduced mobility 1.71 cm*V-’ s-l). This peak is assumed to result from the desorption of unfragmented indole acetic acid ions. An interesting feature of this result is that this single peak could be easily observed with pulse energies as low as 20 ,MJ. As the laser energy was increased, broad peaks corresponding to ions of lower mobility began to emerge. These peaks appear to form a regular series and are probably due to the desorption of clusters containing different numbers of indole acetic acid molecules. As mentioned earlier, indole acetic acid is known to absorb in the region of the laser wavelength used [7,8] but our results show no evidence for fragmen- tation. In an attempt to determine whether there is any wavelength depen- dence in the desorption process, the laser wavelength was varied between 277 nm and 289 nm at fixed laser output energy. Over this wavelength range, no variation was observed in the results.

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0 ms 100 ms

Fig. 5. The spectra obtained from a sample of tryptamine desorbed at pulse energies of: (a) 4Ofl; (b) 60~J; (c) 120~5; (d) 13Od; (e) 2OOd.

In Fig. 5 the ion mobility spectra for tryptamine desorbed at five laser energies are given. The sample was applied to the aluminium plug in the form of a solution made up in methanol and the solvent allowed to evaporate. In contrast to the spectra of the other compounds, at least two peaks at lower drift times were observed over most of the energy range. The two higher mobility peaks were readily observable at low laser energies. At these energies, the contamination peak described above is not observable and this cannot be invoked to explain why we observe two peaks rather than one. As the laser energy was increased, a broad third peak appeared at longer drift times and began to dominate the spectrum. At the highest energy used, the two original peaks were no longer evident and only the broad single peak remained. This broad peak shows some of the behaviour observed in the spectra of indole acetic acid, in particular its dominance at higher laser pulse energies, and, as in that case, it is attributed to the desorption of relatively low mobility cluster ions. The explanation of the two peaks that appear at lower drift times is more problematic. Tryptamine and related compounds with alkylamine side-chains are known to readily undergo photodissociation at the carbon-carbon bond in the side-chain [12]. It is, therefore, tempting to assign these two peaks to the

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molecular ion and a more mobile fragment ion. However, such an assignment is not consistent with the derived mobilities of 1.75 and 1.63 cm2 V-’ SK’ for these two peaks. On the basis of molecular structure, it might be expected that the mobility of tryptamine would be somewhat lower than that of indole acetic acid, but not as low as 1.63 which would make it less mobile than anthracene. It is possible that the lower mobility peak could arise from the desorption of tryptamine ions with residual solvent molecules attached. To check this possibility we have obtained spectra from samples prepared using both methanol and dichloromethane as solvents. In both cases, the spectra obtained were essentially the same. We have also prepared samples by com- pacting solid tryptamine onto the surface of the aluminium plug and by applying tryptamine in the form of a paste made up in hexane. Again, apart from minor differences in the relative heights of peaks, the spectra obtained were very similar to those shown in Fig. 5. Another possibility is that one of the peaks arises from contamination of the sample. According to information provided by the supplier (Aldrich), the purity of the sample was greater than 98%. TLC on silica plates did reveal the presence of one major component and a trace of another. The silica layer containing the major component was scraped from the TLC plate and the component removed by dissolving in methanol. This solution was then applied to the sample plug, the solvent was allowed to evaporate, and a laser desorption mobility spectrum was recorded. The trace obtained showed low amplitude peaks but, as in all other cases, the same two peaks were observed at low drift times suggesting that these are a feature of the pure compound. On the basis of our observations, the most likely origin of the two higher mobility peaks is the photodissociation of the tryptamine molecule, but this conclusion is subject to the reservations out- lined above relating to the mobilities of the species involved. Some support for this interpretation has been provided by a spectrum of Melatonin (N-acetyl-5- methoxytryptamine) that we have obtained. Melatonin has a similar side- chain to that in tryptamine and it is known to exhibit photodissociation [8]. The spectra observed for Melatonin were poorer, and contained broader peaks, than those shown in Fig. 5 but they also showed evidence of the presence of two higher mobility components.

DISCUSSION

The mobilities of naphthalene [5], anthracene, benzanthracene and penta- cene are shown plotted against the molecular weights of these compounds in Fig. 6. Within experimental error, the relationship is seen to be linear over this range. The apparent linearity of this relationship is a consequence of the limited range of masses involved and, clearly, it could not apply over a more extended range. Also, these results cannot be taken as evidence that there is

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Fig. 6. The variation of reduced mobility with molecular mass for naphthalene [5], anthracene, benzanthracene and pentacene.

a simple and general correlation between molecular mass and mobility for ions of this size. Had we plotted mobility against the maximum dimension of the molecules concerned, we would again have found a linear relationship. It is probable that, for ions that are’large relative to the carrier gas molecules, the structure and dimensions of the ion are a more fundamental determinant of mobility than is the mass.

Unambiguous identification of the species involved would require mass analysis. This would clearly be advantageous in the case of tryptamine and it would allow us to be more specific about the nature of the low mobility species that we describe as clusters. Unfortunately, our system is not equipped with facilities that would allow mass analysis and our conclusions must, of necess- ity, involve an element of speculation. The results that we have obtained do, however, allow us to draw some quite firm conclusions regarding the general features of laser desorption under atmospheric pressure conditions. Perhaps the most significant is the apparent absence of fragmentation of aromatic ring systems at power densities up to and beyond 5 x 10’ W cmd2. This was found to be the case even with indole acetic acid which can be resonantly ionized at the laser wavelength. A similar observation has been made by Lubman and co-workers [9,10] in a study of the non-resonant laser ionization of volatile molecules in the gas phase at atmospheric pressure. In our work, higher laser

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energies were not used as they would have resulted in the removal of the sample from the substrate in a relatively short time but it seems likely that, for many compounds, higher energies could be used without fragmentation occurring. It has been suggested [IO] that this resistance to fragmentation is a consequence of rapid relaxation of excited ionic states by collisions with gas molecules, and this seems reasonable. The tendency for laser desorbed ions not to fragment under atmospheric pressure conditions could possibly find application as a source of intact desorbed ions in mass spectrometry.

The use of IMS as a means of identifying desorbed ions on the basis of their mobilities alone would probably be limited by the poor resolution exhibited by this technique. However, the spectra do show other features, such as the ease with which clusters are formed as the laser energy is increased, and these features could be used to characterise samples containing a single component. Certainly, with the materials that we have investigated so far, we would have no difficulty in discriminating between them on the basis of their mobility spectra alone provided that the samples contained only a single component. Mixtures of several components could possibly be analysed qualitatively if the components had significantly different mobilities or if some pre-separation technique such as TLC were to be employed together with laser desorption IMS. We have found that the amount of sample that can be extracted from a TLC plate is adequate to give rise to a reasonable spectrum. The limited resolution afforded by IMS is one of its more serious drawbacks. However, the resolution exhibited by our simple drift tube design is less than optimal and improvements in this area could almost certainly be made.

ACKNOWLEDGEMENTS

We thank the Science and Engineering Research Council and the University of Salford Research Committee for the provision of facilities used in this work.

REFERENCES

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10 J. Zhu, D. Lustig, I. Sofer and D.M. Lubman, Anal. Chem., 62 (1990) 2225. 11 F. Hillenkamp, in A. Benninghoven (Ed.), Ion Formation from Organic Solids, Springer

Series in Chemical Physics, Springer-Verlag, Berlin, 1983. 12 R. Tembreull and D.M. Lubman, Anal. Chem., 59 (1987) 1082.