Reactions of H3O+ and OH− ions with some organic molecules; applications to trace gas analysis in air

ELSEVIER

International Journal of Mass Spectrometry and Ion Processes 145 (1995) 177 186

and Ion Processes

Reactions of H3 O+ and OH- ions with some organic molecules; applications to trace gas analysis in air P a t r i k Span~l 1, M a r t i n Pavlik 2, D a v i d S m i t h *'1 Institut fi~r Ionenphysik, Universitiit Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria

Received 16 February 1995; accepted 16 March 1995

Abstract

The reactions of H3 O+ and OH ions with several hydrocarbons and several oxygen-bearing organic molecules, MH, have been studied using a selected-ion flow tube (SIFT) and a flowing afterglow (FA) apparatus at 300 K. Most of the reactions are fast, proceeding via proton transfer producing a single product, viz. MH~- for the H3 O+ reactions and M for the O H - reactions, but in seven of the twenty-eight reactions two products are observed. In the reaction of H3 O+ with trans-2-hexenal only one product is observed whereas in its reaction with the cis-3-hexenal isomer two products are observed. The significance of these observations and the value of the data obtained in this study to trace gas analysis in atmospheric air and on human breath using H3 O+ and OH- as chemical ionisation agents is discussed. A by-product of these studies is an estimation of the proton affinity of methylcyclohexane. Keywords." FALP; Ion/molecule reactions; Proton transfer; SIFT; Trace gas analysis

1. Introduction

Advances in mass spectrometric techniques during the last few decades have been spectacular, with the development of various forms of electric and magnetic sector devices, quadrupole mass filters, time-of-flight instruments, and a variety of ion traps such as the * Corresponding author. 1 Present address: Department of BiomedicalEngineering and Medical Physics, Postgraduate Medical School, Hospital Centre, University of Keele, Thornburrow Drive, Hartshill, Stoke-on-Trent, ST4 7QB, UK. 2 Permanent address: Department of Plasma Physics, Faculty of Mathematics and Physics, Comenius University, Mlynska dolina F-2, 842 15, Bratislava, Slovak Republic.

ion cyclotron resonance spectrometer and its Fourier variant with its enormous mass resolution. In parallel with these, there have been developments of new forms of ion sources to provide greater ionisation efficiency, the selective ionisation of particular species, and "soft ionisation" sources to minimise the fragmentation of molecules that can occur under electron bombardment. The ingenious methods are numerous, the literature on the subject seemingly endless. Concerning the published literature, we can do no better than refer the reader to two recent special issues of the International Journal of Mass Spectrometry and Ion Processes [1,2] which contain many excellent

0168-1176/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0168-1176(95)04164-8

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reviews of the recent developments in mass spectrometric techniques and ion source developments, together with numerous references. As well as the most important application of mass spectrometry, that of analysis, it has been vital in ion chemistry research. Our contribution to this has been in the conception and development of the selected-ion flow tube (SIFT) technique and its exploitation to study a wide range of ion/neutral reactions at thermal energies [3-6]. The SIFT technique followed the other eminently productive fast flow tube technique, the flowing afterglow (FA), conceived and developed by Ferguson et al. [7] for the study of ion/neutral reactions. Subsequent to this, we developed the flowing afterglow/Langmuir probe (FALP) technique to study electron/ion [8] and ion/ion [9] recombination and electron attachment [10], thus greatly extending the use of fast flow tube techniques for the study of gas phase reaction processes in the thermal energy regime. Until quite recently, the SIFT and FA techniques have not been applied per se to atmospheric gas analysis, but rather for fundamental studies. However, these techniques have obvious potential in this analytical area in that ionic reactions of selected precursor ions can be used to "soft ionise" certain atmospheric trace gases to the exclusion of the majority gases (N2, O2, H20, CO2, Ar, etc.). Using the SIFT method, different reaction processes can be exploited to ionise the trace gases including proton transfer from ions such as H3 O+ or to ions such as O H - (neither of which react rapidly with the gases referred to above), or charge transfer reactions involving precursor ions of low recombination energy such as NO + and O~-. The significant point is that these reactions usually result in a single product and so identification of the trace neutral gas by the mass spectrometer via their product ions is relatively easy (but see the reservations mentioned later).

Following our contacts with clinicians and medical personnel over the last several years, our interest turned to the application of the SIFT and FA techniques to the detection and quantification of trace gases on human breath, and more recently to the identification and quantification of the gases associated with food flavours. Very recently, Lindinger and co-workers [11,12] has carried out analyses of human breath bag samples using a selected-ion flow drift tube (SIFDT), using proton transfer from H 3 0 + ions to "soft ionise" the trace gases, and has thus measured the concentrations of ethanol (after ingestion of wine) and acetone in the breath samples. These vapours are relatively simple to detect because of their high concentrations on the breath (exceeding 1 ppm). There are many other gases and vapours on the breath of healthy persons at much lower levels, including hydrocarbons, aldehydes and other organic compounds [13,14]. So our objective is to develop our SIFT and FA methods, or hybrids of these, to detect these trace gases down to the ppb regime in real time, ideally from a single breath exhalation, and ultimately to produce a portable device for use in indoor and outdoor environments, for example, and in hospital wards and in food processing plants. In this paper we concentrate on the study of proton transfer reactions involving H3 O+ and O H - ions (since these are the most obvious precursor ions for the analysis of trace gases in air) with a variety of hydrocarbons and oxygen-containing organic molecules, most of which are known to be present on breath, some of which are known atmospheric pollutants and some of which are associated with fruit and food flavours. The data base for these proton transfer reactions, whilst significant, is not large and many more such reactions need to be studied. It is known that when proton transfer from positive ions is exothermic it is invariably fast, as a considerable amount of experimental work, in particular

P. Span6l et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 177-186

the thorough work of Bohme [15], has shown (but see also the data compilation [16]). However, this is not such a firm rule for proton transfer to negative ions (including O H ions) [16], some such reactions being slow even when they are exothermic. Significant to gas analysis, also, is the fact that dissociation of the reactant molecule can occur in some of these "soft ionisation" processes, and clearly, for analytical purposes, the products as well as the rate coefficients of the reactions must be known. Hence the need to extend the data base for these reactions. A further reason for the present study is to make a contribution to the further understanding of these ion/ molecule reaction processes.

2. Experimental For analytical purposes it can be appreciated that large precursor ion signals are desirable in chemical ionisation sources. In our analytical work the SIFT and the FA are the ion sources, hence the SIFT technique was used for the study of the H 3 0 - reactions with a simple microwave cavity gas discharge as the ion source. The source gas was argon at a pressure of about 0.1 Torr with a trace of water. The mass-selected H30 + ions were injected at low energy (to prevent break-up) into helium carrier gas (pressure typically 0.5 Torr) as is usual in SIFT experiments, and ion count rates approaching 105 s-I could be obtained at the downstream mass spectrometer sampling system. The SIFT technique has been described in detail in previous papers [3,4] and so it is not necessary to give details here. It is sufficient to say that the reactant gases and vapours were introduced at controlled flow rates into the carrier gas via a mass flow controller and the decay of the primary ion (H30 +) and the appearance of the product ions of the reactions were recorded by the downstream quadrupole

179

mass spectrometer detection system. The rate coefficients and the ion product distributions were determined in the usual way [3,4]. We were not able to extract good signals of O H - ions from the discharge source, as expected, because the space charge fields in the plasma inhibit the flow of negative ions (and electrons) to the walls. We thus used our flowing afterglow (FA) apparatus to study the O H - reactions. Again, helium carrier gas was used with a trace of water passed through the upstream microwave cavity discharge that generated the afterglow plasma (see, for example Ref. [10] for a detailed description of the FA apparatus). By careful adjustment of the water flow, the majority ions in the afterglow could be adjusted to be O H - and H30 + (together with small fractions of the hydrates of these ions). Quite enormous count rates of ions can thus be observed at the downstream mass spectrometer (>> 106 s-l), far too great for the channeltron ion counter to handle, unnecessary for ion/molecule reaction studies, but very useful indeed for analytical studies! The reactant gases were introduced into the afterglow and the rate coefficients and the ion product distributions were determined as for the SIFT experiments. All these measurements were carried out at 300 K. Included in this study were the reactions with H30 + and O H - of six hydrocarbons, viz. cyclohexane, methylcyclohexane, cyclopropane, benzene, toluene and isoprene, and eight oxygen-containing organic compounds, viz. methanol, acetaldehyde, ethanol, acetone, diethyl ether, ethyl acetate, and two isomeric aldehydes, trans-2-hexenal and cis-3-hexenal.

3. Results and discussion The measured rate coefficients, k, and the ion product distributions are given in Tables 1 and 2 together with the calculated collisional

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rate coefficients, kc, according to the formulation of Su and Chesnavich [17] accounting, where necessary, for the permanent dipole moment, #D, of the reactant molecule (these #D are also given in the Tables when they are known). The results for the hydrocarbon reactions are given in Table 1 and those for the other compounds are collected in Table 2. We now consider these two groups separately.

3.1. Hydrocarbon reactions When the proton affinity (PA) of an acceptor molecule exceeds that of a donor, then the reaction between these species at thermal energies is expected to be efficient, i.e. the

measured rate coefficient, k, is expected to be equal to the collisional rate coefficient, k c [15]. PA(H20 ) is 166.5 kcal mo1-1, and since the PAs of cyclopropane, benzene, toluene and isoprene exceed this value (see Table 1) then the k for the reactions of H3 O+ with these molecules are expected to be equal to the respective k c as is indeed the case. All four reactions proceed via non-dissociative proton transfer, i.e. only a single product, the protonated molecule, is observed, consistent with some previous studies of some of these reactions [18-20], e.g. the isoprene reaction proceeds thus: H3 O+ + C5H8 --+ C5H9~ + H 2 0

Our previous study [19] has shown that the

Table 1 The rate coefficients, k, and ion products for the reactions of H3 O+ and OH SIFT and FA apparatuses a Neutral molecules

PA (kcal mol-1 )

#D (Debye)

k

H3 O+

165.3

0.7

kc

[2.2] C3H ~

(0)

1.5

[1.51 C6 H+

Benzene C6H6

(0)

1.8

[1.91 C7 H+

Toluene c-C6HsCH 3

(0.36)

2.3

(0.25)

C7H7 [2.2]

2.3

CsH ~-

Isoprene CH2=C(CH3)CH=CH2

200.4

OH

C7H[3 (95) C7H75 (5)

Cyclopropane c-C3H 6

189.8

k

[1.95]

< 0.0001

Methylcyclohexane c-C6HIlCH 3

181.3

kc

C6H/-1 (~ 50) C6H[3 (~ 50) (0)

180

with some hydrocarbons determined at 300 K using the

Reactant ions

Cyclohexane c-C6H12 160

(1)

2.0

[2.31 CsH 7 (65) C3H 3 (25) C3H 5 (10)

[1.9]

0.65

[2.0]

" To the left in each box are the measured k, and to the right are the collisional rate coefficients I% [17] in units of 10-9 cm 3 s -1 . A dash means that no reaction was observed (i.e. k < 10-12 cm 3 s-l). The percentages of the product ions are given in parentheses, except when there is a single product ion. Given below each neutral molecule is its proton affinity, PA [21] and its dipole moment #D [32]. Note that the proton affinity of methylcyclohexane has been estimated in this study as (165.3 4- 1.3) kcal mo1-1 .

P. Span(l et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 177 186

reaction of H3 O+ with cyclohexane is very

slow, yet it produces two products: H 3 O + + c - C 6 H 1 2 ---+ C 6 H i - 1 + H 2 + H 2 0

C6H+3 + H20

(2a) (2b)

The branching ratio is about 50% into each

181

value [21]. A value for the PA of methylcyclohexane is not available, but our measurements show (see Table 1) that the k for its reaction with H3O+ is less than kc, and that two

product ions are observed: H3 O+ + c-C6HllCH

channel. On the basis of these observations, together with the reactions of the c-C6H12 with several other ions, we have established that PA(c-C6Hl2) is 160 kcal mol -l, some 9 kcal mo1-1 lower than the current published

3

C7H~3 + H2 + H20 ---, C7H~-5+ H20

(95%)

(3a)

(5%)

(3b)

It is not known if the major channel (3a) is a

Table 2 The rate coefficients, k, and ion products for the reactions of H3 O+ and OH- with some oxygen-containing molecules determined at 300 K using the SIFT and FA apparatuses a Neutral molecules PA (kcal mol -j )

Reactant ions #D (Debye)

k

H3 O+

Methanol CH3OH 181.9

2.1

Acetaldehyde CH3CHO

(1.69)

3.9

[2.7]

4.1

2.8

[3.9]

Ethyl acetate CH3COOC2H 5 200.7 (1.78) trans-2-Hexenal C3H7CH=CHCHO (~ 3) cis-3-Hexenal C2H5CH=CHCH2CHO (~ 3)

2.0

4.7

0.9

C4H90 ~ (98) C2H50 ~- (2) 3.2

C6HIIO+

[3.0] C6H90-

[~ 4]

~ 4

C6 H+ (65) C6HI10 + (35) ~ 4

[2.4] C2H30 2 (60) C4H70 2 (40)

[2.9]

~ 4

[4.0] C2 H702- (60) C2H50- (40)

[2.4]

2.7

[2.8] C3H50-

Ca H 11O+ (1.15)

[3.8] C2H50

C3H7 O+ (2.88)

[2.8] C 2H 30

[3.7]

2.7

Diethyl ether C2HsOC2H5 200.2

2.3

C2H70 +

Acetone CH3COCH3

kc

CH30[2.7]

3.5

Ethanol C2HsOH

196.7

OH

C 2H50+ (2.69)

188.3

k

CH5 O+ (1.7)

186

kc

[~ 4] C6H90

[~ 4]

~ 4

[~ 4]

a To the left in each box are the measured k, and to the right are the collisional rate coefficients k~ [17] in units of 10-9 cm 3 s 1. The percentages of the product ions are given in parentheses, except when there is a single product ion. Given below each neutral molecule is its proton affinity, PA [21] and its dipole moment #D [32]. Note that the rate coefficients for the reactions of hexenals could be lower by about a factor of two.

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P. Spangl et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 177 186

proton transfer reaction followed by dissociation or a hydride ion transfer reaction. Channel (3b) is indeed a proton transfer reaction, but the k for this partial reaction is only 5% of the overall k which is 7x 10-l° cm 3 s -l. So the calculated k(3b) is 3.5 x l0 -1l cm 3 S-1. Now assuming that this proton transfer reaction would dominate the overall reaction and proceed with k = kc if it were exothermic, then the endothermicity, AE, for this reaction can be estimated according to k = k c exp(-AE/kBT), where k B is the Boltzmann constant. Thus the AE obtained is 2.4 kcal mo1-1 and therefore the estimated PA(c-C6H11CH3) = (PA(H20) - 2.5) which is 164 kcal mo1-1. However, assuming channel (3a) is dissociative proton transfer and using the same reasoning, PA(c-C6HllCH3) is indicated to be 165.7 kcal mo1-1. Assuming that the CTH~-3 is the methylcyclohexene ion, the heat of formation of which is known [21], then this reaction (3a) according to the thermochemistry is exoethermic by 4 kcal mo1-1. The uncertainty in the mechanism of this reaction could possibly be resolved using isotopic labelling of the reactants. For now, all that can be said is that the PA of methylcyclohexane is between 164 and 166.5 kcal mol-l. As can be seen from Table 1, four of these hydrocarbons do not react with O H - ions, and using the available thermochemical data [22] it is clear that there are no obvious exothermic channels in the c-C6H12 , c-C3H 6 and C6H 6 reactions. We have no information on the thermochemistry of the negative ions derived from c-C6HIlCH 3 so we cannot assess the ergicities of any possible reactions. That no reaction occurs between O H - and c-C6H11CH3, however, most probably means that there are no available exothermic channels in this reaction. The toluene reaction with O H - has been studied previously by Tanner et al. [23] using isotopic labelling. The reaction proceeds at the

gas kinetic rate generating a single ionic product: O H - + C6HsCH3 ---, C6HsCH 2 + H20

(4)

This is a proton transfer reaction, and the H + ion is extracted from the C H 3 group [23]. The results for the O H - reaction with isoprene are somewhat surprising. The measured k for the reaction is only about 0.3kc, and three product channels are observed, two of which, according to the thermochemical data [22] are apparently endothermic: O H - + C5H8 ---' C5H7 + H20 + 5kcalmo1-1

(65%)

(5a)

(25%)

(5b)

(10%)

(5c)

C3H3 + CzHsOH - 10kcalmo1-1 ---, C3H 5 -]- C 2 H 4 0

- 5kcalmol -I

Mechanistically the neutral reaction product favoured would be (C2H4 + H20) and (C2H2 + H 2 0 ) in reactions (5b) and (5c) respectively, but such reactions would be even more endothermic than those indicated. In a previous study of reaction (5) [24] only a single product C5H7 was observed with k = 0 . 6 k c . The majority proton transfer channel (5a) is apparently exothermic and if proceeding without competition would do so without inhibition. However, there are competing channels and so it is difficult to interpret these data with confidence. What can be said is that if channels (Sb) and (5c) were as endothermic as the current thermochemical data indicates, they would not be observed in these thermal energy experiments. So these channels must be at least close to thermoneutral and it seems most likely that the heats of formation of the C3H3 and C3H;- ions are in error by substantial amounts.

P. SpanH et al./ lnternational Journal of Mass Spectrometry and Ion Processes 145 (1995) 177-186

It must be said that our suprising results for this reaction (5) have been kindly confirmed by a new study in the laboratory of C.H. DePuy (personal communication, 1995). 3.2. Reactions of the oxygen-containing molecules All the molecules included in this part of the study (see Table 2) have large proton affinities and quite large dipole moments. The reactions of both H3 O+ and O H - with all of them are very fast, the measured k being close to the calculated k c in all but one of them (i.e. the O H - reaction with diethyl ether). The reactions of H3 O+ with the majority of these molecules proceed by non-dissociative proton transfer producing only the protonated molecule (thus confirming previous results for the methanol [25], acetone, ethanol and acetaldehyde reactions [20]). A minor exception is the ethyl acetate reaction for which, in accordance with a previous study [26], a minor second product appears to be formed:

----+C 4 H 9 0 f ÷ H 2

---+ C2HsOf + C2H4 + H20

(98%)

(6a)

(2%)

(6b)

The ionic product of channel (6b) is most likely to be protonated acetic acid. The most interesting results of this group of H3 O+ reactions are those of the isomeric forms of hexenal, since the trans-2-hexenal reaction proceeds only via non-dissociative proton transfer: H3 O+ + C H 3 C H 2 C H 2 C H = C H C H O ---+ C6HI1 O+ +

H20

cis-3-hexenal reaction: H30 + + CH3CH2CH=CHCH2CHO --~ C6HI1 O+ + H20

(~ 35%)

(8a)

C6H ~- + 2H20

(~ 65%)

(8b)

Channel (8a) is the non-dissociative proton transfer channel, whereas channel (8b), the major channel, involves H20 abstraction from the unsaturated aldehyde producing the linear diene ion with increased conjugation, a not uncommon reaction. The value of this to analysis is clear in that this ion chemistry provides a means of separately identifying these two isomers (and their quantification) when they are both present in a mixture. A plausible mechanism for reaction (8b) (as kindly indicated to us by the reviewer of the manuscript) is the following: H3 O+ + CH3CH2CH=CHCHzCHO -H20

)

[CH3CH2CH=CHCH2CH=O+H] 1,2 hydride shift

H3 O+ + CH3COOC2H 5

(7)

whilst there are two observed products in the

183

)

[CHsCH2CH=CHC+HCH2OH] proton shift [CH3CH=CHCH=CHCHzO+H2] -H20

)

CH3CH=CHCH=CHCHf It should also be noted that these hexenals have low vapour pressures and it was quite difficult to quantify the flow rates of the vapours into the flow tube. Thus the measured rate coefficients for these reactions (and also those of the corresponding OHreactions) have a much greater uncertainty

184

P. SpanH et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 177 186

associated with them, although we believe that they are actually gas kinetic reactions (see Tables 1 and 2). Unfortunately, the dipole moments and polarisabilities of these molecules are not available and so the kc cannot be calculated accurately (only estimates using the polarisabilities and dipole moments of similar compounds [32] are given in Table 2). The O H - reactions with methanol, acetaldehyde, ethanol and acetone (which have been studied previously [23]) proceed via non-dissociative proton transfer at the gas kinetic rate producing only one product (see Table 2). According to the thermochemical data, the acetaldehyde reaction is slightly endothermic (about 0.5 kcal mo1-1 [21]), yet we observe that k = kc for this reaction in accordance with a previous FA study [23] and so the reaction must be exothermic. This does not represent a signficant problem since a small uncertainty in the heats of formation of the product negative ion is to be expected. A previous study also showed that the proton is extracted from the CH 3 group [27]. In the reactions of O H - with both diethyl ether and ethyl acetate two major products are observed: O H - + CzH5OCzH 5 --+ [C2H50- • H20 ] + C2H 4

(9a)

--+ C2H50- 4- C2H 4 + H 2 0

(9b)

It has been argued that the ionic product of channel (9a) is the cluster ion as indicated here because of its large binding energy [28]. Reaction (10) proceeds thus: O H - + CH3COOC2H 5 ---+ CH3CO 2 q- C2H 4 + H20

(10a)

---+-CH2COOCzH 5 + H20

(10b)

The structures of the product ions have been ascertained in previous studies [29] and are as indicated.

The reactions of the two isomeric hexenals (trans-2-, cis-3-hexenal) with O H - are fast, k,-~ kc, and they both proceed by proton transfer: O H - + C6HIoO --+ C 6 H 9 0 - + U20

(11)

Clearly, therefore these O H - reactions, unlike the H30 + reactions of these to hexenals, cannot be used to distinguish between the isomers. Further studies of the reactions of the isomeric forms of some other organic molecules with O H - and H3 O+ would be instructive.

4. Concluding remarks and future directions The work reported in this paper represents both a contribution to the fundamental studies of ion/molecule reactions and an extension of the data base on the kinetics of the reactions of H 3 0 + and O H - at thermal energies. The other major objective of the study was to investigate the potential of these reactions for trace gas analysis. In this respect it can be said that both the H3 O+ and the O H - reactions do indeed have great value in analysis, particularly when combined together to analyse gas samples, as we plan. The essential point is that the reactions of both ions with a wide range of orgaic molecules, MH, generally proceed at the gas kinetic rate to produce one ion product, in the case of the H3 O+ reactions the product is MH~- and in the O H - reactions the product is M - , i.e. the positive and negative ionic products are separated in mass by two mass units, thus facilitating identification in some cases. Of course, all the reactions are not so simple as a glance at Tables 1 and 2 shows; some reactions are slower than gas kinetic (or do not proceed at all with one or both o f H 3 0 + and OH-), and for some there is more than one product. The latter point is actually very valuable in some cases as, for

P. Span6l et al./International Journal of Mass Spectrometry and Ion Processes 145 (1995) 177-186

example, in recognising two different isomers (the hexenals in this study). Obviously, H3 O+ and O H - are not the only precursor ions that can be used for trace gas analysis in air, but whatever ions are used they must not be reactive with the major air constituents. One other such ion is NO+; our preliminary studies have shown that its reactions with many gases are simple (i.e. a single ionic product results), and so we are currently involved in a detailed study of NO + reactions in order to provide the necessary data base for analytical purposes, and the results of this study will be reported in due course.

The value of these fundamental studies is already being demonstrated by our first studies of trace gases on human breath and of the vapours emitted by fruit and food products. Using H3 O+ and O H - ions in both the SIFT and the F A L P apparatuses we can easily quantify on breath, in real time from a single exhalation, acetone (typically present at a partial pressure of 1 p p m for healthy persons, but much higher for diabetics) and isoprene (apparently variable within the range 0.1 to > 1 ppm) [30]. During the course of these measurements it was observed that the large partial pressure of water vapour on warm breath resulted in the clustering of the H 3 0 + and O H - ions with H20 molecules producing cluster ions of the type H30+(H20)n and OH-(HzO)n. These cluster ions then undergo "switching reactions" with the trace gases to be detected, MH, producing ions of the type M H H + ( H 2 0 ) , and M - ( H 2 0 ) , . If the kinetics of these switching reactions are understood, then instead of the presence of these cluster ions being a nuisance they can be of great value in the analytical procedure. Thus, we have carried out a detailed study of the reactions of these water cluster ions with most of the organic molecules included in the present study [31] in order to obtain the necessary kinetic data. The results of these

185

experiments are already being applied to our trace gas analysis research [30]. Many gases are present on human breath at much lower partial pressures than acetone or isoprene [13,14] and it is our aim to be able to detect more of these in connection with our research into respiratory diseases in collaboration with clinicians. With development, our chemical ionisation/SIFT/FA techniques and their hybrids are quite capable of performing breath analysis in real time in the 1 ppb regime. The clinical value of such techniques to noninvasive screening of people is clear. Further fundamental work on ion/molecule chemistry and electron attachment (using the F A L P technique) to back up our developments of practical analytical devices is proceeding.

Acknowledgements We have greatly benefited from our discussions with Professor C.H. DePuy and Dr. J.M. Thompson. We gratefully acknowledge the Fonds zur F6rderung der wissenschaftlichen Forshung Wien, Austria, for financial support of this work. One of us (M.P.) thanks the European Physical Society for a stipend to support his stay in Innsbruck.

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[9] D. Smith, M.J. Church and T.M. Miller, J. Chem. Phys., 68 (1978) 1224. [10] D. Smith and P. Spanel, Adv. At. Mol. Opt. Phys., 32 (1994) 307.

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