GB2324406A - Generating ammonium ions for PER mass spectrometry - Google Patents

Abstract

Method for obtaining an ion flow substantially composed of NH 4 <SP>+</SP> ions from a mixture of different ionisation products produced by ionisation of ammonia, is characterised in that the ionisation products are left in a chamber in which ammonia is present at a pressure above 0.01 torr, until the ionisation products which are initially other than NH 4 <SP>+</SP> are converted by following reactions into NH 4 <SP>+</SP> ions. An electric field prevents formation of cluster ions. The NH 4 <SP>+</SP> ions are then used, perhaps with H 3 O<SP>+</SP>, in proton exchange reaction mass spectrometry to analyse alcohols in inhaled air, hydrocarbons in vehicle exhausts or waste industrial gases.

Description

2324405 1 METHOD OF PERFORMING MASS SPECTROMETRY Conventional gas analysis

by mass spectrometry, as has been used for decades, is based on the gas components to be detected being ionised by impact with high- energy electrons and subsequently being detected in a mass spectrometer. During the ionising impacts of electrons with molecules M, in most cases not only is the mother ion M' produced, but fragmentations also form. In particular with the ionisation of hydrocarbons and hydrocarbon-like molecules, a great many fragmentations form (for example with the ionisation of benzol, there are 16 fragmentations), so with the presence of several hydrocarbons, the quantitative classification of product ions and neutral gas components is difficult, if not virtually impossible, and therefore, in practice, gas analysis by mass spectrometry based on ionisation by electron impact is in any case limited to where only a few neutral gas components are present, each of which, moreover, fragments only slightly.

An improvement to gas analysis by MS could be made in that the ionisation of the gas components to be detected occurs not by electron impact, but instead by means of charge exchanging processes between ions already selected by mass and the neutral gas components to be examined. With such methods of analysis, often described as DAR-MS (ion-molecule reaction mass spectrometry), so called primary ions such as, for example, KrI or Xel ions, are introduced into a reaction chamber, where they collide with the gas to be analysed (Ref. 1: W. Lindinger, J. Hirber, H. Paretzke, International Journal of Mass Spectrometry and Ion Processes, 129 (1993) 79). Here, a small fraction of the primary ions reacts with the gas components present, wherein ideally one type of product ion is produced per gas component, so a simple classification of product ions and gas components present is possible. With this manner of ionisation, fragmentation is indeed greatly reduced, but, particularly with hydrocarbons, it often cannot be entirely avoided. The reason for this is that the difference E between the recombination energy of the primary ion and the ionisation energy of the molecules to be detected leads to dissociation of the ion products in the manner described by the Quasi-Equilibrium Theory (QET). Such 2 a dissociation most often takes place when E is greater than one to two times eV.

A further reduction in fragmentation is achieved by the use of so-called proton exchange reactions. In proton exchanges, the ionisation of a neutral component M takes place by transferring a proton from a proton donor XH' to the neutral gas component M to be detected, wherein the following reaction takes place:

XH+ + M --- MH + + X.

Such a proton exchange occurs when the proton affinity PA(X) of the component X, is less than the proton affinity of M, that is to say when PA(X) < PA(M). Such proton exchange reactions are very fast and are therefore efficient in converting neutrals M into MH' ions. The H30+ ion has proved particularly suitable for ionisation without fragmentation of hydrocarbons and hydrocarbon-like components via proton exchange (Ref.2: A. Lagg, H. Taucher, A. Hansel, W. Lindinger, International Journal of Mass Spectrometry and Ion Processes, 134(1994)55). H20 has a proton affinity of 7.2 eV, a value which is only just lower (by 0.2 to 1.3 eV) than the PA of many hydrocarbon components. A PER-MS method (proton exchange reaction mass spectrometry), in which H30+ ions pre-selected using a mass spectrometer are introduced into a reaction chamber into which inhaled air is also introduced, has recently been successfully developed for examinin trace components of inhaled air, such as methanol, ethanol, acetone and formaldehyde (see Ref'.2). With low energy impacts, these components each take up a proton from H30'without fragmentation. On the other hand, the principle components of inhaled air, such as N., 02, H20 and C02, do not react with H30so the PER-MS method is highly suitable for quantitative analysis of trace components in inhaled air. This method is also useful for analysing a large number of unburned hydrocarbons in the exhaust gases of automotive vehicles. Nevertheless, a disadvantage of the PERNS method described in Ref. 2, is that the apparatus used for it is still relatively expensive, as in addition to the quadrupole mass spectrometer, which detects the product ions, a further MS is necessary ahead of the 3 reaction chamber, for selecting the H30+ ions from the large number of ions in a conventional ion source. H30+ ions are produced by secondary reactions in an electron impact ion source in which H20 under sufficiently high pressure is found (typically 10-3 to approximately 10-2 torr). In the initial electron impact between 5 electrons and H20, the ions H20+, OH+, 0+ and so forth are produced. Subsequently, some of the H20+ ions react with H20 to give H30+. The H30+ ion type is now selected from the large nuinber of ions now present in the source by mass spectrometry and is introduced into the reaction chamber by means of a lens system.

DE-A 195 49 144 describes a way of simplifying and modifying this H30+ ion source system such that the mass spectrometer which filters out the H30+ ions from the various source ions is superfluous. This is done in that all the ions formed initially are converted into H30+ ions by reaction. A three-part arrangement is used for this. In part A, H20 molecules are ionised, whether by impact with electrons in a conventional electron impact ion source or in an electrical discharge (for example, hollow-cathode discharge), or by a particles which are emitted by an et emitter. In all these cases, the ions W, OW, H+ and H2+ are produced in addition to H20+ ions, as described hereinabove. These ions are drawn by means of a weak electric field into an area B, in which H20 at a pressure of 0.01 to approximately

0. 1 torr is found (the pressure is similar or equal to that in the ionising region A).

In area B the reactions 0+ + H20 - OW + OH OH+ + H20 H20+ + OH H20+ + H20 H30+ + HO H+ + H20 - H20+ + H H2+ + H20 - H20+ + H2 take place on the one hand, so that with sufficiently high pressure of H20, all the primary ions ar e converted into H30+ ions, but at the same time, at the high H20 pressures necessary, so-called H30+.(H20), cluster ions are produced by means of 4 association reactions of the type H30+ + 2H20---H30+, H20+ H20 H30+.H20+ 21120-H30+. (H20)2 + H20.

This cluster ion formation prevents or reverses the process. It is also possible to apply a precisely regulated electric field to area C, which can also partially or completely overlap area B, so that a ratio of E/N (E = electric field, N = neutral gas density) of approximately 50 to 150 Td is produced (1 Td = 1 Townsend = 1017 V. CM2). With such E/N values, ions of tile H30+(H201, type gain sufficient kinetic energy between two successive hnpacts with neutral hnpact partners that these impacts are mainly dissociative, that is to say the following dissociative process mainly prevails compared to the association reactions 1130+.(H20)n+M H30+(H20)n-1 + H20 + M where M = H20, Ar, Kr, N2 In this way the formation of 1130+.(H20)n ions is largely reversed, or their formation is prevented from the outset. Nothing changes in the course of the reactions, as the reaction rates of these reactions are not energy dependent. In order to iniprove these dissociation reactions, an additional gas such as Ar, Kr or N2 can be mixed with the H20.

In this way, in the arrangement described, all the ions formed initially are converted into H30+ ions and these cannot quantitatively become H30+. (H20). cluster ions. The ions exiting C through an aperture are composed, in this arrangement, of up to more d= 99% H30+ions, without filtering by mass spectrometry being necessary.

Because of this "reactive filtering", the use of a mass spectrometer for pre-selecting the H30+ ions before their introduction into the reaction chamber, where they will react with the gas components to be analysed, is superfluous. With this, the construction of smaller and cheaper, and also easily transportable gas analysis apparatuses based on the PER-MS method is made possible without having to impose limitations in other areas. Such transportable gas analysis apparatuses are particularly useful for on-line analysis of inhaled air, but also for the analysis of exhaust gases of automotive vehicles and waste gases from industrial installations.

A concrete technical verification of the invention set out hereinabove is described in Ref. 3: A. Jordan, A. Hansel, C. Warnecke, R. Holzinger, P. Prazeller, W. Vogel, W. Lindinger, Ber.nat.-med. Verein Innsbruck, Vol. 84, 7-17, Innsbruck, Oct. 1997; Ref. 4: J. Taucher, A. Hansel, A. Jordan, W. Lindinger, Journal of Agricultural and Food Chemistry, Vol. 44, 12, 3778-3782; Ref. 5: A. Hansel, A. Jordan, R. Holzinger, P. Prazeller, W. Vogel, W. Lindinger, Mass Spectrometry and Ion Processes, 149/150 (1995) 609-619; Ref. 6: C. Warnecke, J. Kuczynski, A. Hansel, A. Jordan, W. Vogel, W. Lindinger, Mass Spectrometry and Ion Processes, 154 (1996) 61-70. These works relate to applications of the PER-MS method in the fields of medical, environmental and food research, wherein VOCs (volatile organic components) could be registered on-line in concentrations down as far as the ppt range.

In particular with the presence of a very large number of VOCs, it can happen that product ions from several of these components have the same mass and therefore cannot easily be identified. There is indeed a range of measuring technologies for dealing with this such as, for example, the observation of isotopic ratios, the decomposition of ions by increasing the impact energy in the drift field, or the observation of the different mobilities of isomeric or isobaric ions. These measures often, though not always, achieve a result.

The object of this invention is to provide additional possibilities for identification and quantifying by the use of a second type of primary ions. As described hereinabove, using H30+ ions, all gas components with a proton affinity greater than that of the water molecules, that is to say greater than 166.5 kcal/mol, can be turned by proton exchange into a state in which they are ionised and thereby detectable by mass spectrometry. Proton affinity is therefore a characteristic which can be used for identifying VOCs. If, for example as often occurs in practice with food research, furfurylthiol and pyrazine are present together in a sample, both 6 components (mass 80) contribute to the ion signal at mass 81 (the respective neutral molecule plus proton) in accordance with their density. Pyrazine has a proton affinity of 209 kcallmol, and furfurylthiol one of 181.9 kcallmol. While both of these neutral components undergo proton exchange by means Of H30+ ions, when a primary ion is used, which occurs by protonation of a neutral molecule which has a proton affinity between 182 and 209 kcal/mol, that is to say of approximately 200 kcallmol, proton exchange takes place only with pyrazine and not with ffirfurylthiol, that is to say at mass 81 only the signal originating from pyrazine would be carried, and the intensity at mass 81 would consequently correspond to the concentration of pyrazine. Although there is a range of such molecules, with suitable proton affmities such as, for example, ethanol (pa 188.3 kcal/mol) or acetone (pa 196.7 kcallmol), these neutral components cannot be used in the same sense as water molecules for obtaining a single type of primary ions in a system without prior separation according to mass. If ethanol or acetone is ionised by electron impact or by alpha radiation, a large number of primary ions are produced which in following reactions with the respective mother gas again produce a large number of further secondary ions, so as a result, a complex spectrum of ions leaves the ion source, which is exactly the opposite to what happens with water molecules, where only protonated water molecules leave the ion source as the single ion type. Even when using a simple molecule such as methane as the source gas, a large number of final ions occur, which also exclude the use of methane for this purpose in a system without prior separation by mass. It is therefore no trivial matter to find a further component in addition to water molecules which, when used as a source gas delivers only a single type of ions which leave the source to subsequently act as reaction ions. One of these clearly very rare components is ammonia, which has a proton affinity of 204 kcallmol, and the initially formed ions of which only produce a single type of ions in subsequent reactions, in a manner similar to water molecules. The following ions are initially produced from NI43: NH3+. NH2+ , NH +, N +. In further reactions with undissociated ammonia, all these ions are finally converted into NH4+ according to the following schema: NH3+ + NH3 - NH4+ + NH2 7 NH2+ + NH3 M3+ __N113+ "2+ + NH3 I'H3+ NH4+ NH+ + NH3 M+ NH4+ N+ + M __ NH+ M+ Thus, in one embodiment the present invention relates to a method for obtaining an ion flow substantially composed of NH4+ions from a mixture of different ionisation products produced by ionisation of ammonia, characterised in that the ionisation products are left in a chamber in which ammonia is present at a pressure above 0.01 torr, until the ionisation products which are initially other than NH4+ are converted by following reactions into NH4+ ions.

In this embodiment it is preferable that to prevent the formation of cluster ions, the ion flow is guided through an electric field, the field strength of which, divided by the number of neutral gas molecules per volume unit is between 50 and 150 Townsend (Td = 10" V.cm).

A furdier embodiment, the invention relates to the use of an arrangement with an evacuatable container which contains an ionising apparatus and which is provided with an outlet aperture for the ion flow produced, wherein between the ionisation apparatus and the outlet aperture there is provided an electric field for obtaining NH4+ ions from ammonia.

In the further embodiment it is preferable that the electric field has a field strength of at least several V1cm at least in area (C) in the proximity of the outlet aperture.

It is also preferable that the H30+ ions from water vapour and N114+ ions from ammonia.

The invention also relates to a method of performing PER-MS, wherein the 30 ionisation gas comprises NH,' ions. Optionally the ionisation gas additionally contains H30+ ions.

8 The arrangement described in Refs. 3 - 6 and DE-A 195 49 144 is consequently also highly suitable for operating with ammonia as the primary gas. If the NH4+ ion is used in this way as the reaction ion, all the components in a gas mixture which have a proton affinity greater than 204 kcal/mol can consequently be detected and quantified. The use alternately of water vapour and ammonia in the ion source consequently offers an additional possibility for identifying molecules. As all the components which have a proton affmity of less than 204 kcal/mol make no contribution to the ion intensities, the whole spectrum of product ions is usually greatly simplified and in many cases observations of isotopic abundance can be used for the remaining components (with a PA greater than 204 kcal/mol) when the subsequent higher masses of the ion masses of the main product are not occupied by further products.

The Figure shows an apparatus known from DE-A 195 49 144 for implementing the method. The apparatus is composed of an evacuatable container 1, into which.qmmonia can be introduced. The purpose of the apparatus is to transform ionisation products produced from N113 in area A such that an ion flow which is principally composed of NH4+ ions is discharged from the aperture 2. A heated cathode 3 is used for ionisation in the example shown, from which cathode electrons migrate towards the anode 4 at a speed such that water vapour present in area A is highly ionised. The ionisation products do not go directly from the ionisation area A to the aperture 2, but instead are guided there by means of an electric field which is produced by a pulling tube or a row of apertures 5. When the density of neutral gas is approximately 1014 particles/cm, the free path length of the ions is in the mm range, and a path length of several cm easily ensures that all the ions are converted into NI-14+ in subsequent reactions. A minimum electric field strength dependent upon the particle density is necessary so that the formation of cluster ions is prevented or reversed. This field strength must prevail at least in area C in the proximity of the aperture 2. The field strength in area B can, however, also be lower if required.

9

Claims (7)

2. 2.

   Method for obtaining an ion flow substantially composed of NIR41 ions from a mixture of different ionisation products produced by ionisation of ammonia, characterised in that the ionisation products are left in a chamber in which ammonia is present at a pressure above 0.01 torr, until the ionisation products which are initially other than NIR4' are converted by following reactions into NH41 ions.

   Method according to claim 1, characterised in that to prevent the formation of cluster ions, the ion flow is guided through an electric field, the field strength of which, divided by the number of neutral gas molecules per volume unit is between 50 and 150 Townsend (Td = 10-17 V. cm2).
3. 3.
4. 4.
5. 5.
6. 6.
7. 7.

   Use of an arrangement with an evacuatable container which contains an ionising apparatus and which is provided with an outlet aperture for the ion flow produced, wherein between the ionisation apparatus and the outlet aperture there is provided an electric field for obtaining N144' ions from ammonia.

   Use of an arrangement according to claim 3, with the proviso that the electric field has a field strength of at least several V1cm at least in area (C) in the proximity of the outlet aperture.

   Use of an arrangement according to claim 3 or 4 for selectively obtaining H30+ ions from water vapour and NH4' ions from ammonia.

   A method of performing PER-MS, wherein the ionisation gas comprises M+ ions.

   A method according to claim 6, wherein the ionisation gas additionally contains H30+ ions.