EP1566829A2 - Method for the production of an output-ion-stream - Google Patents

Abstract

An output ion current of a single ionic species is produced by reacting ions formed in ionization of a source gas and/or ions extracted from ionization region, in a region in which source gas is located until source ionic species that do not react with the source gas are present. A reactant gas different from the source gas reacts with the ions of the source ionic species to convert the ions of the source ionic species into the single ionic species. The production of output ion current comprised of a single ionic species, involves reacting ions formed in ionization of a source gas in an ionization region (A) and/or ions extracted from the ionization region in a region in which source gas is located, until source ionic species are present that do not react with the source gas. A reactant gas different from the source gas is supplied to a reaction region (C) located outside of the ionization region and in which ions of the source ionic species are present. It reacts with the ions of the one or several source ionic species to convert the ions of the source ionic species into the single ionic species forming the output ion current.

Description

The invention relates to a method for obtaining a substantially only from a single type of ion existing output ion current, wherein the Ionization of a source gas in a lonisationsbereich formed ions and / or ions extracted from the ionization region in a region in which There is source gas left to react until essentially only one or more source ion types are present that are not with the source gas react.

Such a method is known for example from AT 001 637 U1. This document describes a process for obtaining an ion stream consisting essentially of H ₃ O ⁺ ions. For this purpose, water vapor is ionized in an ionization region by means of an ion source, whereby various ions are formed (O ⁺ , OH ⁺ , H ⁺ , H ₂ ⁺ , .....). These ions are extracted by means of a weak electric field into a region outside the ionization region and are left in this region, in which H ₂ O is at a pressure above 0.01 Torr, until the first of H ₃ O ⁺ ions have converted various ions by subsequent reactions in H ₃ O ⁺ ions. In this region and / or in an adjoining region, the ion current is also passed through an electric field whose field strength is sufficiently high, so that H ₃ O ⁺ . (H ₂ O) _n cluster ions formed by association reactions with neutral ions occur between two successive bursts Shock partners have acquired sufficient kinetic energy, so that these shocks are predominantly dissociative. It prevents the formation of such cluster ions or largely reversed. To improve these dissociation reactions, H ₂ O may also be admixed with an additional gas, such as Ar, Kr or N ₂ , which serves as cluster partner for the cluster ions but does not react chemically with the H ₃ O ⁺ ions.

Such an ion stream consisting essentially of H ₃ O ⁺ ions can be used in particular as a primary ion stream for the chemical ionization of a sample gas by proton exchange reactions in order to mass spectrometrically examine the ions formed in the sample gas. This proton exchange reaction mass spectrometry, PTR-MS for short, is described in AT 001 637 U1 and the references cited therein. This is a special type of ion-molecule reaction mass spectrometry (IMR-MS), which is also described in AT 001 637 U1 and the references cited therein.

AT 406 206 B further discloses a process analogous to the process known from AT 001 637 U1 for obtaining an ion stream consisting essentially of NH ₄ ⁺ ions. For this purpose, ammonia (NH ₃ ) is ionized as the source gas and the ions formed are left after extraction from the ionization area in a range with an ammonia pressure of above 0.01 Torr (1.33 Pascal) until the substantially only NH ₄ ⁺ ion has formed ion current (to prevent or reverse the formation of cluster ions again a sufficiently high electric field strength is applied to induce shocks).

It is further known from AT 403 214 B to have different ion sources Incorporate source gases and from those in the ion source of various neutral Atoms or molecules of the source gases generated by primary ion species to filter out a filter device all but a primary ion species. The remaining one Primärionenart is allowed to pass into the reaction chamber and in the reaction chamber allowed to react with a sample gas, by ion-molecule reactions (eg proton exchange reactions) resulting reaction products be examined with a mass spectrometer. Disadvantage here is the additionally required, the filter device forming mass spectrometer.

The extraction of only one type of ion existing output ion streams without such a mass spectrometric filtering, as it is known from AT 403 214 B, is possible by the methods of AT 001 637 U1 and AT 406 206 B only for a few types of ions, in particular for H ₃ O ⁺ ions, NH ₄ ⁺ ions and H ₃ ⁺ ions. Only in the case of a few source gases do output-ion streams, which essentially consist of only one single ionic species, form in the manner described in these two publications. Such source gases are described in the literature as "Cl Reagent Gases".

EP 000 865 A1 describes the investigation of a substance gas, which is ionized for this purpose by ion-molecule reactions. The Chemical ionization of the substance gas takes place here as an ionization chamber described space (which is commonly referred to as "drift tube") becomes. The ionization chamber becomes partially ionized primary gas from an ion source fed, which is designed here as a gas discharge chamber. Of the ionization chamber, a reactant gas is supplied in addition to the substance gas, which with the entering from the ion source in the ionization chamber ions and in turn ionizes the substance gas. In the ionization chamber is thus a mixture of the more or less ionized Components of the primary gas, reactant gas and bulk gas. An extraction essentially consisting of only a single ionic species Output ion current is not apparent from this document. To the exit opening The ionization chamber, both the ionized primary particles as well as the Reactant gas and substance ions out.

The object of the invention is to determine the spectrum of producible output ion currents, which essentially consist of only one single ionic species, without the need for a mass spectrometric filtering (as in the AT 403 214 B) is required. This is achieved by the invention a method having the features of patent claim 1.

By the method according to the invention, it is possible in particular to produce substantially only one single ion species existing output ion currents, which by direct addition of the reactant gas in the primary ionization due to the various species present in this area (ions, electrons, atoms, molecules, radicals , excited atoms, excited molecules) would not arise in this form. If, for example, nitrogen (N ₂ ) were added to the source gas H ₂ , then neutral NH ₃ would be formed in the plasma of the primary ionization region (cf Ref1: Fuji et al., Int. Mass Spectrom., 216, 169, 2002). H ₃ ⁺ would thus preferentially react with NH ₃ in the reaction H ₃ ⁺ + NH ₃ → NH ₄ ⁺ + H ₂ and the generation of an N ₂ H ⁺ leaving ion current (as explained below) would not be possible.

The addition of the reactant gas in one of the primary ionization space spatially Separate reaction area has the further advantage that also added gases their presence in the primary ionization region is problematic would be, for. B. NO in filament ion sources (leads to rapid filament breakage) or carbonaceous gases in plasma ion sources (leading to carbon deposits).

Preferably, suitable measures are taken so that a backflow of the reactant gas from the reaction area into the ionization area in the Essentially prevented, d. H. less than 10%, preferably less than 5% of the partial pressure in the ionization region should be from the reactant gas or from it formed products. For this purpose, for example, the lonisationsbereich and the reaction area forming spaces by one or more Partitions to be separated, wherein in a respective intermediate wall a Aperture is arranged, and by appropriate pumping means in the direction from the ionization region to the reaction region pointing gas flow be maintained by at least one of the apertures. Also Zwischenabpumpungen between the areas are conceivable and possible.

In principle, it would be conceivable and possible, at least in some applications, the ions extracted from the ionization region directly into the reaction zone respectively. However, it is preferred that extracted from the ionization area first to lead them to an intermediate area in which they stay be left until the first of the one or more Substantially different (i.e., greater than 90%, preferably greater than 95%) in ions of the one or more source ion species have converted. From this intermediate area can in the Follow ion of one or more source ion species in the reaction area be extracted in that they by adding the reactant gas in the only ionic species of the parent ionic stream substantially (i.e., greater than 90%, preferably more than 95%).

If the extraction of ions from the ionization area directly into the reaction area takes place, the reactions of the ions formed in the ionization are too Essentially, the ions of the one or more source ion species are already in the ionization occurs or these reactions are mainly or to Part in the reaction area instead. It must have source gas with sufficient Pressure (for example, more than 1 Pascal) may be present in the reaction area. Furthermore, the reactant gas should not be allowed to come in contact with ions, which are not yet in ions which have been converted to one or more source ion species. For some combinations of source gases and reactant gases, this is the case. It would be conceivable and possible, in special cases, of the primary ions and / or secondary products interfering ion types (which with the reactant gas to unwanted ion types) by adding a suitable additional gas to transform into non-interfering ions.

As a source gas, a clean gas or a gas mixture can be used. For the Reactant gas is the preferred use of a clean gas, the use of Gas mixtures would also be conceivable and possible.

Hereinafter, embodiments of the invention with reference to the accompanying Drawing explained. In this, the only Fig. Shows a highly schematic Representation of a device with which the inventive method is feasible.

The device shown schematically in the figure for carrying out the inventive Procedure has three areas. The primary ionization area A is fed by a supply 1, a source gas. In the ionization area A is an ion source not shown in detail 2 arranged. The primary ionization of the source gas takes place z. B. by electron emission from a filament, by ionizing radiation (eg α-particles), by an electric discharge or other ionization method. The choice of primary ionization method is not relevant to the subject invention.

The source gas used is a clean gas, for example hydrogen (H ₂ ), or a gas mixture, for example H ₂ argon (Ar) or nitrogen (N ₂ ) nitrous oxide (N ₂ O). Total pressure and partial pressures depend on the choice of ionization method (low pressure or high pressure ion source).

In the primary ionization region A, a multiplicity of species (ions, electrons, Atoms, molecules, radicals, excited atoms, excited molecules).

Im Fall von H₂ als Quellgas besteht der extrahierbare positive lonenstrom einfach geladener lonen aus H⁺, H₂ ⁺, H₃ ⁺ und H₃ ⁺·H₂.

Im Fall eines H₂-Ar-Gemisches besteht der extrahierbare positive lonenstrom einfach geladener lonen aus Ar⁺, Ar₂ ⁺, ArH⁺, ArH₂ ⁺, ArH₂ ⁺, H⁺, H₂ ⁺, H₃ ⁺, H₃ ⁺•H₂.

Im Fall eines N₂-N₂O-Gemisches besteht der extrahierbare negative lonenstrom aus vornehmlich O^--lonen, mit Spuren von NO^--lonen. (vgl. ref2: A.P. Bruins et al., Adv. Mass Spectrom. 7, 355, 1978)

By applying an electric field of suitable polarity, either positive or negative ions are extracted through an aperture 3 in an intermediate wall 4 into the intermediate region B. The generated ionic current is usually non-selective, ie it generally consists of different types of ions:

In the case of H ₂ as a source gas, the extractable positive ionic current of singly charged ions consists of H ⁺ , H ₂ ⁺ , H ₃ ⁺ and H ₃ ⁺ · H ₂ .

In the case of an H ₂ -Ar mixture, the extractable positive ion current of singly charged ions consists of Ar ⁺ , Ar ₂ ⁺ , ArH ⁺ , ArH ₂ ⁺ , ArH ₂ ⁺ , H ⁺ , H ₂ ⁺ , H ₃ ⁺ , H ₃ ⁺ • H ₂ .

In the case of an N ₂ -N ₂ O mixture, the extractable negative ionic current consists primarily of O ^- ions, with traces of NO ^- ions. (ref2: AP Bruins et al., Adv. Mass Spectrom., 7, 355, 1978).

The relative proportions of the exemplified, extractable ion species hang of different source parameters (total pressure of the source gas or partial pressures the various source gas components, temperature, etc.). Next Simply charged ions can - depending on the ion source and the Source gas - also multiply charged ions occur and be extracted.

The intermediate region B is fed with the source gas (total pressure> 0.01 mbar, particle gas density N _B ). The feed can be done by flowing out of the ionization region A in the intermediate region of source gas. There may also be a separate feeder, not shown in the figure. The pressure of the source gas in the intermediate region B may be similar or equal to the pressure of the source gas in the ionization region A. In the intermediate region B, an electric field of field strength E _{B is} applied by electrodes 5. The intermediate region is at a temperature T _B.

In the intermediate region B, the ions extracted from the primary ionization region A interact with the source gas. The spectrum of interactions includes binary ion-molecule reactions (eg H ₂ ⁺ + H ₂ → H ₃ ⁺ + H), ternary ion-molecule reactions (eg H ⁺ + H ₂ + H ₂ → H ₃ ⁺ + H ₂ ), collision-induced dissociation reactions (eg H ₃ ⁺ • H ₂ + H ₂ → H ₃ ⁺ + H ₂ + H ₂ ), as well as activation and deactivation reactions (eg (H ₂ ⁺ ) ^* + H ₂ → H ₂ ⁺ + H ₂ )

The parameters E _B / N _B and T _B define the reaction conditions, ie by varying these parameters it is possible to prefer or suppress certain reaction channels.

By suitable choice of the reaction conditions, the ion stream extracted from the primary ionization region A consisting of numerous ion species is converted into a selective ionic stream substantially of an ion species not reacting with the source gas or an ionic stream of essentially several ion species not reacting with the source gas. These one or more ion species which do not react with the source gas, ie they are "stable" to the source gas, are referred to in this document as "source ion species". At the exit of the intermediate region B, to which the ion current is passed through the electric field E _B , the ionic current is preferably at least 90% of the one or more source ion species, with a value of at least 95% being particularly preferred.

At the exit of the intermediate region, the proportion of ions of the source ion species could also be lower than the stated value of preferably 90% or 95%, for example if a proportion of cluster ions (eg H ₃ ⁺ .H ₂ ) is present, which is converted into dissociation reactions in ions of the one or more source ion species (plus neutral source gas) only in reaction region C described below by applying an electric field with a sufficient field strength in the reaction region C to carry out the required collision-induced dissociation reactions becomes.

The values of E _B / N _B and T _B vary depending on the application example. It would also be conceivable and possible to improve the efficiency of the dissociation reactions taking place in the intermediate region B by adding an additional gas (eg Ar, Kr or N ₂ ) to the source gas which does not react with the ions extracted in the intermediate region via ion molecule reactions but instead only serves as collision partner.

In the case of H ₂ as the source gas, a selective H ₃ ⁺ ionic current is generated. Here are ion-molecule reactions of the following nature: H ₂ ⁺ + H ₂ → H ₃ ⁺ + H H ⁺ H ₂ + H ₂ → H ₃ ⁺ + H ₂

The applied electric field also leads to dissociation reactions, which predominate over the association reactions and which largely reverse the formation of cluster ions, or prevent their formation from the outset: H ₃ ⁺ • H ₂ + H ₂ → H ⁺ + ₃ H ₂ + H ₂

In the case of an H ₂ -Ar mixture as the source gas, a selective H ₃ ⁺ ion stream is also formed (compare ref3: Praxmarer et al., J. Chem. Phys. 100 (12), 8884-8889, 1994). In other source gas mixtures of H ₂ with a pure gas X, whose proton affinity is smaller than that of H ₂ is formed as source ion type H ₃ ⁺ . If the proton affinity of component X is greater than that of H ₂ , XH ⁺ ions are formed as source ion species.

In the case of the N ₂ -N ₂ O source gas mixture, the reaction conditions are chosen so that a selective O ^- ion current is maintained (ref2).

The intermediate region B is of conventional methods and devices for Obtaining a selective ion current already known (he corresponds to the areas B and C of AT 001 637 U1 and AT 406 206 B) and is also referred to as "source drift region. "Intermediate area B could also be divided into two subareas Be split B1 and B2. In the area B1 then the source gas would be present, but the electric field strength is too small for dissociation reactions. In the following Area B2 would have a higher field strength to the dissociation reactions cause.

By applying an electric field, the resulting ions are the one or more sources of ionic species through an aperture 6 in an intermediate wall 7 extracted in the reaction area C.

In the reaction region C, an additional, different from the source gas in its chemical composition, reactive collision partner is added, which is referred to in the context of this document as a reactant gas. The reactant gas can be formed by a clean gas or a gas mixture. The total pressure in the reaction region C is more than 0.01 mbar (particle gas density N _c ). The partial pressures of source gas and reactant gas vary depending on the source and reactant gas used. The admixture of the reactant gas is effected by a feed 8 shown schematically in the figure. By means of electrodes 9, an electric field of the field strength E _{c is} applied. The reaction region C is at a temperature T _c .

By ion-molecule reactions with the reactant gas from the intermediate area B extracted ionic current - preferably consisting essentially from the one or more source ion species - into an output ion stream which, in essence, d. H. to more than 90%, preferably consists of more than 95% of a single ionic species. In practice you can Values of up to more than 99% can be achieved. Consists of the intermediate area B extracted ion stream from more than one source ion type, so the transformation into essentially only one single type of ion succeeds existing output ion current in that from the reactions of the different source ion types with the reactant gas only a single production variety evident.

The parameters E _c / N _c and T _c define the reaction conditions, ie by varying these parameters it is possible to favor certain reaction channels and suppress others to produce a selective ion source ion stream. For example, by suitable choice of the field strength of the field E _c dissociation reactions can be effected in order to reverse the formation of cluster ions or to prevent their formation from the outset. To improve the efficiency of such dissociation reactions, an additional gas could also be mixed into the reaction region C, which does not react with the ions present in the reaction region C via ion molecule reactions, but serves only as a collision partner.

By the electric field E _c , the ions are passed through the reaction region C to the output 10.

By the electrodes 5 in the intermediate region B and / or by the electrodes 9 in the reaction region C, an electrostatic potential is preferably generated. It is hereby preferred that in the intermediate region B and / or in the reaction region C, a homogeneous electric field E _B or E _{c is} generated. Due to the homogeneity of the electric field E _B or E _c , the reaction conditions can be manipulated in an advantageous manner, ie, certain reaction channels are preferred or suppressed.

By changing the reactant gas can in a simple, fast way different selective output ion currents (i.e., essentially only a single type of ion existing output ion currents) are generated, which e.g. used as primary ions for chemical ionization can. Such chemical ionization methods are used, for example, in ion-molecule reaction mass spectrometry (IMR-MS) or proton exchange reaction mass spectrometry (PTR-MS) used. It becomes one substance gas to be examined by means of the output ion current in a drift Tube ionized and subsequently examined by mass spectrometry. The reaction area However, C remains essentially free of the one to be investigated Substance gas, i. the partial pressure of the substance gas in the reaction region C is less than 1/10 of the partial pressure of the substance gas in the drift tube. In the reaction area C are apart from the components of the source and Reaction gas preferably less than 50 ppm of other reactive Components (= reactive impurities) are present (the example of formed back-flowing components of a substance to be analyzed gas ), with a value of less than 25 ppm being particularly preferred. Not On the other hand, reactive components (e.g., nitrogen) can be present at higher levels available.

The type of ion at the exit 10 differs from the one or more Source ionic species.

If BH ₃ ⁺ ions are extracted as source ion species at the exit of the intermediate region, they can be used to generate, for example, initial ion currents which have the following ions in each case as substantially single ion species: N ₂ H ⁺ , H ₃ O ⁺ , NO ⁺ , NH ₄ ⁺ . Reactant gases which react with the H ₃ ⁺ ion stream from intermediate region B to the single ion species forming the parent ion stream are: Nitrogen (N ₂ ) H ₃ ⁺ + N ₂ → N ₂ H ⁺ + H ₂ resulting selective output ion current N ₂ H ⁺ Water (H ₂ O) H ₃ ⁺ + H ₂ O → H ₃ O ⁺ + H ₂ resulting selective output ion current H ₃ O ⁺ Nitric oxide (NO) H ₃ ⁺ + NO → HNO ⁺ + H ₂
ENT ⁺ + NO → NO ⁺ + ENT resulting selective output ion current NO ⁺ Ammonia (NH ₃ ) H ₃ ⁺ + NH ₃ → NH ₄ ⁺ + H ₂ resulting selective ion current: NH ₄ ⁺

A selective OH ^- exit ion current can be obtained from the O ^- ion stream extracted from the intermediate region B by means of the reactant gases methane (CH ₄ ) or H ₂ : O ^- + CH ₄ → OH ^- + CH ₃ O ^- + H ₂ → OH ^- + H

If XH ⁺ ions are present as the source ion species, X being a component of the source gas whose proton affinity is greater than H ₂ , then N ₂ H ⁺ is formed as N ₂ reactant gas as the starting ion current, if the proton affinity of component X is smaller than that of N ₂ . With H ₂ O as the reactant gas, if the proton affinity of X is less than the proton affinity of H ₂ O, then the essentially single ion species of the H ₂ O ⁺ source ion current is formed.

In principle, it would also be conceivable and possible for the intermediate region B eliminated. The reactions of the ions formed in the ionization region to those not with the source gas reactive source ion type or the more not with Source ion species that react to the source gas could then be either already in the ionization region substantially completely run off and / or after extraction the (not or only partially to the one or more source ion types reacted) ions from the ionization in the reaction region C in this continue to run through the existing partial pressure of source gas. It should Such conditions exist that the reactant gas does not interact with the precursor products the one or more source ion types reacts and / or with the Reactant gas-reactive precursor products are mixed with a suitable supplemental gas allowed to react in non-interfering ions.

Even with existing intermediate area B, it would be conceivable and possible that the completion of the reactions to the one or more source ion species takes place only in the reaction region C.

Thus, the areas A and B can at least partially overlap or It may cover the area B and C partially, as far as B not with A covered. In any case, the reaction region C is outside the ionization region A (i.e., outside the range in which this occurs during ionization plasma arising from the source gas is present). The reaction area C is thus spatially separated from the ionization area A and there will be a return of reactant gas from the reaction region C into the ionization region A im Substantially prevented.

Legend to the reference numbers:

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Claims (13)

A process for obtaining a starting ion stream consisting essentially of only a single ion species, wherein ions formed in the ionization of a source gas in an ionization region (A) and / or ions extracted from the ionization region (A) are kept in a region (A, B, C), in which there is source gas, are allowed to react until essentially only one or more source ion species are present which do not react with the source gas, characterized in that a reaction region (C) which extends outside the ionization region (A ) and in which ions of the one or more source ion species are present, a reactant gas different from the source gas is supplied, which reacts with the ions of the one or more source ion species, the ions of the one or more source ion species are essentially converted into the single ion species forming the starting ion stream. A method according to claim 1, characterized in that a return flow of the reactant gas from the reaction region (C) in the ionization region (A) is substantially prevented. A method according to claim 1 or claim 2, characterized in that the ions extracted from the ionization region (A) are guided into an intermediate region (B) in which they are left until they are replaced by ion-molecule reactions with source gas present in the intermediate region Also, the ions initially different from the one or more source ion species have essentially converted into ions of the one or more source ion species or cluster ions thereof. Process according to claim 3, characterized in that ions of the one or more source ion species or cluster ions thereof are extracted from the intermediate region (B) into the reaction region (C). A method according to claim 1 or claim 2, characterized in that the ions extracted from the ionization region are passed directly into the reaction region (C). Method according to one of claims 1 to 5, characterized in that the reaction region (C) and source gas is supplied. Method according to one of claims 1 to 6, characterized in that at least in a the output of the reaction region (C) adjacent section, an electric field (E _c ) of a strength is applied, by which the formation of cluster ions from the ion type of Ausgangssionenstroms is prevented or reversed. Method according to one of claims 3 or 4, characterized in that an electric field (E _B ) of a thickness is applied at least in a section adjacent to the output of the intermediate region (B), by means of which the formation of cluster ions from the one or more source regions lonensorten is prevented or reversed. Method according to one of claims 1 to 8, characterized in that only one source ion type is formed. Method according to one of claims 1 to 9, characterized in that in the region (A, B, C), in which the ions formed in the ionization region (A) and / or from the ionization region (A) extracted ions to the one or several source ion species react, source gas is present at a pressure of at least 1 Pascal. Method according to one of claims 1 to 9, characterized in that for obtaining each substantially only a single ion species existing initial ion streams which differ in the ion type, the reaction region (C), depending on the type of ion to be formed of the starting are supplied to the reactant gases different ion gases, wherein before the supply of a respective reactant gas, the reaction region (C) is pumped to remove previously used different reactant gas. Method according to one of claims 7 to 11, characterized in that in the reaction region (C) applied electric field (E _c ) is electrostatic and homogeneous. Method according to one of claims 8 to 12, characterized in that in the intermediate region (B) applied electric field (E _B ) is electrostatic and homogeneous.

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