The present invention relates to a method for obtaining gaseous ammonium (NH4+) from an ion source. The invention also relates to a method for ionizing a sample with gaseous ammonium, comprising obtaining ammonium and ionizing the sample in a reaction chamber. Furthermore, the invention relates to a method of detecting the ion yield of the mass-to-charge ratio of ions by detecting the ions in an MS-instrument. Finally, the invention relates to an IMR-MS instrument, comprising an ion source; a reaction region connected to said ion source; a mass spectrometer region connected to said reaction region; at least one inlet for source gases; at least one inlet for a sample into the reaction region; an N2-source; a H2O source; and at least one pump.

BACKGROUND OF THE INVENTION

Gas analysis with Ion-Molecule-Reaction-Mass Spectrometry (IMR-MS) has been well established for many decades (see e.g. A. M. Ellis, C. A. Mayhew. Proton Transfer Reaction Mass Spectrometry Principles and Applications. John Wiley & Sons Ltd., UK, 2014). Among the most important techniques in this field are Proton-Transfer-Reaction-Mass Spectrometry (PTR-MS), Selected-Ion-Flow-Tube-Mass Spectrometry (SIFT-MS) and Selected-Ion-Flow-Drift-Tube-Mass Spectrometry (SIFDT-MS). In an ion source so-called primary or reagent ions are generated and injected into a reaction chamber (often called “drift tube” in PTR-MS, “flow tube” in SIFT-MS and “flow-drift tube” in SIFDT-MS), where chemical ionization of the analytes takes place. Typical chemical ionization reactions are:

Proton transfer reactions, either non-dissociative or dissociative, with A being the reagent ion (e.g. H2O.H+, NH3.H+, etc.) and BC being the analyte:
A.H++BC→A+BC.H+
A.H++BC→A+B+C.H+

Charge transfer reactions, either non-dissociative or dissociative, with A being the reagent ion (e.g. O2+, NO+, Kr+, etc.) and BC being the analyte:
A++BC→A+BC+
A++BC→A+B+C+

Clustering reactions, with A being the reagent ion (e.g. H3O+, NO+, O2+, NH4+) and BC being the analyte:
A++BC→BC.A+

In addition other types of reactions can occur (e.g. ligand switching, H+extraction in case of negatively charged reagent ions, etc.).

Eventually the reagent and product ions are separated by their mass-to-charge ratio m/z and detected in a mass spectrometer, amongst others, based on multipole, Time-Of-Flight (TOF) and ion trap technology. In addition, a series of common devices for controlling the various voltages, currents, temperatures, pressures, etc. need to be present in the instrument.

In PTR-MS traditionally H3O+is used as reagent ions. However, recent PTR-MS instruments are additionally capable of utilizing alternative reagent ions, e.g. NO+, O2+, Kr+, NH4+and any other positively or negatively charged reagent ions and thus are sometimes called Selective-Reagent-Ionization-Mass Spectrometry (SRI-MS) instruments. Also in SIFT-MS and SIFDT-MS a variety of reagent ions can be used, with H3O+, NO+and O2+being the most common ones.

All of these reagent ions used in IMR-MS have distinct advantages, which make them particularly suitable for certain applications. A particular beneficial reagent ion is the ammonium cation NH4+. NH3has a Proton Affinity (PA) of 854 kJ/mol, whereas H2O has a PA of 691 kJ/mol. Proton transfer is energetically only possible if the PA of the analyte is higher than the PA of the reagent ion. Thus, some of the advantages of using NH4+are:

Improved selectivity: In IMR-MS often two or more compounds are detected at the same nominal m/z (e.g. isobars or isomers). If they share the same exact m/z (isomers) or if the mass resolution of the mass spectrometer is insufficient to separate isobars, additional measures have to be taken to distinguish them. Pinene (C10H16; PA<854 kJ/mol and >691 kJ/mol) and 2-ethyl-3,5-dimethylpyrazine (C8H12N2; PA>854 kJ/mol) are mentioned as examples in the prior art. The protonated molecules of both compounds share nominal m/z 137 when using H3O+as reagent ions. However, with NH4+as reagent ions, only 2-ethyl-3,5-dimethylpyrazine gets protonated whereas pinene does not react. That is, if only one of these two compounds is present in a sample and ions are detected at m/z 137 with NH4+as reagent ions, the compound can be identified as 2-ethyl-3,5-dimethylpyrazine. If ions are detected at m/z 137 with H3O+as reagent ions, but not with NH4+, the compound can be identified as pinene.

Simplification of mass spectra: If, for example, in a complex sample only compounds with a PA higher than the PA of NH3need to be detected and quantified, using NH4+reagent ions will blank out all analytes with a PA lower than the PA of NH3and thus will lead to a mass spectrum which is considerably easier to interpret than a mass spectrum generated with H3O+reagent ions.

Less fragmentation: Compounds with high PAs (>854 kJ/mol) often show high levels of fragmentation upon PTR ionization from H3O+, which makes identification and quantification difficult. The explosive TATP (C9H18O6) does not form the characteristic ion TATP.H+(or only with a very low abundance), but many fragment ions when reacting with H3O+at a reduced electric field strength (E/N) which is typically used in PTR-MS. When switching to NH4+as reagent ions the protonated TATP molecule as well as TATP.NH4+are formed with high abundance and thus detection and identification of this explosive is improved. Another example is the Chemical Warfare Agent (CWA) sarin (C4H10FO2P), which also shows a high level of fragmentation upon H3O+ionization. Using NH4+as reagent ions effectively suppresses fragmentation and produces the protonated sarin molecule as well as [sarin+NH3].H+clusters.

GB 2 324 406 B describes a method of generating NH4+reagent ions with high purity, so that they can be used without further filtering in a PTR-MS device. In this method NH3is introduced into the first ionization chamber of the ion source. The ionization products are subsequently left in the second ionization chamber of the ion source, together with NH3, until the ionization products which are initially other than NH4+are converted into NH4+ions. This is a method similar to the method described in DE 195 49 144, which is used to generate H3O+from H2O vapor, but with the source gas being NH3instead of H2O.

In SIFT-MS NH4+reagent ions are generated in a similar way, namely by ionization of NH3in the ion source and subsequent ion-molecule reactions between NH3+and NH3, which form NH4+(and NH2).

In AT 413 463 B an extended ion source for PTR-MS is used, which is equipped with an additional ionization chamber. In order to generate NH4+the ion source is operated in a way such that in the second ionization chamber H3+is produced and introduced together with NH3into a third ionization chamber, where H3+and NH3react to NH4+(and H2).

A different method of generating NH4+reagent ions in a PTR-MS instrument is described in DE 10 2011 009 503 A1. There, the PTR-MS instrument is operated so that the ion source produces H3O+reagent ions from H2O source gas, i.e. in the most common way a PTR-MS instrument is being operated. However, NH3is introduced into the drift tube via the sample inlet at a sufficiently high concentration, so that the majority of the H3O+reacts with NH3to NH4+(and H2O). In other words, H3O+reagent ions are converted to NH4+reagent ions in the drift tube by the introduction of NH3.

In summary, all methods for generating NH4+in an IMR-MS instrument require the introduction of NH3into at least one part on the instrument. However, there are a number of considerable disadvantages when using NH3in an IMR-MS instrument:NH3is toxic, corrosive and environmentally hazardous.The use of NH3gas cylinders requires a high level of safety precautions and is prohibited in some areas.The use of ammonia solutions as an NH3source does not provide a stable NH3concentration over time.NH3can damage important parts of the instrument, such as lines, lenses, vacuum pumps, valves, flow controllers, etc.The exhaust of the IMR-MS instrument is contaminated with NH3and needs to be properly disposed.The surfaces inside the IMR-MS instrument get covered with NH3which desorbs very slowly after switching to a different reagent ion, i.e. switching from NH4+to H3O+takes tens of minutes if not hours.

SHORT DESCRIPTION OF THE INVENTION

Due to the large number of disadvantages associated with the generation of NH4+, this ion is rarely used in IMR-MS devices as reagent ion. H3O+is still the standard reagent ion despite its disadvantages such as lower selectivity, more complex mass spectra and higher levels of fragmentation.

The object of the present invention is to provide an ion source with higher selectivity, simpler spectra and less fragmentation when compared to H3O+but with less disadvantages than the known methods involving NH3in the generation of NH4+.

The problem is solved by a method for obtaining gaseous ammonium (NH4+) from an ion source, the ion source comprising a first area and a second area in a fluidly conductive connection, comprising the steps(a) introducing N2and H2O into the first and second area of the ion source;(b) applying an ionization method to the mixture of N2and H2O in the first area;(c) applying at least one electric field or adjusting pressure conditions or a combination of applying at least one electric field and adjusting pressure conditions promoting flow of ions from the first area to the second area and inducing reactions of the ions in the second area,(d) conducting NH4+out of the ion source.

Surprisingly it has been found that by applying an ionization method to a mixture of N2and H2O in the first area of the ion source and then applying at least one field or adjusting pressure conditions or a combination of applying at least one field and adjusting pressure conditions thereby promoting flow of ions from the first area to the second and thereby also inducing collisions and thus reactions of the ions and neutral H2O and N2in the second area, resulted a high yield of NH4+with almost no other ions, in particular no parasitic ions, in the second area. Absolutely no NH3needs to be added to this process at any stage, which is in stark contrast to the prior art, and therefore the negative side effects caused by the use of this dangerous, toxic and corrosive source gas in previous designs are diminished.

The at least one field is an electric field. The pressure and/or the electric field are such as to promote flow of ions resulting from the ionization process in the first area to the second area. Neutral N2and H2O are introduced into the second area either by a flow of remaining neutrals from the first area or by injection into the second area (depending on the type and design of the ionization in the first area). Furthermore, the field and/or pressure are such to induce collisions in the second area and thus to promote NH4+formation. Furthermore, a magnetic field may be applied.

In one embodiment, step (c) includes maintaining the pressure in the second ionization chamber at a pressure below the pressure of the first ionization chamber and applying an electric field in the second ionization chamber to support flow of ions and remaining neutrals from the first ionization chamber to the second ionisation chamber, leading to NH4+formation via ion-molecule reactions in the second ionization chamber.

In one embodiment there is at least one source gas inlet for introducing N2and H2O into the first area.

It turned out that the molar mixing ratio of N2and H2O may be varied over a broad range to allow formation of NH4+. Useful molar mixing ratios of N2to H2O in the first ionization chamber are between 1:9 and 9:1. In a preferred embodiment the molar mixing ratios are between 3:7 and 7:3. Most preferably, the molar ratio between N2and H2O is approximately 1:1.

Though, the N2source may be any gaseous source of N2such as air, in a preferred embodiment the N2source is essentially pure gaseous N2.

In one embodiment N2and H2O are mixed before the introduction into the first ionization chamber.

Alternatively, N2and H2O are introduced into the first area separately and are mixed directly in first area.

In one embodiment N2and/or H2O are introduced in the second area and N2and/or H2O flow to the first area from the second area.

In one embodiment N2and H2O are introduced into the first and the second area.

While the two areas of the ion source may be in a single vessel, there is a preferred embodiment, where the first area is a first ionization chamber and the second area is a second ionization chamber, first and second ionization chamber being connected to allow fluid exchange. The spatial separation of first and second area allows flow control of ions and/or neutrals from the first ionization chamber to the second ionization chamber more easily. Furthermore, the spatial separation allows for simple adjustment of the pressure in the second area without affecting the pressure in the first area. Hence, first area and second area are then first ionization chamber and second ionization chamber, respectively. The ionization source is preferably in the first area/ionization chamber. The source for the (electric) field is preferably in the second area/ionization chamber.

The invention further relates to a method for ionizing a sample with gaseous ammonium, comprising obtaining gaseous ammonium according to the method described above and ionizing the sample in a reaction chamber being connected with the exit of the second ionization chamber.

Furthermore the invention relates to a method of detecting the ion yield of the mass-to-charge ratio of ions produced by the method of the previous paragraph, by detecting the ions in an MS-instrument.

Finally, the inventions relates to an IMR-MS instrument, comprisingan ion source comprising a first area and a second area, an ionization source and at least one source for a electric field;a reaction region connected to said ion source;a mass spectrometer region connected to said reaction region;at least one inlet for source gases;at least one inlet for a sample into the reaction region;an N2-source;a H2O source;at least one pump; anda controlling device controllingflow of N2of the N2-source,flow of H2O of the H2O-source.the least one pump,ionization source andthe source for the electric fieldso as to produce gaseous ammonium (NH4+) in said second area and then conducting the NH4+to the reaction region.

Preferably, the first area and the second area are a first ionization chamber and a second ionization chamber, wherein said second ionization chamber is connected to said first ionization chamber, wherein the first ionization chamber includes the ionization source and the second ionization chamber includes the at least one source for the field.

It is preferred that the controlling device also controls the pressure in the second area.

Additionally, a source for a magnetic field may be present.

