DE10042394B4 - Ion source for ion mobility spectrometer with optical and Teilchenstrahlun gsionisation - Google Patents

Abstract

ion source for ion mobility spectrometer with optical and particle radiation ionization, using as ionization sources both an alpha or Beta emitters as well as a UV light source are used, thereby characterized in that the ion source consists of an ionization space (3) is completed by an output window (2), through the UV radiation of an optical ionization source in the ionization space (3) and on the window surface of the output window (2) Grid (4) is applied, wherein the grid (4) as Teilchenstrahlungsionisationsquelle is trained.

Description

The The invention relates to a device for generating ions for ion mobility spectrometers, which for the sensitive detection of traces of air in air, especially in the process analysis, safety technology and environmental protection.

An ion source for ion mobility spectrometers with optical and particle radiation ionization, in which both an alpha or beta emitter and a UV light source are present as an excitation source, is from the DE 197 30 896 C2 known.

A radioactive ion source for generating low-energy alpha or beta radiation is according to DE 197 30 899 A1 characterized in that it consists of a carrier layer of Halbleitermateiral, on the surface tritium is chemically covalently bound.

A UV light source for an ion mobility spectrometer is used in the US 5,338,931 described.

At one of the DE 195 15 270 C2 In known methods for measuring the mobility spectrum of ions in an ion mobility spectrometer with a beta emitter, the ion current is modulated by a switching element at the beginning of the ion drift path with a coding pattern such that the transmission and blocking times are short compared to the drift time of the slowest ions. Finally, leave the US 5,294,797 an ion source for a mass spectrometer with a beta emitter and a UV light source, with the electrons and photons being pulsed.

• (1) durch Ladungstransfer zwischen Clusterionen der Luft und dem Analyten, wobei die Luftclusterionen durch Bestrahlen von Luft mit Alpha- oder Betateilchen eines radioaktiven Nuklids (z. B. Tritium) gebildet werden, oder
• (2) durch Photoionisation des Analyten infolge Photonenstrahlung im UV-Bereich.

The ionization in ionization spectrometers takes place:

• (1) by charge transfer between cluster ions of the air and the analyte, wherein the air cluster ions are formed by irradiating air with alpha or beta particles of a radioactive nuclide (eg tritium), or
• (2) by photoionization of the analyte due to photon radiation in the UV range.

• – Der Konzentrationsbereich ist für viele Substanzen nach oben begrenzt und nur zu einem kleinen Teil linear.
• – Substanzen wie z.B.: Benzol, Arsin oder Phosphin können im Driftspektrum nicht oder nur schwer von den Clusterionen der Luft unterschieden und folglich nicht nachgewiesen werden.
• – Eine Reihe von organischen Molekülen wie z.B. Alkane und Alkene werden mit nur geringer Empfindlichkeit angezeigt.

Beta-ionization ( 1 ) has become widely used in industrial applications; weak radioactive sources in the exemption range are used. This process has in contrast to photoionization ( 2 ) the following disadvantages:

• - The concentration range is limited for many substances and only to a small extent linear.
• - Substances such as: benzene, arsine or phosphine can not or only with difficulty be distinguished from the cluster ions of the air in the drift spectrum and therefore can not be detected.
• - A number of organic molecules such as alkanes and alkenes are displayed with only low sensitivity.

• – Negative Ionen werden kaum gebildet, da in Luft die Konzentration freier Elektronen nur gering ist. Damit können halogenierte Kohlenwasserstoffe nur bedingt nachgewiesen werden.
• – Im Spektrum fehlt ein ständiger Reaktionsionenpeak (RIP), der zur internen Kalibrierung genutzt werden kann.

Photoionization is used in photoionization detectors (PID) for non-specific detection of trace gas molecules. It allows the detection of molecules in a wide concentration range, which can only be detected with beta-ionization at low sensitivity. The disadvantages of photoionization are:

• - Negative ions are hardly formed, since in air the concentration of free electrons is only small. This halogenated hydrocarbons can be detected only conditionally.
• - The spectrum lacks a constant reaction peak (RIP), which can be used for internal calibration.

It It was therefore an object of the invention to provide a device which the Disadvantages of the known prior art avoids.

To has been an ion source according to the invention developed with the features of the claims, both an alpha or beta source as well as a UV source as excitation sources contains.

following the invention will be explained in more detail with reference to exemplary embodiments:

It demonstrate:

1 an IMS in coaxial source arrangement

2 an ion source in a non-coaxial arrangement

One Ionenmobobilitätsspektrometer usually exists from an excitation device (Ionisationsquelle), to which the ionization space connects, in which in the carrier gas be detected detected trace gas molecules are ionized. This Ionization space is opposite the subsequent Drift path completed by a grid, which serves as a shutter for the Drift process works.

In the inventive device according to 1 the ion source is constructed as follows:
A UV lamp ( 1 ) as an excitation device forms with the front side, the output window ( 2 ), the front end of the ionization space ( 3 ). The UV lamp generates photons of energy 10.6 eV; the output window consists of eg MgF ₂ . For charging prevention is a grid ( 4 ) mounted on the window surface. This grid can be covered with titanium, in which tritium ³ H is stored. The tritium embedded in the lattice is a beta emitter, which acts as a second excitation source.

Beta particles ionize the carrier gas (air) along the grid ( 4 ); a plasma layer is formed, consisting of cluster ions NO ⁺ (H ₂ O) or O ₂ ^- (N ₂ O) and others. At the same time, the UV photons radiate out of the lamp ( 1 ) the grid ( 4 ) and generate ions in the ionization space ( 3 ).

Another grid ( 5 ), which is the shutter for the IMS, is arranged so that ion formation does not take place in the drift space of the IMS. The drift space of the ion mobility spectrometer is determined by the drift rings ( 7 ) formed with the aperture grid ( 8th ) is completed before the ions separated according to their drift time (ion groups) on the collector ( 9 ) of the IMS.

The ion source facing side of the grid ( 5 ) may also be titanium coated and contain tritium, indicating additional emission of beta particles into the space between grids (FIG. 4 ) and shutter grid ( 5 ).

To 2 is the UV lamp ( 1 ) perpendicular to the drift direction on the ionization space ( 3 ), in turn, the corresponding coated grid ( 4 ) and ( 5 ) the front and rear end of the ionization space ( 3 ) in the drift direction.

Advantageously, with this arrangement, it is also possible to position the UV lamp at a decentralized location of the overall device and to detect the UV quanta by means of an optical waveguide arrangement (FIG. 6 ) to the ionization space.

• – Photonen ionisieren alle organischen Komponenten des Gases, deren Ionisierungspotentiale unter der UV-Energie der Lampe (10,6 eV) liegen und ermöglichen einen breiten Bereich des quantitativen Nachweises.
• – Alpha- oder Betateilchen erzeugen negative Sauerstoff- und Wasserclusterionen, die den empfindlichen Nachweis halogenierter Kohlenwasserstoffe ermöglichen.
• – Die Palette nachweisbarer Substanzen wird um solch wichtige wie Benzol, Phosphin, Arsin, Toluol u.a. erweitert.
• – Durch die Auswahl einer geringen Aktivität der Strahlenquelle entsteht ein positiver Clusterionenpeak RIP, dessen Driftzeit gemessen und als Bezugszeit für alle nachzuweisenden Substanzen genutzt werden kann.
• – Der nutzbare Meßbereich wird um mehr als eine Größenordnung erweitert.

The advantages of this arrangement are:

• - Photons ionize all organic components of the gas whose ionization potentials are below the UV energy of the lamp (10.6 eV) and allow a wide range of quantitative detection.
• - Alpha or beta particles produce negative oxygen and water cluster ions that allow sensitive detection of halogenated hydrocarbons.
• - The range of detectable substances is extended to such important as benzene, phosphine, arsine, toluene and others.
• - By selecting a low activity of the radiation source creates a positive cluster ion peak RIP, whose drift time can be measured and used as reference time for all substances to be detected.
• - The usable measuring range is extended by more than an order of magnitude.

The activity the particle radiation source (s) is dimensioned so that by Photoionization formed peak of benzene ions not from the reaction ion peak RIP covered is. That is the case when sources are used in the area of exemption activities become.

wobei e die Elementarladung, g der Geometriefaktor mit g = 0,5 und a die Absorption in der dünnen Schicht mit a = 0,5 sind.For example, with a free-limiting activity A of 5 × 10 ⁶ s ^-1 and a mean energy <ε> of the ionizing beta particles of 5.5 keV and a mean ionization effort for a charge carrier pair in air ω of 30 eV, a signal current I of approximately 37 pA arises :

where e is the elementary charge, g is the geometric factor with g = 0.5 and a is the absorption in the thin layer with a = 0.5.

at Benzene can be displayed well in a RIP of approx. 37 pA.

Claims (4)

Ion source for ion mobility spectrometer with optical and particle radiation ionization, wherein both an alpha or beta emitter and a UV light source are used as ionization sources, characterized in that the ion source from an ionization space ( 3 ) passing through an exit window ( 2 ) is terminated by the UV radiation of an optical ionization source in the ionization space ( 3 ) and on the window area of the output window ( 2 ) a grid ( 4 ) is applied, wherein the grid ( 4 ) is formed as Teilchenstrahlungsionisationsquelle. Ion source according to claim 1, characterized in that the grid ( 4 ) has metallic coatings in which tritium is embedded. Ion source according to claim 1 and 2, characterized in that the grid ( 4 ) Contains ⁶³ Ni. Ion source according to claim 1 and 2, characterized in that the particle radiation source is constructed so that the grid ( 4 ) Contains ^{241 am} or ²²⁶ ra.