WO1999033089A2 - Method and device for detecting sample molecules in a carrier gas - Google Patents

Abstract

A method for detecting the presence of sample molecules in a carrier gas. According to said method, a divergent jet of carrier gas is produced in a vacuum by expanding gas through a nozzle and the sample molecules are ionised into sample molecules in an ionisation area of the carrier gas jet by means of photon absorption. The sample molecules are pulled into a mass spectrometer by an electrical field where they are detected. In order to improve this method, the distance between the discharge opening of the nozzle and the ionisation area can be chosen freely. The ions from the sample molecule are substantially pulled into the mass spectrometer in the direction of the axis of the carrier gas jet.

Description

 Method and device for the detection of sample molecules in a carrier gas

The present invention relates to a method for the detection of sample molecules in a carrier gas, wherein a divergent carrier gas jet is generated by expanding the carrier gas through a nozzle into a vacuum, the sample molecules in an ionization region of the carrier gas jet are ionized to sample molecule ions by absorption of photons, and the sample molecule ions by an electric pull field is drawn into a mass spectrometer and detected in the mass spectrometer.

Furthermore, the present invention relates to a device for detecting sample molecules in a carrier gas, comprising a nozzle for generating a divergent carrier gas jet by expanding the carrier gas into a vacuum, a device for resonant ionization of the sample molecules to sample molecule ions in an ionization region of the carrier gas jet by absorption of photons, a mass spectrometer and a device for generating an electrical drawing field which draws the sample molecule ions into the mass spectrometer with a drawing electrode.

Methods of the type mentioned at the outset, in which sample molecules are selectively ionized by absorption of photons, are known from the literature, for example, under the designation “resonance-enhanced multiphoton ionization (REMPI)”, this designation in the narrower sense only referring to the process used for selective photoionization relates. A method and a device of the type mentioned at the outset are known in particular from German Patent 44 41 972.

In the method known from the cited document, the sample molecule ions are drawn into the mass spectrometer in a direction perpendicular to the axis of the carrier gas jet (so-called "cross-beam" arrangement). In order to avoid distortion of the electrical drawing field by the nozzle, in the known method the electrical drawing field is shielded by an electrostatic shield arranged between the nozzle and the drawing electrode generating the electrical drawing field. The dimensions of the electrostatic shielding dictate a fixed minimum distance between the outlet opening of the nozzle and the ionization area. The distance between the outlet opening of the nozzle and the ionization area can therefore not be chosen freely.

The present invention is therefore based on the object of improving a method of the type mentioned at the outset in such a way that the distance between the outlet opening of the nozzle and the ionization region can be freely selected.

This object is achieved according to the invention in a method having the features of the preamble of claim 1 in that the sample molecule ions are drawn into the mass spectrometer essentially along the direction of the axis of the carrier gas jet.

This is achieved by using an electrical pulling field which is arranged essentially coaxially with the carrier gas jet. In contrast to the known "Cross-beam" arrangement can therefore be briefly referred to as an "axial-beam" arrangement or axial arrangement of the electrical drawing field and the mass spectrometer relative to the carrier gas jet.

The concept according to the invention offers the advantage that the distance between the outlet opening of the nozzle and the ionization region arranged on the axis of the carrier gas jet can be set to any value which can be predetermined, in particular to any value, by moving the nozzle along the axis of the carrier gas jet, without disturbing the rotational symmetry of the electrical drawing field, the axis of symmetry of which coincides with the axis of the carrier gas jet.

This is not possible with a "cross-beam" arrangement of the electric pulling field and the mass spectrometer with respect to the axis of the carrier gas jet, since with such an arrangement the rotational symmetry of the electric pulling field is inevitably disturbed when the nozzle is moved towards the ionization area, which is arranged at the intersection of the axis of the carrier gas jet and the axis of symmetry of the electrical drawing field.

The nozzle is advantageously switched as a repeller, ie placed at an electrical potential, the sign of which corresponds to the sign of the sample molecule ions generated in the ionization region, so that the sample molecule ions generated are accelerated away from the nozzle towards the mass spectrometer. As already stated, in the method according to the invention the sample molecules can be ionized at any distance from the outlet opening of the nozzle, the distance of the ionization area from the outlet opening of the nozzle by moving the nozzle along the axis of the carrier gas jet or by shifting the required photons providing photon source is adjustable relative to the nozzle.

The displacement of the nozzle is preferred, however, since the position of the ionization location relative to the inlet opening of the ion extraction optics of the mass spectrometer is retained.

In a preferred embodiment of the method according to the invention, the sample molecules are ionized within a continuum region of the carrier gas jet, in which the temperature of the carrier gas decreases with increasing distance (x) from the outlet opening of the nozzle.

The continuum region of the carrier gas jet comprises the region of the carrier gas jet between the outlet opening of the nozzle and a certain distance x _τ from the outlet opening of the nozzle, in which the carrier gas jet reaches its minimum temperature when expanded into a vacuum.

In the molecular beam region which adjoins the continuum region of the carrier gas jet at larger distances x from the outlet opening of the nozzle, the temperature of the carrier gas essentially does not decrease any further with increasing distance x from the outlet opening of the nozzle. The temperature of the carrier gas is determined in the usual way from the width of the velocity distribution of the carrier gas particles. For multi-atom carrier gas particles, apart from this "translation temperature", other temperatures can be determined from the occupation of the rotation or vibration levels, which under certain circumstances can differ from the translation temperature and from one another. However, all these temperatures reach their minimum at essentially the same distance x _τ from the outlet opening of the nozzle.

As for the carrier gas particles, different temperatures can be defined for the sample molecules, which can differ from one another and from those of the carrier gas. These temperatures of the sample molecules also no longer decrease substantially from substantially the same distance x _τ from the outlet opening of the nozzle as the temperatures of the carrier gas.

In the following, a distinction is therefore no longer made between the differently defined temperatures of the carrier gas or the sample molecules, but the term “the temperature” is used as a collective term for the translation, rotation and oscillation temperatures.

As can be seen from the above, the area of the carrier gas jet between the outlet opening of the nozzle and the distance x _τ at which the minimum temperature is reached is called the continuum area. The area of the carrier gas jet which adjoins the continuum area at greater distances from the outlet opening of the nozzle is referred to as the molecular jet area. The selectivity of the photoionization increases like the sensitivity with decreasing temperature of the sample molecules in the ionization area of the carrier gas jet and is therefore increased in the molecular jet area compared to the continuum area, but cannot be moved further by moving the ionization area within the molecular jet area to larger distances x from the outlet opening of the nozzle improve.

If the highest possible selectivity of the method for the detection of the sample molecules is desired, the sample molecules are advantageously ionized at a distance x _τ from the outlet opening, since at the lowest temperature reached there essentially all of the sample molecules are in the energetic ground state and thus all sample molecules of a species Absorption of one or more photons with sharply defined energy can be converted into an excited state. However, this requires that a tunable laser be used as the photon source, so that photons of the sharply defined energy required for a desired sample molecule species can be made available. The required tunable laser makes the device required to carry out the method large and expensive. A tunable laser is also comparatively difficult to use and susceptible to interference from the environment, especially if it is operated in an industrial plant rather than under laboratory conditions. In addition, multiple species of sample molecules cannot be measured simultaneously with maximum selectivity. If, on the other hand, only groups of substances are to be separated from one another, for example aromatics on the one hand and aliphatics on the other hand, it can be advantageous to work with reduced selectivity. For this purpose, the sample molecules are ionized at a distance x _x from the outlet opening, which is smaller than the distance x _τ , ie within the continuum area of the carrier gas jet.

In a relation to the distance x _τ reduced Ionisationsabstand x _x is the mean temperature of the sample molecules above the minimum temperature, so that adjacent to the ground state even higher-energy states of the sample molecules are occupied. There is thus a broader spectrum of photon energies available which can be used for the resonant excitation of the sample molecules, so that a photon source with a comparatively broad wavelength spectrum can be used. By ionizing the sample molecules at the temperature in the continuum region of the carrier gas jet which is higher than the minimum temperature of the carrier gas, several species of sample molecules can often be ionized at the same time and thus detected.

It is expedient if the sample molecules are ionized at a distance _x from the outlet opening of the nozzle which is less than approximately 0.9 x _τ , in particular less than approximately 0.8 x _τ , preferably less than approximately 0.5 x _τ . The smaller the distance between the ionization area and the outlet opening of the nozzle, the broader the spectrum of photon energies that can be used to excite the sample molecules. In this case, a fixed frequency laser is preferably used as the photon source, for example a frequency-quadrupled Nd: YAG laser at 266 nm or a KrF laser at 248 nm.

If, on the other hand, the focus is on achieving the highest possible selectivity and sensitivity of the detection method, the sample molecules are advantageously at a distance x _z from the outlet opening of the nozzle between approximately 0.5 x _τ and approximately 3 x _τ , in particular between approximately 0.8 x _τ and about 2 x _τ , preferably between about 0.9 x _τ and 1.5 x _τ , ionized. Because the ionization takes place close to the distance x _τ , the sample molecules are cooled as strongly as possible, which is necessary for highly selective photoionization, without the density of the carrier gas and thus of the sample molecules decreasing more than unavoidably due to the divergence of the carrier gas jet.

In this case, a tunable laser, in particular a dye laser, is preferably used as the photon source, which can be tuned, for example, in a wavelength range from 210 to 400 nm.

In particular, it can be provided that the type of photon source used is selected as a function of the distance of the ionization region from the outlet opening of the nozzle. For example, a fixed-frequency laser can be replaced by a tunable laser at the transition to larger distances κ _z from the outlet opening, which are in the range of the distance x _τ . Conversely, a tunable Lasers at the transition to smaller distances from the outlet opening of the nozzle, in particular at distances smaller than approximately 0.5 x _τ , are replaced by a fixed frequency laser.

The distance of the ionization area from the outlet opening of the nozzle can be adjusted, for example, by moving the nozzle along the axis of the carrier gas jet or by moving the photon source relative to the nozzle.

In a preferred embodiment of the method according to the invention it is provided that the carrier gas jet is generated by means of a nozzle, the outlet opening of which has a diameter which is greater than the mean free path length of the particles of the carrier gas in the region of the outlet opening. As a result of this measure, the carrier gas jet is designed as a supersonic jet with a speed angle distribution that is comparatively narrowly concentrated around the direction of the beam axis and with a comparatively sharp temperature distribution.

In contrast to this, a so-called effusive jet occurs when the outlet opening of the nozzle has a diameter which is smaller than the mean free path of the particles of the carrier gas in the region of the outlet opening.

The advantages of a supersonic jet over an effusive jet are the following: The radiance of the supersonic jet is significantly higher than that of an effusive jet; moreover, the supersonic jet is less strongly divergent due to its narrower velocity-angle distribution, so that the density of the carrier gas jet decreases less strongly with increasing distance x from the outlet opening of the nozzle.

In addition, the temperature distribution in a supersonic jet is much narrower than in an effusive jet, so that desired average temperature values can be set more easily.

In addition, when generating a carrier gas jet designed as a supersonic jet, a valve for closing the nozzle can be arranged directly at the outlet opening of the nozzle, so that the generation of sharply defined carrier gas jet pulses with steep flanks is possible.

In contrast, an effusive jet is usually generated by means of a capillary, which is arranged between a valve for generating the carrier gas jet pulse and the outlet opening. This has the disadvantage that the carrier gas jet pulses emerging from the outlet opening are widened compared to the pulses primarily generated by the valve, since the carrier gas particles require different transit times for the path through the capillary. In addition, the walls of the capillary are covered by carrier gas particles and sample molecules, which are released again in later detection processes and can act as faults (so-called memory effect). The memory effect leads to a deterioration in the achievable temporal Dissolution of the detection method and has a particularly strong effect when using a capillary, since the wall surface of the capillary which can be covered with sample molecules is comparatively large in relation to the internal volume of the capillary.

If a pulsed carrier gas jet is generated by means of a pulsed nozzle, this offers the advantage that a deterioration in the vacuum in the ionization chamber is avoided.

With the axial arrangement of the electric pulling field and the mass spectrometer used according to the invention relative to the carrier gas jet, it is particularly important that the carrier gas jet pulses generated are as short as possible, since in this arrangement the carrier gas jet is not directly on the intake manifold of a vacuum pump which encompasses the ionization area Vacuum chamber can be evacuated, directed and thus the largest part of the carrier gas jet reaches the vacuum pump only after collisions with the walls of the vacuum chamber.

Carrier gas jet pulses with a duration of less than approximately 20 μs are preferably used.

Such short pulses can be generated by using a nozzle with a valve which has a, preferably spherical, valve body which is pushed from a valve seat by an actuating element of an actuating device to open the valve and by means of the fluid flow passing through the valve opening the valve seat is moved back. By separating from Valve body and actuating element is achieved in that the movement of the valve body is determined only by its own (small) mass inertia, so that short valve switching times can be achieved. In addition, the carrier gas jet pulses generated in this way have very short rise times (in the range of a few μs).

A valve of the type mentioned above is described in German Patent 38 35 788, to which reference is made with regard to the structure and function of such a valve and the content of which is hereby made part of the present description.

It is favorable if the actuating device for driving the actuating element has a piezo element.

As an alternative to the quick-switching valve described above, it is also possible to use a nozzle with a valve which has a valve seat which can be closed by a movable valve body and which can be moved away from the valve body faster than the valve body can follow by means of an actuating device. Such a nozzle can also achieve short switching times and high repetition frequencies with a long service life.

A fast-switching valve of the type mentioned above is described in German patent application 197 34 845.9 by the same applicant, to which reference is made with regard to the structure and function of such a valve and the content of which is hereby made part of the present description. Since the sample molecule ions are drawn into the mass spectrometer essentially along the axis of the carrier gas jet in the method according to the invention, there is the possibility that the sample molecule ions accelerated by the electrical pulling field enter into undesired ion-molecule reactions with the neutral carrier gas particles and / or the non-ionized sample molecules.

The likelihood of such undesirable ion-molecule reactions when a pulsed photon source is used, the pulse duration of the photon source being less than the pulse duration of the carrier gas jet, can be reduced in that the rising edge of the photon pulse is essentially at the same time as the start of the stationary one Phase of the carrier gas jet pulse arrives in the ionization region. The stationary phase of the carrier gas jet pulse is understood to mean the area lying between the rising flank and the falling flank of the carrier gas jet pulse, during which the beam density of the carrier gas jet is essentially constant.

The measure mentioned above ensures that the sample molecule ions generated are accelerated from the electric pulling field into a region lying in front of the front of the carrier gas jet pulse, in which there are no or only a few neutral particles with which the sample molecule ions are an undesired ion molecule Reaction could occur. The lowest possible residual gas pressure in front of the carrier gas jet pulse is achieved by using the shortest possible pulse duration for the carrier gas jet pulses (in the order of 10 to 20 μs). In order to enhance the effect of this measure, a high drawing tension is advantageously used to accelerate the sample molecule ions.

To maintain the focusing conditions even at high pulling voltages, it is advantageous if the electric pulling field is designed in such a way that it has a comparatively small field strength in an area surrounding the ionization site and a higher one in an area arranged between the ionization site and the entrance opening of the ion extraction optics of the mass spectrometer Has field strength.

With the axial arrangement of the mass spectrometer used according to the invention relative to the carrier gas jet, the carrier gas jet is directed directly at the ion entry of the mass spectrometer. The penetration of neutral carrier gas particles and non-ionized sample molecules into the mass spectrometer is undesirable, however, since an increased pressure in the mass spectrometer is harmful to the ion detector of the mass spectrometer. It is therefore advantageously provided that the sample molecule ions are drawn through a pinhole in the mass spectrometer, which is preferably pumped differentially.

It has proven to be particularly advantageous if a perforated diaphragm is used, the diaphragm opening of which has a diameter of less than approximately 8 mm, preferably approximately 5 mm.

In principle, any mass spectrometer can be used to analyze the ion masses. It is advantageous to use a time-of-flight mass spectrometer that is synchronized with the pulse train of the photon source.

However, it is particularly favorable if a reflectron is used as the mass spectrometer. Such a reflectron is a time-of-flight mass spectrometer, in which the incoming ions first cross a field-free area at constant speed, then are braked in a braking field until their direction of movement is reversed and the ions are accelerated again so that they return the braking field to its original one Speed, however, leave in the opposite direction, and finally the ions, after they have again crossed the field-free area at a constant speed, are detected by an ion detector.

The principle of the reflectron offers the advantage that ions with the same mass, but when entering the reflectron at different speeds, require essentially the same flight time from an entry opening of the mass spectrometer to the ion detector. Such ions, which have a higher entry speed, need a shorter time to cross the field-free areas, but remain in the braking field for a longer time because they are decelerated with the same deceleration as the initially slower ions, but from a higher entry speed.

With a suitable adjustment of the distance to be crossed in the field-free area to the strength of the braking field, it can be achieved that the total flight time for an area the entry speed of the ions depends only slightly on this entry speed. This enables a high mass resolution to be achieved even if the sample molecule ions absorb different energies from the drawing field. Such a compensation of different entry velocities of the sample molecule ions is particularly advantageous if an electric drawing field with a high drawing voltage is used in order to have the generated sample molecule ions overtake the neutral carrier gas particles and non-ionized sample molecules as quickly as possible.

In order to prevent neutral carrier gas particles or non-ionized sample molecules entering the reflectron from interacting with the sample molecule ions reflected from the braking field of the reflectron or from reaching the ion detector of the reflectron, it is advantageously provided that sample molecule ions entering the reflectron reflect in this way parallel to an ion-optical entry axis that they return longitudinally to the ion detector along tilted ion trajectories. The path of a sample molecule ion in the reflectron thus has the shape of a V. As a result, the ion detector of the reflectron can be arranged at a distance from the entry axis of the reflectron.

A particularly effective shielding of the ion detector from the neutral carrier gas particles and the non-ionized sample molecules is achieved if, on the one hand, particles entering the reflectron parallel to the entry axis and sample molecule ions returning to the ion detector on the other hand, be separated from each other by means of a partition. This partition thus separates the two legs of the V-shaped ion path from one another.

Furthermore, the present invention is based on the object of improving an apparatus for detecting sample molecules in a carrier gas of the type mentioned at the outset in such a way that the distance of the ionization region from the outlet opening of the nozzle can be set as desired.

This object is achieved according to the invention in a device with the features of the preamble of claim 18 in that the device for generating the electric pulling field pulling the sample molecule ions into the mass spectrometer is designed such that it generates an electric pulling field which essentially moves the sample molecule ions along the Direction of the axis of the carrier gas jet pulls into the mass spectrometer.

Advantageous embodiments of the device according to the invention are the subject of claims 19 to 34, the advantages of which have already been explained above in connection with the subjects of claims 2 to 17.

Further features and advantages of the invention are the subject of the following description and the drawing of an exemplary embodiment. The drawing shows:

Fig. 1 is a partially sectioned perspective

Representation of a device according to the invention for the detection of sample molecules in a carrier gas;

FIG. 2 shows a schematic longitudinal section through the device according to the invention from FIG. 1 along the axis of the carrier gas jet; and

3 shows a schematic cross section through a fluid flow-reset ball valve of the device according to the invention from FIGS. 1 and 2.

A device for the detection of sample molecules in a carrier gas, shown in FIGS. 1 and 2 and designated as a whole by 100, comprises a vacuum chamber 102 in the form of a tube cross. This tube cross comprises a first tube 104 with, for example, a vertically oriented axis 106 and a second tube 108 with an axis 110 oriented perpendicular to the axis 106, the axis 106 of the first tube 104 and the axis 110 of the second tube 108 at one point cut so that a central region 112 belonging to the interior of both tubes 104 and 108 is formed.

An upper section 114 of the first tube 104, which extends upward from the central region 112, is closed by a cylindrical cover 116 which is coaxial with the first tube 104 and whose diameter exceeds that of the first tube 104. The cover 116 carries, for example, four cylindrical guide rods 118 on its end face facing away from the first tube 104, the axes of which are aligned parallel to the axis 106 of the first tube 104 and which are located near the circumference of the cover 116 at the same distance from the axis 106 of the first tube 104 and are arranged at an angular distance of 90 ° with respect to this axis.

The guide rods 118 each pass through a guide hole passing through a cylindrical cover 120, which is arranged coaxially to the axis 106, parallel to the axis thereof. This allows the end cap 120 to slide up or down on the guide rods 118. By means of clamping elements 122 provided on the end cover 120, the end cover 120 can be fixed in its vertical position relative to the guide rods 118. By guiding the end cover 120 on the guide rods 118 it is ensured that the axis of the end cover 120 always coincides with the axis 106 of the first tube 104.

A hollow cylindrical bellows 124 coaxial with the first tube 104 is fixed gas-tight to an underside of the end cover 120 with an open upper end and gas-tight to the top of the cover 116 with an open lower end. The wall of the bellows 124 consists at least partially of elastic, pleated material, so that the height of the bellows 124 can be changed as a function of the position of the end cover 120 by pulling apart or compressing the folds. Furthermore, the end cover 120 closes an upper end of a holding tube 126 which is coaxial and has a smaller diameter than the latter, and which extends from a lower side of the end cover 120 downward through the bellows 124, a through opening in the cover 116 and through the upper section 114 of the end extends first tube 104 and opens into the central region 112 near its upper edge.

At its lower end, the holding tube 126 holds a valve nozzle 128 arranged in the interior thereof. An outlet plate 130 which forms a bottom of the valve nozzle 128 is flush with the lower end of the holding tube 126 and closes it.

Furthermore, the outlet plate 130 has a central outlet opening 132 of the valve nozzle 128 with a diameter of, for example, 0.5 mm.

The valve nozzle 128 is provided with a fast-switching ball valve with a fluid flow-resetting valve body, as described in German Patent 38 35 788, to which reference is hereby expressly made.

The basic structure and the mode of operation of this fast-switching ball valve are explained below with reference to FIG. 3.

The inner, upper edge of the outlet opening 132 of the valve nozzle 128 forms a valve seat 134 for a spherical valve body 136, which is in the closed state the valve nozzle 128 sets against the valve seat 134 and thus closes the outlet opening 132 in a gas-tight manner.

The valve body 136 is arranged in a valve body chamber 138, which is separated by a partition 140 with flow channels 142 from the interior 144 of a nozzle prechamber 145 arranged within the holding tube 126. The partition 140 serves to restrict movement of the valve body 136 away from the valve seat 134 to the area of the valve body chamber 138.

In the closed state, the valve body 136 is pressed against the valve seat 134 by the pressure difference between the interior 144 of the nozzle prechamber 145, which contains carrier gas loaded with the sample molecules to be detected, and the evacuated area 112.

To open the valve nozzle 128, the spherical valve body 136 is pushed from the valve seat 134 by an actuating bolt 146, which can be moved parallel to the end plate 130 toward and away from the outlet opening 132 by means of a purely schematically illustrated movement device 148. The actuating bolt 146 is butted in such a way that the spherical valve body 136 is precisely abutted against its equator.

When the spherical valve body 136 has been lifted off the valve seat 134, it is detected by the gas flow which flows through the flow channels 142, through the valve body chamber 138 and through the outlet opening 132 and driven back to the valve seat 134, so that the valve nozzle 128 automatically closes again becomes. A high-strength and abrasion-resistant material, in particular sapphire or a hard metal, is preferably used for the spherical valve body 136 and the valve seat 134. The actuating pin 146 or at least its front, butting part preferably also consists of such a material.

The opening time of the valve depends on the pressure difference between the interior 144 of the nozzle prechamber 145 and the central region 112, on the mass of the spherical valve body 136 and on the diameter of the outlet opening 132. The use of a valve body 136 with a small mass enables short opening times to be realized.

In order to accelerate the actuating pin 146 towards the valve body 136, the movement device 148 can comprise, for example, an electromagnet or a piezoelectric drive element. To move the actuating pin 146 back after the impact with the valve body 136, the movement device 148 can comprise, for example, a return spring acting on the actuating pin 146.

The actuating bolt 146 and the movement device 148 together form an actuating device of the fast-switching ball valve.

The movement device 148 is connected by means of control lines (not shown) to a control device (not shown) which actuates the movement device 148 in an adjustable cycle and thus can open the valve nozzle 128 in this adjustable cycle. The nozzle prechamber 145 of the valve nozzle 128 is connected to a carrier gas reservoir (not shown) via a tubular feed line 150 coaxial to the holding tube 126.

A right section 152 of the second pipe 108, which extends to the right from the central region 112 (in the illustration in FIG. 2), is connected at a right end 154 to an intake port of a first vacuum pump 156.

A left section 158 of the second tube 108, which extends to the left from the central region 112 of the vacuum chamber 102 (in the illustration in FIG. 2), is closed at its left end by a cylindrical cover 160.

A lower section 162 of the first tube 104, which extends downward from the central region 112 of the vacuum chamber 102, is closed at its lower end by an end wall 164 of a reflectron mass spectrometer (reflectron) 166 flanged to the first tube 104.

The reflectron 166 comprises a vacuum tube 168 which is coaxial with the first tube 104 and has the same diameter as the latter, and which is connected at an end remote from the end wall 164 to an intake port of a second vacuum pump 170.

In the half of the vacuum tube 168 facing the second vacuum pump 170, a plurality of ring-shaped brake electrodes 172 are arranged, which are aligned concentrically to a common electrode axis 174, which is tilted by an angle α relative to the common axis 106 of the first tube 104 and the vacuum tube 168. The end wall 164 of the reflectron 166 facing the vacuum chamber 102 bears a proboscis-shaped pulling electrode 176 coaxial with the axis 106 of the first tube 104.

The drawing electrode 176 comprises an essentially hollow cylindrical section 178, which opens at an opening 180 into the interior 182 of the vacuum tube 168 of the reflector 166.

The end of the hollow cylindrical section 178 facing away from the mouth opening 180 is closed by a cone-shaped tip 184 of the drawing electrode 176 which is coaxial with it and has a central entry opening 186 for the passage of an ion beam, the diameter of which corresponds to the diameter of the end face of the hollow cylindrical section 178 frustoconical tip 184 corresponds.

A perforated diaphragm 188 with a central, circular diaphragm opening 190 is arranged inside the hollow cylindrical section 178 of the drawing electrode 176.

Furthermore, an ion optics (not shown) is arranged in the pulling electrode 176, which is designed such that it focuses an ion beam incident along the axis 106 into the pulling electrode 176 onto a focal point 192 in the center of the circular aperture 190.

An ion detector 194 is arranged in the interior 182 of the vacuum tube 168 of the reflectron 166 near the end wall 164 and outside the axis 106 of the vacuum tube 168. Between the opening 180 and the ion detector 194, a partition 196 extends essentially along the direction of the electrode axis 174 through the interior 182 of the vacuum tube 168. This partition 196 divides the interior 182 into an entrance area 182a bordering the opening 180 and an ion detector 194 comprehensive detection area 182b.

As can be seen from FIG. 1, an axis 198 runs perpendicular to the axis 106 of the first tube 104 and perpendicular to the axis 110 of the second tube 108, which axis forms the optical axis of a pulsed laser 200 which is arranged outside the vacuum chamber 102 and whose laser beam 202 passes through a window 204 in a wall of the vacuum chamber 102, passes through the common intersection 206 of the axes 106, 110 and 198 and exits the vacuum chamber 102 again through a second window 208 opposite the first window 204.

The pulsed laser 200 can be controlled via the control device (not shown) of the valve nozzle 128 and can be synchronized with the valve nozzle 128.

Using the device according to the invention for the detection of sample molecules in a carrier gas, the method according to the invention is carried out as follows:

First, the vacuum chamber 102 by means of the first vacuum pump 156 and the vacuum pipe 168 to be evacuated by the second vacuum pump 170 to a pressure of typically each 10 ^"4 Pa. A carrier gas loaded with the sample molecules to be detected is provided in the carrier gas reservoir (not shown). The carrier gas then fills the tubular feed line 150 and the nozzle prechamber 145.

The movement device 148 of the valve nozzle 128 is now actuated by the control device (not shown) in order to accelerate the actuating bolt 146 towards the valve body 136 and to push the valve body 136 away from the valve seat 134. The carrier gas under pressure P ₀ (for example 1.013 · 10 ⁵ Pa (1 atm)) in the nozzle chamber 145 then flows through the outlet opening 132 of the valve nozzle 128 with the diameter D (for example 0.5 mm) into the vacuum chamber 102, whereby in the vacuum chamber 102 a conically widening carrier gas jet 210 is generated, coaxial with the axis 106 of the first tube 104.

The operating conditions, in particular the pressure P ₀ and the temperature T of the carrier gas in front of the valve nozzle 128 and the diameter D of the outlet opening 132 of the valve nozzle 128 are chosen so that the mean free path length λ of the carrier gas particles is significantly smaller than the diameter D of the outlet opening 132. As a result, the carrier gas particles interact with one another as they pass through the valve nozzle 128, with the result that the carrier gas jet 210 is formed as a supersonic jet with a speed-angle distribution that is comparatively closely concentrated around the direction of the jet axis 106 and a comparatively sharp temperature distribution .

If, on the other hand, the operating conditions were chosen such that the mean free path length λ of the carrier gas particles is greater than the diameter D of the outlet opening 132, then the carrier gas jet 210 is formed as an effective jet. Such an effective jet would be very divergent due to its wide angular velocity distribution, which would lead to low jet densities even at small distances x from the outlet opening 132 of the valve nozzle 128. In addition, the temperature distribution in an effective jet is very wide. The formation of the carrier gas jet 210 as a supersonic jet is therefore preferred.

The carrier gas jet 210, which is initially designed as a supersonic jet, comprises a continuum region which extends from the outlet opening 132 to a distance x _τ from the outlet opening 132, and a molecular beam region which adjoins the continuum region at larger distances x from the outlet opening 132.

The continuum area is characterized in that the temperature of the carrier gas jet and thus of the sample molecules decreases with increasing distance x within this area. From the distance x _τ , the minimum temperature of both the carrier gas particles and the sample molecules is reached. The temperature of the carrier gas particles and the sample molecules remains constant in the subsequent molecular beam region.

If the highest possible selectivity of the method for the detection of the sample molecules is desired, the sample molecules are advantageously ionized at a distance x _τ from the outlet opening 132, since at the lowest temperature reached there essentially all of the sample molecules are in the energetic ground state and thus all of the sample molecules are absorbed one or more photons with sharply defined energy can be converted into an excited state. However, this requires that a tunable laser, for example a dye laser, is used as laser 200 so that photons of the required, sharply defined energy can be made available.

However, if the selectivity does not play a decisive role because, for example, only groups of substances are to be dissolved separately, for example aromatics and aliphatics, the sample molecules are advantageously ionized at a distance Xj from the outlet opening 132 that is smaller than the distance x _τ . With an ionization distance that is reduced compared to the distance x _z , the mean temperature of the sample molecules is above the minimum temperature, so that in addition to the ground state, higher energetic states of the sample molecules are occupied with a non-negligible probability. A broader spectrum of energies that can be used to excite the sample molecules is thus available, so that a photon source with a comparatively broad wavelength spectrum can be used.

It is therefore possible to use a less complex, smaller and significantly cheaper fixed-frequency laser, in particular a solid-state laser, as the pulsed laser 200 instead of a complex tunable laser.

If a fixed frequency laser is to be used as the laser 200, the distance κ _{τ of} the intersection point 206 from the outlet opening 132 is chosen to be less than approximately x _τ , in particular less than approximately 0.8 x _τ , preferably less than approximately 0.5 x _τ . If, on the other hand, maximum selectivity and sensitivity of the detection method is required, a tunable laser is used as laser 200 and the ionization distance x _{x is} between approximately 0.5 x _τ and approximately 1.0 x _τ , in particular between 0.8 x _τ and 1, 0 x _τ , preferably chosen between approximately 0.9 x _τ and 1.0 x _τ .

For this purpose, the end plate 130 and thus the holding tube 126 and the valve nozzle 128 are displaced in the vertical direction until the intersection 206 has the desired ionization distance x _x from the outlet opening 132 of the valve nozzle 128.

The distance x _τ can either be determined experimentally by moving the valve nozzle 128 and observing the changes in the ion signal generated by the reflectron 166, or can be estimated using the following theoretical gas-dynamic considerations:

The maximum achievable terminal Mach number M _τ during expansion through the valve nozzle 128 depends, according to Anderson and Fenn, for single-atom gases such as argon as follows on the nozzle diameter D (in cm) and the pressure from P ₀ above the nozzle (in atm) (see For example, SR Goates and CH Lin, Applied Spectroscopy Reviews 25 (1989), pages 81 to 126):

M _τ = 133 (P ₀ D) ⁰ ' ⁴ . (I)

The Mach number M is the ratio of local flow velocity to local sound velocity. It is with the distance x from the outlet opening 132 of the valve nozzle 128 via the relationship

mit dem Adiabatenexponenten γ = 5/3 und dem Proportionalitätsfaktor A = 3,26 für den Fall einatomiger Trägergase, wie beispielsweise Argon oder Helium, verknüpft.

with the adiabatic exponent γ = 5/3 and the proportionality factor A = 3.26 for the case of monatomic carrier gases, such as argon or helium.

The distance x _τ at which the terminal Mach number M _{τ is} reached corresponds to the distance from which no further cooling occurs. It is obtained by replacing the Mach number M with the terminal Mach number M _τ in equation (II) and substituting M _τ with the right-hand side of the equation (I). You can find the relationship like this:

where P _{0 must be given} in atm and D in cm and x _τ results in cm.

Due to the axial arrangement of the reflectron 166 in the device 100 according to the invention for the detection of sample molecules in a carrier gas, any ionization distances κ _z can be realized.

After opening the valve nozzle 128, the control unit (not shown) triggers a laser pulse from the laser 200 such that the laser pulse arrives in the ionization region 212 surrounding the intersection 206 at the same time as the stationary phase of the carrier gas pulse begins.

As a rule, the laser pulse is triggered a few μs after opening the valve nozzle 128. At the same time, a timer (not shown) is reset and started. In the ionization region 212, the sample molecules carried in the carrier gas jet 210 are ionized by resonance-amplified multiphoton ionization (REMPI), wherein one sample molecule in each case changes into an excited state by absorption of one or more photons with suitable energy, from which the sample molecule then absorbs another photon (or several other photons) is ionized to a sample molecule ion.

The sample molecule ions formed in this way are drawn into the interior of the drawing electrode 176 by an electrical drawing field essentially parallel to the axis 106 of the carrier gas jet 210 through the inlet opening 186.

In order to generate the electrical pulling field which is rotationally symmetrical with respect to the axis 106 of the carrier gas jet 210, the rotationally symmetrical pulling electrode 176 is set to an electrical potential, the sign of which is opposite to the sign of the sample molecule ion charge.

In order to prevent the sample molecule ions from returning to the valve nozzle 128 and to strengthen the electrical pulling field, the end plate 130 of the valve nozzle 128 is also switched as a repeller, that is to say set to an electrical potential whose sign corresponds to the sign of the sample molecule ion charge.

In the following it is assumed that positive sample molecule ions are formed during photoionization. In this case, the pull electrode 176 must be set to negative and the end plate 130 of the valve nozzle 128 to a positive potential. Due to the rotational symmetry of the electrical drawing field generated by means of the rotationally symmetrical drawing electrode 176, the orbits of the sample molecule ions intersect at the focal point 192 of the ion optics in the interior of the drawing electrode 176. Neutral carrier gas particles and non-ionized sample molecules contained in the carrier gas jet 210 become the truncated cone-shaped electrode 184 acting as a skimmer 176 as well as largely prevented from entering the reflectron 166 by the perforated screen 188 in the interior of the drawing electrode 176. This prevents the vacuum in the interior 182 of the vacuum tube 168 of the reflectron 166 from deteriorating inadmissibly.

The sample molecule ions which have entered the reflectron 166 through the pulling electrode 176 first cross a field-free area in the half of the vacuum tube 168 facing the vacuum chamber 102 at a constant speed. The time required to fly through this distance is reciprocal to the speed which the sample molecule ions accelerate in the have obtained electrical pulling field, and therefore increases with increasing mass of the sample molecule ions.

After flying through the field-free route, the sample molecule ions reach the area between the brake electrodes 172, which are at positive potentials which increase gradually with increasing distance from the vacuum chamber 102 from one brake electrode 172 to the adjacent brake electrode 172, so that the brake electrodes 172 together form an electric brake field for generate the incoming sample molecule ions. In this electrical braking field, the sample molecule ions are braked until they reach reversal points from which they are accelerated again in the direction of the ion detector 194 and leave the braking field again at the same speed at which they entered it, but in reverse Direction.

Since the electrode axis 174 is tilted with respect to the axis 106 of the vacuum tube 168, the paths 214 of the sample molecule ions are not exactly reflected back in themselves, but instead the sample molecule ions reach the constant velocity after passing through the field-free area in the half of the vacuum tube 168 facing the vacuum chamber 102 the ion detector 194 arranged in the detection area 182b, which delivers a time-resolved electrical ion signal proportional to the current ion flow.

By assigning this ion signal to the time determined with the aid of the timer and elapsed since the triggering of the laser pulse, the dependence of the ion signal on the total flight time of the sample molecule ions can be determined. The total flight time of a sample molecule ion is proportional to the root of its mass.

The reflectron 166 is particularly suitable for achieving a high mass resolution since it minimizes the time-of-flight differences between sample molecule ions which have the same mass but are ionized at different distances from the pulling electrode 176 and therefore absorb different energies from the electrical pulling field. Those sample molecule ions whose ionization sites are further away from the drawing electrode 176 and which are therefore accelerated to a higher speed by the drawing field cover the distances in the field-free regions of the reflectron 166 in a shorter time than those sample molecule ions whose ionization sites are closer to the drawing electrode 176 lie. To do this, however, they remain in the brake field generated by the brake electrodes 172 for a longer time, since they have to be decelerated from the higher initial speed to zero speed at the point of reversal with the same delay as the slower sample molecule ions. Appropriate coordination of the distances to be covered in the field-free region of the sample molecule ions to the strength of the electric braking field can therefore achieve that the entire flight time of the sample molecule ions is essentially independent of the distance of their ionization location from the pulling electrode 176. This makes it possible to enlarge the extent of the ionization region 212 transversely to the axis 106 of the carrier gas jet 210, which in turn increases the number of sample molecule ions generated and thus the sensitivity for the detection of the sample molecules.

The neutral carrier gas particles and the non-ionized sample molecules, which have been stripped from the carrier gas jet 210 by the truncated cone-shaped tip 184 acting as a skimmer or which have been reflected back by the aperture plate 188 into the vacuum chamber 102, pass through the right section 152 of the second tube 108 the first vacuum pump 156, which removes the carrier gas particles and the non-ionized sample molecules from the vacuum chamber 102 to maintain the required vacuum. Those carrier gas particles and non-ionized sample molecules which have entered the entry area 182a in the interior of the vacuum tube 168 of the reflector 166 through the aperture 190 of the pinhole 188 are kept away from the detection area 182b and thus from the ion detector 194 by the partition 196 and reach the second vacuum pump 170, which removes these carrier gas particles and non-ionized sample molecules from the interior 182 of the vacuum tube 168 to maintain the required vacuum.

In order to avoid the build-up of too high a pressure in the vacuum chamber 102, the valve nozzle 128 is designed so that the carrier gas pulse is as short as possible, preferably shorter than approximately 20 μs.

At the end of a pulse, the valve nozzle 128 is automatically closed by the fluid flow through the valve body chamber 138 and the timer is stopped after the maximum ion flight time has elapsed. In the pause following the pulse, the first vacuum pump 156 and the second vacuum pump 170 remove residual carrier gas particles and sample molecules from the vacuum chamber 102 or from the vacuum tube 168 of the reflector 166, whereupon a new measuring cycle begins with the opening of the valve nozzle 128.

In conjunction with a suitable sampling and, if necessary, sample enrichment system, a device 100 according to the invention, which is tailored to the respective area of application, is suitable for numerous measuring tasks in the field of industrial process management and control and in the field of environmental monitoring. The device 10 according to the invention is particularly suitable for on-line measurement of organic compounds in the raw and / or clean gas of a waste incineration plant, in order to regulate the burner conditions in the waste incineration plant on the basis of the measurement results.

Claims

PATENT CLAIMS 1. A method for the detection of sample molecules in a carrier gas, wherein a divergent carrier gas jet is generated by expanding the carrier gas through a nozzle into a vacuum, the sample molecules in an ionization region of the carrier gas jet are ionized to sample molecule ions by absorption of photons, and the sample molecule ions by an electric pull field drawn into a mass spectrometer and detected in the mass spectrometer, characterized in that the sample molecule ions are drawn into the mass spectrometer essentially along the direction of the axis of the carrier gas jet. 2. The method according to claim 1, characterized in that the nozzle is connected as a repeller. 3. The method according to any one of claims 1 or 2, characterized in that the sample molecules are ionized within a continuum region of the carrier gas jet, in which the temperature of the carrier gas decreases with increasing distance (x) from an outlet opening of the nozzle. 4. The method according to claim 3, characterized in that the sample molecules are ionized at a distance κ _τ from the outlet opening of the nozzle, which is less than about 0.9 x _τ , in particular less than about 0.8 x _τ , preferably less is about 0.5 x _τ , where x _{τ is} the distance of the boundary between the Continuity area of the carrier gas jet and a molecular jet area of the carrier gas jet, in which the temperature of the carrier gas does not substantially decrease with increasing distance (x) from the outlet opening of the nozzle, are designated by the outlet opening of the nozzle. 5. The method according to claim 4, characterized in that a fixed frequency laser is used as the photon source. 6. The method according to any one of claims 1 to 3, characterized in that the sample molecules at a distance x _z from the outlet opening of the nozzle between approximately 0.5 x _τ and approximately 3 x _τ , in particular between approximately 0.8 x _τ and approximately 2 x _τ , preferably between approximately 0.9 x _τ and approximately 1.5 x _τ , where x _{τ is} the distance between the boundary between a continuum region of the carrier gas jet, in which the temperature of the carrier gas increases with increasing distance (x) from the Outlet opening of the nozzle decreases, and a molecular beam region of the carrier gas jet, in which the temperature of the carrier gas does not substantially decrease with increasing distance (x) from the outlet opening of the nozzle, from the outlet opening of the nozzle. 7. The method according to claim 6, characterized in that a tunable laser, in particular a dye laser, is used as the photon source. 8. The method according to any one of claims 1 to 7, characterized in that the carrier gas jet is generated by means of a nozzle whose outlet opening has a diameter which is greater than the mean free path of the particles of the carrier gas in the region of the outlet opening. 9. The method according to any one of claims 1 to 8, characterized in that a pulsed carrier gas jet is generated by means of a clocked nozzle. 10. The method according to claim 9, characterized in that a nozzle is used with a valve which has a, preferably spherical, valve body which is pushed to open the valve by an actuating element of an actuating device from a valve seat and by means of which through the valve opening Fluid flow is moved back to the valve seat. 11. The method according to claim 10, characterized in that an actuating device having a piezo element is used. 12. The method according to any one of claims 9 to 11, characterized in that a pulsed photon source is used, wherein the pulse duration of the photon source is less than the pulse duration of the carrier gas jet, and that the rising flank of the photon pulse is substantially simultaneous with the beginning of a stationary phase of the carrier gas jet pulse arrives in the ionization region. 13. The method according to any one of claims 1 to 12, characterized in that the sample molecule ions are drawn through a pinhole in the mass spectrometer. 14. The method according to claim 13, characterized in that a perforated diaphragm is used, the aperture opening has a diameter of less than about 8 mm, preferably of about 5 mm. 15. The method according to any one of claims 1 to 14, characterized in that a reflectron is used as the mass spectrometer. 16. The method according to claim 15, characterized in that parallel to an ion optical entry axis in the reflectron entering sample molecule ions are reflected so that they run back relative to the entry axis of tilted ion trajectories to an ion detector. 17. The method according to claim 16, characterized in that parallel to the entry axis in the reflectron entering particles on the one hand and to the ion detector returning sample molecule ions on the other hand are separated from each other by means of a partition. 18. Device for the detection of sample molecules in a carrier gas, comprising a nozzle for generating a divergent carrier gas jet by expanding the carrier gas into a vacuum, a device for ionizing the sample molecules to sample molecule ions in an ionization region of the carrier gas jet Absorption of photons, a mass spectrometer and a device for generating an electric pulling field pulling the sample molecule ions into the mass spectrometer with a pulling electrode, characterized in that the device for generating the electric pulling field is designed in such a way that it generates an electric pulling field which the sample molecule ions in essentially into the mass spectrometer (166) along the direction of the axis (106) of the carrier gas jet (210). 19. The apparatus according to claim 18, characterized in that the nozzle (128) is connected as a repeller. 20. Device according to one of claims 18 or 19, characterized in that the ionization region (212) within a continuum region of the carrier gas jet (210), in which the temperature of the carrier gas with increasing distance (x) from an outlet opening (132) of the nozzle ( 128) decreases, is arranged. 21. The apparatus according to claim 20, characterized in that the ionization region (212) is arranged at a distance (x _x ) from the outlet opening (132) of the nozzle (128) which is less than approximately 0.9 x _τ , in particular less than is approximately 0.8 x _τ , preferably less than approximately 0.5 x _τ , where x _{τ is} the distance between the boundary between the continuum area of the carrier gas jet (210) and a molecular jet area of the carrier gas jet (210), in which the temperature of the carrier gas coincides increasing distance (x) from the outlet opening (132) of the nozzle (128) essentially does not decrease further, referred to from the outlet opening (132) of the nozzle (128). 22. The apparatus according to claim 21, characterized in that the device (100) comprises a fixed frequency laser (200) serving as a photon source. 23. Device according to one of claims 18 to 20, characterized in that the ionization region (212) at a distance (x _x ) from an outlet opening (132) of the nozzle (128) between approximately 0.5 x _τ and approximately 3 x _τ , in particular between approximately 0.8 x _τ and approximately 2 x _τ , preferably between approximately 0.9 x _τ and approximately 1.5 x _τ , where x _{τ is} the distance between the boundary between a continuum region of the carrier gas jet (210), in which the temperature of the carrier gas decreases with increasing distance (x) from an outlet opening (132) of the nozzle (128), and a molecular beam region of the carrier gas jet (210) in which the temperature of the carrier gas increases with increasing distance (x) from the outlet opening ( 132) of the nozzle (128) essentially does not decrease further, referred to by the outlet opening (132) of the nozzle (128). 24. The device according to claim 23, characterized in that the device (100) comprises a tunable laser (200) serving as a photon source, in particular a dye laser. 25. The device according to one of claims 18 to 24, characterized in that the distance (x _x ) ionization region (212) from an outlet opening (132) of the nozzle (128) can be changed by moving the nozzle (128). 26. Device according to one of claims 18 to 25, characterized in that the nozzle (128) has an outlet opening (132) with a diameter which is greater than the mean free path of the particles of the carrier gas in the region of the outlet opening (132). 27. The device according to one of claims 18 to 26, characterized in that the nozzle (128) is a pulsed nozzle, by means of which a pulsed carrier gas jet (210) can be generated. 28. The apparatus according to claim 27, characterized in that the nozzle (128) comprises a valve which has a spherical valve body (136) which sits on a valve seat (134) in the closed state of the valve, and an actuating device (146, 148) through which the valve body (136) can be pushed by the valve seat (134) to open the valve. 29. The device according to claim 28, characterized in that the actuating device (146, 148) has a piezo element. 30. Device according to one of claims 18 to 29, characterized in that a pinhole (188) is arranged between the ionization region (212) and the mass spectrometer (166). 31. The device according to claim 30, characterized in that the perforated diaphragm (188) has a circular diaphragm opening (190) with a diameter of less than approximately 8 mm, preferably of approximately 5 mm. 32. Device according to one of claims 18 to 31, characterized in that the mass spectrometer (166) is a reflectron. 33. Apparatus according to claim 32, characterized in that the reflectron (166) has a device (172) for braking the sample molecule ions, which is designed such that sample molecule ions entering the reflectron (166) parallel to an ion-optical entry axis (106) are reflected that they run back along an ion path tilted relative to the entry axis (106) to an ion detector (194). 34. Apparatus according to claim 33, characterized in that the reflectron (166) comprises a partition (196) which, on the one hand, particles entering the reflectron (166) parallel to the entry axis (106) and returning to the ion detector (194) sample molecule- ions on the other hand.