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WO1999033089A2 - Procede et dispositif pour detecter la presence de molecules d'echantillons dans un gaz porteur - Google Patents

Procede et dispositif pour detecter la presence de molecules d'echantillons dans un gaz porteur Download PDF

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Publication number
WO1999033089A2
WO1999033089A2 PCT/EP1998/008069 EP9808069W WO9933089A2 WO 1999033089 A2 WO1999033089 A2 WO 1999033089A2 EP 9808069 W EP9808069 W EP 9808069W WO 9933089 A2 WO9933089 A2 WO 9933089A2
Authority
WO
WIPO (PCT)
Prior art keywords
carrier gas
nozzle
outlet opening
gas jet
approximately
Prior art date
Application number
PCT/EP1998/008069
Other languages
German (de)
English (en)
Other versions
WO1999033089A3 (fr
Inventor
Horst-Henning Grotheer
Harald Oser
Reinhold Thanner
Original Assignee
Deutsches Zentrum für Luft- und Raumfahrt e.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Deutsches Zentrum für Luft- und Raumfahrt e.V. filed Critical Deutsches Zentrum für Luft- und Raumfahrt e.V.
Publication of WO1999033089A2 publication Critical patent/WO1999033089A2/fr
Publication of WO1999033089A3 publication Critical patent/WO1999033089A3/fr

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16K—VALVES; TAPS; COCKS; ACTUATING-FLOATS; DEVICES FOR VENTING OR AERATING
    • F16K31/00—Actuating devices; Operating means; Releasing devices
    • F16K31/02—Actuating devices; Operating means; Releasing devices electric; magnetic
    • F16K31/06—Actuating devices; Operating means; Releasing devices electric; magnetic using a magnet, e.g. diaphragm valves, cutting off by means of a liquid
    • F16K31/0644—One-way valve
    • F16K31/0655—Lift valves
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16K—VALVES; TAPS; COCKS; ACTUATING-FLOATS; DEVICES FOR VENTING OR AERATING
    • F16K15/00—Check valves
    • F16K15/18—Check valves with actuating mechanism; Combined check valves and actuated valves
    • F16K15/182—Check valves with actuating mechanism; Combined check valves and actuated valves with actuating mechanism
    • F16K15/1823—Check valves with actuating mechanism; Combined check valves and actuated valves with actuating mechanism for ball check valves
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J49/00—Particle spectrometers or separator tubes
    • H01J49/02—Details
    • H01J49/04—Arrangements for introducing or extracting samples to be analysed, e.g. vacuum locks; Arrangements for external adjustment of electron- or ion-optical components
    • H01J49/0422—Arrangements for introducing or extracting samples to be analysed, e.g. vacuum locks; Arrangements for external adjustment of electron- or ion-optical components for gaseous samples
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J49/00—Particle spectrometers or separator tubes
    • H01J49/02—Details
    • H01J49/10—Ion sources; Ion guns
    • H01J49/16—Ion sources; Ion guns using surface ionisation, e.g. field-, thermionic- or photo-emission
    • H01J49/161—Ion sources; Ion guns using surface ionisation, e.g. field-, thermionic- or photo-emission using photoionisation, e.g. by laser
    • H01J49/162—Direct photo-ionisation, e.g. single photon or multi-photon ionisation

Definitions

  • 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.
  • 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.
  • 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).
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 ⁇ from the outlet opening of the nozzle, in which the carrier gas jet reaches its minimum temperature when expanded into a vacuum.
  • 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.
  • 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 ⁇ from the outlet opening of the nozzle.
  • temperatures of the sample molecules 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 ⁇ from the outlet opening of the nozzle as the temperatures of the carrier gas.
  • the temperature is used as a collective term for the translation, rotation and oscillation temperatures.
  • the area of the carrier gas jet between the outlet opening of the nozzle and the distance x ⁇ 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.
  • the sample molecules are advantageously ionized at a distance x ⁇ 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.
  • 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.
  • 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.
  • the sample molecules are ionized at a distance x x from the outlet opening, which is smaller than the distance x ⁇ , ie within the continuum area of the carrier gas jet.
  • 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.
  • 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.
  • the sample molecules are ionized at a distance x from the outlet opening of the nozzle which is less than approximately 0.9 x ⁇ , in particular less than approximately 0.8 x ⁇ , preferably less than approximately 0.5 x ⁇ .
  • 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.
  • the sample molecules are advantageously at a distance x z from the outlet opening of the nozzle between approximately 0.5 x ⁇ and approximately 3 x ⁇ , in particular between approximately 0.8 x ⁇ and about 2 x ⁇ , preferably between about 0.9 x ⁇ and 1.5 x ⁇ , ionized. Because the ionization takes place close to the distance x ⁇ , 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.
  • 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.
  • 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.
  • a fixed-frequency laser can be replaced by a tunable laser at the transition to larger distances ⁇ z from the outlet opening, which are in the range of the distance x ⁇ .
  • 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 ⁇ 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 ⁇ s 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.
  • 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.
  • the actuating device for driving the actuating element has a piezo element.
  • 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.
  • a nozzle can also achieve short switching times and high repetition frequencies with a long service life.
  • 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 ⁇ s). In order to enhance the effect of this measure, a high drawing tension is advantageously used to accelerate the sample molecule ions.
  • 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.
  • 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.
  • a perforated diaphragm is used, the diaphragm opening of which has a diameter of less than approximately 8 mm, preferably approximately 5 mm.
  • 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.
  • a reflectron is used as the mass spectrometer.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • Fig. 1 is a partially sectioned perspective
  • 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;
  • FIG. 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • valve body 136 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.
  • 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.
  • valve body 136 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.
  • the movement device 148 can comprise, for example, an electromagnet or a piezoelectric drive element.
  • 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.
  • 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 ⁇ 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.
  • 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.
  • 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.
  • the method according to the invention is carried out as follows:
  • 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)
  • P 0 for example 1.013 ⁇ 10 5 Pa (1 atm)
  • D for example 0.5 mm
  • 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 ⁇ of the carrier gas particles is significantly smaller than the diameter D of the outlet opening 132.
  • 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 .
  • 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.
  • 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 ⁇ 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 ⁇ , 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.
  • the sample molecules are advantageously ionized at a distance x ⁇ 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.
  • 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.
  • the sample molecules are advantageously ionized at a distance Xj from the outlet opening 132 that is smaller than the distance x ⁇ .
  • 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.
  • the distance ⁇ ⁇ of the intersection point 206 from the outlet opening 132 is chosen to be less than approximately x ⁇ , in particular less than approximately 0.8 x ⁇ , preferably less than approximately 0.5 x ⁇ .
  • a tunable laser is used as laser 200 and the ionization distance x x is between approximately 0.5 x ⁇ and approximately 1.0 x ⁇ , in particular between 0.8 x ⁇ and 1, 0 x ⁇ , preferably chosen between approximately 0.9 x ⁇ and 1.0 x ⁇ .
  • 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 ⁇ 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 ⁇ 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):
  • 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
  • the distance x ⁇ at which the terminal Mach number M ⁇ 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 ⁇ in equation (II) and substituting M ⁇ with the right-hand side of the equation (I). You can find the relationship like this:
  • any ionization distances ⁇ z can be realized.
  • control unit 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.
  • the laser pulse is triggered a few ⁇ s after opening the valve nozzle 128.
  • a timer (not shown) is reset and started.
  • 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.
  • REMPI resonance-amplified multiphoton ionization
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • valve nozzle 128 is designed so that the carrier gas pulse is as short as possible, preferably shorter than approximately 20 ⁇ s.
  • 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.
  • 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.
  • 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.

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  • Analytical Chemistry (AREA)
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Abstract

L'invention concerne un procédé pour détecter la présence de molécules d'échantillons dans un gaz porteur. Selon ce procédé, un jet divergent de gaz porteur est produit sous vide par expansion du gaz porteur à travers un ajutage, les molécules d'échantillons sont ionisées dans une zone d'ionisation du jet de gaz porteur par absorption de photons pour donner des ions de molécules d'échantillons. Ces derniers traversent ensuite un champ électrique clarificateur dans un spectromètre de masse où ils sont détectés. L'invention vise à améliorer ledit procédé de sorte que la distance entre l'ouverture de sortie de l'ajutage et la zone d'ionisation puisse être sélectionnée librement. A cet effet, les ions de molécules d'échantillons passent dans le spectromètre de masse essentiellement le long de la direction axiale du jet de gaz porteur.
PCT/EP1998/008069 1997-12-18 1998-12-10 Procede et dispositif pour detecter la presence de molecules d'echantillons dans un gaz porteur WO1999033089A2 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE19756444A DE19756444C1 (de) 1997-12-18 1997-12-18 Verfahren und Vorrichtung zum Nachweis von Probenmolekülen in einem Trägergas
DE19756444.5 1997-12-18

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WO1999033089A2 true WO1999033089A2 (fr) 1999-07-01
WO1999033089A3 WO1999033089A3 (fr) 1999-09-30

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DE19957256A1 (de) * 1999-11-27 2001-06-28 Deutsch Zentr Luft & Raumfahrt Verfahren und Vorrichtung zur Steuerung der Zusammensetzung eines Produktstromes
DE10014847A1 (de) * 2000-03-24 2001-10-04 Gsf Forschungszentrum Umwelt Verfahren und Vorrichtung zum Nachweis von Verbindungen in einem Gasstrom
DE10247272B4 (de) * 2002-10-10 2011-11-17 Eads Deutschland Gmbh Spektrometer zur Gasanalyse

Citations (3)

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Publication number Priority date Publication date Assignee Title
US5037062A (en) * 1988-10-20 1991-08-06 Deutsche Forschungsanstalt Fur Luft- Und Raumfahrt E.V Quick acting ball valve
US5206594A (en) * 1990-05-11 1993-04-27 Mine Safety Appliances Company Apparatus and process for improved photoionization and detection
US5629518A (en) * 1994-11-25 1997-05-13 Deutsche Forschungsanstalt Fuer Luft-Und Raumfahrt E.V. Process and apparatus for detecting sample molecules in a carrier gas

Patent Citations (3)

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Publication number Priority date Publication date Assignee Title
US5037062A (en) * 1988-10-20 1991-08-06 Deutsche Forschungsanstalt Fur Luft- Und Raumfahrt E.V Quick acting ball valve
US5206594A (en) * 1990-05-11 1993-04-27 Mine Safety Appliances Company Apparatus and process for improved photoionization and detection
US5629518A (en) * 1994-11-25 1997-05-13 Deutsche Forschungsanstalt Fuer Luft-Und Raumfahrt E.V. Process and apparatus for detecting sample molecules in a carrier gas

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
BOESL U ET AL: "LASER ION SOURCES FOR TIME-OF-FLIGHT MASS SPECTROMETRY" INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES, Bd. 131, 1. Januar 1994 (1994-01-01), Seiten 87-124, XP000446266 ISSN: 0168-1176 *
NEUSSER H J ET AL: "DECAY ENERGETICS OF MOLECULAR CLUSTERS STUDIED BY MULTIPHOTON MASS SPECTROMETRY AND PULSED FIELD THRESHOLD IONIZATION" INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES, Bd. 131, Nr. 1/03, 24. Februar 1994 (1994-02-24), Seiten 211-232, XP000446270 ISSN: 0168-1176 *

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DE19756444C1 (de) 1999-07-08

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