JPS62294961A - Detector - Google Patents

Abstract

PURPOSE:To enable highly accurate detection of a sample gas ionized, by providing a means to turn ON or OFF discharge of ultraviolet rays in a short time than the transition time of a detection gas by ultraviolet rays in a windowless light ionization detector. CONSTITUTION:A gas for generating ultraviolet rays is supplied into a container 11 at an inlet 16 and a potential sufficient to generate ultraviolet rays is supplied between a cathode 18 and an anode 26. At the same time, a sample gas containing a gas to be inspected is introduced together with a carrier gas through an inlet 24. The sample gas is ionized by the ultraviolet rays to be collected with a collector electrode 28. The pulse frequency of a power source which applys a voltage to the cathode 18 and the anode 26 is made shorter than the transition time of ions. This suppresses background as caused by the ionization of released gas from the inside of a container 11 and others.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Field of Industrial Application] The present invention relates to gas chromatography, and particularly to a detection device for detecting a low concentration of a specific gas in a sample gas in gas chromatography.

[Prior art and its problems]

photoionization detector
detector) is one type of detector used in this field and ranges from about 10-4 to IQ-1'G.
The gas to be located can be detected with a concentration of 10. There are two types of photoionization detectors, one of which is a "window-type" detector that can operate at atmospheric pressure.
, the other is a "windowless" detector designed to operate at low pressures, i.e., below 50 Torr.
``indowless-type'' detector)
It is. The present invention primarily relates to "windowless" detectors.

Windowless photoionization detectors typically include a hollow vessel, such as a quartz tube, with an outlet port for connecting a low-pressure vacuum pump, and a gas source at one end of the tube that supplies ultraviolet light. and a sample gas inlet at the other end of the tube.

A collimator is provided between the ends of the quartz tube (glass or a suitable metal can also be used as tube material) to collimate the ultraviolet radiation along the tube toward the sample gas inlet. A cathode is typically located at the gas source, such as the inlet where the source gas enters a quartz tube, and an anode is located at the mouth of the sample gas inlet. An ion chamber separated by a metal cylinder
amber) longitudinally surrounds the mouth of the sample gas inlet and is normally held at ground potential with respect to the anode.

In detector operation, the ion chamber cylinder collects ions generated by radiation acting on the sample gas, and a monitor converts the electrical current due to the collected ions into a signal indicating the presence or absence of a particular gas. and connected to the measuring device.

A drawback that has hindered the wide application of windowless photoionization detectors is the background current (back-gro
and current) is at a relatively high level, which causes noise in the received signal. The observed noise level imposes severe limits on the minimum detectable signal.

The background current is photoemission from electrodes and other internal surfaces described below.

(b) Ionizing collisions between metastable atoms or molecules and the surface or other internal surface of the detector electrode.

(C) Ions entering the ion chamber from the ultraviolet source.

All these effects increase the noise due to the instability of the UV source.

[Purpose of the invention]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned problems and provide a detection device capable of detecting ionized sample gas with high precision.

[Summary of the invention]

The present invention provides a detection apparatus for detecting gas in a sample gas within a carrier gas stream. The apparatus may connect a cathode, a first inlet into which a source gas can be introduced, a second sample gas and carrier gas inlet, and vacuum pump means for supplying ultraviolet light. a photoionization detector consisting of a container consisting of a collector electrode means for collecting the ions produced by the ionization, and vacuum pump means for maintaining a pressure between 0.1 Torr and 50 Torr; means for alternately switching the power supply to the cathode on and off at a rate whose period is shorter than the transition time of the ions to be detected.

The ambient temperature of the photoionization detector described above was increased from 200°C to 30°C.
It is desirable to provide a means to keep it within the range of σ′.

In this device, it is desirable that the port be installed at a position where metastable substances present in the detector can be exhausted.

The cathode is preferably provided in a neutralization chamber housed in the first chamber of the container.

The first and second inlets are collinear to collinear the ultraviolet radiation emitted by the gas in the gas source during use of the detector to produce a beam collinear with the first and second inlets. Preferably, means are provided to enable the ultraviolet light to be directed into the sample gas entering the second chamber of the detector through the second inlet.

Furthermore, by connecting with collector electrode means and collecting ions from behind,
amplification means for amplifying the generated signal; gate means connected to the amplification means for gating the output from the amplification means to identify a signal current due to the signal; low-pass filter means connected to the gate means; and means for monitoring or detecting the output of the filter means to indicate that the presence of a predetermined gas in the sample gas has been detected.

Furthermore, delay means may be provided, which are connected between the gate means and the alternate switching means and provide a feedback loop to stabilize the switching of the power supply.

The present invention further provides means capable of generating a collimated beam of ultraviolet light and a sample gas/carrier gas inlet arranged such that the ultraviolet light can be directed to the gas entering the detector through the inlet. a port to which a vacuum pump means can be connected; and collector electrode means for collecting ions generated by ionization within the detector, the inlet and the collector electrode being spaced apart within the ionization chamber; Means may also be provided for providing a constant potential gradient between the collector electrode and the collector electrode.

In the above-described device, the means for providing a constant potential gradient is further arranged regularly in a coaxial relationship between the inlet and the collector electrode and electrically arranged to control the rate of movement of the electric potential towards the collector electrode. It is desirable to have a plurality of annular electrodes connected by +.

Preferably, the annular electrode is placed within the cylindrical chamber of the detector between the collector electrode and another electrode located adjacent to the inlet.

The means for supplying a constant potential gradient may alternatively comprise ceramic tubes connected thereto with means for causing a low current to flow along the tube.

The present invention further provides a windowless photoionization detector including a first inlet through which ionizing radiation is allowed to enter the chamber, and a first inlet through which sample gas enters the chamber through which the sample gas is ionized by the ionizing radiation. an ionization chamber consisting of a second inlet that can be used, and - two spaced apart electrodes, one of which is a collector electrode, arranged regularly in a coaxial relationship between the two spaced apart electrodes; It includes a plurality of annular electrodes that provide a constant electric field to control the rate of movement of ions toward the collector electrode.

The noise level of the UV source can be lowered, such as by using hollow electrodes or alternatively by using an inductively or capacitively coupled plasma for UV generation.

[Embodiments of the invention]

In the following, two illustrated preferred embodiments of the invention will be described in detail. It is clear that these are examples chosen to illustrate the subject invention in detail and are not intended to limit the interpretation of the subject invention.

In FIG. 1, a windowless photoionization detector suitable for the present invention is shown. It is made of glass or quartz and is itself a tabular por-tion.
) 12 and a conical pedestal 14. The plate-shaped portion 12 has an inlet 16 through which the gas of the gas source supplying ultraviolet light is introduced into the container 11. Inside the plate-shaped part 12 is a hollow cathode 1 which is connected to a power source (shown in FIG. 2) and emits stable radiation with high intensity from a gas source.
Sun is installed. The plate-shaped portion 12 also reduces the pressure inside the container 11 to 0. It is equipped with an output boat 20 connected to a vacuum pump (not shown) that can maintain between ITorr and 50Torr.

A collimator 22 is mounted at or near the junction of the two parts of the container 11 with an opening coaxial with the inlet 16 along the axis of the plate 12 of the container 11 .

The conical portion 14 of the container 11 is provided with an inlet 24 concentric with the collimator opening and inlet 16. Inside the inlet 16 is an anode 2 made of platinum or nickel wire.
6 is installed and extends from the mouth of the inlet 24 into the conical pedestal part 14. A collector electrode 28 circumferentially surrounding the anode 26 also has a truncated conical shape and is arranged to collect the ionizing current. The collector electrode 28 has a ground potential,
In other words, since it is kept negative with respect to the anode 26, the electrode 2
There is an applied electric field between 6 and 28 that is sufficient to collect ions but creates a strong ionizing field (field 1ntensi-fied 1
(onization) The throat is not thick.

When using a photoionization detector, the source gas is supplied through the inlet 16. The gases most commonly used to provide ultraviolet light are helium, argon, and hydrogen. A potential sufficient to supply ultraviolet light is applied between cathode 18 and anode 16 by power supply 30 (shown in FIG. 2). This power supply 30 can be operated to block ultraviolet radiation, such as by modulating or pulsating the electrical energy supplied to it, for reasons explained below.

At the same time as the ultraviolet light is maintained, a sample gas containing the gas to be detected is passed along the detector through inlet 24 to a gas substantially transparent to the ultraviolet light, such as nitrogen,
Intermittently introduce air, water vapor or carbon dioxide into the carrier gas stream. The sample gas contains the most abundant organic vapor (
orgsoc vapoues) as well as C3
It may contain any one of a number of inorganic gases and vapors, including z, NO, NH:l and H2S.

The ionizable components of the sample gas are ionized by the ultraviolet light as the sample gas exits the inlet 24 and moves toward the collector electrode 28;
It generates a current that is collected at collector electrode 28 and applied as a signal to a power amplifier.

The ultraviolet radiation" can also cause photoelectric emission from the electrodes and similar emissions from other surfaces, such as the inner surface of the container 11. Ions can also enter the ion chamber 29 from the ultraviolet source.

Although such emission in the plates 12 of the container 11 can be limited to some extent by the collimator openings, the effect occurring in the frustum 14 causes a charge to collect on the surface of the electrodes 28. In addition, atoms of rare gases, which can be introduced by using ultraviolet light as a gas, can be easily excited to their metastable state by electron bombardment. Ionizing collisions occur between these metastable atoms, or between carrier gas molecules, and between positive and other surfaces, generating ions. This ion is also collected at electrode 28. If the radiation is continuous and uninterrupted, as is the case with conventional techniques, each of these additive effects will generate background noise, e.g. the signal corresponding to the sample gas to be detected. Superimposition of the desired signal can obscure the signal to an extent that seriously affects the detection capability of the detector.

These undesired effects can be overcome by a detection device according to the invention, such as that shown in FIG.

A feature of this device is that it is equipped with a power source that operates the ultraviolet source intermittently. By this simple means, different migration speeds of various emitters and ions (migra
As shown in FIG. 3, the respective generated signals can be separated by taking advantage of the advantages of “flash” the ultraviolet source
The photoelectron current is first detected due to the higher velocity of the electrons, but also due to the metastables and molecules cited above, or the ions from the ultraviolet source. The signal corresponding to the current generated by This is due to the fact that the distance of the gas source from the collector electrode is greater when compared to the distance traveled by the ion, and that if the metastable substance or molecule is heavier than the ion to be detected, the migration speed is 1 slower. Conceivable.

If the pulse rate is maintained such that each pulse period is shorter than the ion's transit time (eg, 1710 milliseconds), each effect can be separated as shown in FIG. In FIG. 3, the signal due to the photoelectric current is shown by the square wave section A, and the signal resulting from the detection and
This is due to the resulting secondary effects.

Any effects due to the presence of metastables can be significantly reduced by bringing the ambient temperature of the photoionization detector to a temperature range of 200°C to 300°C. At such high temperatures, metastable materials rapidly disappear. Another method of reducing the effects of metastables is to place the pump port 20 at a location where the metastables can be evacuated from the container 10.

In FIG. 2, the amplified output signal from amplifier 32 is gated by gating circuit 34 so that the signal current can be isolated from other effects. The circuit has an output to a low pass filter 36 and an output to a delay circuit 38 in a feedback loop to source 30.

Intrinsic noise directly resulting from fluctuations in UV levels can also affect the sensitivity of the instrument, and thus the ability to detect peaks in the observed signal, and thus the presence of gas, can be significantly affected.

To minimize the effects of noise, control the level of radiation by feeding the output from the UV source to the gas source with a feedback loop (not shown) to stabilize the output and maintain it at a constant level. It can be used to

Second, given the speed at which the source gas is introduced into the detector, the radiation emitted by the gas can be kept at a constant intensity along with the noise caused by the radiation.

FIG. 4 shows a second embodiment of the photoionization detector used in the present invention. In FIG. 4, it consists essentially of two chambers 112 and 114 connected by a hollow tube 115. Reference numeral 112 has a cylindrical shape, and as in FIG. A hollow tubular cathode 118 is connected to the negative terminal of 30 in FIG.
A ter-shaped cathode is installed. The hollow tube 115 is provided with an outlet boat 120 for connecting a vacuum pump and may have a diameter such that the tube collimates the ultraviolet light emanating from the cathode 118. Instead, collimator 112 can be
It can be installed within 15.

Chamber 114 has an inlet 124 coaxial with the axis of tube 115 and cathode 118 . This chamber 114 has an upright cylindrical shape closed at both ends and is connected to a polariziB potential source (
A circular, flat plate-shaped electrode 126 is connected to a first terminal (not shown). A second electrode 128 similar to electrode 126 is attached to the top of chamber 114;
It is connected to a second terminal of a source of polarization potential via a conductor 131 extending through an insulator 133.

several annular electrodes 135a, 135b, 135c,
135d, 135e.

135f is installed. These annular electrodes are electrode 1
26 and the wall of the chamber 114, and the electrode 135f is provided on the surface of the electrode 128.

These are suitable resistors 1 connected between adjacent electrodes.
37a, 137b, 137c, 137d, 13
7e, installed by 137f. Electrode 135a~
135 is maintained at a controlled potential gradient, thus controlling the rate of ion transfer between electrodes 126 and 128. When using a detector, two electrodes 126 and 128
The potential between is dependent on the gas being ionized and the pressure maintained within the detector, and is typically 10 to 100 volts.

The greater the pressure and the heavier the ions collected by the electrode, the greater the potential difference. Electrode 126 is held at a positive or negative potential with respect to collector electrode 128 depending on the polarity of the ions being collected. An electric field gradient along which ions move can also be obtained with a slightly conductive ceramic tube along which a current flows. This avoids the use of multiple conductive rings which are quite expensive to assemble.

If a detector is used, the ultraviolet light is transmitted between the cathode 118 and the chamber 114.
Radiation is transmitted along the tube and is emitted by the gas flowing through the population 116 due to the potential difference between the sample and
The gas introduced through gas population 124 is ionized. Ions generated by ionization of the sample gas move toward collector electrode 128 as described in detail with reference to FIG. It may be advantageous to have a constant flow of gas moving against the ions in the drift tube.

〔Effect of the invention〕

As explained above, in the present invention, ultraviolet rays are intermittently supplied to the gas to be detected using a photoionization detector for a time shorter than the transition time, which is considered to be the cause of background noise (ultraviolet rays). photoelectron emission from electrodes or other surfaces, photocurrent peaks due to collisions of free electrons caused by excitation of ultraviolet ray-supplying gas, carrier gas, etc. to a metastable state, and current generated by ionization of the gas being detected. It becomes possible to separate each peak.

Also, by creating a feedback loop,
By controlling the output level of ultraviolet rays to a constant value, it is also possible to remove noise caused by fluctuations in the output level.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a windowless photoionization detector that constitutes the detection device of the present invention. FIG. 2 is a block diagram of a detection device according to the present invention. FIG. 3 is a diagram showing the characteristics of the detection device according to the present invention. FIG. 4 is a schematic diagram of a second bar windowless photoionization detector constituting the detection device of the present invention. 20.120: Exit port, 22,122: Collimator, 18°118: Cathode, 26.126.128:
Electrode, 29: ionization chamber.

Claims (1)

[Claims] In the detection device including a windowless photoionization detector and a pump means, means is provided for controlling the discharge of the ultraviolet rays on/off in a time shorter than the transition time of the detection gas due to the ultraviolet rays in the windowless photoionization detector. A detection device characterized by: