JPH0376415B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an ionization detector used in a gas chromatograph, and particularly to a photoionization type ionization detector.

In general, an ionization type detector ionizes sample components in a carrier gas flow and measures the ion current, and a photoionization type detector uses light energy as a means to ionize the components. There have been various types of ionization detectors in the past, but the following have mainly been put into practical use.

(1) Flame ionization detector. This has already been described in the following document published in 1958 and has been widely put into practical use.

Mc.Williams and Dewar “Gas”
Chromatography" 1958, Desty, London. However, since this detector uses _H2 , it cannot be used in areas where there is a risk of explosion. It also has the weakness of not being able to detect formic acid or formaldehyde because it does not respond to them.

(2) An ionization detector that uses light in the ultraviolet region generated by glow discharge or microwave discharge in He or Ar under atmospheric pressure, and is described in the following literature.

JEL Lovelock “Anal.Chem.33, 163 (1961)”
Or Yamane “J.Chromatography 9192
(1962)" However, it is difficult to obtain a stable signal with this detector. The following research was conducted to improve this.

(3) Photoionization detection using ultraviolet light generated by discharge under reduced pressure.

JGWPvice “Anal.Chem 40541 (1968)” or R.RFreeman and WE Wentworth “Anal.
Chem. 431987 (1971)" However, a vacuum pump is required to maintain a predetermined reduced pressure state at all times, making the device complex and difficult to operate. In addition, this photoionization detector is further developed by JNDricoll “J.Chroma to gvaphy 13449”.
(1977), research continued and it became possible to obtain a stable light source.

The method involves He and _H2 at a pressure of 0.1-10mmHg.
Alternatively, a mixed gas such as He and Ar is sealed in a container provided with a pair of electrodes. A glow discharge is generated between these electrodes to obtain vacuum ultraviolet light, which can be used to generate LiF or
Stable ion current measurements are made by irradiating a carrier gas stream through a MgF ₂ window. However, this method has the problem that the window materials LiF and MgF ₂ deteriorate, resulting in a short lifespan of several hundred hours.

It should be noted that the various ionization detectors mentioned above, except for (2), are all detectors for organic substances, and have the problem that they generally do not respond to inorganic gases.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a photoionization type ionization detector that is highly sensitive to a wide range of substances, has a relatively simple structure, and can provide stable detection signals.

[Summary of the invention]

In the photoionization type ionization detector of the present invention, the discharge cathode is arranged so that the tip of the discharge cathode extending in the light generation chamber is directed toward the ionization chamber,
The other positive electrode for discharge is placed closer to the ionization chamber than the tip of the negative electrode for discharge, the housing wall of the ionization chamber is formed as a collector counter electrode, and a negative potential of 30 to 200 V is applied to the positive electrode for discharge. It is characterized by being equipped with a battery that provides

[Embodiments of the invention]

FIG. 1 is a system diagram of a general gas chromatograph. The flow rate of the carrier gas is kept constant by a flow rate regulator 1, and flows out through a sample injection section 2, a chromatography column 3, and a detector 4. The output signal of this detector 4 is displayed on a recorder 5 as a chromatogram.

FIG. 2 is a block diagram of a photoionization type ionization detector which is an embodiment of the present invention, and the main body portion of the detector is shown in a sectional view. A carrier gas flow leaving the chromatographic column 3 is introduced through a gas conduit 12 fixed to the lower part of the housing 11 and exits through a gas outlet pipe 8 fixed to the upper part of the housing 11. The flow path in the carrier gas housing 11 serves as an ionization chamber 13, and the ions ionized here are supported at the center of the collector support tube 9 by insulating members 10a and 10b, and their tips are placed inside the ionization chamber 13. It is collected by the inserted collector electrode 7.
The ion current from the collector electrode 7 is sent to the current amplifier 28 via the collector contact plug 6, and its output signal is recorded on the recorder 5. Note that the collector support cylinder 9 is fixed to the upper part of the housing 11,
A collector connection plug 6 is attached to its upper end.

A horizontal hole is formed in the lower side wall of the housing 11, and a connecting fitting 14 is fixed to the entrance side of the horizontal hole. Further, a positive electrode 16 sandwiched between a pair of electrical insulating materials 15 is attached to the end face of the connecting fitting 14, and a negative electrode housing 23 is connected to the right side of the positive electrode 16 via a connecting tube 22. The cathode 19 supported by the high voltage connection plug 25 attached to the right end of the cathode housing 23 is connected to the light generation chamber 18.
The tip of the negative electrode 19 is inserted into the positive electrode 16.
It is close to the light passing hole 17 of. Also, this negative electrode 1
The middle part of 9 is supported on the inner surface of the connecting tube 22 by a fixing metal fitting 20 attached via an electrical insulating material 15, and the fixing metal fitting 20 is a connecting screw ring 24 screwed onto the inner surface of the connecting tube 22. It is sealed airtight.

Note that the discharge gas conduit 21 that introduces the discharge gas is attached to pass through the upper wall of the connection tube 22, so that the discharge gas can flow into the light generation chamber 18, and the high-pressure connection plug 25 has direct current. A controlled potential from a potential regulator 27 connected to a power source 26 is applied. Further, the negative electrode 19 and the positive electrode 16 are preferably made of platinum, and the interval between them is 1 to 1.
A light passing hole 17 with a diameter of 1.5 to 2 mm is provided to guide the light generated during that time to the ionization chamber 13.
is provided at the center of the positive electrode 16.

The operation of the photoionization type ionization detector constructed in this way will be described next. First, the gas conduit 12 is connected to the outlet of the chromatocolumn 3 of the gas chromatograph, and a constant flow rate of carrier gas is introduced, passed through the ionization chamber 13, and then discharged from the gas outlet tube 8. On the other hand, Ar or He gas is constantly introduced through the discharge gas conduit 21, passed through the ionization chamber 13 through the light passing hole 17, and discharged together with the carrier gas through the gas outlet tube 8.

Further, when a negative potential adjusted by the DC power supply 26 is applied to the negative electrode 19 via the high-voltage connection plug 25, a discharge occurs between the negative electrode 19 and the positive electrode 16, and strong light is emitted. This light enters the ionization chamber 13 through the light passage hole 17 of the positive electrode 16 and ionizes the sample gas in the carrier gas. Housing 1 surrounding this ionization chamber 13
The wall 1 serves as a counter electrode to the collector electrode 7, and its potential is grounded. On the other hand, the collector 7 is connected to a current amplifier 28 via an electrically shielded collector connection plug 6, and a negative or positive potential of several tens of V to about 100 V is applied to the collector 7. The ion current is recorded on the recorder 5 via the current amplifier 28.

As mentioned above, the components in the sample are ionized using the light energy generated by the gas discharge, and the amount of component gas is measured by measuring the ion current. The energy of ultraviolet light at this time is about 33.9×10 ^-19 J and is shifting toward the lower energy side. Therefore, it is possible to ionize all gas components except Ne.
Incidentally, it goes without saying that He is used as the carrier gas in this case. Next, an example of analysis using a photoionization type ionization detector will be shown.

FIG. 3 is an air chromatogram obtained by the photoionization type ionization detector shown in FIG. In this case, the chromato column 3 uses a molecular sieve 5A, and uses He as a carrier gas and discharge gas to detect Ne, H ₂ and large amounts of O ₂ and N ₂ . In order to ionize Ne in this way, it is necessary that the ionization potential is greater than its ionization potential of about 34.5×10 ^-19 J. On the other hand, the highest excitation energy of He is 33.9×10 ^-19 J as mentioned above, so it is necessary to provide energy larger than this.

Although not all the energy generated by gas discharge has been clearly elucidated, the energy of the primary ionization potential of He varies through about 10 different high energy levels before reaching the ultimate value of approximately 39.38×10 ^-19 J. metastable states exist, and the light energy level produced when these return to the ground state is
This can be interpreted as having enough energy to ionize Ne.

When actually measuring gases, N ₂ , O ₂ and
High sensitivity to inorganic gases such as H ₂ can be inconvenient. For example, this is the case when measuring a trace amount of organic gas present in the air.

FIG. 4 is a chromatogram of air using the photoionization type ionization detector shown in FIG. 2, and is an example of measurement of lower hydrocarbons. Since this organic gas overlaps with the output peak of N ₂ , it reduces the quantitative performance of methane, etc.

FIG. 5 is a chromatogram of air when Ar is used as the discharge gas. The reason why the peaks of N ₂ etc. were eliminated is that the energy of light obtained by discharging in an Ar atmosphere is approximately 18.58 × 10 ^-19 J, which is lower than the ionization energy of inorganic gases and that of CH ₄ organic gases. This is because the energy is higher than the ionization energy. In this case, the chromatography column 3 is a column filled with squalane-coated packing.

As mentioned above, this photoionization type ionization detector is
Detect trace amounts of Ne in the air with He discharge gas,
By replacing the discharge gas with Ar, it is possible to eliminate harmful substances such as N ₂ and detect trace amounts of organic gases, which expands the functionality of this detector and improves its detection ability. be.

Furthermore, an important feature of the present detector is that the light produced by the discharge is stable. That is, the discharge is stable. In the case of FIGS. 3 and 4 in which He is used as the discharge gas, the results are shown in which a limiting resistor for stabilizing the discharge is provided only on the discharge cathode side in the potential regulator 27. However, when Ar is used as the discharge gas, stability of discharge cannot be achieved with this alone, so in the device shown in FIG. Alternatively, a battery is provided that provides a negative potential of 30 to 200 V to the positive electrode 16 for discharge. FIG. 5 shows the results when Ar was used as the discharge gas in such a configuration.

FIG. 6 is a diagram showing the discharge current when a resistor is inserted only on the negative electrode side of the potential regulator shown in FIG. 2 and Ar discharge gas is used. Even if the resistance is changed to 1MΩ, 3MΩ, and 10MΩ, the fluctuation is large. In this case, the anode 17 side is directly grounded without inserting a resistor. To improve this, a resistor of 3KΩ or more is also inserted between the anode 16 and the ground point to stabilize the discharge.

FIG. 7 is a graph showing the relationship between the discharge current and the resistance connected between the positive electrode and the ground in FIG. 2, and shows the stable discharge region. According to experimental results, the discharge is stable when the resistance value is 3KΩ or more and the discharge current is 100μA or more.

The photoionization type ionization detector of this example combines an ionization detection section through which carrier gas passes after passing through a chromatography column, and a light emitting section that emits light that ionizes sample components into this ionization chamber. The positive electrode with a light passage hole facing the electrode is 3KΩ.
Organic and inorganic gases can be detected stably and with high sensitivity by selecting the discharge gas and carrier gas that are grounded through the above resistor and introduced into the light generation chamber where the negative electrode and the positive electrode are present. Furthermore, since there is no need to reduce the pressure using a vacuum pump, the device has advantages such as being simple and inexpensive.

In the above embodiment, the positive electrode of the light emitting part is grounded via a resistor, but it can also be stabilized by applying a negative potential of 30 V or more to the positive electrode side.

FIG. 8 is a graph showing the discharge stability range obtained by a photoionization type ionization detector according to another embodiment of the present invention, and the horizontal axis shows the potential of the positive electrode. That is, by installing a battery in the potential regulator 27 that provides a negative potential of 30 to 200 V to the positive electrode 16 side in FIG. 2, the discharge becomes stable when the discharge current is approximately 75 μA or more. We will consider this a little further.

Research on gas discharges by John Sealy Edward
The basic theory was established by Tounsend (1868-1957) and subsequent researchers. but,
When the pressure of the discharge body is high, such as under normal pressure, an unequal electric field is likely to occur on the discharge electrode surface or space charge, and phenomena that cannot be explained theoretically also occur. As described above, the electric resistance provided in the potential regulator 27 between the anode 16 and the ground point is
Suppresses the uneven electric field caused by the physical shape of the surface or the pressure of the discharge body under normal pressure, as well as the uneven electric field caused by the space charge that occurs at the same time as the discharge starts. It is thought that this action stabilizes the discharge.

The photoionization type ionization detector of this embodiment is capable of discharging even in the case of Ar discharge gas by interposing a resistor between the positive electrode and ground, or by applying a negative potential to the positive electrode and grounding it. A stable range is obtained.
This provides the effect that hydrocarbon gases can also be analyzed with high precision.

FIG. 9 is a chromatogram of a substance detected using the photoionization type ionization detector shown in FIG. As mentioned above, formic acid and formaldehyde cannot be detected with a flame ionization detector, so conventionally they were converted to methane by reduction with hydrogen and then measured indirectly with a flame ionization detector. However, if the photoionization type ionization detector of the present invention is used, direct measurement becomes possible as shown in FIG.

In this case, a glass column is filled with chromosolve coated with ethylene glycol (packing material for a gas chromatography column) as a separation column, and Ar is used as a carrier gas and a discharge gas.

〔Effect of the invention〕

The photoionization type ionization detector of the present invention can obtain a stable detection signal using light energy generated by stable discharge. Further, it is relatively simple and does not require a vacuum pump or the like, and other effects can be obtained.

[Brief explanation of drawings]

Figure 1 is a system diagram of a gas chromatograph, Figure 2 is a block diagram of a photoionization type ionization detector that is an embodiment of the present invention, and Figures 3, 4, and 5 are the apparatus shown in Figure 2. chromatogram, Figure 6 is the second
Insert a resistor only on the negative electrode side of the potential regulator shown in the figure.
A line diagram showing the discharge current when a discharge gas is used, FIG. 7 is a graph showing the relationship between the discharge current and the resistance inserted between the positive electrode and the ground in FIG. 2, and FIG. FIG. 9 is a graph showing the discharge stability range obtained by the photoionization type ionization detector of the example, and is another chromatogram obtained by the apparatus shown in FIG. 2. 1...Flow rate controller, 2...Sample injection section, 3...
Chromato column, 4...detector, 5...recorder,
6... Collector grounding plug, 7... Collector, 8
...Gas outflow pipe, 9...Collector support tube, 10...
... Insulating member, 11 ... Housing, 12 ... Gas conduit, 13 ... Ionization chamber, 14 ... Connection fitting, 15
... Electrical insulating material, 16 ... Positive electrode, 17 ... Light passing hole, 18 ... Light generation chamber, 19 ... Negative electrode, 20 ...
... Fixing fitting, 21 ... Discharge gas conduit, 22 ... Connection tube, 23 ... Cathode housing, 24 ... Connection screw ring, 25 ... High voltage connection plug, 26 ... DC power supply, 27 ... Potential regulator , 28... current amplifier.

Claims (1)

[Claims] 1 It has a light generation chamber equipped with a discharge cathode and a discharge positive electrode, and an ionization chamber equipped with a collector electrode and a collector counter electrode, and the discharge cathode is connected to a DC power supply via a limiting resistor, and the discharge Photoionization type ionization detection in which light generated due to discharge in the light generation chamber in the presence of gas is guided into the ionization chamber, and a sample introduced into the ionization chamber is ionized by the light from the light generation chamber. In the device, the discharge cathode is arranged so that the tip of the discharge cathode is directed toward the ionization chamber, and the discharge positive electrode is placed on a side closer to the ionization chamber than the tip of the discharge cathode. The housing wall of the ionization chamber is formed as the collector counter electrode, and a
A photoionization type ion detector characterized by being equipped with a battery that provides a negative potential of 200V.