GB2147734A - Flow contoured electron capture detector cell - Google Patents
Flow contoured electron capture detector cell Download PDFInfo
- Publication number
- GB2147734A GB2147734A GB08420118A GB8420118A GB2147734A GB 2147734 A GB2147734 A GB 2147734A GB 08420118 A GB08420118 A GB 08420118A GB 8420118 A GB8420118 A GB 8420118A GB 2147734 A GB2147734 A GB 2147734A
- Authority
- GB
- United Kingdom
- Prior art keywords
- cell
- gas
- detector
- electron capture
- tubular
- Prior art date
- Legal status (The legal status 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 status listed.)
- Granted
Links
- 230000005264 electron capture Effects 0.000 title claims abstract description 20
- 230000002285 radioactive effect Effects 0.000 claims abstract description 10
- 239000011888 foil Substances 0.000 claims abstract description 8
- 238000000034 method Methods 0.000 claims description 11
- 239000002184 metal Substances 0.000 claims description 7
- 238000004458 analytical method Methods 0.000 claims description 4
- 238000004817 gas chromatography Methods 0.000 claims description 4
- 238000009877 rendering Methods 0.000 claims description 3
- 239000007789 gas Substances 0.000 abstract description 18
- 239000012159 carrier gas Substances 0.000 abstract description 6
- 238000011109 contamination Methods 0.000 abstract description 2
- 238000013461 design Methods 0.000 description 9
- 230000000694 effects Effects 0.000 description 5
- 239000000919 ceramic Substances 0.000 description 4
- 238000001211 electron capture detection Methods 0.000 description 3
- 239000012212 insulator Substances 0.000 description 3
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 2
- 230000003247 decreasing effect Effects 0.000 description 2
- 229910052739 hydrogen Inorganic materials 0.000 description 2
- 239000001257 hydrogen Substances 0.000 description 2
- 230000035945 sensitivity Effects 0.000 description 2
- YZCKVEUIGOORGS-NJFSPNSNSA-N Tritium Chemical compound [3H] YZCKVEUIGOORGS-NJFSPNSNSA-N 0.000 description 1
- 150000001298 alcohols Chemical class 0.000 description 1
- 150000001350 alkyl halides Chemical class 0.000 description 1
- PNEYBMLMFCGWSK-UHFFFAOYSA-N aluminium oxide Inorganic materials [O-2].[O-2].[O-2].[Al+3].[Al+3] PNEYBMLMFCGWSK-UHFFFAOYSA-N 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000001627 detrimental effect Effects 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 150000004820 halides Chemical class 0.000 description 1
- 229930195733 hydrocarbon Natural products 0.000 description 1
- 150000002430 hydrocarbons Chemical class 0.000 description 1
- 238000012905 input function Methods 0.000 description 1
- 238000003780 insertion Methods 0.000 description 1
- 230000037431 insertion Effects 0.000 description 1
- 150000002576 ketones Chemical class 0.000 description 1
- 238000005259 measurement Methods 0.000 description 1
- 150000002894 organic compounds Chemical class 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000000575 pesticide Substances 0.000 description 1
- 239000012898 sample dilution Substances 0.000 description 1
- 238000004454 trace mineral analysis Methods 0.000 description 1
- 229910052722 tritium Inorganic materials 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
- G01N27/64—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using wave or particle radiation to ionise a gas, e.g. in an ionisation chamber
- G01N27/66—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using wave or particle radiation to ionise a gas, e.g. in an ionisation chamber and measuring current or voltage
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
- G01N30/02—Column chromatography
- G01N30/62—Detectors specially adapted therefor
- G01N30/64—Electrical detectors
- G01N2030/642—Electrical detectors photoionisation detectors
Abstract
A small volume electron capture detector has a cylindrical cell 120 inside which there is provided a generally funnel-shaped insert structure 130 having a cup-shaped section and a cylindrical section, and a radioactive foil 125. The rims of the cup-shaped section nearly touch the inner walls of the cell 120 so as to separate the active volume from the areas at the top corners of the cell 120 which are not actively swept by the carrier gas, thus reducing the tailing of chromatographic peaks. Make-up gas entering through ports 122 sweeps the sample only into the central region of the cell. Contamination of foil 125 by the sample is also reduced. An alternative structure (230) is disclosed (Fig 4). <IMAGE>
Description
SPECIFICATION
Flow contoured electron capture detector cell
This invention relates generally to a small volume electron capture detector cell and in particular to methods of minimizing or eliminating mixing effects in such a cell.
By the electron capture detection technique in gas chromatography, a tritium or Ni63 source ionizes the molecules of a carrier or make-up gas as it flows through the detector and the slow electrons thus produced are caused to migrate to the anode, forming a steady or pulsed current. This current becomes reduced if a sample containing electron absorbing molecules is introduced and this loss of current can be amplified by an electrometer for analysis.
The electron capture detector is extremely sensitive to certain molecules such as alkyl halides, but is relatively insensitive to hydrocarbons, alcohols, ketones, etc. This selective sensitivity to halides makes the detection method especially valuable for the trace analysis of many environmentally important organic compounds, such as pesticides. Electron capture detectors, however, have not been used extensively in conjunction with high resolution capillary columns. They have often been considered to be too large in volume to be suitable for use with high resolution systems, and the detector cell generally contained regions not actively swept by the carrier gas. The latter phenomenon is sometimes called the mixing effects, or mixing volume effects, and such unswept regions are known to cause tailing of chromatographic peaks.The degree to which mixing occurs within the cell is dependent upon cell design and gas flow rate, this problem generally increasing as the length-to-diameter ratio (L/D) of the cell decreases.
According to one aspect of the invention there is provided an electron capture detector cell as set out in claim 1 of the claims of this specification.
According to another aspect of the invention there is provided a method of rendering an electron capture detector suitable for high resolution analysis as set out in claim 6 of the claims of this specification.
Examples of the prior art and of the invention will now be described with reference to the accompanying drawings, in which:
Fig. 1 is a schematic cross-sectional view of a prior art electron capture detector;
Fig. 2 is a schematic cross-sectional view of an electron capture detector of the present invention;
Fig. 3 is a comparison of measured response between electron capture detectors of Fig. 1 and Fig.
2;
Fig. 4 is another embodiment of electron capture detector cell according to the present invention.
There is shown in Fig. 1 the general design of a prior art electron capture detection system (such as the commercially available one disclosed by P.L.
Patterson in J. Chromatogr., 134 (1977) at page 25).
The top portion of a gas chromatography column 11 through which the sample to be analyzed is led into the detector is housed concentrically inside an inlet tube 12 so as to form a passageway 13 having an annular cross section between the inner wall of the inlet tube 12 and the outer wall of the column 11. This passageway 13 is for a make-up gas, the use of which may become necessary, when a capillary column is used, in order to push the column gas (sample with a carrier gas) into the detector. The make-up gas then becomes mixed with the gas from the column 11. A generally cylindrical metal anode 15 is connected to the upper end of the inlet tube 12, separated therefrom by a ceramic insulator 16. The other end of the anode 15 opens into a cylindrical cell 20 (of length L and diameter D), separated therefrom by another ceramic insulator 21.The top end of the cylindrical anode 15 is provided with side ports 22. Thus, the sample from the column 11 and the make-up gas from the passageway 13 become mixed together as they travel upwards through the cylindrical anode 15, entering the interior of the cell 20 from below, some of this mixed gas passing through the side ports 22. On the inner wall of the cell 20 is a radioactive foil 25 which, for example, may be a
Ni63 or H3 source. The top of the cell 20 is connected to an exit tube 26.
The prior art electron capture detector of Fig. 1 has several disadvantages. Firstly, because the sample from the column 11 is made to pass through the metal anode 15 before entering the detector cell 20, there results a sample loss by absorption and this can cause chromatographic peak broadening. Secondly, a sample loss by absorption occurs also on surfaces within the cell 20 especially when they are activated by hydrogen. Even when hydrogen is not used as the carrier gas, the presence of hot metal or ceramic surfaces with which the sample can come in contact should be expected to have detrimental effects. Thirdly, the make-up gas, when its use is necessary, tends to dilute the sample, decreasing the sensitivity of the detector.Fourthly, the detector cell 20, according to the prior art design as shown, includes regions at the top corners which are not actively swept by the carrier gas. An electron capture detector is generally sensitive to oxygen, and it is therefore necessary to prevent its back diffusion by increasing the length-to-diameter ratio of the exit tube 26.
This necessarily tends to enlarge such unswept areas especially when the length-to-diameter ratio (L/D) of the cell 20 is decreased.
An electron capture detection system according to the present invention is shown in Fig. 2 wherein components which are identical or comparable to a component in Fig. 1 are given a three-digit numeral of which the last two are identical to those of the corresponding component in Fig. 1. In this embodiment, an insulating tube 114 of high purity alumina is positioned inside the inlet tube 112 and the anode 115 which are separated by a ceramic insulator 116. This insulating tube 114 extends up to a point just below the side ports 122 which are provided near the top of the anode 115.The gas chromatography column 111 through which the sample is led into the detector extends higher than in Fig. 1 and reaches beyond the side ports 122 so that only the make-up gas introduced through the annular passageway 113 between the outer wall of the column 111 and the inner wall of the insulating tube 114 will enter the detector cell through the side ports 122. These changes in the inlet system from Fig. 1 are intended to cause the make-up gas to sweep the sample only into the central region of the cell, minimizing the sample dilution by preventing the complete mixing with the make-up gas and reducing the contamination by the sample of the radioactive foil 125 which is disposed on the inner wall of the generally cylindrical cell 120. The top of the cell 120 is connected to an exit tube 126.
Placed inside the cell 120 is a metallic structure 130, the purpose of which is to limit the active volume of the detector 120, defined as the region from which electrons are collected for measurement, to below this structure 130, thereby separating the unswept areas from the active volume. For this purpose, this structure 130 is made of a conductive metal and maintained at the same potential as the side walls of the cell 120, or the radioactive foil 125, for example, by electrically connecting to the latter. The structure 130 is generally shaped like a funnel, having a cylindrical section and a cup-shaped section. The cup-shaped section faces the top end of the column 111, while the cylindrical section which serves as a gas conduit, points upward to the exit tube 126.The cup-shaped section is so designed and positioned that its rims are closely adjacent to, but not completely touching, the radioactive foil 125 so that gas can pass through the gap therebetween, although a majority of the gas introduced into the detector cell 120 will be caused to pass through the cylindrical section of the structure 130. Thus, the top corners of the cell 120 which are not actively swept by the carrier gas, and hence previously referred to as the unswept areas, are effectively separated by the structure 130 from the region below which is approximately bounded by the inner surface of the cup-shaped section of the structure 130, a lower portion of the radioactive foil 125 and the top of the inlet system. The inner surface of the cupshaped section may be tapered appropriately so as to streamline the gas flow through the center.With the insertion of this structure 130, therefore, mixing effects in the top corners of the cell 120 near the exit tube 126 are no longer of any significance because they do not occur within the active region.
Fig. 3 shows the effect of inserting the structure 130 on the measured response. Curve 1 is for a flame ionization detector and is assumed to properly represent the actual input function to the detector. Curve 2 is for a 100-microliter electron capture detector of design according to Fig. 2, while Curve 3 is for a commercially available 350microliter electron capture detector of prior art design according to Fig. 1. The flow rate for all three
Curves is 10 millilitersiminute. The reduction in peak tailing in the case of Curve 2 is to be noted with respect to Curve 3.
Fig. 4 shows another design for the insert structure 230. In this design, the cup-shaped section is more cylindrical than conical as in Fig. 2 and there is provided a metal screen 231 at the lower end of the cup-shaped section to further serve to define the active region of the cell.
The present invention has been described above in terms of but a few embodiments. They, however, are intended to be illustrative rather than limiting and the disclosure is accordingly to be construed broadly. For example, the figures are to be understood only as being schematic so that they do not necessarily represent the true or intended dimensional relationships. The radioactive source may be placed differently and the design of the inlet system may be modified. Neither the use of a make-up gas nor a particular design of the inlet system is a requirement. The scope of the invention is limited only by the following claims.
Claims (13)
1. An electron capture detector cell comprising a tubular structure having an entrance end and an exit end for a gas to pass through from said entrance end to said exit end, a radioactive source inlet said tubular structure, and a means for defining an active volume of said cell within said tubular structure, regions inside said tubular structure which are not actively swept by said gas being separated from said active volume.
2. The detector cell of claim 1 wherein said defining means is a funnel-shaped structure having a tubular section and a cup-shaped section, said funnel-shaped structure being so positioned that said cup-shaped section faces said entrance end and that said tubular section faces said exit end.
3. The detector cell of claim 2 wherein the rims of said cup-shaped section are adjacent the inner surface of said tubular structure so that only a small portion of said gas passes between said rims and said inner surface of said tubular structure, a large portion of said gas flowing through said tubular section.
4. The detector cell of claim 1 wherein said radioactive source is a radioactive foil disposed on the inner surface of said tubular structure.
5. The detector cell of claim 2 wherein said cupshaped section has a metal screen.
6. In an electron capture detector for gas chromatography including a tubular cell through which a gas is passed, a method of rendering said detector suitable for high resolution analysis, said method comprising the step of defining an active volume within said cell by separating therefrom unswept regions within said cell which are not actively swept by said gas.
7. The method of claim 6 wherein electrons to be captured and measured by said detector are collected from said gas only inside said active volume.
8. The method of claim 6 wherein said defining step includes installing inside said cell a means for causing a major portion of said gas to pass through said cell without moving through said unswept regions.
9. The method of claim 6 wherein said defining step includes installing inside said cell a funnelshaped structure.
10. The method of claim 9 wherein said structure is so shaped and positioned that a major portion of said gas entering said cell passes through said structure.
11. The method of claim 9 wherein said structure comprises a metal screen for defining a boundary surface of said active volume.
12. An electron capture detector cell substantially as hereinbefore described with reference to and as illustrated in Figures 2 and 3 or Figure 4 of the accompanying drawings.
13. A method of rendering an electron capture detector suitable for high resolution analysis substantially as hereinbefore described with reference to the accompanying drawings.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US52208183A | 1983-08-11 | 1983-08-11 |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8420118D0 GB8420118D0 (en) | 1984-09-12 |
| GB2147734A true GB2147734A (en) | 1985-05-15 |
| GB2147734B GB2147734B (en) | 1987-03-04 |
Family
ID=24079389
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB08420118A Expired GB2147734B (en) | 1983-08-11 | 1984-08-08 | Flow contoured electron capture detector cell |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPH071249B2 (en) |
| DE (1) | DE3429479C2 (en) |
| GB (1) | GB2147734B (en) |
| IT (1) | IT1174647B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0831325A1 (en) * | 1996-09-03 | 1998-03-25 | Hewlett-Packard Company | Method and apparatus for ion discrimination in an electron capture detector |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1186525A (en) * | 1966-09-12 | 1970-04-02 | Gen Electric | Improvements in "Ion Chamber Detector" |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4304997A (en) * | 1979-02-27 | 1981-12-08 | Hewlett-Packard Company | Electron capture detector with thermionic emission electron source |
| JPS5637551A (en) * | 1979-09-05 | 1981-04-11 | Hitachi Ltd | Ionization detector of electron capture |
-
1984
- 1984-08-08 GB GB08420118A patent/GB2147734B/en not_active Expired
- 1984-08-08 JP JP59165025A patent/JPH071249B2/en not_active Expired - Lifetime
- 1984-08-10 IT IT22310/84A patent/IT1174647B/en active
- 1984-08-10 DE DE3429479A patent/DE3429479C2/en not_active Expired - Fee Related
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1186525A (en) * | 1966-09-12 | 1970-04-02 | Gen Electric | Improvements in "Ion Chamber Detector" |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0831325A1 (en) * | 1996-09-03 | 1998-03-25 | Hewlett-Packard Company | Method and apparatus for ion discrimination in an electron capture detector |
Also Published As
| Publication number | Publication date |
|---|---|
| DE3429479A1 (en) | 1985-02-21 |
| IT8422310A0 (en) | 1984-08-10 |
| IT1174647B (en) | 1987-07-01 |
| DE3429479C2 (en) | 1995-11-02 |
| GB2147734B (en) | 1987-03-04 |
| GB8420118D0 (en) | 1984-09-12 |
| JPH071249B2 (en) | 1995-01-11 |
| JPS6060554A (en) | 1985-04-08 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 746 | Register noted 'licences of right' (sect. 46/1977) |
Effective date: 19980921 |
|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 20010808 |