GB2138997A - Dual-flame ionization detector for GC and LC eluent detection - Google Patents

Abstract

A dual-flame ionization detector is used for analysis of compounds in a chromatography column eluent. By properly controlling the chemical environment and combustion conditions of the upper flame 12 and lower flame 11 and by maintaining the upper collector electrode 45 adjacent the upper flame and the top part 26 of the upper burner at proper potentials with respect to the lower burner 16, it is possible to use the detector both in a mode in which response is based on carbon content and in a selective mode based on the presence of hetero atoms. <IMAGE>

Description

SPECIFICATION Dual-flame ionization detector for GC and LC eluent detection Background of the invention This invention relates to a dual-flame burner for use in the flame ionization detection ofchemical substances and more particularly to such a detector which can be used in conjunction with gas chromatography (GC) or liquid chromatography (LC).

The electrical conductivity of a flame is increased when certain chemical compounds are burnt therein and this is the basis of operation of the flame ionization detectors. Ionization detectors with one flame, for example, have been used for the detection of hydrocarbons in GC with carbon-containing compounds eluting from a GC column in vapor phase.

The compounds are decomposed and ionized in the flame and ionic species such as CHO+ are measured at the collector as a variance in electric current. Such single-flame ionization detectors as disclosed, for example, in an article titled "Direct Quantitative Analysis Using Flame lonisation Detection" by C.F.

Simpson and T. A. Gough (J. of Chromatographic Science, Vol. 19, 1981 at page 275), however, cannot be used for both universal and selective detection of chromatographic column eluents. In addition, it cannot be directly connected to an LC column because the LC solvent will also become ionized, resulting in a high background current.

For selective detection of phosphorous nitrogen or other compounds containing hetero atoms, thermionic specific detectors have been used. In such detectors as disclosed, for example, in an article titled "Micro-Column High-Performance Liquid Chromatography And Flame-Based Detection Principles" by V.L. McGuffin and M. Novotny (J. of Chromatography, Vol. 218, at page 179), however, alkali salts are necessary in the charge-transferring process to form ions for measurement. Although these detectors are not sensitive to the hydrocarbons in LC solvents, they may not be connected to an LC column directly because of the poor stability of the bead temperature and high detector noise. A method of using a dual-flame burner for chemical analysis by observing flame coloration has been disclosed in U.S. Patent No. 4,097,239.None of these detectors, however, allows simultaneous detection of eluents from GC and LC columns.

According to the invention there is provided a dual-flame ionization detector as set out in claim 1 of the claims of this specification.

The invention also includes a method of using a dual-flame ionization detector as set out in claim 12 or claim 16 of the claims of this specification.

An example of the invention and various modes of its use will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic sectional view of a dual-flame ionization detector according to the present invention; Figure 2 is an example of phosphorus detection by the detector of Figure 1.

Figure 3 is an example of chlorine detection by the detector of Figure 1.

Figure 4 is an example of nitrogen detection by the detector of Figure 1.

Figure 5shows an alternate mode in which the detector of Figure 1 can be used.

Figure 6shows schematically how the detector of Figure 5 is connected to an LC or a GC column.

Detailed description of the invention Referring now to Figure 1, there is shown schematically in a cross-sectional form a dual-flame ionization detector embodying the present invention. Of the two flames which burn in a vertical relationship with respect to each other, the lower flame 11 may sometimes also be called the primary flame while the upper flame 12 may be referred to as the secondary flame. The lower burner (or inner burner) 16 for the lower flame 11 is a generally cylindrical structure having a centrai bore 17 for receiving the sample to be analyzed and a fuel gas.The sample may be the eluentfrom a GC column our a narrowbore microparticle packed LC column and it is introduced into the central bore 17 through a nebulizer such as an electrospread, ultrasonic, crossflow orfritted disk pneumatic flow micronebulizer or a capillary column 20 the end of which is in contact with a thin disc frit 46 having pore size no greater than 100 Fm. A micronebulizerfor nebulizing and transporting volatile and non-volatile sample species from the end of the column 20 into the lower flame 11, or a frit 46 which can function as a micronebulizer receives the column eluent and functions also as a column end stop guide. A fuel gas such as air is introduced into the bore 17 through a gas conduit 22.The upper burner (or outer burner) 25 for supporting the upper flame 12 is essentially a cylindrical quartz tube which surrounds the top of the lower burner 16 as well as the lowerflame 11. At the top of the upper burner 25 is a cylindrical tube 26 made of stainless steel or nickel. A voltage regulating means 27 (only schematically shown) is provided for adjustably maintaining the tube 26 at a predetermined potential with respect to the housing 30 which supports the burners 16 and 25. During the operation of the detector, the housing is presumed to be at ground potential. The lower and upper burners 16 and 25 are supported by the housing 30 in such a way that there is formed between the outer wall of the lower burner 16 and the inner wall of the upper burner 25 an annular passageway 32 which leads from gas conduit 35.The upper burner 25 is further surrounded by a cylindrical tower 40, forming gas conduit 42 along the outside wall of the cylindrical tube 26. A tubular collector electrode 45 is positioned above the upper burner 25, partially surrounding the upper flame 12. Another voltage regulating means 48 is provided for adjustably maintaining the collector electrode 45 at another predetermined potential.

The detector illustrated in Figure 1 can be operated for universal and selective detection of chromatogra pry column eluents. By the universal detection is meant the mode of analysis in which response is based only on the carbon content. Analyses in which response is based on the presence of halogen or other hetero atoms will be referred to as the selective mode of detection. In both modes of detection, the lower burner 16 is caused to function as a selective pyrolyzer by controlling the chemical environment, catalytic effects, surface ionization, flame temperature, etc. of the lower flame 11.

For universal detection, the energy provided by the lower flame 11 may be so controlled as to be sufficient for converting small molecules such as LC solvent into CO and/or CO2 while the upper flame 12 is so controlled that these carbon oxides created by the lower flame 11 will not be further ionized. Larger molecules such as high-molecular-weight hydrocarbons, on the other hand, will be pyrolyzed by the lower flame 11 only into smaller molecules such as CHO, CH4 and C2H8 but not into carbon oxides. These smaller molecules are ionized subsequently by the upperflame 12 and collected for measurement by the electrode 45. For this purpose, the tube 26 may be typically maintained at a potential a few hundred volts lower than that of the collector electrode 45.

Alternatively, the tube 26 may be maintained at a potential a few hundred volts higher than that of the collector electrode 45 so that the molecules ionized by the upperflame 12 are collected by the tube 26. In either case, therefore, the ions are measured as a current. The current-measuring device is not shown in Figure 1.

In the case of selective detection, the energy and the chemical environment of the lower flame 11 are controlled so that all hydrocarbons will be converted to CO and/or CO2. High electron affinity decomposition species from the lower flame such as HPO, chlorine, etc. are further ionized by the upper flame 12 orwithin the burners and collected by the electrode 45 or the tube 26, as in the case of universal detection.

The combustion energy and chemical environ mentofthe lower flame 11 can be controlled by selecting fuel gases, flow rates, fuel gas ratios, and material for the burners. The material which is used between the lower and upper flames 11 and 12 affects also the selectivity of the detection. Quartz, nickel, ceramic and platinum are among the materials known to be suitable for use. Quartz, for example, enhances the response of phosphoruscontaining compounds.

Examples of phosphorus-, chlorine- and nitrogendetection by such dual-flame detector are illustrated in Figures 2,3 and 4. The samples used for these results were phosdrin, o-dichlorobenzene and azobenzene, respectively. Sample size injected on column is 0.6 ng per test sample. The columns used in these experiments were 25 cm x 0.3 mm ID fused silica LC column packed with 3 Fm C18 reverse phase. The mobile phase was 75% methanol and 25% water, with flow rate of 3 l/min. Fuel gases and their flow rates were 175 ml/min of air through conduit 42,100 ml/min of air through conduit 35 and a mixture 100 ml/min of air and 150 ml/min of hydrogen through conduit 22.

Another detection mode becomes possible with the detector of Figure 1 if the electrodes are arranged as shown in Figure 5 where corresponding components are designated by the same numerals as in Figure 1. In this detection mode, the top section of the upper burner 25 (or tube 26) is used as a separate collector for capturing the ion current generated by the sample species in the lower flame 11. At the same time, the upper flame 12 can be used for selective ionization of hetero containing compounds, these ions being collected by the collector 45. For this purpose, an extra electrode 50 is inserted into the upper flame 12 so as to create a potential difference of a few hundred volts between the electrode 50 and the collector 45, the inserted electrode being typically maintained at ground potential as shown in Figure 5. This mode thus allows simultaneous detection of eluents from both GC and LC column. It is particularly useful in the case of GC eluent detection because of the absence of interference from the LC mobile phase except water, formic acid and inorganic buffer solvents. A typical set-up for this mode of analysis is schematically illustrated in Figure 6 wherein the sample to be analyzed is introduced into the detector 55 shown in Figure 5 either from an LC column 56 or a GC column 61. Nos. 57 and 58 are an LC pump and an injector, respectively, while Nos. 62,63 and 64 are a high pressure GC gas cylinder, a pressure regulator and an injector, respectively. The dotted lines indicate an oven.

Claims (20)

1. A dual-flame ionization detector for analysis of chromatography column eluent both in a universal mode in which response is based only on carbon content and in a selective mode in which response is based on the presence of hetero atoms, said detector comprising: a primary burner for defining a lower flame, a secondary burner for defining an upper flame above said lower flame, said primary burner being disposed within said secondary burner, and a collector electrode adjacent said upper flame.

2. The detector of claim 1 further comprising a means for maintaining said collector electrode at a predetermined potential.

3. The detector of claim 1 wherein said collector electrode is cylindrical and surrounds said upper flame.

4. The detector of claim 1 wherein said primary and secondary burners are of generally cylindrical configuration and are coaxially disposed with respectto each other.

5. The detector of claim 1 further comprising a means for maintaining a predetermined potential difference between said first flame and the top section of said secondary burner so that said top section functions as a collectorforthe molecules ionized by said lower flame.

6. The detector of claim 1 further comprising a means for providing another predetermined potential difference between said upper flame and said collector electrode.

7. The detector of claim 1 further comprising a micronebulizer for nebulizing volatile and nonvolatile sample species and transporting said species into said lower flame.

8. The detector of claim 7 wherein said micronebulizer is an electrospread nebulizer.

9. The detector of claim 7 wherein said micronebulizer is an ultrasonic nebulizer.

10. The detector of claim 7 wherein said micronebulizer is a cross-flow nebulizer.

11. The detector of claim 7 wherein said micronebulizer is a fritted disk nebulizer.

12. A method of using a dual-flame ionization detector for concurrent analysis of eluents from both GC and LC columns, said detector comprising a lower burner for defining a lower flame, an upper burnerfordefining an upperflame above said lower flame, said lower burner being disposed within said upper burner, and an upper collector electrode adjacent said upper flame, said method comprising the steps of maintaining said upper flame at a first potential with respect to said collector electrode and maintaining the top section of said upper burner at a second potential with respect to the top section of said lower burner.

13. The method of claim 12 further comprising the steps of inserting an extra electrode into said upper flame and maintaining a first predetermined potentential difference between said extra electrode and said upper collector electrode.

14. The method of claim 13 further comprising the step of maintaining a second predetermined potential difference between said lower burner and the top section of said upper burner.

15. The method of claim 13 wherein said first and second potential differences are of the order of a few hundred volts.

16. A method of using a dual-flame ionization detector for analysis of chromatography column eluent optionally both in a universal mode in which response is based only on carbon content and in a selective mode in which response is based on the presence of hetero atoms, said detector comprising a lower burnerfor defining a lower flame, an upper burner enclosing said lower burner therein for defining an upperflame above said lower flame and an upper collector electrode adjacent said upper flame for collecting charged particles in said upper flame, said method comprising the step of controlling the chemical environment and combustion conditions of said flames for selective conversion of said eluent in the case of said selective mode of detection to allow the detection of high electron affinity decomposition species from said lower flame, and for the detection of high molecular weight hydrocarbons in the case of said universal mode of detection.

17. The method of claim 16 further comprising the step of adjustably maintaining said upper collector electrode at a first predetermined potential and the upper section of said upper burner at a second predetermined potential.

18. The method of claim 17 wherein said first and second potentials are no greater than a few hundred volts.

19. Adual-flame ionization detector substantially as herein before described with reference to and as illustrated in the accompanying drawings.

20. A method of using a dual-flame ionization detector substantially as hereinbefore described with reference to the accompanying drawings.