The Electrolytic Conductivity Detector ELCD

This detector can be used for halogens, nitrogen and sulfur but not for all three simultaneously. The principle of operation of the ELCD is extremely simple. The gases eluting from the column are passed with hydrogen through a small nickel furnace heated to

Figure 1 FID chromatogramsof gas oil before (A) and after (B) hydrotreating. SCD chromatograms of gas oil before (C) and after (D) hydrotreating. (From Hutte RS. In Chromatography in the Petroleum Industry (1995) Adlard ER (ed.). Reproduced with permission of Elsevier Science.)

about 850-1000°C. Under these conditions halogen compounds are reduced to HX, nitrogen compounds to NH3 and sulfur compounds to H2S. These gases are all very soluble in water and if they are passed into a circulating water stream the electrical conductivity will show major changes as the gases emerge. It is also possible to operate in an oxidizing mode so that sulfur goes to SOx, halogens to HX and nitrogen to N2.

In practice the equipment is complicated by the fact that the system outlined above would give an integral

Table 1 Typical performance characteristics for the NPD

Characteristic

Typical values

Table 1 Typical performance characteristics for the NPD

Characteristic

Typical values

Sensitivity

SN = 0.1-1.0 A • s/g N

SP = 1.0-10.0 A • s/g P

Detectivity

Dn = 5x10-14-2x10~13gNs-1

DP = 1 x 10~14-2 x 10~13 gPs 1

Specificity

SN/SC = 103-105 gC/gN

SP/SC = 104-5x105gC/gP

SN/SP = 0.1-0.5 gP/gN

Linear range

103-105

Source: From PL Patterson in Detectors for Capillary Chromatography (eds. HH Hill and DG McMinn). Reproduced courtesy of John Wiley and Sons Inc.

chromatogram rather than the more familiar differential chromatogram and this is overcome by circulating the electrolyte through ion exchange resins to constantly regenerate pure electrolyte. Selectivity for

Figure 2 Comparison of FID and NPD chromatograms for a sample containing trace amounts of cocaine and heroin. (From Patterson PL. In Detectors for Capillary Chromatography (1992) Hill HH and McMinn DG (eds). Reproduced with permission of John Wiley & Sons, Inc.)

halogens co-emerging with nitrogen or sulfur compounds can be obtained by using other electrolytes rather than water (n-propanol is recommended since other reaction gases are not soluble in it). Another way to obtain selectivity is to use small scrubbers containing chemicals that will remove specific gases. For example an acidic scrubber removes ammonia but allow halogen acids to pass through.

Clearly there are number of opportunities for variation in the chemistries involved (material of reaction tube construction, reaction tube temperature, oxidation or reduction, electrolyte and scrubber) in order to obtain optimum sensitivity and selectivity. For halogens the sensitivity is in the low picogram range with a linearity of five orders of magnitude and a halogen/hydrocarbon selectivity of better than 106. In spite of these impressive performance figures the main use of the ELCD has been in the determination of halogenated compounds in water where its use is specified in US Environmental Protection Agency Method 502.2. Figure 3 shows a chromatogram obtained by purge and trap sampling of water with the ELCD in the halogen mode.

Like the NPD it seems probable that the application of this detector will decline in the future. As separations tend to become faster it is doubtful if the ELCD is capable of providing the rapid response and low dead volume required in such circumstances.

1. Dichlorodifluoromethane

2. Chloromethane

3. Vinylchloride

4. Bromomethane

5. Chloroethane

6. Trichlorofluoromethane

7. 1,1-Dichloroethane

8. Methylene chloride

9. fra/is-1,2-Dichloroethane

10. Chloroform

11. 1,1,1-Trichloroethane

12. Carbon tetrachloride

13. 1,2-Dichloroethane

25 24 26

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Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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