1. Gas Chromatography: Detectors

2. The Ideal Detector n Adequate sensitivity - range 10 ^- 18 to 10^-15 g analyte/s n good stability and reproducibility n a linear response to analyte that extends over several orders of magnitude n a temperature range from room temperature to at least 400 degrees C

3. The Ideal Detector n A short response time that is independent of flow rate n high reliability and ease of use n similarity in response toward all analytes of alternatively a higher predictable and selective response toward + classes of ananlytes n nondestructive of sample

4. Types of Detectors n Thermal Conductivity Detector n Thermionic Detectors n Atomic Emission Detector n Flame Ionization Detector n Electron Capture Detector

5. Thermal Conductivity Detector (TCD) n Q. How does this work? n Answer u Electronically measures the changes in the thermal conductivity of the gas. u The thermal conductivity changes due to the presence of the analyte

6. The Modulate Single Filament (TCD) n A method to get rid of some noise ! n How? u Both the analytical and reference gases are passed altrenately over a tiny filament u This is done at a set frequency, that sets the frequency of the output of the electrical signal u The amplitude fo the signal is proportional to the difference in the thermal conductivity of both gases

7. TCD n What is the best type of carrier gas for the TCD? n He and H have high thermal conductivity therefore they show the presence of other components of the gas because of the large drop in thermal conductivity n Other gases are not used in TCD

8. Advantages n simplicity n large linear dynamic range ( about 10^5) n general response to organic and inorganic samples n non-destructive character

9. Limitations n Low sensitivity (10^-8 solute/mL carrier gas) n Others exceed this by factor of 10^4 to 10^7

10. Thermionic Detectors (TID) n How does this work? u The effluent is mixed with H u Passes through a flame tip assembly to ignite u Hot gas flows around an electrically heated rubidium silicate bead u Heated bead forms a plasma u What occurs in the plasma to produce large amounts of ions for D and N containing molecules is not understood

11. TID n There is a large ion current that results

12. Advantages n Selective for organic compounds with P and N n This is useful for detecting pesticides that contain P

13. Atomic Emission Detection (AED) n Eluent introduced into a microwave - energized helium plasma coupled to a diode-array optical spectrometer n Plasma sufficiently energetic to atomize all of the elements in the sample and excite their characteristic atomic emission spectra

14. AED n Spectra observed using a spectrometer that employs a movable, flat diode array is capable of detecting omitted radiation between 170 to 1780 nm n The positional diode array is capable of measuring 2 to 4 elements at any given setting n Present software allows measurement of the concentration of 15 elements

15. Flame Ionization Detector n Consists of a stainless steel burner assembly installed in the detector compartment and a electrometer system in a separate unit adjacent to the gas chromatograph n Often it is installed in the tandem with the thermal conductivity cell n Effluent form the column enters burner base through millipore filters which remove contaminates

16. Flame Ionization Detector n hydrogen mixed with gas stream at bottom of jet and air or oxygen is supplied axially around the jet n hydrogen flame burns at the tip, which also functions as the cathode and is insulated form the body by a ceramic seal n Collector electrode is above the burner tip and is made of platinum

17. Flame Ionization Detector n In series with flame gases is a selection of resistors 10^7 to 10^10 ohms n vibrating reed electrometer used to provide sensitivities up to 5 x 10^13 Amps n Carbon counting device that produces a current proportional to number of ions or electrons formed in the flamed gases

18. Flame Ionization Detector n Responds to all organic compounds except for formic acid n Response greatest with hydrocarbons and decreases with substitution n Except for vapor of elements in Groups I and II, does not respond to inorganic compounds n Sensitivity high due to low noise level

19. Flame Ionization Detector n Insensitivity to water, the permanent gases, and inorganic compounds simplifies the resolution of components in analysis of aqueous extracts and in air pollution studies

20. Electron Capture Detector n operates similar to proportional counter for measurement of X-radiation n effluent form the column passes over a beta- emitter - I.e.) nickel-63 or tritium

21. ECD n electron from emitter causes ionization of carrier gas (N) n produces a burst of electrons n standing current between electrodes decreases in presence of organic species that capture the electrons

22. ECD n selective in its response and highly sensitive n Hewlett Packard makes one with a detection limit of less than 8 fg/sec for lindane n sensitive toward molecules with electronegative functional groups (halogens, peroxides, quinones, nitro groups) n insensitive towards amines, alcohols and hydrocarbons

23. ECD n important application: detection and determination of chlorinated insecticides n advantages u does not alter the sample significantly (in contrast to flame detection) u quick and easy u relatively cheap

24. ECD n disadvantage : linear response range is usually limited to around 2 orders of magnitude