Split Injection: In the split injector, the injected sample is vaporized into the stream of carrier gas ,and a portion of the sample and solvent ,if any, is directed onto the head of the gc column .The remainder of the sample is vented. Typical split tations range from 10:1 to 100:1 and can be calculated from the equation:
split ratio=(column flow+vent flow)/column flow where the approximate
column flow=3.141(internal column radius in cn)^2(column length in cm)/retention time of argon or butane in min)
On-column injection: with on-column injection ,the sample is injected directly onto the column using a small syrings needle.Obviously, this technique is easier to use with larger bore gc columns,but modern gas chromatographs can precisely control the on-column injection process,including automatic control of heating and cooling of the injector. This method of analysis gives good quantitative results and is especially valuable for wide-boiling range and thermally labile samples.With this technique, a short section of uncoated fused silica capillary analytic column.
Programmed Temperature Injectors: The programmed temperature injector is held near the boiling point of the solvent; after injection of the sample,it is temperature programmed rapidly until it reaches the desired maximum temperature, which is normally higher than that of an isothermal(constant temperature)injector.As the sample components vaporize ,they are transferred onto the head of the gc column.This technique is a variation of on-column injection ,but reduced the peak broadening frequently seen with on-column infections.
concentration decvices for Sample injection:Several concentrationg devices for organic chemical analyses are commercially available. These devices interface with the inlet system of the gas chromatograph and concentrate organics onto adsorbents such as charcoal an /or porous polymers.The sampleis thermally desorbed onto the head of a gc column by reverse fluhsing with the carrier gas. Concentration devices are also used for analyzing off-gases from such materials as polymers. Often, a simple cool stage is sufficient to trap volatiles that are subsequently desorbed by rapidly increasing the temperature of the trapping device.
GC Detectors: Flame Ionization Detector: The anallyte in the effluent enters the flame ionization detector(FID) and passes through a hydrogen/air flame. ions and electrodes in the detector by decreasing the gap resistance .By amplifying this current flow a signal is produced. Flame.
Thermal Conductivity detector: In the thermal conductivity detector(TCD). the tempeature of a hot filament changes when the analyte dilutes the carrier gas. With a constant flow of helium carrier gas,ghe filament temperature will remain constant,but as compounds with different thermal conductivities elute, the different gas compositions cause heat tobe conducte d away from the filament at different rates,which in turn causes a change in the filament temperature and electrical resistance , The TCD is truly a universal detector and cna detect water ,air, hydrogen, carbon monoxide, nitrogen,sulfur diocide,and many other compounds . For most organic molecules ,the sensitivity of the TCD detector is low compared to that of the fid,but for the compounds for which the FID produces little or no signal ,the TCD detector is a good alternative.
Thermionic speciific detector: The thermionic specific detector (TSD)is similar to the FID with the addition of a small alkali salt bead, such as rubidium,which is placed on the burner jet.Nitrogen and phosphorus compounds increase the current in the plasma of vaporrized metal ions.The detector can be optimized for either nitrogen-containing compounds or phosphorus -containing compounds by careflly controlling of the bead temperature and hydrogen and air flow rates .The detector can be tuned using azobenzene for nitrogen and parathinon for phosphorus.
Flame photometric detector:with the flame photometric dectector ,sa with the FID, the samlple effluent is burned in a hydrogen/air flame. By using optical filters to select wavelengths specific to sulfur and phosphorus and a photomultiplier tube ,sulfur or phosphorus compounds can be selectively detected
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