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Sample injection in Gas Chromatography depends on the nature of sample. Sample volume should be kept minimal for best column efficiency. The sample should enter the column as a ‘plug’ of vapour to avoid brand broadening and loss of resolution.
Injection of Liquid Sample
Microsyringe injection is the most commonly used technique for liquid sample injection. Sample is injected through a rubber septum into a flash vaporizer. Temperature of sample port should be atleast 50OC higher than the lowest boiling component in the sample to ensure complete vaporization. Injection volumes generally range between 0.5 to 5.0 µl. Injection precision with a syringe is around ± 1%.
Injection of Gas Samples
Introduction of a constant volume of gas is more difficult with a syringe. Gas sampling valves provide high precision injection (± 0.1%). Gas sampling valves can be manually operated or automated with pneumatic or electric actuators. Gas samples in collection bottles are connected to the switching valves. The carrier gas flow is not interrupted as sample expands into a constant volume sample loop. Upon switching valve position the sample content in the loop is injected into the carrier gas stream.
Gas Chromatography Inlet Types
Column inlet is hardware attached to the column head for introduction of sample into the flowing carrier gas stream.
It is a non vaporizing technique which is particularly useful for analysis of high boiling compounds like petroleum waxes, triglycerides and other thermally unstable compounds. Sample is introduced directly without heating. It enters a heated glass liner which prevents sample degradation by coming in contact with heated metal walls.
Split-splitless injectors are used for introduction of highly concentrated samples into capillary columns. Sample is volatilized by injection into a heated glass liner. The carrier gas then either sweeps the total sample (Splitless mode) or a portion (Split mode) into the column. The split vent controls the amount of sample entering and the other portion is exhausted. This mode is useful for highly concentrated or dirty samples. It helps in producing narrow band widths. Split-less injection is useful for trace level analysis.
Programmed Temperature Vaporizing Injection (PTV)
PTV is the technique of choice for introduction of large sample volumes (up to 250µl) to improve sensitivity. The sample is introduced into the liner at a controlled injection rate. The temperature of the liner is kept below the boiling point of the solvent. Ideal for wide boiling samples as it does not degrade thermally labile compounds.
Purge & Trap
An inert gas bubbles out volatile components in an aqueous solution. The volatiles are trapped on an adsorbent trap which on heating releases the volatiles into the carrier gas stream. Samples requiring preconcentration or purification can be introduced through such a system usually in conjunction with a split/splitless port.
Tip of the day
Flush syringe several times with sample to remove traces of past analysis samples from the needle and also clean or replace glass liner regularly.
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