Question

what are the three types of injections with GC and when they should be used. Explain why cold trapping and solvent trapping are used and what they are.

Answer 1

1.Direct Injection

2.Split Injection

3.Splitless Injection

Are the three types of injections used in GC.

•The injection system should fulfill the following requirements:-

i) The injected amount should not overload the column

ii)The injection bandwidth should be small compared to the band broadening effects in the capillary column.

iii)The column should be able to achieve its optimum separation efficiency.

iv)Sample composition should remain intact. No discrimination should occur on basis of boiling point, polarity, concentration or thermal stability.

v) Applicable to trace analysis as well as high concentration samples.

vi) The injected amount should be reproducible.

Direct Injection-

Volume(s)-- 0.1 µL – 1 µL

Concentration Range-

1 ppm - %-levels

i) Direct injection is the most commonly used injection technique for packed columns and 0.53 mm ID columns. Its major advantages are its ease of use and the wide concentration range that it can cover. It is especially suitable for the lower concentrations in the ppm range, which are below the scope of the split injection. Higher gas flow rates are needed for an efficient and quick sample transfer from the injector liner to the column. The minimum gas flow for direct injection is about 5 mL/min. Direct injection can therefore only be applied to columns, which can cater for higher column flow rates such as, packed columns or 0.53 mm ID columns. These columns can also handle the higher concentration levels because of their increased sample capacity. Direct injection will produce poor peak shapes if too large injection volumes (> 1 µL) are applied. Discrimination effects are low compared to split injections.

ii) split injection-

ii) split injection is the most widely used injection technique for capillary columns. This is despite its tendency for discrimination of compounds with higher boiling points. The reproducibility of the injection is strongly dependent on liner geometry and heat capacity. Sample discrimination during the evaporation from the syringe also occurs due to differences in component volatility. The difference in sample capacity of the various capillary columns (varying internal diameters and film thickness) can easily be addressed by changing the split flow. The relatively high split flow takes care of a quick and efficient sample introduction onto the column. Split injections can exploit the full separation power of the capillary column because of this quick sample introduction. The following table gives some general guidelines; which split-flows should be applied to which column diameter. Split flows and split ratios are related to column sample capacity and the minimum flow needed in the injector to minimize band-broadening effects.

•Volume(s)-0.1µL – 1 µL

•Concentration Range-

50 ppm - %-level

iii) Splitless Injection-

•Volume(s)-0.5 µL – 2 µL

•Concentration Range-0.5 ppm – 50 ppm.

The most widely used technique for low ppm level samples. Splitless is often confused with direct injection. Sample is being introduced on the column during the entire splitless time. Due to this 1 – 2 min period there is a large initial injection bandwidth and refocusing of the analytes on the column is essential in order to obtain good symmetrical narrow peak shapes. This is done in two ways:

A) Cold trapping. The large temperature drop between injector (250 °C) and a low initial oven temperature (50 °C) effectively reduces the mobility of higher boiling compounds to virtually zero. These compounds freeze in a narrow band and only start to migrate again during the temperature program. This cold trapping effect combines both focusing due to thermal condensation as well as focusing as a result of a strong retention in the columns stationary phase.

B) Solvent focusing. Re-concentration of lower boiling components (close to the boiling point of the solvent) takes place by the solvent effect. Low oven temperatures will allow the solvent to condense in the column together with the low boiling sample components. The liquid film formed by the solvent will start to evaporate and the sample components will concentrate in a continuously smaller liquid film, resulting in a narrow band of concentrated sample components. Whether this process really takes place as described here is still a point of discussion among chromatographers. Fact is that both choices of solvent and initial oven temperatures are important in obtaining narrow peak shapes.