Influence of Sample Volume and Temperature on Headspace Sensitivity

The sample volume (VS) is included in the phase ratio (P) in eqn [1], but its influence on headspace sensitivity is not independent of the partition coefficient (K). The latter can vary widely from practically zero in the case of a gas sample up to several thousands, where the applicability of HS-GC ends. The phase ratio (P), and thus the influence of the sample volume, does not generally span such a wide range. For example, a 1 mL sample in a 10 mL vial has a phase ratio of 9, while with a sample volume of 5 mL the phase ratio decreases to 1.

Whether or not this causes an increase in the resulting gas concentration, and thus of the resulting peak area, depends mainly on the partition coefficient. In the case of a high partition coefficient (K >

100, e.g. ethanol in water), a change in the phase ratio from 1 to 5 will barely influence headspace sensitivity; in contrast, where the partition coefficient is very small the sensitivity increases in proportion to the sample volume (e.g. n-hexane in water at 50°C). This result may be surprising since it differs so much from normal GC analysis, where peak areas increase with increasing volumes of injected sample.

The vapour pressure of a compound increases exponentially with temperature. One would therefore expect a similar increase in the volatility, and thus enhanced sensitivity for a headspace compound. Again, however, there is a dependence on the partition coefficient. In the case of nonvolatile compound (K p 00 ), a higher temperature will not alter its non-volatility. In the case of a highly volatile compound (K p 0, already at room temperature), the temperature will not affect the headspace sensitivity either, because in this case nearly all of the compound is already present in the gas phase. Headspace samples generally fall between these two extremes. Table 1 gives typical values of partition coefficients at three temperatures for three compounds with a small (tet-rachloroethylene), a medium (ethyl acetate) and a high (ethanol) partition coefficient.

Solar Panel Basics

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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