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derivatization
Another technique that is often used in headspace analysis is derivatization, which can drastically alter the
partitioning coefficient by changing the structure of the compound. Compounds on which analysts typically choose
to perform this step are alcohols, acids and amines, due to their poor partitioning and the tendency to interact with
the sample pathway. Common derivatization techniques used in the vial are esterification, acetylation, silylation and
alkylation. If derivatization is used, it should be noted that excessive pressures can be generated in the vial resulting
in leakage at the septa compromising sample integrity. For more information on derivatization, please refer to the
Handbook of Analytical Derivatization Reactions by Daniel R. Knapp.
Phase ratio
The adjustment of the phase ratio inside the vial can be used to increase the sensitivity for a particular analyte. The
adjustment of the phase ratio is only effective to increase sensitivity when dealing with low K values. If, for example,
Benzene is being analyzed from an aqueous matrix, the sensitivity can be dramatically increased by adding more
sample to the vial thus increasing the mass in the matrix (MM) and decreasing the gas volume in the vial (VG). The
adjustment of the phase ratio in this particular analysis can result in a 50-fold increase in sensitivity. Changes in phase
ratios on compounds with high K values have a minimal effect. It should be noted that compounds that are affected
by changes in phase ratios must be measured very accurately into the vial to minimize error due to changes in the
phase ratio.
diffusion Characteristics of solid samples
When dealing with solid matrices it is important to consider the diffusion of the analytes of interest from the solid
sample. Shortening the path the analyte has to travel to reach the gas phase can dramatically reduce the diffusion
coefficient. This can be accomplished by grinding the sample. If a sample is to be ground for analysis, it is strongly
recommended that this grinding be done using a cryogenic mill to reduce volatile loss. Another technique that is
widely used is adding solvent to the solid sample to either dissolve the solid or generate a slurry.
reverse solvent dilution
In rare cases, analysis can be done be using a technique called "Reverse Solvent Dilution". This technique is used when
the analytes of interest are soluble in the matrix. A small amount of the matrix can be spiked into a vial containing
a solvent in which the sample is not soluble. An example of this technique would be the analysis of a nonpolar
compound from a nonpolar matrix. The nonpolar sample could be spiked into a polar solvent in a vial such as water,
thus reducing the K value for both the sample matrix and the analytes towards the gas phase. It should be noted
that the use of such a technique can create chromatography problems due to potential matrix or reverse solvent
interferences. This technique is especially useful if the sample matrix has a low volatility.
Method Development • Page 6-3
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