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2007 vol. 3

Clinical/Forensic

Why Derivatize?

Improve GC Separations with Derivatization

By Kristi Sellers, Innovations Chemist

Many laboratories include derivatization as part of their sample preparation for gas chromatography (GC) analysis. So, what is

derivatization? Why is it important and how do you choose a derivatizing reagent? The discussion below answers these questions. By

choosing the right derivatization reagent and procedure you can increase resolution and analyte response, significantly improving

your separations.

What is derivatization?

Derivatization is the process by which a compound is chemically changed, producing a new compound that has properties more

amenable to a particular analytical method. Some samples analyzed by GC require derivatization in order to make them suitable for

analysis. Compounds that have poor volatility, poor thermal stability, or that can be adsorbed in the injector will exhibit nonrepro-

ducible peak areas, heights, and shapes. Other compounds that respond poorly on a specific detector may need to be “tagged” with

a different functional group to improve detection. For example, tagging with chlorine can improve response on an ECD (electron

capture detector). In addition to improving suitability and response, derivatization can improve resolution between coeluting com-

pounds and overlapping peaks.

How do I choose a derivatizing reagent?

A good derivatizing reagent and procedure should produce the desired chemical modification of the compound(s) of interest, and

be reproducible, efficient, and nonhazardous.

For GC, there are three basic types of derivatization reactions: silylation, acylation, and

alkylation. Silylating reagents react with compounds containing active hydrogens; these reagents are the most common type used in

GC. Acylating reagents react with highly polar functional groups such as amino acids or carbohydrates. Alkylating reagents target

active hydrogens on amines and acidic hydroxyl groups.

Multiple derivatizing reagents may be necessary for compounds contain-

ing several different functional groups such as androsterone (Figure 1). In these multi-step derivatization procedures the use of other

types of reagents, such as oxime, hydrazone, methylation, and cyclic derivatives, may be necessary.

A multi-step example

Derivatization can substantially improve chromato-

graphic results, as seen in this example derivatiza-

tion of androsterone (Figure 1). Androsterone con-

tains a hydroxyl group and a carbonyl group and

exhibits poor peak shape and poor separation if

analyzed underivatized by GC (Figure 2b). Using

silylation, active hydrogens on OH, SH, and NH

groups can be replaced.

Since

-trimethylsilylimi-

dazole (TMSI) is a strong silyl donor, it will react

readily with the hydroxyl group on the andros-

terone molecule creating a trimethylsilyl (TMS)

derivative. Because androsterone also contains a

carbonyl group, another derivatizing reagent is needed to improve chromatographic peak shape. Methoxyamine will react with the

carbonyl group forming an oxime derivative (CH

ON). Oxime derivatives not only improve chromatographic performance, but also

alter GC separations. Figure 2a shows the chromatographic result of derivatizing sex hormones using TMSI and methoxyamine;

retention times are increased, separation is increased, and peak shapes and responses are improved.

Conclusion

Derivatizing compounds for GC often is necessary to obtain reproducible chromatographic results. Eliminating this step to save time

can be costly and produce inaccurate and unreliable results. A well-chosen derivatization procedure, based on the chemical compo-

sition of the target compounds, can significantly improve your chemical separations.

Figure 1

Derivatization reaction of androsterone using

TMSI/methoxyamine.

References

1 Knapp D., Handbook of Analytical Derivatization Reactions, Wiley-Interscience, 1979, pp.2-24, 449-453, 482.

www.piercenet.com

3 Grob R., Barry E., Modern Practice of Gas Chromatography, Wiley-Interscience, 2004, pp. 817-818.

• Get better separations with increased resolution and response.

• Learn how to choose proper reagents for desired reactions.