Fundamentals of Sample Preparation for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 182

7.5 DERIVATIZATION OF NON-VOC FOR GAS PHASE
ANALYSIS

Many compounds of environmental interest, particularly high molecular weight
compounds or compounds containing polar functional groups, are difficult to
analyze by GC because they are not sufficiently volatile, tail badly, are strongly
attracted to the stationary phase, thermally unstable or even decomposed. In all
these cases, chemical derivatization is needed prior to analysis. Derivatization is a
technique used in chemistry that transforms a chemical compound into a product
of similar chemical structure, called a derivative. Derivatization in chemical
analysis is used to do the following: (a) increase the volatility and decrease the
polarity of compounds; (b) increase thermal stability to protect the analyte from
thermal degradation; (c) increase detector response by incorporating functional
groups, which lead to higher detector signals, for example, CF 3 groups for
electron capture detectors; (d) improve separation and reduce tailing. Common
derivatization methods can be classified into four groups depending on the type of
reaction applied:

Silylation: Silylation replaces active hydrogens with a trimethylsilyl (TMS)
group. The trimethylsilyl group consists of three methyl groups bonded
to a silicon atom [-Si(CH 3 ) 3 ]. The silyl derivatives produced are more
volatile and more thermally stable. The ease of reactivities of the
functional group toward silylation follows the order: alcohol

> phenol

> carboxyl > amine > amide > hydroxyl.

OSi(CH 3 ) 3
Si(CH 3 ) 3

OSi(CH 3 ) 3

ð7 : 14Þ

RAH + R′ CN R A Si(CH 3 ) 3 + R ′

CNH

Acylation: This derivatization adds the acryl group (RCO ) and targets
highly polar, multifunctional compounds, such as carbohydrates and
amino acids, and converts compounds with active hydrogens into esters,
thioesters, and amides. It reduces the polarity of amino, hydroxyl, and
thio groups and adds halogenated functionalities (electron-capturing
groups) for GC-ECD to be detected.
Alkylation: It reduces molecular polarity by replacing active hydrogens
with an alkyl group (e.g., methyl, ethyl). Through alkylation, the acidic
hydrogens in compounds such as carboxylic acids and phenols form
esters, ethers, and amide. Alkyl esters have excellent stability and can be
isolated and stored for a long period of time.
Esterification: In the presence of a catalyst, an acid reacts with an alcohol
to form an ester with a lower boiling point. An example is:

187 CÞ!C 5 H 11 COOCH 3 ðb 3 CÞ C 5 H 11 COOH ðb : p : : p : 127 : ð7 : 15Þ