2. Derivatization for improving separation in gas chromatography

For GC analysis, the effect of derivatization can be beneficial in a variety of circumstances. Some of the most common uses of derivatization for improving the GC separation are the following:

(a) Derivatization that replaces active (polar) hydrogen atoms in the analyte to decrease its boiling point. The active hydrogens in a chemical compound typically enhance the capability to form hydrogen bonds and increase the compound polarity. For this reason, many compounds containing active (polar) hydrogens are not volatile, the volatility being necessary for using GC or GC/MS as a core analytical method. Derivatization can be used to replace active hydrogens from an analyte Y-H (or Y:H) in functional groups such as OH, COOH, SH, NH, and CONH. These reactions can be written in a simplified form as follows:

$$\mathbf{Y}\mathbf{-}\mathbf{H} + \mathbf{R} - \mathbf{X} \to \mathbf{Y}\text{-}\mathbf{R} + \mathbf{H}\mathbf{X} \tag{1}$$

In reaction (1), the reagent R-X contains an "active" group X and a group R that carries a desired property (e.g., lack of polarity for GC). Group R in the reagent can be a low molecular mass fragment such as CH3 or C2H5, a shortchain fluorinated alkyl in alkylation reactions, Si(CH3)3 or other silyl groups in silylations, COCH3 or short-chain fluorinated acyl groups in acylations, etc. An example of a chromatogram resulting from the GC/MS analysis of a silylated tobacco sample is given in Figure 1. Tobacco contains many hydroxy acids such as malic, trihydroxybutanoic, citric, quinic, glucuronic, and chlorogenic. Also, it contains monosaccharides (e.g., glucose, fructose), disaccharides (e.g., sucrose), and even trisaccharides. None of these compounds are volatile, having numerous active hydrogens. The replacement of these hydrogens with Si(CH3)3 by silylation renders these compounds volatile, and they can be analyzed by GC/MS as seen in Figure 1.

(b) Derivatization for enhancing the separation. Specific moieties added to an analyte may be necessary for enhancing the separation. This is frequently practiced for general GC separations and is also very useful for the separation of chiral molecules (see Section 4). The derivatized analytes may have significantly different properties from each other, for example,

#### Figure 1.

GC/MS chromatogram of a silylated tobacco sample, with separation on a DB-5 MS column from Agilent (Agilent Technologies Inc., Wilmington, DE, USA) (Note: an internal standard I.S. was added to the sample).

regarding polarity and implicitly in their boiling point, allowing separations that are difficult to achieve otherwise. Also, derivatization may generate more significant differences between the analytes and the matrix components.

3. Derivatization for chiral separation in gas chromatography

Derivatization Methods in GC and GC/MS DOI: http://dx.doi.org/10.5772/intechopen.81954

The compounds with structures that are mirror images to each other are indicated as enantiomers, and their molecules are not superimposable, having the property called chirality. Chirality is commonly caused by the existence in the molecule of at least one tetrahedral carbon atom substituted with groups that are different. However, chiral molecules may be generated with a phosphorus or a sulfur chiral atom. Not only chiral centers (such as an asymmetric carbon) generate enantiomers, but a chiral axis or a chiral plane can lead to enantiomers. The chirality in an enantiomer is specified using the symbols R and S based on specific rules. For the assignment of a symbol R or S to a chiral carbon, the substituents are arranged in a sequence

a > b > c > d. For the four atoms directly attached to the asymmetric carbon, a higher atomic number outranks the lower, and a higher atomic mass outranks the lower mass. For the same atoms directly attached to the asymmetric carbon, the priorities are assigned at the first point of difference. After the sequence is established, the molecule is oriented in space with the group "d" of the lowest priority behind the asymmetric carbon. When viewed along the C─d bond (from C) and the three substituents a, b, and c are oriented clockwise, the compound contains an R asymmetric carbon, and it contains an S asymmetric carbon for counterclockwise arrangement. More than one asymmetric carbon can be present in a molecule, as in the case of carbohydrates. The stereoisomers generated by more than one asymmetric carbon can be mirror image one to the other (enantiomers) or may have different steric arrangements being diastereoisomers. These types of molecules are schematically

The (S,S)- and the (R,R)-compounds from Figure 3 are enantiomers, while the (S,R)-compound is a diastereoisomer to both (S,S)- and to (R,R)-compounds (it is an enantiomer to the (R,S)-compound). The gas chromatographic separation of enantiomers can be done only using chromatographic columns having chiral stationary phases. The derivatization of enantiomers with non-chiral reagents generates molecules that remain enantiomers. This type of derivatization may improve the chromatographic separation from other molecules, but the derivatized compounds of remaining enantiomers cannot be separated except on chiral stationary phases. Sometimes, better separation can be obtained even between the enantiomers (on chiral chromatographic columns) after derivatization. One such example is the separation of (R)- and (S)-nornicotine derivatized with isobutyl chloroformate on a chiral Rt-BDEXsm column with separation improved compared to that of underivatized

ð2Þ

enantiomers [6]. The derivatization reaction is indicated below:

shown in Figure 3.

Figure 3.

13

Compounds with two chiral centers.


The choice of the appropriate derivatization is not always a simple task. The replacement of a hydrogen atom with a group of atoms may increase the molecular weight of the derivatized analyte. In such cases, it must be verified that the increase in the molecular weight by derivatization brings no or only a small increase in the boiling point of the analyte. Most of the time, low molecular weight substituents such as CH3 or Si(CH3)3 are preferable for GC analysis to the active hydrogens for achieving the previously described goals. Large substituents may increase the boiling point too much and make the compound not acceptable for GC analysis.

Besides replacement of active hydrogens, other derivatization reactions can be utilized. For example, condensation reactions may decrease the boiling point and improve the thermal stability of an analyte. However, the generation of new active hydrogens must be avoided in condensation reactions or must be followed by a second derivatization.

Figure 2. Peak tailing due to multiple retention mechanisms.

regarding polarity and implicitly in their boiling point, allowing separations that are difficult to achieve otherwise. Also, derivatization may generate more significant differences between the analytes and the matrix

(c) Derivatization that replaces active hydrogens in the analyte to improve the

chromatographic column (e.g., a capillary column coated with a bonded stationary phase) may display additional capability to interact with polar molecules, besides the intended interactions due to its bonded phase. This may come, for example, from the silica wall of the column. Secondary interactions taking place with only a portion of the molecules of the analyte

generate peak tailing. This is exemplified in Figure 2 which shows a hypothetical case of two different types of interaction between the column

(d) Derivatization for the improvement of stability of a compound. This stability may refer to thermal stability, a property which overlaps to a certain extent to what was described at point (a). However, even some volatile compounds may be further thermally stabilized by derivatization. Also, chemical stability can be enhanced by protecting specific groups in the analyte using derivatization. For example, thiols can be protected using derivatization against oxidation by the traces of oxygen in the heated

The choice of the appropriate derivatization is not always a simple task. The replacement of a hydrogen atom with a group of atoms may increase the molecular weight of the derivatized analyte. In such cases, it must be verified that the increase in the molecular weight by derivatization brings no or only a small increase in the boiling point of the analyte. Most of the time, low molecular weight substituents such as CH3 or Si(CH3)3 are preferable for GC analysis to the active hydrogens for achieving the previously described goals. Large substituents may increase the boiling point too much and make the compound not acceptable for GC analysis.

Besides replacement of active hydrogens, other derivatization reactions can be utilized. For example, condensation reactions may decrease the boiling point and improve the thermal stability of an analyte. However, the generation of new active hydrogens must be avoided in condensation reactions or must be followed by a

behavior of the analyte in the chromatographic separation. The

Gas Chromatography - Derivatization, Sample Preparation, Application

components.

and a specific molecular species.

injection port of the GC.

second derivatization.

Figure 2.

12

Peak tailing due to multiple retention mechanisms.
