4. Derivatization for improving gas chromatographic detection with other detectors than MS

Gas chromatography (not coupled with mass spectrometry, GC/MS being separately presented) used as an analytical technique can involve various detectors. The variety of such detectors is rather large, and several types include the following: thermal conductivity detector (TCD), flame ionization detector (FID), nitrogenphosphorus detector (NPD), electron capture detector (ECD), flame photometric detector (FPD), photoionization detector (PID), electrolytic conductivity (Hall), sulfur chemiluminescence, nitrogen chemiluminescence, aroyl luminescence detector (ALD), atomic emission detector (AED), helium ionization detector (HID), vacuum ultraviolet (VUV) absorbance, infrared Doppler (IRD) absorption, FID with catalytic conversion of all analytes in CH4 (e.g., Polyarc system [9]), etc. The derivatization with the purpose of improving detectability in GC is determined by the type of detector utilized. Most derivatizations are performed precolumn, even if they are applied only with the purpose of improving detection. However, it is important that the derivatization for improving detection does not deteriorate the separation. Preferably, both the detection and the chromatographic separation are improved by the same derivatization. Some specific postcolumn reactions applied to the analytes are part of certain types of detectors such as chemiluminescence detectors, atomic emission detectors (AED), and FID with catalytic conversion into CH4. Some of these chemical changes in the analytes are not necessarily classified as derivatization reactions.

No specific derivatization is usually recommended to improve sensitivity when using nonselective detectors such as TCD and FID. However, in some cases when the detector is not sensitive to a specific analyte, such as formaldehyde or heavily halogenated compounds, derivatization can be used to enhance detection.

In case of NPD detector, derivatization with nitrogenous compounds can be done, which should give a higher sensitivity. However, this type of derivatization is not very common. An adverse result occurs for the NPD detectors when silylation is performed on the sample. Besides a possible reduction in the NPD response on silylated compounds containing nitrogen, a drastic decrease in the lifetime of the detector may occur, probably due to the excess of silylating reagent that commonly is injected with a derivatized sample and affects the alkali active element of the NPD.

The response of the photoionization detector (PID) depends on the ionization potential of the analyte, and compounds with higher ionization potential are not sensitive in PID, while those with lower ionization potential may have excellent sensitivity, as low as 10�<sup>12</sup> mg of sample. A derivatization resulting in lowering the ionization potential of the analyte may be beneficial for PID detection. However, derivatization for enhancing PID response is not frequently used.

Some detectors such as electron capture detectors (ECD) may benefit very much from certain derivatization types. ECD (as well as negative chemical ionization mass spectrometry or NCI-MS) can be extremely sensitive, but they are selective to compounds that are able to form more stable negative ions. ECD, for example, can have sensitivity as low as 10�<sup>13</sup> mg of analyte in the detector compared to the best sensitivity of FID that can be 10�<sup>8</sup> to 10�<sup>11</sup> mg of analyte. The efficiency of the process seems to be related to the ease of attaching an electron on the molecule. In ECD this process can be written as follows:

$$\mathbf{A} + \mathbf{e}^- \to \mathbf{A}^- \tag{5}$$

With some exceptions, ECD response can be correlated with the electron affinity of the analyte [4]. In general, the halogen substituents increase the sensitivity in ECD in the order I > Br > Cl > F. Multiple substitutions seem to have a cumulative effect. Besides halogens, nitro groups seem to have an effect similar to chlorine groups. For aromatic compounds, the substituents affect the sensitivity of the ECD according to their electron withdrawing capability. Strong electron withdrawing groups such as NO2 increase the sensitivity of the detection, while electron donating groups reduce it.

A variety of substitution groups containing electronegative elements (halogens) or nitro groups can be attached to an analyte. The procedure to attach these groups is in most cases the typical substitution of an active hydrogen in the analyte Y-H with a group R from a reagent R-X that has the appropriate active X group. Some groups used for enhancing ECD (as well as NCI-MS) sensitivity following an alkylation or aryl derivatization reaction are shown in Figure 4, and several substitution groups introduced by acylation, chloroformylation, or sulfonation used for the same purpose are shown in Figure 5. Besides alkylation or aryl derivatization, other derivatization techniques used to replace an active hydrogen are applied to introduce into a molecule as a substituent containing halogens or nitro groups enhancing

significantly the detectability of the derivatized analytes by ECD (as well as NCI-MS). Silylation, for example, can be used for this purpose when silyl groups used for derivatization contain halogens. Several silyl groups containing halogens that can be attached to an analyte by silylation with special reagents are given in Figure 6 [4].

Substitution groups used in silylation for enhancing ECD (and NCI-MS) detectability.

5. Derivatization for improving GC/MS qualitative and quantitative

column and can be easily coupled with a gas chromatograph. Most analyses performed with MS detection (GC/MS or GC/MS/MS) are using EI+ ionization mode with electron impact at 70 eV. The electrons interact with the molecule A to eject an additional electron leaving a positively charged species (with an odd num-

impact and the excess of energy induces fragmentation. For most molecules, this

trons. The formation of molecular ions takes place with a range of internal energies, and more than one fragmentation path is possible for a given molecule. Also, the fragments can suffer further fragmentations. In general, the most abundant fragment ion results from the fragmentations that form the most stable products (ion and neutral radical). The abundance of a fragment ion is affected by its stability. For this reason, the intensity of the response of a mass spectrometric detector can be very different for different molecular species, and the prediction of this intensity is

<sup>A</sup> <sup>þ</sup> <sup>e</sup>– ! <sup>A</sup>▪<sup>þ</sup> <sup>þ</sup> 2e– and A▪<sup>þ</sup> ! Bi

difficult. As a result, the improvements in the sensitivity in EI + �type mass spectrometry (in GC/MS using EI+ ionization) are not usually sought (but not

Derivatization for enhancing sensitivity is, however, frequently applied in NCI-MS. In this technique, the electrons interact with the molecules of the CI gas which is lowering their energy but without forming ions. The ionization of analyte molecules takes place by interaction with the low-energy electrons or with already formed negative ions by electron capture, dissociative electron capture, ion pair formation, or ion molecule reaction. The ionization process with the formation of

The most powerful tool used for compound identification purposes is very likely mass spectrometry (spectroscopy). This technique is capable to provide information from very low amounts of material such as that eluting from a chromatographic

. The ions also receive energy during electron

<sup>+</sup> are commonly but not always with an even number of elec-

<sup>þ</sup> þ Ci

▪ (6)

analysis

Figure 6.

ber of electrons) of the type A▪<sup>+</sup>

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

process can be written as follows:

impossible) through derivatization.

The fragments Bi

17

Figure 4.

Substitution groups used in alkylation and aryl derivatization for enhancing ECD (and NCI-MS) detectability (the masses are considered only for the most abundant isotope.).

#### Figure 5.

Substitution groups used in acylation chloroformation and sulfonation for enhancing ECD (and NCI-MS) detectability.

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

Figure 6.

in the order I > Br > Cl > F. Multiple substitutions seem to have a cumulative effect. Besides halogens, nitro groups seem to have an effect similar to chlorine groups. For aromatic compounds, the substituents affect the sensitivity of the ECD according to their electron withdrawing capability. Strong electron withdrawing groups such as NO2 increase the sensitivity of the detection, while electron donating groups reduce it. A variety of substitution groups containing electronegative elements (halogens) or nitro groups can be attached to an analyte. The procedure to attach these groups is in most cases the typical substitution of an active hydrogen in the analyte Y-H with a group R from a reagent R-X that has the appropriate active X group. Some groups used for enhancing ECD (as well as NCI-MS) sensitivity following an alkylation or aryl derivatization reaction are shown in Figure 4, and several substitution groups introduced by acylation, chloroformylation, or sulfonation used for the same purpose are shown in Figure 5. Besides alkylation or aryl derivatization, other derivatization techniques used to replace an active hydrogen are applied to introduce into a molecule as a substituent containing halogens or nitro groups enhancing

Gas Chromatography - Derivatization, Sample Preparation, Application

Substitution groups used in alkylation and aryl derivatization for enhancing ECD (and NCI-MS) detectability

Substitution groups used in acylation chloroformation and sulfonation for enhancing ECD (and NCI-MS)

(the masses are considered only for the most abundant isotope.).

Figure 4.

Figure 5.

16

detectability.

Substitution groups used in silylation for enhancing ECD (and NCI-MS) detectability.

significantly the detectability of the derivatized analytes by ECD (as well as NCI-MS). Silylation, for example, can be used for this purpose when silyl groups used for derivatization contain halogens. Several silyl groups containing halogens that can be attached to an analyte by silylation with special reagents are given in Figure 6 [4].
