Derivitisation

Derivatisation is a technique used in chemistry to transform a compound into another with similar structure, called a derivative.

The derivative has different chemical and physical properties than the parent compound. 

The derivative chemical structure contains the original compound structure untouched – No rearrangement – No internal alteration.Making it special compared to other chemical reactions.

This change in functionality without rearrangement is achieved by attaching another molecule to a reactive functional group during the derivatisation process.

It is important to not alter the original molecule particularly in a forensic context as an identification or confirmation technique requires considering the entire target molecule. Therefore the core molecule has to be preserved. 

Derivatisation is used in GC because the compound of interest cannot be analysed as it is.

 Too polar  

 Not volatile enough 

 Heat-sensitive 

 Naturally unstable  

 Show a very short lifetime

-As a result derivsatisation increases volatility by reducting absorption in system

-To improve peak separation and peak symmetry

-Also to improve detector response

Derivitasation in MS Improve analytical efficiency 

In mass spectrometry analysis reduction of specificity of ions occur this also lower their value as pieces of evidence – the question of "are they really fragments of the compound analysed for?" arised

Thefore increasing the molecular weight of the target analyte by derivatisation yields bigger specific fragments.

Derivitisation in ms also increase the molecular weight of very volatile compounds (invariably small).

which in turn generates a more complex mass spectrum and increases confidence for drug identification.

 Based on which functional groups requiring derivatisation – taking in account which other functional groups are present and the reason the derivatisation is performed:

Characteristic of a good derivatisation reagent:

 Produces a reaction that is 95+% complete

 Does not induce rearrangement or internal structural alterations

 Produces a derivative which does not interact with the column

 Produces a stable derivative – shelf life

Alkylation is the transfer of an alkyl group from one molecule to another 

Commonly used to derivatise molecules presenting acidic hydrogens (Carboxylic acids and phenols)

Can be used alone or coupled with other types of derivatisation reagent 

An acylation is the process of adding an acyl group to a compound 

Good at derivatising highly polar multifunctional molecules (Carbohydrates and amino acids)

Silylation involves the replacement of an acidic hydrogen on the compound with an alkylsilyl group.