Chemical Ionization

CI – also known as Chemical Ionization is a ‘soft ionization’ process as it utilizes lesser energy as compared to EI. CI has two step procedures. Firstly, reagent gases like methane, isobutene or ammonia are introduced in the source where they go through EI. Then, these ionized gas reagents tend to react with the analytes which results in spectrum formation that is simpler than EI and has lesser fragment formation. This process forms both the positive and the negative ions. The most common ion found in this spectrum is the pseudo-molecular ion (M+1) or (M-1) which can be easily identified.

In a lot of Q-MS procedures, CI needs to have a dedicated source which is different physically from EI source having larger openings. CI analyses need to have high vacuum capacity and therefore, are equipped with turbo pumps. But, due to high concentration of reagent’s gas present in the MS, the CI systems normally run under lesser pressure. The high concentrations of gas also ensure that more collisions take place amongst the analyte and the gas ions, thereby increasing the efficiency of the ionization process.

The steps below show the CI procedure with methane gas. The formation of primary ion with the high energetic EI of methane takes place as follows:

CH4 + e              CH4+* + 2 e

The secondary reagent reaction will happen between methane molecules to create:

CH4 + CH4+* CH5+ + CH3*

CH4 + CH3+ C2H5+ + H2

At last, ion formation would take place by several reactions such as hydride abstraction, protonation, charge transfer and adduct formation. Protonation takes place when a proton is accepted by the analyte and is commonly seen as:

Protonation:                               CH5+ + M                 CH4 + [M + H] +

In the hydride abstraction, hydrogen atom transfers between the analyte and the reagent gas.

Hydride Abstraction:              CH3+ + MH                  CH4 + M+

In both of these reactions, even electron ions are produced which are comparatively stable and can produce pseudo molecular ions such as [M + 1] + or [M – 1] + ions. These ions are abundantly produced even if the EI spectrum of the compound doesn’t display any molecular ions.

In adduct formation, the reactions are low and not present in many applications other than LC-MC. An exception though is polar molecule with very strong base groups. The reaction of adduct formation is as follows:

Adduct formation:                      CH5+ + M                 [M + CH5] +

In the charge transfer reaction, radical molecules are produced that are not stable, defragment easily and quite extensively. The reaction is as follows:

Charge transfer:                     CH4+ + M                     CH4 + M+

Quite often, a spectrum is produced due to this reaction which is quite similar to fragmentation that occurs by EI but with relatively lesser energy. Transfer gases that are commonly used are nitrogen and argon.

Two kinds of CI can occur: PCI and NCI i.e. positive and negative chemical ionization. This is so as majority of the neutral molecules can become positively charged with the above mentioned reactions. Most commonly, protonation is being used especially in PCI which results in the production of positive pseudo molecular ion [M + 1] +. This is especially important while finding the molecular weight of the analyte which gives excessive fragmentation with EI. In forensic toxicology, PCI is very extensively applied and it commonly used to analyze cocaine and its metabolites, ketamine, opiates and other drugs such as MDMA.

NCI is used for limited molecules that are able to create a negative charge. These molecules should have strong electro negative groups like nitric acid or halogens. The compounds which have halogens are excellent for NCI reactions. Those which are not halogenated can be derivatized with halogenated agents like PFPA – pentafluoropropionic anhydrides or HFBA – heptafluorobutyric anhydrides. The NCI reaction is comparatively easier than PCI or EI as many biological endogenous materials would interfere and would not ionize with NCI. This would decrease the background, increase the S//N which gives better sensitivity. Many NCI regimes are used while analyzing pesticides in environmental sciences. However, some applications are used for forensic sciences which include the analyzing of designer drugs, amphetamines and ∆9-tetrahydrocannabinol in different matrices like oral fluids, plasma and sweat patches. The NCI is therefore, quite useful and sensitive for benzodiazepines as they contain halogens.

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