Chemical Ionization in Mass Spectrometry

Chemical ionization (CI) is a soft ionization method, which is the direct extension of molecule and ion reaction research findings in analytical chemistry. Electron bombardment ionization was the first ionization method used. The electron ionization (EI) product contains several pieces that are difficult to evaluate. CI, which produces extremely few pieces, was developed in the 1950s and has enormous promise in analytical chemistry.

Ionization is required since mass spectrometry (MS) observations are dependent on the mass-to-charge ratio (m/z). Sample components become negatively or positively charged ions as a result of ionization. The ions are passed to a mass analyzer, where they are filtered and identified as per their mass-to-charge ratios (m/z).

To form reagent ions, electrons first assault the reagent gas in the CI process. After that, the sample molecules are ionized by reagent ions through a molecular and ion interaction mechanism. The 1970s was seen as a watershed moment in the evolution of CI. At the time, researchers fixed the flaws in CI’s operation in a vacuum condition, enabling CI to operate under atmospheric settings. Atmospheric chemical ionization uses corona discharge to generate energy and does not need a vacuum environment, broadening the spectrum of CI applications. CI is now commonly employed in the mass spectrometry method.

Chemical Ionization in Mass Spectrometry

Mass spectrometry (MS) is a method for determining the mass-to-charge ratio of ions. The findings are shown as a mass spectrum, which is a plot of intensity vs mass-to-charge ratio. Mass spectrometry is utilized in various disciplines and may be used for both pure samples and complicated mixtures.

A mass spectrum is a plot of an ion represented as a function of the mass-to-charge ratio. These spectra are used to identify a sample’s elemental or isotopic signature, particle and molecule masses, and the chemical identities or composition of macromolecules and other chemical substances.

Another ionization technique often utilized for GC-MS applications is CI. It is classified as a soft ionization approach because of the generation of predominantly molecular ions. A proton is transported to or extracted from a gas-phase molecule like methane, ammonium or isobutane in CI. The transfer produces protonated ions with a net positive charge (MH+). 

CI may also make negative ions by extracting protons from basic gases. These negative ions are generated when external and internal environment electrons are caught by electron-deficient functional groups inside the molecule of interest, such as chlorine or fluorine atoms. The measurement of benzodiazepines and other pharmaceuticals is frequently done by negative ion CI.

A sample, which might be solids, liquids or vapour, is ionized in a conventional MS method, for example, by blasting it with an electron beam. Some of the molecules in the sample may disintegrate into positive charge pieces as a result of this, or they may just become positive ions without fragmenting. These ions (fragments) are then sorted based on their mass-to-charge proportion, for example, by accelerating them and exposing them to an electrostatic field: ions with much the same mass-to-charge ratio will deflect in the same amount. 

A machine able to detect charged particles, including an electron multiplier, detects the ions. The results are shown as spectra of detected ion signal strength as a function of the mass-to-charge ratio. The atoms and molecules in the specimen may be recognized by comparing known masses (e.g., a whole molecule) to the determined masses or by observing a distinctive fragmentation pattern.

Applications

CI mass spectrometry is a valuable method for determining the structure of organic molecules. With CI, this is possible since the production of [M+1]+ removes a stable molecule that may be used to predict the groups present. Furthermore, because of less widespread fragmentation, CI improves the capacity to identify the molecular ion peak. By combining chromatographic separation methods (such as gas chromatography (GC), phase high-performance liquid chromatography (HPLC) and capillary electrophoresis) with CI, CI may be utilized to detect and quantify an element present in a sample (CE). This enables the selective ionization of one analyte from a range of molecules, resulting in accurate and exact readings.

Pros and Cons of Chemical Ionization in Mass Spectrometry

The magnification of the CI spectra is straightforward, and the analyte’s precise molecular weight may be acquired. The result of CI comprises minimal pieces and is mostly molecules or ions of the substrate. Selecting a suitable reagent ion may readily boost the selectivity of CI. For instance, the reagent ion H3O+ will only react with an organic compound that has a higher proton affinity than H3O+. Furthermore, CI has high sensitivity and a quick reaction time (15s). However, if the responsive ions are impure and a number of chemical ionization processes occur concurrently, mass spectrometry becomes challenging.

Conclusion

CI is used in conjunction with EI to ionize compounds that would fragment unduly by EI or ionize molecules without disintegration to form a molecular ion that may be used to calculate the molecular mass of sample components. The reagent gas is injected at a high concentration compared to the sample, and CI rather than EI is used to ionize the sample. 

Ionization responses include movement of ions, proton indirection and adduct formation based on the sample particle and the reagent gas. CI is a soft ionization method compared to EI because reagent gas reactions significantly minimize the power consumed by sample molecules, resulting in much less fragmentation of primarily molecular ions.