Question

Why do we analyze positive ions in MS? Is it theoretically possible to analyze negative ions? Give an example, when it is desirable to monitor negative ions?

Answer 1

Dear Friend

                With most ionisation methods there is the possibility of creating both positively and negatively charged sample ions, depending on the proton affinity of the sample. Before embarking on an analysis, the user must decide whether to detect the positively or negatively charged ions.

Positive or negative ionisation?

If the sample has functional groups that readily accept a proton (H⁺) then positive ion detection is used
e.g. amines R-NH₂ + H⁺ = R-NH₃⁺ as in proteins or peptides.

If the sample has functional groups that readily lose a proton then negative ion detection is used
e.g. carboxylic acids R-CO₂H = R-CO₂^- and alcohols R-OH = R-O^- as in saccharides or oligonucleotides

Negative chemical ionization: Negative ions are formed by the electron capture. Resonance electron capture refers to the capture of an electron by a neutral molecule to produce a molecular anion. The electron energy is very low, and the specific energy required for electron capture depends on the molecular structure of the analyte. Electron attachment is an endothermic process, so the resulting molecular anion will have excess energy. In negative –ion chemical ionization, a buffer gas (usually a common chlorine gas such as methane) is used to slow down the electrons in the electron beam until some of the electrons have just the right energy to be captured by the analyte molecules. The buffer gas can also help stabilize the energetic anions and reduce fragmentation.
CH₄ + e^- --> CH₃^- + H⁺
Benefits: Efficient ionization, high sensitivity
Greater selectivity for environmentally or biologically important compounds.

Limitations: Not all volatile compounds produce negative ions

(a) Negative ion mode mass spectrometry is an analytical technique, it gives information about the molecular structure of organic and inorganic compounds.

(b) It is an accurate method for the determination of molecular mass of the components and its elemental composition.
(c) It also used to investigate reaction mixtures (gases, liquids and solids) and also used in understanding reaction kinetics and mechanisms of unimolecular decomposition reactions.
(d) Sugar and sugar alcohols such as sorbitol and glucose will be successfully analyzed by mass spectrometry using atmospheric pressure ionization in negative ion mode.
(g) ) ESI in the negative ion mode has been employed to determine the structure of glycosides of anthraquinones,coumarins, flavonoids, iridoids, aromatics, cardenolides,isoflavonoids and triterpenes.
(h) Chemical warfare agents, explosives, dyes are analyzed by using both positive and negative ion mode by using liquid chromatography-Mass spectrometry..
(i) By using negative chemical ionization we can measure the MDA(3,4-Methylene dioxyamphetamine), MDMA(3,4-Methylene dioxy methamphetamine), MDEA(3,4-Methylene dioxy ethyl amphetamine) in blood.

Mass spectrometer is operated in positive and negative ion mode. Generally now a days positive ion mode is used in many cases and Negative ion mode spectroscopy is also being used in some specific and sensitive cases, in such cases negative ion mode is more efficient than the positive ion mode .In future it can be used in other fields also.

Hope you got the answer my friend, still if you have doubt please fell free to ask me in comments section.

Thank you, all the best.