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

Mass spectrometry (MS) is an analytical chemistry technique which helps identify the amount and type of chemicals present in a sample by measuring the mass-to-charge ratio and abundance of gas-phase ions.

Many important compounds of environmental or biological interest can produce negative ions under the right conditions. For such compounds, negative ion mass spectrometryis more efficient, sensitive and selective than positive-ion mass spectrometry.

Negative ions can be produced by:

• Resonance electron capture: capture of an electron by a neutral molecule to produce a molecular anion.

The sample having 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.

The sample having 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