Cracking Patterns; Ion Types; Molecular Ion; Isotope Ions - Thermo Scientific Prima PRO User Manual

Mass spectrometers

Table of Contents

Cracking

Patterns

Ion Types

Molecular Ion

Isotope Ions

Thermo Fisher Scientific

The sequence of peaks representing a particular gas species is a

direct result of how that species reacts under electron impact in the

source. Depending on the species, the molecules can undergo a

variety of reactions within the ion source, and the resultant set of

peaks is referred to as the "cracking pattern."

A single gas species may produce a variety of different ions giving

rise to several peaks of different mass-to-charge ratio and intensity.

The relative intensities of these various peaks are characteristic of the

gas species in question, although the peak intensities may be affected

by changes in source conditions (such as electron energy). The

relative intensities of the different peaks can be used to de-convolute

the composition of a gas stream containing components of the same

molecular weight.

Some of the different types of ions that can be produced within the

ion source are described below.

This is the simplest reaction to understand and results from the gas

molecule acquiring a single positive charge as a result of electron

impact in the source. An example of this type of ion would be the

ion for nitrogen at m/e 28 amu.

The molecular ion peak may not always be the peak with the highest

intensity in the mass spectrum for a given component. This is

especially true for hydrocarbons, where the fragment peaks are

usually statistically more likely to occur than the molecular ion.

Isotope ions are produced when one or more of the atoms that

comprise a molecule are of a different isotope from that most

commonly found. The relative intensities of these ions are defined by

the relative abundance of the different isotopic species for the

component atoms in question. The relative intensities of isotope

peaks are not normally affected by changes in source conditions.

An example of an isotope peak is the m/e 29 amu peak observed in

the nitrogen mass spectrum. This peak results from a single charge

on a nitrogen molecule where one of the two nitrogen atoms is the

N isotope. The relative intensity of this peak to the molecular ion at

m/e 28 amu is twice the natural abundance of the

N atom could be either of the two atoms).

Prima PRO & Sentinel PRO Mass Spectrometers User Guide

Technical Description: System