Home > Technical Articles > Mass Spectrometry

Mass spectrometry (MS) is a very powerful tool utilized for molecular structure determination and detection. It is often coupled with other instrumentation such as gas or liquid chromatographs to enhance specificity of an analysis or to identify unknown compounds.

Introduction

Mass spectrometry (MS) is an analytical chemistry technique used for molecular structure determination and detection. In general, molecular species are made to form ions which are then analyzed for their mass/charge ratio. In many cases, the parent ion formed is the molecular ion. Under various conditions the parent ion forms fragment ions characteristic of the parent. Mass spectrometers usually consist of a source where the molecules are ionized, a magnetic or electric field where ions are separated by their mass/charge ratio, and a detector where ions are counted. To preserve molecular information, a high vacuum is used to prevent intermolecular gas phase reactions. The various types of mass spectrometry are usually categorized by the modes of sample introduction, mechanisms of ionization, analysis of ions, or even modes of detection.

Categories of Mass Spectrometry

Electron Impact (EI) ionization uses high energy electron bombardment to knock electrons off molecules creating positive ions. This usually results in excess kinetic energy left behind with the molecular ion which causes it to fragment. Fragmentation patterns (mass spectra) are very reproducible even between instruments, and spectra can be used to create libraries which can be used to identify unknowns.

Chemical Ionization (CI) uses gas phase reagents to mediate the energy from EI. Parent ions therefore contain much less kinetic energy, form fewer fragments, and are generally some type of adduct between the chemical reagent and the molecular species. While molecular ions may not survive EI, CI generally produces molecular ions. CI can produce either positive or negative ions.

There are many other modes of ionization such as those from thermospray, electrospray, or plasma sources. The primary techniques used to separate ions by mass/charge ratio are based on magnetic or electric fields and are usually referred to as magnetic or quadrapole mass spectrometry.

There are many types of mass spectrometry used. Most people are familiar with gas chromatography-mass spectrometry (GCMS) for the analysis of volatile and semi-volatile organic compounds. Components separated by the GC enter the source of a mass spectrometer and are ionized using either EI or CI. Liquid chromatography can be interfaced using Particle Beam (PB), Thermospray (TSP), or Electrospray interfaces for the analysis of nonvolatile organics, even biomolecules. Plasma MS (such as ICPMS) is used for elemental determinations, primarily metals down to the parts per trillion level.

Most mass spectrometers use quadrapole analyzers, magnetic sectors, or ion trap technology to separate ions of different mass/charge ratios. Elaborate high resolution mass spectrometers can distinguish various elemental compositions having the same nominal mass by measuring that mass to 4 or more decimal places. MS/MS systems focus parent ions from the first MS into a collision cell where they are further fragmented and analyzed by a second MS. This aids in interpreting the mass spectral fragmentation pattern and molecular structure.  For a quotation......