Mass Spectrometry

1.0 Introduction

Mass spectrometry (MS) is an analytical technique that measures the mass-to-charge ratio of charged particles.[1] It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and other chemical compounds. MS works by ionizing chemical compounds to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios. In a typical MS procedure:

1. A sample is loaded onto the MS instrument and undergoes vaporization
2. The components of the sample are ionized by one of a variety of methods (e.g., by impacting them with an electron beam), which results in the formation of charged particles (ions)
3. The ions are separated according to their mass-to-charge ratio in an analyzer by electromagnetic fields
4. The ions are detected, usually by a quantitative method
5. The ion signal is processed into mass spectra

MS instruments consist of three modules:

1. An ion source, which can convert gas phase sample molecules into ions (or, in the case of electrospray ionization, move ions that exist in solution into the gas phase)
2. A mass analyzer, which sorts the ions by their masses by applying electromagnetic fields
3. A detector, which measures the value of an indicator quantity and thus provides data for calculating the abundances of each ion present

The technique has both qualitative and quantitative uses. These include identifying unknown compounds, determining the isotopic composition of elements in a molecule, and determining the structure of a compound by observing its fragmentation. Other uses include quantifying the amount of a compound in a sample or studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in a vacuum).

MS is now in very common use in analytical laboratories that study physical, chemical, or biological properties of a great variety of compounds. Mass spectrometry is an analytical tool used for measuring the molecular mass of a sample.For large samples such as biomolecules, molecular masses can be measured to within an accuracy of 0.01% of the total molecular mass of the sample i.e. within a 4 Daltons (Da) or atomic mass units (amu) error for a sample of 40,000 Da. This is sufficient to allow minor mass changes to be detected, e.g. The substitution of one amino acid for another, or a post-translational modification.

For small organic molecules the molecular mass can be measured to within an accuracy of 5 ppm or less, which is often sufficient to confirm the molecular formula of a compound, and is also a standard requirement for publication in a chemical journal.Structural information can be generated using certain types of mass spectrometers, usually those with multiple analysers which are known as tandem mass spectrometers. This is achieved by fragmenting the sample inside the instrument and analysing the products generated. This procedure is useful for the structural elucidation of organic compounds and for peptide or oligonucleotide sequencing.

Mass spectrometers are used in industry and academia for both routine and research purposes. The following list is just a brief summary of the major mass spectrometric applications:

1. Biotechnology: the analysis of proteins, peptides, oligonucleotides
2. Pharmaceutical: drug discovery, combinatorial chemistry, pharmacokinetics, drug metabolism
3. Clinical: neonatal screening, haemoglobin analysis, drug testing
4. Environmental: PAHs, PCBs, water quality, food contamination
5. Geological: oil composition

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