THE MASS SPECTROMETER: HOW IT WORKS?

Year 2017, Volume: Volume 2 Issue: İssue 1 (1) - 2.İnternational Congress Of Forensic Toxicology, 97 - 97, 16.02.2017

Muhittin Serdar

Abstract

The mass
spectrometer is an analytical tool which can measure the masses and relative
concentrations of atoms and molecules. It makes use of the basic magnetic force
on a moving charged particle.

The sample has
to be introduced into the ionization source of the instrument. Once inside the
ionization source, the sample molecules are ionized, because ions are easier to
manipulate than neutral molecules. These ions are extracted into the analyzer
region of the mass spectrometer where they are separated according to their
mass (m) -to-charge (z) ratios (m/z). The separated ions are detected and this
signal sent to a data system where the m/z ratios are stored together with their
relative abundance for presentation in the format of a m/z spectrum.

The four
essential sections of a mass spectrometer, and the associated components, are:

1.  Sample Introduction: Gas or liquid
chromatography column, flow injection

2.  Ionisation: Gas phase ions are made in the
ion source

3.  Mass Anayser: Ions are separated by their
mass to charge ratio (m/z) in the mass analyzer

4.  Detector: Only charged molecules (ions) are
detected

Sample
Introduction

The method of
sample introduction to the ionization source often depends on the ionization
method being used, as well as the type and complexity of the sample. The sample
can be inserted directly into the ionization source, or can undergo some type
of chromatography in route to the ionization source. This latter method of
sample introduction usually involves the mass spectrometer being coupled
directly to a high pressure liquid chromatography (HPLC), gas chromatography
(GC) or capillary electrophoresis (CE) separation column, and hence the sample
is separated into a series of components which then enter the mass spectrometer
sequentially for individual analysis.

Ionisation

Many ionization
methods are available and each has its own advantages and disadvantages the
ionization method to be used should depend on the type of sample under
investigation and the mass spectrometer available.

Ionization
methods include the following:

Electrospray
ionization (ESI), Atmospheric Pressure Chemical ionization (APCI), Electron
Impact (EI), Chemical Ionization (CI, Fast Atom Bombardment (FAB), Field
Desorption / Field Ionization (FD/FI), Matrix Assisted Laser Desorption
Ionization (MALDI)

The ionization
methods used for the majority of biochemical analyses are ESI, APCI, EI
(especially GC MS) and MALDI.

With most ionization
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.

Mass Anayser

The main
function of the mass analyzer is to separate, or resolve, the ions formed in
the ionization source of the mass spectrometer according to their
mass-to-charge (m/z) ratios. There are a number of mass analyzers currently
available, the better known of which include quadrupoles, time-of-flight (TOF)
analyzers, magnetic sectors, and both Fourier transform and quadrupole ion
traps.

These mass
analyzers have different features, including the m/z range that can be covered,
the mass accuracy, and the achievable resolution. The compatibility of
different analyzers with different ionization methods varies.

Tandem (MS-MS)
mass spectrometers are instruments that have more than one analyzer and so can
be used for structural and sequencing studies. Two analyzers have all been
incorporated into commercially available tandem instruments, and the analyzers
do not necessarily have to be of the same type, in which case the instrument is
a hybrid one. More popular tandem mass spectrometers include those of the
quadrupole-quadrupole, magnetic sector-quadrupole, and more recently, the
quadrupole-time-of-flight geometries.

Detector

The detector
monitors the ion current, amplifies it and the signal is then transmitted to
the data system where it is recorded in the form of mass spectra. The m/z
values of the ions are plotted against their intensities to show the number of
components in the sample, the molecular mass of each component, and the
relative abundance of the various components in the sample.  The type of detector is supplied to suit the
type of analyzer; the more common ones are the photomultiplier, the electron
multiplier and the micro-channel plate detectors.

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:

Biotechnology:
the analysis of proteins, peptides, oligonucleotides

Pharmaceutical:
drug discovery, combinatorial chemistry, pharmacokinetics, drug metabolism

Clinical:
neonatal screening, hemoglobin analysis, drug testing, steroid hormones,

Environmental:
Polycyclic aromatic hydrocarbons, polychlorinated biphenyls, water quality,
food contamination.

Geological: oil
composition 

Keywords

THE MASS SPECTROMETER: HOW IT WORKS?

References

• Muhittin SERDAR Medical Biochemistry Department, Faculty of Medicine, Acıbadem University, Istanbul, Turkey