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Introduction
Overview
Mass spectrometry is an analytical technique used for determining the masses of chemical species based on
their mass-to-charge ratio (m/z). The mass spectrometer converts the sample into ions that can be
manipulated by an electrical and/or magnetic field. In general this process occurs by the following steps:
1. The sample is ionized: the sample is usually ionized to a cation which is caused by the loss of
an electron or addition of a proton.
2. The mass is analyzed: the ions are then separated according to their mass-to-charge ratio by
an electromagnetic field.
3. The ions are detected: the separated ions are detected and measured quantitatively and the
results are then processed and displayed as a mass spectrum.
Because the ions have such a short lifetime, the experiment must be performed in a vacuum. There are a
variety of ways in which the three steps listed above can be accomplished depending on the desired
resolution and what phase the sample originates in (solid, liquid, solution).
The net charge on an ion enables it to be guided by the electric and/or magnetic fields in a mass
spectrometer. The path an ion follows is based upon its mass-to-charge ratio. Under specific conditions, one
mass-to-charge ratio is selected and ions that have this ratio are directed to a detector where the relative
abundance of ions are measured.
A high vacuum is necessary to prevent collision of the ion with other gas-phase molecules or ions. Collisions
of the ions can cause them to neutralize, scatter, react, or fragment—all of these processes interfere with
the m/z determination and data interpretation. A high vacuum reduces the pressure inside the mass
spectrometer to around 10-2 Pa to 10-5Pa (10-8 to 10-11 atm or 10-8 to 10-5 torr) by removing the neutral gas
molecules from the controlled flight path of the ions. This reduces the probability of a collision and increases
the sensitivity of the instrument. High vacuum pumps must be operated continuously because new neutral
molecules are constantly introduced through small leaks and by collision of an ion with a detector which will
produce a neutral molecule.
Applications
The technique has both qualitative and quantitative uses that include: (1) identifying unknown compounds;
(2) determining the isotopic composition of elements in a molecule; (3) determining the structure of
inorganic, organic, and biological molecules by observing their fragmentation patterns; (4) quantifying the
amount of a compound in a sample; (5) studying the fundamental principles of gas-phase ion chemistry (the
chemistry of ions and neutrals in a vacuum). It can also be used to examine the composition of solid
surfaces. It is the most direct method for measuring the molecular weight of a compound. The disadvantage
to this technique is that the sample is destroyed once it is ionized in the machine.
In biochemistry, a mass spectrometer can be used to characterize proteins by sequencing them and going
step by step through the amino acid chain using a tandem MS. In the petroleum and petrochemical industry
MS is frequently used to monitor the content and composition of gasses and also to determine the final
product purity in processing plants. With mass spectrometry, the starting materials and by-products can be
identified during the process of synthesis.
In the sections that follow, details about ion sources, mass analyzers, and other instrumental aspects will be
presented.