Mass Spectrometry is a powerful technique for identifying unknowns, studying molecular
structure, and probing the fundamental principles of chemistry. Applications of mass
spectrometry include identifying and quantitating pesticides in water samples, it identifying
steroids in athletes, determining metals at ppq (Parts Per Quadrillion) levels in water samples,
carbon-14 dating the Shroud of Turin using only 40 mg of sample (1), looking for life on Mars,
determining the mass of an 28Si atom with an accuracy of 70 ppt(2), and studying the effect of
molecular collision angle on reaction mechanisms.
Mass spectrometry is essentially a technique for "weighing" molecules.* Obviously, this
is not done with a conventional balance or scale. Instead, mass spectrometry is based upon the
motion of a charged particle, called an ion, in an electric or magnetic field. The mass to charge
ratio (m/z)** of the ion effects this motion. Since the charge of an electron is known, the mass to
charge ratio a measurement of an ion's mass. Typical mass spectrometry research focuses on the
formation of gas phase ions, the chemistry of ions, and applications of mass spectrometry.
A variety of ionization techniques are used for mass spectrometry. Most ionization
techniques excite the neutral analyte molecule which then ejects an electron to form a radical
cation (M+)*. Other ionization techniques involve ion molecule reactions that produce adduct
ions (MH+).** The most important considerations are the physical state of the analyte and the
ionization energy. Electron ionization and chemical ionization are only suitable for gas phase
ionization. Fast atom bombardment, secondary ion mass spectrometry, electrospray, and matrix
assisted laser desorption are used to ionize condensed phase samples. The ionization energy issignificant because it controls the amount of fragmentation observed in the mass spectrum. .
Although this fragmentation complicates the mass spectrum, it provides structural information
for the identification of unknown compounds. Some ionization techniques are very soft and only
produce molecular ions,* other techniques are very energetic and cause ions to undergo extensive
fragmentation. Although this fragmentation complicates the mass spectrum, it provides
structural information for the identification of unknown compounds.
Electron Ionization. Electron Ionization (EI) is the most common ionization technique
used for mass spectrometry.** EI works well for many gas phase molecules, but it does have
some limitations. Although the mass spectra are very reproducible and are widely used for
spectral libraries, EI causes extensive fragmentation so that the molecular ion is not observed for
many compounds. Fragmentation is useful because it provides structural information for
interpreting unknown spectra.
The electrons used for ionization are produced by passing a current through a wire
filament (Figure 2). The amount of current controls the number of electrons emitted by the
filament. An electric field accelerates these electrons across the source region to prodce a beam
of high energy electrons. When an analyte molecule passes through this electron beam, a valence
shell electron can be removed from the molecule to produce an ion.
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