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Mass Spectrometry

 

Definition

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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