Introduction to Mass Spectrometry

In this chapter, we will discuss the mass spectra of alkanes, halogen-containing compounds, alcohols, and amines. We will discuss the mass spectra of other functional groups when we consider them in later chapters. 

Infrared spectroscopy gives information about the functional groups in a molecule, but it tells little about the size of the molecule or what heteroatoms are present. To determine a structure, we need a molecular weight and a molecular formula. Molecular formulas were once obtained by careful analysis of the elemental composition, and a molecular weight was determined by freezing-point depression or some other technique. These are long and tedious processes, and they require a large amount of pure material. Many important compounds are available only in small quantities, and they may be impure. 

Mass spectrometry (MS) provides the molecular weight and valuable information about the molecular formula, using a very small sample. High-resolution mass spectrometry (HRMS) can provide an accurate molecular formula. The mass spectrum also provides structural information that can confirm a structure derived from NMR and IR spectroscopy. 

A mass spectrometer ionizes molecules in a high vacuum, sorts the ions according to their masses, and records the abundance of ions of each mass. A mass spectrum is the graph plotted by the mass spectrometer, with the masses plotted as the x axis and the relative number of ions of each mass on the y axis. Several methods are used to ionize samples and then to separate ions according to their masses. We will discuss only the most common techniques, electron impact ionization for forming the ions, and magnetic deflection for separating the ions. 

Electron Impact Ionization In the ion source, the sample is bombarded by a beam of electrons. When an electron strikes a neutral molecule, it may ionize that molecule by knocking out an additional electron. 

When a molecule loses one electron, it then has a positive charge and one unpaired electron. The ion is therefore a radical cation. The electron impact ionization of methane is shown next. 

In the beginning of this chapter, we defined spectroscopy as the study of the interaction between matter and electromagnetic radiation. In contrast, mass spectrometry is the study of the interaction between matter and an energy source other than electromagnetic radiation. Mass spectrometry is used primarily to determine the molecular weight and molecular formula of a compound. 

The radical fragments are not deflected by the magnetic field and are therefore not detected by the mass spectrometer. Only the molecular ion and the cationic fragments are deflected. Smaller ions are deflected more than larger ions, and ions with multiple charges are deflected more than 

Mass spectrometry is an incredibly important tool that has found a broad array of applications. Following is a summary of some major applications of mass spectrometry, categorized by the industry in which they are applied  

Pharmaceutical drug discovery, drug metabolism, reaction monitoring 

Biotechnology amino acid sequencing, analysis of macromolecules 

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Hill, H.C., Introduction to Mass Spectrometry, Heyden, London, 1972. 

Watson, J. T. Introduction to Mass Spectrometry. New York Raven Press, 1985. What is Mass Spectrometryl The American Society for Mass Spectrometry (ASMS), 815 Don Gaspar Drive, Santa Fe, NM 87501. 

Watson, J. T. Introduction to Mass Spectrometry Biomedical, Environmental Forensic Application. New York Raven Press, 1975. 

Kiser, Introduction to Mass Spectrometry and its Applications , Prentice-Hall, Englewood. 

J. T. Watson and O. D. Sparkman, Introduction to Mass Spectrometry Instrumentation, Applications, and Strategies for Data Interpretation, 4th Edition, Chichester, Wiley, 2007. 

Beynon, J.H. and Brenton, A.G. (1982). An Introduction to Mass Spectrometry. University of Wales Press, Cardiff. 

Watson, J.T. Introduction to Mass Spectrometry 3rd ed. Lippincott-Raven New York, 1997. 

The introductory chapter is brief but provides an ample introduction to mass spectrometry and leaves one comfortable as he/she moves on to the historical and instrumentation chapters that follow. A few of the basic equations are given as part of the review of basic concepts. In these few pages Dr Becker clearly introduces the concepts of atomic mass units relative to carbon, isotopes and isotope abundance. Figures 1.1 and 1.2 go hand in hand in providing the reader with the three major parts of a mass spectrometer (source, ion separation, detection) and show various alternatives for each of these. The subtle use of color in these and subsequent figures adds an attractive benefit for the reader. 

K. Doward, Mass Spectrometry A Foundation Course (Cambridge Royal Society of Chemistry, 2004) C. G. Herbert and R. A. W. Johnstone, Mass Spectrometry Basics (Boca Raton, FL CRC Press, 2002) J. Barker, Mass Spectrometry, 2nd ed. (Chichester, UK Wiley, 1999) J. T. Watson, Introduction to Mass Spectrometry, 3rd ed. (Philadelphia Lippincott-Raven, 1997) R. A. W. 

Watson JT, Sparkman OD (2007) Introduction to mass spectrometry instrumentation, applications and strategies for data interpretation, 4th edn. Wiley-Blackwell, Chichester... 

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