Gas Chromatography and Mass Spectrometry: A Practical Guide

O. David Sparkman, Zelda Penton, Fulton G. Kitson

• Editore: Academic Press
• Anno: 2011
• ISBN:
  9780080920153
• Edizione: 2
• Argomento: Physical Sciences
• Categoria: Spectroscopy & Spectrum Analysis

The second edition of Gas Chromatography and Mass Spectrometry: A Practical Guide follows the highly successful first edition by F.G. Kitson, B.S. Larsen, and C.N. McEwen (1996), which was designed as an indispensible resource for GC/MS practitioners regardless of whether they are a novice or well experienced. The Fundamentals section has been extensively reworked from the original edition to give more depth of an understanding of the techniques and science involved with GC/MS. Even with this expansion, the original brevity and simple didactic style has been retained. Information on chromatographic peak deconvolution has been added along with a more in-depth understanding of the use of mass spectral databases in the identification of unknowns. Since the last edition, a number of advances in GC inlet systems and sample introduction techniques have occurred, and they are included in the new edition. Other updates include a discussion on fast GC and options for combining GC detectors with mass spectrometry. The section regarding GC Conditions, Derivatization, and Mass Spectral Interpretation of Specific Compound Types has the same number of compound types as the original edition, but the information in each section has been expanded to not only explain some of the spectra but to also explain why certain fragmentations take place. The number of Appendices has been increased from 12 to 17. The Appendix on Atomic Masses and Isotope Abundances has been expanded to provide tools to aid in determination of elemental composition from isotope peak intensity ratios. An appendix with examples on "Steps to follow in the determination of elemental compositions based on isotope peak intensities" has been added. Appendices on whether to use GC/MS or LC/MS, third-party software for use in data analysis, list of information required in reporting GC/MS data, X+1 and X+2 peak relative intensities based on the number of atoms of carbon in an ion, and list of available EI mass spectral databases have been added. Others such as the ones on derivatization, isotope peak patterns for ions with Cl and/or Br, terms used in GC and in mass spectrometry, and tips on setting up, maintaining and troubleshooting a GC/MS system have all been expanded and updated.

• Covers the practical instruction necessary for successful operation of GC/MS equipment
• Reviews the latest advances in instrumentation, ionization methods, and quantitation
• Includes troubleshooting techniques and a variety of additional information useful for the GC/MS practitioner
• A true benchtop reference
• A guide to a basic understanding of the components of a Gas Chromatograph-Mass Spectrometer (GC-MS)
• Quick References to data interpretation
• Ready source for information on new analyses

Chapter 1. Introduction and History

Gas chromatography/mass spectrometry (GC/MS) is the most ubiquitous analytical technique for the identification and quantitation of organic substances in complex matrices. The gas chromatograph-mass spectrometer (GC-MS) is indispensable in the fields of environmental science, forensics, health care, medical and biological research, health and safety, the flavor and fragrances industry, food safety, packaging, and many others. The instrumentation ranges in price from nearly 1 million dollars to just a few thousand. The size is large enough to require a 4-m×4-m room to that of an average briefcase (Figure 1.1).

Figure 1.1 The Thermo Scientific DFS GC/MS system (left), a high-resolution double-focusing magnetic-sector GC-MS that plays a significant role in the high-resolution (HR) selected ion monitoring (SIM) analysis of trace levels of dioxins; and the Torion Technologies, Inc., GUARDION-7™ (right) used by environmentalists and first-responders to hazardous waste spills.

GC/MS is the synergistic combination of two powerful microanalytical techniques. The gas chromatograph separates the components of a mixture in time, and the mass spectrometer provides information that aids in the structural identification of each component. This combination has several advantages [1]. First, it separates components of a complex mixture so that mass spectra of individual compounds can be obtained for qualitative purposes; second, it can provide quantitative information on these same compounds. Mass spectrometry ionization techniques that require gas-phase analytes are ideally suited to GC/MS because sample volatility is a requirement of gas chromatography (GC). The gas chromatograph, the mass spectrometer, and the interface linking these two instruments are described in the following chapters.

GC/MS can provide a complete mass spectrum from a few femtomoles of an analyte; ideally, this spectrum gives direct evidence for the nominal mass and provides a characteristic fragmentation pattern or “chemical” fingerprint that can be used as the basis for identification along with the gas chromatograph retention time.

Mass spectrometry had its origin ca. late 1800s with the work of John Joseph Thomson [2] and Wilhelm (Willy) Carl Werner Otto Fritz Franz Wien [3]. Mass spectrometry was dominated by the measurement of the various nuclides ^* that made up the known elements of the time until the mid-part of the 20th century when the mass spectrometer’s use for the analysis of petroleum products and other organic compounds began to gain momentum.

^*A nuclide is an atomic species characterized by the total number of neutrons and protons in the nucleus. Although not synonymous with isotope, for the purposes of this book, a nuclide is one of the many atomic species that are characterized by both their atomic number and their mass number.

Chromatography began about the same time as mass spectrometry (ca. 1900) with the seminal publication by Mikhail Semenovich Tsvet (two other papers appeared in German that are often mistakenly referenced as the beginning of chromatography: Tswett MS) 4.5.6. and 7.. The early practice of chromatography consisted of the application of liquid samples to short homemade columns of various absorbents or to absorbent paper. The report of partition chromatography by Archer John Porter Martin and Richard Laurence Millington Synge 8. and 9. in 1941 led to the development of GC by Martin and Anthony Tarfford James 10. and 11. in 1950.

Very soon after the development of GC, attempts to interface the gas chromatograph with the mass spectrometer began. This was a natural development as the gas chromatograph separates organic compounds, and they eluted from the column in a purified state in the gas phase; and the mass spectrometers of that time required pure gas-phase analytes for ionization. However, the original gas chromatographs used packed columns with flow rates (20–30mLmin⁻¹) that overwhelmed the required low pressures of the mass spectrometer. One of the main obstacles to the technique of GC/MS was this incompatibility in pressure requirements. Today’s instrumentation is faced with far fewer such problems because of the use of capillary columns with flow rates that are usually 1.5mLmin⁻¹ or less, and much better pumping systems to maintain the vacuum required for the mass spectrometer.

As will be forever debated, the actual first attempt to interface the gas chromatograph and the mass spectrometer was accomplished by either Joseph C. Holmes and Francis A. Morrell at Philip Morris, Inc., in Richmond, Virginia, who published their work on the interfacing of a gas chromatograph with a Consolidated Engineering Corporation (CEC) Model 21-103B magnetic-sector mass spectrometer in 1957 [12] or Roland S. Gohlke and Fred McLafferty (both at Dow Chemical Company in Midland, Michigan, at that time) who presented their work on interfacing a gas chromatograph with a time-of-flight (TOF) mass spectrometer at the 129th National American Chemical Society (ACS) meeting in April of 1956 in a symposium on Vapor Phase Chromatography[13]. This work was first published in a paper authored only by Gohlke in the April 1959 issue of Analytical Chemistry, almost a year after it was received by the journal on May 31, 1958 [14] and almost 3 years after the 129th ACS meeting. The Holmes/Morrell work was first presented at the Fourth Annual Meeting of American Society for Testing and Materials (ASTM) Committee E-14 on Mass Spectrometry and Allied Topics in Cincinnati, Ohio, in May of 1956.

The GC-MS of today is a unique instrument. Gohlke/McLafferty and Holmes/Morrell treated their systems as a gas chromatograph being used as an inlet to a mass spectrometer; there are some who would treat the mass spectrometer as a detector for the gas chromatograph. Neither of these is true. The mass spectrometer is not a gas chromatograph detector, and the gas chromatograph is not an inlet for the mass spectrometer. It is important to remember that GC/MS is as different from either GC or mass spectrometry as GC and mass spectrometry are from one another. This is because an elevated pressure is required to separate the components of a mixture in a gas chromatograph and a greatly reduced pressure is required to separate the ions of various mass-to-charge ratios (m/z values) that characterize a pure component of that mixture.

Both the Gohlke/McLafferty and the Holmes/Morrell attempts at interfacing the gas chromatograph and the mass spectrometer involved splitting a small portion of the gas chromatograph eluate to the mass spectrometer, with the remainder being diverted to either a conventional gas chromatograph detector or the atmosphere. This was necessary to circumvent the conflicting high-/low-pressure needs of the two instruments. In the 1960s, devices were developed to enrich the eluate from packed gas chromatograph columns with respect to the analyte. These devices, for the most part, have now fallen into disuse because of the use of capillary gas chromatograph columns that produce eluates much richer in analyte concentration than packed columns and improved vacuum systems. Today’s modern instrument has the exit end of the gas chromatograph column placed directly in the ion source of the mass spectrometer.

A very significant factor in the evolution of GC/MS was the development of data systems. When GC/MS was first being explored, it was readily seen that the potential for the volume of data was overwhelming. A 10-minute chromatographic run, acquiring spectra at the rate of one per second, would result in a total of 600 spectra. Extracting the spectra associated with various chromatographic peaks and then dealing with the presence of mass spectral peaks that were due to background associated with the sample or the gas chromatograph column was quite daunting. ^* It was not until the development and commercialization of the minicomputer (ca. 1965) that it was possible to bring the computer to the GC-MS. Before that time, the data had to be brought from the mass spectrometer to the computer and input manually. When Digital Equipment Corp. (acquired by Compaq Computer, which was then acquired by Hewlett–Packard) introduced the first commercial minicomput...