Analytical Gas Chromatography

Walter Jennings, Eric Mittlefehldt, Phillip Stremple

• Publisher: Academic Press
• Year: 1997
• Print ISBN: 9780123843579
• eBook ISBN: 9780080527208
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Gas chromatography remains the world's most widely used analytical technique, yet the expertise of a large proportion of chromatographers lies in other fields. Many users have little real knowledge of the variablesin the chromatographic process, the interaction between those variables, how they are best controlled, how the quality of their analytical results could be improved, and how analysis times can be shortened to facilitate the generation of a greater numberof more reliable results on the same equipment. An analyst with a more comprehensive understanding of chromatographic principles and practice, however, can often improve the quality of the data generated, reduce the analytical time, and forestall the needto purchase an additional chromatograph or another mass spectrometer. The Second Edition of Analytical Gas Chromatography is extensively revised with selected areas expanded and many new explanations and figures. The section on sample injection has been updated to include newer concepts of split, splitless, hot and cold on-column, programmed temperature vaporization, and large volume injections. Coverage of stationary phases now includes discussion, applications, and rationale of the increased thermal and oxidative resistance of the newly designed silarylenepolysiloxane polymers. Conventional and"extended range"polyethylene glycol stationary phases are examined from the viewpoints of temperature range and retention index reliabilities, and the chapter on"Variables"has been completely rewritten. The ways in which carrier gas velocity influences chromatographic performance is considered in detail, and includes what may be the first rational explanation of the seemingly anomalous effects that temperature exercises on gas viscosity (and gas flow). The practical effects that these changes cause to the chromatography is examined in pressure-, flow-, and"EPC-"regulated systems."Column Selection, Installation, and Use"has been completely rewritten as well. The accuracy of theVan Deemter plots has been greatly enhanced; a new program corrects for the first time for the changes in gas density and diffusion that occur during the chromatographic process because of solute progression through the pressure drop of the column. A new section has also been added on meeting thespecial requirements of columns destined for mass spectral analysis. The chapter on"Special Applications"has been expanded to include considerations of"selectivity tuning,"of fast analysis, and the section of Applications has been thoroughly updated and expanded. - Incorporates nearly 60% new material - Covers the newest concepts and materials for sample injection and stationary phases - Presents detailed consideration of the influence of carrier gas velocity on practical aspects of chromatographic performance - Contains a chapter on "Special Analytical Techniques" which includes consideration of selectivity tuning and fast analysis - Provides a new section addressing the special requirements of columns to be used in mass spectral analysis - Includes an improved program that greatly enhances the accuracy of the Van Deemter plots by more accurately depicting localized chromatographic conditions at each point in the column

CHAPTER 1

INTRODUCTION

1.1 General Considerations

In the late 1800s, Mikhail Tswett separated natural pigments into colored zones by percolating plant extracts through adsorbent-packed columns. He later used the word “chromatography” to describe this process [1, 2]. Our use of the word has broadened, and “chromatography” is now used for a number of processes in which the substances to be separated are subjected to equilibrium partitioning between two phases. In most cases, one of those phases is stationary and the other is mobile. The principles of liquid-liquid chromatography (LLC) are employed by the separatory funnel at one end of the spectrum, and by the Craig countercurrent distribution apparatus at the other extreme. Applications of liquid–solid chromatography range from paper—through column—to some forms of thin-layer chromatography.

Their work on liquid-solid chromatography (LSC) earned Nobel Prizes for A. J. P. Martin and R. L. M. Synge. It was in his award address that Martin suggested a gas might be used as the mobile phase in chromatographic processes. Some years later, James and Martin [3] subjected to the passage of ethyl acetate vapor a mixture of fatty acids that had been affixed to an adsorbent. In doing so, they demonstrated the sequential elution of the fatty acids. Coupled with an automated titration system, this generated a graph composed of a series of “steps” depicting the sequential additions of base as each successively eluted acid was neutralized by automated titration.

In 1954, Ray [4] inserted the sensing filament of a thermal conductivity cell, constituting one leg of a Wheatstone bridge, into the outlet of a gas chromatographic column. He generated the first modern-day “chromatogram” where each eluting substance generated a Gaussian-type peak. The schematics and chromatograms that Ray published stimulated a number of workers to enter what promised to become a new and exciting field. Within a decade, some hundreds of individual scientists were engaged in both basic and applied research in gas chromatography. Many of these contributions have been detailed elsewhere (e.g., [5, 6]), but several fundamental steps in the development of modern analytical gas chromatography deserve special mention. These include Golay’s invention of the open tubular column [7], Desty’s elegantly simple design for a glass-capillary-drawing machine [8], and the concept of a thin-walled fused silica column [9].

When a gas is employed as the mobile phase, either a liquid or a solid can be utilized as the stationary phase. These processes are “gas-liquid chromatography” (GLC) and “gas-solid chromatography” (GSC), respectively. The former has greater general utility and is more widely used, while the latter is especially useful for the separation of highly volatile compounds, including fixed gases (see later chapters). In popular usage, the term “gas chromatography” and the abbreviation “GC” are often applied to both processes.

1.2 A Simplistic Approach

In the process of gas chromatography, a thin film of the stationary phase is confined to the column, and continuously swept by a stream of mobile phase (i.e., carrier gas). The two extremes in column types are packed columns and open tubular columns.

Packed columns are typically 2–5 m long, 1–5 mm in internal diameter (ID) (d_c), and are filled with an “inert” granular support, each particle of which is coated with the stationary phase. As implied by the name, micropacked columns are a smaller version of the packed column, usually having IDs of less than 1 mm, and smaller packing granules. The length of a packed column is practically limited by the pressure drop generated by the resistance it offers to gas flow.

There are three general types of open tubular columns. The most widely used is the wall-coated open tubular (WCOT) column, in which the stationary phase exists in the form of a uniform thin film affixed to the inner periphery of an open tube, the column. In porous layer open tubular (PLOT) columns, a porous layer exists on the inner wall of the column, while the central portion is open. Porosity of that layer is sometimes achieved by chemical means such as etching of the wall per se, and in other cases by deposition of the porous particles from a suspension. The porous layer may serve as a support for a stationary phase, or as the “stationary phase” per se. SCOT (support coated open tubular) columns are a form of PLOT column. Commonly used sorbents include porous polymers, aluminum oxide, and selected zeolites. In some open tubular columns, the d_c may be as large as 0.5–0.75 mm. While these are open tubular columns, they should not be regarded as true “capillaries.” We will consider all of the above columns in this book, but the use of the word “capillary” will be restricted to columns whose inner diameters do not exceed 0.35 mm.

Whether it is packed or open tubular, the column, which in the normal GC system is connected to the inlet of the gas chromatograph at one end and to the detector at the other, is adjusted to some suitable temperature and continuously swept with the mobile phase (carrier gas). When a mixture of volatile components is introduced to the inlet end of the column, each solute in that sample engages in a highly dynamic equilibrated partitioning between the stationary phase and the mobile phase in accordance with its distribution constant (K_c = c_s/c_m). Let us consider a single band of solute at some one point in time: as the solute molecules in the gas phase are swept forward by the carrier gas, those in the stationary phase are carried downcolumn a finite distance. At that instant, the equilibrium distribution K_c is violated at the rear of the band (where c_s is finite and c_M is zero) and at the front of the band (where c_s is zero and c_M is finite). To reestablish the distribution constant throughout the band, the dominant partitioning is from stationary phase to mobile phase at the rear of the band, and from mobile phase to stationary phase at the front of the band. In other words, the flow of carrier gas disrupts the equilibrium distribution at the front and rear of each chromatographing solute band, causing continuous evaporation at the rear and reestablishment at the front of each solute band as it chromatographs through the column (Fig. 1.1). Because all solutes are injected simultaneously, separation is obviously contingent on differences between the K_c values of the individual solutes. The proportion of a solute that is in the mobile phase at any given time is a function of the “net” vapor pressure of that solute; molecules of those components exhibiting higher vapor pressures partition more toward the mobile phase. They are swept toward the detector more rapidly and are the first solutes eluted from the column. Other solutes exhibit lower vapor pressures, either because they are higher-boiling or because they engage in interactions with the statio...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• PREFACE
• ABOUT THE AUTHORS
• Chapter 1: INTRODUCTION
• Chapter 2: THE OPEN TUBULAR COLUMN
• Chapter 3: SAMPLE INJECTION
• Chapter 4: THE STATIONARY PHASE
• Chapter 5: VARIABLES IN THE GAS CHROMATOGRAPHIC PROCESS
• Chapter 6: COLUMN SELECTION, INSTALLATION, AND USE
• Chapter 7: INSTRUMENT CONVERSION AND ADAPTATION
• Chapter 8: SPECIAL ANALYTICAL TECHNIQUES
• Chapter 9: SELECTED APPLICATIONS
• Chapter 10: TROUBLESHOOTING
• Appendix: ABBREVIATIONS, TERMS, AND NOMENCLATURE
• INDEX