OVERVIEW

3.1 CENTRAL ROLE PLAYED BY COLUMN

The gas chromatographic column can be considered to be the central item in a gas chromatograph. Since the early 1970s the nature and design of the column has changed considerably from one containing either a solid adsorbent or a liq- uid deposited on an inert solid support packed into a length of tubing to one containing an immobilized or crosslinked stationary phase bound to the inner surface of a much longer length of fused-silica tubing. With respect to packing materials, solid adsorbents such as silica gel and alumina have been replaced by porous polymeric adsorbents while the vast array of stationary liquid phases in the 1960s was greatly reduced in number by the next decade to a smaller number of phases of greater thermal stability. These became the precursors of the chem- ically bonded or crosslinked phases of today. Column tubing fabricated from

copper, aluminum, glass, and stainless steel served the early analytical needs of gas chromatographers. Presently, fused-silica capillary columns having a length of 10–60 m and an inner diameter of 0.20–0.53 mm are in widespread use.

Although gas chromatography (GC) may be viewed in general as a mature analytical technique, improvements in column technology, injection, and detec- tor design steadily appear nonetheless. Innovations and advancements in gas chromatography since the mid-1980s have been made with the merits of the fused-silica column as the focal point and have been driven primarily by the environmental, petrochemical, and toxicological fields as well as by advances in sample preparations and in mass spectrometry.

3.2 JUSTIFICATION FOR COLUMN SELECTION AND CARE

The cost of a gas chromatograph can range from $6000 to over $100,000 depend- ing on the type and number of detectors, injection systems, and peripheral devices such as data system, headspace and thermal desorption units, pyrolyzers, and autosamplers. When one also factors in the purchase of high-purity gases on a regular basis required for operation of the chromatograph, it quickly becomes apparent that a sizable investment has been made in capital equipment. For example, cost-effectiveness and good chromatographic practice dictate that users of capillary columns should give careful consideration to column selection. The dimensions and type of capillary column should be chosen with the injection system and detectors in mind, considerations that are virtually nonissues with packed columns. Careful attention should also be paid to properly implemented connections of the column to the injector and detector and the presence of high boilers, particulate matter in samples, and other factors.

The price of a column ($200–$800) may be viewed as relatively small com- pared to the initial, the routine, and preventive-maintenance costs of the instru- ment. In fact, a laboratory may find that the cost of a set of air and hydrogen gas cylinders of research grade purity for FID operation is far greater than the price of a single conventional capillary column! Consequently, the column should be carefully selected for an application, handled with care following the sugges- tions of its manufacturer, and installed as recommended in the user’s instrument manual to derive maximum performance from a gas chromatographic system.

The introduction of inert-fused silica capillary columns in 1979 markedly changed the practice of gas chromatography, enabling high-resolution separations to be performed in most laboratories. Previously such separations were achieved with reactive stainless-steel columns and with the glass columns. After 1979 the use of packed columns began to decline. A further decrease in usage of packed columns occurred in 1983 with the arrival of the megabore capillary column of 0.53 mm inner diameter, which serves as a direct replacement for the packed column. These developments, in conjunction with the emergence of immobilized or crosslinked stationary phases specifically tailored for fused silica capillary columns and overall improvements in column technology, have been responsible for the greater acceptance for capillary GC.

Trends The results of a survey of 12 leading experts in gas chromatography appeared in 1989 and outlined their thoughts on projected trends in gas chro- matographic column technology, including the future of packed columns versus capillary columns (1). Some responses of that panel are

1. Packed columns are used for approximately 20% of gas chromatographic analyses.

2. Packed columns are employed primarily for preparative applications, fixed gas analysis, simple separations, and those separations where high resolu- tion is not required or not always desirable (PCBs).

3. Packed columns will continue to be used for gas chromatographic methods that were validated on packed columns where time and cost of revalidation on capillary columns would be prohibitive.

4. Capillary columns will not replace the packed column in the near future, although few applications require packed columns.

Majors, shortly thereafter in 1990, summarized the results of a more detailed survey on column usage in gas chromatography, this one, however, soliciting response from LC*GC readership (2). Some conclusions drawn from this survey include

1. Nearly 80% of the respondents use capillary columns.

2. Capillary columns of 0.25 and 0.53 mm i.d. are the most popular as are columns lengths of 10–30 m.

3. The methyl silicones and poly(ethylene glycol) stationary phases are the most preferred for capillary separations.

4. Packed columns are used mostly for gas–solid chromatographic separations such as gas analyses.

5. The majority of respondents indicated the need for stationary phases of higher thermal stability.

Although no additional surveys have been published since 1990, the above men- tioned trends still prevail to date for several reasons: (1) description of packed columns and related supplies and accessories have substantial presence in catalogs and Websites of the major column vendors and (2) the usage of packed columns users is abundant and has become apparent to the editors of this text after discus- sions with attendees in their GC short course offered at professional meetings.

Column manufacturers rely on the current literature, results of their own mar- keting surveys, the number of clicks on their Websites and so forth to keep abreast of the needs of practicing gas chromatographers. The fused silica capillary col- umn has clearly emerged as the column of choice for most gas chromatographic applications. A market research report (3) showed that $100 million were spent on capillary columns worldwide and, at an estimated average cost of $400 for a column, this figure represents about 250,000 columns, just in 1993 alone. The

number of columns and users has considerably increased since then. Despite the maturity of capillary GC, instrument manufacturers continue to improve perfor- mance of gas chromatographs, which has diversely extended the applications of gas chromatography.

Chromatographers can expect to see continued splendid efforts by capillary column manufacturers on producing columns having lower residual activity and being capable of withstanding higher column temperature operation with reduced column bleeding. With the increasing popularity of high-speed or fast GC (Chapter 5) and increasing presence of GCMS (Chapter 7) in the analytical laboratory, especially for environmental, food, flavor, and toxicological analyses, improvements in column performance that affect the MS detector have steadily evolved, namely, columns with reduced column bleed. There is also an increased availability of capillary columns exhibiting stationary phase selectivity tuned for specific applications obtained by synthesis of new phases (4), the blending of stationary phases, and preparation of phases with guidance from computer modeling (5).

3.3 LITERATURE ON GAS CHROMATOGRAPHIC COLUMNS

The primary journals where developments in column technology and applica- tions are published in hardcopy format and online versions include Analytical Chemistry, Journal of Chromatography A, Journal of Chromatographic Science, Journal of Separation Science (formerly the Journal of High Resolution Chro- matography, including the Journal of Microcolumn Separations) and LC*GC magazine. The biennial review issue of Analytical Chemistry —Fundamental Reviews (published in even-numbered years) contains concise summaries of developments in gas chromatography. An abundance of gas chromatographic applications may be found in the companion issue, Application Reviews (pub- lished in odd-numbered years) covering the areas of polymers, geologic materials, petroleum and coal, coatings, pesticides, forensic science, clinical chemistry, environmental analysis, air pollution and water analysis.

Most industrial and corporate laboratories as well as college and universities have access to literature searching through one of a number of online computer- ized database service, such as Sci Finder Scholar. Although the location of articles on gas chromatography in primary Journals is relatively easy, finding publica- tions of interest in lesser known periodicals can be a challenge and prove to be tedious at times. CA Selects and Current Contents are convenient alternatives.

The biweekly CA Selects —Gas Chromatography topical edition available from Chemical Abstracts Service, is a condensation of information reported throughout the world. Current Contents in media storage formats provides weekly coverage of current research in the life sciences, clinical medicine, the physical, chemical, and earth sciences as well as agricultural, biology, and environmental sciences.

The periodic commercial literature and annual catalog of column manufactur- ers describing applications for their columns also contains more and more useful

technical information of a generic nature with each passing year. In addition, this author strongly recommends LC*GC Magazine as a valuable resource in which not only timely technical articles appear but also sections devoted to “Column Watch” and troubleshooting for GC. However, the Internet has emerged as most extensive source of chromatographic information, particularly the Websites of the column manufacturers as described in the next section.

3.4 GAS CHROMATOGRAPHIC RESOURCES ON THE INTERNET The World Wide Web (WWW) has provided us with copious amounts of infor- mation via retrieval with search engines offered by an Internet service provider (ISP) (6); The Net has impacted our everyday activities with convenience of com- munication by e-mail, online placement of orders for all types of items and many other tasks. There are numerous Websites on gas chromatography in general, gas chromatographic columns, gas chromatographic detectors, and so on; all one has to do is locate them via “surfing the Net.” All manufacturers of gas chromato- graphic instrumentation, columns, and chromatographic accessories and supplies maintain and update their Website, which is also clearly indicated on their com- mercial literature. This author strongly suggests that you identify and regularly visit the Internet addresses of column manufacturers, for example, and “book- mark” the corresponding Websites. Internet addresses may change slightly from time to time or perhaps dramatically as the nature of any business changes today via expansion or consolidation. For example, there has been some consolidation in the GC column industry where J&W was purchased by Agilent Technolo- gies, Chrompack by Varian, and Supelco by Sigma-Aldrich. Therefore, prudence suggests that it is impractical to list here the exact Web addresses of vendors because they probably will ultimately change with time. However, “homepages”

are easily searchable and continually updated, serving as an outstanding source of reference material for the practicing chromatographer. Convenient listing of Websites and addresses of vendors may be found in the annual Buyers’ Guide in American Laboratory.

A listing of GC resources available on the Internet is simply too overwhelming a task. Nonetheless, a sampling of resourceful guides and information which one can find on gas chromatographic sites includes

Free downloads of software: retention time locking, method translation Technical libraries of chromatograms searchable by solute or class of solutes Column cross-reference charts

Application Notes

Guides to column and stationary phase selection Guides to column installation

Guides to derivatization Troubleshooting guides

Guides for syringe, septa, ferrule, and inlet liner selection

Guides for setting up a gas chromatograph

Past presentations at professional meetings such as Pittcon

Of the plethra of informative, significant ^.com and ^.org. sites, one site deserves special mention because it serves as a path both for immediate assis- tance for an analyst and also for the continuing education of users of GC and HPLC, namely, the Chromatography Forum maintained by LC Resources (www.lcresources.com.) There are several message boards, including a GC message board (and a liquid chromatography message board along with several others) where one can post anonymously a chromatographic problem or question while another individual(s) can post a response, initiating a dialog of commu- nications on the topic. This site offers broadening of one’s knowledge of the technique, even for the experienced user and is a particularly valuable asset for an analyst working in an environment where an individual is the sole chro- matography user or does not have access to other resources or assistance with technical problems.

PART 2 PACKED-COLUMN GAS CHROMATOGRAPHY