INTRODUCTION

The following guidelines can prolong the lifetime of a column:

1. Any gas chromatographic column, new or conditioned, packed or capillary, should be purged with dry carrier gas for 15–30 min before heating to a final elevated temperature to remove the detrimental presence of air.

2. A column should not be rapidly or ballistically heated to an elevated temperature but should be heated by slow to moderate temperature pro- gramming to the desired final temperature.

3. Excessively high conditioning and operating temperatures reduce the life- time of any gas chromatographic column.

4. Use “dry” carrier gas or install a moisture trap in the carrier-gas line. Do not inject aqueous sample on a column containing a stationary phase intolerant of water.

5. The accumulation of high-boiling compounds from repetitive sample injec- tions occurs at the inlet end of the column and results in discoloration of the packing. It is a simple matter to remove the discolored segment of packing and replace it with fresh packing material. This action prolongs the column lifetime.

6. Do not thermally shock a column by disconnecting it while it is hot. Allow the column to cool to ambient temperature prior to disconnection. Packings are susceptible to oxidation when hot.

7. Cap the ends of a column for storage to prevent air and dust particles from entering the column. Save the box in which a glass column was shipped for safe storage of the column.

when he illustrated that a long length of capillary tubing having a thin coat- ing of stationary-phase coating the narrow inner diameter of the tube offered a tremendous improvement in resolving power compared to a conventional packed column (43). Such a column is also often referred to as a wall-coated open tubu- lar column (WCOT). The high permeability or low resistance to carrier-gas flow of capillary column enables a very lengthy column to generate a large number of theoretical plates.

In contemporary practice, separations of high resolution are attainable by cap- illary GC, as illustrated in the chromatogram in Figure 3.14, which was generated with a conventional fused-silica capillary column. An exploded view of this chro- matogram of a gasoline-contaminated jet A fuel mixture with a data acquisition system indicates the presence of over 525 chromatographic peaks. Because of its separation power, capillary gas chromatography has become synonymous with the term “high-resolution gas chromatography.”

3.8.2 Chronology of Achievements in Capillary GC

The first column materials employed in the developmental stage of the technique were fabricated from plastic materials (Tygon and nylon) and metal (aluminum, nickel, copper, stainless steel, and gold). Plastic capillaries, which are thermo- plastic in nature, had temperature limitations, whereas metallic capillary columns had the disadvantage of catalytic activity. Rugged, flexible stainless-steel columns rapidly became state-of-the-art, and were widely used for many applications, mainly for petroleum analyses. The reactive metallic surface proved to be unfa- vorable in the analysis of polar and catalytically sensitive species. In addition,

FIGURE 3.14 Chromatogram of a sample of jet A fuel contaminated with gasoline on 30-m×0.25-mm-i.d. HP-1 (0.25-µm film). Column temperature conditions: 30^◦C (5 min), 2^◦C/min to 250^◦C; split injection (100 – 1). Det: FID.

only the split-injection mode was available for quite some time for the introduc- tion of the small quantities of sample dictated by a thin film of stationary phase within the column. As the surface chemistry of glass was gradually studied and understood, capillaries made of borosilicate and sodalime glass became popular in the 1970s and replaced metal capillary columns (44). Here the metal oxide content and presence of silanol groups on the glass surface necessitated carefully controlled deactivation and coating procedures, but separations obtained on glass capillaries were clearly superior to those obtained with metal capillaries. The fragility of glass often proved to be problematic, requiring restraightening of a capillary end upon breakage with a minitorch followed by a recoating of the straightened portion with a solution of stationary phase to deactivate the straight- ened segment. Patience was also helpful! Today equivalent or superior separations with a fused-silica capillary column can be generated with the additional feature of ease of use.

The most significant advancement in capillary gas chromatography occurred in 1979 when Dandeneau and Zerenner of Hewlett–Packard (at the time) introduced fused silica as a column material (45,46). The subsequent emergence of fused silica as the column material of choice for high-resolution gas chromatography is responsible for the widespread use of the technique and has greatly extended the range of gas chromatography. In the next two decades, there were several other major developments in capillary gas chromatography. Instrument manufacturers responded to the impact of fused-silica columns by designing chromatographs with injection and detector systems optimized in performance for fused-silica columns. There were also concurrent advances in the area of microprocessors.

Reporting integrators and fast data acquisition systems with increased sampling rates now are available to be compatible with the narrow bandwidths of cap- illary peaks. The stature of the capillary column has been further enhanced by continuing improvements in the performance and thermal stability of the sta- tionary phase within the column. A column containing a crosslinked phase, a silarylene phase, or silphenylene phase, for example, has an extended lifetime because it has high thermal stability and can tolerate large injection aliquots of solution without redistribution of the stationary phase. Inlet discrimination was addressed with the development of on-column injection, the programmed tem- perature vaporizer, electronic pressure-controlled injection and, more recently, the large volume injector with cool on-column inlet mode.

Since Golay’s proposal of the use of the capillary column, capillary gas chro- matography has exhibited spectacular growth, maturing into a powerful analytical technique. Some of the more notable achievements in capillary gas chromatog- raphy are listed in Table 3.13.

3.8.3 Comparison between Packed and Capillary Columns

Three stages in the evolution of the capillary-column technology are presented in Figure 3.15: a packed-column separation and two separations with a stainless- steel and glass capillary column. Better resolution is evident with the capillary

TABLE 3.13 Advancements in Capillary Gas Chromatography

Year Achievement

1958 Theory of capillary column performance, GC Symposium in Amsterdam

1959 Sample inlet splitter

1959 Patent on capillary columns by Perkin – Elmer 1960 Glass drawing machine developed by Desty 1965 Efficient glass capillary columns

1975 First capillary column symposium 1978 Splitless injection

1979 Cold on-column injection

1979 Fused silica introduced by Hewlett – Packard

1981 – 1984 Deactivation procedures and crosslinked stationary phases

1983 Megabore column introduced as an alternative to the packed column 1981 – 1988 Interfacing capillary columns with spectroscopic detectors (MS,

FTIR, AED)

1992 – 2002 Programmed temperature vaporizer electronic pressure- controlled sample inlet systems; MS-grade columns, solid-phase

microextraction sampling techniques, large-volume injectors, advances in GCMS, more affordable bench – top GCMS systems Source: Some data here courtesy of Agilent Technologies.

chromatograms because more peaks are separated and smaller peaks can be detected. The superior performance of the glass capillary column is clearly apparent.

In addition to providing a separation where peaks have narrower bandwidths compared to a packed-column counterpart, a properly prepared fused-silica cap- illary column, which has an inert surface (less potential for adverse adsorptive effects toward polar species), yields better peak shapes; bands are sharper with less peak tailing, which facilitates trace analysis as well as provides more reliable quantitative and qualitative analyses. Sharp, narrow bands of the trace com- ponents present in a capillary chromatogram such as that in Figure 3.14 have increased peak height relative to the peak of the same component at identical concentration in a packed-column chromatogram where the peak may be unre- solved or disappear in the baseline noise. Moreover, because of the low carrier-gas flowrate, greater detector sensitivity, stability, and signal-to-noise levels are pos- sible with a capillary column. One drawback of the capillary column, though, is its limited sample capacity, which requires dedicated inlet systems to introduce small quantities of sample commensurate with a low amount of stationary phase.

Operational parameters of packed and capillary columns are further contrasted in Table 3.14.

The superior performance of a capillary column can be further viewed in the following manner. Because of the geometry and flow of a gas through a packed bed, molecules of the same solute can take a variety of paths through the column

FIGURE 3.15 Optimized separations of peppermint oil on (a) 6-ft×0.25-in.-i.d.

packed column, (b) 500-ft×0.03-in.-i.d. stainless-steel capillary column; and (c) 50-m× 0.25-mm-i.d. glass capillary column. Stationary phase on each column was Carbowax 20M. [W. Jennings, J. Chromatogr. Sci. 17, 637 (1977). Reproduced from the Journal of Chromatographic Science by permission of Preston Publications, a Division of Preston Industries, Inc.]

TABLE 3.14 Comparison of Wall-Coated Capillary Columns With Packed Columns

Packed Capillary

Length (m) 1 – 5 5 – 60

Inner diameter (mm) 2 – 4 0.10 – 0.53

Plates per meter 1000 5000

Total plates 5000 300,000

Resolution Low High

Flowrate (mL/min) 10 – 60 0.5 – 15 Permeability (10⁷ cm²) 1 – 10 10 – 1000 Capacity 10 µg/peak >100 ng/peak Liquid film thickness 10 0.1 – 1 (µm) Source: Data obtained in References 47 and 48.

enroute to the detector (via eddy diffusion), whereas in a capillary column all flow paths have nearly equal length. The open geometry of a capillary column causes a lower pressure drop, allowing longer columns to be used. Since a packed column contains much more stationary phase, often thickly coated on an inert solid support, there are locations in the packing matrix where the stationary phase spans or spreads over to adjacent particles (Figure 3.2a). Some molecules of the same component encounter thinner regions of stationary phase, whereas other molecules have increased residence times in these thicker pools of phase, all of which create band broadening. On the other hand, a capillary column contains a relatively thin film of stationary uniformly coated on the inner wall of the tubing. These factors, collectively considered, are responsible for the sharp band definition and narrow retention time distribution of molecules of a component eluting from a capillary column.

At higher column oven temperatures with increased linear velocity of carrier gas, capillary separations can be achieved that mimic those on a packed column but with a shortened time of analysis. The reduced amount of stationary phase in a capillary column imparts another advantage to the chromatographer, namely, one observes less bleed of stationary phase from the column at elevated temperatures and this means less detector contamination. Theoretical considerations of the capillary column are discussed in Section 3.10.

3.9 CAPILLARY COLUMN TECHNOLOGY