Analysis of the State of the Art: Gas Chromatography Column Technology

In honor of LCGC's celebration of 30 years covering the latest developments in separation science, we asked a panel of experts to assess the current state of the art of gas chromatography (GC) column technology, and to try to predict how the technology will develop in the future. This article is part of a special group of five articles covering the state of the art in sample preparation, GC column technology, GC instrumentation, liquid chromatography (LC) columns, and LC instrumentation.

Recent Advances

We started by asking our experts what they considered the most important advances in GC column technology in the last decade. Two themes emerged: enhanced selectivity, with particular mention of ionic liquids, and improved column stability and inertness.

GC Column Technology Expert Panel

"Stationary phase selectivity is often the most important factor in the separation of compounds, and the marketplace has seen an increase in offerings, including columns designed for specific targeted separations," said Frank Dorman of Penn State University. "Inertness has been improved by a few of the largest manufacturers, resulting in new lines of columns enabling lower detectability of reactive compounds."

For Paola Dugo, Luigi Mondello, and Peter Tranchida of the University of Messina, ionic compounds were high on the list of valuable developments. "Ionic liquids are characterized by interesting properties, such as high thermal stability and a dual-type selectivity," they commented. "Specifically, the same ionic liquid can show a high selectivity toward both apolar and polar compounds."

Philip Marriott of Monash University, in turn, feels the most significant recent improvement is the enhanced selectivity achieved with coupled-column technology, largely based on microfluidic methods. "Smart coupled-column technologies should be able to provide both improved separation and an increased sensitivity if properly implemented, and these address the goals that have been searched for since GC was invented," he said.

John Seeley of Oakland University, however, was most impressed by another technology development: direct column heating strategies. "Several studies have shown how these modules allow high-resolution multidimensional separations to be produced with far greater ease than if conventional column ovens were used," he said. "I think these low thermal mass heating modules will have a large impact on the coupled-column separations developed of the future."

The Biggest Problems, and How to Overcome Them

In spite of recent advances, however, challenges remain. John Hinshaw, of BPL Global and the editor of LCGC's "GC Connections" column, believes that the areas of greatest advance are also on the list of ongoing improvements still needed: deactivation and thermal stability. Other key issues cited by panelists included improving column ruggedness and stability over its lifetime, better low-bleed columns, and the limited chromatographic background of many end users.

"The continual development of low-bleed stationary phases for gas chromatography–mass spectrometry (GC–MS) will mark important advances in column technology," notes Jared Anderson of the University of Toledo.

Another area with room for improvement is thin-film coatings, particularly for high-speed and GC×GC separations. "Problems can be encountered with thin stationary phase films, namely in relation to a non-uniform coating across the column length," note the group from the University of Messina. Marriott seconded that idea. "Making reproducible, uniform, thermally stable, very thin film phases in narrow-bore format with adequate deactivation of the support (capillary wall) will be of interest to technologists," he notes.

Another important challenge is the need for users to have available, and understand, the use of multiple stationary phases. "As interest in multidimensional GC moves from academic laboratories to industry, classification and utilization of orthogonal stationary phases will be important," noted Anderson. "Therefore, new stationary phases will continue to be developed for application-specific separations."

Seeley agrees. "I think it is important that theoretical tools are developed that allow users to accurately predict the retention of analytes on a wide range of stationary-phase combinations," he said.

Dorman sees user understanding of columns to be important generally, even for one-dimensional GC, particularly given that today's end users of GC often do not have a high degree of specialized training. "Users are less interested and able to optimize separations, and often try to separate everything using the same GC column," he notes. "In many cases, simple installation can be a major source of poor analytical performance." He wonders whether these challenges can be overcome by changing the format of the GC column to allow for easier installation, or alternatively, if hardware improvements could reduce the potential for incorrect installation.

Improving GC Columns

We also asked what challenges must be overcome to improve GC columns. Dorman responded that the efficiency of many current GC columns, particularly polydimethylsiloxane-based stationary phases, is already near the theoretical maximum, but that the analytical performance of other stationary phase materials would benefit from technological improvements. "Materials like polar stationary phases, ionic liquids, and shape-selective stationary phases are far from the performance of the more common siloxanes, and for these materials to achieve general-purpose use, they will need to be improved," he said.

The development of novel stationary phases is particularly important for multidimensional GC, our panelists point out. "Real-world samples are often of high complexity and, hence, a change in a single-column stationary phase does little to improve the final number of totally isolated compounds," the Messina group said. "With multidimensional GC approaches (both heart-cutting and comprehensive), however, the combination of stationary phases that have entirely different retention mechanisms can greatly improve the separation of complex samples."

Seeley also sees the need for a suite of stationary phases that exploits the full range of analyte intermolecular interactions. "For example, right now we do not have many stationary phases that interact with an analyte's hydrogen bond basicity," he says. "For instance, separating ethers from hydrocarbons is quite challenging with our current columns, but the hydrogen bond basicities of these classes are completely different."

Hinshaw sees the catalytic decomposition of thermolabile compounds as an important area that needs improvement. Marriott agrees. "In GC×GC methods, there is no 'hiding' from the effects of volatile degradation products, that in a 1D GC experiment tend to be accepted as an inevitable consequence of doing the business of GC analysis," he says. "Modulated bleed peaks can be seen as streaks throughout the 2D GC plot." Improved phase stability should receive much more attention from manufacturers, he concludes.

The Relationship Between Column Technology and Instrument Development

When considering the potential effects of GC column technology on new instrument development, the experts foresee a variety of possible paths.

Hinshaw believes that better deactivation and stability may eliminate the need for some of the old-style inlets (classical split–splitless).

Anderson foresees continued instrument innovation being driven by continued expansion of multidimensional GC.

Seeley sees that development going in the opposite direction, with new technologies facilitating the growth of GC×GC. Improved GC column technology will spur the development of new devices that are capable of fully exploiting the resolving power of the column, he says. "For example, the high resolution and wide temperature range of today's GC columns have led to the development of new monolithic fittings and fluidic devices that provide low internal volume, leak-free connections. Such devices will be important components in new multidimensional GC instruments."

The Messina group, in turn, feels that new GC instrument development will be focused on two aspects. The first is the reduction of extracolumn band broadening, which may be possible to achieve by improving injection systems and detectors. The other is more efficient GC column heating and cooling. "Technological evolution will enable the development of more efficient GC ovens," they predict.

Marriott has a different view. "The only significant effect that column technology can have on instrument design is miniaturization," he says. And that should spur improvements in field deployable instruments. "It is now decades since true microscale GC column dimensions were first demonstrated, and there seems to be increased activity in this area," he says. "As columns get smaller, having a classical dimension GC instrument makes no sense, and so commensurate shrinkage of all components of the gas chromatograph will be logical — the oven housing, detectors, heating methods, and related power demands."

Dorman, on the other hand, does not see a strong push for instrumentation improvements. "Since most separations are not run near optimal conditions, the columns and the hardware are not being pushed to the maximum right now, so there is no real pressure to change anything significant," he says. "Manufacturers change things like software and packaging instead." Because most end users are unaware of the shortcomings of current columns, he says, it takes a long time for changes in column formats to be widely adopted. "This results in less pressure to adapt instrumentation, and becomes a cyclical argument."

Looking to the Future

When asked to predict the future of GC column technology over the next five years, our experts had a variety of perspectives.

Anderson anticipates a higher demand for field analysis, as a result of the increasing use of GC for environmental analysis. "This requires GC column technology that possesses small diameter capillaries and stationary phases that are rugged and durable, even when they are introduced to fairly harsh environmental conditions such as water," he adds.

Dorman foresees price pressure from Asian manufacturers, which could spark Western manufacturers to develop column formats that are not as easily copied, and that allow for simpler use. There will be three sources of pressure to do this, he says. "It should allow for simpler installation and use, and it may allow for an increase in price due to the improvement in performance. Finally, it might be platform-specific, thereby 'forcing' loyalty to a specific vendor."

Seeley and Marriott see new phase chemistries, such as ionic liquids, as one area with potential to drive innovation. "For example, if ionic liquid phases become much more popular, then other manufacturers will have to respond in the phase innovation battle," says Marriott.

Hinshaw and Marriott both foresee increased emphasis on columns that are useful for high-speed, high-resolution operation (narrow-bore columns with thin films). Marriott feels that such an increased focus on fast GC methods should not be hard to accomplish, either, because current hardware is well suited to the narrow-bore columns needed for fast GC. "That fast GC should still be an underutilized technology is surprising, and perhaps suggests that GC users are rather conservative," he notes.

Seeley believes that high-speed and high-resolution separations will also see improvements from the direct column heating technology he cited as an important recent development. "It will be interesting to see if the independent heating of specific column segments will allow new separation strategies to be developed," he adds.

Small-Diameter Capillary Columns

The experts also foresee an increase in use of small-bore capillary columns, particularly as high-speed and GC×GC applications increase. "The main route towards faster GC analysis, and comprehensive 2D GC analysis, is by using microbore capillaries," note the Messina group. "With regards to fast GC, it is now possible to shorten analyses times by a factor of 4–5 times [by using small-bore columns], with no price to pay in terms of resolution." They also note that in comprehensive 2D GC analysis, the use of small-bore columns can provide "astonishing" levels of resolving power.

Marriott agrees that the move to narrow-bore columns is natural. "Perhaps it is surprising that a question like this should even need to be asked," he says. "It highlights the fact that such technology is not as widely used as it should be within the GC community!"

Both Dorman and Seeley, however, point out that the improved separation efficiency of narrow-bore columns comes at the cost of sample capacity, which relegates them to use in niche applications. "This is a compromise situation that currently limits the practical column internal diameter for most separations to 150–180 μm," Dorman says.

Our experts also note other issues with small-diameter columns. "The interface of these small-diameter columns to current capillary injectors is also an issue; the injectors were not designed with these small internal diameters in mind," says Dorman. "For the use of 100-μm columns to become more common, the injectors will likely need to be modified to allow for more optimal interface of these products." Seeley also points out another limitation: "Conventional sample inlets and detectors often produce enough extracolumn broadening to diminish much of the benefits of small-diameter columns," he notes. "That said, small-diameter columns have been shown to be highly effective secondary columns for GC×GC analysis."

Packed Vs. Capillary Columns

All the experts agree that packed columns cannot compete with capillary columns in terms of resolving power, but that packed columns will not go away, particularly for industrial use.

"There are too many applications that are too difficult to perform on wall-coated open-tubular (WCOT) columns," says Dorman. "Also, packed columns offer many stationary phase selectivities that are not accessible using WCOT columns." He even believes that as the overall performance of the materials used in packed columns is improved, that they could see an increase in use for some applications that can be run on either column type.

Acknowledgment

I would like to extend a special thank you to Daniel W. Armstrong for serving as the chair of this GC Column Technology section of our special coverage for the 30th anniversary issue of LCGC. Armstrong is the Robert A. Welch Professor of Chemistry and Biochemistry at the University of Texas at Arlington, and a member of the Editorial Advisory Board of LCGC.