Fast GC Part 2: Techniques and Tradeoffs

Article written by Jamie Hoban, Organics QC Technician, Analytical Products Group, Inc.

Complete article from Edition 30 APG eNewsletter
To read the Part 1 of Jamie's article, Fast GC Part 1: History and Hardware, click here

Fast GC techniques range from relatively simple to the exotic, with costs and additional considerations increasing across this spectrum as well, providing a wide selection of possibilities for improvement in nearly any laboratory setting. This article will focus on a core set of knowledge and applications, with the intent to introduce Fast GC techniques that can be applied to existing methods or with a minimal investment in additional materials.

A basic understanding of related theory should be a requirement for any GC analyst. Attempts to adjust common methods should be approached with a more intimate grasp of what happens during the GC run, and why, as haphazard trial-and-error experiments can lead to hours of frustration without demonstrable improvement. Exhaustive explanations are beyond the scope of this presentation, so the outline here will be to resume from last month's article with data analysis and then proceed backwards through the instrument cycle to sample introduction.

GCs do not produce chromatograms, although the contradictory conception is seldom detrimental in standard work. This becomes an important point to remember, however, when fighting against decreases in resolution and peak shape in the enhanced analysis. Inappropriate data sampling and poor integration in the data analysis software can mask or terribly degrade adequate data points collected by the instrument. Chromatograms should have approximately ten data points available across each peak to ensure proper representations. It is always better to error on the side of a data collection rate that is too high, as this can easily be adjusted down later. Data rates, described in Hertz (or data points per second), are compared to peak widths in seconds to determine the number of points for each peak.

As the title of this article is Techniques and Tradeoffs, it is time to start owning up to some of the bad news. As a start, many mass spectrometers are unable to handle full scan modes while collecting sufficient data in high-speed analysis. For this reason, it is best to rely on the three dimensional separations of mass spec as the primary aid in increasing instrument throughput. Additionally, high data rates in other GCs can reduce the ratio of signal-to-noise, the first indication that Fast GC is not always compatible with analyses already operating near detection limits. High rates of data collection can also lead to storage of larger data files. This difficulty is largely negated when considering that while data collection may lead to larger files, the total length of each run, and therefore the file overhead, is reduced.

Many method improvements are achieved through simple adjustments of the GC column and oven. A number of different programs that model or predict GC separations are available, and at least one form of this software should be in the toolkit of any serious developer. While experience and research can relinquish trial-and-error method development to a small number of specific experiments, modeling programs are capable of analyzing millions of flow and temperature possibilities in a matter of minutes. Use of this advantage also allows experimentation on many different phases without the need to invest thousands of dollars in a wide array of GC columns. It is important to keep in mind the realistic limits of the column and GC when using modeling software, as the programs will use any range of values supplied. Consult the appropriate manufacturers for information on maximum temperature and temperature ramps, and adhere to reasonable flow rates for carrier gas.

The commonly depicted GC triangle involves speed, sensitivity, and resolution as the three most important competing factors. Speed can be maximized while maintaining resolution in a number of ways. Maintaining phase ratio, or the comparison of a column's diameter to its film thickness, is one way to shorten analysis time while producing a chromatogram with an identical pattern of retention. Focusing method development with allowance for adequate, not excessive, resolution of the critical pair of analytes is another way to increase speed. As reductions of column diameter or film thickness are often an important aspect of the move towards Fast GC, sensitivity suffers once again. Smaller columns have smaller capacities and peak presentation will suffer quickly if too much material is presented into the column in effort to improve analyte response.

The column trend continues as we move to the GC inlet: smaller is better, and faster. While four-millimeter inlet sleeves are common for many GC methods, they can cause band broadening and incomplete transfers when column parameters are designed for high-speed analysis. Two-millimeter sleeves increase the flow of carrier gas through the inlet and help to compress the initial analyte band to reduce broadening during the rest of the instrument run.

Although small diameter sleeves can increase sensitivity by focusing more of the injected sample onto the column, the samples injected must be considerably smaller than a four-millimeter sleeve will allow. This punctuates the major points to be considered when pursuing Fast GC: Can the method under review absorb some loss of resolution without greatly affecting quantitation and reproducibility, and is sensitivity going to be an issue when smaller amounts of sample are presented to the GC detector? These are the wages of speed in Fast GC.