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Numerous articles have been published in the scientific literature
regarding faster methods for gas chromatography (GC), yet confusion
remains on how best to speed up separations. A significant source of
this confusion is the fact that authors often neglect to define the terms
"analysis speed" and "analysis time". Does the analysis time include
sample preparation time? Or is it just the run time between injection
and last time point on the chromatogram? Does it include recondition-
ing, paperwork, or interpretation? Is it the instrument time or the oper-
ator time? Numerous questions often are left unanswered and it is these questions that are
to blame for the chaos in fast GC. Here I will try to clarify this confusion.
A chromatographic analysis consists of four steps: sample preparation, chromatographic
separation, detection, and data interpretation. Clearly these steps are related and can not
be considered in isolation. Changes in the sample preparation might affect the perform-
ance of the separation, and more sensitive and selective detectors may allow simpler sample
preparation. It is these very strong interactions among the four steps that make it very dif-
ficult to describe the consequence of a change somewhere in the procedure on the total
analysis time. The next problem to consider is the fact that the term "total analysis time"
also is not very well defined. Is it the time-to-result for a sample, or is it the total operator
time for the analysis of 100 samples divided by 100? Because of all this confusion, infor-
mation from the literature on how to speed up GC analyses should be interpreted and
used with great care. It is the author’s sincere belief that these undefined terms have been,
and still are, major obstacles, to the success of faster GC. People have tried solutions
towards faster GC that too often did not work. This made people lose their confidence in
fast GC. However, we should not forget there are almost 20 methods for speeding up a GC
separation!1 If one selects the wrong route, all too often the conclusion is that fast GC does
not work, rather than that the analyst was wrong in his or her selection. Fast GC works
if—and only if—the correct route is selected. Doing that is much simpler than one might
expect. Simple guidelines can be followed to select the best option, if we restrict ourselves
to the chromatographic separation itself.
The selection of the best route to speed up a separation starts with an understanding of
why a chromatographic separation takes time. The total time a chromatogram takes is the
sum of all empty baseline segments plus the sum of the width of all baseline peaks. How
can we minimize the total time? Very simple: Get rid of the baseline, only separate those
peaks that need to be separated and make the peaks as narrow as possible. This sentence
summarizes the three main routes to faster GC. In correct scientific terms, and in the
correct order of implementation, one would describe them as 1) minimize resolution to
a value just sufficient, 2) maximize the selectivity of the chromatographic system, and
3) implement a method that reduces analysis time while holding resolution constant.
If your chromatogram contains baseline or over-resolved peaks, the first step in making
the separation faster is to eliminate this over-resolution. The options to do this include:
• shortening the column.
• working at an above optimum carrier gas velocity.
• increasing the initial temperature or the temperature programming rate.
• converting an isothermal separation to a programmed method.
• using flow programming.
• using a thinner film.
Only after having eliminated all baseline and situations of over-resolution should one
continue to step 2. But more importantly, if one does not have baseline or over-resolved
peaks, do not even consider using these options! Faster temperature programming has
been described as a universal solution for faster GC. But if your chromatogram is full of
peaks all just separated without any excess resolution, faster programming will ruin your
Continued on page 23
Achieving Faster GC
Hans-Gerd Janssen, Ph.D., Unilever Food and Health Research Institute
Editorial
Achieving Faster GC . . . . . . . . . . . . . . . . . . . . . 2
Petrochemical
Eliminate Column Breakage in High
Temperature Biodiesel Analysis . . . . . . . . . . 3
Environmental
Reliably Detect Pesticides Down to
10pg with Sensitive SIM GC/MS
Multiresidue Method . . . . . . . . . . . . . . . . . . . 6
PTV On-Column Liner Gives
You Two Inlets in One . . . . . . . . . . . . . . . . . . . 8
Air Monitoring
Early Detection of Structural Mold with
SilcoCan™ Air Sampling Canisters. . . . . . . 10
Foods, Flavors & Fragrances
Prepare Samples
in Half the Time Using a Fraction
of the Solvent with dSPE . . . . . . . . . . . . . . . 12
Prevent Fraud in Egg Pasta
with Simple Analysis of
Cholesterol and Glycerides . . . . . . . . . . . . . 14
Clinical/Forensic/Toxicology
Fast Screening and Confirmation
of Gamma-Hydroxybutyrate (GHB)
in Urine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Pharmaceutical
Beyond C18—Increase Retention
of Hydrophilic Compounds Using
Biphenyl Columns . . . . . . . . . . . . . . . . . . . . . 18
Two Options for Analyzing Potential
Genotoxic Impurities in Active
Pharmaceutical Ingredients . . . . . . . . . . . . 20
Bioanalytical
Reduce Downtime with Robust
Lipidomics Method . . . . . . . . . . . . . . . . . . . . 22
Restek Trademarks
Crossbond, Integra-Gap, MXT, Pinnacle, Press-Tight, Resprep,
Restek logo, Rtx, Rxi, SilcoCan, Uniliner
Erratum:
In Advantage 2008.02, Figure 1 on page 19 was incorrect.
The corrected figure can be seen at
in this issue
2008.03
Editorial