Chromatography Techniques » June 2013
Harvesting the power of application-specific phases, as opposed to general purpose columns,
allows both performance and productivity gains.
» by Kristen Parnell, Brand Manager for GC Columns and Accessories, Phenomenex, Inc., Torrance, Calif.
Figure 1: The master resolution equation.
Gas chromatography (GC) remains one of the most important and widely used analytical testing techniques performed today across a number of industries. Growing demands on
the speed and sensitivity of modern testing methods for GC has led to more sophisticated analytical
instrumentation and the increasing prevalence of gas
chromatography mass spectrometry (GCMS) and
other GC techniques. Though this is a common thread among a varied base
of analytical testing laboratories, unique challenges are faced in each industry.
Traditional approaches to dealing with these challenges through the use of
general-purpose GC column technologies have typically yielded only small
strides, such as a 15 percent improvement in run time or sensitivity. Selectivity
is of extreme importance in advancing GC testing methodologies. Application-specific GC column stationary phases in the forensics, fuels and environmental
industries can lead to potential performance and productivity gains.
The end goal in gas chromatography separations is accurate identification or quantification of a laboratory’s compounds of interest. This is
achieved through several means, one of which is resolution. Resolution (R)
is impacted by efficiency (N), selectivity (a) and retention (k), as seen in the
master resolution equation (Figure 1).
Traditional approaches to improving resolution have often focused on
the efficiency of GC columns through manipulation of a column’s length.
Efficiency is directly proportional to length, so longer columns will provide
higher resolution. Based on the master equation;
however, resolution is proportional to the square
root of efficiency. This means that large increases
in efficiency will not necessarily result in a signifi-
cant improvement in separation, although they
will have direct impact on run times and resulting
lab throughput. For example, doubling a column’s
length will approximately double the analysis time,
A key to unlocking the true separation power of the GC column lies in the
stationary phase. A GC column's stationary phase selectivity has the largest,
most direct impact on resolution. This selectivity depends on the nature of
the stationary phase, the nature of the components and the oven temperature
at the time of elution. It is the most influential GC column criterion, as it not
only determines the final resolution obtained, but also influences virtually
every column selection parameter. By increasing the resolution between two
compounds through optimization of the stationary phase selectivity, other
parameters may be altered to allow significant reductions in the total analysis
time, while maintaining or improving the original accuracy and sensitivity of
a method. This allows laboratories to more quickly and effectively reach their
separation goals and improve overall productivity.
Unlike pure samples (such as a bag of cocaine or a pseudoephedrine pill),
biological samples (from blood, urine or hair),
generally used in drugs of abuse testing, contain
contaminants from proteins, red blood cells or
salts that interfere with the analysis and yield
false or inaccurate results. These compounds are
also inherently difficult to analyze because of
their chemically reactive properties, and peaks
often tail on general purpose columns.
Chemists performing these tests have traditionally used varying GC phases, from a non-polar 100 percent dimethylpolysiloxane phase
to a more polar 50 percent phenyl phase.
With these general purpose GC stationary
phases, long run times are traditionally needed to fully resolve the drugs of interest from
contaminants of similar molecular weights
or masses, adding to sample backlogs and
Figure 2: An application-specific stationary phase compared to a traditional 5 percent phenyl-arylene phase.