Packed or capillary?
Packed columns have higher sample
capacity than capillary columns, although the difference has
been greatly reduced by the large-bore 530µm capillaries
invented by Agilent. Improvements in detector sensitivity have also
reduced the need for large samples. The one area in which packed
columns may have an advantage is in analysis of gas samples.
For almost all other samples,
capillaries provide much better efficiency (narrow peaks) which
leads to greatly improved peak separation. In fact, the
separating power is so great that many analyses can be done on
surprisingly short columns in very brief runs. This time saving
translates directly into reduced turnaround time and increased
For new or updated methods, we
recommend capillary columns unless there is some overwhelming
reason for using packed columns.
It must be as inert as possible,
particularly for trace analysis work or for compounds that tend
to tail badly, for example active compounds such as mercaptans
or alike. For capillaries, fused silica is the material of
There are two basic types of
fused silica capillary columns: The Wall Coated Open Tubular or
WCOT columns and the Porous Layer Open Tubular or PLOT columns.
The stationary phase in WCOT columns is a liquid film coated to
the deactivated wall of the column. These are the most widely
used columns in gas chromatography. In the PLOT columns the
stationary phase is a solid substance that is coated to the
Packed columns may be glass or
metal, usually stainless steel. Metal, although inherently
active, is durable and suitable for nonpolar materials. But if
samples with polar components are to be analyzed, select glass.
If even this is too active (peak tailing, sample loss), try a
When selecting capillary columns
the first decision to be made is whether a PLOT column is
needed. Here are the typical application areas for the three
types of PLOT columns:
gases, sensitive to water
resolution of C1 to C3 isomers,
only partial resolution of isomers of C4 and
higher (up to C14), polar compounds, volatile
will tolerate water
of isomers of C1 to C10, sensitive
If none of the above mentioned
applications is what you are interested in then you will be able
to use a WCOT type column.
When faced with an unknown
sample, first try the column that is presently in your GC. If
that does not give satisfactory results, consider what you know
about the sample. The basic principle is that analytes like to
interact with stationary phases of similar chemical nature. This
means that the more you know about your sample the easier it is
to find the optimum separation phase.
The most important step is to
consider the polar character of your analytes:
- Nonpolar Molecules - generally
composed of only carbon and hydrogen exhibit no dipole
moment. Straight-chained hydrocarbons (n-alkanes) are common
examples of nonpolar compounds.
- Polar Molecules - primarily
composed of carbon and hydrogen but also contain atoms of
nitrogen, oxygen, phosphorus, sulfur, or a halogen. Examples
include alcohols, amines, thiols, ketones, nitriles, organo-
- Polarizable Molecules -
primarily composed of carbon and hydrogen, but also contain
unsaturated bonds. Examples include alkenes, alkynes and
Hewlett-Packard offers you the
right stationary phase for your specific separtion needs: Is
your sample a mixture of non polar components of the same
chemical type, such as hydrocarbons in most petroleum fractions?
Try a nonpolar column such as HP-1 which separates them in
(approximate) boiling point order. Perhaps you suspect some
aromatic components; try a column such as HP-5 or HP-35 with
Samples with polar or polarizable
compounds often resolve well on the more polar and/or
polarizable stationary phases that contain phenyl groups and
alike. Examples are the HP-210 or HP-225 columns. If even more
polar phases are required consider the polyethylene glycol (PEG)
phases, also often called the wax phases.
Please see the phase
selection chart for stationary phase suggestions that are
based on application and analyte polarity.
Bonding creates chemical bonds
between the phase and the column tubing. Crosslinking
polymerizes the phase in place to increase its molecular weight.
Both processes are happening simultaneously during the
manufacturing process of bonded/crosslinked columns and have the
desirable effects of increasing temperature stability and
reducing column bleed. Bonded/cross-linked columns can be rinsed
to remove contamination that might build up over time and allow
larger volume injections. Where there is a choice, we recommend
the bonded/crosslinked phases over the standard coated version.
The general rule is that thin
films elute components sooner with better peak resolution and at
lower temperatures than thick films. This indicates that they
are well suited to samples with high-boiling components,
closely-spaced components, or temperature sensitive components.
The "standard" film
thickness is 0.25 to 0.5 m m. These work well for most samples
(including waxes, triglycerides, and steroids) eluting up to 300
° C. For components eluting at higher temperatures, thin films
(0.1 m m) are available.
While standard or thin films are
appropriate for high-boiling components, thicker films are
needed to resolve low-boiling materials. Film of 1 to 1.5 m m
work well for components eluting between 100 and 200 ° C.
Extremely thick films (3 to 5 m m) are needed for gases,
solvents, and purgeables to increase their interaction with the
Another reason for using a
thicker than normal film is to maintain resolution and retention
times when changing to a wider bore column. For this reason,
wide bore columns tend to be available only with thicker films.
Thick films mean more material in
the column and therefore more bleed. Temperature limits must be
lowered as film thickness rises.
As a general practice, 15m
columns are used for fast screening, simple mixtures, or very
high molecular weight compounds. The 30m length has become the
most popular one for most analyses. Very long columns (50, 60
and 105m) are for extremely complex samples.
Column length is not a very
strong parameter in column performance. For example, doubling
column length doubles isothermal analysis time but increases
peak resolution by only about 40%. If an analysis is almost but
not quite good enough, there are better ways than length to
improve it. Consider a thinner film, optimizing the carrier flow
through the column, and using temperature programming if you are
not already doing so.
One special situation is the
analysis of samples with extremely active components. These will
tail severely if they contact the column material. Relatively
short columns with thick films reduce the chance of interaction
by having less column material and smothering it with stationary
phase to conceal active sites.
Increased diameter means more
stationary phase, even with the same thickness, for greater
sample capacity. It also means reduced resolving power and
Narrow columns provide the
resolution needed for complex samples, but typically require a
split injection because of the low sample capacity. Wider
columns avoid this if the loss of resolution can be tolerated.
When sample capacity is a major consideration, as with gases,
very volatile samples, and purge and trap or headspace sampling,
large id or even PLOT columns may be appropriate.
Also consider the limitations and
needs of your instrumentation. An adapted packed column inlet
can use the larger bore capillary column but not the narrow
ones. Inlets designed specifically for capillary columns
generally handle the entire id range. GC/MS and MSD with direct
coupling may require narrow columns because the vacuum pumps
cannot handle the high flows used with larger columns. Look at
your entire system to discover which parts limit your choice of