An Overview of Basic Principles of Analytical Methods - Field Sampling: Principles and Practices in Environmental Analysis - page 267

is performed in columns using only the pull of gravity to move the mobile phase. The
columns used have an inner diameter of 1 cm or greater and a stationary phase with
particles between 100 to 300 µm in diameter. The second kind of LC uses high pressure
to move the mobile phase through the column. Because of this it is called high pressure
or high performance LC (HPLC). High performance LC uses narrow columns 2 to 4 mm
in inner diameter with particles of 3 to 10 µm in diameter as the stationary phase and
mobile phase pressures up to 4 MPa. High performance LC achieves considerably higher
separation efficiencies than does column chromatography.
As with the SPE method discussed above, particle-free samples are required for either
of these methods. Suspended solids will be “filtered” out on the top of the column and
prevent the movement of mobile phase through it.
The other chromatographic method commonly used in environmental analysis is GC.
In this method, a carrier gas such as helium, nitrogen, or hydrogen is used as the mobile
phase to drive the sample through a column that contains the stationary liquid phase. The
sample—which must be in either a gaseous or a vapor state, vaporization being
accomplished by heating the sample when it is injected into the GC—is then separated by
partitioning between a liquid phase and the carrier gas. A limitation of this method is that
the sample must be able to be converted to a gas or vapor state with minimal loss or
degradation.
The sensitivity of modern gas chromatographic methods cannot be overemphasized.
Solvents being used in a part of a laboratory several rooms—or in some cases several
floors—away may be detected in a sample being analyzed. Thus great care must be taken
to ensure that samples are not exposed to any vapors during sampling, transportation, and
storage. This is also why both positive and negative controls are needed for all methods.
In both HPLC and GC it is possible to “identify” a compound by its retention time
(e.g., the time between when a compound is introduced into a column and when it
emerges). If repetitive analysis of samples that all have the same constituents and the
instrument is set up the same way retention times can be a very good indication of the
type and amount of compound present. However, two compounds can have the same
retention times, and so an independent separate identification method is always
appropriate. Gas chromatography/MS is extremely valuable to the analyst and laboratory
because the compounds can be identified conclusively during the analysis and it does not
require costly and time-consuming conformation using a second method.
Introduction of samples that will not vaporize or decompose when heated will destroy
the chromatographic column. Any material in the sample having these characteristics will
invalidate the analytical results, prevent further analysis, and prevent the use of the
instrument for several days. Also, gas chromatographic columns are not cheap,
sometimes costing over $1000, and thus care needs to be exercised in the preparation and
introduction of samples into a GC to ensure its suitability and continued operation [14,
15].
10.7.6. Combined Methods
In modern instrumental analysis it is common to use a combination of instrumentation in
tandem or sequentially. The most common is GC coupled with MS (GC/MS). As