Agriculture Reference
In-Depth Information
tion, cleanup, and concentration and also the introduction of contaminants
during extraction, drying, cleanup, and other procedures. However, these
methods are not universally applicable to all soil analysis procedures, and
caution must be used in application to new or untested analysis or analytical
procedures [1,2].
9.2.6.
Gas Chromatography-Mass Spectroscopy
Gas chromatograph-mass spectroscopy (GC/MS) consists of a gas chromato-
graph where the chromatographic column exits into a mass spectrometer (see
Figure 9.5) that acts as a detector for the GC. As the GC carrier gas exits the
GC column, the carrier gas is stripped from separated components that enter
the MS, where it is ionized, fragmented, and scanned and the mass of the com-
ponent it contains along with its fragmentation pattern is recorded. The frag-
mentation pattern is used to identify the compound using a computer to match
the pattern found with standard fractionation patterns for known compounds.
The computer then produces a list of compounds that come close to match-
ing the unknown along with a percentage (%) that indicates how well the pat-
terns match.
Caution must be exercised with these types of matching pattern computer
programs as they can easily misidentify compounds, especially if the program
is set up for the wrong analysis or assumed composition of the unknown [2-4].
9.3.
HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY
In high-performance liquid chromatography (HPLC) the mobile phase is a
liquid, in which the sample must be soluble, and detection is most often
accomplished by ultraviolet absorption. It is generally a slower process than
gas chromatography; however, its advantage is that the compounds to be sep-
arated are not limited by their boiling points, although low-boiling compounds
are almost never separated by HPLC. Solid mixtures, as long as they are
soluble in the mobile phase, can be chromatographed.
9.3.1.
Sample Introduction
L syringe is used
to fill the sample loop that holds a specific volume of sample solution. The
valve is switched to the RUN position, and the elutant carries the sample out
of the sample loop and into the column. A recording of the detector output
is automatically begun and produces a chromatogram of the separated
components.
For HPLC the injector is a valve. In the charge position a 50-
m
