Atomic-Emission Detector (AED)
Introduction
As capillary
column based gas
chromatography takes its place as the major, highest resolution separation
technique available for volatile, thermally stable compounds, the requirements
for the sensitive and selective detection of these compounds increases.
Since more and more complex mixtures can be successfully separated, subsequent
chromatograms (output of a chromatographic separation) are increasingly
more complex. Therefore, the need to differentiate between the sample components
using
the GC detector as a means of compounds discriminating is more and
more common. In addition, each detector has its own characteristics (selectivity,
sensitivity, linear
range, stability, cost, etc.) that helps in a decision about which
detector to use.
One of the newest additions to the gas chromatographer's arsenal is
the atomic emission detector (AED). This detector, while quite expensive
compared to other commercially available GC detectors, is an extremely
powerful alternative. FOR INSTANCE, Instead of measuring simple gas phase
(carbon containing) ions created in a flame as with the flame ionization
detector, or the change in background current because of electronegative
element capture of thermal electrons as with the electron capture detector,
the AED has a much wider applicability because it is based on the detection
of atomic
emissions.
The strength of the AED lies in the detector's ability to simultaneously
determine the atomic emissions of many of the elements in analytes that
elute from a GC capillary column (called eluants or solutes in some books).
As eluants come off the capillary column they are fed into a microwave
powered plasma (or discharge) cavity where the compounds are destroyed
and their atoms are excited by the energy of the plasma. The light that
is emitted by the excited particles is separated into individual lines
via a photodiode
array. The associated computer then sorts out the individual emission
lines and can produce chromatograms made up of peaks from eluants that
contain only a specific element.
Instrumentation
The components of the AED include 1) an interface for the incoming capillary
GC column to the microwave induced plasma chamber, 2) the microwave chamber
itself, 3) a cooling system for that chamber, 4) a diffraction
grating and associated optics
to focus then disperse the spectral atomic lines, and 5) a position adjustable
photodiode
array interfaced to a computer. The microwave cavity cooling is required
because much of the energy focused into the cavity is converted to heat.
Schematic of a gas chromatographic atomic emission detector
These notes were written by Dr.
Thomas G. Chasteen at Sam Houston State University, Huntsville, Texas.
Return
to The Chemiluminescence Home Page
