The Hollow Cathode Lamp
The hollow cathode lamp (HCL) uses a cathode made of the element of interest with a low internal pressure of an inert gas. A low electrical current (~ 10 mA) is imposed in such a way that the metal is excited and emits a few spectral lines characteristic of that element (for instance, Te 214.3 nm and a couple of other lines; Se 196 nm and other lines, etc.). The light is emitted directionally through the lamp's window, a window made of a glass transparent in the UV and visible wavelengths.
Hydride Generation and Waste
The reaction of many metalloid oxyanions with sodium borohydride and HCl produces a volatile hydride: H2Te, H2Se, H3As, H3Sb, etc. As with
AAS, the oxidation state of the metalloid is crucial and care must be taken to produce the specific metalloid oxidation state before the sample is introduced into the hydride generation system.
The time from reagent mixing and when the volatile hydride is separated from the liquid and sent to the optical cell is also
important. The timing of that process is controlled by flowing reagents together using a peristaltic pump and tubing of specific lengths. After being mixed together the liquid mixture flows through a tube of a specific length (read this as a controlled reaction time) and is ultimately flowed into a gas/liquid separator where
the hydride and some gaseous hydrogen (produced by the NaBH4 + H2 reaction) bubble out and are purged (via a high purity inert gas) into the optical cell via a gas transfer line.
Most of the reagents introduced into the system flow to a waste container, and since the acid content is very high, often approaching 50%, as with
AAS, the waste container is glass and must be handled carefully and labeled well.
The Optical Cell and Flame
The optical cell is fused silica glass tube (transparent in the visible and UV wavelengths and thermally stable at high temperatures) through which the HCL's beam passes on the way to the monochromator and PMT. In some instruments it sits on top of the normal
AAS air/acetylene flame. The gaseous, metalloidal hydride flows into the optical cell from the hydride generation module pushes
by an inert purge gas. In the optical cell it decomposes into the elemental form which can absorb the HCL's beam.
The Monochromator and PMT
Tuned to a specific wavelength and with a specified slit width chosen, the monochromator isolates the hollow cathode lamp's analytical line. Since the basis for the HG
AAS process, like
AAS, is
atomic ABSORPTION, the monochromator seeks to only allow the light not absorbed by the analyte atoms in the optical cell to reach the PMT. That is, before an analyte is aspirated, a measured signal
is generated by the PMT as light from the HCL passes through the
optical cell. When analyte atoms are present in the cell from
hydride decomposition--while the sample is aspirated--some of
that light is absorbed by those atoms (remember only volatile
hydride gets to the optical cell and then only decomposed
hydride produces the elemental form). This causes a decrease in
PMT signal that is proportional to the amount of analyte. This
last is true inside the linear range for that element using that slit
and that analytical line. The signal is therefore a decrease in measure light: atomicabsorption spectroscopy.