氢化物发生原子吸收 Hydride Generation Atomic Absorption Spectroscopy

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 HGAAS 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.

Acidic Content and Oxidation State of Samples and Standards
The samples and standards are often prepared with duplicate acid concentrations to replicate the analyte's chemical matrix as closely as possible. In HGAAS, acid contents of samples and standards
of 10% to 50% are common; this is much much higher than in normal AAS.
The oxidation state of the analyte metalloid is important in HGAAS. For instance, HGAAS analysis of selenium requires the Se(IV) oxidation state (selenite). Se(VI), the more highly oxidized state of
the element (selenate), responds erratically and non reproducibly in the system. Therefore, all selenium in Se calibration standards and Se containing samples must be in the Se(IV) form for analysis.
This can be accomplished by oxidizing all Se in the sample to selenate using a strong oxidizer such as nitric acid or hydrogen peroxide (decomposing the excess oxidant) and then reducing the contained selenate to selenite with boiling HCl. After that reduction step, the final acid content is made
up to the required content before the sample is introduced into the hydride generation module. The literature also suggests that the time from reduction to introduction into the hydride module is important:Shorter is best.
Also important is the concentration of sodium borohydride and hydrochloric acid reagents feed into the hydride generation reaction vessel: optimization of this is important and may be different for
different elements. Example concentrations are 0.35% NaBH4 and 50% HCl. Note that this acid content is not necessarily identical with the acid content of the samples and standards themselves.
The reagent acid's content is aimed at producing a reproducible amount of hydride in the module.

Double Beam Instruments
The light from the HCL is split into two paths using a rotating mirror: one pathway passes through the optical cell and another around. Both beams are recombined in space so they both hit the PMT
but separated in time. The beams alternate quickly back and forth along the two paths: one instant the PMT beam is split by the rotating mirror and the sample beam passes through the cell and hits
the PMT. The next instance, the HCL beam passes through a hole in the mirror and passes directly to the PMT without passing through the optical cell. The difference between these beams is the amount of light absorbed by atoms in the optical cell.
The purpose of a double beam instrument is to help compensate for drift of the output of the hollow cathode lamp or PMT. If the HCL output drifts slowly the subtraction process described immediately
above will correct for this because both beams will drift equally on the time scale of the analysis.Likewise if the PMT response changes the double beam arrangement take this into account.

Ignition, Flame conditions, and Shut Down
The process of lighting the AAS flame involves first putting the optical cell in place and connecting the hydride gas transfer line. Next the fuel and the oxidant are turned on and then the flame is lit
with the instrument's auto ignition system (a small flame or red-hot glow plug). After only a few minutes the flame is stable. Deionized water or a dilute acid solution can be aspirated between
samples (but experimentation is required to ascertain what produces the best reproducibility). An aqueous solution with the correct amount of acid and no analyte is often used as the blank.To stabilize the HGAAS system the acidic blank is often flowed through
the sample inlet tube for 5 or 10 minutes; although the longer this
goes on, the more acidic waste is produced. Careful control of the fuel/air mixture is important because each element's response depends on successful decomposition of the volatile hydride in the heated optical cell. Remember that the flame's heat must break down the hydride and reproducibly create the elemental form of the analyte atom. Optimization is accomplished by aspirating a solution containing the element (with analyte content about that of the middle of the linear response range) and then adjusting the fuel/oxidant mix until the maximum light absorbance is achieved. Also the position of the burned head, optical cell, and sample uptake rate are similarly "tuned." Most computer controlled systems can save variable settings so that methods for different elements can be easily saved and reloaded.

Shut down involves aspirating deionized water through all three inlet tubes (borohydride, acid, and sample inlets) for a short period and then closing the fuel off first. Most modern instruments control the ignition and shutdown procedures automatically. The plastic tubing that is stretched around the peristaltic pump head is loosened to length its lifetime. Finally the purge gas is turned off.

Hydride Generator

真的是好东东，我下了，保存了

我也下了,先谢了!

发的真是时候，谢谢了！