Atomic absorption spectroscopy

F-AAS (Flame AAS)

In the flame technique, the dissolved sample is first converted into an aerosol. For this purpose, the sample is atomized with a pneumatic atomizer into a mixing chamber and swirled with fuel gas and oxidant. A fine mist, the aerosol, is formed. To make the droplet size even smaller and more uniform, the aerosol first hits a ceramic baffle ball and then, if necessary, a mixing blade that allows only fine droplets to pass through. A small portion of the original aerosol finally passes from the mixing chamber into the flame. There, first the solvent evaporates and the solid sample components melt, vaporize and finally dissociate. Too high flame temperatures can lead to ionization interference, especially with alkali and some alkaline earth elements, which can be controlled by adding an ionization buffer (CsCl or KCl). Too low flame temperatures lead to chemical interferences. In flame AAS, the flame can alternatively be operated with two different flames.

  • Air-acetylene flame: Usually used, this flame uses air as the oxidant and acetylene as the fuel gas.
  • Nitrous oxide-acetylene flame: The compounds of some elements (e.g. Al, Si, Ti, but also Ca and Cr) require higher temperatures for dissociation. In this case, the gas nitrous oxide (laughing gas) is used as oxidant instead of compressed air. At approx. 2800 °C, this flame is about 500 °C hotter than the air-acetylene flame. Due to its reducing effect, oxides of e.g. Cr, Ca and Al can also be atomized.

Graphite furnace AAS

Graphite furnace AAS takes advantage of the fact that graphite conducts electricity and heats up due to its electrical resistance when an electric current is applied. First, 5 to 50 microliters of the sample solution are placed in a graphite tube furnace and heated in several steps. The program depends largely on the element to be analyzed as well as its chemical environment. In addition, it plays a major role in what kind of instrument and in what kind of graphite tube furnace system (longitudinally heated/transversely heated graphite tube furnace) is used. In general, it can be said that in the transversely heated graphite furnace, approximately 200 °C lower pyrolysis temperatures and 200 to 400 °C lower atomization temperatures are used. The "Recommended Conditions" of the graphite furnace manufacturer should serve as a guide for the selection of the correct temperature/time program. From this, temperatures and times should be optimized to give maximum signal area to the measurement signal with minimum background signal. The sample composition may require a deviation from standard program.

  1. Drying 1: for about 30 s the oven is heated to 120 °C to constrict and nearly dry the sample.
  2. Drying 2: the oven is heated to 400 °C for about 20 s to dry the sample completely (if water of crystallization is present).
  3. Pyrolysis: for about 30 s the oven is heated to 1300 °C to 1700 °C to remove the organic components. This is done by pyrolysis or ashing
  4. Atomization: at 1500 to 2500 °C the sample is atomized for about 5 s (depending on the element)
  5. Bake-out: finally, at the end of the analysis, the sample is heated at 2500 to 2800 °C for about 3 more s to remove residuals of the sample.

Each step includes a rise time (ramp) within which the specified temperature is reached. Step two may be omitted for simple samples (drinking water); it is more commonly used for samples with complex matrices (body fluids or highly saline wastewater). For the atomization step, a ramp of 0 seconds is usually selected; here, the maximum power of the furnace power pack is applied to the graphite furnace tube to achieve a maximum heating rate. The temperatures are of course dependent on the analyte and can vary considerably. The advantage over the flame technique is that the sample can be brought quantitatively into the beam path and remain there longer (up to 7 s). Furthermore, often interfering matrix components can be separated by different evaporation temperatures; either they evaporate beforehand, or they remain. The detection limits are therefore up to 3 orders of magnitude better than with the flame technique, or ICP-OES. However, interference can occur if not working under specific measurement conditions. The summary of all measures that lead to interference-free analysis in graphite furnace AAS is referred to as the STPF (Stabilized Temperature Platform Furnace) concept. STPF concept

  • Pyrolytically coated graphite furnace (better durability and sensitivity)
  • Platform in the graphite furnace (atomization into a constant temperature gas phase in the graphite furnace)
  • Peak area evaluation (less dependence on the time of maximum atomization of the analyte)
  • Gas stop during atomization (atom cloud remains longer

Hydride technique

For some elements, mainly tin, arsenic, antimony, bismuth, selenium, tellurium and germanium, detection limits comparable to those in the graphite furnace can be achieved with the relatively simple hydride technique. If the element to be determined forms gaseous hydrides such as AsH3, SnH4 or H2Se with nascent hydrogen, these can be carried out of their solution by inert gas (usually argon) and transferred to a heated glass cuvette. The cuvette is made of quartz glass, since simple glass will glass out over time at the temperature used (up to 1000 °C). The heater can be either an electric heater or the flame of a flame AAS. An advantage of electric heating is better temperature control, since not every element has its sensitivity maximum at the same temperature. In the cell, hydrides decompose back into hydrogen and the element of interest at temperatures around 1200 K. This reaction is not only temperature controlled, but also temperature controlled. This reaction is not only temperature controlled, but it also depends on the surface properties of the cell. The hydride technique is not limited exclusively to AAS; it is also used in ICP-OES.

Cold vapor technique

The cold vapor technique (CV-AAS) is a subtype of the hydride technique. Here, a reducing agent is used to generate atomic mercury rather than hydride. The reducing agent can be sodium borohydride NaBH4, as above, but more commonly tin(II) chloride is used, which offers higher sensitivity and is less prone to foam formation. In the case of mercury, the technique is referred to as cold vapor, since the quartz cell does not need to be heated and no activation energy is required for decomposition of the hydride. Nevertheless, slight heating to 50 to 100 °C is advantageous so that no water vapor settles in the cell, which can interfere with the sensitivity conductivity.