Atomic vapour generation techniques

A surprising addition has recently been made to the list of elements which may be usefully determined by vapour generation techniques, namely cadmium.5 Sodium tetraethylborate was used to produce a volatile cadmium species, with citrate being used to mask interference from nickel and copper. Using an argon-diluted hydrogen diffusion flame as atomizer, the detection limit by AFS was 20 ng l-1. 

The technique of flame emission spectroscopy is used to determine the concentration of Ba, K, and Na ions by measuring the intensity of emission at a specific wavelength by the atomic vapour of the element generated from calcium acetate i.e., by introducing its solution into a flame. 

To implement an easy and automated means for chemical vapour generation procedures (hydride generation for arsenic, selenium, etc., and cold vapour mercury), which allows for a reduction on the interferences caused by first-row transition metals (such as copper and nickel). FI methods may be readily coupled with almost all the atomic-based spectroscopic techniques (including graphite furnace atomisers). 

We have already seen in Chapter 2 that choice of atomizer system to be used may have a dramatic effect upon sensitivity, and thus upon signal-to-noise ratio. It is necessary to choose not only between flames, electrothermal atomization (ETA), and cold vapour and hydride generation techniques (which are discussed in Chapter 6), but sometimes also between different flames. Those elements which tend to form thermally stable oxides, such as Al, Ti, Si, Zr, may only be determined in a hotter, reducing nitrous oxide-acetylene flame. They cannot be determined with useful sensitivity in the air-acetylene flame. Some elements, Ba and Cr for example, may be determined in air-acetylene, but are more efficiently atomized in nitrous oxide-acetylene. 

Analysis of samples with complex matrices may be afflicted by severe spectral interferences, due to atomic or molecular emission lines (Schramel, 1988 Olesik, 1991). In case of mercury, the sensitivity can be considerably increased and the relative impact of the interfering elements minimized by use of CV-generating technique. By nebulization of a sample solution, mixed with a reducing agent into a spray chamber, also a fraction of the sample solution is aspirated simultaneously with the mercury vapour into the plasma, and the multi-element capability is maintained. 

Tsalev, D., 1999. Hyphenated vapour generation atomic absorption spectrometric techniques. J. Anal. Atom. Spectrom. 14, 147-162. 

AAS determinations based on hydride and cold vapour generation, electrothermal atomization with graphite furnaces have also been used successfully with FI systems to improve the overall performance of these techniques. Special requirements on the atomization-detection systems for hydride and cold vapour generation will be discussed in Chapter 5, and for the graphite furnace, in Chapter 4. 

Hydride generation technique. Hydride generation is an analytical technique to separate volatile hydride-forming elements from the main sample matrix before their introduction into the light path of the instrument, and to convert them into an atomic vapour once they are there. 

Hydride generation methods involve three or four successive steps depending on the technique used (i) The hydride is generated by chemical reduction of the sample (ii) The formed hydride may be collected in the batch type methods (iii) The hydride is entrained in a gas stream into the atomizer (iv) The hydride is decomposed in the atomizer to form the atomic vapour, and the absorption signal is measured. A number of methods in use are based on this principle, but they differ in the means of reduction, atomization, and sample introduction. 

Initially hydride generation and cold vapour techniques were developed for the quantitative determination of the hydride-forming elements and mercury by atomic absorption spectrometry (Chapters, Sections 6.2 and 6.3), but nowadays these methods are also widely used in plasma atomic emission spectrometry. In the hydride generation technique, hydride-forming elements are more efficiently transported to the plasma than by conventional solution nebulization, and the production and excitation of free atoms and ions in the hot plasma is therefore more efficient. Spectral interferences are also reduced when the analyte is separated from the elements in the sample matrix. Both continuous (FIA) and batch approaches have been used for hydride generation. The continuous method is more frequently used in plasma AES than in AAS. Commercial hydride generation systems are available for various plasma spectrometers. 

A novel technique of atomisation, known as vapour generation via generation of the metal hydride, has been evolved, which has increased the sensitivity and specificity enormously for these elements [5-7,9]. In these methods the hydride generator is linked to an atomic absorption spectrometer (flame graphite furnace) or inductively coupled plasma optical emission spectrometer (ICP-OES) or an inductively coupled plasma mass spectrometer (IPC-MS). Typical detection limits achievable by these techniques range from 3 pg/1 (arsenic) to 0.09 pgd (selenium). 

As described previously, although techniques to utilize AFS were developed several decades ago, until recently they were not widely used. However, a new generation of simple AFS instruments have been developed to specifically detect the vapour-forming elements, such as those that form hydrides (As and Se) and mercury, which forms an atomic vapour. All of these analytes have a primary line below 260 nm and, since the analytes may be readily separated from the bulk matrix and concomitant elements, dispersion is not necessary. Instead these basic instruments originally used a simple interference filter, although these have now been superseded by the more efficient multi-reflectance filter. 

As aheady mentioned, the simplest way of generating metal nanoparticles in the gas phase is to produce atoms that are subsequently allowed to coalesce under controlled conditions. This so-called metal-vapour synthesis requires more or less expensive equipment. Numerous modifications of this well established technique have been successfully applied. However, a detailed description of all the devices would exceed the scope of this article. For a summarizing overview, see Reference 7. Some more recent and relevant results shall be mentioned here. 

Maintaining the quality of food is a far more complex problem than the quality assurance of non-food products. Analytical methods are an indispensable monitoring tool for controlling levels of substances essential for health and also of toxic substances, including heavy metals. The usual techniques for detecting elements in food are flame atomic absorption spectroscopy (FAAS), graphite furnace atomic absorption spectrometry (GF AAS), hydride generation atomic absorption spectrometry (HG AAS), cold vapour atomic absorption spectrometry (CV AAS), inductively coupled plasma atomic emission spectrometry (ICP AES), inductively coupled plasma mass spectrometry (ICP MS) and neutron activation analysis (NAA). 

There are several design parameters which distinguish film growth techniques from one another, namely generation of the source atom/molecule, delivery to the surface and the surface condition. The source molecule can be generated in a number of ways including vapour produced thermally from solid and liquid sources, decomposition of organometallic... 

---