Semi-flame methods

In semi-flame methods the atomization of the analyte takes place in a flame, but the pretreatment of the sample is outside the flame. 

---

A number of methods have been described for improving the sensitivity of conventional FAAS in order to allow the analysis without resorting to more expensive techniques. Best known of these techniques are hydride generation, cold vapour, semi-flame (Delves cup, tantalum boat), and slotted tube atom trap (STAT) methods. 

A. Cuesta, J.L. Todoli, J. Mora, A. Canals, Rapid determination of chemical oxygen demand by a semi-automated method based on microwave sample digestion, chromium(vi) organic solvent extraction and flame atomic absorption spectrometry. Anal. Chim. Acta 372 (1998) 399-409. 

Another popular method to estimate overpressures is the Baker-Strehlow method. This method is based on a flame speed, which is selected based on three factors (1) the reactivity of the released material, (2) the flame expansion characteristics of the process unit (which relates to confinement and spatial configuration), and (3) the obstacle density within the process unit. A set of semi-empirical curves is used to determine the overpressure. A complete description of the procedure is provided by Baker et al.18 The TNO multi-energy and Baker-Strehlow methods are essentially equivalent, although the TNO method tends to predict a higher pressure in the near field and the Baker-Strehlow method tends to predict a higher pressure in the far field. Both methods require more information and detailed calculations than the TNT equivalency method. 

The first studies of the kinetics of the NO-F2 reaction were reported by Johnston and Herschbach229 at the 1954 American Chemical Society (ACS) meeting. Rapp and Johnston355 examined the reaction by Polanyi s dilute diffusion flame technique. They found the free-radical mechanism, reactions (4)-(7), predominated assuming reaction (4) to be rate determining, they found logfc4 = 8.78 — 1.5/0. From semi-quantitative estimates of the emission intensity, they estimated 6//t7[M] to be 10-5 with [M] = [N2] = 10 4M. Using the method of Herschbach, Johnston, and Rapp,200 they calculated the preexponential factors for the bimolecular and termolecular reactions with activated complexes... 

Other analyzers include gas chromatography, which is a sensitive, automated, and semi-continuous technique in which CO is separated from water, CO2, and hydrocarbons (other than methane) by a stripper column and CO is passed through a catalytic reduction tube where it is converted to methane. The converted CO is passed through a flame ionization detector its sensitivity range is 0.026 to 43.7 ppm (IPCS, 1999). Other methods such as small personal exposure monitors (PEMs) can measure CO concentrations on a continuous basis and store data on internal digital memories (Ott et ah, 1986). 

Gas chromatography on capillary columns coated with Apiezon L has been demonstrated to be very suitable for determination of alkyl-PCBs. Pure model standard substances are missing in most cases. However, rather good semi-quantitative data are obtained with a specific flame ionization detector method with anthracene as internal standard [64], For environmental levels,mass spec-trometric detection methods are necessary. 

Human biological materials to be investigated include (a) hard calcified tissues, e.g. bone, teeth, other calcified formations (b) semi-hard tissue, e.g. hair, nails (c) soft body tissues and (d) various biological fluids and secretions in the human body. The treatment of each of these materials varies from one material to another and, as stated earlier, is often determined by the instrumental method to be employed for measuring the analytical signal, the elements to be determined and the concentration levels at which these are present. For the purposes of this discussion, it shall be generally assumed that the analytical techniques employed include atomic absorption spectrometry both with (F-AAS) as well as with a furnace (GF-AAS), neutron activation analysis (NAA), flame emission spectrometry (FES) voltammetric methods and the three inductively coupled plasma spec-trometric methods viz. ICP-atomic emission spectrometry, ICP-mass spectrometry and ICP-atomic fluorescence spectrometry. The sample preparation of biological methods for all ICP techniques is usually similar (Guo, 1989). 

In natural waters rubidium and caesium ions occur at the most in traces. Either flame photometry (method 3.3.. 1) or A AS (method 3.3.4.2) following concentration by coprecipitation with potassium as tetraphenylborate is recommended for the quantitative determination of rubidium and caesium ions in water or for their semi-quantitative estimation (in the case of caesium, AAS-method 3.3 A A and method with concentration 3.3.4.5). 

---