Discrete Sample Nebulization Techniques

In the early days of flame spectrometry, some very elaborate accessories were designed to give reproducible discrete sample nebulization.16,17 However, as the technique became more widely employed, the devices used became progressively simpler, often taking the form of small funnels with a capillary bore outlet connected directly to the nebulizer capillary.17 Even this is not really necessary, because all that is required is a small (1-2 ml capacity) beaker with a conical depression in the bottom. Conventional Auto Analyser sample cups work very well. The end of the flexible nebulizer aspiration tube is simply dipped into the droplet of solution in the cone. This is especially useful if, for example, such sample cups have been used for evaporative pre-concentration of water samples in a vacuum desiccator.19 

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Small samples (25-200 mm ) may be introduced using the technique of pulse nebulization (also known as discrete sample nebulization, direct-injection cup nebulization, gulp sampling, and Hoescht cup nebulization). This technique may also be employed for higher concentrations than normally nebulized. A cup or funnel made of an inert material (e.g., polytetrafluoroethylene) is attached to the nebulizer tubing and the sample is put into the cup as a discrete aliquot using a micropipette. The sample is totally consumed and the transient peak signal recorded. 

Earlier FAAS techniques for measuring serum copper levels which included protein precipitation with trichloroacetic acid (Olson and Hamlin, 1968) and/or solvent extraction have been superseded by simpler procedures using either large sample dilution or viscosity adjusted reference solutions with minimal dilution. The analyses are made mainly using continuous nebulization, discrete sample injection or by flow injection techniques with little advantage gained from flame adaptors to increase sensitivity. 

From the late 1960s onwards, a number of research groups around the world began to investigate alternatives to pneumatic nebulization for sample introduction, in an attempt to overcome transport efficiency limitations. The most successful approaches were those which involved heating small, discrete liquid samples, and sometimes even solid samples, directly on a metal filament, boat, or cup which could be positioned reproducibly into a flame. However, since the temperature of the metal would be lower than that of the flame itself, the techniques were confined to the determination of relatively easily atomized elements such as arsenic, bismuth, cadmium, copper, mercury, lead, selenium, silver, tellurium, thallium, and zinc. 

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