Discrete sample nebulization

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... 

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. 

Figure 3 Effect of discrete sample volume aspirated and of continuous nebulization on the photomultiplier signals in AAS for a l mg l 1 magnesium solution...

Apart from continuous sample aspiration also flow injection and discrete sampling can be applied (see Section 3.1), both of which deliver transient signals. In the latter case 10-50 pi aliquots can be injected manually or with a sample dispenser into the nebulization system, as was first proposed by Ohls et al. [125] and described by Bemdt et al. (see Ref. [126]). The approach is especially useful for preventing clogging in the case of sample solutions with high salt contents, for the analysis of microsamples as required in semm analysis or when aiming at the... 

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included protein precipitation with trichloroacetic acid (Olson and Hamlin, 1968) and/or solvent extraction have been superseded by simpler procedures using either large sample dilution or viscosity adjusted reference solutions with minimal dilution.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. 

The next logical step in improving micro FAAS procedures was to use discrete sample injections into a flowing carrier liquid being pumped into the nebulizer of a flame atomic... 

One of the application areas that high-efficiency nebulizers are very well suited to is in the handling of extremely small volumes being eluted from an HPLC or flow injection analyzer (FIA) system into an ICP-MS for doing speciation/microsampling work. The analysis of discrete sample volumes encountered in these types of applications allows for detection lunits equivalent to a stan- FIGURE 3.9 Spray pattern of a PFA dard concentric nebulizer, while consuming microflow concentric nebulizer (cour-10—20 times less sample. tesy of Elemental Scientific Inc.). 

In pneumatic nebulization for ICP-OES, continuous sample feeding requires a sample aspiration time of about 30 s so as to attain a stationary signal, a measurement time of around 5-10 s, and a rinsing time again of 30 s at minimum. However, discrete sampling is also possible with injection systems known from flame AAS [140, 141] and by flow injection. Work with sample aliquots of down to 10 xL then becomes possible, which is particularly useful, for example, in work with microsamples [156] or for the analysis of solutions containing high salt contents [443]. 

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. 

In spite of the sensitivity of the determination, because of the low concentrations of cadmium in most environmental samples, the element is still often preconcentrated. For example, discrete nebulization flame AAS has been used to measure foliar cadmium after extraction of the APDC complex into chloroform.15 Cobalt was extracted at the same time. Many other solvent extraction procedures have been described.1 Alternatively resins such as a chelating polydithiocarbamate resin have been employed to concentrate cadmium prior to determination.16 Extractions onto solid phase materials for preconcentration may be made more convenient by automation, for example using flow injection methodology.17... 

Uchida, T., Kojima, I., lida, C. Determination of metals in small samples by atomic absorption and emission spectrometry with discrete nebulization. Anal. Chim. Acta 116, 205-210 (1980)... 

This may be either a continuous process, used when the sample size is relatively large (1 ml or more), or a discrete process, used with samples of less than 20 /il. Continuous-flow systems are simpler to use and more precise, but they are less sensitive. They employ a nebulizer in association with a flame or gas plasma, and either a rotating electrode (Rotrode) or drip-feed to the electrode with the arc or spark. The pneumatic nebulizer has an efficiency of 5-10% and generates an inhomogeneous aerosol. Efiiciency can be improved by proper design of the nebulizer and spray chamber (N4), by use of heated nebulizer gas (R6) or ultrasonic devices (S23). The maximum improvement is a 5- to 10-fold increase in sensitivity. There is also an increase in the complexity and cost of the instrument which usually offsets these benefits. The effect... 

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