Continuous sample nebulization

Flow injection procedures are very useful for performing trace analyses in highly concentrated salt solutions. Fang and Welz [270] showed that the flow rate of the carrier solution can be significantly lower than the aspiration rate of the nebulizer. This allows even higher sensitivities than with normal sample delivery can be obtained. Despite the small volumes of sample solution, the precision and the detection limits are practically identical with the values obtained with continuous sample nebulization. The volume, the form of the loop (single loop, knotted reactor, etc.) and the type and length of the transfer line between the flow injection system and the nebulizer considerably influence the precision and detection limits that are attainable. 

Figure 28-5 Continuous sample introduction methods. Samples are frequently introduced into plasmas or flames by means of a nebulizer, which produces a mist or spray. Samples can be introduced directly to the nebulizer or by means of flow injection (FIA) or high-performance liquid chromatography (HPLC). In some cases, samples are separately converted to a vapor by a vapor generator, such as a hydride generator or an electrothermal vaporizer.

Figure 28-6 Processes leading to atoms, molecules, and ions with continuous sample introduction into a plasma or flame. The solution sample is converted into a spray by the nebulizer. The high temperature of the flame or plasma causes the solvent to evaporate, leaving dry aerosol particles. Further heating volatilizes the particles, producing atomic, molecular, and ionic species. These species are often in equilibrium, at least in localized regions.

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... 

Apart from continuous pneumatic nebulization, all sample introduction techniques known from ICP-AES have been used and are of use for ICP-MS. Similar to ICP-AES, the analysis of organic solutions is somewhat more difficult [525]. 

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]. 

The solution to be nebulized can be a one-off sample, pumped or drawn into the nebulizer at a rate varying from a few microliters per minute to several milliliters per minute. Alternatively, the supply of solution can be continuous, as when the nebulizer is placed on the end of a liquid chromatographic column. 

Atmospheric Pressure Chemical Ionization. As its name reveals, APCI[16] is a Cl carried out at atmospheric pressure instead of under vacuum, as occurs for classical Cl. As for ESI, the sample must be in a solution that is continuously flowing into the APCI source (flow rate between 0.2 and 2 ml min ). The solution passes through a pneumatic nebulizer and is desolvated in a heated quartz tube or heating block, thus producing vaporization of solvent and analyte molecules (Figure 2.4). 

The introduction of samples via nebulizers requires that they are either pneumatically or peristaltically pumped into the nebulizer for aerosol formation. This restricts the range of viscosities that can be easily handled by the nebulizer. For example highly saline or oil samples may well have to be diluted by an order of magnitude or greater. This dilution can be carried out either in a batch mode or continuously. Batch systems are quite complex in design but the rate of analysis is high. It is often the case that where dilution is required, in addition, a fast rate of analysis is also desirable. Some batch systems have been introduced commercially, notably to monitor wear metals in the oil industry. 

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