Continuous sample aspiration

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... 

Other types of flow analyzers have also been in parallel development in recent decades. Besides laboratory flow analyzers, which are the main subject of this chapter, there are also different types of chemical analyzers for process analysis. Usually, in such instruments the volume of analyzed sample is not a significant factor, and measurements are carried out with continuous sample aspiration and during the flow of sample through the detector that is used. The variety of commercial offerings of such instrumentation and the variety of construction types are vast (Liptak, 2003). Scientific journals also... 

The qualifiers continuous and discrete as applied to pervaporation refer to different aspects of the process. In fact, analytical pervaporation is a continuous technique because, while the sample is in the separation module, mass transfer between the phases is continuous until equilibrium is reached. Continuous also refers to the way the sample is inserted into the dynamic manifold for transfer to the pervaporator. When the samples to be treated are liquids or slurries, the overall manifold to be used is one such as that of Fig. 4.18 (dashed lines included). The sample can be continuously aspirated and mixed with the reagent(s) if required (continuous sample insertion). Discrete sample insertion is done by injecting a liquid sample, either via an injection valve in the manifold (and followed by transfer to the pervaporator) or by using a syringe furnished with a hypodermic needle [directly into the lower (donor) chamber of the separation module when no dynamic manifold is connected to the lower chamber]. In any case, the sample reaches the lower chamber and the volatile analyte (or its reaction product) evaporates, diffuses across the membrane and is accepted in the upper chamber by a dynamic or static fluid that drives it continuously or intermittently, respectively, to the detector — except when separation and detection are integrated. 

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. 

To this end, loop-based injection is preferred and the sample aspiration tube (Fig. 6.9) behaves as a sampling probe, as demonstrated in the landmark work of Thomsen et al. who determined reactive silicate in coastal waters during a cruise from Monterey Bay to San Francisco Bay in the USA [3]. The sample was continuously aspirated from a water layer about 2 m below the sea surface and a situation of "infinite sample volume" was attained in the main channel of a shipboard reagent injection (reversed flow) flow injection system. A reagent aliquot was injected into the flowing sample every 45 s and the resulting coloured zone was quantified by spectrophotometry. The height of the recorded peak was directly related to the reactive silicate concentration at the specified location and depth. 

As stated above, the sampling operation is carried out with the aid of a moving aspirating tip in continuous segmented systems. However, unlike in other configurations, some air is also withdrawn between sample aspirations. The volume taken by the tip can be quantized In two ways, namely ... 

Figure 8.2. Continuous monitoring based on sample injection.(a) The sample, aspirated from a process reactor (process) by pump PI is intermittently injected by means of valve S into carrier stream C which is merged downstream with reagent solution P, propelled by pump P2. (b) Prior to the monitoring sequence (monitoring) the system is calibrated by injecting a series of standards (calibration), which procedure is repeated after the monitoring period.. Note that the baseline is reached between each injection, thereby allowing control of the performance of the analyzer itself.

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]. 

Continuous segmented methods avoid carry-over by use of air bubbles establishing physical separations (segments) along the continuous flowing stream. These methods were invented by Skeggs [1] and formed the basis of the Technicon AutoAnalyzer. They are now also implemented on Skalar assembhes. Samples are introduced sequentially by aspiration with a moving articulated pipette. 

Flow-injection and continuous-flow systems are very similar. The major differences are outhned here. Continuous-flow systems are characterized by a relatively long start-up time prior to instrument stabilization, whereas the flow-injection approach requires little more time than that needed to stabilize the detector output. Tubing diameters on a flow-injection manifold are usually much smaller and the samples are injected into the flow line rather than aspirated. No wash cycle is employed in the flow-injection regime, since the sample is a discrete slug. Flow rates in continuous-flow manifolds are often larger than in the flow-injection regime. 

In broad terms, a flow-through sensor is an analytical device consisting of an active microzone where one or more chemical or biochemical reactions, in addition to a separation process, can take place. The microzone is connected to or incorporated into an optical, electric, thermal or mass transducer and must respond in a direct, reversible, continuous, expeditious and accurate manner to changes in the concentrations of chemical or biochemical species in the liquid or gaseous sample that is passed over it, whether forcefully (by aspiration or injection) or otherwise (gases). 

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