Flow-injection approach

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. 

Gaseous sample introduction into an ICP-MS presents different problems. Owing to its extremely sensitive nature, Dean et al. [13] introduced the sample as the gaseous hydride by a flow-injection approach. This was reasonably effective because lower volumes of samples and reagents were in use. They utibzed nitric acid as a carrier stream to prevent the formation of argon chloride species in the plasma. Argon chloride has the same mass as arsenic which is mono-isotopic, and this severely bmits arsenic determination. An additional problem was that the sensitivity was extremely dependent on the purity of reagents. 

Many of the apphcations described in the hterature do httle more than reduce the volume of sample taken. However, Perkin Elmer introduced a product particularly designed to use the flow-injection approach. At present this seems httle more than a marketing ploy to gain advantages over competitors. However, in reahty, many ICP systems, and indeed some AA systems, do not make it easy to hnk to FIA systems because the data-processing side of these instruments cannot cope with a transient rather than a continuous signal. 

Another interesting area of apphcation is where flow systems are used in conjunction with the flow-injection approach to select particular components from a sample, i.e. organic or inorganic mercury samples, or to preconcentrate particular elements of interest. McLeod... 

Pitts, L., PI. Worsfold, and J. Hill. 1994. Selenium speciation—a flow injection approach employing online microwave reduction followed by hydride generation-quartz furnace atomic absorption spectrometry. Analyst 119 2785-2788. 

River, stream, and lake water groundwater as well as atmospheric precipitation are commonly analyzed by using ICP-MS [303], Often the samples can be run directly or after simple filtration or centrifugation to remove suspended particulates [304]. Typically samples can be preserved by the addition of 1% to 2% concentrated nitric acid by volume [305]. Nitric acid is preferable to hydrochloric or sulfuric acid because of the lack of molecular ion spectral overlaps from nitric acid or its reaction products in the ICP, as discussed earlier. In most cases preconcentration or separation is unnecessary. In some cases preconcentration and removal of chlorides from the sample are advantageous and can be done by using a simple flow injection approach [306]. 

Preliminary studies conducted in the 1980s by the authors research group [9] using oontinuous flow injection approaches clearly exposed the following effects of US on analytioal systems ... 

Fig. 15.5 The on-bead typical response against Mg2+ of a l,2,10,13-tetraoxa-7,16-diaza-cyclocta-decane-dansyl library member in a flow-injection approach...

R.C. Prados-Rosales, J.L. Luque-Garcia, M.D. Luque de Castro, Propelling devices the heart of flow injection approaches, Anal. Chim. Acta 461 (2002) 169. 

J.-H. Wang, E.H. Hansen, Coupling on-line preconcentration by ion-exchange with ETAAS. A novel flow injection approach based on the use of a renewable microcolumn as demonstrated for the determination of nickel in environmental and biological samples, Anal. Chim. Acta 424 (2000) 223. 

W. R. Seitz and M. L. Grayeski, The Flow Injection Approach for Making Analytical Chemiluminescence Measurements. Biolumin. Chemilumin. Instrum. AppL, 2 (1985) 95. 

Velasco-Arjona A and Luque de Castro MD (1996) A robotic flow injection approach to the fully automated determination of starch in Fdo. Analytica Chimica Acta 333 205-213. 

A flow injection system for the analysis of reactive phosphate in seawater was introduced by Johnson and Petty [169]. This involved the concept of reverse or reagent injection FIA, which they showed to be inherently more sensitive than the conventional sample injection flow injection approach. A major advantage of the system was that it could be used for underway analysis. 

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