Continuous flow analysers

Various approaches to the analysis of dissolved silicon have been tried. Most of them are based on the formation of /J-molybdosilic acid [ 199-203 ]. Dissolved silicon exists in seawater almost entirely as undissociated orthosilicic acid. This form and its dimer, termed reactive silicate , combine with molybdosilicic acid to form a- and /I-molybdosilicic acid [180]. The molybdosilicic acid can be reduced to molybdenum blue, which is determined photometrically [206]. The photometric determination of silicate as molybdenum blue is sufficiently sensitive for most seawater samples. It is amenable to automated analysis by segmented continuous flow analysers [206-208]. Most recent analyses of silicate in seawater have, therefore, used this chemistry. Furthermore, reactive silicate is probably the only silicon species in seawater that can be used by siliceous organisms [204]. 

Figure 6.6 Diagrammatic representation of the function of the continuous flow analyser modules.

A separation step is sometimes an essential part of an analytical method and may be as diverse as distillation, filtration, digestion, extraction, phase-separation or dialysis. These can all be performed by continuous flow analysers either by adding a specially designed glass fitting to the manifold or analytical cartridge or by the addition of a separate module to the analyser. Many biological samples contain protein and dialysis is often used to remove this protein, which would otherwise affect the analysis. 

Table 1.2 gives some of the reasons for the LGC setting up its automation team. The primary motivation was economic. LGC was often subject to constraints on staffing in parallel with large increases in analytical commitments. The introduction of cost-effective analyses, using mechanical or automatic instruments, reduces staff involvement and allows well qualified people to be released for the development of new analytical requirements. The analysis of beer samples by multi-channel continuous flow analyser [S, 6, 7] and the introduction of a mechanical solvent extraction and identification system to analyse and measure levels of quinizarin in gas oil, both for duty purposes, were prime examples [8], Both systems involved commercially available components and/or instruments integrated with modules designed and built in-house. 

J. Ruzicka, E.H. Hansen, Flow injection analyses. Part I. A new concept of fast continuous flow analyses, Anal. Chim. Acta 78 (1975) 145. 

B = steady states related to the plateau and baseline S and W = sampling arm in the sample and blank solution cups a = lag phase two.s = half wash time. Adapted from Anal. Chim. Acta 78 (1975) 145, J. Ruzicka, E.H. Hansen, Flow Injection Analyses. Part I. A new concept of fast continuous flow analyses, with permission from Elsevier (Ref. [6]). 

Unsegmented-flow analysers (Fig. 1.3b) can be classified according to whether samples are Injected or continuously Inserted Into the system, Into flow-injection, analysers (FIA) [16,17] and completely continuous flow analysers (CCFA) [18], respectively. Both are described In detail In Chapters 6 and 7, respectively. 

As far as separation techniques are concerned, they can be implemented on automatic continuous analysers or robot stations as ancillary modules (dia— lysers, ion exchangers, liquid-liquid extractors). As stated in Chapter 12, chromatographic processes —particularly column (HPLC and GC), but occasionally also planar chromatographic purposes— are commonly the subject of automation. A conventional chromatograph furnished with a system for sequential introduction of samples —which can even be partially treated in a continuous fashion before of after column separation (derivatlration and post-column techniques)— markedly resembles continuous flow analysers. Gas and liquid chromatographs are often used as separative-determinative modules in robot-stalons. 

Fia. 4.25 Different ways of coupling continuous liquid-liquid extraction to air-segmented (SFA), flow-injection (FIA) and completely continuous flow analysers (CCFA). 

Fig. 7.2 (a) Scheme of a completely continuous flow analyser for uninterrupted determination of chemical oxygen demand, (b) Results obtained in the monitoring of laboratory waste throughout an afternoon. (Reproduced from [1] with permission of the Royal Society of Chemistry). 

Continuous-flow analysers in their segmented version prevailed in the automation of clinical chemistry for over a decade, until they began to be displaced by batch analysers which, oddly enough, where Introduced by the same manufacturers who launched and popularized SFA. This type of analyser was commented on In Chapter 5, which described in detail the Analyzers II, SMA 12/60 and SMAC, as well as a series of non-cllnlcal applications grouped according to the type of detection system used. 

These analysers are less prone to carry-over than continuous-flow analysers. In addition, the allow the use of strong acids and/or organic solvents. However, they do not permit the incorporation of continuous separation... 

Modern continuous flow analysers arc generally mechanically simpler and less expensive than their discrete counterparts. Indeed, in many continuous flow systems, the only moving parts arc pumps and switching valves. Both of these componem.s arc inexpensive and reliable. In contrast, discrete systems often have a number of moving parts such as syringes, valves, and mechanical devices for transporting samples or packets of reagents from one part of the system to another. In the most sophisticated discrete systems, unit operations are performed hy versatile computerized robots, in much the same way human operators would. 

Immobilized D-glucose oxidase has been used for the determination of serum samples containing D-glucose in a commercially available continuous flow analyser.The method appears to be a convenient and economical alternative to the use of soluble enzyme reagents. The D-glucose oxidase-4-aminophenazone-phenol procedure for the determination of D-glucose has been evaluated for use in an autoanalyser.The method compared well with two other available routine methods using the same autoanalyser. 

Hydrazine (Bernhard and Wilhelms, 1967) or ascorbic acid are the common reductants in the flow-analysis of phosphate. The hydrazine method seems to reduce dye coating of the flow cell window but is equivalent to the ascorbic acid method described below. In continuous flow analysers (CFA) the same manifold can be used for both methods. 

Fig. 10-13. Mobile 4-channel Continuous Flow Analyser for shipboard use (Institut fOr Meereskunde, Kiel).

An instrument for the measurement of individual amino acids in an amino acid mixture. The amino acids are separated on an ion-exchange column and estimated colorimetrically on a continuous flow analyser after their reaction with ninhydrin. In some instruments the amino acids are measured fluori-metrically. 

A type of automatic analysis in which samples are mixed with reagents by means of plastic tubes, the solutions then being continuously pumped through a number of modules in which reaction takes place, finally passing to a flow-through colorimeter where the intensity of the colour is measured. The Technicon company manufactured the first commercial continuous flow analyser and since then have produced more sophisticated machines. It is these that will be described ... 

This enables faster sampling rates to be attained on continuous flow analysers. The curve regenerator is connected between the colorimeter and the chart recorder of continuous flow analysers, and, by detecting the rate at which the peaks rise rather than the height of the peak as in conventional continuous flow analysis, it enables the faster sampling rates to be attained. 

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