The Sequential Injection Analyser

The parameters inherent to the analytical procedure can be modified without alterations to the manifold architecture, and this aspect makes the sequential injection analyser very attractive. 

Sequential injection systems were conceived [89] to simplify flow-based analytical procedures and enhance versatility and robustness for process monitoring. The confluence connections usually present in segmented flow and flow injection analysis are eliminated and different analytical applications can be implemented without the need for multiple manifold re-configuration. 

The basic components of the system are a liquid driver with only one carrier stream, a multi-port selection valve and a detector (Fig. 2.9). The valve is the heart of the sequential injection system and normally comprises 6—10 peripheral ports and a central port in a multi-position valve configuration. The central port is linked to a holding coil and the peripheral ports are connected to different solution aspiration tubes and transmission lines that are linked to different manifold components, e.g., detector and mixing chamber. Only one peripheral port is connected to the central port at any one time. Stream management inside the holding coil is accomplished by a bi-directional piston (or peristaltic) pump. The analyser is fully computer controlled and the injection volumes, residence times, delivery of solutions and analytical path lengths are selected based on a valve timing sequence and related flow rates. 

Flow reversal plays a beneficial role in reducing sample dispersion [92], The iso-concentration concentric cylinders typical of laminar flow are stretched during sample transport inside the holding coil and tend to be compressed again after flow reversal. As a result, a process analogous to peak de-convolution occurs (Fig. 5.18). 

The sequential injection system is extremely versatile because modifications to the conditions required for development of the physicochemical processes involved are achieved through software control of the system parameters rather than physical changes to the hardware. In fact, 

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Development of the sequential injection analyser, as well as intensive studies exploiting beads (including magnetic beads) [80—82], led to the appearance of flow analysers incorporating bead injection [83] and lab-on-valve [84] devices. 

In single-line (also called straight or single channel) flow systems, the required reactants are present in the sample carrier stream and are added to the sample zone as a consequence of dispersion. The configuration is associated with the inception of flow injection analysis and is characteristic of the sequential injection analyser. Flow injection systems comprising two or more streams that converge to form the main carrier stream into which the sample is inserted [134] are also considered as single-line flow systems. 

The Lab-on-Valve flow system resembles the sequential injection analyser but the main components are assembled on a multi-position valve. Exploitation of optical fibres permits spectrophotometric measurements to be performed very close to the main reaction coil. The compact geometry of the system makes it attractive for, e.g., in situ assays. For more details of this system, see 2.4.2. 

It is important to stress that a typical sequential injection system cannot be considered as a flow system exploiting a sandwich technique, as sequential aspiration of different solutions, accommodation of the sampled aliquots inside a holding coil and flow reversal are inherent only to the sequential injection analyser. 

The multi-port selection valve of a sequential injection analyser can be regarded as an advanced commuting device, which selects one inlet port to be commuted to any one outlet port at a time. Also, the entire manifold can be accommodated in such a valve, leading to the lab-on-valve concept [84]. 

There are many variants of analytical flow systems, e.g., segmented flow analysis, flow injection analysis, sequential injection analysis, multisyringe flow injection analysis, batch injection analysis, mono-segmented flow analysis, flow-batch analysis, multi-pumping flow analysis, all injection analysis and bead injection analysis, all of which have acronyms [176]. In view of the existence of several common features, however, all flow analysers can be broadly classified as either segmented or unsegmented, with the most common example of the later mode being the flow injection analyser. 

Mini-columns for analyte separation/concentration can also behave as reactors, resembling the packed bed reactor. In this context, organic polymer monoliths, largely used in the medical and biological fields [73], should be highlighted. Monolithic mini-columns consist of continuous beds with macropores and mesopores which are characterised by low back-pressure effects. These columns offer several other advantages [74], as emphasised in Chapter 8. In the context of flow analysis, monolithic mini-columns were implemented in a sequential injection analyser in 2003 [75] and the potential and limitations of the approach, called Sequential Injection Chromatography, were recently reviewed [76]. 

Sandwich techniques can also be implemented in a sequential injection analyser, as demonstrated by the spectrophotometric determination of nitrate and nitrite in natural waters [12]. The sample plug was set between two reagent plugs and the assembly was aspirated towards the holding coil of an ordinary sequential injection analyser. Two Griess reaction zones were established at both ends of the sample zone, and only one was... 

Gemfibrozil Drug dissolution samples UV-Vis/ HPLC 6 to 10 (in the first subsystem) 2.28 mg IA1 First and second subsystems sequential injection analyser and HPLC with monolithic column [122]... 

Placement of commercial cartridges with powdered reagents (often lyophilised or crystallised) into the flow manifold enables in-line reagent dissolution, as demonstrated by Marshall et al. [101] who designed a sequential injection analyser with a cartridge tray attached to the main selecting valve and applied it to the chemiluminometric determination of adenosine-5 -triphosphate (ATP). They described the preparation and... 

Spectrophotometric determination of iron in synthetic solutions. SAM was implemented in a sequential injection analyser with monosegmentation. Concentration gradients were established at the interfaces between the different sequential zones [357],... 

Sequential injection analysis was conceived in 1990 by Ruzicka Marshall [75] as a novel approach to process analysis and laboratory assays, as well as to the development of sensors. A flow diagram of the simplest analyser is shown in Fig. 2.8. 

Multi-commuted flow analysers are characterised by enhanced flexibility and easy operation and combine some of the attractive characteristics of both segmented and unsegmented flow analysis. In this way, segmented flow, flow injection and sequential injection analytical procedures are efficiently implemented in multi-commuted flow manifolds. 

The spectrophotometric determination of Fe(II), Fe(III), nitrate and nitrite ions in natural and wastewaters in a sequential injection system [298] is an interesting example of this innovation. In-line tangential filtration was performed prior to the sample insertion port as part of the sampling step. Particulate material was not aspirated towards the holding coil and several samples could be analysed without the need for frequent cleaning of the filtration unit. Analytical figures of merit were similar to those for the corresponding flow-based analytical procedure with manual sample filtration prior to flow analysis. 

Trojanowicz, M., ed. 2008. Advances in Flow Analysis. Weinheim Wiley-VCH. Tzanavaras, P. D., E. Thiakouli, and D. G. Themelis. 2009. Hybrid sequential injection-flow injection manifold for the spectrophotometric determination of total sulfite in wines using o-phthalaldehyde and gas-diffusion. Talanta 77 1614-1619. van Staden, J. F., M. G. Mashamba, and R. S. Stefan. 2002. An on-line potentiometric sequential injection titration process analyser for the determination of acetic acid. Anal. Bioanal. Chem. 374 141-144. 

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