Chapter 1

Evolution and Description of the Principal Flow Techniques

Abstract

Advances in Science and Technology have resulted in an increasing demand for control analyses and posed various challenges to analytical chemists such as the need to develop new methods exhibiting as much selectivity, sensitivity, sample and reagent economy, throughput, cost-effectiveness, simplicity and environmental friendliness as possible. Automation and miniaturization of solution-based analysis are essential to make them fast and efficient for routine and research tasks.

Flow techniques have undoubtedly aroused especial interest in relation to many other automatic methodologies of analysis. Ever since, segmented flow analysis was developed by Skeggs in 1957, flow techniques have been in continuous evolution toward new developments. They have gained importance for clinical, industrial and environmental purposes as they allow highly reproducible fast determinations. There is no solid argument in favor of using any particular flow technique separately; rather, substantial advantages can be derived from their combination.

Thus, a detailed description of flow techniques, their evolution, their hyphenation advantages and a critical comparison between them are presented in this chapter.

Keywords

Chip on valve (ChOV)Cross-injection analysis (CIA)DispersionFlow injection analysis (FIA)Flow techniquesLab on valve (LOV)Multicommutated flow injection analysis (MCFIA)Multipumping flow systems (MPFS)Multisyringe flow injection analysis (MSFIA)Segmented flow analysis (SFA)Sequential injection analysis (SIA)

Chapter Outline

1.1 The need for analysis automation

1.1.1 Automatic and automated methods

1.1.2 Flow techniques contribution to greener analytical chemistry

1.2 Skeggs’ solution: SFA

1.3 Flow injection analysis

1.4 Sequential injection analysis

1.5 Multicommutated flow analysis

1.6 Multisyringe flow injection analysis

1.7 Multipumping flow systems

1.8 Lab on valve

1.9 Chips

1.10 Lab in a syringe

1.11 Combined use of flow techniques

1.12 Theoretical foundations of flow techniques

1.12.1 Graphical representation of signals provided by flow systems

1.12.2 Variation of the sample profile along its travel: convective and diffusive phenomena

1.12.3 Dispersion coefficient

1.12.4 Influence of the injected sample volume

1.12.5 Relationship between the dispersion coefficient and injected sample volume

1.12.6 S1/2 and peak overlap

1.12.7 Influence of tubing length on dispersion

1.12.8 Influence of the flow rate on dispersion

1.12.9 Influence of tubing diameter

1.12.10 Influence of coil diameter

References

Advances in science and technology have resulted in an increasing demand for control analyses and posed various challenges to analytical chemists such as the need to develop new methods exhibiting as much selectivity, sensitivity, sample and reagent economy, throughput, cost-effectiveness, simplicity and environmental friendliness as possible. Automation and miniaturization of solution-based analysis are essential to make them fast and efficient for routine and research tasks [1]. Flow techniques have undoubtedly aroused especial interest in relation to many other automatic methodologies of analysis. Ever since, segmented flow analysis (SFA) was developed by Skeggs in 1957, flow techniques have been in continuous evolution toward new developments. They have gained importance for clinical, industrial and environmental purposes as they allow highly reproducible fast determinations. Since the beginnings of automation of analytical methods, various different flow techniques have been developed and used for analytical or monitoring applications. There is no solid argument in favor of using any particular flow technique separately; rather, substantial advantages can be derived from their combination. Since flow-based methods are nonseparative tools, advantages of combining flow techniques with separation techniques are also noteworthy (see Chapter 3). Among the benefits of automation of analytical procedures, the increase of sample frequency, minimization of sample contamination or alteration, miniaturization of the analytical system, analyst security improvement and lower reagent and sample consumption, implying less personal and consumable costs should be highlighted. Thus, a detailed description of flow techniques, their evolution, their hyphenation advantages and a critical comparison between them are presented in this chapter.

The need for analysis automation

The need to seriously consider the development of automatic methods of analysis arose in the 1950s, when clinical tests started to be increasingly used for diagnostic purposes in medicine. This led to a rapid increase in the demand for laboratory tests which, for obvious economic reasons, could not be met simply by hiring more laboratory staff. The large number of samples with which analysts can be confronted imposes the use of expeditious analytical methods, such as automatic ones. Among the benefits of automation of analytical procedures, the increase of sample frequency, minimization of sample contamination or alteration, miniaturization of the analytical system, and lower reagent and sample consumption, implying less personal and consumable costs should be highlighted. Furthermore, automation improves methods’ repeatability and reproducibility, since human error is avoided and the analytical system is more isolated from the environment. Automation is of great value also, when dealing with samples or chemicals which are harmful, e.g. radioactive samples or cancerogenic chemicals. Thus, analyst safety is also improved by automation, e.g. radiochemical methods automation is of high interest since this reduces analyst exposure to radiation.

Automatic and automated methods

Despite the major conceptual and operational differences between automated and automatic, these terms are frequently confused. Automated is referred to partly automated methods and automatic to fully automated ones. A fully automated method should allow the whole analytical process to be completed with no intervention from the analyst; that is to say it should also be capable of making decisions by itself, including operating conditions alteration in response to the analytical results. All too frequently, methods are deemed automatic simply because one or several steps of the analytical process are performed in an automated manner. However, an automatic method should be capable of completing all steps including sampling, sample preparation and dissolution, interference removal, aliquot withdrawal, analyte measurement, data processing, results evaluation and decision making, and also of restarting the whole process in order to adapt it to the particular needs of a new sample if necessary.

Table 1.1

Steps of the Analytical Process and Equipment Used to Automate the Operations Involved

Sampling Solids Robot Unusual Dedicated microprocessor computers hosts Liquids Dispensers Low Gases Pumps Low Mixtures Robot Unusual Physical conditioning Size reduction Dispenser, robot and mill Low Drying Robot and stove or lamp Unusual Dissolution Robot and vortex, heater Unusual Filtration Robot and filtering unit Unusual Centrifugation Robot and centrifuge Unusual Weighing Robot and balance Unusual Aliquot withdrawal Dispenser, robot and balance Unusual Injection Dispenser and valve Low Robot and syringe Usual Chemical conditioning Extraction Robot Unusual pH adjustment Dispenser Low Masking Dispenser Low Derivatization Special sampler Usual Measurement Transducing Transducer Low Signal conditioning Noise filter Low Amplification Amplifier Low Data storage Graph...