Flowing-stream analysis

One of the most active areas of electrochemical research and development in the last decade has been the use of electrochemical detectors for flowing stream analysis. Of particular interest is the analysis of complex solutions containing several chemical species. The use of high performance chromatography coupled to electrochemical detection (LCEC) is an important development in this regard. In most applications of conventional LCEC, such as in clinical, biomedical or organic analysis, discrete samples are analysed. 

In gas-solid extractions the sample is passed through a container packed with a solid adsorbent. One example of the application of gas-solid extraction is in the analysis of organic compounds for carbon and hydrogen. The sample is combusted in a flowing stream of O2, and the gaseous combustion products are passed through a series of solid-phase adsorbents that remove the CO2 and 1T20. 

There are many potential advantages to kinetic methods of analysis, perhaps the most important of which is the ability to use chemical reactions that are slow to reach equilibrium. In this chapter we examine three techniques that rely on measurements made while the analytical system is under kinetic rather than thermodynamic control chemical kinetic techniques, in which the rate of a chemical reaction is measured radiochemical techniques, in which a radioactive element s rate of nuclear decay is measured and flow injection analysis, in which the analyte is injected into a continuously flowing carrier stream, where its mixing and reaction with reagents in the stream are controlled by the kinetic processes of convection and diffusion. 

Flow injection analysis (FIA) was developed in the mid-1970s as a highly efficient technique for the automated analyses of samples. °> Unlike the centrifugal analyzer described earlier in this chapter, in which samples are simultaneously analyzed in batches of limited size, FIA allows for the rapid, sequential analysis of an unlimited number of samples. FIA is one member of a class of techniques called continuous-flow analyzers, in which samples are introduced sequentially at regular intervals into a liquid carrier stream that transports the samples to the detector. ... 

Dynamic headspace GC/MS. The distillation of volatile and semivolatile compounds into a continuously flowing stream of carrier gas and into a device for trapping sample components. Contents of the trap are then introduced onto a gas chromatographic column. This is followed by mass spectrometric analysis of compounds eluting from the gas chromatograph. 

Palen, J. W. and Taborek, J., Solution of Shell-Side Flow Pressure Drop and Heat Transfer by Stream Analysis Method (Heat Transfer Research, Inc., Alhambra, CA), AICHE Chemical Engineering Progress Symposium Series No. 92, Vol. 65 (1969), pp. 53-63. 

Palen, J.W. and TABORAK, J. Chem. Eng. Prog. Sym. Ser. No. 92, 65 (1969) 53. Solution of shell side flow pressure drop and heat transfer by stream analysis method. 

Titration. This method obviously requires physical removal of a sample from the plant and results in the sample being thrown away. In modem equipment samples can be taken at specified intervals from a flowing sample stream using Flow Injection Analysis, enabling the analysis time to be shortened. 

The required volume for the analysis of water samples from a drainage flow, stream, or river is collected from a depth of up to 50 cm at the center of a flow using an appropriate sampling bottle. A sample size of 1000 mL should be sufficient for the usual type of determination. The sampling bottle and bottles for storage and shipment should be well washed with an appropriate organic solvent and distilled water so that the sample is not contaminated, and keeping those bottles in a clean container is recommended. It is recommended that samples taken are kept below 5 °C and shipped to the laboratory as soon as possible. 

Another interesting development, in which continuous flow was combined with discrete sample titration, is continuous flow titration by means of flow injection analysis (FIA) according to Ruzicka and co-workers70. Fig. 5.16 shows a schematic diagram of flow injection titration, where P is a peristaltic pump, S the sample injected into the carrier stream of diluent (flow-rate fA), G a gradient chamber of volume V, R the coil into which the titrant is pumped (flow-rate fB), D the detector and W waste. 

The complex flow pattern on the shell-side, and the great number of variables involved, make it difficult to predict the shell-side coefficient and pressure drop with complete assurance. In methods used for the design of exchangers prior to about 1960 no attempt was made to account for the leakage and bypass streams. Correlations were based on the total stream flow, and empirical methods were used to account for the performance of real exchangers compared with that for cross flow over ideal tube banks. Typical of these bulk-flow methods are those of Kern (1950) and Donohue (1955). Reliable predictions can only be achieved by comprehensive analysis of the contribution to heat transfer and pressure drop made by the individual streams shown in Figure 12.26. Tinker (1951, 1958) published the first detailed stream-analysis method for predicting shell-side heat-transfer coefficients and pressure drop, and the methods subsequently developed... 

This technique differs from flow injection analysis in the sense that whereas in the latter technique the sample plug is injected into a flowing stream of reagent, in the former technique plugs of reagent are injected into a continuous stream of the sample. Under these conditions the amount of sample in the zone of the reagent will increase as the dispersion increases. The sample will become well... 

Flow injection analysis is a rapid method of automated chemical analysis that allows for quasi-continuous recording of nutrient concentrations in a flowing stream of seawater. The apparatus used for flow injection analysis is generally less expensive and more rugged than that used in segmented continuous flow analysis. A modified flow injection analysis procedure, called reverse flow injection analysis, was adopted by Thompson et al. [213] and has been adapted for the analysis of dissolved silicate in seawater. The reagent is injected into the sample stream in reverse flow injection analysis, rather than vice versa as in flow injection analysis. This results in an increase in sensitivity. 

FTA [5-7] is a version of continuous-flow analysis based on a nonsegmented flowing stream into which highly reproducible volumes of sample are injected, carried through the manifold, and subjected to one or more chemical or biochemical reactions and/or separation processes. Finally, as the stream transports the Anal solution, it passes through a flow cell where a detector is used to monitor a property of the solution that is related to the concentration of the analyte as a... 

A more recent development is a technique known as flow injection analysis, in which a discrete volume of a liquid sample is injected into a carrier stream. Reagents required for the development of the analytical property of the analyte, e g. colour developing reagents for spectrophotometry, are already present in the stream. The stream then flows straight to the detector and the technique depends upon the controlled and reproducible dispersion of the sample as it passes through the reaction zone. Thus the reaction does not necessarily need to develop to completion,... 

Flow injection analysis is a fast-developing technique with many potentialities. Particular attractions are the relative simplicity of operation and automation, together with sample throughputs which may exceed 100 per hour. Thus routine monitoring of process streams and pollution control are obvious areas for application. 

Flow-injection analysis is based on the introduction of a defined volume of sample into a carrier (or reagent) stream. This results in a sample plug bracketed by carrier (Fig. 1 (a)). 

In this technique, which was developed in the 1970s, microlitre volumes of liquid sample are injected, at intervals, into a continuously flowing carrier stream which is not air-segmented. Various reagent streams are introduced as required and controlled mixing of reagents and sample occurs. The fact that flow injection analysis does not involve air-segmented streams makes it possible to include such separation steps as solvent extraction and gas diffusion. 

Figure 6.11 Examples of Tecator Chemifold types for flow injection analysis. S, sample injection port C, carrier stream R1 R2, R3, reagent streams D, detector W, waste.

The elutriation method is really a reverse sedimentation process in which the particles are dispersed in an upward flowing stream of fluid. All particles with terminal falling velocities less than the upward velocity of the fluid will be carried away. A complete size analysis can be obtained by using successively higher fluid velocities. Figure 1.4 shows the standard elutriator (BS 893)(6i for particles with settling velocities between 7 and 70 mm/s. 

Flow injection analysis is based on the injection of a liquid sample into a continuously flowing liquid carrier stream, where it is usually made to react to give reaction products that may be detected. FIA offers the possibility in an on-line manifold of sample handling including separation, preconcentration, masking and color reaction, and even microwave dissolution, all of which can be readily automated. The most common advantages of FIA include reduced manpower cost of laboratory operations, increased sample throughput, improved precision of results, reduced sample volumes, and the elimination of many interferences. Fully automated flow injection analysers are based on spectrophotometric detection but are readily adapted as sample preparation units for atomic spectrometric techniques. Flow injection as a sample introduction technique has been discussed previously, whereas here its full potential is briefly surveyed. In addition to a few books on FIA [168,169], several critical reviews of FIA methods for FAAS, GF AAS, and ICP-AES methods have been published [170,171]. 

A schematic diagram of the automatic system is shown in Fig. 4.7a. The sample -to-gas-chromatograph interface and requirements for the injection of a hquid mixture onto a gas chromatographic column from a flowing stream present the greatest problem, in contrast to sample transfer and dilution techniques in automatic analysis, which are well documented. 

---