Continuous flow analysis
Continuous-Flow Analysis ( CFA ) is an analysis technique that automatically carries out wet chemical analysis processes. The process steps of photometric- analytical methods are processed in a continuous flow of sample and reagents in the intended sequence.
Such an analysis method and its time sequence is usually built on a hardware method manifold , matched to the concentration range to be measured and the type of sample. The composition of the samples to be measured (sample matrix) and existing interfering components can make a user-specific structure necessary for the same determinations.
The CFA analysis system draws in liquid samples from an automatic sampler, forwards each sample to the existing method manifolds and divides them into aliquots for the simultaneous execution of all established determination methods in each sample.
Today, CFA analysis methods are state of the art in various application areas as ISO standards, such as water quality, soil quality (extracts), tobacco (extracts) or food analysis. In the DIN EN ISO standards for water quality , there is the umbrella term flow analysis , under which the CFA technology is run together with the related FIA technology.
Note: Other standards use the term "Continuous flow analysis" in the German translation, which can be misunderstood as the analysis of a flow. The use of unclear names makes it much more difficult to find specific information through search engines.
The CFA technique works with regular subdivision (segmentation) of the continuous reaction stream by means of gas bubbles (air, N2). To distinguish it from the FIA technique (flow-injection analysis, injection-flow analysis), CFA is also referred to in the literature as segmented flow analysis SFA . The segmentation in the CFA technique enables even more complex analytical process steps to be strung together without the carryover (dispersion, carry-over) between successive samples becoming inadmissibly large. Modern Microflow CFA systems achieve a sample throughput of 30 to 40 samples per hour even with complicated methods, even if the passage from the sampler through all process steps to the detector is 10 to 25 minutes. This means that the CFA analyzer simultaneously feeds 6 to 12 samples through the analytical system without one sample influencing the next sample in an unacceptable manner.
More about the technique of a single CFA method
The structure of a wet chemical analytical method on the CFA analyzer is shown in a flow diagram and documented for the user. In the picture shown you have to imagine a constant flow from left to right. By using a multi-channel peristaltic pump, the ratio of sample and continuously conveyed reagents is always constant, regardless of the speed of the pump. The absolute amount of the individual solution is defined by the pump hose used; the color coding has been established worldwide for flowrated pump tubes for over 40 years . In the case shown of a 12-position peristaltic pump for a CFA Microflow Analyzer, the respective dosing tubes give the stated amount in µl / min (number).
The method manifold ( Analytical Manifold ) as a term that has been established over the decades comprises all components for performing the analytical method steps that are located between the peristaltic pump (= numbers) and the measuring cell (= colorimeter). This can be a module with injectors, mixing and reaction spirals, but also an incubator (heating bath), a dialysis unit, a flow-through microdistillation module, a UV digestion unit or a high-temperature hydrolysis unit.
The sequence of the analytical treatment stages of the method manifold can be referred to as the reaction path. The continuous flow is regularly segmented at the beginning of this reaction section by means of gas bubbles (air, N2), every 1-2 seconds depending on the technology. The reagents are metered into the ready-segmented stream, that is, into the individual segments flowing past the respective metering point after a time determined by the structure. The regular segmentation of a sample sucked in for 60 seconds every 1 second now means that in this case (40 samples / h, 2: 1), in principle, a 60-fold repeated measurement is carried out for each sample. The CFA reaction line can be imagined as a conveyor belt with a reaction tube every second, the contents of which are then precisely added to further solutions that run through a matrix separation (dialysis), through a digestion unit, through a reaction stage with incubation, and so on . At the end of the analytical process, these reaction solution segments flow continuously through a measuring cell, the air segmentation usually being removed beforehand ( debubbling ).
The continuous signal of the photometric detection now shows as a graphic the course of the measured values of the successively arriving segments, all of which have passed through the entire analytical sequence independently of one another and in the same way. This makes the reproducibility of the overall analytical system visible on the photometer graphic, both at the moment and for each sample individually at the time of its individual run. This unique property of the CFA technique, to show the stability of an analytical system and to make the plausibility of a single measurement fundamentally comprehensible, is the reason for the decade-long unbroken success of the CFA technique in the chemical laboratory in the opinion of the author. The second flow analysis technique, FIA, as an unsegmented technique, does not offer this quality information.
CFA analyzers are used in today's analytical laboratory for ion analysis and conventional parameters, mostly as a system for the simultaneous determination of 2 to 5 parameters in each sample (perception of the author). With 50 samples per hour and 4 parameters simultaneously, i.e. 200 determinations per hour, the CFA technique outperforms even large discrete analyzers in sample throughput , with higher analytical sophistication and significantly lower costs.
Examples
- In the case of total cyanide, acid is first added to the sample and the complex cyanide is split up using UV, then the cyanide is distilled in a flow-through, after which the distillate forms a color with reagents, the strength of which is measured. The run through the analytical system takes about 15–20 minutes, depending on the conditions.
- In the case of total phosphorus, acid is first added to the sample and organophosphorus is converted using UV light, then the sample is hydrolyzed at 95 ° C to convert inorganic phosphorus compounds, the orthophosphate then present forms a color with reagents, the strength of which is measured. The run through the analytical system takes about 20-25 minutes.
- In the case of soil extracts, the samples for the determination of ammonium or nitrate nitrogen are first passed through a dialyzer, which separates the interfering sample matrix (color, humic substances ...), then diffused ions react with reagent to form a color which is measured. The run to the detector takes 6–12 minutes, depending on the method. With a sample throughput of 60 / h, there are 6–12 samples in transit through the analytical system
- In the case of fertilizer digestion solutions, the samples are first diluted 1:10 to 1:20 in a dilution loop on the manifold, then dialyzed and then measured by means of a color reaction. The run to the detector takes about 8-14 minutes.
Other uses
In addition to the described application of CFA technology for performing photometric analysis processes, there are also applications with other detectors such as fluorimeters, flame photometers or sensor technology. CFA is used as a transport technology, which enables the rapid transport of samples one after the other through a sample preparation, without inadmissible concentration carry-over from one sample to the next sample.
History of CFA analysis technology
The biochemist Dr. Leonard T. Skeggs (1918-2002) first built a prototype of an automatic analyzer for his laboratory samples in Cleveland, Ohio in 1951. The revolutionary idea was - not to try to move samples individually from one work step to the next, as was previously the case - but to connect the individual stations with a thin hose system and to pump the samples through sequentially. In order to prevent the samples from mixing with one another, air bubbles were introduced at regular intervals. Hence the term "segmented flow analysis", also CFA (Continuous Flow Analysis).
Originally nobody was interested in commercialization until Edwin C. "Jack" Whitehead, who had founded Technicon Instruments Corporation with his father in 1939, bought the development in 1954. The AutoAnalyzer was fully developed in 1957 and was the first commercially available analysis system to be offered. This laid the foundation for "automatic chemical analysis. This machine, later called AutoAnalyzer I, was originally capable of one, later two parallel parameters, e.g. glucose and BUN (urea nitrogen) at a rate of initially 20 samples per hour determine.
The samples were taken from the autosampler with 40 positions and mixed one after the other with the corresponding reaction solutions. The quantity measurement was effected by selecting different pump tubing diameters and thus flow rates in the peristaltic pump. Mixing took place in glass spirals, protein fractions were then removed in a dialyzer and the color reaction was completed in a heating bath. The concentration of the analyte was determined in a photometer and could be read off on the scale of the recorder (initially with a logarithmic scale, later linearized).
The next step was the combination of several methods (typically 12) in one device and the optimization of the components, which increased the sample frequency of the individual determinations to 60 per hour; the Technicon SMA 12/60 (Sequential Multiple Analyzer) was created, which has become a standard system for clinical laboratory analysis. At the same time, the AutoAnalyzer II was created, which was presented in 1972 and brought this form of automation to industrial laboratories. In 1979 Technicon and with it the AutoAnalyzer lines were sold to Revlon.
Since its development, the CFA technique has flourished for decades as “the” standard technique in clinical analysis. Countless publications on photometric, chemical CFA processes exist from the 1960s to the 1990s. The device technology developed from the first generation of autoanalyzers to the widespread macroflow technology and to compact and fast microflow CFA systems.
At the end of the 1970s, the FIA technology was developed from the CFA principle, omitting segmentation, which has become well established in the university sector for individual determinations and simpler procedures. For larger routine laboratories with several simultaneous determinations, considerable sample throughput and security against the influences of a fluctuating sample matrix, the CFA technology still offers the right solution. Low costs for consumables, fast processing of sample series and simple quality management guarantee further use of this technology in the future.
swell
- ↑ Determination of nitrite nitrogen and nitrate nitrogen and the sum of both by flow analysis (CFA) and spectrometric detection, ISO 13395: 1996.
- ↑ Segmented Flow Analysis, Encyclopedi of Analytical Science, Vol 8, Academic Press.
- ^ The use of Microcontinuous Flow Analysis and FIA in Water Analysis, Straka M., International Laboratory, 09-1990, p. 33.
- ↑ Water quality - Chemical analysis method Determination of total cyanide and free cyanide by continuous flow analysis, ISO 14403: 2002 (E), First edition March 1, 2002.
- ↑ Determination of orthophosphate and total phosphorus contents by flow analysis, Part 2: Method by continuous flow analysis (CFA), ISO 15681-2.
- ↑ Determination of ammonium by flow analysis and spectrometric determination, EN ISO 11732.
literature
- LT Skeggs Jr .; Analytical Chemistry, 38, 32A (1966)
- Edwin C. Whitehead; Technicon Corporation Worldwide - Review and Outlook; AutoAnalyzer Innovations; Technicon Symposium 1978; Vol. 1, page 22ff
- Morris H. Shamos; The Development of Laboratory Automation; Technicon International Symposium on Advances in Automated Analysis; April 1974, Sydney
- L Lewis, 'Leonard Tucker Skeggs - a multifaceted diamond', Clinical Chemistry , 27/10 (1981), pp 1465-1468
- Coakly, William A .: Handbook of Automated Analysis: Continuous Flow Technique, Marcel Dekker Inc., 1981, ISBN 0824713923
- R. Stanley; A multidimensional approach to analytical science; Journal of Automatic Chemistry, Vol. 6, No. 4; Oct-Dec. 1984, page 175ff.
Web links
- Principle of the CFA ( Flash ; 232 kB)