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Flow Injection Or Segmented Flow Analysis – What Are the Similarities and Differences?
Reason for using SFA or FIA
The reason to automate is that you can run more samples. Running more samples equals increasing your productivity. Having the instrument run samples for you reduces your labor costs. All this tedious mixing and shaking is instrumental in increasing your accuracy day by day. It improves quality and really helps in that QA/QC program. Whether you are a research facility, municipal laboratory, or commercial laboratory, you can benefit from automation.
Almost any method that can be done manually can be automated. Time-consuming steps such as manual titration can be replaced by using instrumentation. Instruments can perform digestion, distillation, dilution and filtration for you. Most importantly, these methods are always duplicated exactly to ensure quality control procedures are met.
Your most expensive cost is labor, followed by supplies. A flow analyzer will allow you to do more samples in less time and free up the time you or your staff spend on important things like running other samples, reporting results, and doing that dreaded paper work. Automated equipment uses fewer reagents and generates less waste than manual methods. It’s not about getting rid of employees. It’s about freeing up time so that employees have time to create and run more samples. The beauty of AutoAnalyzer is that it is running samples while you are doing something else.
History of Flow Analysis
The original automated analyzer was the auto analyzer invented by Leonard Skaggs in 1954. It took Technicon Corporation three years to perfect it and develop a commercial product. The concept of auto-analysis evolved over time and has its roots in almost every automated device we see today. Flow injection analysis was a way to buy a Technicon auto-analyzer because one could make his own device with a peristaltic pump, some Teflon tubing, and flow through a spectrometer cell. In fact, the first FIA equipment was put together with Lego blocks. The first commercial FIA instrument was introduced by Tekator, the analytical arm of Perstorp.
All continuous flow analyzers share commonality of parts with similar functions. All consist of autosamplers, pumps, reagent mixing manifolds with tubing, flow through detectors, and a signal collection path. The signal is always expressed as a peak with a maximum and a baseline, and the peak is either bell-shaped or rectangular.
Fractured Flow Analysis (SFA)
Segmented flow analysis is the first autochemistry and is what Skeggs initially demonstrated to Technicon Corporation. Segmented Flow has over 50 years of success as a proven technology. Discontinuous flow is the basis of multiple automated EPA and many other regulatory approaches.
A typical discontinuous flow analyzer pumps the sample into a cartridge using peristaltic pump tubing. Sample air is mixed with a discontinuous carrier stream, reagents are added, and mixing occurs by end-over-end mixing in the coil as the sample is carried along the tubing path. Depending on the volume of internal tubing and the time required for transport from injection to detection, there may be multiple samples in the tubing at a given time. For example, if the delay time is 10 minutes and samples are injected every minute, 10 samples will travel through the tubing.
Flow Injection Analysis (FIA)
Flow injection analysis is considered an acceptable alternative to equivalent sectional flow methods. In other words, although USEPA Method 350.1 is a discontinuous flow method, USEPA considers the FIA method equivalent. FIA methods have thousands of literature references and multiple ATP approvals. Since EPA considers FIA to be equivalent to SFA, OI did not receive EPA ATP letters. There are several FIA methods that are EPA approved without an SFA equivalent, for example OIA1677 through CN.
In flow injection analysis the sample is injected into the carrier stream through a valve. The sample solution does not pass through the peristaltic pump tubing before the valve. Mixing occurs as the samples travel through a tightly wound Teflon mixing coil. Unlike discontinuous flow in which tubing can contain multiple samples at once, flow injection injects one sample and detects it before injecting the next sample.
Comparison of SFA and FIA
Flow injection is basically a derivative of SFA without air splitting. Both SFA and FIA mix the sample and reagents in a continuous flow of reagents. The reaction that occurs, the size of the peak, the sensitivity, and so on are all determined by how the tubing is configured, where the reagents are titrated, and the length of the tubing in which the reaction is taking place. Instrument manufacturers configure tubing “cartridges” according to published methods, or develop methods based on R&D and published work.
The analyst can easily change and/or improve method performance by modifying any aspect of the analytical cartridge. SFA’s segmentation limits dispersion, SFA is slightly more forgiving than FIA in tubing length. For example, adding extra tubing on an FIA method can significantly change the peak shape and sensitivity, while adding extra tubing on an SFA system makes no difference. For example, in my previous lab we routed an SFA method in which a non-working heater cartridge was routed through several feet of tubing into a fecal coliform bath and then back to the detector. Adding several feet of tubing would have been detrimental to the FIA, but was not noticeable in the SFA method.
The literature is misleading in sample size because SFA methods use a smaller sample than FIA. Throughput is not faster with FIA, and in fact it has been shown that well-run SFA systems outperform FIA in samples per hour. The SFA is mixed to one end such that the sample and reagent are placed in the vial and inverted. FIA calls Ruzicka “controlled dispersion” which is difficult to explain and is ultimately the result of solution flowing faster in the center than at its sides and then rapidly hitting the mixing coil and other flow walls. Obstacles in his way. It is not necessary to maximize the SFA signal, but the theoretical SFA should always be and the resulting peak will be rectangular. Since the FIA current “tunnels”, the FIA peak is Gaussian.
It is difficult to say exactly what the maximum time for SFA incubation is because the only limiting factor is the minimal carryover that occurs as the solution “transfers” as it travels down the tube from one section to the next. Most FIA materials set a maximum of 2 minutes for FIA. In fact, Ruzicka and Hansen gave a maximum of 1 minute for the FIA’s reaction.
Advantages of flow method
Many methods are written specifically for continuous flow analysis. Some examples are ammonia by EPA 350.1 or block digestion TKN by EPA 351.2. An example of an FIA method written specifically for FIA is OIA 1677. These EPA methods specify automated chemistry within the method itself. Other methods, such as the manual fluoride by ISE method, can be converted to automated methods in accordance with 40 CFR part 136.6. Continuous flow methods reduce manual labor, and reduced manual labor improves accuracy and reduces potential contamination. Flow methods have a better chance of lower MDLs and fewer QC failures than manual methods.
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