The reason for using SFA or FIA
The reason to automate is so you can run more samples. Running more samples is the same as increasing your productivity. Having an instrument analyze samples for you reduces your labor costs. Having the instrument do all those boring mixes and shakes increases day-to-day precision. This improves quality and really helps with that QA/QC program. Whether you are a research center, 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 titrations can be replaced with instrumentation. The instruments can do digestions, distillations, dilutions, and filtrations, all for you. More importantly, these methods are always exactly duplicated to ensure quality control procedures are followed.
Your most expensive cost is labor, and then supplies. A flow analyzer will allow you to run more samples in less time and increase the amount of time you or your staff spend on more important things like running other samples, reporting results, and doing that dreaded paperwork. Automated equipment uses fewer reagents and generates less waste than manual methods. It’s not about getting rid of staff. This is to free up time so staff can have time to prepare and analyze more samples. The beauty of the Auto Analyzer is that it analyzes samples while you do something else.
History of flow analysis
The original auto analyzer is the Auto Analyzer invented by Leonard Skeggs in 1954. It took the Technicon Corporation three years to perfect it and develop a commercial product. The concept of self-testing eventually branched out and has roots in almost every automated device we see today. Flow injection analysis was a way to avoid buying a Technicon autoanalyzer because a person with a peristaltic pump, some Teflon tubing, and a flow spectrometer cell could make their own device. In fact, the first FIA instruments were held together with Lego blocks. The first commercial FIA instrument was introduced by Tecator, the analytical arm of Perstorp.
All continuous flow analyzers share common elements with similar functions. They all have an autosampler, pumps, reagent mix manifolds made up of tubing, a flow detector, and a way to collect the signal. The signal is always expressed as a peak with a maximum and a baseline, and the peak is bell-shaped or rectangular.
Segmented Flow Analysis (SFA)
Segmented flow analysis is the first automotive chemistry and is what Skeggs initially demonstrated to the Technicon corporation. Segmented flow enjoys more than 50 years of success as a proven technology. The segmented flow is the basis of multiple automated EPAs and many other regulatory methods.
A typical segmented flow analyzer pumps the sample into the cartridge using a peristaltic pump tubing. The sample is fused with an air-segmented carrier stream, reagents are added, and mixing is produced by mixing end-to-end in spirals as the sample is transported down the path of the tubing. Depending on the internal volume of the tube and the amount of time it takes to transport from injection to detection, there could be multiple samples inside the tube at any one time. For example, if the delay time is 10 minutes and samples are injected every minute, there will be 10 samples traveling inside the tube.
Flow Injection Analysis (FIA)
Flow injection analysis is considered an acceptable alternative to equivalent segmented flow methods. In other words, although the USEPA 350.1 method is a segmented flow method, the USEPA considers it an equivalent FIA method. There are thousands of literature references to FIA methods and multiple ATP approvals. Since the EPA considers FIA to be equivalent to SFA, OI did not obtain ATP letters from the EPA. There are also several FIA methods that are EPA-approved without an SFA equivalent, eg CN per OIA1677.
In flow injection analysis, the sample is injected through a valve into a carrier stream. The sample solution does not pass through the peristaltic pump tubing before the valve. Mixing occurs as the sample travels through tightly wound Teflon mixing coils. Unlike segmented flow which can hold multiple samples inside the tube at the same time, flow injection injects one sample and detects it before the next sample is injected.
Comparison of SFA and FIA
Flow Injection is basically a derivative of SFA without air segmentation. Both SFA and FIA mix samples and reagents in a continuous stream of reagents. The reaction that occurs, the shape of the peak, the sensitivity, etc., is determined by the configuration of the tube, where the reagents are connected, and the length of the tube in which the reactions occur. Instrument manufacturers configure tube “cartridges” according to published methodology or develop methods based on R&D and published work.
An analyst can easily change and/or modify method performance simply by modifying any aspect of the analytical cartridge. Since SFA segmentation limits dispersion, SFA is a bit more lenient than FIA when it comes to tube lengths. For example, the addition of additional tubes in an FIA method can significantly change the shape and sensitivity of the peak, while the additional tubes in an SFA system do not really matter. For example, in one of my previous labs we routed an SFA method that had a non-functioning heater cartridge through several feet of tubing to the fecal coliform bath and then back to the detector. Adding several feet of pipe would have been detrimental to FIA, but it was not noted in the SFA method.
The literature is misleading regarding sample size, as SFA methods tend to use fewer samples than FIA methods. FIA’s throughput is not faster, and in fact, a well-operated SFA system has been shown to far outperform FIA in samples per hour. SFA is mixed end to end similar to putting sample and reagent in a vial and inverting. FIA mixes by what Ruzika called “controlled dispersion,” which is harder to explain than end-to-end, and is the result of solution flowing faster through the center than the sides, then rapidly colliding with the mixing coil walls and other flows. interruptions placed in its path. SFA signals do not have to be maxed out, but the theoretical SFA should always be and the resulting peak would be rectangular. Due to the “tunneling” that occurs in the FIA flow, an FIA peak is Gaussian.
It is difficult to say exactly what the maximum time for an SFA incubation is because the only limiting factor is the minimal carryover that occurs as the solution “transfers” from one segment to the next as it travels down the tube. Most of the FIA literature places 2 minutes maximum for the FIA. In fact, Ruzika and Hansen gave a maximum of 1 minute for the FIA’s reaction.
Advantages of flow methods
Many methods are written specifically for continuous flow analysis. Some examples are ammonia per EPA 350.1 or Block digestion TKN per EPA 351.2. An example of an FIA method written specifically for FIA is OIA 1677. These EPA methods specify automated chemistry in the methodology itself. Other methods, such as the manual ISE fluoride method, can be modified in accordance with 40 CFR Part 136.6 and converted to automated methods. Continuous flow methods reduce manual labor, and that reduction in manual labor tends to improve accuracy and reduce potential contamination. Flow methods also tend to have lower MDLs than manual methods and a better probability of quality control failures.
