The reason to use SFA or FIA
The reason to automate is so that you can run more samples. Running more samples is the same as increasing your productivity. Having an instrument run samples for you decreases your labor costs. Having the instrument do all those boring mixings and shakings increases your day to day precision. This improves quality and really helps with that QA/QC program. Whether you are a research facility, a municipal laboratory, or a commercial lab, 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 using instrumentation. Instruments can do digestions, distillations, dilutions, and filtrations all for you. More importantly, is that these methods are always duplicated exactly ensuring that quality control procedures are met.
Your most expensive cost is labor, and after that is supplies. A flow analyzer will allow you to do more samples in less time and increase the amount of time you or your staff spends on more important things like running other samples, reporting results, and doing that dreaded paper work. Automated equipment uses less reagents and generates less waste than manual methods do. This is not about getting rid of staff. This is about freeing up time so that staff can have time to prepare and run more samples. The beauty of the auto analyzer is that it is running samples while you are doing something else.
History of Flow Analysis
The original automated 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 auto-analysis eventually branched out and has roots in almost every automated device we see today. Flow Injection Analysis was a way to get out of buying a Technicon Auto-Analyzer because a person with a peristaltic pump, some Teflon tubing, and a flow through spectrometer cell could fabricate 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 branch of Perstorp.
All continuous flow analyzers share a commonality of parts with similar functions. All have an autosampler, pumps, reagent mixing manifolds comprised of tubing, a flow through 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 either bell shaped, or rectangular.
Segmented Flow Analysis (SFA)
Segmented Flow analysis is the first auto chemistry and is what Skeggs initially demonstrated to the Technicon corporation. Segmented flow enjoys over 50 years of success as a proven technology. Segmented flow is the basis of multiple automated EPA and many other regulatory methods.
A typical segmented flow analyzer pumps the sample into the cartridge using peristaltic pump tubing. The sample merges with an air segmented carrier stream, reagents are added and mixing occurs by end over end mixing in coils as the sample is transported down the path of the tubing. Depending on the internal tubing volume and the amount of time the transport from injection to detection takes there could be multiple samples inside the tubing at a given time. For instance, if the delay time is 10 minutes and samples are injected every minute there will be 10 samples traveling within the tubing.
Flow Injection Analysis (FIA)
Flow Injection Analysis is considered as an acceptable alternative to equivalent segmented flow methods. In other words, though USEPA method 350.1 is a segmented flow method, the USEPA considers a FIA method equivalent. There are thousands of literature references to FIA methods, and multiple ATP approvals. Since the EPA considers FIA equivalent to SFA, OI did not obtain EPA ATP letters. There are also several FIA methods that are EPA approved without an SFA equivalent, for instance CN by OIA1677.
In Flow Injection analysis the sample is injected by a valve into a carrier stream. The sample solution does not pass through peristaltic pump tubing prior to the valve. Mixing occurs as the sample travels through tightly wound Teflon mixing coils. Unlike segmented flow that may contain multiple samples within the tubing all at the same time, Flow injection injects a 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 sample and reagents in a continuously flowing stream of reagents. The reaction that occurs, the peak shape, the sensitivity, and so forth is all determined by how the tubing is configured, where the reagents are teed in, and the length of the tubing that the reactions are happening in. Instrument manufacturers configure tubing “cartridges” according to published methodology, or develop methods based on R&D and published works.
An analyst can readily change and/or modify method performance simply by modifying any aspects of the analytical cartridge. Since the segmentation of SFA limits dispersion, SFA is a bit more forgiving than FIA in the length of tubing. For instance, the addition of extra tubing on a FIA method can significantly change peak shape and sensitivity, while additional tubing on a SFA system does not really matter. For example, at one of my previous laboratories we routed an SFA method that had a non-working heater cartridge through several feet of tubing to the fecal coliform bath and then back to the detector. Adding several feet of tubing would have been detrimental to FIA, but was un noticeable in the SFA method.
Literature is deceiving in sample size as SFA methods tend to use less sample than FIA. Throughput by FIA is not faster, and in fact it has been demonstrated that a well operated SFA system far exceeds FIA in samples per hour. SFA mixes by an end over end similar to putting sample and reagent in a vial and inverting. FIA mixes by what Ruzika termed “controlled dispersion” which is harder to explain than end over end, and is a result of solution flowing faster through the center than at its sides, and then rapidly colliding with the walls of the mixing coils and other flow interruptions placed in its path. SFA signals do not have to be brought to a maximum, but theoretical SFA should always be and the resulting peak would be rectangular. Because of the “tunneling” that occurs in FIA flow, a FIA peak is Gaussian.
It is hard to say exactly what the maximum time for a SFA incubation is because the only limiting factor is the minimal carryover that occurs as solution “transfers” from one segment to the next while traveling down the tube. Most FIA literature places 2 minutes at the maximum for FIA. In fact, Ruzika and Hansen gave a 1 minute maximum for FIA reaction.
Advantages of Flow Methods
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 a 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 fluoride by ISE method, can be modified according to 40 CFR Part 136.6 and made into automatic methods. Continuous flow methods reduce manual labor and that reduction of manual labor tends to make precision better and reduce potential contamination. Flow methods also tend to have lower MDL’s than manual methods and a better chance of fewer QC failures.