Being a new resident in the United States, installing water filters on my water faucet was one of the first things that I had to do after moving into my apartment. Filtration is truly one of the most commonly used sample preparation techniques, not only in the lab, but also very frequently at home. Many coffee lovers use filtration every morning. And would the pleasant afternoon swim lose its appeal if the pool filter stopped working? Of course, these are generally filtering out larger particles than those in our laboratory experiments.
Microfiltration and ultrafiltration are commonly used techniques. The key difference is the membrane pore sizes; the pore size of microfiltration ranges from 0.1 to 10 microns, while that of ultrafiltration is smaller than 0.1 microns. Of the two, microfiltration is more commonly used in ion chromatography applications, as required or mandated by International Standard Organization methods, such as ISO 10403-1, ISO 14-11, and ISO 5667-3. The ISO methods recommend or mandate microfiltration at the time of sampling. After all, this is the cheapest methodology to remove biological activity from a sample. By performing microfiltration on your samples at the time of sampling, you remove particulates (which may otherwise clog your system), and you also maintain sample integrity by removing biological activity. However, if filtration at the time of sampling is not possible, then it is always recommended to keep the samples away from direct sunlight, and keep them cooled while delivering them to their final destination as fast as possible. These are measures that can minimize sample deterioration, but it is a deviation from the standard methods.
With modernization, everyone is interested in automating every process. Years ago, the concept of automated filtration was coined. This is a nice solution if, and only if, there are only particulates (that need to be removed), and biological activity is not a concern. However, automated filtration, when used as a secondary filtration, is useful for samples. This is particularly true for those samples high in iron, as they can form precipitates during storage even after an initial filtration.
Thermo Fisher Scientific offers two primary, complementary, and markedly different approaches to automated filtration for water samples. The first is the use of the Dionex AS-DV autosampler with dedicated vials and filter caps. The second is the use of the inline high-pressure filters. The Dionex AS-DV filtration approach can be used for a wide range of samples, including those which are heavily loaded with particulates. Filtration occurs from the top down, so filtration efficiency is assisted by gravity, which acts to deposit large particles in the bottom of the tube; thus, the filter acts on the least contaminated part of the sample. Because each sample has its own individual filter, sample carryover (i.e. cross contamination) due to the filter is eliminated.
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The second approach involves the use of two inline high-pressure filters with a back flush. This approach can be implemented where particulate loading of samples is heavier or if there are concerns about sample carryover. The filters are installed on an auxiliary valve with an additional pump. The back flush flow traverses the filter in the opposite direction from that of sample introduction. For instance, while the first filter is in use, the second filter is being back flushed by the additional pump and, vice versa, when the second filter is in use. By doing so, the particulates are flushed off the frit to waste and the lifetime of the filters is extended dramatically.
Despite all the benefits and available solutions, not every sample will benefit from filtration. For instance, filtration is not recommended for ultrapure water analysis, where all analytes are in the single digit µg/L, or even ng/L, concentrations. Such samples are more than likely to be contaminated by filter materials. Henceforth, if used suitably, microfiltration leads to less system breakdown from pressure-related issues by removing particulates, as well as increased reporting accuracy by maintaining sample integrity.
Everything has its pros and cons, likewise for filtration. Filtration, when done correctly, will definitely save a lot of unnecessary work. So, the next time when you are musing in the morning over how smooth your coffee is, or enjoying your cooling swim on a steaming hot summer day, thank filtration!