Filter selection prefilters

Prefiltering samples prior to SPE in a standardized manner using glass-fiber filters having no organic binders and testing the analytes of interest to establish that they are not adsorbed on the filter selected is recommended [43], Alternatively, Simpson and Wynne [76] present the counter viewpoint... 

Filter manufacturers provide easy-to-follow selection tables and charts to enable the determination of the correct efficiency. By referring to these charts a proper filter selection can be made, but it should always be borne in mind that the life of coalescing elements can be extended by the inclusion of a prefilter. 

In spectrophotometric analyzers, interference filters are selected for desired wavelengths, as determined from the spectral relationship curves. Photodetectors are least sensitive in the blue end of the spectrum. This can be dealt with by using prefilters or narrow spectral ranges, which are calibrated for more sensitivity. Improvements in spectrophotometers include a flashed xenon light source with dual-beam measurement. Dual-beam machines measure the spectrum of both the light source and the reflected light for each measurement. 

The experimental procedure used by the author places various prefilters on top of the selected membrane in 47 mm holders which are run in parallel on the process stream to be filtered. Usually, the test is carried out at constant flow rate and the rise in pressure drop is recorded versus the volume processed until the membrane plugs. The combinations providing the highest throughput are then evaluated with respect to cost of the media. 

The search for the optimum prefilter media is often facilitated by testing the membrane and prefilter separately after the combination has reached the limiting pressure drop. If the final membrane is plugged but the prefilter shows a low pressure drop (see Figure 2.28), the prefilter is too coarse and a more retentive prefilter should be selected to protect the final filter. 

---