Sample filtration plays a major role in analytical chemistry. Filtration is a practice where a liquid is freed from suspended solid impurities. The technique is commonly employed in wet chemistry for separation of precipitates for further quantitative analysis or for removal of interfering suspensions in spectroscopic studies.
In chromatographic analysis sample filtration before injection becomes all the more important for different reasons. After injection the sample is carried to the chromatographic column and subsequently to the detector. Any particle impurities if present in the sample will start blocking the chromatographic column leading to irregularities in flow rate which can shift retention times of peaks or to complete stoppage of mobile phase flow. Such problems may be overcome with column washing but in severe cases the expensive column needs to be replaced.
Solid suspensions can also deposit in the detector cell which can lead to excessive noise and appearance of ghost peaks. Excessive deposition can even lead to pressure build up and damage to cell walls.
In classical analysis filtration is carried out using coarse paper filters but due to strict restriction on particle size in chromatographic analysis special membrane filters are required. Generally 0.45 µ size membrane filters are fine but for small diameter capillary columns packed with packings under 3μm will require 0.2 μm size filters. These membrane filters are wetted with the mobile phase and placed in the filtration syringe. Filtered samples are collected into the sample vials and capped properly.
Teflon filters are useful for organic solvents. Nylon filters can be used for both aqueous as well as organic samples but acidic samples should be filtered using PTFE-based membrane filters.
It goes without saying that the objective of sample filtration would get defeated if the mobile phase itself is not filtered. For this purpose the inlet tubing immersed in the mobile phase container should be fitted with stainless steel or ceramic particle filtration frits of specified sizes.
Sample and mobile phase filtration is strongly recommended as it will not only improve the quality of results but will also extend column life and reduce maintenance costs and time for the system components
Syringe filters are available for HPLC sample purification from several sources. A few of the renowned suppliers are mentioned here: