Reducing Sample Loss During the Filtration of Protein Precipitates Deproteinization of biological samples is essential in the analysis of several low-moleculsr-weight substances and in most enzyme assays. For this purpose the proteins are usually precipitated with a suitable acid and the precipitate removed either by filtration or centrifugation. Centrifugation is generally preferred, as filtration requires additional glassware and involves the risk of sample loss due to adherence to glass surface andior evaporation. Samples rich in lipids, however, necessitate filtration, as the lipids may float on centrifugation. This note describes a simple and inexpensive strategy far cutting down sample loss during filtration of protein and other precipitates, that is suitable for relatively small volume samples.
A small circular filter paper approximately 3 4 em in diameter is folded as shown in the figure. The tip ofthe pipet is placed in the middle of the paper, and the paper is wrapped around it as indicated. The paper is then fixed firmly on the pipet tip with a smallrubber band. The pipet with the attached filter paper is then introduced into the sample containing the Gatein or the other precipitate, and the appropriate volume is withdrawn. Only filtered solution enters the pipet, as it passes through the filter paper. The pipet along with the filter paper is taken out ofthe sample, and the filter paper is carefully removed after detaching the rubber band. Appropriate aliquots of the filtrate are finally transferred to a second tube. Sample loss due to adsorption on glass is minimized, as the sample enters the pipet directly without coming in contact with a funnel or a second tube. Evaporation of the sample is also minimized, as only small surface area of the solvent is exposed to air during filtration. Sample loss due to adsorption on paper can be minimized by selecting small-sized paper far filtration. The paper should not, however, be too small for proper fixation on the pipet or tip. While all types offilter papers can be used, sample loss is hieher with thicker p a w r s like Whatrnan no. 42. The procedure works quite \V well witGboth the conven&nal and automatic pipets. The filtration procedure can be very useful for undergraduate laboratories with modest centrifugation facilities, especially when many samples must he handled. We have been successfully using the procedure far filtration of protein precipitates during studies on the kinetics of proteolytic enzymes' in undergraduate and pastgraduate courses. By eliminating the need for centrifugation, the entire assay can A B be performed i n simple test tubes. The procedure also appears very useful in the filtration of solvent extracts, which stand high risk of evaporation, during lipid Preparation 0f.a pipet for obtaining analysis. filtered aliquots. A34-cm diameter Laskowisky, M. In Methods in Enzymology; Colowick, S.P.; Kaplan, N.O., Eds; filter oaoer is folded (A),and a pipet lor diokt tiol is introduced in the Academic: New York, 1955; Vol. 2, pp 26-35. hid& of the oaoer (8). The oaoer is wrapped iround ihk pip& and M. Saleemuddin fixed on the pipet (or pipet tip) with a rubber band (C). The pipet with Depanmenl of Bocnem slry attached filter paper is used for obFacAy of f e Sc ences taining filtered aliquots from soluAlgarn MLS m L r vers I, tions containing the precipitate.
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522
Journal
of Chemical Education
