Uremic ultrafiltrates (and normal serum, for comparison) were fractionated by means of gel filtration. The collected fractions were further investigated by combined analytical techniques: "high-performance" liquid chromatography, gas chromatography, mass spectrometry, and isotachophoresis. Ultrafiltrate fractions in the so-called middle molecular mass region (Mr 500-2000) contained a considerable amount of substances of low molecular mass, such as carbohydrates, organic acids, amino acids, and ultraviolet absorbing solutes. Ultraviolet absorbance in the "middle molecular mass region" of the gel chromatogram is mainly due to the presence of these rather low-molecular-mass solutes. Therefore this signal is not a quantitative measure of molecules with a "middle" molecular mass.
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Schoots et al. (1982) studied this question.
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