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Mass spectrometry (MS) provides analyte identification over a wide molar-mass range. However, particularly in the case of complex matrices, this ability is often enhanced by the use of pre-MS separation steps. A separation, prototype technique for the “gentle” fractionation of large/ultralarge analytes, from proteins to whole cells, is here described to reduce complexity and maintain native characteristics of the sample before MS analysis. It is based on flow field-flow fractionation, and it employs a micro-volume fractionation channel made of a ca. 20 cm hollow-fiber membrane of sub-millimeter section. The key advantages of this technique lie in the low volume and low-cost of the channel, which makes it suitable to a disposable usage. Fractionation performance and instrumental simplicity make it an interesting methodology for in-batch or on-line pre-MS treatment of such samples.

Determination of total cholesterol (TC), HDL and LDL cholesterol, and triglycerides (TG) is used to assess lipoprotein abnormalities and coronary artery disease (CAD) risk. Furthermore, studies have demonstrated that small, dense LDL particles penetrate more easily into the arterial intima (1), exhibit increasing binding to arterial wall proteoglycans(2), and are more prone to oxidative stress(3). Small, dense LDL particles also have a prolonged plasma half-life because of their lower binding affinity for the LDL receptor(4). The presence of small, dense LDL particles in plasma is, therefore, considered to be proatherogenic. Analytical methods for LDL size characterization are therefore expected to improve classification, diagnosis, and therapy of dyslipidemic patients. However, simple methods for routine LDL size measurement are not yet available. LDL size is mainly measured by nondenaturing PAGE (5), which is time-consuming and may be unsuitable for large numbers of samples. Easier methods for measuring LDL size have been proposed, including high-performance gel-filtration chromatography (HPGC)(6). Both PAGE and HPGC can give accurate size estimations only if appropriate standards are available. Moreover, recent studies have shown that LDL particles may be discoidal, with diameter and height that are not significantly correlated(7). PAGE and HPGC cannot give information on particle conformation. Flow field-flow fractionation (FlFFF) is a separation technique in which macromolecules and particles are separated by the combined actions of an axial flow and a perpendicular cross-flow (8). No stationary phase is present, and the separation mechanism is sufficiently gentle not to alter the native structure of proteins and protein complexes(9). Proteins are fractionated according to their difference in diffusion, with retention time that is inversely proportional to the analyte diffusion coefficient. In principle, with the use of FlFFF it is possible to obtain the hydrodynamic radius (Rh) of proteins from their retention time …