A generic LC-MS approach for the absolute quantification of undigested peptides in plasma at mid-picomolar levels is described. Nine human peptides namely, brain natriuretic peptide (BNP), substance P (SubP), parathyroid hormone 1-34 (PTH), C-peptide, orexines A and B (Orex-A and -B), oxytocin (Oxy), gonadoliberin-1 (gonadothropin releasing-hormone or luteinizing hormone-releasing hormone, LHRH) and α-melanotropin (α-MSH) were targeted. Plasma samples were extracted via a 2-step procedure: protein precipitation using 1vol of acetonitrile followed by ultrafiltration of supernatants on membranes with a MW cut-off of 30 kDa. By applying a specific LC-MS setup, large volumes of filtrates (e.g., 2×750 μL) were injected and the peptides were trapped on a 1mm i.d.×10 mm length C8 column using a 10× on-line dilution. Then, the peptides were back-flushed and a second on-line dilution (2×) was applied during the transfer step. The refocalized peptides were resolved on a 0.3mm i.d. C18 analytical column. Extraction recovery, matrix effect and limits of detection were evaluated. Our comprehensive protocol demonstrates a simple and efficient sample preparation procedure followed by the analysis of peptides with limits of detection in the mid-picomolar range. This generic approach can be applied for the determination of most therapeutic peptides and possibly for endogenous peptides with latest state-of-the-art instruments. 相似文献
Ceramic–analcime zeolite composite membranes have been synthesized by hydrothermal crystallization of zeolite over clay supports. The zeolite layer is characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and pore size distribution determined using the bubble point technique. The XRD pattern of the zeolite is found to match with JCPDS file #19-1180 which is an analcime-o zeolite of molecular formula NaAlSi2O6·H2O having orthorhombic crystal structure with lattice parameters: a=13.72 Å, b=13.714 Å and c=13.714 Å. In this paper, we report a gas phase nitration scheme, in which we show through the elemental analysis that the modification of entire matrix (and not limited to pores and channels) occurs. The nitrated zeolite was reduced to aminated zeolite membrane by reacting it hydrazine hydrate.
Separation of surfactant (CPC) was performed using these three types (unmodified, nitrated and aminated) of membranes and it showed about 300% increase in the retention of the surfactant after its modification. Its hydrophilic nature also increases as shown by the increase in the permeate flux. In order to find the reason for this enhanced performance, structural analysis of the modified membranes was carried out. The XRD patterns of these were found to be identical and they did not match with those of any of the compounds given in the JCPDS files. The patterns were therefore indexed, using first principle, to find their crystal structure and it was found that the structure changed from orthorhombic for unmodified to tetragonal geometry for the modified zeolites. This also caused about 10% increase in the unit cell volume of the modified zeolites. Anion exchange capacity and the elemental analysis showed that the nitration occurred over the entire zeolite matrix (not restricted to the pores and channels) and on an average about one amine group was present in every second formula unit of the zeolite. This extensive presence of nitrate or amine group in the zeolite matrix makes the modified zeolite membrane highly hydrophilic and may be responsible for the increase in the retention of the surfactant and permeate flux for the modified membranes. 相似文献
Membrane surface roughness alters the surface area accessible to foulants and may influence macroscopic properties, such as zeta potential. It is usually quantified by atomic force microscopy (AFM) at a single scan size. This would be appropriate if roughness is independent of scale. This study shows that the root-mean-square roughness, RRMS, is scale (or scan size, L × L) dependent through the power law RRMS = AL3−D. The coefficient, A, is the roughness at a scan size of 12 μm2. D is the fractal dimension that relates the increase in roughness to the increase in scan size. Values for A and D were determined for a range of micro- and ultrafiltration membranes using an AFM scan series covering at least three orders of magnitude in L. They were also determined for nanofiltration membranes by re-analysis of data in the literature. The results suggest that using the power law expression allows potentially greater discrimination among membrane types and provides a way to quantify membrane roughness over a range of scales. It was further observed that the coefficients A and D of PVDF membranes showed positive and negative correlations, respectively, with the molecular weight cut-off. Additionally, zeta potentials of PVDF membranes measured by the tangential streaming potential method became more negative with increasing A and more positive with increasing D, suggesting possible significant influence of roughness on hydrodynamic transport of ions. 相似文献
Membrane cleaning is a key point for the implementation of membrane technologies in the dairy industry for proteins concentration. In this study, four ultrafiltration (UF) membranes with different molecular weight cut-offs (MWCOs) (5, 15, 30 and 50 kDa) and materials (polyethersulfone and ceramics) were fouled with three different whey model solutions: bovine serum albumin (BSA), BSA plus CaCl2 and whey protein concentrate solution (Renylat 45). The purpose of the study was to evaluate the effect of ultrasounds (US) on the membrane cleaning efficiency. The influence of ultrasonic frequency and the US application modes (submerging the membrane module inside the US bath or applying US to the cleaning solution) were also evaluated. The experiments were performed in a laboratory plant which included the US equipment and the possibility of using two membrane modules (flat sheet and tubular). The fouling solution that caused the highest fouling degree for all the membranes was Renylat 45. Results demonstrated that membrane cleaning with US was effective and this effectiveness increased at lower frequencies. Although no significant differences were observed between the two different US applications modes tested, slightly higher cleaning efficiencies values placing the membrane module at the bottom of the tank were achieved. 相似文献
The issue of evaluating equivalent pore diameter distributions in membrane microfilters from gas-liquid (g-l) porosimetry data has been critically examined. Experiments performed with one isotropic and one composite anisotropic membrane in both possible orientations revealed conspicous dependence of the obtained (g-l) porosimetry peaks on imposed pressure ramp rates, p. Interference of this kinetic effect can be eliminated from the measured data by extrapolation to p = 0. The ramp rate effect is most likely caused by tortuous pore length distribution, and relatively long times required for liquid expulsion. For two experiments, the observed effects of p could be reconciled with predictions of the Schlesinger-Bechhold theory [Bechold et al., Kolloid Z., 55 (1931) 172–198]. The data obtained with the thin top layer of the composite membrane facing intruding air directly did deviate somewhat from the theory. Pores characterized by (g-l) porosimetry are likely of the “throat type”, and their size distribution is considerably more narrow than that obtained for the “node-type” pores by SEM-image analysis [Zeman and Denault, J. Membrane Sci., 71 (1992) 221–231]. A single bivariate distribution function was constructed for these two distinct pore populations. Flow-weighted or number fraction distributions can be calculated from the extrapolated porosimetry data. For narrow ranges of “throat” diameters, these distributions are fairly similar. 相似文献
Hydrophilic polysulfone ultrafiltration (UF) membranes were prepared from blends of cellulose acetate with carboxylated polysulfone of 0.14 degree of carboxylation. The effects of blend polymer composition on compaction, pure water flux, water content and membrane hydraulic resistance (Rm), have been investigated to evaluate the performance of the membranes. The performance of the blend membranes of various blend polymer compositions were compared with that of membranes prepared from pure cellulose acetate and blends of cellulose acetate and pure polysulfone. The hydrophilic cellulose acetate-carboxylated polysulfone blend UF membranes showed better performance compared to membranes prepared from pure cellulose acetate and blends of cellulose acetate and pure polysulfone. 相似文献