Packings octadecyl

Using a different approach, Oleschuk et al. succeeded in packing octadecyl silica beads from a side channel into a specifically designed chamber of the microchip. 

The temperature with large columns may not be homogenous. A mathematical model of the effect of a radial temperature gradient has been developed and validated on octadecyl-packed columns of 11-15 cm diameter... 

Although open tubular columns have been used for RPLC,42 most applications use packed columns. The most widely used column packings are formed by chemically bonding butyl (C4), octyl (C8), or octadecyl (C18) chains to a silica surface. Phenyl (Ph), cyano (CN), and amino (NH2) functionalities are also used. Polymeric packings which are finding broad acceptance include alkyl-grafted poly(methylmethacrylate) and alkyl-grafted or unmodified... 

A polyethylene-coated (PEE) silica column was used with water-methanol eluents to achieve the separation and retention of 27 pesticides.40 The retention times of 33 commercial pesticides were determined on an octadecyl (ODS)-derivatized alumina column using water-methanol eluents and compared with retention properties on an ODS-silica column packing.41 More recently, RP-HPLC was used in combination with diode array detection for the identification and quantification of 77 pesticides (acidic, basic, and neutral) in groundwater samples.42... 

One column can be used for different types of liquid chromatography by changing the eluent components. As an example, a column packed with octadecyl-bonded silica gel has been used for size-exclusion liquid chromatography with tetrahydrofuran (THF), normal-phase liquid chromatography with n-hexane, and reversed-phase liquid chromatography with aqueous acetonitrile. Examples of the chromatograms are shown in Figure 1.4. 

For example for the preparation of a 15 cm long, 4.6 mm i.d. stainless tube column, 2.5 g of octadecyl-bonded silica gel was suspended in 25 ml of hexanol-methanol mixture, and kept in an ultrasonic bath for a few minutes to remove air. After the reservoir was filled with the slurry, methanol was pumped in at 10 ml min -1 under constant pressure, 45 MPa (450 bar). After the replacement of slurry solvent by methanol, the flow was stopped and the pressure allowed to drop. When 0 MPa was reached the reservoir was removed. Then, 20 ml of water was added and methanol was again pumped in under the same conditions as before. Again, the flow was stopped and the pressure allowed to drop until it reached 0 MPa. The pre-column was removed and the analytical column closed. The maximum pressure that can be applied in the filling stage is based on the pore size, particle shape, and purity of the silica gel. This reproducible packing procedure is performed at constant temperature by using a water bath (60-BO °C). 

The efficiency of a well packed column should be about 95 000 m 1 5% for a 5 pm octadecyl-bonded silica gel column. The equations for the calculation of the number of theoretical plates (AO and peak asymmetry (.. s) are as follows. A model chromatogram is given in Figure 3.5. 

The most widely used support substance for the manufacture of packing materials in analytical HPLC columns is silica. Silica can be treated with organochlorosilanes or similar reagents to produce siloxane linkages of any derived polarity similar to what is done for GC columns (stationary phases). The most popular materials are octadecyl silane (ODS), which contains a carbon loading of CIS groups and octyl, which contains C8 groups materials such C2, C6, and C22 are also available. 

FIG U RE 1.13 Gradient separation of polypeptides on silica rod column and particle-packed columns. Mobile phase velocity 4mm/s, gradient 5%-60% ACN in the presence of TFA, gradient time 5min, columns (a) silica rod column, (b) Capcellpak SG (5 pm), (c) LiChrospher WP 300 RP-18e (5 pm), (d) nonporous NPS-ODS-1 HPLC column (1.5pm) (e) polymer-based TSKgel Octadecyl-NPR (2.5pm). (Reprinted from Minakuchi, H. et al., J. Chromatogr. A, 828, 83, 1998. Copyright 1998, with permission from Elsevier.)... 

HPLC methods can be ntilized for the pre-concentration of aromatic amines from polluted waters on silica gel or octadecyl silica (ODS) colnmns [55], The determination is then performed by RP HPLC using ODS packings as the stationary phases and a mixture of methanol, isopropanol, and water as the mobile phase [55], RP HPLC with diode array detector (DAD) methods coupled on-line with a continnons seqnential anaerobic/aerobic reactor system have been employed in wastewaters treatments [56], A continnons monitoring of the possible presence of aromatic amines in azo dyes wastes is based on indncing in the waste, the reaction of a reduction of the dye, followed by HPLC/ UV or HPLC/MS analysis [57-59], The redncing agent solutions are sodium dithionite or tin(II) chloride in an aqneons acidic medinm at 70°C, followed by SPE [58,59], LLE [60,61], or SEE [60-62],... 

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