Porous silica layer

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

Figure 25-10 shows a rapid separation of proteins on superficially porous particles, which consist of a 0.25-p.m-thick porous silica layer on a 5-pm-diamctcr nonporous silica core. A stationary phase such as Clg is bonded to the thin, porous outer layer. Mass transfer of solute into a 0.25-p.m-thick layer is 10 times faster than mass transfer into fully porous particles with a radius of 2.5 pm. enabling high efficiency at high flow rate. Superficially porous particles are especially suitable for separation of macromolecules such as proteins, which diffuse more slowly than small molecules. 

Capillary had porous silica layer (0.70 pm) with chemically bonded Cl8... 

As an alternative to wholly porous sub-2-pm particles, 1.7-pm fused-core particles surrounded by a 0.5-pm porous silica layer with 90 A pores, have emerged. In a comparative study, substantially lower back pressure was reported when the fused-core particles were used. This allowed for columns to be coupled in series, which increased the peak efficiency up to 92,750 plates (164). Wider-pore fused-core particles have an average pore diameter of 160 A. The wider-pore particles are particularly useful for increased sample loading and the rapid separation of peptides using volatile mobile phases (165). 

It should be mentioned that there is another type of relatively new column that is made from the 2.7-pm fused-core silica particles, bonded with C18 alkyl chains, by fusing a 0.5-pm porous silica layer onto 1.7-pm non-porous silica cores. The selectivity of the fused-core particle columns is very similar to that of certain <2-pm C18 columns and has the advantage of a substantially lower back-pressure at much higher flow rates, which allows rapid separations to be performed even routinely on a conventional LC system without significant loss in efficiency or resolution. The fused-core columns are new to antibiotic analysis and may serve as good alternatives to <2-pm columns in the field. 

Another variant of IR spectroscopic technique is known as Fourier Transform IR Attenuated Total Reflection Spectroscopy (ATR FT-IR) has been described by Poston et al. [123] to study adsorbates at the silica-solution interface. The authors reported the preparation of ZnSe internal-reflection elements coated with a porous silica layer of ca. 700 nm thick. They studied the adsorption of ethylacetate from n-heptane solutions. This technique allows the determination of IR spectra in-situ and its dependence on the solution concentration. They found a nonlinear adsorption isotherm of ethylacetate on... 

Formation of porous silica layer in ALOT columns... 

Tock and co-workers [101] suggested a very interesting method for preparation of porous silica layers in open tubular columns. This method was worked out for fused silica capillaries used in liquid chromatography. However, in our opinion it is also promising for ALOT columns in gas chromatography. 

Tock and co-workers [101] suggested preparation of ALOT column by precipitation of a thick porous silica layer on the inner wall of fused silica capillaries. Precipitation of silica was from polyethoxy-siloxane solution dynamically coated on the inner wall of the capillary. This investigation was carried out for capillary HPLC however, the results are undoubtedly valuable for gas capillary ALOT columns too. 

Corasil I and II (particle range 37 - 50 microns). Waters Associates, Framingham, Mass, (solid glass bead with either single (Type I) or a double (Type II) porous silica layer). 

When comparing the 2.7 p,m Ascends Express or Halo pardcles (0.5 p,m porous silica layer) to the 2.6 p,m Kinetex particles (0.35 p,m porous silica layer), the... 

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