Page 178 - Illustrated Pocket Dictionary of Chromatography

180 SILICA, SILICA GEL

silica, silica gel A material composed of silicon dioxide, SiO 2.
Silica is a typically support material used in HPLC and GC columns.
It is stable except at high pH or in hydrofluoric acid (HF) solutions.
For HPLC silica is prepared from the controlled polymerization of
silicic acid, Si(OH) 4. Final HPLC-grade silica is porous, pellicular or
nonporous, spherical or irregular in shape, derivatized or underiva-
tized, activated or base deactivated.

silicic acid A commonly used starting material used in the forma-
tion of silica with the chemical formula Si(OH) 4.

siloxane bonds The siloxane bond is the principle structural bond
in silica gel: ∫Si—O—Si∫.
silylation Describes the reaction between a silica derivatizing
reagent and the surface silanol group.

sineresis The process that occurs during the postpolymerizarion
aging of silica; water and unbound ions are exuded from the silica gel,
leaving a smaller more rigid structure called a hydrogel.

single-beam detector In a single-beam absorbance detector the
source light passes through the sample directly to the detector device.
Changes in source intensity, noise, and drift are uncompensated. See
dual-beam.

slope The slope (m) of a plot is the change in the dependent vari-
able (Dy) per change in the independent variable (Dx), or m =Dy/Dx.
In analytical work plots of detector response (R) versus analyte
concentration (C) give rise to a slope of DR/DC. This represents
the sensitivity of the analysis to the analyte and is also called the
response factor for the analyte. The general equation for a linear rela-
tionship between x and y is:

y = mx b
+
where b is the y-intercept.
size-exclusion chromatography (SEC) An LC technique that
generates a separation based solely on analyte size. This technique is
commonly used to characterize polymers. The molecular weight is
determined through the creating of a MW versus retention time plot.
See molecular weight calibration curve. SEC techniques include gel