Effizienz kommerziell erwerblicher Silikagele (dp=5 μm und 10 μm) in der Flüssigkeits-Chromatographie

Translated title of the contribution: Efficiency of commerically available silicas in HPLC

R. Ohmacht, I. Halász

Research output: Contribution to journalArticle

13 Citations (Scopus)

Abstract

Eighteen commercially available silicas having nominal particle sizes (dp) of 10 μm and 5 μm, have been systematically studied. The chromatographically determined average particle size (d) is about 35% greater than the average particle size as estimated using a Coulter Counter (dc). The particle sizes quoted by the manufacturers mostly lie between d and dc. The d values for the silicas with a quoted dp of 10 μm lay between 7.6 μm and 12.2 μm. The constants of the simplified (for 10 μm particles) or the complete (for 5 μm particles) van Deemter equation were measured and tabulated for varying capacity ratios (k′=0-3) as were the asymmetry factors As for an inert substance. When u=1 mm/s the reduced plate height (h/d) was less than 3 for substances with 0≤k′<3 when dp=10 μm. The capacity ratios of individual samples on a given stationary phase were reproducible with an accuracy of ±0.05 for all the silicas studied. The A, B and C terms of the van Deemter equation (dp=5 μm) were reproducible to within ±10%, even when the column was stored dry for a week. The decrease in umin with increasing k′ value depended both on the quality of the stationary phase (e.g., its geometrical form, the narrowness of the sieve fraction) and on the packing method. The reduced plate heights (h/d)min were bigger for smaller particles. In consequence of the definition of d, the permeability of a column is always proportional to d2 and the pressure drop in columns with smaller size particles increased substantially. It is a matter of experience that the column life decreases with increasing pressure drop. The extracolumn band broadening of commercial instruments is often so large that 10 cm long columns (i.d.=4 mm) packed with 5 μm particles are scarcely more efficient than identical columns packed with 10 μm particles. Although the definition of d favours spherical particles if reduced plate heights are to be calculated, the efficiencies of columns packed with spherical particles are very similar to those packed with irregular particles. The preference for spherical packing material over the cheaper irregular silica does not seems to be supported by experiment. The A term always increases with increasing k′ values, although this is theoretically unexpected. The C terms seems to be overwhelmingly determined by the speed of mass transport in the mobile phase. The dependence of the C term on k′ is, when k′ is >0.75, well described using the exclusively k′ dependent factions of the Cm term of the Golay equation. However, the increase in the C term when k′ is increased from 0 to 0.75 is about a factor three less than the theoretically calculated value. The stability of the columns has been demonstrated. Although columns (20 cm long in length) were stored dry for three months the efficiency only decreased slightly. With a 26 bar pressure drop 130 theoretical plates per second were obtained. The average pore diameters, pore volumes and specific surface areas of the commerical silicas influenced the k′ values of the sample substances. These factors however scarcely affected the column efficiencies even though their range of variation was not very narrow in the 18 silicas studied.

Translated title of the contributionEfficiency of commerically available silicas in HPLC
Original languageGerman
Pages (from-to)216-226
Number of pages11
JournalChromatographia
Volume14
Issue number4
DOIs
Publication statusPublished - Apr 1 1981

Keywords

  • Asymmetry factors
  • Average particle size
  • Commercial silicas (5-10 μm)
  • Constants of the van Deemter equation
  • Efficiency
  • Efficiency and surface properties of the support
  • Reduced plate height
  • Speed of analysis
  • Spherical and irregular particles

ASJC Scopus subject areas

  • Analytical Chemistry
  • Biochemistry
  • Clinical Biochemistry
  • Organic Chemistry

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