Q1(b). HPLC against GLC; particle size of packed GLC columns

Answer in brief

(i) Non-volatile, thermally unstable, ionic and high-molar-mass analytes. (ii) 60 to 120 mesh = 0.25 to 0.12 mm (250–120 µm); usual grades 60–80 and 80–100 mesh.

Key points

• GLC: the analyte must be volatile and thermally stable at the column temperature (50 to 300 °C); HPLC needs only that it dissolves in the mobile phase.
• HPLC alone separates non-volatile, thermally labile, ionic and highly polar species and macromolecules (proteins, peptides, nucleic acids, synthetic polymers).
• Packing: a solid support of diatomaceous earth (Chromosorb) coated with 5 to 10 % of the liquid stationary phase.
• Size: 60–80 mesh (0.25–0.18 mm) or 80–100 mesh (0.18–0.15 mm); at the finest 100–120 mesh (0.15–0.12 mm).
• Small, uniform particles give more plates; still smaller particles make the pressure drop across the column too high.