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UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper III covers analytical and organic chemistry.

UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper III

Q2(a). Eluent composition for retention factor 5 in reversed-phase HPLC

Question

In an HPLC measurement, a solute has a retention time of 21.0 min and an unretained species is eluted in 1.0 min using a mobile phase comprising 20 % (vol by vol) methanol and 80 % water. Determine the composition of methanol and water to elute the solute corresponding to retention factor 5.0. Given: (i) polarity index of water is 10.2, methanol is 5.1; (ii) log 2 = 0.301.

Answer in brief

43.6 % methanol and 56.4 % water by volume (about 44 : 56). P′ of the eluent falls from 9.18 to 7.98 and k from 20 to 5.

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Topic notes: chromatography and solvent extraction (plates, resolution, van Deemter, HPLC, GLC, ion exchange, Craig), with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversHPLCretention factorpolarity indexreversed-phase chromatographysolvent strengthmobile phase composition

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