A retention time is the elapsed time between sample introduction and a detected peak under specified operating conditions. Compare an unknown with standards measured with the same stationary phase, mobile phase, flow and temperature. Matching time supports a tentative identity; it does not exclude a different co-eluting compound. Two resolved analyte peaks indicate at least two detected components.
For a validated linear detector response through the origin, concentration is proportional to peak area. Use standards to determine the area-per-concentration factor before calculating an unknown. Peak-area percentages equal composition percentages only when the supplied response factors justify that conversion. A single peak does not prove absolute purity because undetected or co-eluting components may remain.
Example 1
Standards X,Y and Z have retention times 2.4,4.1 and 5.6min. A mixture has peaks at 2.4 and 5.6min under matched conditions. State supported identities.
The peaks match X and Z. These are tentative identifications supported by the standards, not proof that other substances could not co-elute.
Example 2
A 4.0 mg L−1 standard gives area 200 units. Assume linear response through zero. Find concentration for area 150.
Response factor =4.0200=50 area units per mg L−1. Unknown concentration =50150=3.0 mg L−1.
Watch out:Different substances can have different detector responses; response factors are needed before converting relative areas to composition.
Retention standards and chromatogram data practice | Year 12 Chemistry | Ascent Arcade