Chromatographic fraction purity calculation: if a solute is collected between times t1 and t2, how should the purity of this collected fraction be computed from the eluted amounts?

Introduction:
During chromatography, fractions are often pooled between time windows t1 and t2. The purity of a pooled fraction should express the proportion of desired solute relative to all detectable components in that fraction, typically solute plus co-eluting impurities.

Given Data / Assumptions:

• Collected fraction contains the target solute and possible impurities.
• Amounts can be determined by peak integration with appropriate response factors.
• Solvent contribution is not included in purity by mass of analytes.

Concept / Approach:
Purity is defined as the fraction of desired component mass within the total analyte mass of the collected fraction. Therefore, a ratio using solute in the numerator and the sum of solute plus impurities in the denominator reflects true purity on a mass (or molar) basis.

Step-by-Step Solution:
Step 1: Quantify the amount of solute and impurities between t1 and t2.Step 2: Exclude solvent volume from analyte purity calculations.Step 3: Compute purity = solute / (solute + impurities).Step 4: Report as fraction or percentage as needed.

Verification / Alternative check:
Chromatography software commonly reports purity as the percent area of the main peak over the sum of all peak areas in the selected window after response factor corrections, matching the stated formula.

Why Other Options Are Wrong:

• Difference (solute − impurity): Not normalized; can be misleading and unit-dependent.
• solute / impurity: A ratio, not a fraction of the total; fails when impurities approach zero or are multiple species.
• Including solvent terms: Solvent volume is not part of analyte purity by mass.

Common Pitfalls:
Ignoring detector response factors, baseline drift, or overlapping peaks; always apply calibration or relative response factors for accurate mass-based purity.

Final Answer:
amount of solute eluted / (amount of solute eluted + amount of impurity eluted)