BPS2032 Chap.2 Reading Peaks and Resolution
Reading Peaks and Resolution
Resolution is a property of a named peak pair. It combines the distance between peak centres with the widths of both peaks, so it cannot be judged reliably by time separation alone. Broad peaks can erase the benefit of a large centre gap, while narrow peaks can make a modest gap analytically useful.
Correct measurement starts with peak identity and a shared width convention.
Label the earlier peak as 1 and the later as 2, confirm that retention times and widths use one unit, and identify whether the formula expects baseline or half-height widths. Select adjacent peaks when evaluating a critical pair.
Non-adjacent peaks may produce a large number that says nothing about the unresolved neighbour between them.
The unit investigation uses a resolution target of at least 1.4 for specified peak pairs. Some general chromatography material uses 1.5 as a baseline-separation convention, but that generic value should not replace the unit specification.
A complete result names the pair, shows the calculation, states the width convention and compares the value with the applicable target. If the pair fails, the trace guides the next diagnosis: broad peaks point toward efficiency, narrow crowded peaks toward selectivity, and peaks close to dead time toward retention control.
What this chapter covers
- 01
Pairwise meaning of chromatographic resolution
- 02
Peak centres, baseline widths and consistent measurement
- 03
Elution order and labelling of peak 1 and peak 2
- 04
Resolution equation with common time units
- 05
Unit-specific target of Rs at least 1.4 for defined pairs
- 06
Generic 1.5 convention kept distinct from the unit target
- 07
Diagnosis through centre spacing, width and dead-time proximity
- 08
Reporting a number together with identity, convention and decision
Worked example · free
Measure a pair against the unit target
- 1Find the centre gap: 6.70 - 6.15 = 0.55 min.
- 1Double the gap: 2 × 0.55 = 1.10 min.
- 1Add the baseline widths: 0.31 + 0.35 = 0.66 min.
- 2Divide: Rs = 1.10 / 0.66 = 1.67.
- 2Compare 1.67 with 1.4 and state the assumptions about identity and width measurement.
Key terms
- Critical pair
- The identified neighbouring peaks whose separation controls an important method decision.
- Peak centre
- The retention-time location used to represent the centre or apex of an identified peak.
- Baseline width
- Peak width measured using the baseline convention required by the selected resolution or efficiency equation.
- Half-height width
- Peak width measured at half of the peak height, used only with equations defined for that convention.
- Adjacent peaks
- Neighbouring peaks in elution order, the appropriate candidates for a pairwise resolution assessment.
- Elution order
- The sequence in which identified compounds leave the column under a specified method.
- Pair specification
- A stated requirement linking a named peak pair with a measurable acceptance threshold.
- Measurement convention
- The defined rule used to locate peak centres or widths consistently across comparisons.
Reading Peaks and Resolution FAQ
Why does resolution include both peak widths?
Both bands contribute to overlap. A pair with broad peaks needs a larger centre gap to create the same usable valley as a pair with narrow peaks. Ignoring either width discards part of the separation evidence.
Which resolution target should I use?
Use the target stated for the unit investigation and the named pair. The captured unit material uses at least 1.4. A generic 1.5 convention can provide background context but must not overwrite the unit specification.
What if the calculated value is just above the threshold?
Review peak identity, width placement, integration quality and replicate behaviour. Report the unrounded calculation and a sensible final precision. A marginal pass deserves verification rather than a claim of abundant separation.
How do I choose a corrective lever after a failed result?
Inspect the trace. Excessive width suggests an efficiency investigation, narrow crowded peaks suggest selectivity, and poor retention near the unretained marker suggests solvent-strength control. Confirm the pattern before changing conditions.
Can I calculate resolution for non-adjacent peaks?
A number can be computed, but it may conceal an unresolved peak between them and therefore fail to answer the analytical question. Method decisions normally focus on identified neighbouring pairs and the specified critical pair.
Assessment move
Practise peak labelling before arithmetic. For each chromatogram, mark the earlier and later members of the critical pair, record both centres and write the width convention beside the values. Solve the formula in three visible moves: centre gap, width sum and final division. Then add a decision sentence comparing the result with the unit target.
Keep an error log for missing factors of two, mixed time units, non-adjacent pair choices and unsupported claims that a passing number proves the entire method. Alternate numeric drills with diagnosis drills in which the widths or positions indicate whether retention, selectivity or efficiency deserves the first experiment.
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