An analyst extends a gradient to dramatically improve resolution between two closely eluting peaks. What quantitative consequence might this produce?
ABetter quantitation in all cases, since baseline resolution eliminates peak overlap bias
BPotentially worse quantitation — very long gradients broaden peaks, reducing peak height and signal-to-noise, which can decrease quantitative precision
CNo effect on quantitation, since peak area is conserved regardless of peak width
DImproved linearity range, since wider peaks are easier to integrate accurately
The misconception is that more resolution is always better for quantitation. While adequate resolution is necessary to prevent peak overlap bias, pushing resolution beyond baseline (Rs ≥ 2.0) via very long gradients or highly retentive conditions broadens peaks. A broader peak has lower height and can decrease signal-to-noise, reducing the ability to detect and accurately quantify low-concentration analytes. Optimization balances sufficient resolution against peak shape.
Question 2 Multiple Choice
A pharmaceutical analyst is quantifying an active ingredient whose recovery from tablet extraction varies between 85–95% across preparations. Which calibration approach best corrects for this variability?
AExternal standard calibration, since it directly relates peak area to known concentrations in a standard solution
BInternal standard calibration, because adding a structurally similar compound to every sample and plotting the area ratio corrects for variable recovery and injection volume differences
CStandard addition, because it eliminates the need for a calibration curve entirely
DSingle-point calibration at the expected concentration, since the variability is small enough to ignore
Variable recovery means the amount of analyte reaching the detector is not consistently proportional to the original sample concentration. Internal standard calibration corrects for this: because the internal standard undergoes the same extraction and injection as the analyte, the area ratio (analyte/internal standard) remains accurate even when absolute recovery varies. External standard calibration assumes consistent recovery and injection volume, making it vulnerable to exactly this type of variability.
Question 3 True / False
System suitability testing must be passed before unknown samples are analyzed in a validated HPLC method.
TTrue
FFalse
Answer: True
System suitability testing verifies that the instrument is performing acceptably on the day of analysis — checking injection repeatability, tailing factor, theoretical plate count, and resolution between critical peak pairs. These tests catch instrument problems (degrading column, air bubble, leaking valve) before they corrupt results. Pharmacopeial methods (USP, EP) require system suitability criteria to be met before results are considered reportable, not just during method development.
Question 4 True / False
A calibration curve verified to be linear from 10–100 µg/mL can be safely extrapolated to quantify samples at 150 µg/mL without additional verification.
TTrue
FFalse
Answer: False
Quantitation outside the verified linearity range is unreliable. Detector response can become nonlinear at higher concentrations (detector saturation, stray light effects in UV detection) or curve downward due to matrix effects. Method validation establishes the linear range; samples falling outside that range should either be diluted back into range or the linearity range should be re-established and validated for the new concentration.
Question 5 Short Answer
Why is internal standard calibration preferred over external standard calibration when sample preparation recovery is variable, and what properties should the internal standard have?
Think about your answer, then reveal below.
Model answer: Internal standard calibration adds a known amount of a reference compound to every sample before sample preparation. Because the internal standard undergoes identical extraction, cleanup, and injection steps as the analyte, the area ratio (analyte peak area / internal standard peak area) remains proportional to the original analyte concentration even when absolute recovery varies. The ideal internal standard is structurally similar to the analyte (similar polarity, ionization, and recovery) but chromatographically resolved from it and absent from real samples.
External standard calibration assumes constant recovery and injection volume — assumptions violated by variable extraction or injection drift over a long sequence. Internal standard calibration makes the measurement self-normalizing against those sources of variation, which is why it is the preferred approach in regulated pharmaceutical testing.