The plot
A chromatogram is a line graph. The horizontal axis is time in minutes from the moment the sample was introduced. The vertical axis is detector response, usually absorbance in milli-absorbance units at a stated wavelength. Each component of the sample reaches the detector at a characteristic time and shows up as a peak. The plot is the raw evidence behind the purity figure on the front page of the report.
Reading it left to right
- Solvent front. A small disturbance in the first minute or two is the unretained solvent passing the detector. Laboratories normally exclude it from integration. If it has been counted as a peak, the purity figure is being diluted by something that is not part of the sample.
- Baseline. The flat line between peaks. A stable, flat baseline means the instrument was equilibrated and the integration is trustworthy. A drifting or noisy baseline makes small peaks hard to separate from noise and is a reason to read the integration table with more caution.
- Main peak. The tallest peak, which should be the material named on the label. Its position on the time axis is the retention time. A symmetric, sharp peak indicates good chromatography; a shoulder or a split top suggests a closely eluting impurity that the method has not fully separated.
- Secondary peaks. Smaller peaks before or after the main one are other components: incomplete sequences, oxidation products, or process residues. Their retention times relative to the main peak are consistent from run to run under the same method, which is how a laboratory recognises a known impurity.
- Late region. Peaks well after the main one are more strongly retained components, sometimes aggregates or highly hydrophobic residues. A long tail of tiny peaks here is common and usually accounts for a fraction of a percent in total.
The integration table
Below or beside the plot, the report lists every integrated peak with its retention time, area, and area percentage. The purity figure is the area percentage of the main peak. Two checks make the table useful:
- The area percentages should add up to 100%. If they do not, something was excluded and the report should say what.
- The main peak's retention time should match the reference standard's retention time stated in the method, or the laboratory should explain the difference. Retention time alone does not prove identity, but a mismatch is a reason to look at the mass-spectrometry result.
What to compare between lots
When two reports describe different lots of the same material under the same method, lay the chromatograms side by side. The main peak should sit at the same retention time within a few hundredths of a minute. The pattern of secondary peaks should look alike; a new peak that did not exist in the earlier lot is the most informative difference a chromatogram can show, more so than a small change in the headline percentage. See what a purity percentage means for how the figure itself is calculated.
| Feature | Looks like | Reading |
|---|---|---|
| Sharp, symmetric main peak | Narrow triangle, single apex | Method resolved the main component cleanly |
| Shoulder on the main peak | Bump on one side of the apex | Closely eluting impurity, incompletely separated |
| Flat baseline | Straight line between peaks | Integration of small peaks is reliable |
| Rising baseline | Line climbs across the run | Gradient or detector drift; small-peak areas are approximate |
| Unlabeled early spike | Narrow peak in the first minute | Solvent front; should be excluded from the calculation |