What a chromatogram shows
An HPLC chromatogram is a graph plotting detector response (y-axis) against time (x-axis). Each peak represents a distinct component in the sample. The position of a peak along the x-axis is its retention time — a characteristic property used for identification. The area under the peak is proportional to the quantity of that component.
The main peak
The main peak represents the target peptide. In a high-purity peptide sample, this peak will dominate the chromatogram, typically representing 99% or more of the total peak area. The retention time of this peak should match the known retention time of the reference standard.
Impurity peaks
Smaller peaks before or after the main peak represent impurities — synthesis byproducts, degradation products, or residual reagents. The percentage purity is calculated as: (main peak area ÷ total peak area) × 100. A purity of 99.3% means the main peak accounts for 99.3% of all detected signal.
What to look for on a COA
A thorough COA will reference the chromatographic method (column type, mobile phase, flow rate), report the retention time of the main peak, and list any significant impurity peaks with their relative percentages. This level of detail allows independent verification and comparison across lots.