When a Certificate of Analysis lists a purity percentage and a confirmed molecular weight, those two figures come from two different instruments answering two different questions. Understanding what each one measures makes a COA far easier to trust.
HPLC — high-performance liquid chromatography — answers "how pure is it?" The sample is dissolved and pushed through a column packed with material that interacts with different molecules at different rates. Compounds separate as they travel through, emerging at distinct times. A detector records each one as a peak, and the relative size of the main peak versus everything else gives the purity percentage. A tall, dominant peak with little else around it indicates a clean sample; a cluster of smaller peaks signals by-products or impurities.
Mass spectrometry answers a different question: "is this actually the right compound?" The instrument ionizes the molecule and measures its mass-to-charge ratio with high precision. Because every compound has a known theoretical molecular weight, comparing the measured mass to the expected value confirms identity. HPLC can tell you a sample is 99% one thing — but mass spectrometry tells you that one thing is the molecule you intended.
This is why reputable documentation reports both. Purity without identity could describe a very pure version of the wrong compound. Identity without purity could confirm the right molecule buried among contaminants. Used together, they cover both failure modes.
When you read a COA, look for the two in tandem: an HPLC purity figure and a mass spectrometry confirmation that the measured weight matches the theoretical weight. That pairing is the analytical backbone of research-grade material.