A certificate of analysis written in Spanish usually opens with a reassuring line: Pureza: 98%. Readers in Costa Rica see it on supplier pages, in PDF attachments and in WhatsApp screenshots, and most of them stop reading right there. That is a mistake. The pureza line is the easiest number on the page to produce and the hardest to interpret.
This guide reads a Spanish-language certificate the way an analytical chemist would, line by line, with the question each line should answer. It is about documents, and about the gap between what a document states and what a vial contains. It is not a recommendation to use any compound in people.
The percentage answers a narrower question than it appears to
Pureza, on the certificates you will meet, almost always means chromatographic purity. A sample is dissolved and pushed through a column by high-performance liquid chromatography (HPLC, sometimes written CLAR in Spanish). A detector records what comes off the column over time. The software then reports the area of the main peak as a share of the total area of all peaks, and that share becomes the percentage.
Three assumptions hide inside that sentence. The detector has to see every impurity, the method has to separate each impurity from the main peak, and peak area has to stand in for amount. Peptide chemists usually set the detector between about 214 and 220 nanometres, where the peptide bond absorbs light. An impurity that absorbs poorly there, or that leaves the column at the same moment as the main peak, never counts against the figure.
So 98% on one certificate and 98% on another are not the same measurement. They are the same number produced by two methods, and only the method line tells you how much weight it can carry.
Pureza: what a chromatogram shows and what it skips
A certificate that prints the chromatogram gives you more than one that prints only the result. Look for a tall main peak on a quiet baseline, and for the retention time, written tiempo de retención. Small peaks beside the main one are impurities, and their number matters as much as their combined size. A shoulder on the main peak can conceal a closely related sequence error that the integration folds into the target.
Then find the method line, método de análisis. A usable one names the column type, the solvent gradient and the detection wavelength. “HPLC” on its own is a technique, not a method. Two laboratories can run the same sample with different gradients and report purities several points apart, both honestly.
A chromatogram is also a snapshot of one sample on one day. Peptides can degrade in storage, so the purity printed on paper describes the past, and the date on the certificate decides how far into the present it can reach.
Identidad: the mass spectrum should come first, not last
Purity says nothing about whether the main peak is the compound named on the label. A perfectly clean peak can be the wrong peptide. Identity is established by mass spectrometry (espectrometría de masas, EM or MS), which measures the molecular mass of what is in the sample. The certificate should print the measured mass beside the theoretical mass for the stated sequence, and the two should agree within the tolerance of the instrument.
Two details separate a careful certificate from a decorative one. The first is that the measured value is an actual number rather than the word conforme or a tick in a box. The second is that the spectrum itself, or at least the charge states it was calculated from, is attached. Peptides usually show several charge states at once, and a laboratory that understands its own data will say which ones it used.
Mass confirms identity. It does not confirm purity, and HPLC does not confirm identity. Each test covers the blind spot of the other, which is why a certificate carrying only one of them is half a certificate.
Contenido, agua y contraión: where the rest of the powder goes
Here is the line most readers have never seen, because many certificates leave it out: contenido de péptido, or net peptide content. The powder in a vial is not all peptide. It also holds water, and it holds counterions, the charged partners left behind by synthesis and purification. Trifluoroacetate and acetate are the usual ones.
Chromatographic purity ignores all of that. It compares peptide-related peaks with one another and says nothing about the water and salt that never reach the detector as peaks. A powder can read 98% by HPLC and still be a good deal short of 100% peptide by weight. Net peptide content is normally determined by amino acid analysis or an equivalent method, and water by Karl Fischer titration (contenido de agua).
The reason this matters in a laboratory is plain arithmetic. If the label states one mass and the powder holds less peptide than that, every concentration calculated from the label is off by the same fraction. Two researchers comparing results from two suppliers can end up working with different effective amounts and never find out. Pharmacopoeial monographs for peptide drugs generally treat purity and assay as separate tests for exactly this reason.
When a certificate lists purity, water and peptide content together, the three figures can be checked against one another. When it lists purity alone, you are being asked to assume the rest.
Lote, fecha and laboratorio: the lines that tie paper to powder
Número de lote, the lot number, should match the one printed on the vial. A certificate with no lot number describes a product in general. A certificate with one describes a batch, and only that kind can be matched to what you hold. Compare the two strings character by character. One certificate reused across several batches is a risk that a lot match catches in seconds.
Where a certificate is not posted beside the product, ask for it by lot. On a catálogo de péptidos de investigación that lists concentrations and stock status for each product, the lot number has to come from the seller. That is a normal arrangement, and it makes the lot the first question to ask.
Fecha de análisis comes next. A certificate dated long before the batch was made, or after it was sold, deserves a second look. An old date does not prove the contents changed, but it does mean the paper cannot speak for the present.
Then the laboratory. The certificate should say who performed the testing: a name, a location and, ideally, an accreditation. ISO/IEC 17025 is the international standard for testing and calibration laboratories, and an accredited laboratory has a published scope listing the tests it is accredited to run. Accreditation for some other test does not cover the one on your certificate, so read the scope instead of the badge. Also ask whether the testing laboratory is independent of the seller. A certificate issued by the company selling the product is a declaration. One issued by a separate laboratory is evidence.
Certificates also cross languages. A report can arrive in English from a laboratory abroad while the seller’s listing is in Spanish. That is ordinary. A translation that changes a number is not, so compare the figures and not only the labels.
A worked example with a mitochondrial peptide
Take MOTS-c, a 16-amino-acid peptide encoded in mitochondrial DNA. Lee and colleagues described it in Cell Metabolism in 2015, and it has been studied in laboratory models of metabolism since. It is a research compound, and nothing here is a claim about what it does in people. It makes a good practice case because it is short, many suppliers list it, and its certificate can be tested against every line above.
A Costa Rican reader can practise on any catalogue organised by research category, and one that lists mitochondrial research compounds as its own section is a convenient place to start. Request the certificate for one product and run it through six questions, in the order a chemist would ask them:
- Does the pureza line come with a named method, a column, a gradient and a wavelength?
- Is a chromatogram attached, with a retention time and a quiet baseline?
- Does the measured mass appear as a number beside the theoretical mass for the stated sequence?
- Are water content and net peptide content reported, or is purity standing alone?
- Does the lot number match the vial, and does the analysis date fit the batch?
- Is the testing laboratory named, independent of the seller, and accredited for this test?
No certificate answers all six on the first try, and that is fine. The value is in the pattern of what a document includes and what it leaves out. Peptides CR says on its about page that documentation is available for the products it lists. The practical test of that sentence is the one this article has been building: ask for the lot, the method and the measured mass, in that order. A seller who can answer all three in one message has told you more than any percentage ever will.
