Analytical methods

RP-HPLC — how much of the vial is one substance

Reversed-phase high-performance liquid chromatography pushes the dissolved sample through a packed column under pressure: what sticks to the packing moves slowly, what prefers the solvent moves fast, so the mixture arrives separated in time. The detector draws a chromatogram — time along the bottom, signal up the side, humps rising from a baseline. Each hump is a peak, and a peak means one thing: molecules arrived together at that moment. The target peak’s area over the total is purity; PDA (photodiode array) detection adds a spectral fingerprint.

What it cannot answer: Which molecule. A peak has a retention time, not a name — a cleanly made wrong peptide gives one sharp peak and an excellent purity number. Area percent is a ratio, never a weight.

Mass spectrometry — which molecule is in the vial

A mass spectrometer weighs molecules. Electrospray ionization charges each molecule; the instrument sorts by mass-to-charge ratio. High resolution returns that mass to several decimal places, and a peptide’s exact mass follows from its formula. Observed and theoretical agree within a tight tolerance, or they do not. That is why mass, not purity, is the evidence for identity. LC-MS/MS goes further: it breaks the peptide apart and weighs the fragments, which fall along the backbone and spell out the sequence.

What it cannot answer: How much. Signal depends on how readily a molecule ionizes, not on weight. Mass also cannot separate species that weigh the same — a scrambled sequence, a D-amino acid substitution, some isomers — which is what fragmentation and chromatography are for.

qNMR and amino acid analysis — counting the milligrams

This pair answers the most commercially consequential question on a certificate, the one buyers assume the label answered: how many milligrams of peptide the vial holds. The label states intent; net peptide content states fact. Quantitative NMR (nuclear magnetic resonance) reads the radio signal atomic nuclei give back in a magnetic field. Signal area is proportional to the number of nuclei present, so comparing the sample with a weighed certified standard gives peptide mass by arithmetic. Amino acid analysis gets there another way: hydrolyze the peptide into its amino acids, quantify each against standards, and read mass back from the sequence.

What it cannot answer: Purity or identity. They count milligrams, and would not object if those milligrams were the wrong molecule.

Karl Fischer — how much of the weight is water

Freeze-dried peptide pulls water from the air, and water has weight. Karl Fischer titration measures it by chemistry, not by drying: a reagent consumes water in a reaction of known stoichiometry, and the amount consumed converts to water mass.

What it cannot answer: Anything about the dry remainder — though every percent of water is weight that is not peptide, which is how a high purity figure and a low content figure share a page.

Ion chromatography — what salt the mass includes

Peptides leave synthesis as salts, and which salt matters: trifluoroacetate (TFA) is purification residue, acetate is generally preferred. Ion chromatography separates charged species on an ion-exchange column, naming the counterion and its amount.

What it cannot answer: Identity or purity. Counterion is mass that is not peptide, and it never shows as an impurity peak.

GC-MS headspace — leftover synthesis chemicals

Residual solvents are volatile chemicals of synthesis that did not fully leave. Headspace sampling: seal the powder in a vial, warm it, and sample the air above, where volatiles concentrate. Gas chromatography separates them; a mass spectrometer names each.

What it cannot answer: Anything that does not evaporate — nonvolatile impurities, salts and metals never reach the detector.

ICP-MS — metals, including palladium

Inductively coupled plasma mass spectrometry injects the sample into an argon plasma hot enough to strip molecules into charged atoms, then counts them element by element. It is the reference method for elemental impurities: heavy metals, and catalyst residues such as palladium.

What it cannot answer: Chemistry. It reports how much of an element is present, not what it was bound to.

LAL endotoxin and bioburden — bacterial residue

Endotoxins are fragments of the outer membrane of gram-negative bacteria, which outlast the bacteria. The LAL (Limulus amebocyte lysate) test uses a reagent whose clotting cascade responds to endotoxin, read out as a concentration by USP <85>. Bioburden testing, USP <61> and <62>, counts viable microorganisms.

What it cannot answer: Sterility, viruses, other pyrogens — or anything about identity, purity or content.

SEC-MALS — has the peptide clumped

Size-exclusion chromatography runs the sample through a porous packing that small molecules wander into and large ones cannot, so large arrive first. MALS (multi-angle light scattering) measures scattered light in several directions, giving absolute molar mass. Together they show whether the peptide sits as single molecules or has clumped into dimers and aggregates.

What it cannot answer: Sequence, impurities, or why clumping happened.

Peptide mapping and Ellman’s reagent

Mapping cuts the peptide and weighs the pieces, showing which cysteines are joined; Ellman’s develops color in proportion to unpaired thiols.

Answers: Is the molecule wired correctly.

Not: How much is there.

SPR and functional assays

Surface plasmon resonance watches binding at a sensor surface.

Answers: How this lot compares with a reference standard.

Not: What a compound does in a living system — that is claims territory.

CD and FTIR

Circular dichroism reads the difference in absorption of left- and right-circularly polarized light, which tracks helix and sheet content; FTIR reads bond vibrations.

Answers: Does a molecule that should have a fold have one.

Not: Identity or content.

USP <905> content uniformity and vacuum-decay closure integrity

Uniformity compares vials across the lot, so a good average cannot hide a bad spread; vacuum decay changes the pressure around a sealed package and watches for drift.

Answers: Is every vial alike, and does it stay sealed.

Not: What is inside.