What HPLC does not see: the rest of the vial at 214–220 nm

A certificate of analysis says a lot is 99.4% pure by HPLC, and a reader takes that as a description of the vial. It is a description of a chromatogram. Reversed-phase HPLC with ultraviolet detection at 214–220 nm reports the main peak as a share of everything that absorbed light at that wavelength and came off the column inside the integration window. A great deal of what can be in a vial does neither. Water, salts, counter-ions, residual solvents, scavengers, metals, endotoxin, polymer fragments and plasticizers are either transparent at 214 nm, swept out in the solvent front before integration starts, or never leave the column at all. This article lists them, explains why each is invisible to the purity method, and shows what each looks like on a mass spectrum, where many of them do appear, scattered across the m/z range as adduct ladders, repeating series and background rather than as a peak anyone labels.

What a UV detector at 214–220 nm sees

The peptide bond absorbs around 190–220 nm; 214 nm is the usual compromise between sensitivity and the opacity of the mobile phase. A molecule registers in proportion to how many absorbing groups it carries, so a 30-residue peptide gives a strong signal, a dipeptide a faint one, and a molecule with no amide bond, no aromatic ring and no conjugated double bond gives nothing. The purity figure is a ratio of peak areas in that signal: it cannot include anything that does not absorb, anything that elutes outside the integrated range, or anything that stays on the column.

Three ways to be absent from a purity chromatogram

Reason	What it means	Typical of
No chromophore at 214 nm	The molecule does not absorb; whatever its amount, it adds nothing to any peak	Water, inorganic salts, sugars, polyethylene glycol, many solvents, silanes, metals
Elutes in the void or solvent front	Small polar molecules pass through unretained and come off with the sample solvent, in the first minutes most methods exclude from integration	Counter-ions, residual solvents, free amino acids, buffer components
Does not elute	Strongly retained or insoluble material stays on the column or the frit, or was filtered out before the sample was loaded	Aggregates, lipids and endotoxin, resin fragments, particulates

What is commonly in the vial, and why HPLC misses it

The usual non-peptide contents of a lyophilized research peptide

Component	Where it comes from	Typical amount	Why HPLC at 214 nm misses it
Water	Incomplete freeze-drying; uptake from air after opening (the powder is hygroscopic)	3–8% by weight; more once opened	No absorbance; elutes with the solvent
Trifluoroacetate	Cleavage from the resin and HPLC purification, both in trifluoroacetic acid; one anion per protonated basic group	5–20% by weight, more for short basic peptides	Weak absorbance only below 210 nm; elutes in the void. Present in the mobile phase itself, so it is the baseline
Acetate, chloride	Salt exchange after purification	2–10% by weight	Transparent at 214 nm; void volume
Inorganic salts	Buffers used in purification or exchange; sodium and potassium from glassware and reagents	Traces to a few percent	No chromophore; not retained
Residual solvents	Acetonitrile and trifluoroacetic acid from purification; dimethylformamide, dichloromethane, diethyl ether, piperidine from synthesis	Tens to thousands of ppm	Acetonitrile and ether do not absorb; others elute in the front
Scavengers and their adducts	Triisopropylsilane, water, ethanedithiol and thioanisole added at cleavage to trap reactive cations	Traces	Silanes have no chromophore; the aromatic ones do and can appear as small early peaks
Free amino acids and very short fragments	Hydrolysis during cleavage or storage; incomplete coupling	Traces	One or no peptide bond, so almost no signal per molecule; elute early
Polyethylene glycol and detergents	Tubing, vial closures, laboratory plastics, some resins	Traces	No chromophore; PEG elutes as a broad smear
Phthalates and other plasticizers	Plastic tubing, pipette tips, storage vials	Traces	Absorb at 230 and 275 nm rather than 214; elute late or not at all
Siloxanes	Silicone tubing, septa, vial caps, antifoam	Traces	No chromophore; strongly retained
Metals	Copper, iron, nickel and zinc from reagents, water and equipment; copper in GHK-Cu is intended	ppm	No chromophore; not separated by reversed-phase chromatography
Endotoxin	Gram-negative bacteria in water, glassware and the lyophilizer; survives filtration and autoclaving	Picograms to nanograms per milligram	Lipopolysaccharide has no useful chromophore and is present far below the detector's range; it also adsorbs to the column
Excipients	Mannitol, sucrose or trehalose added before freeze-drying by some suppliers to bulk the cake	Can exceed the peptide by weight	Sugars do not absorb; they elute in the void
Aggregates and particulates	Self-association during drying or storage; insoluble material	Variable	Removed by the 0.22 µm filter before the sample is loaded, or retained on the column

Two consequences follow. First, the purity figure and the weight of peptide in the vial are different quantities, which is the whole subject of net peptide content and purity: 5 mg of powder at 99% purity is commonly 3.5–4.5 mg of peptide, with the rest being water and counter-ions that the chromatogram never saw. Second, a vial can be 99% pure by HPLC and still carry something that matters to an experiment, such as endotoxin in a cell-based assay or a metal that catalyzes oxidation, because those are not what the purity method measures.

What the mass spectrometer makes of them

Electrospray mass spectrometry does not need a chromophore; it needs something that will ionize. Many of the invisible components ionize readily, and what they produce is not a tidy peak at a diagnostic mass but a pattern spread across the spectrum. Reading these patterns is most of what separates a clean interpretation from a confused one.

How the non-peptide contents appear in electrospray mass spectrometry

Component	Appearance in the spectrum	Effect on the peptide's signal
Sodium and potassium salts	Adduct ladders: [M+Na]+ 22 u above [M+H]+, [M+K]+ 38 u above, and mixed adducts on every charge state; salt cluster ions at low m/z	Signal divided among adducts; ion suppression at high salt
Trifluoroacetate	[M+TFA+H]+ 114 u above the molecular ion; in negative mode a strong ion at m/z 113 and clusters at 227	Strong suppression in positive mode; TFA pairs with the peptide and lowers its ionization
Acetate, formate	Adducts 60 or 46 u above the molecular ion; mild compared with trifluoroacetate	Little suppression; the reason analytical methods prefer formic acid
Polyethylene glycol	A repeating series of singly and doubly charged ions spaced 44 u apart (22 for doubly charged), across hundreds of m/z	Crowds the spectrum; easily mistaken for a complex sample
Phthalates	A sharp ion at m/z 149, with 279 (dibutyl) and 391 (bis(2-ethylhexyl)) as the parent species	Background; persistent in a contaminated instrument
Siloxanes	A series spaced 74 u apart, prominent at m/z 371, 445 and 519	Background; from caps and tubing rather than the sample
Residual solvents	Mostly below the scanned range or lost in the source; dimethylformamide and dimethyl sulfoxide give protonated and sodiated cluster ions at low m/z	Negligible, unless they change ionization efficiency
Metals	Copper and iron give adducts offset by the metal's mass less the displaced protons, and a characteristic isotope pattern (copper: 63 and 65 u, 69:31)	Split signal; the copper adduct of a histidine-containing peptide can rival the protonated ion
Free amino acids, short fragments	Small ions at low m/z, below the range most peptide methods scan	None
Endotoxin	Not seen: too large, too heterogeneous and far too dilute for a routine spectrum	None
Water, inorganic buffers, sugars	Water adducts (+18) on adjacent ions; sugars as sodium adducts at low m/z; phosphate as clusters in negative mode	Suppression at high buffer concentrations

Two things are worth holding on to. A mass spectrum full of scattered small ions is not evidence of an impure peptide; it is usually evidence of salts, plastics and the instrument's history, and the test is whether the ions fit one of the series above. And ion suppression means the mass spectrometer's intensities are not amounts: a strong trifluoroacetate background can make a clean peptide look weak, and a sodium-laden sample spreads one molecule across five adducts. Mass spectrometry for peptide identity covers the adducts and charge states in detail; the mass match tool predicts the protonated, sodiated, potassiated and trifluoroacetate adducts for any sequence so the ladder can be assigned rather than guessed at.

The methods that do see them

Each class of invisible material has a method built for it, and a complete characterization of a lot uses several. None of them is HPLC at 214 nm.

Methods for what the purity chromatogram leaves out

What	Method	Reported as
Water	Karl Fischer titration, coulometric, often with an oven	Percent by weight
Trifluoroacetate, acetate, chloride	Ion chromatography; 19F NMR for trifluoroacetate	Percent by weight; moles per mole of peptide
Residual solvents	Headspace gas chromatography, by ICH Q3C	ppm against the class limits
Metals	Inductively coupled plasma mass spectrometry, by ICH Q3D and USP <232>/<233>	ppm per element
Endotoxin	Limulus amebocyte lysate or recombinant Factor C assay	Endotoxin units per milligram
Peptide content	Amino acid analysis, nitrogen determination, quantitative NMR or mass balance	Percent peptide by weight
Non-absorbing impurities in general	HPLC with a universal detector: evaporative light scattering or charged aerosol	Area percent, by mass rather than by absorbance
Aggregates	Size-exclusion chromatography; light scattering at 320–350 nm	Percent high-molecular-weight species

A certificate that reports only HPLC purity and a mass is reporting on the peptide and nothing else in the vial. That is normal for research material, and it is adequate for many purposes, but the reader should know which questions it has not answered. How to read a certificate of analysis sets out what a fuller certificate contains; water content, salt forms, endotoxin and related substances each take one of these components in depth.

Frequently asked questions

If HPLC says 99% pure, what is the other 1%?

Other peptide-like material that absorbed at 214 nm: deletion sequences, oxidized or deamidated forms, diastereomers. It says nothing about water, counter-ions, salts or solvents, which are outside the measurement entirely and are usually 10–30% of the powder's weight.

Why does my mass spectrum show a comb of peaks 44 units apart?

Polyethylene glycol, from tubing, vial closures or a detergent. It is a contaminant of the sample's handling, not a property of the peptide, and it ionizes well enough to dominate a spectrum at trace levels.

Can trifluoroacetate be seen on a certificate?

Not by HPLC purity, where it is the mobile phase. It is measured by ion chromatography or fluorine NMR, and a certificate that states a counter-ion content has done one of those. Net peptide content accounts for it by difference.

Would endotoxin show up anywhere on a standard certificate?

Only if an endotoxin test was run and reported, in endotoxin units per milligram. Neither HPLC nor mass spectrometry detects it at the levels that matter in cell-based work.

Is a spectrum with many small unexplained ions a sign of a bad lot?

Usually not. Assign the ions to the salt, trifluoroacetate, PEG, phthalate and siloxane series first. What is left after that, if anything, is the question to ask the supplier.

Sources & Further Reading

1. Kuipers BJH, Gruppen H. Prediction of molar extinction coefficients of proteins and peptides using UV absorption of the constituent amino acids at 214 nm to enable quantitative reverse phase high-performance liquid chromatography–mass spectrometry analysis. Journal of Agricultural and Food Chemistry 2007;55(14):5445–5451. doi:10.1021/jf070337l
2. Keller BO, Sui J, Young AB, Whittal RM. Interferences and contaminants encountered in modern mass spectrometry. Analytica Chimica Acta 2008;627(1):71–81. doi:10.1016/j.aca.2008.04.043
3. Annesley TM. Ion suppression in mass spectrometry. Clinical Chemistry 2003;49(7):1041–1044. doi:10.1373/49.7.1041
4. Roux S, Zékri E, Rousseau B, Paternostre M, Cintrat JC, Fay N. Elimination and exchange of trifluoroacetate counter-ion from cationic peptides: a critical evaluation of different approaches. Journal of Peptide Science 2008;14(3):354–359. doi:10.1002/psc.951
5. Vehovec T, Obreza A. Review of operating principle and applications of the charged aerosol detector. Journal of Chromatography A 2010;1217(10):1549–1556. doi:10.1016/j.chroma.2010.01.007
6. International Council for Harmonisation. ICH Q3C (R8): Impurities: Guideline for Residual Solvents, 2021; ICH Q3D (R2): Guideline for Elemental Impurities, 2022.
7. United States Pharmacopeia. General Chapters <85> Bacterial Endotoxins Test, <232> Elemental Impurities: Limits, <233> Elemental Impurities: Procedures, <921> Water Determination. USP–NF, current edition.
8. Westwood S, Choteau T, Daireaux A, Josephs RD, Wielgosz RI. Mass balance method for the SI value assignment of the purity of organic compounds. Analytical Chemistry 2013;85(6):3118–3126. doi:10.1021/ac303329k

Cosmic Peptides. (2026, November 1). What HPLC does not see: the rest of the vial at 214–220 nm. Cosmic Peptides Research Library. Retrieved November 1, 2026, from https://cosmicpeptides.com/research/methods/what-hplc-does-not-see

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  author       = {{Cosmic Peptides}},
  title        = {What HPLC does not see: the rest of the vial at 214–220 nm},
  howpublished = {Cosmic Peptides Research Library},
  year         = {2026},
  month        = nov,
  url          = {https://cosmicpeptides.com/research/methods/what-hplc-does-not-see},
  urldate      = {2026-11-01},
  note         = {Last reviewed 2026-11-01}
}

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