What do AU, mAU and full-scale annotations mean on a peptide HPLC chromatogram?
Reversed-phase HPLC detectors used for synthetic peptides most often report ultraviolet absorbance as a function of time. The vertical axis of the chromatogram is therefore an absorbance axis, not a purity axis. Two unit systems appear on Australian research certificates of analysis: absorbance units (AU) and milli-absorbance units (mAU). By definition, 1 AU equals 1000 mAU. A main-component apex of 0.42 AU is the same signal as 420 mAU; the two traces are identical if the scale is converted correctly. What changes is the printed tick labels and, too often, the researcher’s impression of peak size.
Full-scale annotations compound the unit question. A footer or axis caption may state full scale = 500 mAU, 1.000 AUFS, or simply show a numeric maximum without a unit. AUFS (absorbance units full scale) is a legacy integrator phrase meaning the absorbance value that fills the plotted pane. If AUFS is 1.000 and the main peak reaches about 40 per cent of the pane, the apex is approximately 0.40 AU. If the same file is reprinted at 100 mAU full scale, that peak is clipped and the pane is dominated by baseline. Neither reprint changes the underlying digital chromatogram; both change what a casual reader sees.
Offset and zero are part of the same reading task. Some data systems apply a baseline offset so that the solvent front does not sit on the x-axis. Others plot a slightly negative baseline after blank subtraction. A reader who treats the x-axis as absorbance zero will mis-estimate small peaks. The correct zero is the annotated baseline, not the bottom edge of the graphic. Wavelength and bandwidth (for example 214 nm, 4 nm) belong beside the axis because absorbance is wavelength-specific. Suggestions on how to read experimental material emphasise that axes, units and conditions are part of the result, not decoration (DOI:10.1002/asi.5090180408 — Suggestions on how to read experimental material in information science — American Documentation — 1967).
How do autoscale and attenuation hide or exaggerate related-substance peaks?
Autoscale is the display choice that most often produces a false visual impression of chromatographic purity. When a data system sets the y-maximum to the apex of the main peak, related-substance peaks at 0.1 to 0.5 area per cent occupy only a few pixels. They remain in the integration table, but they disappear from a glance-reading of the PDF. Conversely, if a laboratory prints an expanded impurity pane (for example 0–10 mAU) beside the main-component pane, those same peaks look large. Neither pane is more true than the other; they are two windows onto one dataset. Never infer chromatographic purity from peak height on an autoscaled plot.
Attenuation, still printed on some older integrator reports, is a related control. Historical integrator workflows used attenuation factors so that a large peak remained on scale. A change of attenuation between the sample run and the system-suitability run can make two chromatograms look dissimilar when the chemistry is unchanged. If a COA pack mixes an autoscaled sample plot with a fixed-scale system-suitability plot, the reader must not compare them as if they shared an axis. The correct comparison is the peak table, the stated units, and the method identifier.
Truncation is the opposite artefact. A fixed y-maximum below the main-peak apex clips the peak and can hide a shoulder on the front or tail. Shoulders are integration events, not aesthetic flaws. If the main peak is flat-topped on the PDF, the reader should look for an off-scale note, a second expanded pane, or a peak table that still reports a finite area. A clipped picture with a complete peak table is a documentation inconsistency to query with the supplier. How (not) to read the graphic is how not to treat it as a cleanliness score (DOI:10.5840/acpq199569241 — How (Not) To Read Heidegger — American Catholic Philosophical Quarterly — 1995). The plot is a scaled instrument record; misreading scale is a documentary error before it is an analytical one.
How should a peak-table area-percent column be reconciled with the plotted trace?
The area-percent column in the peak table is calculated from integrated detector response, not from the height of the printed peak. Display scale does not enter the formula. A peak that is almost invisible on an autoscaled 0–500 mAU pane can still be 0.35 per cent of total integrated area. A peak that dominates an expanded 0–5 mAU pane can still be 0.08 per cent. Reconciliation therefore runs from the table to the trace, not from the trace to a guessed percentage.
Three table conventions must be read explicitly. First, some reports normalise all integrated peaks to 100 per cent (area-percent purity). Second, some exclude solvent, void and designated blank peaks before normalisation; the caption should say so. Third, some reports list relative area against the main peak rather than against the total; 2.0 per cent relative to main is not 2.0 per cent of total if other peaks are present. These conventions are independent of y-axis units, which is why a reader who only studies the picture cannot recover them.
Discordance between picture and table is informative. If the expanded trace shows a peak that the table omits, the peak may sit below the reporting threshold or ignore limit, be marked as solvent or spike, or fall outside integration start and stop times. If the table lists a peak that cannot be found even on an expanded pane, the PDF graphic may be a different run, the time axis may be cropped, or a blank feature may still have received an identifier. Either discordance is a reason to request the native data file or a reprint at a stated scale, not a reason to invent a purity number from the picture. Tabulated results require structured reading of headings, coding rules and footnotes before the headline percentage (DOI:10.7816/ulakbilge-05-15-06 — HOW TO BE READ PIAAC RESULTS FOR ADAPTATION TO NEW AGE? — Ulakbilge Dergisi — 2017). The chromatogram is a concordance check, not a substitute for the table.
How are dual-wavelength HPLC panes at 214 nm and 280 nm read on one COA page?
Many peptide COAs print two chromatograms, or two panes, acquired at different ultraviolet wavelengths—commonly a peptide-bond region near 214 nm or 220 nm and an aromatic region near 280 nm. Each pane has its own y-axis. Absorbance at 214 nm is not interchangeable with absorbance at 280 nm; a large 214 nm peak with no 280 nm counterpart is expected for sequences that lack tryptophan and tyrosine. Reading the two panes as if they shared a scale is a unit error of a different kind.
Bandwidth and reference wavelength, where a diode-array method uses them, should be transcribed beside each pane. A 4 nm bandwidth centred at 214 nm is not the same photometric window as 8 nm at 220 nm. If the COA states dual-wavelength confirmation, the reader should confirm that both panes carry the same sample identifier, the same run time axis and the same method version. A 280 nm pane plotted at 0–20 mAU next to a 214 nm pane at 0–800 mAU will make aromatic impurities look dramatic; that is scale, not composition. The correct comparison is presence or absence of a peak at a stated relative retention time, not the relative height of the two traces.
Spectral match or peak-purity index values, when printed, are not chromatographic area-percent purity. They are diode-array comparisons along the peak apex. They belong in a different column of the analytical pack and must not be copied into a purity specification. The y-axis of the chromatogram does not display those indices, and autoscale does not change them. If only one wavelength is reported, the reader should record that fact. Absence of a 280 nm pane is not a defect for a non-aromatic peptide; it is a method choice. What is a defect is a y-axis without a wavelength, because then the absorbance numbers cannot be interpreted at all. A cover-sheet purity value that omits wavelength cannot be reconciled with either pane.
Which laboratory peptide documentation fields must match the HPLC chromatogram scale?
A chromatogram that cannot be tied to a lot is not a result; it is an unlabelled figure. Before any discussion of mAU or autoscale, the reader should verify that the PDF header or footer repeats the lot number, the internal sample or LIMS identifier, the method identifier and revision, the detection wavelength, the column identity, the analysis timestamp and the instrument identifier that appear on the COA cover sheet. Australian research groups assembling batch files for audit should treat mismatch among those fields as a documentation failure even when the peak table looks complete.
Scale-specific fields are easy to skip. They include y-axis units, full-scale or autoscale statements, offset, and—where two panes are printed—the scale of each pane. If the cover sheet states purity 98.7 per cent (HPLC, 214 nm) and the only chromatogram is an unlabelled trace, the file is incomplete. If the chromatogram is labelled 220 nm and the cover sheet says 214 nm, the file is inconsistent. Finding those fragments in a multi-page pack is a documentation-navigation task: page numbers, bookmarks and run identifiers are part of the record. Australian scientific documentation guidance likewise treats how to find information as a first-order problem, not an afterthought (DOI:10.3897/tdwgproceedings.1.19941 — Documentation about Atlas of Living Australia tools: how to find information — Proceedings of TDWG — 2017).
Signatory fields (analyst, reviewer, authorised-by, dates) do not change absorbance units, but they establish that the scaled plot and the table were issued together. A chromatogram dated after the COA approval date, or a method revision that does not match the listed procedure, is a filing problem. Structured reading of scientific documents starts with methods and identifiers, not with the headline number (DOI:10.5694/j.1326-5377.1992.tb137249.x — 13. How to read a journal article — Medical Journal of Australia — 1992). Local Australian stock and tracked dispatch establish which physical vials should match which lot file; they do not replace this concordance check. Researchers should also confirm that vial labels, packing lists and the COA lot string use the same character sequence, including prefixes and suffixes, before the chromatogram is interpreted.
What checklist should an Australian laboratory use before filing a peptide COA?
A practical filing checklist keeps the scale-reading rules operational. First, confirm that the vial-label lot, the COA lot and the chromatogram lot are identical character-for-character. Second, confirm that the method identifier, wavelength and analysis date on the cover sheet match the chromatogram header. Third, record y-axis units (AU or mAU) and whether autoscale or a stated full scale was used. Fourth, if dual panes are present, record both wavelengths and both scales; do not compare heights across panes. Fifth, read the peak-table normalisation rule (total area, solvent-excluded total, or relative to main). Sixth, look for peaks that appear on an expanded trace but not in the table, and for table rows that have no corresponding feature; query reporting threshold and integration events. Seventh, confirm analyst and authorised sign-off. Eighth, file the PDF with page completeness checked: no missing even pages, no cropped time axis, and no header that belongs to another sample.
The checklist is laboratory practice, not a specification. Acceptance criteria live in the purchaser’s research quality system and in the supplier’s stated limits; this article does not set those limits. It only requires that the chromatogram be read as a scaled record and that laboratory peptide documentation remain internally consistent.
Research-use framing should be explicit in the filed note: the material is a research reagent, the COA is an analytical identity and purity document, and no biological claim is attached to area-percent values. Australian groups that keep lot-traceability logs should store the scale annotations alongside the purity number so that a later reader does not reconstruct a false visual memory of the trace. A COA PDF without its peak table, without units, or without a lot string is not a complete analytical record, regardless of how clean the plot appears on an autoscaled pane.
Apply this checklist to documented stock
You now have a practical way to read purity figures, method notes, and lot traceability. When you source materials, hold suppliers to that same checklist — ClaraScience issues batch documentation with every order and dispatches from Australian warehouses with Express tracked shipping.
Start with a retail order to review documentation end-to-end, or register for wholesale if you restock multiple compounds.
Frequently asked questions
Does a taller peak on the HPLC PDF mean the peptide lot is chromatographically purer?
No. Peak height on a printed chromatogram is controlled by y-axis scale, attenuation and autoscale. Chromatographic purity is taken from the integrated area-percent table after the stated normalisation rule, not from how large the main peak looks on the page. Always read units, full scale and the peak table together.
What is the difference between AU and mAU on laboratory peptide documentation?
They are the same absorbance quantity on different scales: 1 AU equals 1000 mAU. A COA may label the axis in either unit. Convert before comparing two chromatograms, and do not treat a 400 mAU apex as larger than a 0.40 AU apex. Record the unit in the lot file beside the purity number.
Why does a related-substance peak appear in the table but not on the trace?
Autoscale to the main peak can shrink small peaks to a few pixels while integration still records them. Expand the y-axis or read the table. If a peak is absent from both an expanded pane and the table, check reporting threshold, solvent exclusion and whether the graphic is the same run as the table.
Should 214 nm and 280 nm panes use the same y-axis scale?
Not necessarily. Each wavelength has its own absorbance range, so comparing heights across panes is not valid. Confirm both panes share lot, method and time axis, and read each scale independently. Sequences without aromatic residues may show little 280 nm response; that is expected photometry.
Which fields in laboratory peptide documentation must match the chromatogram?
Lot number, sample identifier, method revision, detection wavelength, analysis timestamp, instrument identifier, y-axis units and sign-off should agree with the COA cover sheet. Mismatch is a documentation failure even if the peak table is complete. Local Australian stock and tracked dispatch identify which vials must match which file.
Is visual inspection of an autoscaled chromatogram enough to file a COA?
No. Visual inspection without units, wavelength and the peak-table normalisation rule is not a complete reading. File the graphic, the table, the scale annotations and the lot concordance note together. Area-percent values remain analytical identity and purity data for research use only.
References
- DOI:10.1002/asi.5090180408 — Suggestions on how to read experimental material in information science — American Documentation — 1967
- DOI:10.5840/acpq199569241 — How (Not) To Read Heidegger — American Catholic Philosophical Quarterly — 1995
- DOI:10.7816/ulakbilge-05-15-06 — HOW TO BE READ PIAAC RESULTS FOR ADAPTATION TO NEW AGE? — Ulakbilge Dergisi — 2017
- DOI:10.3897/tdwgproceedings.1.19941 — Documentation about Atlas of Living Australia tools: how to find information — Proceedings of TDWG — 2017
- DOI:10.5694/j.1326-5377.1992.tb137249.x — 13. How to read a journal article — Medical Journal of Australia — 1992
Research use only
This article is provided for laboratory research and educational purposes only. Products referenced are not for human or veterinary use. ClaraScience makes no therapeutic, medical, or efficacy claims, and nothing here constitutes medical advice.
