What is a chromatogram?
For research use only. Not for human or animal consumption.
A chromatogram is the record a chromatography run leaves behind. In most modern labs it is a line chart: time along the bottom, detector signal up the side, and a peak each time something passes the detector.
It looks that way because of what chromatography does.
Chromatography in one paragraph
Every chromatography method has two parts: a stationary phase that stays put, and a mobile phase that moves past it carrying the sample. Different molecules spend different amounts of time stuck to the stationary phase, so they travel at different speeds and come out separated. The first version, in the early 1900s, was a column of chalk that pulled plant pigments apart into colored bands. That is where the name comes from: "color writing."
Today the same idea runs in several forms:
| Technique | Mobile phase | Typical use |
|---|---|---|
| Thin-layer chromatography (TLC) | Solvent creeping up a coated plate | Quick checks; the "chromatogram" is the plate itself |
| Gas chromatography (GC) | Inert gas | Volatile compounds, residual solvents |
| High-performance liquid chromatography (HPLC) | Liquid under pressure | Peptides, most non-volatile organics |
| Ion chromatography | Aqueous buffer | Counterions such as TFA, acetate, chloride |
| Size-exclusion chromatography (SEC) | Liquid | Separating by molecular size, such as aggregates |
What the chart shows
The x-axis is time. Usually minutes since the run started. Where a peak sits along it is called its retention time.
The y-axis is detector response. For HPLC with a UV detector, that is absorbance, often in mAU. For mass spec detection it is ion counts.
The baseline is the line drawn when nothing is coming off.
Each peak is a component leaving the column. Its position says when, its height and area say how much signal it gave.
What "interpreting" a chromatogram means
Interpreting a chromatogram usually comes down to four questions:
- How many peaks are there? One large peak with small neighbors is the typical picture for a purified peptide.
- Where are they? Retention time, compared within the same method.
- How big are they? Peak area, which a peak table converts into area percent.
- Are they well separated? Peaks that run into each other are harder to measure accurately.
The answers depend on the detector. A UV detector sees only what absorbs at its wavelength. Water and most salts are invisible to it, which is why a chromatogram alone cannot tell how much of a powder is peptide.
The chromatogram report
When a lab prints a chromatogram on a COA, it usually adds:
- a header with the method: column, mobile phases, gradient, flow rate, wavelength
- the chart itself
- a peak table with retention time, area and area percent for each integrated peak
That bundle is what people mean by a "chromatogram report." The purity figure on the COA is read straight off the peak table.
FAQ
Is a chromatogram the same as a spectrum?
No. A chromatogram plots signal against time. A spectrum plots signal against wavelength or m/z. LC-MS produces both: a chromatogram over time, and a mass spectrum at each moment.
Can two chromatograms of the same material look different?
Yes, if the method differs. Column, gradient and wavelength all change where and how peaks appear.
Research use only
Peptira supplies materials for laboratory research. For research use only. Not for human or animal consumption. Not for diagnostic or therapeutic use. This page explains an analytical chart. It is not medical advice.