MolDraw › Chemistry tools › Mass Spectrum Viewer
Free Mass Spectrum Viewer
Open a mass spectrum, label the peaks, and compare it with the isotope pattern of your formula, SMILES or peptide. Check adducts, ppm error and neutral losses. Your files stay on your device.
or drop it here. JCAMP-DX (.jdx, .dx), CSV/TXT (m/z, intensity), mzML or mzXML (first spectrum).
Examples are simulated and free to reuse. Download the sample .jdx.
Main peaks
Tap a row to use it as the measured m/z.
Neutral losses
ppm error
Common adducts
Formula, SMILES or peptide
How to use the mass spectrum viewer
- Open your file, or press an example button.
- Pick Stick or Profile, zoom to the area you want and tick Base peak = 100% to normalise.
- Type a formula, SMILES or peptide and pick the ion, such as [M+H]+. The red sticks show the theoretical isotope pattern.
- Tap the measured peak to get the ppm error. The adduct table shows which common ions are present.
- Read the neutral loss labels between the main peaks.
- Export a PNG or CSV.
Reading a mass spectrum
Each peak is an ion at its m/z (mass divided by charge). The tallest peak is the base peak and is set to 100%. In EI spectra the molecular ion M+• is often small and fragments carry the story. In ESI you mostly see [M+H]+, [M+Na]+ or [M-H]-, and big molecules such as peptides also show 2+ and 3+ ions, with isotope peaks 0.5 or 0.33 apart.
Isotopes: carbon-13 adds a peak 1 above M, about 1.1% per carbon. Chlorine and bromine add strong peaks 2 above M. Neutral losses such as −18 (H2O), −28 (CO), −44 (CO2) or −35 (Cl) between peaks show which groups break off.
Frequently asked questions
Is my data uploaded?
No. Files are read and drawn in your browser only.
Which files can I open?
JCAMP-DX mass spectra (peak tables and continuous data), CSV or TXT with m/z and intensity columns, and mzML or mzXML. For mzML and mzXML the first spectrum is shown. Compressed (zlib) arrays work in current browsers.
What is the difference between monoisotopic, average and exact mass?
Monoisotopic mass uses the most common isotope of each element (12C, 1H, 35Cl). Average mass uses natural abundances and is what you weigh on a balance. Exact m/z is the monoisotopic mass of the ion you see, with the added or removed atoms and the electron mass.
How do I check for chlorine or bromine?
Enter the formula and look at the pattern. One Cl gives an M+2 peak about one third of M (3:1). One Br gives M and M+2 about equal (1:1). Two Br give 1:2:1. The red sticks show the theory over your data.
How is the ppm error worked out?
ppm = (measured − theoretical) / theoretical × 1 000 000. Tap a peak to use it as the measured value. For high-resolution data, within about 5 ppm is a good match.
What are the neutral loss labels?
The tool looks at the gaps between the main peaks and names common small neutral losses such as H2O, NH3, CO, CO2, CH3, HCl, Cl and Br. Isotope peaks are skipped. At unit resolution CO and C2H4 look the same, so both may be listed.
Where do the isotope numbers come from?
Isotope masses and natural abundances are public reference values from IUPAC (CIAAW) and NIST. The pattern is worked out by combining the isotopes of every atom.