Calculator Inputs
Formula Used
Charge from isotope spacing: n = isotope reference mass / isotope peak spacing.
Corrected m/z: corrected m/z = observed m/z - calibration offset.
Neutral molecular weight: MW = n × corrected m/z - n × adduct mass + neutral loss - neutral gain.
Error: Dalton error = calculated MW - expected MW. PPM error = Dalton error / expected MW × 1,000,000.
How To Use This Calculator
Enter the observed m/z value from your spectrum. Add a known charge n if it is already assigned. Leave charge n as zero when you want the tool to estimate n from isotope spacing. Enter the isotope spacing between two adjacent isotope peaks. Set the adduct mass for the ion form. Use proton mass for common positive ions. Use negative values for removed charge carriers when needed. Add calibration offset, neutral losses, or neutral gains only when your method requires them. Press calculate to view the neutral molecular weight above the form.
Example Data Table
| Sample | m/z | Spacing | n | Adduct | Calculated MW |
|---|---|---|---|---|---|
| Peptide A | 500.2500 | 0.5017 | 2 | 1.007276 | 998.485447 |
| Protein B | 1200.7500 | 0.3345 | 3 | 1.007276 | 3599.227171 |
| Polymer C | 875.4100 | 1.0033 | 1 | 22.989218 | 852.420782 |
About Molecular Weight From Spectral Peaks
Why Neutral Weight Matters
Mass spectrometry often reports mass to charge ratio, not direct molecular weight. This calculator converts the observed peak into a neutral molecular weight estimate. It also estimates charge n from isotope spacing. That helps when a peak envelope appears with multiple charge states.
Charge State Logic
The charge state controls the final answer. A doubly charged ion carries two charges, so its measured m/z is roughly half of its ion mass. Isotope spacing gives a strong clue. For common carbon isotope spacing, adjacent peaks are about 1.0033548378 Da apart before charge division. If spacing is near 0.5017, the charge is about two.
Adduct Correction
Many ions include added or removed particles. A protonated ion includes proton mass. A sodium adduct includes sodium related mass. The calculator subtracts the chosen adduct mass for every charge. This gives the estimated neutral molecule. You can enter a negative adduct mass for special negative mode cases.
Calibration And Losses
Small calibration errors can move a peak. The offset field corrects the measured m/z before mass conversion. Neutral loss and neutral gain fields support fragmentation, solvent loss, salt effects, and method specific corrections. Use these fields carefully. They should match your instrument notes and chemistry model.
Reporting Results
The result table shows corrected m/z, charge n, ion mass, neutral molecular weight, Dalton error, and ppm error. PPM error is useful because large and small molecules can be compared on a relative scale. Use the CSV export for spreadsheets. Use the PDF export for quick records. Review isotope assignment before final reporting.
FAQs
1. What does charge n mean?
Charge n is the ion charge state. It tells how many charges the detected ion carries. Higher n values reduce the observed m/z for the same neutral molecule.
2. How does isotope spacing calculate n?
The calculator divides the isotope reference mass by the measured isotope spacing. For many organic molecules, the reference is 1.0033548378 Da. The answer is rounded for charge use.
3. What adduct mass should I enter?
Use the mass added per charge carrier. For protonated ions, use about 1.0072764666 Da. For sodium adducts, use about 22.989218 Da. Match the value to your ion type.
4. Can I use negative ion mode?
Yes. Enter an adduct mass that matches your negative ion model. Some cases may use a negative correction. Always confirm the chemistry behind the detected ion.
5. What is ppm error?
PPM error is relative mass error. It equals Dalton error divided by expected mass, then multiplied by one million. Smaller absolute ppm values usually mean better agreement.
6. When should I enter neutral loss?
Enter neutral loss when the measured ion lost a known neutral group before detection. The calculator adds that loss back into the neutral molecular weight estimate.
7. What does calibration offset do?
Calibration offset adjusts the measured m/z before conversion. Use it only when your instrument or calibration check shows a known systematic m/z shift.
8. Are CSV and PDF exports included?
Yes. Submit the same entered values with the CSV or PDF button. The file contains the main inputs, calculated molecular weight, charge n, and error values.