Calculator
Example data table
Example neutral monomer mass: 500.1234 Da. The following table shows how common adduct choices shift the expected observed signal.
| Adduct | Mass shift (Da) | Charge state | Output m/z |
|---|---|---|---|
| [M+H]+ | 1.007276 | +1 | 501.130676 |
| [M+Na]+ | 22.989218 | +1 | 523.112618 |
| [M+K]+ | 38.963158 | +1 | 539.086558 |
| [M+2H]2+ | 2.014553 | +2 | 251.068976 |
| [M-H]- | -1.007276 | -1 | 499.116124 |
| [M+Cl]- | 34.969402 | -1 | 535.092802 |
Formula used
Forward calculation: m/z = ((n × neutral mass) + adduct shift + correction mass) / |z|
Reverse calculation: neutral mass = ((m/z × |z|) - adduct shift - correction mass) / n
Where: n is the number of analyte units, z is the signed charge state, and the correction mass lets you include neutral losses or extra gains.
Positive adduct shifts increase the ion mass, while negative shifts reduce it. Charge affects the divisor only through its magnitude because m/z is reported as a positive ratio.
How to use this calculator
- Select whether you want to predict observed m/z or recover neutral mass.
- Enter the known mass value for your chosen direction.
- Pick a common adduct or switch to a custom shift.
- Set analyte units for monomers, dimers, or larger clusters.
- Add any correction mass for losses or gains.
- Choose your preferred decimal precision and submit the form.
- Review the result summary, formula context, and exported report.
Frequently asked questions
1. What does an adduct mass calculator do?
It estimates how an ion’s observed signal changes after protonation, metal attachment, deprotonation, or cluster formation. It can also reverse the process to recover neutral mass from measured m/z.
2. Why does charge state matter so much?
Charge changes the denominator of the m/z ratio. A larger absolute charge spreads the ion mass across more charges, reducing the observed m/z for the same ion.
3. When should I use analyte units above one?
Use values above one when the detected ion contains multiple analyte copies, such as dimers, trimers, or higher clusters. This helps model ions like [2M+H]+ accurately.
4. What is the correction mass field for?
It lets you include extra mass changes beyond the selected adduct. Enter negative values for neutral losses, such as water loss, or positive values for additional attachments.
5. Can I use custom adducts?
Yes. Enter any custom label, exact mass shift, and signed charge state. This is useful for unusual solvents, buffer-related ions, derivatization products, or instrument-specific annotations.
6. Why are some negative-mode shifts positive numbers?
Some negative ions gain a species with positive mass, such as chloride or formate. The ion is still negative overall because its charge state is negative.
7. Does this calculator use monoisotopic values?
The included common adduct library uses exact-style adduct masses suited to high-resolution interpretation. You can still enter your own custom values if your workflow uses different conventions.
8. Can I export the result for reports?
Yes. After calculation, use the CSV or PDF buttons in the result section to save a compact summary of inputs, assumptions, and the final computed value.