Mass Spectrometry m/z Calculator
Calculate the mass-to-charge ratio (m/z) of an ion from its neutral mass, charge number and adduct mass. Handles protonated, sodiated, ammoniated and multiply charged ions.
Find the m/z of an Ion
Mass Spectrometry m/z Formula
Where:
- \(M\) — neutral mass of the analyte in daltons (numerically the same as g/mol)
- \(\Delta\) — mass added by the adduct, in daltons (e.g. \(+1.007276\) Da for H⁺, \(-1.007276\) Da for [M−H]⁻)
- \(z\) — charge number, the absolute number of charges on the ion (1, 2, 3…)
- \(m/z\) — mass-to-charge ratio, reported in Da/e (often just called m/z or Th)
The formula gives the position where the ion appears in a mass spectrum: the total mass of the ion (analyte plus any adducts) divided by the number of charges it carries.
Mass Spectrometry m/z – Calculation Example
Example: a peptide with a neutral monoisotopic mass of 573.35 Da is observed as a singly protonated ion, [M+H]⁺.
\[ m/z = \frac{M + \Delta}{z} = \frac{573.35 + 1.007276}{1} = \frac{574.357276}{1} = 574.3573 \text{ Da/e} \]
Result: the ion appears at m/z 574.3573, and the total ion mass is 574.3573 Da. Since z = 1, the isotope peaks are spaced about 1 Da apart.
How to Use the Calculator
- Enter the Ion (Analyte) Mass — the neutral monoisotopic mass of your molecule in daltons (the default is 573.35 Da).
- Set the Charge Number to the absolute number of charges, e.g. 2 for a doubly protonated ion [M+2H]²⁺. It must be a whole number of 1 or more.
- Enter the Adduct Mass (optional): 1.007276 for H⁺, 22.989218 for Na⁺, 38.963158 for K⁺, 18.033823 for NH₄⁺, or 0 if there is no adduct. Use a negative value (−1.007276) for deprotonated ions [M−H]⁻.
- Read the Mass-to-Charge Ratio and Total Ion Mass in the results, together with the calculation details and isotope spacing.
What Is m/z and Why It Matters
In mass spectrometry, an instrument does not measure mass directly — it separates ions by their mass-to-charge ratio, \(m/z\). That is why two ions with quite different masses can appear at the same position on a spectrum if their charges differ. Understanding \(m/z\) is essential for interpreting spectra, identifying unknowns, and planning experiments, because the peak position is the primary piece of information a mass spectrometer provides.
The charge number \(z\) has a dramatic effect. A 10,000 Da protein carrying a single proton appears at m/z ≈ 10,001, which is beyond the range of many mass analyzers. The same protein with 20 protons, [M+20H]²⁰⁺, appears near m/z 501 — comfortably within range. This is why electrospray ionization, which readily produces multiply charged ions, made large biomolecules accessible to instruments with modest mass ranges.
Charge also changes the appearance of isotope patterns. Every ion gives a cluster of peaks from naturally occurring isotopes (mainly ¹³C). These peaks are spaced by \(1/z\) Da: 1 Da apart for singly charged ions, 0.5 Da for doubly charged ions, 0.33 Da for triply charged ions, and so on. Reading the isotope spacing is a standard way to determine the charge state of an unknown peak — the calculator reports this spacing automatically whenever \(z > 1\).
The adduct mass accounts for whatever is attached to the analyte during ionization. In positive mode this is most often a proton, but sodium, potassium and ammonium adducts are common in LC-MS. In negative mode, deprotonation [M−H]⁻ is typical, so you enter a negative adduct mass. Because \(m/z\) depends on the exact adduct, specifying the right one matters for matching measured spectra to predicted values.
Assumptions, Limitations and Practical Tips
- Use monoisotopic masses for accurate results. Average (chemical) masses will shift the predicted m/z by a fraction of a dalton for small molecules and by more for large ones.
- Use the proton mass, not the hydrogen atom mass. H⁺ is 1.007276 Da; a neutral hydrogen atom is 1.007825 Da. The 0.00055 Da difference matters at high resolution.
- Adduct masses should include all attached species. For [M+2H]²⁺, enter the analyte mass once, charge 2, and adduct mass 1.007276 (a single H⁺) — the calculator multiplies nothing; if you prefer, enter 2.014552 for two protons. For [M+H+Na]²⁺ you would enter 23.996494 (1.007276 + 22.989218) with z = 2.
- The calculator assumes a point charge. In reality, electrons have negligible mass (about 0.00055 Da each), so losing or gaining electrons changes the mass far less than typical measurement accuracy — it is safely ignored.
- Negative-ion mode: enter the adduct as a negative number. The charge number stays positive because \(z\) is the absolute number of charges; polarity affects the sign of the spectrum, not the \(m/z\) value.
- Common reference masses: H⁺ +1.007276, Na⁺ +22.989218, K⁺ +38.963158, NH₄⁺ +18.033823, Cl⁻ −34.969402 (for [M+Cl]⁻ adducts, added as a positive adduct with z = 1).
If you are working out ion structures and charge distributions, the Formal Charge calculator helps you determine which atom carries the charge, and the Empirical Formula calculator is useful for confirming the composition of the neutral analyte. For solutions used in sample preparation, see the Molarity calculator and the Ionic Strength calculator.
Frequently Asked Questions
How do I calculate m/z by hand?
Add the adduct mass to the neutral mass of your molecule, then divide by the number of charges. For example, a 573.35 Da peptide with one proton (1.007276 Da) and one charge gives (573.35 + 1.007276) / 1 = 574.3573. If it carries two protons, use (573.35 + 2 × 1.007276) / 2 ≈ 287.68 Da. The division by charge is the step people forget most often.
How can I determine the charge state of an unknown peak in a spectrum?
Look at the spacing between the isotope peaks in the cluster. Naturally occurring ¹³C makes adjacent isotope peaks appear 1/z Da apart: 1 Da spacing means z = 1, 0.5 Da means z = 2, 0.25 Da means z = 4. Many instrument data systems can zoom in on the isotope envelope, so this measurement is usually quick and reliable for peptides and proteins in electrospray spectra.
How do I convert a measured m/z back to the neutral mass?
Rearrange the formula: M = (m/z × z) − Δ. For a peak at 574.3573 assigned as [M+H]⁺, the neutral mass is 574.3573 − 1.007276 = 573.35 Da. For a doubly charged ion at m/z 287.68 with two protons, M = 287.68 × 2 − 2 × 1.007276 ≈ 573.35 Da. You must assume both the charge and the adduct correctly.
What units is m/z reported in, and are Th and Da/e the same?
Yes — the thomson (Th) and Da/e are equivalent ways of expressing mass-to-charge ratio, and most papers simply write the dimensionless 'm/z'. Note that m/z is not a mass in daltons, even though the numbers coincide for singly charged ions. Strictly, z counts elementary charges, not coulombs, which is why the ratio stays numerically convenient.
Why does my calculated m/z not match the measured peak?
The three usual culprits are: using average (chemical) masses instead of monoisotopic masses, which shifts small molecules by a fraction of a dalton and proteins by several; using the hydrogen atom mass (1.007825 Da) instead of the proton (1.007276 Da); and assuming the wrong adduct — a Na⁺ adduct sits about 22 Da above the protonated peak. Also confirm the charge state from isotope spacing before comparing values.
What is a typical m/z range for common mass spectrometers?
Typical ranges are roughly m/z 50–2,000 for quadrupole and ion-trap instruments, up to 4,000 for many Q-TOFs, and 10,000–80,000 or more for Orbitraps and FT-ICR instruments. Large biomolecules exceed these limits as singly charged ions, but electrospray produces multiply charged ions: a 66,000 Da protein at 30+ charges appears near m/z 2,200, well within standard ranges.
Do I need to account for the mass of the electrons lost or gained during ionization?
No, in practice electron mass is negligible. An electron weighs about 0.00055 Da, so even a 10-charge ion differs from the point-charge approximation by under 0.006 Da — far below the accuracy of most instruments and within the noise of even high-resolution measurements. Standard m/z calculations ignore electron mass entirely, and this calculator does the same.