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Mass Spectrometry Adduct Calculator

Chemistry

Formula C₈H₁₀N₄O₂ · monoisotopic 194.080376 · average 194.194

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Adduct set

15 adducts active (both tabs use this set).

Custom adducts

Write adducts as [M+Na]+, [2M+H]+, [M+2H]2+ or [M+H-H2O]+. Aliases: ACN, MeOH, IsoProp, DMSO, FA, Hac, TFA. Press Enter to add (up to 10).

Adduct map

M 194.0804+17.0265+21.9819+37.9559100120140160180200220m/z[M+H]⁺[M+H]⁺ m/z 195.0877[M+NH₄]⁺[M+NH₄]⁺ m/z 212.1142[M+Na]⁺[M+Na]⁺ m/z 217.0696[M+K]⁺[M+K]⁺ m/z 233.0435[M+H−H₂O]⁺[M+H−H₂O]⁺ m/z 177.0771[M+2H]²⁺[M+2H]²⁺ m/z 98.0475
1+ / 1−
2+ / 2−
3+ and higher
hollow cap: 2M or 3M
neutral M

Stick heights are not intensities. Which adducts actually appear depends on solvent, additives, salts and instrument. Click or press Enter on a stick to select it. Dimers and trimers are outside this range; switch to All ions to see them.

Adduct ions

AdductIon formulam/zzΔ (u)
[M+H]⁺
C₈H₁₁N₄O₂⁺195.0877+1+1.007276
[M+NH₄]⁺
C₈H₁₄N₅O₂⁺212.1142+1+18.033826
[M+Na]⁺
C₈H₁₀N₄NaO₂⁺217.0696+1+22.989221
[M+K]⁺
C₈H₁₀KN₄O₂⁺233.0435+1+38.963158
[M+H−H₂O]⁺
C₈H₉N₄O⁺177.0771+1−17.003288
[M+2H]²⁺
C₈H₁₂N₄O₂²⁺98.0475+2+2.014553
[2M+H]⁺
C₁₆H₂₁N₈O₄⁺389.1680+1+1.007276
[2M+Na]⁺
C₁₆H₂₀N₈NaO₄⁺411.1500+1+22.989221

Isotope pattern of [M+H]⁺ C₈H₁₁N₄O₂⁺

0 %50 %100 %194.5195.0195.5196.0196.5197.0197.5m/zM100.0M m/z 195.0877 100.00 %M+110.3M+1 m/z 196.0902 10.32 %M+20.89M+2 m/z 197.0922 0.89 %

Hover over or focus a stick to read its m/z.

Peaks 1.00 apart → charge 1.

Unit (nominal-mass) resolution with abundance-weighted centroids: fine structure such as ¹³C vs ¹⁵N at M+1 (0.0063 Da apart) is merged.

Peakm/zRelative %Absolute %
M195.0877100.0089.872
M+1196.090210.329.272
M+2197.09220.890.803

Diagnostic spacings

These differences are the same for every molecule, so a pair of peaks this far apart points to the adducts named.

NH₄⁺ − H⁺

17.026549

Na⁺ − H⁺

21.981944

K⁺ − H⁺

37.955882

K⁺ − Na⁺

15.973937

About This Tool

Mass Spectrometry Adduct Calculator – ESI Adduct m/z, Isotope Patterns and Neutral Mass

A soft-ionisation spectrum rarely shows the molecule on its own. Electrospray (ESI) and APCI produce a family of adduct ions: the molecule plus a proton, a sodium ion, an ammonium ion, a chloride, or two copies of the molecule sharing one charge. This mass spectrometry adduct calculator works in both directions. Give it a formula or a neutral mass and it lists the exact m/z of more than 45 ESI adducts with the isotope pattern of each ion. Give it the peaks you observed and it finds the neutral mass that explains them.

Why ESI makes adducts rather than M⁺•

Electrospray transfers ions that already exist in solution into the gas phase. A basic site picks up a proton, giving [M+H]⁺, while acids lose one and appear as [M−H]⁻. Sodium and potassium are everywhere: they leach from glassware, arrive with buffers and salts, and sit in solvents at trace level. That is why [M+Na]⁺ and [M+K]⁺ show up next to the protonated ion even when nobody added salt. Ammonium acetate or formate buffers give [M+NH₄]⁺, and formic or acetic acid in the mobile phase gives formate and acetate adducts in negative mode.

The electron mass matters at high resolution

Every adduct here is built from its composition: ion = n·M + mass(added) − mass(removed) − z·mₑ, and m/z = ion / |z|. The electron term is easy to forget. For caffeine, C₈H₁₀N₄O₂ with a monoisotopic mass of 194.080376, the correct [M+H]⁺ is 195.087652. Adding a whole hydrogen atom instead of a proton gives 195.088201, which is 0.55 mDa or 2.8 ppm too high, enough to fail a 2 ppm acceptance window on an Orbitrap or TOF.

Reading the 21.98 and 37.96 spacings

Because every singly charged adduct carries the same molecule, the gap between two of them does not depend on M. Sodium sits 21.9819 above the protonated ion, potassium 37.9559 above it, and ammonium 17.0265. In negative mode, chloride is 35.9767 above [M−H]⁻ and formate 46.0055. When you see two peaks 21.98 apart, you have almost certainly found the [M+H]⁺/[M+Na]⁺ pair, and the adduct map draws these brackets for you.

Dimers and the 2M versus 2+ ambiguity

At higher concentrations two molecules can share one charge, giving [2M+H]⁺ or [2M+Na]⁺. This creates a real ambiguity: a peak at m/z 195.09 could be [M+H]⁺ of a 194 Da compound, [M+2H]²⁺ of a 388 Da one, or [2M+H]⁺ of a 97 Da one. The peaks tab ranks all of these readings by how many of your peaks each explains, and flags any explanation that relies on a charge state it cannot confirm.

Isotope patterns and charge state

Each carbon carries a 1.07 % chance of being ¹³C, so the M+1 peak grows by about 1.1 % per carbon: roughly 10 % for caffeine and 60 % for a 50-carbon peptide. Chlorine gives a 3:1 M:M+2 pair per atom and bromine an almost 1:1 pair, which is unmistakable. The spacing of the cluster gives the charge: ¹³C peaks are 1.003 apart for a 1+ ion, 0.502 apart for 2+ and 0.334 apart for 3+. The calculator uses this spacing to lock each observed peak to the right charge and to set isotope peaks aside.

Monoisotopic versus average mass

High-resolution instruments resolve individual isotope peaks, so the monoisotopic mass is the right basis. For large molecules, the monoisotopic peak shrinks as the chance of containing at least one heavy isotope grows: in human insulin it is only about 15 % of the tallest peak, and for big proteins it disappears into the noise. There the average mass, built from standard atomic weights, describes the centroid of the envelope that a low-resolution instrument reports.

Positions, not intensities
Which adducts actually appear, and how strongly, depends on the analyte, solvent, additives, salts and source settings. The calculator tells you where each ion would be, not whether it will form.

Frequently Asked Questions

Is the Mass Spectrometry Adduct Calculator free?

Yes, Mass Spectrometry Adduct Calculator is totally free :)

Can I use the Mass Spectrometry Adduct Calculator offline?

Yes, you can install the webapp as PWA.

Is it safe to use Mass Spectrometry Adduct Calculator?

Yes, any data related to Mass Spectrometry Adduct Calculator only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

What is an adduct ion in ESI mass spectrometry?

Electrospray does not usually knock an electron off the molecule. Instead the neutral molecule M picks up or loses a small charged species: a proton gives [M+H]⁺, a sodium or potassium ion gives [M+Na]⁺ or [M+K]⁺, and losing a proton gives [M−H]⁻. Each of these adduct ions appears at its own m/z, so one compound can produce several peaks.

Why is my [M+Na]⁺ peak 21.98 above [M+H]⁺?

Both ions carry the same molecule and one positive charge, so the gap is simply the mass of Na⁺ minus the mass of H⁺: 22.989221 − 1.007276 = 21.981944 u. The same 21.98 spacing appears for every compound, which is why analysts use it to recognise sodium adducts at a glance. K⁺ sits 37.956 above [M+H]⁺ and NH₄⁺ 17.027 above it.

Why does the calculator subtract the electron mass?

A cation has lost an electron, so its mass is the mass of its atoms minus 0.000549 u per charge, and an anion has gained one. Ignoring it shifts caffeine's [M+H]⁺ by 0.55 mDa, which is 2.8 ppm: larger than the 2 ppm acceptance window many high-resolution labs use.

Should I use monoisotopic or average mass?

Use monoisotopic mass for any instrument that resolves isotope peaks, such as an Orbitrap, TOF or FT-ICR, because each peak you see is a single isotopic composition. Average mass is for very large molecules whose monoisotopic peak is too small to see, and for low-resolution instruments that report the centroid of the whole isotope envelope.

How can I tell the charge state of a peak?

Look at the spacing of its ¹³C isotope peaks. A singly charged ion has peaks 1.003 apart, a 2+ ion 0.502 apart and a 3+ ion 0.334 apart, because the 1.003 u mass difference is divided by the charge. The peaks tab uses this spacing automatically to restrict each peak to adducts of the right charge.

How do I find the molecular mass from several adduct peaks?

Paste the peak list into the Peaks → molecule tab. For each peak and adduct it computes the neutral mass that would explain it, then looks for one mass that explains as many peaks as possible within your tolerance, for example [M+H]⁺, [M+Na]⁺ and [M+K]⁺ all pointing to M = 194.0804. The best few explanations are ranked by peaks explained and RMS error.