Exact Mass Calculator

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Exact Mass & Monoisotopic Mass Calculator

Calculation Results

Monoisotopic Mass (M):
[M + H]⁺ (Positive Ion):
[M + Na]⁺ Adduct:
[M – H]⁻ (Negative Ion):
function calculateMass() { var formulaInput = document.getElementById("chemicalFormula").value.trim(); var errorBox = document.getElementById("errorBox"); var resultsBox = document.getElementById("results"); errorBox.style.display = "none"; resultsBox.style.display = "none"; if (!formulaInput) { errorBox.innerHTML = "Please enter a valid chemical formula."; errorBox.style.display = "block"; return; } // Exact masses of most abundant isotopes (Monoisotopic masses) var isotopeTable = { "C": 12.0000000, "H": 1.0078250, "O": 15.9949146, "N": 14.0030740, "P": 30.9737620, "S": 31.9720707, "F": 18.9984032, "Cl": 34.9688527, "Br": 78.9183376, "I": 126.904473, "K": 38.9637066, "Na": 22.9897693, "Ca": 39.9625909, "Mg": 23.9850417, "Fe": 55.9349421, "Zn": 63.9291422, "Si": 27.9769265 }; // Adduct masses var protonMass = 1.007276466; // H+ var electronMass = 0.0005485; var totalMass = 0; var regex = /([A-Z][a-z]*)(\d*)/g; var match; var isValid = true; var atomsFound = false; while ((match = regex.exec(formulaInput)) !== null) { atomsFound = true; var element = match[1]; var count = match[2] === "" ? 1 : parseInt(match[2]); if (isotopeTable[element]) { totalMass += isotopeTable[element] * count; } else { errorBox.innerHTML = "Unknown or unsupported element: " + element + ". Please use standard symbols like C, H, O, N, P, S, Cl, etc."; errorBox.style.display = "block"; isValid = false; break; } } if (isValid && !atomsFound) { errorBox.innerHTML = "Invalid formula format. Use standard notation like C2H5OH."; errorBox.style.display = "block"; isValid = false; } if (isValid) { document.getElementById("resMonoisotopic").innerText = totalMass.toFixed(6); document.getElementById("resMH").innerText = (totalMass + protonMass).toFixed(6); document.getElementById("resMNa").innerText = (totalMass + 22.989769).toFixed(6); document.getElementById("resML").innerText = (totalMass – protonMass).toFixed(6); resultsBox.style.display = "block"; } }

Understanding Exact Mass in Mass Spectrometry

In analytical chemistry, specifically mass spectrometry (MS), the exact mass refers to the calculated theoretical mass of an individual molecule based on the specific isotopes of its constituent atoms. Unlike the average mass found on a periodic table, which accounts for the natural abundance of all isotopes, the exact mass uses the mass of the most abundant stable isotope (monoisotopic mass).

Monoisotopic Mass vs. Average Mass

It is crucial to distinguish between these two values when performing high-resolution mass spectrometry:

  • Monoisotopic Mass: The sum of the masses of the most abundant isotopes (e.g., Carbon-12 at 12.000000, Hydrogen-1 at 1.007825). This is the mass observed in high-resolution MS for small to medium-sized molecules.
  • Average Mass: The weighted average of all naturally occurring isotopes (e.g., Carbon at ~12.011). This is typically used for stoichiometric calculations in "wet chemistry."

Common Isotopic Masses Used in This Tool

The following table lists the precise values used by our calculator for the most common elements in organic and biological chemistry:

Element Symbol Exact Mass (Da)
CarbonC12.000000
HydrogenH1.007825
NitrogenN14.003074
OxygenO15.994915
SulfurS31.972071

How to Calculate Exact Mass

To calculate the exact mass manually, you multiply the count of each atom by its specific monoisotopic mass and sum them up. For example, for Water (H₂O):

Mass = (2 × 1.007825) + (1 × 15.994915) = 18.010565 Da

In Electrospray Ionization (ESI), molecules are typically observed as ions. A positive ion is usually the molecule plus a proton ([M+H]⁺), while a negative ion is the molecule minus a proton ([M-H]⁻). Our calculator automatically provides these common adduct values for your convenience.

Practical Application Example

If you are analyzing Glucose (C₆H₁₂O₆), the calculator will provide:

  • Monoisotopic Mass: 180.063388
  • [M+H]⁺: 181.070664 (This is what you would look for in a positive mode MS spectrum).
  • [M+Na]⁺: 203.052615 (A very common salt adduct in mass spectrometry).

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