Exact Mass Calculator
Calculate monoisotopic mass (exact mass) from any chemical formula — with MS adduct m/z values [M+H]⁺, [M+Na]⁺, [M−H]⁻, isotope pattern simulation, isotope abundance tables, and complete step-by-step working for mass spectrometry analysis.
| Element | Isotope | Count | Mono Mass (each) | Contribution (Da) | Avg Contribution |
|---|
| Adduct | Formula | m/z Value | Mode |
|---|
Uses proton mass 1.007276 Da (not hydrogen atom 1.007940 Da) — required for correct adduct m/z.
Select an element to view all isotopes with masses and natural abundances, plus the calculated average atomic mass. Or enter a custom element symbol.
| Isotope | Mass Number (A) | Exact Mass (Da) | Fractional Abundance | Percent Abundance (%) | Contribution to Avg |
|---|
Click any row to load that element formula into the exact mass calculator. All monoisotopic masses are for the most abundant isotope of each element.
| Symbol | Element | Isotope | Mono Mass (Da) | Abundance (%) | Avg Mass (Da) |
|---|
What Is Exact Mass and Monoisotopic Mass?
This exact mass calculator computes the monoisotopic mass of any chemical formula — the value required for high-resolution mass spectrometry data interpretation. It also generates m/z values for common MS adducts ([M+H]⁺, [M+Na]⁺, [M+K]⁺, [M−H]⁻, [M+2H]²⁺), displays approximate isotope patterns for M, M+1, and M+2 peaks, and calculates isotope abundances and average atomic mass for individual elements.
The terms exact mass and monoisotopic mass are used interchangeably in mass spectrometry. Both refer to the mass calculated using the mass of the most abundant isotope (not the lightest) of each element in the molecule.
Key distinction: Monoisotopic mass uses the most abundant isotope — not the lightest. For most elements these coincide (¹H, ¹²C, ¹⁴N, ¹⁶O), but for elements like iron, the most abundant isotope ⁵⁶Fe is not the lightest (⁵⁴Fe).
Monoisotopic Mass vs Average Mass
| Property | Monoisotopic Mass | Average Mass |
|---|---|---|
| Definition | Uses most abundant isotope mass of each element | Weighted average of all isotope masses |
| Glucose C₆H₁₂O₆ | 180.0634 Da | 180.1559 Da |
| Use case | High-resolution MS peak assignment | Molarity, solution preparation, weighing |
| Decimal places | 4–6 significant decimal places | 2–4 decimal places typical |
| Instrument type | HRMS (Orbitrap, TOF, FTICR) | Low-resolution MS, analytical balances |
Nominal Mass
The nominal mass is the integer mass — the monoisotopic mass rounded to the nearest whole number. For glucose: nominal mass = 180. It is used for low-resolution mass spectrometers that cannot resolve individual isotope peaks. The mass defect = monoisotopic mass − nominal mass. For organic molecules (H, C, N, O only) the mass defect is typically positive and small; for halogens it can be negative (e.g. Cl, Br).
How to Calculate Exact Mass — Step-by-Step
Calculating the monoisotopic mass (exact mass) of any molecule from its molecular formula follows a four-step method. Every high-resolution mass spectrometer uses this calculation internally to generate theoretical m/z values for spectral matching.
- Step 1 — Parse the formula: Break the molecular formula into element symbols and atom counts. Handle parentheses (e.g. (CH₃)₂SO₄ → C₂H₆SO₄).
- Step 2 — Look up monoisotopic masses: Find the mass of the most abundant isotope for each element. ¹²C = 12.00000 Da (by definition), ¹H = 1.00783 Da, ¹⁶O = 15.99491 Da, ¹⁴N = 14.00307 Da.
- Step 3 — Multiply by atom count: Multiply each isotope mass by the number of atoms of that element.
- Step 4 — Sum all contributions: Add all contributions. Append units (Da or u). This is the exact mass / monoisotopic mass.
Worked Example — Caffeine C₈H₁₀N₄O₂ (monoisotopic mass calculation)
- C × 8: 12.00000 × 8 = 96.00000 Da
- H × 10: 1.00783 × 10 = 10.07825 Da
- N × 4: 14.00307 × 4 = 56.01228 Da
- O × 2: 15.99491 × 2 = 31.98983 Da
- Monoisotopic mass = 96.00000 + 10.07825 + 56.01228 + 31.98983 = 194.08035 Da
- Nominal mass = 194 · Mass defect = +0.0804 Da
- [M+H]⁺ m/z = 194.08035 + 1.00728 = 195.08763
Technical Checkpoint — Glucose C₆H₁₂O₆ Verification
- C × 6: 12.00000 × 6 = 72.00000 Da
- H × 12: 1.00783 × 12 = 12.09396 Da
- O × 6: 15.99491 × 6 = 95.96947 Da
- Monoisotopic mass = 180.06339 Da ✓
- [M+H]⁺: 180.06339 + 1.007276 = 181.07067 m/z ✓
- Note: proton mass = 1.007276 Da, NOT hydrogen atom mass 1.007940 Da
Isotope Abundance and Isotope Patterns in Mass Spectrometry
Every element in nature consists of a mixture of isotopes — atoms with the same number of protons but different numbers of neutrons. These natural isotope abundances are fixed values and create characteristic patterns in mass spectra, allowing element identification directly from the isotope pattern shape.
Natural Isotope Abundances of Common Elements
| Element | Isotope | Mass (Da) | Natural Abundance (%) |
|---|---|---|---|
| Carbon | ¹²C | 12.00000 | 98.93% |
| Carbon | ¹³C | 13.00335 | 1.07% |
| Hydrogen | ¹H | 1.00783 | 99.99% |
| Hydrogen | ²H (D) | 2.01410 | 0.01% |
| Chlorine | ³⁵Cl | 34.96885 | 75.76% |
| Chlorine | ³⁷Cl | 36.96590 | 24.24% |
| Bromine | ⁷⁹Br | 78.91834 | 50.69% |
| Bromine | ⁸¹Br | 80.91629 | 49.31% |
| Sulfur | ³²S | 31.97207 | 94.99% |
| Sulfur | ³⁴S | 33.96787 | 4.25% |
The Chlorine M+2 Pattern
Chlorine has two dominant isotopes: ³⁵Cl (75.76%) and ³⁷Cl (24.24%). The ³⁷Cl is exactly 2 mass units heavier. Any molecule containing one chlorine atom shows two peaks: M (monoisotopic, ³⁵Cl) and M+2 (³⁷Cl) in approximately a 3:1 intensity ratio. Two chlorines produce M:M+2:M+4 in approximately a 9:6:1 ratio. This distinctive pattern is one of the most useful diagnostic tools in mass spectrometry.
The Bromine M+2 Pattern
Bromine's two isotopes (⁷⁹Br 50.69%, ⁸¹Br 49.31%) are nearly equal in abundance, producing M and M+2 peaks of approximately equal intensity (≈1:1 ratio). This is instantly recognizable in a mass spectrum and confirms the presence of one bromine atom. Two bromines give M:M+2:M+4 ≈ 1:2:1.
The Carbon M+1 Pattern
Each carbon atom contributes approximately 1.1% to the M+1 peak intensity (due to ¹³C natural abundance 1.07%). For a molecule with n carbons, the M+1 peak relative abundance ≈ n × 1.1%. For caffeine (8 carbons): M+1 ≈ 8.8%. This allows estimation of carbon count directly from the relative M+1 intensity in the mass spectrum.
Mass Spectrometry Adducts — [M+H]⁺, [M+Na]⁺, and More
Mass spectrometers do not measure molecular mass directly — they measure the mass-to-charge ratio (m/z) of ions. In electrospray ionization (ESI-MS), neutral molecules must acquire a charge to be detected. This happens through adduct formation, where the molecule combines with a small ion during the ionization process.
Critical: MS adduct calculations use the proton mass = 1.007276 Da, not the hydrogen atom mass (1.007940 Da). The difference (0.000549 Da = electron mass) matters at high resolution (4+ decimal places).
Common ESI-MS Adducts
| Adduct | Ion Mode | Added Mass (Da) | m/z Formula | Common in |
|---|---|---|---|---|
| [M+H]⁺ | Positive | +1.007276 | (M + 1.007276) / 1 | Most ESI-MS positive mode |
| [M+Na]⁺ | Positive | +22.989218 | (M + 22.989218) / 1 | Carbohydrates, lipids |
| [M+K]⁺ | Positive | +38.963158 | (M + 38.963158) / 1 | Carbohydrates |
| [M+NH₄]⁺ | Positive | +18.034374 | (M + 18.034374) / 1 | Lipids, with ammonium acetate |
| [M−H]⁻ | Negative | −1.007276 | (M − 1.007276) / 1 | Acids, negative mode ESI |
| [M+2H]²⁺ | Positive | +2×1.007276 | (M + 2.014552) / 2 | Peptides, proteins |
Adduct Table for Glucose C₆H₁₂O₆ (Monoisotopic mass = 180.0634 Da)
| Adduct | Calculation | m/z |
|---|---|---|
| [M+H]⁺ | 180.0634 + 1.007276 | 181.0707 |
| [M+Na]⁺ | 180.0634 + 22.989218 | 203.0526 |
| [M+K]⁺ | 180.0634 + 38.963158 | 219.0265 |
| [M−H]⁻ | 180.0634 − 1.007276 | 179.0561 |
| [M+2H]²⁺ | (180.0634 + 2×1.007276) / 2 | 91.0380 |
| [M+NH₄]⁺ | 180.0634 + 18.034374 | 198.0977 |
Percent Abundance and Average Atomic Mass
The percent abundance of an isotope is the fraction of atoms of that isotope in a naturally occurring sample of the element, expressed as a percentage. The fractional abundance is the same value expressed as a decimal (0 to 1). These values are determined experimentally by mass spectrometry and are fixed constants of nature.
Worked Example — Chlorine Average Atomic Mass
- ³⁵Cl: mass = 34.96885 Da, fractional abundance = 0.7576
- ³⁷Cl: mass = 36.96590 Da, fractional abundance = 0.2424
- Average = (34.96885 × 0.7576) + (36.96590 × 0.2424)
- = 26.4982 + 8.9554 = 35.4536 Da ≈ 35.453 g/mol ✓
- Verify: 0.7576 + 0.2424 = 1.0000 (abundances must sum to 1)
Worked Example — Bromine Average Atomic Mass
- ⁷⁹Br: mass = 78.91834 Da, fractional abundance = 0.5069
- ⁸¹Br: mass = 80.91629 Da, fractional abundance = 0.4931
- Average = (78.91834 × 0.5069) + (80.91629 × 0.4931)
- = 40.003 + 39.892 = 79.895 Da ≈ 79.904 g/mol ✓
Worked Examples — Exact Mass Calculations
1. Aspirin C₉H₈O₄ — Monoisotopic Mass
- C×9: 12.00000 × 9 = 108.00000 Da
- H×8: 1.00783 × 8 = 8.06264 Da
- O×4: 15.99491 × 4 = 63.97965 Da
- Monoisotopic mass = 180.04229 Da
- [M+H]⁺ = 180.04229 + 1.007276 = 181.04957 m/z
2. Cholesterol C₂₇H₄₆O — Monoisotopic Mass
- C×27: 12.00000 × 27 = 324.00000 Da
- H×46: 1.00783 × 46 = 46.36018 Da
- O×1: 15.99491 × 1 = 15.99491 Da
- Monoisotopic mass = 386.35509 Da
- [M+H]⁺ = 387.36237 m/z · [M+Na]⁺ = 409.34431 m/z
3. Sucrose C₁₂H₂₂O₁₁ — Monoisotopic Mass
- C×12: 12.00000 × 12 = 144.00000 Da
- H×22: 1.00783 × 22 = 22.17219 Da
- O×11: 15.99491 × 11 = 175.94405 Da
- Monoisotopic mass = 342.11624 Da
- [M+Na]⁺ = 342.11624 + 22.989218 = 365.10546 m/z
4. Isotope Abundance — Carbon ¹³C
- ¹²C: mass = 12.00000 Da, fractional abundance = 0.9893, percent abundance = 98.93%
- ¹³C: mass = 13.00335 Da, fractional abundance = 0.0107, percent abundance = 1.07%
- Average atomic mass = (12.00000 × 0.9893) + (13.00335 × 0.0107) = 11.8716 + 0.1391 = 12.0107 Da ✓
5. Chloroform CHCl₃ — Identifying Cl Pattern
- Monoisotopic (all ³⁵Cl): C=12.00000, H=1.00783, Cl×3=35.96707×3=107.90121
- Monoisotopic mass CHCl₃ = 117.91453 Da
- M+2 peak (one ³⁷Cl): relative abundance ≈ 3 × (24.24/75.76) × 100 ≈ 96% (very large M+2 due to 3 Cl atoms)
- Characteristic 3-chlorine pattern: M:M+2:M+4:M+6 ≈ 27:27:9:1
6. Acetic Acid CH₃COOH — Full Calculation
- Formula: C₂H₄O₂
- C×2: 12.00000 × 2 = 24.00000 Da
- H×4: 1.00783 × 4 = 4.03132 Da
- O×2: 15.99491 × 2 = 31.98983 Da
- Monoisotopic mass = 60.02115 Da
- Average mass = (12.0107×2) + (1.00794×4) + (15.9994×2) = 60.0524 Da
- [M+H]⁺ = 61.02843 · [M−H]⁻ = 59.01388