181.070276
Da
181.070276
m/z
182.073631
m/z
183.076986
m/z
1.007276
Da
1.003355
m/z
181.070276
Da
181.070276
m/z
182.073631
m/z
183.076986
m/z
1.007276
Da
1.003355
m/z
The Mass Spectrometry m/z Calculator determines the mass-to-charge ratio (m/z) for ions generated in various ionization modes used in mass spectrometry (MS). Mass spectrometry is the premier technique for molecular weight determination, structural elucidation, quantitative analysis, and proteomics. The fundamental measurement in MS is the m/z ratio, which depends on the molecular mass of the analyte, the type of ionization (protonation, deprotonation, cation adduct formation), and the charge state. This calculator supports common ionization modes including ESI (electrospray ionization) protonation/deprotonation, sodium and potassium adducts, and neutral species. It also predicts the positions of M+1 and M+2 isotope peaks, which are essential for confirming molecular formulas and charge states in high-resolution mass spectrometry.
The mass-to-charge ratio depends on the ionization mode:
Positive mode — protonation:
$$m/z = \frac{M + n \times 1.00728}{z}$$
where M is the molecular mass, n is the number of protons added, and z is the total charge. For singly protonated species, m/z = M + 1.00728.
Negative mode — deprotonation:
$$m/z = \frac{M - n \times 1.00728}{z}$$
Alkali metal adducts:
$$m/z = \frac{M + n \times m_{\text{adduct}}}{z}$$
where m_adduct is 22.989 for Na⁺ or 38.964 for K⁺. Isotope peaks appear at approximately:
$$\text{M+1}: m/z + \frac{1.00335}{z}, \quad \text{M+2}: m/z + \frac{2.00671}{z}$$
The spacing between isotope peaks (1/z Da) directly reveals the charge state of multiply charged ions, which is crucial in protein mass spectrometry.
The calculated m/z value tells you where to expect the molecular ion peak in the mass spectrum. In ESI-MS of small molecules (MW < 1000), the most common ions are [M+H]⁺ (positive mode) and [M-H]⁻ (negative mode), both singly charged. For larger molecules and proteins, multiple charging produces a series of peaks at m/z = (M + nH)/n, creating a characteristic charge-state envelope. Sodium adduct peaks [M+Na]⁺ appear 22 Da higher than [M+H]⁺ and are common contaminants in ESI. The isotope peaks help confirm molecular formulas — the M+1 intensity is approximately proportional to the number of carbon atoms (1.1% per carbon), while M+2 intensity indicates the presence of Cl, Br, S, or Si.
Inputs
Results
Glucose (C₆H₁₂O₆, MW = 180.063) produces [M+H]⁺ at m/z = 181.07. The sodium adduct [M+Na]⁺ at m/z = 203.05 is often more abundant in ESI due to trace sodium in solvents.
Inputs
Results
Lysozyme (MW ~14,305 Da) with 10 protons (+10 charge state) appears at m/z ≈ 1431.6. The 0.1 Da spacing between isotope peaks confirms the +10 charge state. The charge-state envelope typically spans +8 to +15 for this protein.
m/z is the mass-to-charge ratio, where m is the mass of the ion in daltons (Da) and z is the number of elementary charges (unitless integer). It is measured in Thomson (Th), where 1 Th = 1 Da/e. All mass spectrometers actually measure m/z, not mass directly — the charge state must be determined to calculate the true molecular mass.
For multiply charged ions, examine the spacing between isotope peaks: Δm/z = 1.003/z. If adjacent isotope peaks are ~0.5 apart, z = 2; if ~0.33 apart, z = 3; etc. In ESI, adjacent charge states in the envelope can also be used: z₁ = (m/z₂ - H)/(m/z₁ - m/z₂), where H is the adduct mass (typically 1.008).
Sodium ions are ubiquitous contaminants in solvents, glassware, and even the analyst's skin. Compounds with multiple oxygen atoms (sugars, polyethers) readily form sodium adducts due to favorable coordination. To suppress sodium adducts, use plastic containers, add ammonium salts, or use chelating agents like EDTA.
The molecular ion peak is the peak corresponding to the intact molecule with a charge. In EI-MS, it is M⁺• (radical cation formed by electron removal). In ESI/MALDI, it is typically [M+H]⁺ or [M+Na]⁺. The molecular ion peak provides the molecular weight, the most fundamental piece of information in MS.
High-resolution MS (HRMS) measures m/z to 4-5 decimal places, enabling determination of the exact molecular formula. Each element has a unique exact mass (¹²C = 12.000000, ¹H = 1.007940, ¹⁶O = 15.994915), so precise mass measurement distinguishes molecules with the same nominal mass but different formulas (e.g., CO vs. N₂: 27.9949 vs. 28.0061).
Natural elements contain multiple stable isotopes. ¹³C (1.1% natural abundance) causes the M+1 peak, with intensity approximately 1.1% × number of carbons. ³⁷Cl (24.2%) and ⁸¹Br (49.3%) produce distinctive M+2 patterns. ³⁴S (4.2%) also contributes to M+2. These isotope patterns are molecular fingerprints.
ESI (Electrospray Ionization) produces multiply charged ions from solution, ideal for proteins and polar molecules. MALDI (Matrix-Assisted Laser Desorption/Ionization) produces predominantly singly charged ions from a solid matrix, ideal for large biomolecules and polymers. ESI gives charge-state envelopes; MALDI gives simpler spectra with one peak per species.
From two adjacent ESI charge states: M = z₂ × (m/z₂ - H), where z₂ = (m/z₁ - H)/(m/z₁ - m/z₂) and H = 1.00728 for protonation. Modern software performs deconvolution automatically, reconstructing the true mass spectrum from the multiply charged envelope.
The base peak is the most intense peak in the mass spectrum, assigned a relative intensity of 100%. It may or may not be the molecular ion peak. In EI mass spectra, extensive fragmentation often makes a fragment ion the base peak. In ESI of small molecules, the molecular ion is often the base peak.
Yes. Native mass spectrometry (native MS) uses gentle ESI conditions to preserve non-covalent interactions in the gas phase. Protein-protein complexes, protein-ligand binding, and even intact viral capsids (millions of Da) have been analyzed by native MS, providing stoichiometry and binding information.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
How helpful was this calculator?
Be the first to rate!
