Mass Spectrometry Calculator

The Mass Spectrometry Calculator computes the theoretical mass-to-charge ratio (m/z) for any compound — the value used to identify molecules when analyzing them with a mass spectrometer. Enter your molecular formula, charge, adduct type (such as [M+H]+ or [M+Na]+), and instrument resolution to get the theoretical m/z and exact mass. Optionally enter an observed m/z value to calculate the mass error in both ppm and Da.

Enter chemical formula using standard notation

Optional: for mass error calculation

Results

Theoretical m/z

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Exact Mass

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Mass Error

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Mass Error (Da)

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Results Table

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Frequently Asked Questions

What is m/z ratio in mass spectrometry?

The m/z ratio is the mass-to-charge ratio of an ion, which is the fundamental measurement in mass spectrometry. It represents the mass of the ion divided by its charge state.

How do I calculate mass error in ppm?

Mass error in ppm is calculated as: ((observed m/z - theoretical m/z) / theoretical m/z) × 1,000,000. It measures the accuracy of mass measurements.

What are adducts in mass spectrometry?

Adducts are ions formed when molecules associate with other species like protons (H+), sodium (Na+), or potassium (K+) during ionization. Common positive adducts include [M+H]+, [M+Na]+, and [M+K]+.

Why do I see multiple peaks for the same compound?

Multiple peaks occur due to isotopes of the same element having different masses. For example, carbon-13 creates additional peaks alongside the main carbon-12 peak.

What is the difference between exact mass and molecular weight?

Exact mass is the precise mass of a molecule calculated using exact atomic masses of specific isotopes. Molecular weight is the average mass considering natural isotope abundances.

How does instrument resolution affect mass measurements?

Higher resolution instruments (like FT-ICR) can distinguish between very close m/z values and provide more accurate mass measurements compared to lower resolution instruments (like quadrupoles).

What molecular formula format should I use?

Use standard chemical notation like C6H12O6 for glucose or H2SO4 for sulfuric acid. Include all atoms with their counts, and use capital letters for element symbols.