Mass Accuracy Calculator Formula
Understand the math behind the mass accuracy calculator. Each variable explained with a worked example.
Formulas Used
Mass Error (ppm)
error_ppm = (measured_mass - theoretical_mass) / theoretical_mass * 1e6Mass Error (mDa)
error_mda = abs(measured_mass - theoretical_mass) * 1000Variables
| Variable | Description | Default |
|---|---|---|
measured_mass | Measured m/z | 180.0637 |
theoretical_mass | Theoretical m/z | 180.0634 |
How It Works
Mass Accuracy in Mass Spectrometry
Mass accuracy is the difference between the measured and theoretical mass of an ion, expressed in parts per million (ppm). It determines how confidently you can assign a molecular formula.
Formula
Error (ppm) = (m_measured - m_theoretical) / m_theoretical × 10⁶
Modern high-resolution instruments achieve < 2 ppm (Orbitrap) or < 1 ppm (FT-ICR). Time-of-flight (TOF) instruments typically achieve 2-5 ppm.
Worked Example
Glucose [M+H]+ measured at 180.0637, theoretical 180.0634.
- 01Error = (180.0637 - 180.0634) / 180.0634 × 10⁶
- 02Error = 0.0003 / 180.0634 × 10⁶ = 1.67 ppm
- 03Error in mDa = 0.3 mDa
Frequently Asked Questions
What mass accuracy is needed for formula assignment?
At < 1 ppm, formulas can be assigned with high confidence for small molecules. At 1-5 ppm, a few candidate formulas may exist. Above 10 ppm, too many formulas are possible for unique identification.
Does mass accuracy improve with calibration?
Yes. External calibration typically gives 2-5 ppm accuracy. Internal calibration (lock mass) improves this to < 1-2 ppm. Some instruments continuously calibrate during acquisition.
How does mass accuracy relate to resolving power?
Resolving power (m/delta_m) determines whether two close masses can be distinguished. Mass accuracy determines how precisely the center of a peak is measured. Both are important but independent instrument specifications.
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