The performance of mass analyzers is typically quantitated in terms of resolution and mass accuracy.  At a minimum, the resolution of the mass analyzer should be sufficient to separate two ions differing by one mass unit anywhere in the mass range scanned. Such resolution or a little bit higher is provided by quadrupole and ion trap analyzers and it is sometimes referred as unit mass resolution.

The following definition is typically used to calculate resolution in Mass Spectrometry:

R = m/z / Δm /z

• Usually, the peak width is measured as the full width at half maximum (FWHM) of the peak, which is nowadays accepted as a general definition of mass spectrometric resolution.
• Different definition is used for magnetic sector analyzers: R = ( (m/z)₂ - (m/z)₁ ) / (m/z)₁, where (m/z)₁ and (m/z)₂ are mass-to-charge ratios of two very close peaks with the same height and peak valley overlap corresponding to 10%.

  The typical values of resolution for low resolution mass analyzers (e.g. quadrupoles and ion traps) are below 5000. High resolution instruments have a resolution 15000.

The second parameter important to mass spectrometry, mass accuracy, is defined as:

mass accuracy = (m/z(exp) - m/z(theor)) / m/z(theor) *10⁶

We can calculate mass accuracy on line with the help of .

 Notice that the values are expressed in ppm units.

The principal advantage of high mass accuracy is the possibility to determine the elemental composition of individual molecular or fragment ions, which is a powerful tool for the structural elucidation or confirmation.

• For exact mass measurements, the minimum acceptable mass accuracy is 5 ppm.
• For higher m/z values or when the presence of many different elements in a given ion is suspected, the mass accuracy <5 ppm is not sufficient for a clear indication of the elemental composition.

There are several ways to help select the correct elemental composition:

• Increasing resolution,
• Consideration of fragment ions,
• Consideration of the sample history.

Obviously, correct m/z scale calibration is essential to exact mass determinations. Mass spectrometers can be either internally or externally calibrated.

• Internal calibration means that the calibration mixture is present in the ion source as the same time as the sample. Hence, conditions for the mass analyses of the sample and calibrant are absolutely identical. Internal calibration provides the highest precision, but the introduction of calibrant together with the sample may cause ion suppression effects or may introduce interfering peaks into the spectrum.
• External calibration, on the other hand, measures the calibrant before or after the analyte, mimicking the unknown’s analysis conditions as closely as possible. External calibration may provide results almost identical to internal calibration if the instrument is very stable and the calibration is performed immediately before or after the analyte analysis.

HPLC/MS coupling allows several different approaches to mass calibration:

• Internal calibration solutions can be added to the mobile phase via a T-piece after the chromatographic column (dead volumes must be reduced to avoid peak broadening), before the column (the calibrant may destroy the separation), or via a second sprayer in the ion source.
• The dual spray solution is used with electrospray but not for other API techniques.

In my opinion, the best solution is external calibration with calibrant introduction during the system dead volume (then there is no interference with chromatographic peaks) and again immediately after the analysis. For long analysis, the calibrant may be sampled periodically during the middle part of chromatogram in regions without important peaks. Such an approach eliminates ion suppression effects and mass interferences caused by the calibrant, while maintaining mass accuracy.