Quality and Accuracy

Mass accuracy describes the deviation between the m/z measured by the instrument and the theoretical true value, and is usually expressed in ppm (parts per million). For example, if the theoretical m/z of an ion is 500.020 and the measured value is 500.000, the deviation is 0.020; converted to a relative deviation, this is 0.020/500 × 10⁶ = 40 ppm. The smaller the ppm value, the more accurate the measurement. It answers a quantitative question: to what extent can I trust this measured mass?

The significance of mass accuracy lies in ‘identifying molecules’. There is a one-to-one correspondence between molecular formulae and exact masses; the more accurately the mass is measured, the fewer candidate molecular formulae there are. High mass accuracy (e.g. better than 5 ppm) can often narrow down the candidate molecular formulae from several dozen to just one or two, significantly reducing the probability of misidentifying spectral peaks; conversely, when accuracy is poor, multiple different molecular formulae fall within the error range, making it impossible to determine which one it is based on mass alone.

It is important to distinguish between mass accuracy and mass resolution, as the two are often confused. Resolution addresses the question of ‘whether two adjacent peaks can be separated’—it is about ‘distinguishability’; precision addresses the question of ‘how far the measured m/z value deviates from the true value’—it is about ‘accuracy’. High resolution is a necessary condition for high precision (if peaks cannot be separated, accuracy is naturally out of the question), but it is not a sufficient condition—precision also depends on whether the mass axis calibration is accurate. It is entirely possible for an instrument to have very high resolution yet poor precision due to calibration drift.

Neo Source INSTRUMENT’s ProC series and its two imaging ion sources both utilise a dual-field reflection time-of-flight (TOF) mass analyser. TOF compensates for ion kinetic energy dispersion via reflectors, maintaining high resolution across a wide mass range. Combined with mass-axis calibration to ensure high mass accuracy, this enables the precise identification of compounds such as lipids and metabolites within complex biological matrices, thereby reducing misidentification of substances caused by m/z deviations. In mass spectrometry imaging, this capability directly determines the reliability of the identification of each substance within an ‘image’—only when identifications are accurate can subsequent biological interpretations be valid. MSI DPI and MSI LDPI can be retrofitted onto mass spectrometers from Agilent, AB SCIEX and Thermo.