When choosing a time-of-flight mass spectrometer, should one prioritise mass resolution or mass accuracy?

Mass resolution and mass accuracy are two distinct aspects: resolution determines ‘whether adjacent peaks can be separated’, whilst accuracy determines ‘the difference between the measured m/z and the true value’. When selecting a mass spectrometer, one should first consider whether the sample is frequently surrounded by interfering peaks (requiring high resolution), and whether element/formula identification without standard samples is required (requiring high mass accuracy); the relative importance of these factors varies depending on the application, and it is not a case of choosing one over the other.

What are the two parameters?

Mass resolution is the ability to distinguish between two closely spaced m/z values, commonly expressed as R = m/Δm; mass accuracy is the degree to which a measured value deviates from the true value, typically expressed in ppm. High resolution does not necessarily imply high accuracy, as the two stem from different design objectives.

When is resolution more important

When a sample contains a large number of compounds with similar masses (such as lipid isomers or metabolite mixtures), peaks are prone to overlap; in such cases, resolution determines whether you can ‘see clearly’. The New Ceyuan ProC-2 has a resolution of >2000 at m/z=92, enabling it to separate many adjacent peaks, which is particularly crucial for complex imaging spectra.

When is accuracy more important

When inferring molecular formulae (deriving elemental composition from exact mass) or identifying unknowns without standard samples, high mass accuracy (low ppm error) is indispensable. Accuracy depends on calibration masses, frequency stability and acquisition linearity, and is relatively independent of resolution design.

How to Prioritise in Imaging Scenarios

In imaging, each point corresponds to a full spectrum, and with a vast number of points, spectral overlap can contaminate the entire image; therefore, resolution is of more ‘obvious’ importance for imaging. However, high accuracy enhances the reliability of substance annotation, and is particularly valuable in metabolomic identification. Of course, it would be ideal to have both; however, when budgets are limited, trade-offs should be made based on the primary challenges: first, draw up a list—the mass density of the analytes, whether structural inference is required, whether imaging is to be performed, and the budget. If mass density is high or imaging is the primary focus → prioritise resolution; if the identification of unknown compounds is the primary focus → prioritise accuracy; if conditions permit, select a platform that offers a balance between the two (such as dual-field reflection TOF, which provides practical resolution whilst maintaining accuracy).

Can high resolution replace accuracy?

No. Resolution merely addresses ‘peak separation’; without knowing the exact mass of each peak, reliable structural identification remains impossible. The two are complementary: high resolution reduces overlap, whilst high accuracy ensures identity.

How is mass accuracy ensured?

Through high-quality calibration (internal standards or known peaks), a stable sampling clock, a time-to-digital converter with good linearity, and standardised vacuum and temperature control; these factors are not necessarily tied to high resolution.

Is high accuracy essential for imaging?

Not necessarily. If the aim of imaging is simply to examine the distribution of known substances, resolution and repeatability are more critical; however, if the intention is to incidentally discover and identify unknown substances during imaging, high accuracy significantly enhances the reliability of the interpretation.