Mass resolution: Can it separate two peaks that are ‘very close together’?

Mass resolution describes the ability of a mass spectrometer to distinguish between ions of similar mass. The most commonly used definition is: when two adjacent peaks can just be separated, the ratio of the mass m of one peak to the mass difference Δm between the two peaks, i.e. R = m/Δm. The larger the value of R, the better the instrument is able to separate two ions with only a slight difference in mass, and the clearer the spectrum. For example, at m/z = 92, R = 3500, which means that a mass difference of approximately 92/3500 ≈ 0.026 can be resolved—this is sufficient to distinguish between many closely spaced molecular and isotopic peaks.

Why is resolution so crucial? Real samples often contain a large number of molecules with closely similar masses (i.e. isobars). When resolution is insufficient, two peaks that should be separate may merge into one, leading to the misidentification of two substances as one, or causing a weak peak to be masked by the tail of a strong peak. When inferring elemental composition, insufficient resolution also prevents the narrowing down of candidate molecular formulae. It can be said that resolution determines how ‘clear’ a spectrum is, and is a prerequisite for qualitative identification.

It is important to distinguish that resolution is not the same as mass accuracy. Resolution addresses the question of ‘whether adjacent peaks can be separated’, whilst mass accuracy addresses ‘how much the measured m/z differs from the theoretical value’. An instrument may have high resolution but poor accuracy due to a lack of calibration, and vice versa. Only when the two work in tandem—high resolution to separate the peaks and high accuracy to determine the m/z precisely—can molecules be reliably identified.

Neo Source INSTRUMENT’s online time-of-flight mass spectrometer, the ProC-1, as well as the ProC-1C in-situ catalysis/pyrolysis system and the ProC-1S flue gas analysis system, all achieve a mass resolution of 3,500 (at m/z = 92); the benchtop ProC-2 achieves 2,000. These specifications stem from their dual-field reflective time-of-flight (TOF) mass analyser—which achieves time focusing by compensating for ion kinetic energy dispersion via reflectors, thereby maintaining high resolution across a wide mass range. The same TOF architecture is utilised in two imaging sources, aiding in the differentiation of mass-close lipids and metabolites within complex biological matrices. MSI DPI and MSI LDPI can be retrofitted to mass spectrometers from Agilent, AB SCIEX and Thermo.