Mass spectrometer resolution

Definition and explanation:

Several different definitions of resolution are used in mass spectrometry.

Understanding the differences between these definitions is useful for understanding the characteristics of different mass spectrometers.

Unit resolution means that you can distinguish each mass from the next integer mass. In other words, you can distinguish between mass 50 and mass 51, and you can also distinguish between mass 1000 and mass 1001. This definition is commonly used when discussing the resolution of quadrupole and ion trap mass spectrometers.

The peak shape of a quadrupole mass spectrometer is typically ‘flat-topped’.

Magnetic sector mass spectrometers define resolution as:

That is to say, the mass number of the observed mass divided by the difference between two masses that can be resolved.

Magnetic-sector mass spectrometers produce peaks with triangular or Gaussian distributions.

In a magnetic sector mass spectrometer, a peak is typically defined as a 10 per cent trough, i.e. one-tenth of the height of the higher of the two peaks. If you have only one peak, you can estimate the resolution by dividing the peak width at the 5 per cent level by the mass of the observed peak. The resolution value defined above is constant across the mass range. The 10 per cent valley definition is generally considered sufficient to resolve small isotopic peaks.

Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometers use the same resolution definition as magnetic sector mass spectrometers. However, a resolution defined as 50 per cent of the valley is typically used, as the peak shape of the de Lorentz curve widens near the baseline without expansion. In FTICR, resolution is inversely proportional to mass; therefore, it is important to know the mass at which a given resolution is achieved in order to determine the resolution for another mass. Time-of-flight mass spectrometers also typically use the 50 per cent peak height definition. In time-of-flight instruments, the peak shape follows a Gaussian distribution.

Consider the distinction between the unit resolution definition and the resolution definition as defined in magnetic sector mass spectrometers. If we have a resolution of 5,000 on a magnetic sector mass spectrometer, we can separate m/z 50.000 and m/z 50.010, or m/z 100.000 and m/z 100.0200, or m/z 1000.000 and m/z 1000.200 (all within a 10 per cent valley between the two peaks). Unit resolution will allow you to distinguish between m/z 50 and m/z 51, or between m/z 100 and m/z 101, or between m/z 1000 and m/z 1001.