Mass Spectrometers: Defining Resolution

Since several different definitions of resolution are used in mass spectrometry, it is useful to understand the distinctions between the different definitions to understand the characteristics of different mass spectrometers.

Unit resolution means that you can separate each mass from the next integer mass. That is, you can distinguish mass 50 from mass 51, and you can distinguish mass 1000 from mass 1001. This definition is commonly used when discussing resolution on quadrupole and ion trap mass spectrometers. Peak shapes in quadrupole mass spectrometers are usually "flat-topped."

Flat-topped peaks, roughly unit resolution at m/z 500

Magnetic sector mass spectrometers define resolution as m/Dm, that is, the difference between two masses that can be separated divided by the mass number of the observed mass. Magnetic sector mass spectrometers have triangular or Gaussian peak shapes.

Peak shapes from a magnetic sector MS (resolution ~ 1,000)

In magnetic sector mass spectrometers, peaks are usually defined to be separated down to a 10% valley, that is, a point that is 1/10 of the height of the higher of the two peaks. If you only have one peak, then you can estimate the resolution by using the peak width at the 5% level divided by the mass of the observed peak. The resolution value as defined above is constant across the mass range. The 10% valley definition is usually considered adequate for resolving small isotope peaks.

Fourier transform ion cyclotron resonance (FTICR) mass spectrometers use the same resolution definition as magnetic sector mass spectrometers. However, it is common to use a 50% valley definition for the resolution because of the broadening near the baseline of the unapodized magnitude-Lorentzian peak shape. Resolution is inversely proportional to mass in FTICR, so it is important to know the mass at which a given resolution was obtained in order to determine what the resolution should be at another mass.

Time-of-flight mass spectrometers also usually use the 50% peak-height definition. Peak shapes in time-of-flight are Gaussian.

Consider the difference between the definition of unit resolution and resolution as defined in a magnetic sector mass spectrometer. If we have 5000 resolution on a magnetic sector mass spectrometer, we can separate m/z 50.000 from m/z 50.010, or separate m/z 100.000 from m/z 100.020, or separate m/z 1000.000 from m/z 1000.200 (all down to a 10% valley between the two peaks). Unit resolution would allow you to distinguish m/z 50 from m/z 51, or distinguish m/z 100 from m/z 101, or distinguish m/z 1000 from m/z 1001.

Conclusion

We've learned to think of resolution, in general, as peak width at half-height, but as you now know resolution can mean very different things on different mass spectrometers. Knowing your instrument and the implications of resolution for it will help you work smarter and more efficiently.

For more information: Robert B. Cody, product manager for Mass Spectrometry, JEOL USA, Inc., 11 Dearborn Rd., Peabody, MA 01960. Telephone: 978-535-5900. Fax: 978-536-2205.