************************************************************************ From: "Manfred Schwarz" Date: Thu, 24 Aug 2000 16:13:13 +0200 Subject: Definition of Mass Resolution Organization: T-Online Hello, could anyone give me a definition of mass resolution and how it is calculated for a quadropole mass spectrometer. And please give me an example or the resolution for the 5973N Agilent Mass Spectr. Is there a difference in the calculation between Ion Trap and Quadropole? Best Regards Manfred ************************************************************************ From: Rob Bossio Date: Thu, 24 Aug 2000 12:12:57 -0400 Subject: Re: Definition of Mass Resolution Organization: Florida State University I'll give this one a try. Mass resolution is defined as m2/(m2-m1) where m2 is the higher m/z ratio, and m1 is the lower m/z ratio. It seems pretty simple, but, there are permutations. The two permutations are where you measure where m2 and m1 start or stop. The two criterion I know of are at 10% and 50% of the height of the peaks you are looking at. So if you were to see a distinct peak jutting out at half of one peaks intensity, you would measure your m2 and m1 from there, if you were using the 50% criterion. If you were using the 10% criterion, you would say these two "peaks" are unresolved. There is another criterion, called the mass resolving power, which is measured as m/(delta m), usually at 50% of the peak height. It is easy to confuse the two, but they are distinctly different criterion for figures of merit in mass spectrometry. Delta m is the width of that individual peak at half the height. Mass resolving power is a measure of how "fat" a peak is, the bigger the number, the skinnier the peak. Mass resolving power tells you about one peaks ability to "hide" another underneath it. Mass resolving power and mass resolution are certainly related; the higher your mass resolving power, the more likely you will have resolved two peaks. I unfortunately haven't worked with the agilent instrument you mentioned, hopefully someone else here has. Hope I answered your question Rob Bossio ************************************************************************ From: "O_DavidS" Date: Thu, 24 Aug 2000 16:03:20 -0700 Subject: Re: Definition of Mass Resolution Organization: * The Resolving Power of a mass spectrometer, R, is M1/delta M, where M1 is the lower value of two adjacent mass spectral peaks or the m/z value of the peak used to calculate R. Delta M can be calculated by the comparison of two adjacent mass spectral peak of about equal height. The two peaks will have an overlay (valley) of 10% or 50%. In the 10% valley definition, the height from the baseline to the junction point of the two peaks is 10% of the full height of the two peaks. Each peak is contributing 5% to the height of the valley. In the 50% definition, this height is 50% of the full height of the peak. Because it may be difficult to get two mass spectral peaks of equal height adjacent to one another, another method of calculating delta M is to use the full width at half maximum (FWHM) of a single m/z value peak. Resolving power calculated using the FWHM method gives values for R that are about twice that determined by the 10% valley method. You can also calculate the resolving power by using the full width at 5% full maximum. This will result in an R value close to the 10% valley method. However, this is very difficult because of the background signals. Resolution is simply Delta M. Resolution represents the smallest differences in m/z values that can be separated. Quadrupole ion traps(QITs) and transmission quadrupole (TQ) mass spectrometers, like the Agilent instrument, operate at constant resolution. This means that the ability to separate ions at m/z 100 and m/z 1000 is the same. If delta M is 1 m/z unit at m/z 100, the resolution at m/z 100 is 1 and the resolving power (R ) is 100/1 or 100. If delta M is 1 at m/z 1000, the resolution at m/z 1000 is also 1, but the resolving power (R ) at m/z 1000 is 1000/1 or 1000. QIT and TQ mass spectrometers operate at constant resolution with increasing resolving power with increasing m/z value. The resolution specification for the Agilent MSD is +/- 0.3 m/z throughout the m/z scale. Time-of-flight (TOF) mass spectrometer and instruments using both electric and magnetic fields to separate ions, like the JEOL double-focusing GCMate and LCMate, operate at constant resolving power (R ). At a resolving power of 1000 throughout the m/z scale, these instruments will separate ions of m/z 1000 and m/z 1001. In this example delta M is 1 and M is 1000; therefore, R = 1000/1 or 1000. This property of constant resolving power over the entire m/z scale means that with a resolving power of 1000 values of 0.1 m/z can be separated at m/z 100, e.g. R = 1000 = M/[delta M] = 100/0.1. This means that the resolution increases with decreasing m/z values in these types of instruments. If the isotope peaks of a single-charge ion are separated at m/z 1000 (where the m/z difference in the isotope peaks is 1) in a TOF or a double-focusing mass spectromter, then the isotope peaks of an ion of the same mass with 10 charges will be separated at m/z 100 (1000/10), where the m/z difference of the isotope peaks is 0.1 m/z. You can read more about this in my book "Mass Spectrometry Desk Reference", Global View Publishing, Pittsburgh, 2000 (http://www.LCMS.com) Regards; David -- O. David Sparkman Consultant-At-Large ods@compuserve.com