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Also see "How Do Isotopes Affect My Mass Spectrum?" Resolution maybe defined by instrument's ability to resolve two adjacent peaks. To examine this concept, one can look at molecular isotopes. The following mass spectra are of chlorophyll-a at different resolutions.
At the highest resolution shown all the different isotopic distributions are well separated and the MWmono 892.5 u is easily distinguished.
At resolution two or the intermediate resolution shown, the isotope distributions are just distinguishable and the MWmono 892.5 u has been distorted by the blending of the isotope peaks. The apex is now shifted to 892.6 u.
At the lowest resolution shown, the isotopic distributions have merged and only a broad peak is observed. Notice now the apex of the peak is approaching MWave 893.5 u. and if the centroid of the peak was calculated instead of the apex, this number matches MWave of 893.5 u.
As the molecular weight (MW) increases so does the difficulty in obtaining isotopic resolution. When studying large molecular weights such as human P450 3A4 most instruments are incapable of providing isotopic resolution as seen below.
Rather, a nearly Gaussian shaped curve as shown below is observed in the mass spectra from most instruments.
Even though most instruments are incapable of obtaining high resolution at high mass, low resolution mass spectra provide surprising accurate average molecular weights (MWave). As discussed in the isotope section, MWmono at high mass is an impractical measurement, so typically low resolution at high mass is adequate for most applications.
These spectra are computer generated using Bruker software. |
Resolution One
Resolution Two
Resolution Three
Human P450 3A4 high resolution