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Unformatted text preview: Laboratory 2: Accurate Mass Techniques Introduction A Couple of Definitions There are few terms that will be referred to in this laboratory that may not have been defined in lecture, thus they will be described here. You can read more about the concepts explored during this experiment in Gross chapters 3.3 (accurate mass techniques), various portions of 6 (nitrogen rule, DBE calculations), and 11 (electrospray ionization) Resolving Power and Resolution The ability of a mass spectrometer to differentiate between ions very close in m/z ratio is referred to as resolution. Resolution is almost always less than 1 and is computed as follows: m m R / = where R is the resolution, m is the smallest resolvable difference in m/z in Th, and m is the m/z of the ion in question in Th. It should be noted that many mass spectrometrists use the full width at half maximum (FWHM) for the peak as m. Smaller resolution values imply improved precision. Mass spectrometry companies and mass spectrometrists prefer to use the resolving power figure of merit instead. This number is almost always larger than 1 and is simply the inverse of the resolution. Thus: m m RP = / where RP is the resolving power, m is the smallest resolvable difference in m/z in Th, and m is the m/z of the ion in question in Th. It should be noted that many mass spectrometrists use the full width at half maximum (FWHM) for the peak as m. Resolving powers are preferred because they represent the highest m/z for which a difference of 1 can be resolved. For example, if a mass spectrometer has a resolving power of 5,000, the smallest mass difference that can be resolved at m/z 5,000 is 1 Th. Also, resolving powers increase as resolution improves, so marketing executives can put bigger and bigger numbers in their sales brochures. In this experiment, resolving powers will be computed and m will be the peak width at half height. Although it may not appear so it first glance, resolving power does impact mass accuracy. A narrow peak can have its centroid defined more precisely (remember, the peak width at halfheight for a guassian distribution equal to the standard deviation of the mean). Thus, higher resolving power instruments are capable of making more precise measurements. Precise measurements can be accurate measurements if the instrument is well calibrated. Electrospray Ionization Electrospray ionization (ESI) is a method for generating ions that involves dissolving the analyte in a slightly conductive, polar solution and infusing it through a needle in a large electric field (usually several tens of kV/m). The electric field generates a very fine spray of very highly charged droplets. These droplets dry and eventually ions evaporate from their surfaces. Ions are generated by charge transfer (usually protons or alkali metals). ESIgenerated ions of organic species are even electron ions. Thus, it is important to remember that for even electron ions, the nitrogen rule is inverted. rule is inverted....
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 Spring '08
 DavidE.Clemmer

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