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After a mass spectrum is acquired, the analyst is responsible for the interpretation of that spectrum. In this section, the following topics will be presented:
Accountability
Two methods used by the analyst for spectral evaluation are library search and spectral interpretation. In either case, it is the responsibility of the mass spec operator (or data reviewer) to make an assessment of the data.
Every analyst should have some ability to interpret spectra even if it is used only to confirm the quality of library matches.
Initial EvaluationWhen comparing two spectra, one should expect to see the most abundant peaks in the library spectra present in the acquired spectra. The peaks should be in the same relative abundance.
m/z=194 base peak in both spectra
m/z=109 relative abundance 45 percent in both spectra
In addition, the analyst must determine if the acquired spectrum has sufficient sensitivity for comparison to the library spectrum.
Library Search
In a library search, a computer is used to match an acquired spectrum to a database of spectra by peak matching. This is a brute force method that uses the only two variables available, the mass-to-charge ratios in the spectrum and their relative abundances.
Using a computer to do a library search is one way to eliminate a lot of possibilities from a large database of compounds. A computer search does not guarantee the "correct answer."
Spectral Interpretation
Interpreting a mass spectrum is an intellectual exercise where the analyst applies a series of rules to solve a puzzle. Spectral interpretation is done by a chemist and is based on that person's knowledge of chemistry and their knowledge of fragmentation to determine the identity of the molecule.
Target Compound Analysis
Target compound analysis is the analysis for the presence, absence or quantification of specific compounds. One requirement is the presence of a major quantitating ion, which in most cases is the base peak. In addition, the presence of at least two qualifying ions, which are chosen on the basis of their specificity for confirming the identity of the compound, is required in target compound analysis.
Becoming Proficient
The mass spectrometer operator or the data reviewer ultimately is responsible for making the assessment is this the correct match. To become proficient at interpreting mass spectra requires practice and experience.
Summary
There are multiple mass spectrometer parameters that must be adjusted in order to obtain quality spectra. The analyst or data reviewer is responsible for ensuring the quality of the MS results.
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