how close the masses are. The closer those compounds match (ppm or Da), the more likely it is that the target
compound is a match.
Retention Time
Most accurate mass spectrometry systems use some type of chromatography. The SCIEX X500 QTOF System uses
liquid chromatography (LC). A set retention time using a known standard for the compound on the LC system is
used to identify a known target compound in a sample. If the suspected compound is in an unknown sample, then
the closer the retention time is to the set retention time from the standard, the more likely the unknown compound
can be identified. Retention times can change and must be updated.
Isotope Pattern
The full scan mass spectrum from a compound on an accurate mass system has a distinct isotope pattern based
on its molecular formula.
shows the isotope pattern for imazalil.
Figure 2-7 Isotope Pattern
Library Searching
Comparing acquired MS/MS spectra from unknown samples to a database of compounds with reference spectra
is one of the most powerful tools in qualitative analysis. Library search algorithms compare the unknown spectra
from the sample and then try to match the spectra to the known compounds and spectra in the database. The
closer the match and the higher the reported score are, the more likely it is that the compound was identified.
The purity, fit and reverse fit are calculated as follows:
• If there is a peak at a given mass in both the (reduced) library spectrum and the (reduced) unknown spectrum
whose intensity ratio is within the limits specified by the user, the intensity of the peak in the library spectrum
is set equal to that of the unknown spectrum
System User Guide
SCIEX X500 QTOF System
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Principles of Operation
