• The purity is calculated as:
100.0 (UL
total
)
2
/ (U
total
•L
total
) where:
U
total
= ∑ U
m
•U
m
L
total
= ∑ L
m
•L
m
UL
total
= ∑ U
m
•L
m
The sums include all masses where the intensities U
m
and L
m
are the square roots of the mass-weighted, that
is reduced; unknown; and library entries. The purity is guaranteed to fall in the range between 0 to 100 and
is a measure of the similarity of the library spectrum and the unknown spectrum.
• The fit is calculated in exactly the same manner as the purity, except that only masses which occur in the library
spectrum are included in the sums. This has no effect on L
total
or UL
total
because no terms are deleted from
these sums. The fit is a measure of the degree to which the library spectrum is contained in the unknown
spectrum. A high fit and a low purity indicates that the unknown spectrum is likely impure, but contains the
library compound.
• The reverse fit is also calculated in the same manner as the purity, except that only masses that occur in the
unknown spectrum are included in the sums. The reverse fit is a measure of the degree to which the unknown
spectrum is contained in the library spectrum.
Formula Finding
Using an accurate mass number, the formula finding algorithm tries to predict the possible chemical formula for
the compound, based on the MS and MS/MS spectrum generated from an accurate mass, mass spectrometer. It
is important to know that a high formula finder score does not necessarily mean that the compound in the sample
is the one identified by formula finder as there are often several formula that match within a few ppm of mass
error. Care must be taken and other confirmatory testing must be done before a compound is identified using
formula finding.
A list of possible formula is determined using precursor mass accuracy, isotopic pattern, and MS/MS fragmentation.
Proposed formulas are scored based on precursor mass accuracy and average MS/MS mass accuracy of matching
fragments.
SCIEX X500 QTOF System
System User Guide
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RUO-IDV-05-2334-E
Principles of Operation
