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TOF 算法TOF 算法.pdf
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TOF 算法TOF 算法.pdf
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Superior Molecular Formula Generation
from Accurate-Mass Data
Technical Overview
Abstract
When using mass spectrometry to analyze samples containing unknowns, it is
often necessary to derive elemental compositions (molecular formulas) for the
unknowns based on the mass spectral data. For lower-mass compounds,
accurate-mass measurements may be sufficient to produce a single, conclu-
sive molecular formula or a small number of possible formulas. With increasing
mass, however, the number of possible formulas increases dramatically.
Additional constraints must be applied to reduce the list of candidate formulas
to a manageable number. Agilent MassHunter Workstation software includes
proprietary molecular formula generation (MFG) software. This software takes
full advantage of the mass accuracy of the data, and then uses additional mass
spectral information to logically narrow the list of possible formulas. It reduces
ambiguity and delivers a list of candidate molecular formulas with scores
based on the relative probability that each formula is the correct one. This
significantly reduces data interpretation time and increases the value of
accurate-mass analysis.
Authors
Ed Darland, Doug McIntyre, David Weil,
Frank Kuhlmann, and Xiangdong Li
Agilent Technologies
Our measure is your success.
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2
Superior Molecular Formula Generation from Accurate-Mass Data
The challenge of molecular formula
generation
Molecular formula generation from MS
data is important for identification of
endogenous and exogenous metabolites,
drug impurities, synthesis products,
pharmaceutical degradation products,
and for many other applications. The
task is difficult because the number of
plausible elemental compositions grows
very rapidly with increasing molecular
mass, and with the number of possible
elements. For example, at a mass of
509.978 u, with only C, H, N, O, S, Cl,
and F allowed in the composition, there
are more than 250 possible formulas
within a ±2.5 ppm mass window. Such
a large number of possibilities makes it
very hard for researchers to ascertain
the molecular formula using mass
information alone.
A recent study
1
concluded that even
1-ppm mass accuracy is not, by itself,
sufficient to narrow the list of possible
molecular formulas. For molecules with
higher masses, even expensive orbital
trapping and Fourier transform mass
spectrometers will fail to provide a
concise list of possible formulas based
solely on accurate mass. The same
study, however, concluded that addi-
tional constraints such as isotope
pattern matching can rule out most of
the candidate formulas.
Overview of molecular formula
generation
As noted previously, accurate-mass
measurements are, by themselves, often
insufficient to conclusively determine
the elemental composition of unknown
compounds. The unique Agilent molec-
ular formula generation software,
however, can incorporate additional
information such as:
• Isotope masses, abundances, and
spacing
• User knowledge of sample
composition (allowed elements)
• Accuracy of the mass measurements
• Masses of MS/MS fragment ions and
neutral losses (if available)
Mass accuracy defined
Mass accuracy refers to the difference between a measured mass and a
theoretical or calculated mass. Mass accuracy (also called mass error) is
typically expressed in parts-per-million (ppm). It is calculated as follows:
(m
measured
– m
calculated
) x 10
6
/ m
calculated
For example, if the theoretical mass is 500 u and the measured mass is
500.005 u, the mass error is:
(500.005 – 500) x 10
6
/ 500 = 10 ppm
into its calculations (see Figure 1). By
doing so, it can generate a much smaller
and more relevant list of candidate
molecular formulas. For each compound,
the software scores the possible
molecular formulas based on their
relative probabilities and assigns each
formula a relative rank.
MFG also works in concert with the
MassHunter software’s proprietary
molecular feature extraction (MFE)
algorithms. MFE does an outstanding job
of identifying the relationship between
multiple forms of a single compound
(e.g. adducts). If multiple forms of the
same compound are present, MFG
combines their scores into a single
“cross score.”
www.agilent.com/chem/ms
3
Technical Overview
6500 Series Quadrupole Time-of-Flight
LC/MS systems. Conversely, the super-
lative mass accuracy and consistency of
the 6200 Series and 6500 Series data
make it easier for the MFG algorithms to
generate the best possible results.
MFG is applicable across a wide variety
of applications that require identification
of unknown compounds. Accordingly,
Agilent has incorporated this sophis-
•
Accurate mass
•
Isotope abundance
distribution
•
Mass spacing between
isotope peaks
•
Allowed elements
•
Minimum and maximum
number of each element
•
Maximum mass error
•
Accurate mass of each
fragment ion
•
Accurate mass of neutral loss
from precursor to each
fragment ion
Isotope
pattern
MS/MS
fragments
(when
available)
C12 H14 N9 CL
C13 H23 N3 P S CL
C14 H24 N2 S2 CL
C14 H22 N O3 S CL
C15 H25 O P S CL
C16 H19 N3 P CL
Molecular
formula
generation
MS/MS
MS
User input
Figure 1. Agilent’s molecular formula generation software uses multiple dimensions of information to generate and score lists of possible molecular
formulas. It has been optimized for analysis of accurate-mass data from Agilent 6200 Series TOF and 6500 Series Q-TOF LC/MS systems.
ticated algorithm into the following
Agilent software:
• MassHunter Workstation software
(qualitative analysis module)
• MassHunter Metabolite ID software
• MassHunter Profiling software
• GeneSpring MS software
By dramatically reducing the number of
plausible formulas, and by using unique
scoring methods, the MFG software
reduces data interpretation time and
increases the value of the accurate-mass
analysis.
The MFG algorithms have been designed
specifically to produce optimum results
from the accurate-mass data generated
by Agilent 6200 Series Time-of-Flight and
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