Mass scale alignment of time-of-flight mass spectra
View Patent ↗The invention generates mass scale comparability between mass spectra which are acquired in time-of-flight mass spectrometers, particularly with ionization by matrix-assisted laser desorption. Always slightly distorted mass scales of different mass spectra from the same type of sample can be aligned. The flight times of identical ions always differ slightly from one mass spectrum to the next due to non-reproducible processes in the ionization method. Thus the apparent mass values of ion signals of identical substances in different mass spectra do not match even if the flight times are converted into mass values with the identical calibration equation. After alignment of the mass scales, mass spectra can be reliably compared with respect to deviations in intensities of bio-makers, or be added together without deterioration in the mass resolution, and improved reference spectrum libraries can be created. Furthermore, the invention allows more reliable library searches to be carried out.
1. Method for generating comparability between two time-of-flight mass spectra, comprising the following steps:
a) selecting a number of solitary characteristic ion signals each within a predetermined mass region throughout the first mass spectrum,
b) selecting characteristic ion signals in the second mass spectrum, each within a small tolerance interval around one of the characteristic ion signals of the first spectrum,
c) comparing the intensity pattern of the two sets of characteristic ion signals with predetermined intensity tolerance values for the intensity ratios, and eliminating those ion signals which do not match,
d) calculate shift and expansion parameters from the two sets of characteristic ion signals, and
e) shifting and expanding either the second mass spectrum to match the first spectrum, or shifting and expanding both mass spectra to form an average mass spectrum.
2. Method according to claim 1 , wherein the mass spectra are represented as lists with mass values and intensity values.
3. Method according to claim 2 , wherein the lists additionally contain the widths of the ion signals in mass units.
4. Method according to claim 3 , wherein checking the widths of the characteristic ion signals is used to avoid the inclusion of overlapping ion signals.
5. Method according to claim 1 , wherein the mass spectra are represented as lists with flight time values and intensity values for the ion signals.
6. Method according to claim 5 , wherein the lists additionally contain the widths of the ion signals in time units.
7. Method according to claim 6 , wherein checking the widths of the characteristic ion signals is used to avoid the inclusion of overlapping ion signals.
8. Method according to claim 1 , wherein the time-of-flight mass spectra are acquired by ionization by matrix-assisted laser desorption.
9. Method according to claim 1 , wherein the alignment of the ion signals is iteratively improved by using more characteristic ion signals.
10. Method according to claim 1 , wherein an additional transformation with a quadratic term is used to obtain mass spectra which better match at the lower and higher mass end.
11. Method according to claim 1 , wherein mass spectra are grouped together to form average spectra after they have been aligned.
12. Method according to claims 1 , wherein a spectrum library contains library mass spectra, and wherein each library mass spectrum already contains a list of characteristic ion signals for assisting the alignment of further spectra.