PEAK  MATCH

Department Of Chemistry, University College CorkIreland's No.1 for Accurate MassDouble Focussing Electron Impact, Mach3 Data System

Tel. +353 21 902015     Fax. +353 21 903110        Email: mach3@mssun.ucc.ie


Peak Matching (on a Kratos Profile High Resolution Mass Spectrometer)

Peak Matching (or peak match) is the most accurate method of mass assignment. It can produce measurements to better than 1 ppm deviation from the (calculated) accurate mass of an ion. This method is used for single ions only. In most cases the molecular ion is to be intensively investigated. For example, a low-resolution mass spectrum shows a molecular ion of 436 m/e (i.e. a mass of 436.0). To obtain the exact mass of this ion a high resolution (accurate mass) experiment is required. Instead of scanning a full mass range (e.g. mass 20-800) the mass spectrometer is set up to measure only a very small mass range around the molecular ion or ion of interest. Technically this is achieved by setting the magnet to a specific value and (only) scanning the ESA (electrostatic analyser) and accelerating voltage (which is linked to the ESA). To improve the preciseness of the result two mass peaks of a reference compound (whose exact masses are known) are used to bracket the molecular ion of interest (one reference mass above, one below). This bracketing sets two precise reference points when monitoring the molecular ion to allow for possible small drifts of the magnet to higher or lower masses (see graph below).

The experimental procedure is as follows: After setting the magnet in the region of the molecular ion, MI, by entering the nominal mass (e.g. 436) or an expected accurate mass (e.g. 436.09243), two suitable reference masses are (automatically) selected. The ESA is initially set to the low reference mass and scanned within a very small band, typically 500 ppm below to 500 ppm above, i.e. 1000 ppm (~MI ±0.2 mass units, mu). 10-15 of such scans are then recorded. The ESA is then set to the high reference mass and scanned in the same fashion. Finally the molecular ion is scanned also in the same fashion. If the molecular ion is visible in this region the scanning band is further decreased, typically to 150-600 ppm (~MI ±0.08 mu) and the whole procedure repeated. The results are then treated by a special computer program that averages and centroids the 10-15 scans (for further increase of accuracy) and then calculates the exact mass of the molecular ion. The exact mass can now be treated by a further computer program that calculates the molecular formula and its derivation from the theoretically expected molecular mass for that specific molecular formula. The obtained deviation is typically smaller than ± 5 ppm, which is acceptable for publication reasons.

The method for the calculation of the molecular composition of the molecular ion is best demonstrated with an example:

C10H10O3 has an exact mass of 173.06299 (main isotope), no other combination of atoms (within certain limitations) delivers the same precise mass. The computer program is then supplied with the possible atoms present in the molecule and their estimated numbers (e.g. C:10-20, H:10-40, N:0-3, S:0-1 etc.). It then calculates all possible combinations of the selected atoms and displays the results, i.e. molecular formulae with their deviations. Only one formula should result in a deviation smaller than ~1 ppm. A maximum deviation can be selected prior to computation, to narrow the list of displayed results.

Results for example:

Individual scans obtained for low reference, 
      high reference and molecular ion peak. Scan width lowered to 150 ppm in succesive groups
2. Averaged, centroided and calibrated peaks:
Individual scans of above graph are averaged, centroided and calibrated. The rightmlost graph shows the accurate (precise) mass of the measured molecular ion.

Formula search on scan 1 of xxx_yy#187

Mass

intensity

Deviation in ppm

C

H

O

178.06288

96287970

-0.63

10

10

3

Accurate
Mass

% Rel.
Abundance

12
C

13
C

1
H

2
H

16
O

17
O

18
O

1

178.06299

89.9082

10

0

10

0

3

0

0

2

179.06635

10.0918

9

1

10

0

3

0

0

Compiled/written by M.Jauch 12/97

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