Software Manual Section 24

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Manual Mass Peak Centering
	Introduction This program can be used to perform a mass peak centering routine manually. It is fairly flexible and automatically calculates the mass resolution. This program is also called several times by the analysis programs Iso and Trace, but the user interface is limited in these automated measurements. The main difference between this program and the 'High Mass Resolution Scan' (Section 8) is that this routine is optimized only for finding the center of a given mass peak in the shortest possible time. It does not perform a complete scan across the peak as the 'High Mass Resolution Scan' does.
	Manual vs. Automated Mode When this program is called up manually from the Menu window, it looks the way it is shown above. All of the buttons and numeric entry field are available in this mode. However, when this program is called as a subroutine from one of the analysis programs, only the bottom two panes (graph and curve statistics) are visible as shown here on the left. In this automatic mode a direct user interaction is not necessary/possible. In the individual description below all items that refer to the bottom two panes are valid for both modes.
	Measurement Conditions: B-Field (Old Center) The current magnetic field value is shown here. This display is both active input and output: It is possible to change the current B-field value both here and with the B-field knob on the ims3f. This magnetic field value is also called "Old Center" because it is the starting value of the peak centering routine. From here the programs moves to the left and right (i.e. to lower and higher B-field values) to find the 50% edges. Once these points are found, the new center is determined as the midpoint of the edges.
	Curve Statistics: Old Center Position The old center position is also shown in the graph and its B-field value is displayed here. This display and the graph itself is constantly being updated while this mass peak centering window is open.
	Measurement Conditions: Mass The corresponding mass is displayed here. Note that this is for display only; it is not possible to directly enter a value here. For a similar program that does offer the option to directly enter a mass value, see section 8 (High Mass Resolution Scan). The accuracy of the mass value calculation depends on the current mass calibration.
	Measurement Conditions: Mode By default, the program tries to go from the starting magnetic field value to the left and right to find both edges of the peak. This is the default mode "L+R" shown here. It is also possible to find a peak center by detecting only one edge (modes "L" and "R"). For this to work, however, the width of the peak (in M/dM) cannot be entered as 'zero'.
	Measurement Conditions: Width (M/dM) If there is information about the approximate width of the peak (expressed as mass resolution M/dM with dM measured at 50% of maximum), enter it here. A good estimate here will accelerate the automatic peak centering routine significantly. After the centering routine is started, the count rate at the 'old center' is measured first. Then the program jumps by half a peak width to the left and gets the count rate again. Depending on whether the count rate here is more or less than 50% of the maximum (the value measured in the 'old center'), it then moves more to the outside or inside to find the 50% point. Once this point is found, the programs jumps by a whole peak width to the other side and repeats the measurement steps there to find the other 50% point. If there is no information about the peak width, enter 'zero' here and the program will move to the outside in small steps starting directly at the 'old center'.
	Measurement Conditions: Detector Choose the detector that should be used for the mass peak centering (EM or FC).
	Measurement Conditions: Time/Step This is the counting time for each measurement step. In the automated mode, the counting time is increased if the count rate at the first measurement (old center) is too low. In the manual mode it is up to the operator to select a useful value.
	Measurement Conditions: Maximum Number of Steps How many steps is the program allowed to make (in each direction) before it should give up? A value of 100 may be necessary if the instrument is set to low mass resolution and no information is entered about the peak width. In the middle of an automated analysis, the peak center is usually found with less than 5 steps.
	Curve Statistics: Old Center Count Rate Once the peak centering routine is started, the count rate at the starting point (old center) is displayed in the graph and numerically shown here.
	Curve Statistics: 50% Count Rate Once the count rate at the 'old center' is determined, the corresponding 50% value is displayed as a red horizontal line in the graph. The routine then tries to find those two points where the peak outline intersects with this line.
	Curve Statistics: Edge Positions Once found, the B-field values for the left and right edge are shown here.
	Curve Statistics: New Center Position The new center position is displayed with a vertical green line in the graph and its magnetic field value is shown here.
	Curve Statistics: Mass Resolution M/dM (at 50%) The mass resolution, based on the peak width at 50%, is automatically calculated and displayed here. This is the value that should be entered as a starting value in the 'Width' field for the next run of the mass peak centering routine (assuming of course that the slit settings on the ims3f are not changed in the meantime).
	Curve Statistics: Peak Shift The peak shift is the difference between the old center and the newly determined one. It is displayed here after the new center has been found.
	Graph: Vertical Axis Scaling Choose between a logarithmic and a linear scale for the (vertical) count rate display in the graph. The minimum and maximum values of the axis are scaled automatically.
	Graph: Horizontal Axis Note that the horizontal axis show the B-field values and not the mass. For a similar program that does show the horizontal axis in mass units, see section 8 (High Mass Resolution Scan). The minimum and maximum values of the axis are scaled automatically.
	Start Click here to start the automatic peak centering routine. The current magnetic field setting is taken as the 'old center' starting position. During the centering routine it is possible to stop the measurement by clicking on the 'Abort' button that shows up after clicking on Start.
	Number of Steps The number of steps needed to find the left and right edges of the mass peak are shown here. Note that these numbers cannot get larger than the chosen 'Maximum # Steps'.
	Exit To exit this mass peak centering program, click here. The 'New Center Position' will be retained as current magnetic field setting.

FJ Stadermann	Back to Top	Next Section