Software Manual Section 08

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High Mass Resolution Scan
	Introduction This high resolution mass spectrum program scans a small range around a mass peak to show its shape and to determine the mass resolution. Using this program is essential during tune-up of the instrument for high mass resolution measurements. Right now only a single mass range can be scanned at a time. Multiple ranges (e.g. 12 +/- 0.01 and 13 +/- 0.01) may be added in the future.
	Mass Enter the mass (in amu) here that will be in the center of the mass range. Alternatively, you can manually adjust the B-Field to the desired center of the scanning range and this display will update accordingly.
	Scan +/- Enter the desired range of the mass scan (in amu). Note that the value shown here will result in a scan from (mass - 0.005) to (mass + 0.005), i.e. the total range will be 0.01 amu (2 x 0.005).
	Direction The usual way to acquire a mass spectrum is from low to high masses ("Up") and the results shouldn't really be different if the scan was made in the other direction ("Down"). In some cases, however, it may be helpful to go from high masses to low masses, e.g. if a 'memory' effect of the detector is suspected or if the sample sputters away rapidly.
	# of steps Enter the number of mass steps for the entire scan range. Entering a very low number (e.g. 10) here will result in a faster measurement, but the determination of the mass resolution will be less accurate.
	Time/Step (s) Enter the desired counting time per step. Low counting times will lead to faster measurements, but more statistical 'noise'.
	Detector Mass spectra can be acquired with either the electron multiplier or the secondary Faraday cup.
	Reset This button will reset all measurement conditions (mass range, step sizes, detector etc.) to the ones used during the previous scan. This is important for the following reason: After a mass scan has been made, the curve will be shown in the plot area. In many cases you may now find that the area was not well centered or that the scanning range was too small or too wide. It is now possible to change the values of the measurement conditions with the controls described above in order to optimize everything for the next scan. While these changes are being made to the controls, the plot area will automatically update to reflect these changes without actually re-scanning the mass range.Thus, it is e.g. possible to change the scanning range until the peak(s) fit just perfectly into the plot area. Only then a new mass spectrum needs to be acquired to reflect the new, optimized measurement conditions. And finally, if everything gets messed up, a simple click on "Reset" brings everything back to its original setting.
	Start Click here to start a measurement. The "Start" button will change to "Running" for as long as the measurement is active.
	Abort This button will only show up during a measurement. Clicking "Abort" will immediately stop a measurement. More precisely, the measurement will be stopped after the current measurement step. If a long counting time per step is selected, you may notice some delay.
	Horizontal Axis The horizontal axis shows the mass scale in amu. Remember that in many cases this mass axis is not absolutely calibrated, while the relative calibration is typically quite exact. For example: in the picture shown on the left, you cannot be certain that the "13.0025" mark is actually at 13.0025 amu. However, you can usually assume that the relative distance from the "13.0025" mark to the "13.0050" mark is exactly 0.0025 amu.
	Vertical Axis There are different controls for the vertical axis: "Auto Scale" automatically adjusts the Y axis to show the entire range of measurements and "Manual" can be used for a customized range. It is also possible to switch from "Linear" to "Logarithmic" scaling. The four up/down buttons next to vertical axis can be used to change the upper and lower limits of the displayed range. Pressing one of these buttons will switch the scaling mode to "Manual".
	Previous Curve Choose whether you want to see the previous curve in the plot area. This function can be useful during tune-up of the instrument to see whether you have made any progress in improving the shape of the mass peak. The previous curve will be shown in pale pink 'behind' the current curve. If the previous curve has been acquired under different measurement conditions, only that part of the previous curve will be shown that fits into the current mass range.
	Cursor This display has two different modes of operation: During a measurement it displays the current mass and the last reading of the signal. The signal is displayed as counts ("Actual Counts") and as counts/second. After a measurement, a black vertical line (the cursor, hence the name of this display) will show up in the plot area. This cursor can be moved around by just clicking somewhere in the plot area. The displayed mass and count rate are always from the current position of the cursor. It is thus possible to get exact numerical information for any part of the plot.
	Curve Statistics After each scan, several basic parameters are automatically calculated and displayed: The Maximum Count Rate in counts/second is shown and its location is highlighted by a small blue circle in the plot area. The Position of the Center of the largest peak is identified by its mass and is shown in the plot by a vertical orange line. Note that the center is not necessarily identical with the maximum of the peak. Instead, the center is calculated as the midpoint between both 10% marks (see below). The Mass Resolution M/dM is automatically calculated. For this, the program first locates the two points on the left and right edge of the peak where the count rate is 10% of the maximum count rate. These 10% marks are highlighted by green crosses in the plot. From these point the mass resolution is calculated. In addition, the program also calculates a the Width of the Slope on one side of the largest peak, a number which may be helpful for determining the qualitity of the tune-up. Incidentally, this number is closely related to CAMECA's definition of the mass resolution. Here, the program first finds the 90% and 1% points on the edge of the peak. These points are highlighted in the plot with black dots. The slope value is then calculated with this formula: Slope = (m90+ m1) / (m90 - m1) / 4 This formula brings the value of the slope into the same range as the value of the mass resolution. A steeper (i.e. better) slope leads to a higher value. All of these curve statistics may be helpful, but it is still necessary (*sigh*) to keep the own brain turned on. In cases where the 1%-, 10%- or 90%-points are outside the selected scanning range or when neighboring peaks overlap, the values calculated here are most likely meaningless.
	Exit Click here to leave the HiRes program. If a measurement is running you have to click "Abort" first and then "Exit".
	Print & Save Buttons for printing the plot and saving the data to a file still have to be added.

FJ Stadermann	Back to Top	Next Section