How To Use An Ionsonic Microscope

Transcription

1 Sample Analysis Design Element2 - Basic Software Concepts

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4 Scan Modes Magnetic Scan (BScan): the electric field is kept constant and the magnetic field is varied as a function of time the BScan is suitable for scanning large, continuous mass ranges to obtain a complete spectrum from an unknown sample. for repeated scanning, a fast flight-back follows each scan depending on the (mass) distance for the flight-back, a settling time for the magnet must be allowed the settling time takes into account the time needed for the magnet to recover from the fast flight-back of each cycle this scan mode is also used for Mass Calibration in order to account for the non-linear behavior of the magnet

5 Scan Modes Electric Scan (EScan): the magnetic field is kept constant and scanning is performed electrically by varying the accelerating voltage and the ESA high voltage this scan mode is the fastest scan the EScan starts with setting the magnet to a mass at the lower end of the Mass Range the available mass range for the electric scan depends on the magnet mass; it ranges from -6% to +30% of the magnet mass

6 Scan Modes EScan advantages: fast scans can be conducted over the detectable mass range without huge energy transfer in the magnet = this results in settling times that do not have to be observed even when performing the fastest repeated scans thus possible to scan the mass ranges of the isotopic pattern and to skip the ranges of no interest possible to carry out replicate measurements in quick succession because of the low energy transfer the advantages of EScan make it the preferred and default scan mode

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9 Integration Window (%): Expressed as a percentage of the peak width, is used to calculate peak intensity. A value of 100% means that the intensity will be calculated from samples, measured in a window of ± ½ peak width of peak center Peak Search Window (%): This specifies the mass window, where the software performs a search (calculation) for the peak center. For example, a value of 100% means that all samples are used in a window of ± ½ peak width of the accurate mass as specified during the elements/isotope selection.

10 Samples per Peak In order to establish a minimum level of precision, the ion signal (peak) must be measured several times during the scan

11 Samples per Peak Recommended values for Samples per Peak as a function of the mode of resolution: -10 samples for low resolution measurements - 20 samples for medium resolution measurements - 30 samples for high resolution measurements

12 Detection Modes

13 Detector Modes Counting Mode: is a digital measurement (counting) and counts events instead of ion signal height it is therefore very sensitive and useful especially for the detection of low signals (concentrations). during acquisition, the number of occurrences is used to generate the intensity information that is stored in a data file this mode of operation can be used in the detection range from 0 to 5 x 10 6 counts per second (cps)

14 Detection Modes Analog Mode: is a standard detection mode each time an ion signal exceeds a certain threshold, the height above the threshold delivers the intensity information (that is stored in a data file) the detection range is between 10 4 and 10 8 cps

15 Detection Modes Both Mode: the analog and the counting signals are monitored by the hardware continuously should the ion signal exceed the security threshold ( trip ) of the ion counter (5 x 10 6 cps), then the protection device disables the counting part until the intensities are below the trip level (if applicable) Note a cross calibration between modes is done automatically

16 Element2 Software Concepts MASS CALIBRATION

17 Mass Calibration The expectation of an ICP-MS user is to see the exact information about peaks, mass numbers, etc. when looking at the computer screen Unfortunately, the computer hardware has no idea about mass and intensity of ions or peaks in a mass spectra

18 Mass Calibration The computer knows DAC (Digital-to-Analog Converter) steps for setting the magnetic field of the instrument and the ADC steps (Analog-to- Digital Converter) for a received intensity The Mass Calibration program translates and adjusts the information available by the computer hardware into the information an ICP- MS user really needs; i.e. mass numbers and intensities for single ions and isotopic patterns

19 Mass Calibration The computer translates DAC values into mass numbers. The DAC values are used for setting the strength of the magnetic field, which modifies the direction of the ions. The hardware is referred to as Magnet DAC or MDAC The actual translation table, the Mass Calibration Table, is stored on the computer s disk in the data directory as <name>.mcl

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