Trapping frequency calibration

Usually, ions of tn/z 69 and 414 from the calibration chemical PFTBA are used to find the values for the two parameters, A and B. The trapping frequency calibration is carried out near = 0.845 at a fixed RF trapping field. A linear relation between q xm and RF c is true only under ideal conditions but, as a first-order approximation, it works well for this two-step isolation method in a non-ideal quadrupole ion trap. Typically, in an ion trap for which the oscillation frequency of the RF potential is 1 MHz, the frequency error of the calibration is less than 1 kHz, which corresponds to an error of < 1 Th in the high-mass isolation step. The amplitude of the broadband waveform is determined empirically by the manufacturer and can be accessed by users. 

SFI is simple in principle, requiring application of the resonant excitation voltage at the axial secular frequency for the isolated ion of selected miz value. While this frequency may be readily calculated theoretically, the actual secular frequency of ions focused near the center of the trap in a relatively dense ion cloud is subject to frequency shifts arising from space charge effects (that vary with the concentration of analyte in the ion source and thus the number of ions in the trap), nonideal trap geometry, and the inadequacies in automated frequency calibration. Thus, in practice, SFI involves a laborious procedure in which an applied frequency waveform is matched empirically to the actual secular frequency of the isolated ions. 

We have since verified that there has been no noticeable loss of mass resolution or sensitivity for the Esquire 3000+ using a modified ring electrode that has 2x1.5 mm laser holes and a single 2 mm fluorescence hole. Mass calibration and tuning the phase between the drive frequency and the auxiliary AC were necessary to re-establish ion trap performance. Our experimental peak width (both with and without ring electrode holes) is about 0.3 Th for rhodamine 101, or about half that of the simulation. Possible reasons for this discrepancy include inaccuracies in the shapes of the model electrodes, insufficient cooling time in the model prior to ion ejection (only ca 2 ms was used to shorten the length of the simulation), or an incomplete description of the ion ejection waveform. 

Figure 11.7. (A) After excitation, the summation of all ions trapped within the analyzer cell produces a transient signal that is digitized and stored by the data system. A fast Fourier transform (FFT) is applied to the transient (B), producing a frequency spectrum that is then converted into a mass spectrum via a calibration equation. (Reprinted from Ref. 161 with permission.)...

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