Digital ion trap
The timing of the high voltage switch is controlled precisely with digital electronic circuitry.
[2] Sheretov[3] also implemented the pulsed waveform drive for the quadrupole ion trap working in mass-selective instability mode, although no resonance excitation/ejection was used.
The idea was substantially revisited by Ding L. and Kumashiro S. in 1999,[4][5] where the ion stability in the rectangular wave quadrupole field was mapped in the Mathieu space a-q coordinate system, with the parameters a and q having the same definition as the Mathieu parameters normally used in dealing with sinusoidal RF driven quadrupole field.
The secular frequency dependence on the a, q parameters was also derived thus the foundation was laid for many modern ion trap operation modes based on the resonance excitation.
Reilly suggested to Ding at that time that they should focus the DIT for analysis in the high mass range where other instruments could not compete.
However, work published by Ding and Shimadzu over the years following the 2001 meeting were focused on development of square wave driven DIT's in the conventional mass range of commercial instrumentation.
[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Over the course of eighteen years, the Reilly group contributed substantially to the development of modern digital waveform technology (DWT), its implementation and characterization, methods of waveform generation,[22][21] and general theory which includes but is not limited to stability diagrams,[18] the pseudopotential model,[19] and more recently digital quadrupole acceptance.
[26][27][28][29] In parallel to Reilly's achievements but also working separately, the Ding group at the Shimadzu Research Lab continued to implement their digital drive technology for a 3D ion trap.
The DIT technology has also been developed and implemented in the linear and 3D quadrupole ion traps by many other groups around the world.
The latter interpretation is explained by the change to the stability diagram that results when the duty cycle moves away from d = 0.5.
The stability of ion motion in a digitally driven quadrupole can be calculated from the analytical matrix solutions of Hill's equation:[41][42]
In a digital system that is operated without a physical DC offset the waveform potential reduces to the value
In the general case, the final matrix of a waveform cycle defined by n constant potential steps is:
Stable motion simply means that the secular oscillation of the ion has a maximum displacement.
Ion trajectories in a linear or 3D DIT as well as in a digital mass filter, may also be calculated using a similar procedure.
[27][44] Unlike stability calculation it is advantageous for the purpose of resolution and accuracy to represent each period of the waveform with an adequate number of constant voltage steps.
A stability diagram may be generated by calculating the matrix trace for each axis over a defined range of q and a values.
[46][47] The DIT is a versatile instrument because it is capable of operating at constant AC voltage without a DC offset for any conceivable duty cycle and frequency.
When a = 0 there will be a finite range of stable q values for each quadrupole axis that will depend on the duty cycle.
Fig 3 (a) shows a Mathieu space stability diagram for the duty cycle d = 0.50 of a linear DIT.
When the duty cycle is increased to d = 0.60 the range of completely stable q values decreases (see Fig 3 (b)) as indicated by the reduction of green that the horizontal line intersects.
In this representation the total range of stable q values along the x-axis, that is defined by the intersection of the line through the blue and green regions, is greater than the total range of stable q values along the y-axis that is defined by the intersection of the line through the yellow and green regions.
If the frequency of the linear DIT is decreased to cause a particular ion to have a q value that corresponds to right hand side boundary of the completely stable green region, then it will excite and ultimately eject in the y direction.
Digital devices use a duty cycle which allows them to operate completely independent of a DC voltage and without resonant excitation.
[18] For a particular duty cycle, the operator can quickly reference the range of stable masses at each frequency of a scan.
Normally the depth of the 'effective potential' well, or the pseudopotential well, is used to estimate the maximum kinetic energy of ions that remain trapped.
Mass range of DIT up to 18,000 Th was demonstrated by use of an atmospheric MALDI ion source[50] and was later expanded to cover m/z of a singly charged antibody at about 900,000 Th by Koichi Tanaka etc.
[46] Since rectangular waveforms are employed in the DIT, electrons can be injected into the trap during one of the voltage level without being accelerated up by the varying electric field.
[31][23] Two sets of switch circuitry were normally used to generate 2 phases of rectangular pulse waveform for two pairs of rods in case of the linear digital ion trap.
Hexin Instrument Co., Ltd (Guangzhou, China) commercialized a portable ion trap mass spectrometer DT-100 in 2017 for VOC monitoring.
