A Conjoined Rectilinear Collision Cell and Pulsed Extraction Ion Trap with Auxiliary DC Electrodes

Abstract

Ion traps are routinely directly coupled to mass analyzers, where they serve to suitably cool and shape an ion population prior to pulsed extraction into the analyzer proper. Such devices benefit from high duty cycle and transmission but suffer slow ion processing times caused by a compromise in the buffer gas pressure range that suitably dampens the ion kinetic energy without causing excessive scatter during extraction or within the analyzer. A rectilinear RF quadrupole ion trap has been characterized, conjoining a pressurized collision region with a pumped extraction region, and an unbroken RF interface for seamless ion transfer between them. Auxiliary electrodes mounted between the RF electrodes provide DC voltage gradients that serve to both guide ions through the device and position them at the extraction slot. The influence of the auxiliary DC upon the trapping RF field was measured, and suitable parameters were defined. A mode of operation was developed that allowed parallel processing of ions in both regions, enabling a repetition rate of 200 Hz when the device was coupled to a high-resolution accurate-mass analyzer.

Figure 1

Schematic diagram of the Ion Processor including high-pressure (HP) and low-pressure (LP) regions, auxiliary DC electrode profiles.

Figure 2

(a) Isometric view of Ion Processor architecture. (b–d) Cross-sectional views of the high and low-pressure regions of the Ion Processor, and at the point of ion extraction, respectively.

Figure 3

Low-pressure (LP) region auxiliary DC electrodes with dimensions in millimeters.

Figure 4

Simulated pressure profile for the Ion Processor with and without conductance-restricting cylinders between electrodes.

Figure 5

Potential and processing sequence of ions as they traverse the Ion Processor.

Figure 6

Oscilloscope traces showing (a) 4 kV voltage lift over 1.5 ms, with 1000 V_p–p RF applied throughout, and (b) quench of RF and application of 500 V push/pull extraction voltages. (c) 200 Hz cycle of the high- and low-pressure regions, along with activation of the ion injecting “split lens” that admits ions from the quadrupole filter.

Figure 7

(a) Ion Processor characterization setup. (b) Time spread of the detector signal (solid), with the detector’s spread deconvoluted (dashed), simulated results (crosses).

Figure 8

IonCCD measurement of extracted ion beam width with 2.5 mm trapping electrode.

Figure 9

Layout of the experimental Orbitrap Astral instrument with the Ion Processor as a pulsed ion source.

Figure 10

Number of detected ions vs Inject Time during which the ions were accumulated in the Ion Processor. (a) Full mass range of FlexMix ions and a fraction of MRFA ions cotrapped. (b) Comparison of the number of MRFA ions in two mass selection scenarios—isolation and cotrapping with the full mass range.

Figure 11

Space-charge related suppression of different m/z peaks vs the total number of trapped ions.

Figure 12

Ultramark m/z 1522 MS/MS spectrum comparison between (a) Ion Processor fragmentation following measurement in Astral and (b) IRM fragmentation following measurement in the Orbitrap analyzers.

Figure 13

(a) Transfer time scan of various m/z ions into low-pressure region, at approximately 1.75 × 10^–3 mbar. (b) Transfer time scans of m/z 2722 ions at a range of pressures.

Figure 14

HP-LP ion transfer efficiency vs the DC offset for the 800 V_pp RF waveforms applied to HP and LP in phase or antiphase.

Figure 15

Myoglobin mass spectrum and zoomed spectrum of the 23+ charge envelope. 100× averages measured at 200 Hz repetition rate.