The rates of production of the ionized molecules are low in a QqTOF. This characteristic means that a time-to-digital converter is an effective means for acquiring the time-of-flight spectrum; click here for more information. A TDC notes the arrival time of each ion at the detector and records how many ions arrived with the same flight time over a large number of acceleration pulses. The result of this histogramming process is a time-of-flight spectrum. To avoid distortion of the spectrum with this scheme, the probability of detecting an ion in the most intense peak from each acceleration pulse must be kept «10%; click here for more information. Higher ion rates must be avoided, because pulses from two or more ions are likely to pile up and be counted as a single ion, thus causing a distortion of the spectrum.

Data acquisition electronics for a TOF-MS having low to medium ion rates, such as a QqTOF

The diagram above illustrates a Model 9353-P Time Digitizer (TDC) in use with a QqTOF mass spectrometer. The acceleration pulse generator initiates an injection of ions into the flight tube, as well as starting the model 9353-P's timing cycle via its Start Input. Each ion that arrives at the microchannel plate detector causes a small pulse signal which is amplified by the Model 9327-P and immediately applied to a timing discriminator. This in turn generates the Stop Input pulses for the 9353-P. The 9353-P records the arrival time of all the Stop pulses following each Start pulse, from which the software can calculate a histogram of number of events (y-axis) vs. flight time (x-axis). This is, of course, the required TOF-MS spectrum.

The timing discriminator built into the model 9327-P uses a zero-crossing timing technique that virtually eliminates changes in pulse amplitude causing changes in pulse timing, thereby improving peak symmetry and resolution in the resulting time spectrum. Click here for more information on this topic

Microchannel Plates with Segmented Anodes
One way of improving the maximum ion-rate limit in a QqTOF is to use multiple detectors. The figure below shows an example of a detector with four anode segments. In this case, the four separate amplifier channels in a Model FTA420C-P are used to feed four separate inputs on the Model 935-P Quad Constant-Fraction Timing Discriminator. The Model 935-P uses constant-fraction timing on the leading edge of the pulses to eliminate sensitivity of timing to pulse amplitude Click here for more information on this topic. Its four outputs feed the Stop inputs of four separate 9353-P Time Digitizer cards, all mounted in the same computer. This approach increases the maximum ion rate limit by a factor of four when compared to using a single detector.

Electronics for a QqTOF, featuring a 4-segment microchannel plate detector