Time-to-Digital Converters (TDC) or Time Digitizers have certain advantages that make them the best choice when very low ion rates are encountered, or when exceptionally narrow peaks are anticipated in the TOF-MS spectrum. TDCs measure the arrival times of single ions when the detector pulse crosses a discriminator threshold. Thus, there is no correlated noise floor to hamper detection limits. Additionally, the threshold crossing time can be measured with a jitter that is small compared to the detector pulse width. The low jitter in measuring the pulse arrival time, means that the ultimate time resolution and mass resolution will be limited only by the ion optics in the flight tube. Furthermore, techniques available with TDCs permit digital time resolutions as low as a few picoseconds.

The major weakness of a TDC is its restriction to single-ion pulses. This limits its use to low ion rates, such that the probability of detecting an ion in the most intense peak in the spectrum is «10% for each TOF-MS acceleration pulse (see Application Note AN57; click here to locate it). At higher ion rates two or more pulses have a significant probability of piling up on each other and causing spectrum distortion. Consequently the TDC is best suited for mass spectrometers that have low ion rates, such as the QqTOF.

Digital Signal Averagers (DSA), on the other hand, thrive on high ion rates, because the sampling ADC responds proportionally to the number of ions in each pulse. However, compared to a TDC, the techniques available for a digital signal averager do not enable digital resolutions below circa 250 ps. Furthermore, the width of the pulse from the detector and preamplifier sets the minimum peak width that can be achieved. With a DSA. much greater care is required in delivering the analog signal from the microchannel plate detector to the DSA. Impedance mismatches are inevitable, and result in some ringing following each pulse. That ringing is duplicated in the recorded spectrum.

For very low ion rates and a high number of records/spectrum, the correlated noise in a DSA can contribute to detection limits. Automatic correlated noise subtraction can render this issue moot up to 65,535 records/spectra.

A digital signal averager is the better solution when ion rates are high, such as encountered with LC/TOF MS, GC/TOF-MS, IT/TOF-MS and MALDI TOF-MS. With automatic correlated noise subtraction its applicability can be extended to the QqTOF for all but the extremely low ion rates.

For more details on the operating characteristics of TDCs and DSAs, click here.

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