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Multiple Stop Advantages over a Time-to-Amplitude Converter (TAC)
Both an MCS and a time digitizer can operate as a multiple-stop time
spectrometer. On each scan through the selected time span, both
instruments can record multiple stop events. In the case of the MCS
this is achieved by counting "stop" events in the appropriate bin as
they arrive. For the time digitizer, the arrival time for each event
is recorded as the event occurs. Clearly, both instruments can
record multiple stop events following each start trigger. In
contrast, a time-to-amplitude converter (TAC) can record only one
stop event for each start trigger. (See the Time-to-Amplitude
Converters introduction.) The multiple stop capability allows the
Model 9308-pci Picosecond Time Analyzer to acquire data at much
higher rates than a TAC/MCA system for time spans >250 ns. The 9353
has this same advantage over a TAC for time spans ≥50 ns. For
example, on a time span of 81.92 µs, a 9353 can acquire data 42,000
times faster than a TAC, and a Model 9308 picosecond TIME ANALYZER
can exceed the TAC data acquisition rate by a factor of 1600.
These comparisons are based on minimizing spectrum distortion by
maintaining <1% dead time losses. The advantages are even greater
for longer time spans.
Multichannel scalers and time digitizers have an upper limit on the
event rate as a result of the dead time caused by the pulse-pair
resolving time. Generally, the probability of detecting a single
event within the pulse-pair resolving time must be kept less than 2%
during each scan, in order to limit the dead time losses and
distortions to less than 1%. In the application Time-of-Flight Mass
Spectrometry (TOF-MS) analyzing the output of a chromatograph (LC or
GC), this places an unproductive limit on the ion rates that can be
accommodated. As a result, TOF-MS spectra collected for 100-ms time
intervals contain circa 20 counts in the largest peaks. This leads
to statistical errors in excess of 22%, and correspondingly high
detection limits. A Digital Signal Averager would be a much more
appropriate solution for this type application. |
