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Initial Autozero = YES allows the system to balance the bridge automatically
before each run for maximum dynamic range. The current status of the
balance control, from -99% to +99%, is shown on the display.
A continuous drift in the balance value over time may indicate you have an
air leak which is causing one set of filaments to oxidize, changing their
resistance and unbalancing the detector bridge.
Range
Setting
Use the 0.05 mV range in the following cases:
•
Low currents (100-150 mA)
•
High Currents (300 mA)
•
Output Signal Fed to Computer or Integrator
Low Currents
(100-150 mA)
Analyses that had required a high filament current and a less sensitive range
can often be performed at lower currents and a more sensitive range. For
example, a filament current of 120 mA and a range of 0.05 mV yields about
the same output signal as a filament current of 300 mA and a range of 0.5
mV. At the lower current, the detector noise is lower, the stability is higher,
and the filament lifetime is extended. Thus, Varian recommends that,
whenever possible, you operate the TCD at the lowest practical filament
current.
The chromatograms in Figure 19 illustrate the high sensitivity and detectivity
(18 ppm of Neon in air) that is possible with the TC Detector. Also, a
comparison of chromatogram A with chromatogram B shows that with the
use of the bridge output signal amplifier, you can obtain the same analytical
results at half the filament current, thereby extending the lifetime of the TCD
filament. The Neon peak in chromatogram C illustrates how you can use a
high filament current (300 mA) with a high amplification setting to increase
the effective sensitivity of the TCD.
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