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HTEC-500
11-4. Time Constant
There are three settings for the time constant. These are 1.0, 1.5 and 3.0 sec. These influence the peak
height and the noise level. The time constant determines if the signal response speed will reach 60% of
the real signal. For example, when the time constant is set at 1.5 sec and 1 nA real signal is generated in
the cell, the ECD integrator output generates 0.06 mV (0.1 nA = 1 mV) in 1.5 second.
1 nA × 0.1 mV/nA × 60% = 0.06 mV
The optimal setting for the time constant should be based on the peak shape, the signal height and the
flow rate. Generally, when the lower theoretical platelets column is selected, select a longer time
constant. When using Eicom applications, please follow the application notes. 1.0 sec is rarely used.
Please select 1.5 sec or 3.0 sec for our applications.
11-5. Background Current
When the columns are set up and mobile phase is allowed to flow at a certain condition before
injecting samples, the ECD monitors the current, which is called background current. Generally
speaking, lower background current allows higher sensitivity. This is because there is less background
noise and more stability.
11-6. If Background Current is High
• The surface of the working electrode could be dirty. In this case, disassemble the cell (see Fig. 23) and
wipe the surface with acetone using a Kimwipe.
• Resistance of the working electrode could be increased. In this case, change it for a new one.
11-7. Caution for the Platinum Working Electrode
Alcohol or hydrogen peroxide can be detected with the platinum working electrode.
• If hydrogen peroxide is to be detected, do not use alcohol in the mobile phase.
• If the mobile phase contains little or no alcohol, bacteria can generate. To avoid this, do not use
mobile phase for more than one week. Make fresh mobile phase every week.
• A new platinum electrode should be conditioned with mobile phase under applied potential for one
day and analysis is then possible. This conditioning is required in order to form a thin oxidized layer on
the surface of the electrode.
• Background current is sometimes very low with the platinum electrode. This situation could be the
cause of low sensitivity. This is caused by the surface of the platinum electrode becoming covered
with non-electrolyte contamination mainly from the mobile phase or the columns. Please open the
cell and wipe the surface of the working electrode (see fig. 23)
