Thermo Scientific 074114, Руководство оператора

Страница: 61 / 148

3 • Operation and Maintenance
Doc. 065378-01 1/12
53
step determines the sampling rate at which the detector stores spectra.
Ideally, the step is no smaller than the interval suggested for the rise time
(see the table in
“Data Collection Rate” on page 45 ). The advantage of a
larger step size is that it reduces the amount of data stored; for example,
selecting a step twice as large as that listed in the table reduces the data
file size for the 3D field by 50%.
If you select a larger step size when the average parameter is on (see
“Average (Chromeleon 6.8 only)” on page 53 ), the system response is a
combination of the selected rise time and the average of the data points
between steps. If you select a larger step size when the average parameter
is off, the selected rise time is valid, but some data points will be skipped.
Average (Chromeleon 6.8 only)
The Average parameter operates in conjunction with the Step parameter (see “Step
(Chromeleon 6.8 only)” on page 52 ).
When Average is On , Chromeleon 6.8 averages the data points between step
intervals, reports that value, and noise is reduced. This setting is recommended for
most applications.
When Average is Off , Chromeleon 6.8 reports the data points at the step interval.
The data points between steps are skipped and noise is not reduced.
Negative Absorbance
Negative absorbance is the result of decreased baseline absorption of the mobile
phase after the autozero routine (which usually occurs at the beginning of a run).
At the default setting of 1, the Dionex PDA becomes saturated when the light
transmission is 20% above the autozero level. This results in a flat, truncated
baseline.
If desired, increase the negative absorbance level in order to continue seeing the
actual baseline and peaks. This is especially helpful when running a gradient
application in which the absorbance decreases more than 100 mAU. As the
following table indicates, the extent of negative absorbance depends on the
wavelength. (The values in this table are intended as guidelines only.)
If Average is On and the step size is more than double the rise time,
the system response is dominated by the step size. The system
response time is approximately the larger of (a) the rise time
or (b) two
times the step size (when Average is On).