74 Understanding Your Agilent ChemStation
4 Integration
Overview
Before the integrator can integrate peaks, it must establish a baseline point.
At the beginning of the analysis, the integrator establishes an initial baseline
level by taking the first data point as a tentative baseline point. It then
attempts to redefine this initial baseline point based on the average of the
input signal. If the integrator does not obtain a redefined initial baseline point,
it retains the first data point as a potential initial baseline point.
Tracking the Baseline
The integrator samples the digital data at a rate determined by the initial peak
width or by the calculated peak width, as the run progresses. It considers each
data point as a potential baseline point.
The integrator determines a baseline envelope from the slope of the baseline,
using a baseline-tracking algorithm in which the slope is determined by the
first derivative and the curvature by the second derivative. The baseline
envelope can be visualized as a cone, with its tip at the current data point. The
upper and lower acceptance levels of the cone are:
• + upslope + curvature + baseline bias must be lower than the threshold
level,
• - upslope - curvature + baseline bias must be more positive (i.e. less
negative) than the threshold level.
As new data points are accepted, the cone moves forward until a break-out
occurs.
To be accepted as a baseline point, a data point must satisfy the following
conditions:
• it must lie within the defined baseline envelope,
• the curvature of the baseline at the data point (determine by the derivative
filters), must be below a critical value, as determined by the current slope
sensitivity setting.
The initial baseline point, established at the start of the analysis is then
continuously reset, at a rate determined by the peak width, to the moving
average of the data points that lie within the baseline envelope over a period
determined by the peak width. The integrator tracks and periodically resets
the baseline to compensate for drift, until a peak up-slope is detected.
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