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User Manual MON20/20 Software for Gas Chromatographs
3-9000-745 OCTOBER 2010
B-2
B.2 Peak detection
For normal area or peak height concentration evaluation, the
determination of a peak's start point and end point is automatic. The
manual determination of start and end points is used only for area
calculations in the Forced Integration mode. Automatic determination of
peak onset or start is initiated whenever Integrate Inhibit is turned off.
Analysis is started in a region of signal quiescence and stability, such
that the signal level and activity can be considered as baseline values.
Note
The controller assembly software assumes that a region of signal quiescence and
stability will exist.
Having initiated a peak search by turning Integrate Inhibit off, the
controller assembly performs a point by point examination of the signal
slope. This is achieved by using a digital slope detection filter, a
combination low pass filter and differentiator. The output is continually
compared to a user-defined system constant called Slope Sensitivity. A
default value of 8 is assumed if no entry is made. Lower values make
peak onset detection more sensitive, and higher values make detection
less sensitive. Higher values (20 to 100) would be appropriate for noisy
signals, e.g. high amplifier gain.
Onset is defined where the detector output exceeds the baseline constant,
but peak termination is defined where the detector output is less than the
same constant.
Sequences of fused peaks are also automatically handled. This is done by
testing each termination point to see if the region immediately following
it satisfies the criteria of a baseline. A baseline region must have a slope
detector value less than the magnitude of the baseline constant for a
number of sequential points. When a baseline region is found, this
terminates a sequence of peaks.
A zero reference line for peak height and area determination is
established by extending a line from the point of the onset of the peak
sequence to the point of the termination. The values of these two points
are found by averaging the four integrated points just prior to the onset
point and just after the termination points, respectively.