Emerson Daniel Danalyzer 1000, Hardware Reference Manual

Page: 26 / 320

SEP 2005
MODEL 1000
DESCRIPTION
1-14
1.6.3 Peak Detection
For normal area or peak height concentration evaluation, the determination of a peak's start, peak
point, and end 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. It is
important that this be the case because the assumption is made by the GC Controller software.
Having initiated a peak search by turning Inhibit off, the GC Controller performs a point by point
examination of the signal slope. This is achieved by using a digital slope detection filter which is
a combination low pass filter and differentiator. The output of this detector is constantly compared
to a system constant entered by the operator 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.
Peak termination is determined by the same application of this detector to the signal, but in the
reverse sense. Onset is defined where the detector output exceeds the baseline constant, but
termination is defined subsequently 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. The zero reference line will, in general, be non-horizontal, and thus
compensates for any linear drift in the system from the time the peak sequence starts until it ends.