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Model 1000A
DESCRIPTION
1-11
where PW is given in seconds. All the various details in the analysis
process are independent of the value of N. Allowable values of N are 1 to
63, which corresponds to values of PW from 2 to 63 seconds.
The variable N is known as the integration factor. This term is used
because N determines how many points are averaged or integrated to
form a single value. The integration of data upon input, before storing,
serves two purposes. First, the statistical noise on the input signal is
reduced by the square root of N. In the case of N = 4, a noise reduction of
two would be realized. Secondly, the integration factor controls the
bandwidth of the chromatograph signal. It is necessary to match the
bandwidth of the input signal to that of the analysis algorithms in the
GC. This prevents small, short duration perturbations from being
recognized as true peaks by the program. It is therefore important to
choose a Peak Width corresponding to the narrowest peak in a group
under consideration.
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 software.
Having initiated a peak search by turning Inhibit off, the GC 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
JUNE 2008
Peak Detection