4-21
2000-OSM, F1
method has forced gating and slope detection on at the same time, the controller uses the one turned
on first, ignoring the second one and sending an alarm to tell you that both peak detection methods
were on.
Forced Gating
When a component's elution time is known, forced gating can measure the peak. You specify the
gating time on and off, which lets you measure only that amount of peak area you desire. In the
method table, forced gating has four commands associated with it.
On @ Base
Off @ Base
On @ Valley
Off @ Valley
“On @ Base” tells the controller it is at the base and to start measuring. The controller continues to
measure until it reads “Off @ Base” or “Off @ Valley” regardless of whether there are no peaks, one
peak, or multiple peaks. The other functions work similarly.
Slope Detection: Introduction
Slope detection determines a peak’s starting and ending points using the analyzer’s computer. With
slope detection active, the controller constantly checks the chromatographic signal for changes which
would indicate the start of a peak, a peak’s maximum, and the end of a peak. Slope detection utilizes
several other functions: noise calculation, peak lumping, tangent skim, baseline definitions, peak
width, and suspend end. In order to explain how these functions interact, the chromatogram in Figure
4-20 shows the basic functions, and the following paragraphs describe the functions.
T-END
T0
TIME
SLOPE
DETECT
OFF NOW
SLOPE
DETECT
ON NOW
NOISE
CALC
ON
NOISE
CALC
OFF
PEAK
WIDTH
Figure 4-20. EXAMPLE OF BASIC SLOPE DETECTION FUNCTIONS
Slope Detection: Basic Functions
The basic slope detection functions used in a method table are Noise Calculation, Peak Width, and
Slope Detect. Slope Detect On and Slope Detect Off must always appear in the method table when
you want to use slope detection. If noise calculation and peak width are not specified, the controller
will use its stored default values.
Noise calculation determines the noise level in the analysis during a period of inactivity. Shortly after
the start of the analysis, Noise Calc is turned on for at least two seconds, and then turned off. This
noise level measured by Noise Calc is used to decide when the slope of the chromatogram indicates
a peak change and not just random fluctuation due to noise.
Peak width is used to calculate the minimum slope of the start of a peak. A lower number causes the
system to respond more quickly to the start of a peak but is less sensitive to long term change. A
