Collecting Fractions
Entering More Peak Detection Parameters (Peak Detection Options)

Use the PGM Wizard (see The Control Program  The Program Wizard) to enter the basic peak detection parameters as described in  Detecting the Peak Start, Peak Maximum, and Peak End (Peak Detection Options). In addition, you can enter special parameters on the Peak Detection Options page.

Derivative Step

Use this parameter to determine for which period the signal values are evaluated to determine the slope and curve of the signal. The slope and curve are determined at the associated retention time t for the range (t - DerivStep/2) to (t + DerivStep/2). You can have Chromeleon display the retention time on the control panel via the RetentionTime variable (see  Checking the Fraction Collection Status on the Control Panel).

(Dimension: s; range: 0.02 to 60.00)

 Note:

The higher the DerivStep value is, the lower the noise for the Slope and the Curve.

PeakStartTrueTime, PeakMaxTrueTime, and PeakEndTrueTime

Use these parameters to set the time in seconds for which the different conditions must be fulfilled. If the conditions are not fulfilled within this time period, the peak start, peak maximum, and peak end cannot be detected.

(Dimension: s; range: 0.0 to 4.0)

 Notes: 

If you perform smoothing when using a mass spectrometer, calculating a signal value for a channel may take up to 5 seconds. If you do not allow for this time before the peaks reach the fraction collector, peaks cannot be collected. Therefore, set the respective parameter to at least 5 seconds.

In addition, a peak can be detected only after the DerivStep and PeakStartTrueTime (or PeakEndTrueTime) have expired. To make sure that a peak is completely collected as a fraction on an MS detection channel, the following must apply:

5 + PeakStartTrueTime (or PeakEndTrueTime) + DerivStep < DelayTime.

(The Delay Time is the time between the detector (here: an MS) and the Fraction Collector. If necessary, install a longer loop before the fraction collector.)

Shoulder detection

Use the PeakStartCurve and PeakEndCurve parameters to determine the curve in [Signal]/s^2 that must be exceeded so that a shoulder is detected after the peak start (PeakStartCurve). If the value is below this value, a shoulder can be detected after the peak maximum (PeakEndCurve).

(Dimension: [Signal]/s^2; range: 0 to 10^10; default setting: Off)

 Tip:

Chromeleon supports these parameters only for the Extended Fraction Collection license.

Threshold for PeakEndSlope enabling and disabling

Use the ThresholdDoNotResolve parameter to disable peak end detection via the PeakEndSlope parameter. Determine the threshold above which PeakEndSlope is disabled.

(Dimension: [Signal]/s; range: -10^10 to 10^10; default setting: Off)

If the minimum between these peaks is above the threshold value, a peak end is not detected before. Instead, a new peak is detected and a tube change is triggered when the signal slope is 0.0. The advantage is that the fraction for the first peak is collected until the minimum; the fraction for the second peak is then collected from the minimum.

In the picture, the red horizontal line indicates the threshold value. The solid lines indicate the fractions for which the minimum between the peaks is above the threshold value.

If the minimum is below the threshold value, a peak end is detected before the minimum (via the PeakEndSlope parameter; red descending line). Thus, the first fraction is collected only until the first dotted line. Then, a peak start is detected (red ascending line), so that the second peak is collected only from the second dotted line.

The area between these two lines contains a compound of the two substances. This area is collected as a separate fraction only if the program contains the following line:

 CollectOutsidePeaks =  Yes

Peak starts and peak ends that occur underneath the ThresholdDoNotResolveLine are detected as usual (indicated in the picture by the green diagonal lines).

Drift compensation

In the Baseline Drift field, enter the baseline drift that is used to correct the signal value. Enter the related baseline offset in [Signal] in the Baseline Offset field. The related thresholds (Peak Start Threshold, Peak End Threshold, and Threshold No Peak End) are compared to the signal value:

Signal value - (BaselineOffset + Dt*BaselineDrift)

Whenever Baseline Drift changes, Dt is set to 0. In addition, Baseline Offset is set to the current value of the correction term. Thus, the BaselineDrift value always matches the actual drift.

For an overview of the fraction collection topics, refer to  Collecting Fractions.