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Understanding the Zero-Distortion density in a chromatographic model is important in using the wide-range of chromatographic models in PeakLab.

The ZDD button in the three fitting options controls the default starting estimates for the ZDD parameters:


Please note that you may only need to use two items in the dialog, the GenHVL and the GenNLC asymmetry defaults, if you are fitting analytical GC and non-gradient LC peaks.

Most of the ZDD parameters control the asymmetry or skew of the zero-distortion density. The Gen2HVL, Gen2NLC, [Y], and [T] ZDDs also have a kurtosis adjustment, and the [V] density has a second skew adjustment. The [Q] and [S] ZDDs only adjust the kurtosis or fourth moment.

The initial values are defaults the program will use when automatically placing peaks to begin a fit. The initial values in the program come from fits of high-resolution IC data which represent only a starting point. These values may or may not be especially applicable to your peaks. Although these are only starting estimates for an iterative fitting process, you should set these values as close to the expected fitted values as possible. The fits will be faster, and this also minimizes the possibility of fits where the global minimum (the optimum solution) is not automatically achieved.

Assigning New ZDD Default Parameter Values

Use the dropdown to select any fit from the current file. The ZDD from that that specific fit will be shown with the average value of the ZDD parameter across all data sets fitted. The count of values in the average will also be shown. Simply select the fit to insert this value into into the fields associated with that specific ZDD.

Generate/V5_8985.gif Use the Select a Different Data File button to use fits stored in a different PeakLab file as a source of the ZDD values.


The Save option can be used to save a configuration [zdd] file to disk containing these ZDD starting estimates (this file uses a [zdd] extension). This option allows this specific ZDD configuration to be recalled at any time. A PeakLab [pfd] data file automatically saves a current ZDD configuration and that is used when a given pfd file is opened. Those values will be overridden when the Read option is used to open one of these [zdd] configuration files. When such an ZDD configuration is imported, all values in the dialog are updated.

The Clear option erases the values in all of the fields.

The Recall option restores the values saved in the current [pfd] file. If the values in the various fields are those loaded from the current pfd file, there will be no observed change when selecting this option.

The Defaults option restores the initial program defaults based on this high S/N IC data. If the values in the various fields are presently the program's defaults, there will be no observed change when selecting this option.

The MinToSec option converts the time-sensitive ZDD parameters from a minutes to a seconds time scale. The SecToMin option does the reverse, converting all x-scale parameters from a seconds time scale to a minutes one. In general, you will probably find it easiest if you consistently use only a retention scale, or specifically a minutes or seconds time scale. The program's Transform Data option can easily convert the time scale once the data have been imported.

The ZDD Models

The following ZDD models are presently in the program:

Default Asymmetric Normal

Default Asymmetric Error

[Z] Asymmetric Normal

[Y] Asymmetric Error Model

[G] Half-Gaussian Modified Gaussian (GMG)

[E] Exponentially-Modified Gaussian (EMG)

[Q] Error Model (Symmetric)

[S] Student's t (Symmetric)

[T] Generalized Student's t (Asymmetric)

[V] Generalized GMG

[Yp] Asymmetric Error Model (High Overload Shapes Only)

[YpE] Generalized Laplace (High Overload Shapes Only)
[Yp2] Generalized Error 2-Width (Overload Shapes)

[Yp2E] Generalized Laplace 2-Width (High Overload Shapes Only)

[K] Generalized Two-Sided Kinetic Orders 0,1 (High Overload Shapes Only)


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