Global Nonlinear Least-Squares Equilibrium Analysis:

Part 3: Scan Diagnostics Previous: Model Selection Next: Next: Scan Editing

After selecting a model, the "Scan Diagnostics" button will be activated. This function will apply a series of tests prior to fitting to determine which of the available scans are suitable for inclusion in the fit. You have to execute this function before proceeding to the next functio. The following tests will be employed:

  1. Slope Check: This test will measure the slope at the beginning and the end of the scan. It will then calculate a ratio of the slope at the end over the slope at the beginning of the scan. If the ratio is too small, a warning will be produced. If the ratio is too small, check for aggregation, or a speed that is too slow for the experiment. In addition, warnings will be produced if either slope is negative. This is usually a sign of excessive noise at the ends of the scan. Further editing will probably be required (see next section for more information on how to edit scans in the control panel).

  2. Point Number Check: If the number of datapoints in this scan is below 50, a warning is issued. Check to make sure that the smallest datapoint increment was used in the Beckman Data Acquisition software to make sure sufficient datapoints have been collected. Also, make sure that the column was at least 3 mm in height.

  3. OD Range: The XLA allows you to collect data between zero and one OD and higher. For most wavelengths the Xenon flashlamp of the XLA produces sufficient intensity to obtain a linear response up to 0.9-1.0 OD. Above this optical density you probably reach into a nonlinear range. Therefore, it is prudent to at least have exploited all data in that absorbance range. If the total range is below 0.9 OD, a warning is issued. Check for aggregation, slow rotor speed or improper scan cropping/editing.

If three or more warnings were generated, the scan will be marked with a blue triangle () in the Scan Listbox and automatically excluded from the fit. After reviewing the reason for the exclusion and assigning initial parameter estimates, you can override the exclusion by double-clicking on the scan in the scan listbox. In that case, the scan will be labeled with a red triangle () to indicate manual selection of this scan. Scans that were found acceptable for inclusion into the fit will be marked with a green triangle (). These warnings are for your information only, they have no effect on the rest of the program since there are valid exceptions to these warnings when including such scans in a fit is appropriate. Please refer to the global equilibrium analysis tutorial for more information.

After the scan check a listing identifying any problems found will be generated and displayed in an information window. If any warnings were generated, a note will be printed on the screen. All collected information will also be written to a file in the UltraScan root directory, called "diagnostics.res". This file can also be printed from the file menu of the information window.

After the scan diagnostics have completed, and all scans have been assigned as either included (green arrow) or excluded (blue arrow), the Concentration Histogram button will be activated. Clicking on this button will generate a concentration histogram of all included scans. The program will sample each datapoint of each included scan and count the frequency of occurence for each concentration. The program will subdivide the concentration range defined by the lowest and highest concentration occuring in any included scan into fifty equally spaced bins and plot the frequencies of each bin.

The purpose for this plot is to provide a qualitative idea of the signal strength for each concentration present in the fit. Since self-associating models depend on a concentration range which leads to the presence of multiple species, the signal provided by each species is hence directly correlated to the concentration. If you have a self-associating system present, the signal strength ideally should be equally represented for each concentration/species. If the data includes data taken at multiple wavelengts, a warning will be printed to alert you that you should correct the extinction coefficients for each wavelength so that all concentrations can be displayed, and corrected for differences in absorptivity. In that case, only a histogram of the optical densities is possible. This can be handled in the model control module which is described in more detail here.

Previous: Model Selection Scan Diagnostics Next: Next: Scan Editing

www contact: Borries Demeler

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Last modified on January 12, 2003.