DynaFit Scripting Manual

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Offset

Under the term offset we mean a constant value of instrumental signal that is added to the simulated reaction progress curve. For example, a typical UV-VIS absorption spectrophotometer shows readings of absorbance that are proportional to the concentration of certain reactants, plus a ``baseline'' value that may have been introduced by the solvent or by the spectrophotometric cell in which the reaction is taking place. Using the mathematical formalism,

$$A = a_0 + \epsilon \times c$$

where $A$ is the experimental signal (e.g., absorbance), $\epsilon$ is the molar response coefficient, $c$ is the concentration of a spectroscopically visible reactant, and $a_0$ is the baseline offset.

Baseline offset in DynaFit is given by the keyword offset followed by a numerical value in proper instrumental units (e.g., absorbance units or relative fluorescence units).

ExampleIn this example each simulated progress curve has the value 0.01 (e.g., absorbance units) added to each data point.

[progress]
   delay  3
   offset 0.01
   file   ff.txt

Adjustable offset

In the statistical analysis of experimental data, it is often advantageous to treat the instrument offset as an adjustable parameter. In this case the numerical value of the baseline offset is followed by the question mark.

Example : Adjustable offsetIn this example the initial value of the baseline offset (0.01) is optimized in the least-squares regression.

[progress]
   delay  3
   offset 0.01 ?
   file   ff.txt

Automatic offset

DynaFit can treat the first data point on each experimental reaction progress curve as the baseline offset. The value of the independent variable (e.g., absorbance or fluorescence) is automatically set to zero for the first data point, and the remaining data points are adjusted accordingly. This kind of automatic baseline adjustment is indicated by the keyword auto.

Example : Automatic baseline offsetsThe experimental data in file FF.TXT are adjusted by setting the first data point to zero on the vertical axis, and adjusting accordingly the remaining data points in the set.

[progress]
   delay  3
   offset auto
   file   ff.txt

Automatic adjustable offset

It is possible to treat the automatic baseline offset as an adjustable parameter. In this case the keyword auto is followed by the question mark.

Example : Automatic baseline offsetsThe experimental data in file FF.TXT are adjusted by setting the first data point initially to zero, and subsequently optimizing the baseline adjustment in the least-squares regression.

[progress]
   delay  3
   offset auto ?
   file   ff.txt

Locally adjustable automatic offset

If multiple files are being analyzed simultaneously (``global analysis''), it is possible to treat the automatic baseline offset as a parameter adjustable separately for each data set. In this case the keyword auto is followed by the question mark and the keyword local.

Example : Automatic baseline offsets adjusted separately for each datasetThe experimental data in file F1.TXT, F2.TXT, and F3.TXT are adjusted by setting the first data point initially to zero, and subsequently optimizing the baseline adjustment in the least-squares regression. A separate best-fit value of the baseline offset is obtained for each dataset.

[progress]
   delay  3
   offset auto ? local
   file   f1.txt
   file   f2.txt
   file   f3.txt

Example : Globally adjustable offsetThe experimental data in file F1.TXT, F2.TXT, and F3.TXT are adjusted by setting the first data point initially to zero, and subsequently optimizing the displacement of progress curves from the origin. The same best-fit value of the baseline offset is obtained for all dataset.

[progress]
   delay  3
   offset auto ?
   file   f1.txt
   file   f2.txt
   file   f3.txt

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Petr Kuzmic | Jul 12 2005