Searching Reference Spectra
Searching Single Reference Spectra

To facilitate substance identification, the peak spectrum, which is displayed on the Spectra Plots of the Integration or PPA method, can be compared to the UV spectra of a spectra library.

Based on the available library, Chromeleon creates a spectra list sorted by the degree of similarity, the Hit List. The number of possible hits can be limited via comprehensive search criteria.

1. If the pointer is in a spectra window, select Library Search on the context menu of the UV spectrum to start the spectra search.

   In the edit box, specify the library to be searched. The following are minimum entries required to receive a valid search result:

2. Select a spectra library (LIB file) from the Spectra Library to be searched in list. If no LIB file is displayed, click Browse to search for the file.

3. In the Match Criterion field, determine the mathematical method based on which the original spectrum and the library spectrum are compared with each other. The best search results are generally received with Least Squares.

4. Click OK to start a spectra search.

 Note:

To be able to compare UV spectra with each other, the spectrum and the reference spectrum should be recorded under identical conditions. For best results, compare spectra to your own spectra library.

Result

A list of possible candidates is displayed. The top spectrum has the highest match value and the best similarity to the original spectrum.

Detailed Search

To accelerate the search and to receive very specific results, there are numerous options:

• Normally, the Spectrum Derivative option is disabled, that is, spectra comparison is based on the actual curve shape. If the 1´st Derivative option is selected, the comparison of the two spectra is performed based on the first derivative. Consequently, the curve characteristics are more significant (shoulders become real extremes), which allows a more precise comparison. The drawback of this option is the reduction of the signal-to-noise ratio, which causes sections with weaker signals to lose significance.

• Select Hit Threshold and enter a value between 0 and 1000, for example, 950. Only the spectra with a match above 950 is displayed. Spectra with a lower match value is not displayed. Spectra with a match value below 900 are usually spectra of other substances (if derivatives are not used for the comparison). Exceptions to this are acceptable only if, for example, the signal-to-noise ratio is low.

Select the following options to ensure precise search results:

• Enter the number of relative extremes the spectrum should have (Number of Relative Extrema). This option is useful to exclude spectra that are similar but have an additional side maximum.

• Select Check Greatest Relative Maximum to use only spectra with the greatest relative maximum at the same position. Select Allowed Deviation to define a tolerance range. The range should not be more than 10 nm to avoid that the criterion weakens.

• Via Retention Time Window, define a time window that includes the retention time of the peak. Use this option to exclude substances with very similar spectra, but which are eluted at very different retention times.

Additional criteria are only required in special cases. However, they are useful for searching large libraries containing numerous spectra of the same substance, but extracted under different conditions; for example, different solvents and detectors, different users, different date, etc.

 Note:

When creating your own spectra library, please enter information in all fields, even in the fields that may seem unnecessary. In the course of time and with a growing library, this may become a major advantage.