Qualitative Analysis and Search-Match Basics

In qualitative analysis you try to find out which phases are actually present in your sample. Match! makes this task as easy as possible by providing some proven and elaborate tools for it:

One of the core functions of Match! is the so-called "search-match" routine. It compares the diffraction pattern of an unknown sample to the patterns of known compounds stored in the reference pattern database.

To be exact, Match! compares the peak data found in the experimental diffraction pattern to the peak data in the reference database. For each reference database pattern, Match! calculates a numerical value describing the quality of the agreement with the pattern of the unknown sample, the so-called "figure-of-merit" (FoM).

Once the search-match calculation has finished, the resulting so-called "candidate entries" are ranked according to their FoM values; the entries with the highest FoM are the ones that are most likely to be present in the sample. The ranked list of candidate entries is then presented to the user in the candidate list on the lower left-hand side of the Match! window. Afterwards, the user can evaluate the entries at the top of the list and select those which are most likely to be present in the sample ("matching entries/phases").

Note that the search-match calculation can either be performed for all peaks of the diffraction pattern or for marked peaks only.

A common strategy for qualitative analysis goes like this:

  1. Process your raw diffraction data carefully, until the profile calculated from the experimental peak data is in reasonable agreement with the experiment. It is also essential to eliminate any potential 2θ axis errors (e.g. zero point or specimen displacement) in this step.
  2. Start with a full search-match calculation (menu command "Search/Search-Match", shortcut <Ctrl+M>, toolbar button ) and the selection of a first (typically major) phase. The latter can e.g. be performed by double-clicking on the corresponding line in the candidate list.
  3. Afterwards, a common strategy is to focus on experimental peaks that are not covered by identified phases yet. In order to make this easier, you can activate the "Show uncorrelated peaks" option, e.g. by pressing <Ctrl+Shift+K>.
  4. Mark (click on) a peak that cannot be "explained" with the already selected phase(s), i.e. which has not been correlated to an identified phase yet.
  5. Run the menu command "Search/Search-Match (marked peaks only)" (or press the keyboard shortcut <Ctrl+Shift+M> or the toolbar button ). Wait until the update of the candidate list (figure-of-merit values) has been completed.
  6. Investigate the entries/phases at the top of the candidate list as usual, and select one (or more) phases that you assume to be present, based on the results for the marked peak(s).
  7. Match! will automatically run a new full search-match calculation (for all peaks) at this point, in order to update the figure-of-merit values for all reference database entries with regard to the new "situation" (list of selected phases as well as the peak intensities covered by them).
  8. Repeat the steps 4-7 until the profile calculated from the selected phases (and the background) is in reasonable agreement with the experimental diffraction profile, so that it is rather likely that all phases in the sample have been identified.

If no reasonable phase is found when running a search-match calculation for one (or more) marked peaks, this could be due to one of the following reasons:

Please pay attention to the following hints, too: