Catalytic Selectivity is an amount of the catalyst’s capability to express the conversion to the desired products. In practice, a catalytic process involves desired product and also by products (arise from side reaction, which is not desired), catalytic ability towards conversion to desired product is a measure of selectivity.

Dear Mehri Sanati! Thank you very much.

What is the mathematical expression ?

This question is not related to measure theory! Measure theory does not deal with such things.

selectivity is defined as percentage of the consumed reactant that formed the desired product.

We have another engineering term, which is call yield. yield refers to the quantity of product (desied and undesierd products) formed per quantity of feedstock in production routes.

Percentage of comsumed reactant / desiered product = selectivity

I believe Joseph is talking about selectivity in the analytical determination/quantitation framework. As such, and according to IUPAC "selectivity of a method refers to the extent to which it can determine particular analytes under given conditions in mixtures or matrices, simple or complex, without interferences from other components" (Pure Appl. Chem., Vol. 73, No. 8, pp. 1381–1386, 2001)

Selectivity and specificity are often seen as similar although they are not. See for instance the definition of specificity in the ICH Q2 guideline "Specificity is the ability to assess unequivocally the analyte in the presence of components which may be expected to be present. Typically these might include impurities, degradants, matrix, etc. ".

They both say the same thing although the terminology is different. In a specific method, the analytical signal is only generated when the analyte is present (for instance a ISE-ion selective electrode). In a non-specific method the analytical signal is generated for other substances than the (intended) analyte. For instance in HPLC with UV spectrophotometric detection you will have UV absorption for other substances present in the sample (therefore a non-specific method). In this case you can confer selectivity by separating the (several) chemical compounds by using a chromatographic column – so we will have a non-specific yet selective method. As a conclusion, a specific method is always selective but a selective method may be non-specific.

Take a look at the Vessman paper (Pure Appl. Chem., Vol. 73, No. 8, pp. 1381–1386, 2001).

Hope it helps.

Dear Luis Gouveia

The classical definition of selectivity was criticized.

Happy New Year!

Joseph

Dear Joseph, could you elaborate as I missed the critics?

Regards and Happy New Year

Luis