Separating the Analyte from Interferents

Separating the Analyte from Interferents

When a method shows a high degree of selectivity for the analyte, the task of per- forming a quantitative, qualitative, or characterization analysis is simplified. For ex- ample, a quantitative analysis for glucose in honey is easier to accomplish if the method is selective for glucose, even in the presence of other reducing sugars, such as fructose. Unfortunately, analytical methods are rarely selective toward a single species.

In the absence of interferents, the relationship between the sample’s signal, S_samp, and the concentration of analyte, C_A, is

S_samp = k_AC_A                   7.9

where k_A is the analyte’s sensitivity.* In the presence of an interferent, equation 7.9 becomes

           7.10

where k_I and C_I are the interferent’s sensitivity and concentration, respectively. A method’s selectivity is determined by the relative difference in its sensitivity toward the analyte and interferent. If k_A is greater than k_I, then the method is more selec- tive for the analyte. The method is more selective for the interferent if k_I is greater than k_A.

Even if a method is more selective for an interferent, it can be used to deter- mine an analyte’s concentration if the interferent’s contribution to S_samp is insignifi- cant. The selectivity coefficient, K_A,I, was introduced as a means of characterizing a method’s selectivity.

                   7.11

Solving equation 7.11 for k_I and substituting into equation 7.10 gives, after simplifying

                 7.12

An interferent, therefore, will not pose a problem as long as the product of its con- centration and the selectivity coefficient is significantly smaller than the analyte’s concentration.

When an interferent cannot be ignored, an accurate analysis must begin by separat- ing the analyte and interferent.