FROM UNIVARIATE TO MULTIVARIATE CALIBRATION: EXPERIENCES IN THE ANALYTICAL CHEMISTRY LABORATORY COURSES

The concept of calibration plays a key role in Analytical Chemistry. It may be considered as the central idea behind instrumental analysis which, in turn, accounts for a great deal of what is done in practical Analytical Chemistry nowadays. Thus, its correct establishment is of capital importance starting from the first university courses in Analytical Chemistry.

At the Faculty of Science and Technology of the University of The Basque Country, calibration is first introduced, theoretically, to the students in the 3rd (out of a total of a 5) year in the subject 'Analytical Chemistry' and, practically, in the subject 'Experimental Analytical Chemistry II'. To begin, the concept of calibration is kept in the univariate realm, that is, by means of a simple linear regression of functions of the type S = b0 + b1 * C, where S stands for the analytical signal, C for the concentration of the analyte to be determined and b0 and b1 are the calibration parameters. At this level, the students have their first practical experiences in 'external' calibration using simple analytical methods such as, for instance, the spectrophotometric determination of tiocyanate with Iron (II) or in 'internal' calibration by means of the standard addition calibration method in the fluoride determination in seawater using a fluoride selective electrode.

The concept of calibration gets extended to the multivariate realm in the 4th year subject 'Advanced Analytical Chemistry'. In that course, the notion of using more than one predictor variable and the possibility of simultaneously determining more than one analyte at once is introduced and its advantages and drawbacks are discussed. From the practical point of view, students are faced with the use of methods such as principal component regression (PCR) or partial least square (PLS) regression through, for instance, the simultaneous determination of a mixture of salicylic acid, caffeine and paracetamol in commercial analgesic formulations.

However, this extension of the calibration concept seems to get disregarded sometimes in cases where it should be successfully applied. This is what has many times happened in the 5th year practical course 'Experimental Analytical Chemistry III'. There, students are faced with the challenge of setting up their own proposals for the analytical determination of common substances (metal ions, common anions, etc.) in matrices such as waste, drink or sea water, food, environmental samples, etc. Our experience shows that most of the times the students head for 'simple' instrumental methods based on univariate calibration when the use of the multivariate approach is feasible.

This work shows our experience, carried out in the mentioned 5th year course, in the extension of the univariate to the multivariate approach through the example of the turbidimetric determination of sulphate in seawater by means of common turbidimetry and ultraviolet-visible spectroscopy once it is precipitated with barium chloride. Experimental details of the experiences developed as well as the calculation methods and results obtained are provided.

As a result of these experiences, it can be concluded that multivariate calibrations poses some advantages over the still more popular univariate approach that should be emphasized to the students in order for them to use them later in their professional careers.