Precise Determination of the Absorption Maxima in Wide Bands In analytical chemistry and spectroscopy, the analys~rof ~olutionrhy spectroscopic methods, one frequentlv m counters compcmndn w h ~ r hhave vrn. wide nnd mund hands, where it is \cry difficult tu fund the precm posltron of the mn*imum. Thw i t u a r i m becomes eqwcinllv rnticnl whm onr worki uirh the expanded ~ c a l r s .which rs the adequate way ofwarking when one requires a precise position. We describe here a simple method which can he used to solve this problem, and makes use of a double beam spectmmeter. In this type of spectroscopy the spectra in solution is measured by placing the sample in one beam and the solvent in the other beam (See curve A, Fig. 1). Interchanging the sample cell and the blank generates a transposition spectrum, which far convenience is called a trans spectrum (curve A', Fig. 1). The curve of the trans spectrum has a minimum where the normal spectrum has a maximum and vice versa. Manipulating conveniently the positions of zem and 100% transmission generates intersections of the curves of the t r a n s s p e c t n h and the spectrum:
Figure 1. A sample spectrum and its inverse.
Figure 2. Analysis of a family d transposed spectra to determine the maximum.
1) Measure three spectra and three trans spectra with the maximum expansion an the horizontal scale and preferably on the same graph paper. See Figure 2. 2) Next draw straight lines through the intersections of curve A with curve A', B' and C'. 3) Find the center points for each of the three lines and connect them by a line which is extended until it cmsses line A. This intersection point corresponds to Amax. 4) Repeat steps 2 and 3 with intersection of A' with A, with B, and with C; this gives the determined A,., on curve A'. ln this way one obtains the values far A., whose average is highly reproducible. Using this method with a Perkin-Elmer 450 spectrometer, one can determine the absorption maximum a t 300 nm for 4-nitrobiphenyl with a reproducibility of +0.3 nm which on e regular spectra has a maximum, which is -10 nrn wide. The theoretical basis for this method lies in the fact that i t permits one to define a family of lines which necessarily converge a t the maximum sought. The last curve of the family has zero longitude and its middle point coincides with the maximum.
Observation Usually the line which unites the average points of the curves is a straight line which makes its extension easy. If a cnlc nrnsri n hew thri h e is not stralxht, thenone ha? to rnrwase the n u m h c r d rransspectralcurvei used. Thi, mt.thud la e..pecmll) useful when one Iner to dlrt~ngurshbetween two bands whirh arr very eloae, for rxnmplr, in rhc rhnrartcrirouon of iaomers which have similar iprctra. i r ia also useful for studymg small shifts in the bands, resulting from a change of solvent or the other solutes present. National University of Columbia Bogota. Columbia
J a i m e De La Zerda L. Piero De M i l h i P. Jose Luis Villaveces C.
Volume 52, Number 6, June 1975 / 415
