Construction and evaluation of ion selective electrodes for nitrate with asumming operational amplifier. Application to tobacco analysis

In this paper, the construction and evaluation of an electrode selective to nitrate with improved sensitivity, constructed like a conventional electro de (ISE) but using an operational amplifier to sum the potentials supplied by four membranes (ESOA) is described. The two types of electrodes, without an inner reference solution, were constructed using tetraoctylammonium bro mide as sensor, dibutylphthalate as solvent mediator and PVC as plastic mat rix, the membranes obtained directly applied onto a conductive epoxy resin support. After the comparative evaluation of their working characteristics they were used in the determination of nitrate in different types of tobacc o. The limit of detection of the direct potentiometric method developed was found to be 0.18 g kg(-1) and the precision and accuracy of the method, wh en applied to eight different samples of tobacco, expressed in terms of mea n R.S.D. and average percentage of spike recovery was 0.6 and 100.3%, respe ctively. The comparison of variances showed, on all ocassions, that the res ults obtained by the ESOA were similar to those obtained by the conventiona l ISE, but with higher precision. Linear regression analysis showed good ag reement (r = 0.9994) between the results obtained by the developed potentio metric method and those of a spectrophotometric method based on brucine, ad opted as reference method, when applied simultaneously to 32 samples of dif ferent types of tobacco. (C) 2001 Elsevier Science B.V. All rights reserved .