External gravimetric calibration

Monitoring the MMA/DMAAm reaction is challenging becanse both monomers have very similar NIR spectra, and because other interfering snbstances are present in the reaction mixtnre. The anthors prepared calibration samples gravimetrically and made the NIR measurements at reaction temperatnres. The calibration sets consisted of only five or six samples, which is considerably fewer than standard recommendations for NIR chemometric model development. The final models for MMA and DMAAm were validated with an internal validation set as well as an external reaction validation. The performance of the models is summarized in Table 15.6. This table inclndes a measurement of the standard deviation of an external GC method... 

Nitrogen and Carbon Dioxide are analyzed on a Caibosieve S column (10 x 1/8") at 30°C isothermally using a TCD detector. Both analyses used Helium carrier gas at 30 ml/min., a 0.25 cc loop and a heated valco valve. An external gravimetric standard was used for calibration throughout the process of analysis. 

Related to the above-described inter-elemental correction method, the need for a proxy element can be obviated at the expense of resorting to sequential analysis of the sample and the calibrator, that is, with external gravimetric calibration. Although this technique is not favored by all, it has played a major role, for example, in the recent re-evaluation of the atomic weight of zinc [13]. 

The isotope dilution analysis method provides a definitive technique that does not depend on the comparison to one or more external calibration standards for quantitation. The method is similar to the standard addition technique in that a known quantity of analyte material is added to the sample to be analyzed, thereby performing the quantitation in situ. This approach ehminates matrix interference problems. The method is also similar to the internal standardization procedure, with the exception that a form of the analyte itself serves as the internal standard. AU of these factors combine to provide analysis characteristics comparable to the most basic gravimetric methods. 

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