Species concentrations, chemometrics

Finally, the development of chemometrics over the past 20 years has also aided in the use of UV-vis technology for more complicated chemical matrices than was possible at earlier times. Chemometrics allows large quantities of spectral data to be analyzed and reduced to a useful bit of information such as the concentration of one or more chemical species. In addition, through analysis of residuals, it is possible to detect when something unexpected occurs in a process. 

To shed light on the mechanism of formation of silsesquioxane a7b3, to identify the species formed during the process, and to try to explain the high selectivity towards structure a7b3 of the optimised synthetic method described above (64% yield in 18 h), the synthesis of cyclopentyl silsesquioxane a7b3 was monitored by electrospray ionisation mass spectrometry (ESI MS) [50-52] and in situ attenuated total reflection Fourier-transform infrared (ATR FTIR) spectroscopy [53, 54]. Spectroscopic data from the latter were analysed using chemometric methods to identify the pure component spectra and relative concentration profiles. 

Fig. 9.13 Relative concentration profiles of the three species identified by chemometric deconvolution of the complex ATR FUR spectra reported in Fig. 9.11.

Conducting polymer sensors can be operated either to quantitatively measure the concentration of a target vapor species or to qualitatively analyze a complex mixture of vapors. For single vapors, the detection limits can be in the low-ppm region. Exposure to a mixture of vapors results in a unique pattern of responses, which is usually deciphered using standard chemometric techniques. The pattern can be used like a fingerprint to identify certain products, or to establish the quality of foodstuffs, wines, perfumes, etc. The electronic nose has similar components as the natural nose this is illustrated in Figure 1.15. 

The participation of cluster species in these catalytic syntheses of heterocycles can find some support in a chemometric optimization of the Ru3(CO)i2-catalyzed cyclization of 2-nitrostilbene to 2-nitrophenylindole.l ° The results obtained suggest the existence of two different mechanisms, one based on a Ru(CO)5-catalyzed process (favored at low concentrations of the catalyst) and another based on a Ru3(CO)i2-catalyzed process (favored at high catalyst concentrations). This is related to the well known equilibrium between monomeric and cluster species under CO pressure (previous section) The Ru(CO)5-catalyzed process is first-order with respect to the substrate and electron transfer from the complex to the nitro compound is probably the rate-determining step. The Ru3(CO) 12-catalyzed process seems to be zero-order with respect to the substrate and possibly dissociation of CO is rate-determining. 

Aqueous CoMo solutions with a Co/Mo ratio of 0.5 containing only H4Co2Moio038 anions were prepared and their speciation with respect to pH was determined by Raman spectroscopy from chemometric methods. It was shown that at molybdenum concentration of 0.8M, H4Co2Moio038 dimers remain the main species up to pH 7. 

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