Signal Polarization Techniques

Polarization techniques have also been used to determine mechanisms by which microorganisms induce localized corrosion in the forms of pitting or crevice corrosion. In most cases itpit was determined in the presence and absence of bacteria, itpit provides data as to the tendency for pitting, but not the rate for pit propagation. Salvarezza et ah and De Mele 

Ringas and Robinson performed electrochemical tests on stainless steels and mild steels in three cultures of SRB. In all cases pitting resistance was lower in cultures of SRB. Potentiodynamic polarization 

in I2th International Corrosion Congress, NACE International, Houston, TX, 1993. 

Mansfeld, R. Tsai, H. Shih, B. Little, R. Ray, P. Wagner, in CORROSlON/90, paper no. 109, NACE International, Houston, TX, 1990. 

Ferrante and D. Feron, m Microbially Influenced Corrosion and Biodeterioration, Ed. by N. J. Dowling, M. W. Mittleman, and J. C. Danko, University of Tennessee, Knoxville, TN, 1991, pp. 3-55-3-63. 

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On metals in particular, the dependence of the radiation absorption by surface species on the orientation of the electrical vector can be fiilly exploited by using one of the several polarization techniques developed over the past few decades [27, 28, 29 and 30], The idea behind all those approaches is to acquire the p-to-s polarized light intensity ratio during each single IR interferometer scan since the adsorbate only absorbs the p-polarized component, that spectral ratio provides absorbance infonnation for the surface species exclusively. Polarization-modulation mediods provide the added advantage of being able to discriminate between the signals due to adsorbates and those from gas or liquid molecules. Thanks to this, RAIRS data on species chemisorbed on metals have been successfidly acquired in situ under catalytic conditions [31], and even in electrochemical cells [32]. 

The problems involved in quantitative analysis using NMR spectroscopy, have been discussed by several authors and it is evident that it still causes a lot of problems as especially pointed out by Hays55 in his excellent review on the subject. Thus in liquid state NMR spectroscopy the quantitative estimation of atoms and groups involves the use of normal analytical method. In the case of solid state NMR spectroscopy, however, the application of the cross-polarization technique results in signal enhancements and allows repetition rates faster than those allowed by the carbon C-13 Tl. Therefore, the distortion of relative spectral intensities must always be considered a possibility, and hence quantitative spectra will not always be obtained. 

JMOD (APT) is usually performed with broadband H decoupling and is relatively sensitive to the precise matching of the delay D2 to the J,., coupling constant, and so is less easier to use than the polarization techniques DEPT and INEPT. On the other hand, only one single experiment is necessary to measure the signals of all carbon multiplicities. 

Sum-frequency generation (SFG) at second-order and the nonlinear Raman spectroscopy BioCARS at fourth-order can also probe chiral molecules. They have no analog in linear optics. We show that both are only symmetry allowed in a fluid, if the fluid is chiral. However, in contrast to optical activity phenomena, these processes arise entirely from induced electric-dipoles (without magnetic or quadrupolar transitions) and they are not circular differential. All laser beams can be linearly polarized and no polarization modulation is required as the detection of a sum-frequency (yiz. five-wave mixing) photon is in itself a measure of the solution s chirality. Since an achiral solvent can not contribute to the signal, these techniques are sensitive, background-free probes of molecular chirality. The SFG... 

When the nucleus under study has poor abundance, such as for carbon-13, the cross polarization technique is used. This uses a complex pulse sequence to transfer the polarization from a nucleus with high abundance (normally hydrogen, but phosphorus and aluminium are also used) to the second nucleus, enhancing its signal. 

As shown in Fig. 11, the decrease in the Al MAS NMR signal of tetrahedrally coordinated framework aluminum at about 60 ppm is accompanied by the appearance of signals at about 0 ppm and 30 ppm. Furthermore, in the spectra recorded by the application of the cross-polarization technique (right-hand side), the intensities of the signals at 0 ppm and 30 ppm increase inversely to the intensity of the signal at 60 ppm. Therefore, the authors suggested that H- Al CP/MAS NMR experiments preferentially monitor non-framework aluminum atoms. In addition, this observation indicates that the 30-ppm line is an independent signal [90]. 

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