Correlation techniques, wavelength

In the gas correlation techniques, gas-filled cells mounted on a rotating disk cross the analyzing infrared beam in turn. One correlation cell is filled with a gas that will not absorb infrared light, such as nitrogen (N2). The other cell (or cells) are filled with a high concentration of the gas to be measured. The wavelength range is selected at the absorption band of the gas to be measured by an optical band-pass filter. 

Several methods have been proposed to overcome multiple scattering. One simple solution proposes the use of thin samples, but flare effects and wall interactions complicate data interpretation [II], Alternative solutions involve cross correlation [13,36,126,1271 and two color cross correlation techniques [ 11,13,36) employing simultaneous illumination of the sample by two laser beams with differing wavelengths. 

One of the most important considerations is the concentration of the component being measured. The larger the concentration, the more options for the analyst. When considering vibrational spectroscopic analyzers, a major component will have numerous wavelengths at which it may be anal)rzed. Minor components require the analyst to seek wavelengths at which they have major absorbances and, almost invariably, use multiple wavelength correlation techniques such as partial least squares (PLS) or principle component analysis. 

Infrared (IR) spectroscopy is probably the quickest and cheapest of the spectroscopic techniques in determining the functional groups of the sample. The samples can be soUds, liquids, or gases and can be measured in solution or as neat liquids mulled with KBr or mineral oil. Comparison of IR spectra of substances of known structure has led to many correlations between wavelength (or frequency) of IR absorption and features of molecular structure. Certain structural features can easily be established. For example, in an organic compound that contains only C, H and O, the oxygen can only be present as C=0,0—H, or C—O—C or a combination of these, such as the ester or carboxylic acid group. 

Smith (NASM), Mazeh, Zucker (Tel-Aviv U.), and Latham (CfA) have used CSHELL to seardi for brown dwarf companions to nearby stars. They are now analyzing spectra of brown dwarf companion candidate stars with a 2-D cross-correlation technique that can extract the velocity of a companion whidi contributes only 1% of the total flux. Contrast between primary and secondary stars is mazdmumin the near-infrared wavelengths they observed. 

Pressure. Standard atmospheric pressure is defined to be the force exerted by a column of mercury 760-mm high at 0°C. This corresponds to 0.101325 MPa (14.695 psi). Reference or fixed points for pressure caUbration exist and are analogous to the temperature standards cited (23). These points are based on phase changes or resistance jumps in selected materials. For the highest pressures, the most rehable technique is the correlation of the wavelength shift, /SX with pressure of the mby, R, fluorescence line and is determined by simultaneous specific volume measurements on cubic metals... 

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