Spectral Standards

Obvious species such as CO provide useful spectral standards and are plentifully abundant in space. The Doppler shift for CO can then be applied to other unidentified transitions to see if they are coincident with known transitions in the laboratory spectrum of a molecule. Different molecular environments may complicate matters, with some CO molecules along the line of site of the telescope having different Doppler shifts (Figure 3.12). The CO transition at 115 GHz may then appear to be split into several lines associated with a different Doppler shift in each cloud. The identification problem now also has to decide to which cloud the unknown transition belongs. 

Despite the difficulty imposed by the attenuation and dispersion of the atmosphere on subpicosecond THz pulses, there has been some effort to extract spectroscopic signatures of explosives, such as RDX, from standoff ranges up to 30 m [91], However, the measured spectral absorption peak of RDX artificially broadens, due to the absorption and dispersion in the atmosphere, as the distance to the target increases. This broadening at standoff distances suggests that spectroscopic identification of explosives such as RDX might be problematic because the apparent spectral shape cannot be directly compared to a spectral standard curve. To circumvent the atmospheric attenuation and dispersion... 

Guo C, Shah RD, Dukor RK et al (2006) Fourier transform vibrational circular dichroism from 800 to 10000 cm(—1) near-IR-VCD spectral standards for terpenes and related molecules. Vib Spectrosc 42 254-272... 

Connetquot, New York 0.4-/am filtered (river water HA as spectral standard) Precipitation with acetic HA 0-1.2 (1978) Hair and Bassett... 

To aid in identification of aldehydes and ketones in tobacco smoke and in cellulose smoke, over 90 2,4-DNPH derivatives were prepared to serve as melting point and spectral standards. 

A.J. Davies, Spectral Data Standard Exchange Formats. CSA23. 

The peak at mje = 98 is taken as the arbitrary standard. The height of the other peaks is measmed relative to it. Once this matrix has been established, ordered sets of mass spectral peak heights at mfe = 69, 83, 84, and 98 constitute the experimental b vector for an unknown mixture that contains or may contain the four... 

With the availability of computerized data acquisition and storage it is possible to build database libraries of standard reference spectra. When a spectrum of an unknown compound is obtained, its identity can often be determined by searching through a library of reference spectra. This process is known as spectral searching. Comparisons are made by an algorithm that calculates the cumulative difference between the absorbances of the sample and reference spectra. For example, one simple algorithm uses the following equation... 

Accuracy When spectral and chemical interferences are minimized, accuracies of 0.5-5% are routinely possible. With nonlinear calibration curves, higher accuracy is obtained by using a pair of standards whose absorbances closely bracket the sample s absorbance and assuming that the change in absorbance is linear over the limited concentration range. Determinate errors for electrothermal atomization are frequently greater than that obtained with flame atomization due to more serious matrix interferences. 

Atomic emission is used for the analysis of the same types of samples that may be analyzed by atomic absorption. The development of a quantitative atomic emission method requires several considerations, including choosing a source for atomization and excitation, selecting a wavelength and slit width, preparing the sample for analysis, minimizing spectral and chemical interferences, and selecting a method of standardization. 

Eatty acids from commercial fats and oils, such as peanut oil, are extracted with methanolic NaOH and made volatile by derivatizing with a solution of methanol/BE3. Separations are carried out using a capillary 5% phenylmethyl silicone column with MS detection. By searching the associated spectral library students are able to identify the fatty acids present in their sample. Quantitative analysis is by external standards. 

Transmission Fourier Transform Infrared Spectroscopy. The most straightforward method for the acquisition of in spectra of surface layers is standard transmission spectroscopy (35,36). This approach can only be used for samples which are partially in transparent or which can be diluted with an in transparent medium such as KBr and pressed into a transmissive pellet. The extent to which the in spectral region (typically ca 600 4000 cm ) is available for study depends on the in absorption characteristics of the soHd support material. Transmission ftir spectroscopy is most often used to study surface species on metal oxides. These soHds leave reasonably large spectral windows within which the spectral behavior of the surface species can be viewed. 

EPA.INIH Mass Spectral Data Base, Vol. 1, U.S. National Bureau of Standards, Wasliiagton, D.C., 1978, p. 5. 

Spectra. The abiHty to consult collections of standard spectra is cmcial in the analysis of unknown compounds. A long history of data collection efforts has been aimed at these appHcations. Among the best known of the pubHshed handbooks are the Sadtkr Spectral Data Sheets which include ir,... 

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