Multiple scanning

Fig. 21. Raman spectra showing improvement of signal-to-noise using multiple scans with computer time averaging over single scan. Lower traces single scan upper traces multiple scans (10 scans) and computer output, (a) i of CCU (b) Hg emission line and n of LiCh (c) n of Na02 with oxygen isotopic counterparts (89).

A full-range IR spectrum can be recorded in about 1 second and sensitivity can be enhanced by accumulating multiple scans which are then computer-processed to increase the signal-to-noise ratio. 

NMR has been used comparatively little for quantitative analysis although peak areas are directly proportional to concentration. The principal drawbacks are the expensive instrumentation and a lack of sensitivity. The latter can be improved with the aid of computers to accumulate signals from multiple scans or by using a pulsed (Fourier transform) technique. Relative precision lies in the range 3-8%. 

Additionally, with the inclusion of computers as part of an instrument, mathematical manipulation of data was possible. Not only could retention times be recorded automatically in chromatograms but areas under curves could also be calculated and data deconvoluted. In addition, computers made the development of Fourier transform instrumentation, of all kinds, practical. This type of instrument acquires data in one pass of the sample beam. The data are in what is termed the time domain, and application of the Fourier transform mathematical operation converts this data into the frequency domain, producing a frequency spectrum. The value of this methodology is that because it is rapid, multiple scans can be added together to reduce noise and interference, and the data are in a form that can easily be added to reports. 

Single scan DSC information is the most commonly used instrumental mode, but multiple scans performed on the same sample can be used to obtain additional information about the characteristics of a material, or of the reproducibility associated with a given process. 

The solution CPPL spectrum of the polymer was also successfully obtained, using a home-constructed dynamic flow-through cell, so that multiple scans could be obtained to increase the signal/noise ratio. Ten scans were recorded and afforded a very similar, though noisier, CPPL profile to the aggregate case. 

Like always in NMR 118), when the single-scan signal-to-noise ratio SjN) is insufficient for a precise evaluation of the acquired data, it can be enhanced by averaging or, more frequently, accumulating, of data sets obtained in a number of repeated scans. Multiple-scan data accumulation also opens a number of novel experimental possibilities extending beyond... 

Figure 17.49 Cyclic voltammery of Co(DTB)32 + at a transparent PEDOT modified FTO. Scan speed 100 mV/s, potential referred to SCE. The smaller peak centered at about —650 mV is determined by the intrinsic polymer electroactivity. PEDOT was grown by multiple scan deposition (three scans), cycling the potential between —0.7 and +1.5 V versus SCE with a scan speed of 20mV/s. From Bignozzi et al., unpublished results.

A nickel surface in contact with an alkaline solution is spontaneously covered with nickel hydroxide Hereby a-nickel hydroxide is formed which on aging is converted to the p-form At about 0.63 V (NHE) a-nickel hydroxide is oxidized to y-nickel oxide hydroxide. At a 80 mV more positive potential the P-form is transformed to p-nickel oxide hydroxide In the cathodic scan nickel oxide hydroxide is reversibly reduced to nickel hydroxide at about 0.54 V. In multiple scan cyclovoltammetry the current increases with each scan indicating an increase of the active electrode surface. 

Low, M. J. D., Epstein, L. R., and Bond, A. C., Infrared spectra of methyldiboranes. The examination of unstable substances using multiple-scan interference spectroscopy, Chem. Comm., 226 (1967). 

Self-assembly is a massively parallel process, and can normally involve very large numbers of components (a large crystallization might involve 1027molecules). Robotic pick-and-place methods for placement are limited by the fact that they are serial. Although they can be accelerated by using a number of robotic devices in parallel (for example, the multiple scanning probe heads of the IBM millipede 131), they cannot approach the number of molecules in a test tube, for example. 

The determination of TCDD in the final extract was by high resolution mass spectrometry multiple scan averaging. No gas chromatography was employed the sample was vaporized via the direct insertion probe (12.13). This method is inherently more prone to interferences than GC-MS (17.18) the response observed may not be due to TCDD. The results do serve as an indication, however, that if TCDD is present in human milk it is at 1 ppt or less. Thus any methods developed must be capable of this detection limit. 

A full-range ir spectrum can be recorded in about 1 second and sensitivity can be enhanced by accumulating multiple scans which are then computer-processed to increase the signal-to-noise ratio. One of the major uses for FT-ir spectrometers is in coupling them to gas chromatographs in what is called a coupled or hyphenated technique, i.e. gc-ir (p. 111). This enables the spectra of eluting peaks to be recorded without the need to trap the component or stop the gas flow. Computer enhancement and manipulations of the recorded spectra are additional features of both stand-alone FT-ir spectrometers and gc-ir systems. 

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