Spectral scan

Muller R H and Farmer J C 1984 Fast self-compensating spectral-scanning ellipsometer Rev. Sol. Instrum. 55 371-4... 

Fig. 1. Serial spectral scans following rapid neutralization of an alkaline solution of 6-hydroxy-2-methylpteridine.

Figure L Narrow range mass spectral scan of molecular ion region during GC elution of NDMA, A, Recorded at retention time of NDMA beer sample con-taining 0.6 fig/kg NDMA (0.15 ng). B. Background 1 min before elution of NDMA. C. Standard solution containing 0.4 ng NDMA. D. Background.

The most widely regarded approach to accomplish the determination of as many pesticides as possible in as few steps as possible is to use MS detection. MS is considered a universally selective detection method because MS detects all compounds independently of elemental composition and further separates the signal into mass spectral scans to provide a high degree of selectivity. Unlike GC with selective detectors, or even atomic emission detection (AED), GC/MS may provide acceptable confirmation of the identity of analytes without the need for further information. This reduces the need to re-inject a sample into a separate GC system (usually GC/MS) for pesticide confirmation. Through the use of selected ion monitoring (SIM), efficient ion-trap or quadrupole devices, and/or tandem mass spectrometry (MS/MS), modern GC/MS instruments provide LODs similar to or lower than those of selective detectors, depending on the analytes, methods, and detectors. 

The main advantages of plasma-source mass spectrometry (PS-MS) over other analytical techniques, such as PS-AES and ETAAS, are the possibilities of quantitative isotope determination and isotope dilution analysis the rapid spectral scanning capability of the mass spectrometer and semiquantitative determinations to within a factor of two or three. Several labelling methods are used for the quantification of analytes present in complex mixtures. In these methods, the sample is spiked... 

Figure 9.2 Typical spectral scan of a fluorescent compound showing its absorbance peak or wavelengths of most efficient excitation and its emission peak or wavelengths where light emission occurs. The Stoke s shift is the distance in nanometers between the absorbance peak and the emission peak. The larger the Stoke s shift, the less interference that will occur from excitation light when measuring fluorescence emission.

In a Fourier transform IR instrument the principles are the same except that the monochromator is replaced by an interferometer. An interferometer uses a moving mirror to displace part of the radiation produced by a source (Fig. 5.4) thus producing an interferogram which can be transformed using an equation called the Fourier transform in order to extract the spectrum from a series of overlapping frequencies. The advantage of this technique is that a full spectral scan can be acquired in about 1 s compared to the 2-3 min required for a dispersive instrument... 

Deactivated catalysts (5.4-5.5 mg) were loaded at ambient conditions into a 0.1 cm id X 2.0 cm quartz tube having one sealed end. Catalyst was held in place with a plug of Amersil quartz wool. Both quartz wool and tubes were baked out at 500°C for one hour prior to use. The loaded tube was placed into the electrically heated tip of the solids probe of a DuPont model 21-491 mass spectrometer operated at 1 X 10 7 torr. The solids probe was located in the mass spectrometer so that its tip, through which desorbing hydrocarbon molecules passed, was immediately adjacent to the ion source. The probe temperature was raised from 38° to 385°C in 22 minutes. Spectral scanning was conducted automatically every 30 seconds from approximately mje 16 to ra/e 650 at 10 sec/decade. 

Meat discoloration studies typically involve a maximum of 5 days, with discoloration analy-sis being performed every day or on alternate days. The actual experimental time involved in the objective assessment of discoloration is not extensive and depends on the number of samples being analyzed. Colorimetric measurements with hand-held colorimeters are very rapid (three measurements per meat surface in < 1 min). Spectral scans of meat surfaces require 1 to 2 min. Extraction and analysis of ground meat products has the added step of homogenization and filtration prior to spectrophotometry, but relative to many laboratory procedures, this is relatively quick. Isolation and purification of preparative amounts of myoglobin requires only 2 to 3 days once appropriate preparations are made. Finally, metmyoglobin can be reduced to oxymyoglobin in 15 to 20 min. 

Figure 10. Excitation (left) and emission (right) spectra optimized for aleurone tissue showing intensity differences between aleurone, endosperm, and pericarp tissues. The emission monochromator was set at 445 nm for excitation spectral scans and the excitation monochromator was set at 350 nm for emission spectral scans. RFI = relative fluorescence intensity. (From [29])...

The influence of acid concentration on the absorbance of methanolic GA3 at 253 m/x as a function of time at 23° C. is shown in Figure 1. Because at least two species contribute to this absorbance, the curves give only a crude indication of the initial rate of degradation of GA3. Spectral scans of the solutions show that after ithe initial rise, the absorbance at 253 m begins to contain appreciable contributions from a number of additional components. The number of additional components and their rate of appearance and disappearance increase markedly with increasing acidity. GA3 appears to be relatively stable in neutral methanol. 

Signal Averaging Addition of successive spectral scans for... 

The temperature was indicated and controlled to 0.1°C with a platinum resistance probe which extended 1 mm inside the inner surface of the cell. The pressure was adjusted using a 100 cc Ruska syringe pump and was measured to within 0.1% with a 710A Heise digital pressure gauge which is traceable to an NBS standard. The pressure varied less than 0.15 bar (2 psi) during a spectral scan. The wavelength accuracy was 0.2 nm for the Varian (Cary) 2290 spectrophotometer. 

The latter permits spectral scanning of the entire fluorescence spectrum using boxcar averaging. In Figure 15 is shown a comparison of the normal CH flame emission from an oxy-acetylene slot torch and the laser induced fluorescence spectrum. The CH was... 

For MS work, the electron impact (El) mode with automatic gain control (AGC) was used. The electron multiplier voltage for MS/MS was 1450 V, AGC target was 10,000 counts, and filament emission current was 60 pA with the axial modulation amplitude at 4.0 V. The ion trap was held at 200°C and the transfer line at 250°C. The manifold temperature was set at 60°C and the mass spectral scan time across 50-450 m/z was 1.0 s (using 3 microscans). Nonresonant, collision-induced dissociation (CID) was used for MS/MS. The associated parameters for this method were optimized for each individual compound (Table 7.3). The method was divided into ten acquisition time segments so that different ion preparation files could be used to optimize the conditions for the TMS derivatives of the chemically distinct internal standard, phenolic acids, and DIMBOA. Standard samples of both p-coumaric and ferulic acids consisted of trans and cis isomers so that four segments were required to characterize these two acids. The first time segment was a 9 min solvent delay used to protect the electron multiplier from the solvent peak. 

This report will discuss the results of a study in which an optical multichannel analyzer (OMA) was coupled to standard spectrometers to record both the UV/VIS absorption and fluorescence emission spectra of complex mixtures of PAH s separated by HPLC techniques "on-the-fly" (i.e., one second spectral scans of the HPLC effluent stream) and stored on a floppy disc for subsequent retrieval and data analysis. The system described has the capability of storing 250 (500 point) spectra and can readily be used to increase the effectiveness of HPLC analysis by allowing both quantitative and qualitative data to be obtained. 

If the flame background around the wavelength to be used is variable, as a consequence of variable amounts of potentially interfering matrix elements, it is advisable to scan the emission spectrum in the vicinity of each wavelength of interest for each sample and standard in routine analysis. Spectral scan rate, the wavelength interval studied for each determination, and electronic damping must all then be carefully optimized. 

Each 13C nucleus is magnetically coupled to the protons bonded to it. Under the right circumstances, this magnetic coupling allows the transfer of polarization from the protons to the carbon nucleus. The number of protons bonded to the 13C nucleus determines how this polarization transfer occurs. A DEPT experiment usually includes three spectral scans ... 

Monochromator-based detection, however, can perform spectral scanning that is useful for the assay development and for spectral determinations of fluorophores and compounds. Monochromator types eliminate the need to buy different filter sets for each wavelength. Our laboratory uses several filter-based plate readers and one monochromatic type to accommodate the needs of assay development and compound screening (Table 11.2). 

Operating Conditions Collect not less than 64 FTIR spectral scans of the standards and sample in the absorbance mode. Boxcar apodization and 2 cm-1 resolution are recommended parameters. Spectral normalization should be done on the 4333 cm-1 peak to account for varying sample thicknesses. Use identical operating conditions for the standards and for the sample. 

Collect not less than 64 spectral scans of the standards. Constmct a calibration matrix containing infrared absorbance values for unsaturation types in the standards and their known concentrations. Confirm the validity of the calibration matrix model as recommended in the software manual. A recommended method is cross-validation for all standards by sequentially excluding one of the standards from the calibration matrix, then using the remaining standards to predict the concentrations. After validation, determine the optimum number of factors, or loading vectors, needed to minimize the deviation between actual and predicted concentrations. This determination is automated in most multicomponent analysis... 

Although standard IR spectrometers are used for studying the amide bands, FTIR spectrometers are more accurate and reliable. FT-IR spectrophotometers are based upon the Michelson interferometer. A typical instrument (Fig. 7.1) comprises an optical bench housing the interferometer, sample, infrared source and detector, coupled to a computer, which controls the spectral scanning, analysis and data processing (for review see Griffiths, 1980). 

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