Spectra from plasma sources

Mass spectrum from Y. Su, Y. Duan, and Z. Jin, Helium Plasma Source Time-of-Flight Mass Spectrometry Off-Cone Sampling tor Elemental Analysis, Anal. Chem. 2000, 72, 2455. 

The use of MRM methods for quantitative bioanalysis often reduces sample preparation and analysis time. The MRM method that used LC/ESI-MS/MS for the quantitative analysis of an anticancer drug, Yondelis (Ecteinascidin 743, ET-743, trabectedin. Scheme 9), in human plasma was demonstrated by Rosing et al. [103]. The full-scan mass spectrum of ET-743 (MW 762) contained an abundant [MH+ - H2O] ion at m/z 744 as a result of loss of water molecules from in-source CID (spectrum not shown). The internal standard, ET-729 (Scheme 9, MW 747), exhibited similar performance in the full-scan mass spectrum an abundant [MH+ - H2O] ion at tn/z 730 was produced. The product ion spectra of ET-743 and ET-729 exhibited the most abundant fragment ions at m/z 495 and m/z 479, respectively (spectra not shown). The product ion at m/z 495 (C27H31N2O7) was formed in the collision cell after cleavage of the sulfur bond and ester binding at C-11 [103]. 

Apart from the atomic and ion lines of the species present in a plasma source an emission spectrum has a continuum on which the emission lines are superimposed. This extends over the whole spectrum. It is due to the interactions between free electrons ( Bremsstrahlung ) and to the interaction of free and bound electrons ( recombination continuum ). The former is particularly important in the UV spectral region, whereas the latter is important at longer wavelengths. The spectral intensity distribution for the continuum radiation is given by ... 

ICP/OES can be conducted either simultaneously or sequentially. Simultaneous instruments rely on a polychromator or direct-reading spectrometer to read up to 60 elements from the same sample excitation. Sequential analyses use a computer-controlled, scanning monochromator system. The light emitted by the sample in the plasma source is focused on the entrance slit of the monochromator and the spectrum is scanned through the region of interest. Typically, it is possible to determine several elements per minute in the sample in a sequential spectrometer. 

Fig. 21. High-resolution spectrum of the A (0,5,0)<—A" (0,0,0) vibrational band of CF2 near 252 nm. Also shown is a theoretical spectrum of the same band simulated with a rotational of 400 K. (From Booth et al.. Plasma Sources Sci. Technol., in press.)...

The spectrometer used to measure the VUV reflectivity spectrum has been described in detail elsewhere [20]. A continuum of VUV and soft X-ray radiation from a laser induced plasma source is focussed onto the polymer film, and the light reflected from the film dispersed by a 1 meter polychromator onto a optical multichannel analyzer. The... 

Fig. 1. The above pathways for fatty acid synthesis have been demonstrated to be present in the aorta. The thickness of the arrows denotes the author s interpretation of the relative contribution to total synthesis made by the three intracellular sites. The mitochondrial pathway has the largest capacity to utilize acetate for the elongation of available acyl units. The latter are derived from plasma free fatty acid (FFA) and lipolysis of tissue triglyceride (TG). The cytosol has a limited capacity to synthesize fatty acids from acetate because of minimal acetyl-CoA carboxylase (ACC) activity. The significance of fatty acid synthetase (FAS) activity is dubious in the absence of a source of malonyl-CoA. A microsomal elongation-desaturation pathway can synthesize a spectrum of saturated (SAT) and unsaturated (UNSAT) long-chain fatty acids, similar to the products of the mitochondrial system.

The Einstein derivation assumes an idealized situation in which the excitation and de-excitation processes are controlled by the radiation in the cavity. This situation is achieved in reality only inside stars or very high temperature plasmas. In a low pressure discharge there is usually no thermal equilibrium between the atoms and the radiation because the light readily escapes from the source. Consequently we observe a line spectrum rather than a continuum, Collisions play the dominant role in maintaining the equilibrium populations in a low pressure discharge. However, this does not invalidate the arguments used here since the A- and B-coefficients depend only on the internal structure of the atoms and not on their external environment. 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

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