CS molecular absorption

Figure 6.11 CS molecular absorption of 1% v/v sulfuric acid solution recorded by the high-resolution simultaneous absorption spectrometer (ARES) using a fuel-rich air/acetylene flame...

Huang et al. [70] investigated in detail the determination of sulfur at CS molecular absorption lines in a fuel-rich air / acetylene flame, as described in Section 6.41. As sulfur may be present in a variety of chemical forms in a sample, the authors investigated sulfates, sulfites and sulfides under differenf conditions regarding fheir analytical sensitivity. 

Table 8.2 Determination of sulfur in cast iron CRM (BAM, Berlin, Germany) using the CS molecular absorption at 257.96 nm and HR-CS F AAS with a fuel-rich air/acetylene flame...

M.D. Huang, H. Becker-Ross, S. Florek, U. Heitmann, M. Okruss, Application of High-Resolution CS Molecular Absorption Spectra to the Direct Determination of Sulphur using a Continuum Source Atomic Absorption Spectrometer and an Air-Acetylene Flame, J. Anal. At. Spectrom. (2005) submitted. 

Figure 4.10. Molecular absorption of the CS radical over the wavelength range 257-260 nm (from Welz et al. [10]).

Fourthly, as a continuous radiation source is used in HR-CS AAS, any line of the spectrum is available, and even molecular absorption lines can be used for quantitative determination, as shown for the determination of P at PO bands and sulfur at CS bands using FAAS. 

The high-intensity continuum source offers a number of advantages over narrow line sources. Secondary lines (nomesonance lines) can be used in order to reduce sensitivity, thereby avoiding dilntions. All spectral lines are available, including lines for which no HCL or EDL sources are available, such as fluorine and chlorine. In addition, molecular absorptions, such as those of diatomic fragments like CS or PO, can be used to measure sulfur and phosphorus (Huang et al.)... 

In Chapter 7 several electron excitation spectra of diatomic molecules that have been recorded in a flame were shown. Some of these spectra are only observed in the presence of specific mafrices or acids, such as those of the PO and CS molecules, whereas others, particularly ihe spectrum of OH, is present in any spechoscopic flame, even when no sample is introduced. The concentrations of radicals such as O, OH, CN and H in a flame are determined by the reactions between the natural components of the flame. The influence of any sample constituents on these components is negligible, since every reaction that leads to a reduction or an increase in the concentration of such a species is immediately counteracted by infinitesimal shifts in the equilibria of the main components. This means that these molecular absorption structures in a flame should remain constant, and be eliminated in the calibration process. However, this is not exactly the case, as will be shown in the following examples. 

One of the characteristics of HR-CS AAS is that no element-specific radiation source is required, and that, in principle, absorption can be measured at any wavelength, including molecular absorption bands. 

One possibility that HR-CS AAS offers in order to cope with this potential interference is least-squares BC (see Section 5.2.2), in cases where the source of the molecular absorption is known and a reference spectrum can be produced. In the present example the reference spectrum could be generated by the vaporization of KHSO4, which is shown in Figure 8.16. 

Schlemmer et al. [124] continued this research using HR-CS AAS, where the nature of the background absorption becomes apparent, as shown in Figure 8.50. It is quite obvious from the structure of the peak that the interference is not due to an atomic line, but to some molecular absorption, although the kind of molecule that causes the absorption is yet unknown. It also becomes clear that part of the problems observed in LS AAS is the rapid change of the absorbance with time, which is difficult to follow with sequential correction systems. 

Molecular Cl(H)Si=S (126) was also formed in an argon matrix in a photochemically induced reaction of SiS with HC1. From the isotopic splittings (H/D and 35C1/37C1) of the IR absorptions the Cs structure of the species with silicon as the central atom is deduced. By a normal coordinate analysis a value of 4.83 mdynA-1 is obtained for the SiS force constant, a value which was confirmed by ab initio SCF calculations of the IR spectrum51. 

Here multiplier 2 approximately accounts for doubling of integrated absorption due to spatial motion of a dipole, which is more realistic than motion in a plane to which LCs(Z) corresponds. For representation (235), only one (Debye) relaxation region with the relaxation time rD is characteristic. At this stage of molecular modeling it was not clear (a) why the CS potential, which affects motion of a dipole in a separate potential well, is the right model of specific interactions and (b) what is physical picture corresponding to a solid-body-like dipole moment pcs. 

Figure 4.8. Least-squares BC for molecular spectra with rotational fine structure determination of Pb in the BCR 186 Pig Kidney CRM at 217.001 nm using HR-CS ET AAS and direct solid sample analysis (a) absorbance over time and wavelength after correction for continuous absorption (b) reference spectrum absorbance over wavelength integrated over time for NH4H2P04 (the dotted line represents the center pixel) (c) absorbance over time and wavelength after subtraction of the reference spectrum using least-squares BC.

Since the QRLPP has a fairly high electron temperature, the normal equilibrium between the Cs2 molecules and the Cs atoms is affected in the QRLPP. This means that the bound Cs2 (singlet) density is decreased while the unbound Cs2 (triplets) density is increased due to the QRLPP. We have indeed observed that the transient destruction of molecular state by the pulsed QRLPP can be measured by positioning the cw probe beam close to or in coincidence with the pulsed laser beam. The transient absorption signals observed in the probe beam, with long transient time constants (for example, on the order of 1 msec) provide a new method to study diffusion dynamics in a vapor system (33). 

In Eq. (3), is the relevant molecular transition frequency, y is a dam >ing rate, is a polarizability, and (/) is the z-component of the total electric field in the vicinity of the molecule. If (t) were simply of the form i)Cos(fijr), then Eq. (3) is the well-known phenomenological Lorentzian oscillator model of absorption which leads to an approximate Lorentzian form for the absorption cross section [1]. Similar remarks hold for the SP dipole, fi/f), if E t) = ocos(mr), where E t) is the z-component of the total electric field near the SP dipole. The parameters 04,74 and a, in this case are chosen such that the resulting Lorentzian cross section proximates the known exact sur ce plasmon absorption cross section or its appropriate form in the quasistatic (a A=2 tic/cs) limit. Note that I am using a simplified notation compared to the various notations of Refs. [13-15]. (Relative to Ref. [13], for example, my definitions of surface plasmon dipole... 

Analyses of the photolysis products has been achieved by mass spectrometryand by gas chromatography, and absorption of O2 during photo-oxldatlon has been indicated by differential manometry Cs ). Structural changes in the polymer residue have been monitored spectroscopically (I.R., U.V., N.M.R. Z.), e.s.r. and fluorescenceand by molecular weight... 

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