Element-selective excitation

This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

Excitation of sample atoms by primary radiation from a high-intensity broad spectrum or element selective source. Samples atomized in a chemical flame using a circular burner. Optics to isolate emission line and photomultiplier to measure its intensity. 

As we will demonstrate, luminescent properties, radiative transition characteristics as well as emission under site selective excitation depend on the local environment s)rmmetry of the luminescent center. Therefore it is necessary to take into account and to describe the different local symmetry. There are two systems commonly used in describing symmetry elements of punctual groups ... 

To understand an electron—atom collision means to be able to calculate correctly the T-matrix elements for excitations from a completely-specified entrance channel to a completely-specified exit channel. Quantities that can be observed experimentally depend on bilinear combinations of T-matrix elements. For example the differential cross section (6.55) is given by the absolute squares of T-matrix elements summed and averaged over magnetic quantum numbers that are not observed in the final and initial states respectively. This chapter is concerned with differential and total cross sections and with quantities related to selected magnetic substates of the atom. 

In the mid-1970 s, with the availability of intense X-ray synchrotron sources, a powerful new technique. X-ray absorption spectroscopy (XAS), emerged. This is a local structural probe, the information content of which derives from electron diffraction. For a metalloprotein, the electron source and detector is the metal atom that is probed, because selective excitation is achieved by scanning a range of X-ray wavelengths particularly appropriate to the element of central interest. The selectivity and the local nature of the diffraction process give the technique its major strength. For example, metal-ligand distances can be determined to an accuracy of approximately 0.02 A. In addition, XAS does not require crystalline materials thus, aqueous protein samples are readily probed under a variety of conditions. 

Elements.—The use of a tunable narrow-frequency laser to produce an isotopically selected chemical reaction has been described by Leone and Moore 1 this approach should permit efficient isotope separations to be performed. In the example reported, natural Br2 (79Br/81Br = 1) is photo-predissociated by selective excitation into the 3n0+ state. Bromine atoms, enriched in one isotope, react with HI before scrambling occurs, to produce 80—85% enriched H81Br. 

Instead of exciting the sample by the general radiation from the x-ray tube, one can use the almost monochromatic fluorescent radiation from a secondary fluorescer, as in Fig, 15-11. Proper selection of the secondary fluorescer can then restrict the number of fluorescing elements in the sample, because only those with Z less than that of the secondary fluorescer are excited. Thus an Fe secondary fluorescer will excite Cr in a stainless steel sample but not Fe or Ni. As a result, the sensitivity for Cr detection is increased, because selective excitation has decreased the load (total count rate) on the detector/analyzer system. 

Most types of selective excitation can be modified for simultaneous excitation of n slices or volume elements. Such an approach is advantageous when a limited number of slices or volume elements, but not the entire 3D object, needs to be investigated. By suitable coding of the volume information in n experiments an improvement in signal-to-noise ratio of can be gained [Boll, Miill]. Compared to 3D volume imaging, multislice and multi-volume techniques (cf. Section 9.1) suffer from the lack of achieving well-defined boundaries. 

Apart from preparation of magnetization in slices and lines, selective excitation can also be used for point selection. Here the objective normally is not to scan an image in a pointwise fashion, but rather to localize a selected volume element to acquire a spectroscopic response from it [Auel] (cf. Section 9.1). 

This scheme can be repeated for different gradient directions. Without repetition a slice is selected, with one repetition a line and two repetitions a voxel (Fig. 5.3). Thus, selective excitation is needed to prepare individual volume elements in an extended object for subsequent investigation by NMR spectroscopy [19j. This type of NMR with spatial resolution is called volume-selective spectroscopy. 

Figure 9.18. Selective excitation sequences based on (a) a single and (b) a double pulsed field gradient spin-echo. The element S represents any selective 180° inversion pulse or pulse sequence.

When implementing this sequence it may be necessary to add attenuation to the transmitter to increase the duration of each pulse so that the shorter elements do not demand very short (< 1 xs) pulses (note the similarity with the requirements for the DANTE hard-pulse selective excitation described above). The binomial sequences can be adjusted to provide an arbitrary overall tip angle by suitable adjustment of the tip angles for each element. For example, inversion of all off-resonance signals can be achieved by doubling all elements relative to the net 90 condition. Exactly this approach has been exploited in the gradient-echo methods described below. 

Simulate the 1D selective COSY spectrum of glucose using the configuration file ch5333.cfg. The experiment uses a DANTE-Z element for the selective excitation of the proton at 3.9 ppm. Note depending upon the computer speed this simulation may take several minutes. 

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