Origin of the Signals

Ultraviolet-visible spectroscopy records absorbances in the range from 200 to 700 nm with the visible range starting at about 400nm. Absorption of light at these wavelengths excites an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). 

As an example, ethene absorbs light at 165 nm, just outside the measure-able UV range. Using Equation 10.2, we see that this absorption corresponds to an energy of 725 kJ/mol (Eq. 10.3) and occurs as a result of an electron from a % orbital transferring to an orbital, called a n-n transition (Fig. 10.1). Conjugation of the % system is necessary to lower the energy of the electronic transitions sufficiently to be recorded by instruments in common use. Butadiene, for example, shows an absorbance band at 2 217, 21,000. 

Region Wavelength (nm) Freqnency (Hz) Energy (kJ/mol) Molecnlar Transitions 

Visible 400 7.4x10 300 Electronic transitions, colored componnds 

Numbers in boldface represent typical instrument ranges. 

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Figure 11.4 Schematic of a phospholipids unit (A) showing the polar head group that holds the charge in the electrospray (B) a phospholipids bilayer that is the possible origin of the signals at around 1400 mlz in the DIESMS spectra of bacteria (Figure 11.3) (C) the micellar arrangement that is stabilized in a polar solvent (D) the reverse micellar arrangement that is likely to be encountered in nonpolar environments.

With this group of electrochemical sensors, information is obtained from the current-concentration relationship. The two most important issues to discuss are (1) the origin of the signal for various types of amperometric sensors and (2) the origins of selectivity. To begin our examination of these issues, we briefly reiterate some of the information presented in the Introduction to Electrochemical Sensors (Chapter 5). 

The discussion of the origin of the signal helps in the classification of conduc-tomettic sensors. Chemiresistors (Fig. 8.1a) are normally measured in DC mode and the capacitances in the equivalent circuit can be neglected. Thus, the analytical information is obtained from the modulation of surface (Rs), contact (Rc), or... 

Figure 5. Logarithmic plot of the square root of the HRS intensity, recorded for both the input fundamental and the output harmonic intensities polarized along the Ox direction, versus the diameter of the particles. The slope is 1.9-1-/—0.2 in agreement with a surface origin of the signal (Reprinted from 44 with permission from the American Physical Society)...

On routine calibration, at first zero gas is released and the output signal is set at zero [1,2]. Calibration gas is then admitted. Next, the analyzer is adjusted to the concentration value of the calibration gas (compare the squares in Figure 16-1). The origin of the signal/concentra-tion diagram is found with zero gas. Ideally, zero gas should not contain the component to be measured, but it should be identical in all other respects. This is a requirement that, for the most part, cannot be fulfilled. 

Fig. 2. Photo-electron spectra of compounds of lanthanides, hafnium and tantalum in the region from I between 0 and 40 eV. The origin of the signals from various shells is indicated the intensities are not on the same scale going from one compound to another, and an arbitrary height of background has been removed. Signals due to Kas photons with 9.8 eV higher energy than the main exciting line are marked 3C3...

Fig. 2.13 Interaction of the electron beam in a SEM with the surface layers of the sample. The grey area shows the interaction volume and the location of the origin of the signals used in a SEM to construct an image...

As we have seen, collisions are important for the signal generation in LEI. In low-pressure experiments photoionization instead is the principal origin of the signal. The term Resonance Ionization Spectroscopy (RIS) is then frequently used. Several examples of opto-galvanic detection schemes for different atoms are shown in Fig.9.11. If multi-photon excitation of the atoms to be studied is used the technique is referred to as REMPI (REsonance Multi-Photon Ionization) spectroscopy. The selectivity of RIS and REMPI can be further enhanced by using a mass spectrometer to ana-... 

As we have seen, collisions are important for the signal generation in LEI. In low-pressure experiments photoionization instead is the principal origin of the signal. The term Resonance Ionization SpectToscojjy (RIS) is then frequently used. Several examples of opto-galvanic detection schemes... 

Understanding the origin of the signal/s measured in SIMS can be as simple as comparing masses of the signals measured to those listed in natural abundance tables such as that listed in Appendix A.2. This applies to atomic and simple molecular ions. 

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