Sample Excitation

Direct current arc excitation is preferred for qualitative analysis since it is simple to use and is more sensitive than flame or spark excitation. It is common practice to use 1-5 mg of sample placed in a cup electrode that serves as the lower electrode and is the anode (plus electrode). The counter electrode is mounted immediately above the anode and a dc voltage of 200-250 V is applied. A current of 10-15 A is desirable. Electrode spacing should be maintained constant at about 3-5 mm. Current and voltage also should be maintained constant. 

---

Visible lasers are typically used for sample excitation, although near-IR lasers can be used when visible excitation sources cause sample fluorescence, obscuring the Raman scatter. 

Figure 23 Fluorescence excitation and emission spectra, (a) virgin EVA sample (excitation = 280 nm emission = 360 nm) (b) EVA sample degraded for lh at 180°C (excitation = 239 nm emission = 390 nm) (c) EVA sample degraded for 1 h at 180°C (excitaton = 301 nm emission = 361 nm) (d) EVA sample degraded for 2 h at 180°C (excitation = 238 nm emission = 388 nm). Reprinted from Allen [67]. Copyright 2000, with permission from Elsevier. (This figure has been reproduced from the original in reference [67], however it would appear that the labels excitation and emission have been incorrectly inserted and should be switched for parts (b), (c) and (d).)...

Compared to flame excitation, random fluctuations in the intensity of emitted radiation from samples excited by arc and spark discharges are considerable. For this reason instantaneous measurements are not sufficiently reliable for analytical purposes and it is necessary to measure integrated intensities over periods of up to several minutes. Modern instruments will be computer controlled and fitted with VDUs. Computer-based data handling will enable qualitative analysis by sequential examination of the spectrum for elemental lines. Peak integration may be used for quantitative analysis and peak overlay routines for comparisons with standard spectra, detection of interferences and their correction (Figure 8.4). Alternatively an instrument fitted with a poly-chromator and which has a number of fixed channels (ca. 30) enables simultaneous measurements to be made. Such instruments are called direct reading spectrometers. 

The effects of inhomogeneity in various NMR sequences are well-known and there are a number of ways to combat them. In FFC relaxometry, Bi inhomogeneity is actually not much of a problem since it does not directly affect Ti measurements. Whatever effect it has consists essentially in a loss of signal due to imperfect sample excitation and/or imperfect refocusing (in sequences using spin echoes and/or magnetization inversion). 

With this spectrometer, a difference mid-IR spectrum at a selected time after sample excitation is recorded by sweeping from 1640 to 940 cm in steps that may be as short as approximately equal to the spectral resolution of the spectrometer—in this case, 8 cm. The sample solution is pumped through a flow cell that has IR-transmitting Cap2 windows set with a 0.1-mm optical pathlength. The Bap2 windows have also been used for the sample cell. ... 

X-ray fluorescence is a spectroscopic technique of analysis, based on the fluorescence of atoms in the X-ray domain, to provide qualitative or quantitative information on the elemental composition of a sample. Excitation of the atoms is achieved by an X-ray beam or by bombardment with particles such as electrons. The universality of this phenomenon, the speed with which the measurements can be obtained and the potential to examine most materials without preparation all contribute to the success of this analytical method, which does not destroy the sample. However, the calibration procedure for X-ray fluorescence is a delicate operation. 

As the temperature stimulation is switched off, the static kinetics is governed by equation (4.1.40) with the initial distribution function y(r) from equation (4.2.11). However, all attempts [102] to describe in such a way the experimental tunnelling luminescence decay for F and in KBr (Fig. 4.18) were unsuccessful. Both this observation and the absence of the plateau of 7(f) during the temperature stimulation, characteristic for the quasi-steady states, argue that the tunnelling recombination takes place in correlated pairs. This is in line with the conclusion [107] that for ordinary defect concentrations 1016 cm-3 (X-ray sample excitation for minutes) and the time 105 s the slope is dose-independent but 7(f) oc dose [95]. 

A Spectrace QuanX energy dispersive X-ray fluorescence spectrometer was used, which employed a rhodium target X-ray tube, fundamental parameters software, and pure element standards. Sample excitation conditions were 30kV, 0.10mA, 100 sec count, KaP for Fe, Co, Ni, Cu, Zn, As, Pt, Au, Bi, and Pb, followed by 50 kV, 0.72 mA, 60 sec count, for Pd, Ag, Sn, and Sb. Certified brass samples were run each day prior to sample runs to ensure instrument accuracy and precision. 

Measurements of dynamics in the subnanosecond regime are possible using pump-SH probe experiments where an initial pulse causes either a photo- or thermal excitation of the sample and the SH probe beam monitors the transient surface properties [69, 72, 73, 118-120]. Although experiments of this type have yet to be reported for an electrochemical system, experiments on Si samples excited under ambient and vacuum conditions have been published [69, 72, 73, 120]. 

Fig. 13. a Rheological model for the cantilever response on applying a displacement modulation to a transducer underneath the sample b the solution to the model gives the ratio of the amplitudes of the tip response Zt to the sample excitation Zc as a function of the logo)/Q)0 for different ratios between the sample stiffness kj and the spring constant of the cantilever kc. Reproduced after [122]... 

To minimise the friction effect, it has been proposed to use smaller amplitudes and higher frequencies [122,137]. The so-called scanning local-acceleration microscopy (SLAM) is another modification of contact-mode SFM which was implemented by vibrating the sample at a frequency above the highest tip-sample resonance (region III in Fig. 13b). In this frequency range (around 1 MHz), the cantilever response to the sample excitations becomes independent of the cantilever stiffness and depends linearly on the contact stiffness and reciprocally on the cantilever mass m (Fig. 13b) ... 

A liquid prism was created on a PDMS chip for detection based on absorption and refractive index shift. The liquid prism was formed by filling a hollow triangular-shaped chamber with a liquid sample. Excitation and emission were arranged at the minimum deviation configuration. At a low concentration of fluorescein (< 100 pM), excitation light from an optical fiber was absorbed by the molecule, but there was no shift in the excitation maximum. In this absorption-only mode, the LOD of fluorescein was 6 pM. At higher concentration (i.e., 100-1000 pM), there is an additional shift in the excitation maximum This leads to a much sharper decrease in the measured intensity, which is more than can be accounted for simply by the absorption effect [714]. 

Figure 2. Transient absorbance and baseline spectra for benzophenone in ethanol solution. The lowest trace is a baseline obtained with the 353 nm excitation pulse blocked. The dotted line spectrum was taken 10 ps after sample excitation the solid line spectrum was taken at a delay of 22 ps. These two transient spectra are drawn normalized to the same peak height to facilitate bandshape comparisons.

The Fourier transform Raman spectrometer is constructed around an interferometer (see Figure 4.20) [57], Normally, a continuous wave Nd YAG laser (1064nm) is used for the sample excitation. In relation to the sample arrangement inside the spectrometer, there are two fundamental geometries in which a sample is tested in Raman spectroscopy, that is, the 90° geometry, where the laser beam... 

Fig. 1.19. Quenching of the coherent vibrational oscillations of MDMO-PPV upon photoinduced charge transfer. The AT/T dynamics for pure MDMO-PPV (continuous line) and for MDMO-PPV/PCBM (1 3 wt. ratio) (dashed line), excited by a sub-10-fs pulse, was recorded at the probe wavelength of 610 nm. The inset shows the Fourier transform of the oscillatory component of the MDMO-PPV signal, the nonresonant Raman spectrum of MDMO-PPV (excitation 1064 nm) and the resonant Raman spectrum of an MDMO-PPV/PCBM sample (excitation 457 nm). For the resonant Raman spectrum of MDMO-PPV, it was necessary to quench the strong background luminescence by adding PCBM...

X-ray photoelectron spectroscopy was performed on a PHI 5400 ESCA system, in which Mg Ka radiation was used for sample excitation. Peak positions were corrected for charging of the samples by comparison with the 01a and Clt peak position. After being used in the oxidation of 1-butene the catalysts were cooled in a stream of nitrogen and unloaded from the reactor. Subsequently, the samples were pressed into an indium foil and immediately analyzed with XPS. 

Sample excitation with an alternating voltage instead of a d.c. voltage has several advantages ... 

The macromode spectra described here are acquired with an Instruments SA Jobin Yvon Ramanor HG.2S system. Sample excitation is done with either argon or krypton ion lasers. This scanning spectrometer has a thermoelectrically cooled PMT detector and is fitted with a modified Nachet 400 microscope accessory for Raman microprobe work. The microprobe is capable of providing information from domains as small as 1 // in diameter. 

---