Comparison Between the Different Methods

The different sensitive techniques of Doppler-limited laser spectroscopy, discussed in the previous sections, supplement each other in an ideal way. In the visible and ultraviolet range, where electronic states of atoms or molecules are excited by absorption of laser photons excitation spectroscopy is generally the most suitable technique, particularly at low molecular densities. Because of the short spontaneous lifetimes of most excited electronic states Eij, the quantum efficiency reaches 100% in many cases. For the detection of the laser-excited fluorescence, sensitive photomultipliers are available which allow, together with photon counting electronics (Sect.4.5), the detection of single fluorescence photons with an overall efficiency of lO- rlO-i (Sect.6.3.1). 

Excitation of very high-lying states close below the ionization limit, e.g., by ultraviolet lasers or by two-photon absorption, enables the detection of absorbed laser photons by monitoring the ions. Because of the high collection efficiency of these ions ionization spectroscopy represents the most sensitive detection method, superior to all other techniques in all cases where it can be applied. 

For infrared spectroscopy in molecular beams opto-thermal spectroscopy is a very good choice (Sect.6.3.3). 

For the spectroscopy of atoms or ions in gas discharges, optogalvanic spectroscopy (Sect.6.5) is a very convenient and experimentally simple alternative to fluorescence detection. In favorable cases it may even reach the sensitivity of excitation spectroscopy. For the distinction between spectra of ions and neutral species velocity modulation spectroscopy (Sect.6.6) offers an elegant solution. 

All these methods represent modifications of absorption spectroscopy, whereas LIF spectroscopy is based on emission of fluorescence from selectively populated upper levels. The absorption spectra depend on both the upper and lower levels. The absorption transitions start from thermally populated lower levels. If their properties are known (e.g. from microwave spectroscopy) the absorption spectra yield information on the upper levels. On the other hand, the LIF spectra start from one or a few upper levels and terminate on many rotational-vibrational levels of a lower electronic state. They give information about this lower state. 

The different sensitive techniques of Doppler-limited laser spectroscopy, discussed in the previous sections, supplement each other in an ideal way. 

In the infrared region excitation spectroscopy is less sensitive because of the lower sensitivity of infrared photodetectors and because of the longer lifetimes of excited vibrational levels. These long lifetimes bring about either at low pressures a diffusion of the excited molecules out of the ob- 

---

The colour can be determined by various methods such as the Gardner, Barrett, iodine colour number or US Colophonium standard methods. Usually, the Gardner or the Barrett standards are used. A comparison between the different methods to determine the colour of resins is given in Table 9. In both methods, the colour is evaluated in resin solutions. A 50 wt% resin solution in toluene is used as the Gardner standard, and a solution of 2 g resin in 25 ml toluene is used in the Barrett... 

An overall comparison between the different methods used for surface modification of polymer membranes is presented in Table 24.5, which was prepared by Nady et al. (2001). Please note that it is not always straightforward to interpret and compare results, because many parameters may be influenced simultaneously by one modificrttion method. Thus, Table 24.5 gives a general impression only. 

Both of these equations can be universally expressed through the Stern-Volmer constant k. The latter one has been used already in Section XII.B as a standard for comparison between the different theories of irreversible quenching. In the case of the reversible reaction (3.703), the problem is more difficult, especially at high concentrations of B molecules. After the dissociation, other B molecules can be involved in the reaction with A. This many-particle competition for the partner couples the motion of the molecules, making the problem unsolvable analytically. Thus only approximate solutions were obtained by means of different methods and assumptions whose validity very often remains unclear. 

Comparisons between the different intra-arterial thrombolysis trials and between intraarterial thrombolysis and intravenous thrombolysis is hampered by differences in methodology and type of thrombolytic therapy. In addition, within the intra-arterial thrombolysis trials, thrombolytic deUvery has varied between regional into a parent vessel of the thrombosed vessel, local into the affected artery and into the thrombus itself, or combinations of these methods. In addition, the infusion process has been variable, ranging from continuous to pulsed infusion. Some studies have allowed physical clot dispersion using the tip of the microcatheter while this was prohibited in others, for instance in the PROACT trials. 

It seems interesting to use these parameters in a more extensive calculation involving effects other than a ones. When a effects are dominant this method not only accounts for them, but also gives a more reliable a value since it includes the other effects quantitatively this allows us to establish a comparison between the different compounds. 

This figure gives a rough comparison between the different filter standards (filter classes should always be connected to the standard test method). 

A comparison between the DSC method (using the Perkin-Elmer DSC-1B instrument) and the premelting method of Johnston and Giauque (51) is shown in Table 10.3 (52). Johnston and Giauque (51) came to the conclusion that the nitric oxide used in their measurements contained less than 10"3 mole-% of eutectic impurities, or the so-called purity is of the order of 99.999%. The authors excluded the possibility of noneutectic impurities. It should be noted that the difference between the two methods is not in thermodynamics but rather in instrumentation and the properties of the methods of measurement. The disadvantage of the calorimetric method is the extremely long measurement time of 2-4 days, which is due to the large sample masses and the necessity for equilibrium to be attained at each temperature. The penalty for a shorter analysis time is, of course, a lower accuracy in purity measurements. 

Use of the a-relaxation of amorphous polymers as a standard A simple method to calibrate the temperature is to compare the loss peaks related to the glass-transition dynamics (a-relaxation) of pure amorphous polymers, obtained in a well calibrated apparatus, with the data obtained in the paratus in which the temperature is not calibrated. The calibration tests in the uncorrected equipment must be done under similar conditions (e.g. heating rate) to the ones that are to be performed in future experiments using that equipment. This method was tested using polycarbonate films [30], where a global comparison between the different strategies proposed in this section is also discussed. 

At this stage we can t conclude that the electronegativity scales are not correctly chosen, even that their analytical forms display an approximation per se, but the method one-to-one of comparison between the different... 

The main part of the book presents various applications of lasers in spectroscopy and discusses the different methods that have been developed recently. Chapter 6 starts with Doppler-limited laser absorption spectroscopy with its various high-sensitivity detection techniques such as frequency modulation and intracavity spectroscopy, cavity ring-down techniques, excitation-fluorescence detection, ionization and optogalvanic spectroscopy, optoacoustic and optothermal spectroscopy, or laser-induced fluorescence. A comparison between the different techniques helps to critically judge their merits and limitations. 

Qualitative and quantitative comparisons between the different transgenic plants synthesizing resveratrol and related stilbenes are difficult, as different analytical methods are used to assay these compounds. These analyses have shown various stilbene levels and spatiotemporal distributions, leading to a considerable variability in terms of relative amounts of different forms. 

---