The Basic Experiment

As the name XPS implies, the analytical sample is irradiated with x-ray light and the energy distribution of the emitted photoelectrons is recorded. Fig. 1 shows a comparison between the four different processes which are caused by the interaction between x-ray photons and matter, and which can be observed by four different spectroscopic techniques 17,I8). 

In the photoionisation process the x-rays liberate electrons in the various orbitals of the chemical compound. Depending on the experimental set-up and especially the photon energy Ex = hv, these electrons are either absorbed in the valence band near the Fermilevel, or they leave the atom as free electrons with a kinetic energy EK, 

After the primary process we have a cation with a vacancy in one of its inner orbitals. In the following relaxation process this hole will be filled with 

Which of these two competing relaxation mechanisms can be observed, is independent of the primary excitation and only a function of excited ion. For light elements the Auger process prevails, 9), while the heavier atoms exclusively show fluorescence spectra. 

Thus we have three possibilities to determine the electron binding energies by means of x-ray excitation  

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This chapter simnnarizes the interactions that affect the spectrum, describes the type of equipment needed and the perfomiance that is required for specific experiments. As well as describing the basic experiments used in solid-state NMR, and the more advanced teclmiques used for distance measurement and correlation, some emphasis is given to nuclei with spin / > dsince the study of these is most different from liquid-state NMR. 

For large molecules, such as proteins, the main method in use is a 2D technique, called NOESY (nuclear Overhauser effect spectroscopy). The basic experiment [33, 34] consists of tluee 90° pulses. The first pulse converts die longitudinal magnetizations for all protons, present at equilibrium, into transverse magnetizations which evolve diirhig the subsequent evolution time In this way, the transverse magnetization components for different protons become labelled by their resonance frequencies. The second 90° pulse rotates the magnetizations to the -z-direction. 

It is the aim of this chapter (1) to discuss the basic experiments for the detection of trans H-bond couplings and to indicate experimental problems, (2) to briefly describe the range of groups for which such couplings have been observed and to summarize their sizes, and (3) to present a number of recent applications which characterize changes in H-bond geometries of biomacromolecules. 

When attempting to use NMR to measure a dissociation constant, the basic experiment will be to vary the ligand concentration in the presence of a fixed concentration of protein. (The converse experiment, varying the protein concentration, may sometimes be carried out, but is generally less satisfactory because of problems with protein solubility and aggregation.) What one sees in this experiment will depend critically on the rate of... 

Summary The free-response portion of the AP exam will contain a question concerning an experiment, and there may also be a few multiple-choice questions on one or more of these experiments. This chapter reviews the basic experiments that the AP Exam Committee believes to be important. You should look over all of the experiments in this chapter and pay particular attention to any experiments you did not perform. In some cases you may find, after reading the description, that you did a similar experiment. Not every AP class does every experiment, but any of these experiments may appear on the AP exam. 

X-Ray powder diffraction is a powerful tool for characterization of zeolites. The basic experiment is relatively easy to perform and can be done in most labs on standard diffractometers and the data obtained is easy to analyze for many applications. Powder diffraction can be used to determine whether a new zeolite has been synthesized, whether a desired zeolite has been made or whether a crystallization process has completed. As noted in Section 4.2, X-ray powder diffraction can be an integral tool in determining the details of the structure of a newly synthesized zeolite. In addition, it is a critical characterization technique that can be routinely used, for example, to identify contaminants present in a synthesis, to determine how much zeolite has been bound into a catalyst or adsorbent pellet, or to ascertain if heat treatment alters the zeolite structure. Of the techniques described in this chapter, powder diffraction is probably the most commonly used. Additional details can be found elsewhere [15-19]. 

Smagorinsky, J. 1963. General circulation experiments with the primitive eqna-tions. 1. The basic experiment. Monthly Weather Review 91(3) 99-164. 

Fig. 3. (a) Comparison of subspectra A and A as obtained from equal number of scans within the same total measuring time with the modified and the basic experiment respectively. 

We have implemented the principle of multiple selective excitation (pulse sequence II in fig. 1) thereby replacing the low-power CW irradiation in the preparation period of the basic ID experiment by a series of selective 180° pulses. The whole series of selective pulses at frequencies /i, /2, , / is applied for several times in the NOE build-up period to achieve sequential saturation of the selected protons. Compared with the basic heteronuclear ID experiment, in this new variant the sensitivity is improved by the combined application of sequential, selective pulses and the more efficient data accumulation scheme. Quantitation of NOEs is no longer straightforward since neither pure steady-state nor pure transient effects are measured and since cross-relaxation in a multi-spin system after perturbation of a single proton (as in the basic experiment) or of several protons (as in the proposed variant) differs. These attributes make this modified experiment most suitable for the qualitative recognition of heteronuclear dipole-dipole interactions rather than for a quantitative evaluation of the corresponding effects. 

The basic experiment consisted of simply treating an l-UNCA with a base and monitoring the formation of the d-UNCA. The most straightforward analysis is accomplished by polarimetry. For example, Boc-Phe-NCA was dissolved in THF at a concentration of 0.33 M, and 1.5 equivalents of TEA were added. The resulting solution was placed in a polarimeter cell and the optical rotation was monitored over time (Figure 1). 

Some of the basic experiments with UPD have been carried out with Ag onto Au (hkl).94 This is because the radii of Ag and Au atoms are almost the same, so that the interpretation does nothave to involve a steric displacement effect. Figure 7.143 shows the underpotential deposition of Ag on Au (100). The A and D marks on the figure denote a number of underpotential processes. Using STM to study this system indicates that at AE> 650 mV, a stable Ag surface is formed with normal interatomic distance. Between 200 < AE < 550 mV, stable domains exist, but now (see Fig. 7.144) they are expanded in respect to their interatomic distance and in comparison with those formed at AE > 650. 

In the linear sweep technique, a recording of the current during the potential sweep (say, from 0.0 V on the normal hydrogen scale to 1.2 V positive to it in a 1 M H2 S04 solution) completes one act of the basic experiment. However, and hence the title of this part of the chapter, the electronics can be programmed so that when the electrode potential reaches 1.20 V, it begins a return sweep, going from 1.2 to 0.00 V, NHS. Completion of the two sweeps and back to the starting point is one act in what is called cyclic voltammetry.16 The current is displayed on a cathode ray oscilloscope screen on an X Y recorder, and it is normal to cany out not one but several and often many cycles. Much information is sometimes contained in the difference between the second and other sweeps in comparison with the first (Fig. 8.10). 

In the basic experiment, which is now rarely used, a variable, time-dependent voltage with increments (scan rate) in the order of 1 or 2mV/s with respect to the initial potential Ex is applied to the mercury droplet. 

Every chemistry textbook describes the basic experiments that relate the volume of a gas sample to its pressure and Kelvin temperature. For a given quantity of gas it may be shown, by combining Bovle s law (PV = k", at constant T) and Charles law (V = k T, at constant/5), at... 

Useful modifications of the basic experiment include CH correlations with proton-proton couplings removed in the t (<5H) domain [66] and long-range carbon-proton shift correlations with the mixing period in Fig. 2.53 adjusted to an average value of 2/CH and... 

The two-dimensional INADEQUATE experiment also suffers very much from low sensitivity given by the low natural abundance of carbon-13 (about 10 2), so that only 0.01 % or 10-4 of all carbon-carbon bonds contribute to the satellite signals. In fact, the basic experiment can be modified in order to reduce the data matrix and to save measuring time [72], giving COSY-like square correlations as shown in Fig. 2.60. Nevertheless, the two-dimensional INADEQUATE experiment requires several hours of measuring... 

When the rank curve has the shape as in Fig. 2.15, factors from the first half of the graph enter the basic experiment ... 

Based on the results of a screening experiment and the objective of the research problem, the researcher decides about including a system factor and response into the design of the basic experiment. This does not involve all the information of a selective experiment ... 

Information on factor-variation intervals may be drawn from an active experiment by the random balance method. Thus, linear effects of factors XjX2 in Example 2.10 considerably exceed the affects of other factors. This simultaneously may mean that the selected factor-variation intervals X, (- +) X2(- +) are too high. If this is so, then they should be cut in half in the basic experiment. 

Factor space may be obtained from the matrix of random balance and analysis of variance. Information on number of replications of design points-trials in the basic experiment is obtained from analysis of variance, and some proofs about linear or nonlinear relationships between variables of the research subject from correlation analysis. 

In this way the basic experiment is defined for the linear model, and the gradient that indicates the direction of the fastest response increase or decrease is obtained. When a response maximum or minimum is searched for, the experimental center is moved that way and a new experiment for the linear model performed. The procedure is repeated until moving along the gradient has an effect. When this has no effect, it means we are close to the optimum. Polynomials of higher order, mostly the second, are used in the optimum region. 

All this clearly shows that selection of the subdomain for performing the basic experiment is very important and it will be discussed in the coming sections. 

These variation intervals were chosen to realize the basic experiment ... 

Six design points in the experimental center had to be done due to Table 2.164. For the sake of economy only one design point in the experimental center was set in this example, since the reproducibility variance was obtained in the basic experiment. By processing all 15 design points, the following regression coefficients for second-order model were obtained ... 

In the case of replications of only FUFE 2s in the first phase when the basic experiment was defined, this value of reproducibility variance was obtained Sy = 0.32. With 95% confidence it is evident that the obtained value does not statis-... 

A 1 /4-replica of the full factorial experiment 26"2 has been selected for the basic experiment, with these defining contrasts ... 

In the optimization of conditions of hafnium extraction from tributylphosphate (TBF), these factors were analyzed X,-concentration of nitric acid in outlet water solution, [N] X2-concentration of TBF in e-xylol, % X3-ratio of phases, [1] and X4-time of extraction, min. The coefficient of hafnium separation was determined as the system response. 1/2-replica of full factorial experiment 24 (X4=X3X2X3) was chosen as the basic experiment. The outcomes of the experiment are given in Table... 

Let us analyze the previous case by taking into account the third factor X,. The outcomes of FUFE 23 and the results of application of method of steepest ascent are given in Table 2.216. Thirteen trials were necessary to reach the maximal yield of 85.2%. The outcomes of the simplex method are in Table 2.217. Maximal yield after 14 trials is 85.0%. Approximately the same number of trials has been necessary by both methods to reach the optimum. It should be stressed once again that FUFE requires replications, so that to reach optimum by the method of steepest ascent, we need at least twice as many trials. Evidently, a half-replica instead of FUFE in the basic experiment may reduce the number of trials. However, there is a possibility of wrong direction of the movement to optimum due to the possible effects of interactions. 

The experiment shown in Figure 5.6 shows the basic experiment defining the depolarization ratio. Here incident light, E(. polarized parallel to the y axis is brought to the sample... 

The basic experiment is very simple, comprising [96] adsorption onto the sample at a relatively low temperature, normally 300 K. Subsequently, the sample is heated in a controlled fashion, that is, linearly in time, at rates between 0.5 and 20K/s, and at the same time the evolution of species desorbed from the material into the gas phase are monitored. 

The development of pulse radiolysis in 1960 and the observation of the intense transient optical absorption of e q in 1962 led to a large number of studies concerning the rate constants and the mechanism of reactions of e q. By the end of that decade a huge amount of information had been accumulated and summarized in a book by Hart and Anbar (1970). The reactions of organic compounds with hydrated electrons were specifically reviewed by Anbar (1969). The basic experiment in many of these studies involved the measurement of the lifetime of the transient ejq absorption (Xmax = 715 nm, e = 18,500 m -1 cm-1) in the presence of varying concentrations of a substrate. By 1967 Anbar and Neta had compiled some 600 rate constants. More recently, the rates for some 700 compounds measured mostly by pulse radiolysis, have been summarized (Anbar et al., 1973). 

In the basic experiment, a first optical pulse (pump) is absorbed by the sample. A second, time-delayed pulse of weaker intensity is used to probe the change in optical response. Commonly, the test pulse probes the change in transmission, given in the small signal limit as... 

Caffeine (32mg/kg p.o.) and chlorpromazine (16mg/kg p.o.), administered under the same experimental conditions, are used as reference substances. The basic experiment therefore includes 7 groups. 

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