Possible Artifacts

Determination of a reliable P(x) requires a determination of I(s) from zero to infinity. It is impossible because 000 is inaccessible, and i is limited to 2/X (see Section 1.1.2.1.2). Therefore, a termination effect is produced. 

Artifacts produced by terminatioii are most often in the form of spurious P(x) peaks or a loss of resolution that prevents peak separation. An interesting example is given in the literature [11]. The authors calculated P(x) for a graphite crystal limiting artificially I(s) at s = 1 (the range of selected area diffraction patterns) which is a strong termination effect. They thus demonstrated the production of spurious peaks. 

Ergun [12,13] and Franklin [14-16] reported spurious P(x) peaks at 0.5,0.9,1.95, and 3.31 A for highly disordered carbonaceous substances. 

The second determining factor is the dampening factor 5. It includes all accidental limitations able to broaden I(s), notably the instrumental response (e.g., X fluctuations, mechanical vibrations, strains). It also greatly impedes the resolution of l s), preventing peak separation in P(x). As an example, 5 = 0.115 introduces in P x) an arbitrary increase in the width of the peaks of 0.38 A. As a result, peaks distant from less than 0.4 A cannot be separated. 

Introduction of an artificial 8 in the calculus of P(x) helps to determine the real number of peaks among which spurious ones may be present due to the termination effect. In the limit, an excess of dampening leads to the resolution of only the first peak. 

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Since no background correction can be made, dot maps of minor and trace constituents are subject to possible artifacts caused by the dependence of the bremsstrahlung on composition, particularly with EDS X-ray measurement. 

In certain areas, particularly the rapidly developing area of organo-metalhc spedation, concern has been expressed that artifacts may lead to false results. One example are the doubts about the accuracy and suspicion of possible artifact formation of methyhner-cury (MeHg) duriri analytical procedures, mainly distillation and alkaline dissolution, which were expressed for the first time at the Conference Mercury as a Global Pollu-tanf in 1996 (Hintelmann and Evans 1997 Hintelmann et al.1997). 

Sampling artifacts. The use of in situ pumps to collect water samples for " Th analysis permits simultaneous collection (and separation) of different particle fractions as well as dissolved Th. As pumping systems have been modified to permit determination of POC on the pump filters, it became possible to compare POC determined from the pump samples with conventional POC determinations made on small volume samples (0.5 - 2 L) taken from hydrocasts. The JGOFS data from multiple studies show large discrepancies between these two sample collection methods, with pump POC values 3 to 100 times lower than bottle POC values. Possible artifacts with each approach have been identified. For example Moran et al. (1999) have suggested that DOC is adsorbed onto... 

Figure 3.70 Room temperature optical absorption spectra of a 45(X)A-lhick film of neutral polypyrrole doped with at0.03 torr. (a) before exposure to I2, conductivity <10 6fl 1cm"1 (b) after 2 minutes I2 exposure, conductivity 4.8 ft em 1 (c) after 7 minutes exposure, conductivity 6,7 ft em 1 (d) after 22 minutes I2 exposure, conductivity 32ft cm The three structures seen on the low-energy side of (a) (c) arc possibly artifacts due to interference effects in the films. From Pfluger et at. (1983).

A report on the binding of the anesthetic propofol to human serum albumin and to plasma presents a dataset that challenges simple notions of equilibria [70]. The unbound fraction of propofol was found to increase sharply at low drug concentrations. The authors appear to have carefully eliminated possible artifacts. Explanations based on cooperative binding modes are discussed though no clear explanation emerges. 

The aim of this section is to give a concise description of the nse of traps, note the most popnlar and newly recommended traps, as well as underline the possible artifacts connected with the trap applications to ion-radical reactions. There are traps for ion-radicals and neutral radicals resulting from ion-radical disintegration. 

Next, we used an in-house library design software (see details in Chapter 15) to enumerate the virtual libraries and then calculated various physical properties. Products were removed from consideration if MW is > 300, number of rotatable bonds > 3, and ClogP > 3. For solubility, two in-house model calculations were applied as filters turbidimetric >10 mg/mL and thermodynamic solubility >100 xM. The resulting cherry-picked library was then reviewed by NMR spectroscopists to remove compounds with possible artifacts, likely to be insoluble, or likely to be false positive. These included some conjugated systems and compounds with likelihood of indistinct NMR spectra. 

This paper described a number of the means for measuring the piezoelectric coefficients of bulk materials and thin films. In bulk materials, excellent references are available. Numerous means have been used over the years to measure the piezoelectric coefficients, which can be loosely grouped as charge-based and displacement-based. Accurate data can be obtained by many of the techniques, and agreement between measurement types is usually reasonable, provided that comparable excitation levels are utilized. In contrast, for thin films attached to substrates, the mechanical boundary conditions differ in charge and displacement based techniques. As a result, the direct and converse coefficients are not identical. In addition, perhaps because of the relative immaturity of the field, the numerous possible artifacts are not always accounted for, which can lead to erroneous results in thin film measurements. 

However, in any examination of the FAB-MS profile of these vinyl ether-linked compounds, one should be aware of some possible artifact formation... 

Although it is simple to record spectra, great care is needed to obtain spec with as few artifacts as possible. Some possible artifacts are described here. 

Recently, Castiglioni, et al. have devised a new method, diffuse reflectance CD (DRCD), to measure the solid-state CD of a pure microcrystallite sample or one mixed with KBr [42,43] and checked their performance. An integrating sphere used for UV-Vis and IR spectroscopy was introduced (Fig. 6). The advantage of this technique is to remove possible artifact elements such as reactions with the KBr, the effect of pressure required for the pellet preparation, or sample dissolution in nujol. Further, samples can be recovered after the spectral measurement, in contrast to both KBr pellet and nujol mull methods. 

In this section, recent developments of the FFF methodology are presented which might have a significant impact on the possibilities of FFF separations. On the other hand, problems which are associated with FFF measurements in general are discussed. These problems and possible artifacts have to be kept in mind when interpreting FFF results, regardless of the technique or detection system used. 

Figure 9.1. Pectenotoxins whose structures have been definitively identified. Key +, reported not reported , possible artifact found only as an acid-induced artifact (see Figure 9.2).

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