Reproducibility Problems

Possible applications of MIP membranes are in the field of sensor systems and separation technology. With respect to MIP membrane-based sensors, selective ligand binding to the membrane or selective permeation through the membrane can be used for the generation of a specific signal. Practical chiral separation by MIP membranes still faces reproducibility problems in the preparation methods, as well as mass transfer limitations inside the membrane. To overcome mass transfer limitations, MIP nanoparticles embedded in liquid membranes could be an alternative approach to develop chiral membrane separation by molecular imprinting [44]. 

The reproducibility of the electrodeposition of conducting polymer films has been a very difficult issue. It has long been realized that each laboratory produces a different material and that results from different laboratories are not directly comparable.82 We have experienced reproducibility problems with almost all of the electrochemically polymerized materials used in our work. 

Initial Situation A product contains three active components that up to a certain point in time were identified using TLC. Quantitation was done by means of extraction/photometry. Trials to circumvent the time-consuming extraction steps by quantitative TLC (diffuse reflection mode) had been started but were discontinued due to reproducibility problems. The following options were deemed worthy of consideration ... 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

Variations on the spectral peaks from different species of the same genus were also observed. Three species of Pseudomonas produced the spectra shown in Figure 14.2. These spectra are clearly unique and were used to correctly identify unknown samples. Because of peak ratio reproducibility issues in bacterial protein profiles obtained by MALDI MS,11 a fingerprint approach that had been used for other mass spectrometry approaches has not been used. The profile reproducibility problem was first recognized by Reilly et al.12,13 and later researched by others in the field.14,15 As a later alternative, a direct comparison of the mass-to-charge ratio (m/z) of the unknown mass spectral peaks with a database of known protein masses has been used to identify unknown samples.14... 

Disadvantages of monolithic silica columns include the labor-intensive preparation of individual columns with possible reproducibility problems, limited availability, and relatively short retention caused by the smaller amount of silica existing in a column... 

Dow/Chirotech [35, 36], Topcro Pharma [37] as well as by Solvias [38] using Rh-DuPhos catalysts (Fig. 37.6). Besides these successful examples, a process using Rh-DuPhos was abandoned because of reproducibility problems due to impurities carried over from the preceding step, and because of concerns about the toxicity of 2-nitropropane, even though ee-values of 99% were achieved [39]. 

It is known that relatively subtle solvent properties (. . the presence of trace metal ions or dissolved oxygen) can have a pronounced effect on Tj values (1 ). For this reason, we have emphasized studies based on comparing relative Tj values of resonances taken from the same spectrum of a given compound, rather than comparing absolute Tj values taken from different spectra. To insure reproducibility, duplicate Tj determinations were made in all cases. Monomer Tj values (e.g. methyl a-D-gluco-pyranoside) can be obtained in less than an hour. However, we have experienced difficulty in obtaining consistent absolute Tj values for successive samples of the same monosaccharide. Such reproducibility problems have not been observed for the polysaccharides, and we have observed no successive Tj value differences which can be attributed to solvent or sample preparation. 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobility of individual molecules is restricted. This lack of mobility is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibility problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quality. Alkyl silanes remain, however, ideal materials for surface modification and functionalization applications, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

Although microbiological methods are widely used for quantitation of total niacin, they tend to be time consuming and labor intensive (100). Reproducibility problems have also been reported (96). The HPLC methods for foods generally determine total niacin rather than the individual vitamers. Table 14 summarizes a recent HPLC method for quantitating total niacin in foods. The simultaneous determination of niacin with one or more other B vitamins is covered later in this chapter in Sec. XI. 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

In 1970, Delves15 described the use of 10 mm diameter nickel metal foil micro-crucibles for the atomization of lead in blood samples, after a partial pre-oxidation with hydrogen peroxide at 140 °C. The technique, which became widely known as the Delves Cup Technique , was extensively used for more than a decade in many laboratories around the world, and was also applied to environmental analyses such as the determination of lead in water. A flame-heated nickel tube was again used to overcome the reproducibility problems otherwise caused by the variability in the construction of individual disposable cups. 

In the table, the differences between the columns result from the change in the values of the factors and the differences between the lines give the reproducibility problems of the experiments. The total variance (s ) associated to the table data, here given by relation (5.151), must be divided according to its components the variances of inter-lines (or reproducibility variances) and variances of inter-columns (or variances caused by the factor). 

Coupling CPE to flow injection analysis (FIA) has also been exploited in both on-line and off-line configurations [73, 84]. The use of FIA to introduce the SRP into various analytical devices facilitates the dissolution of the SRP in small volumes (increased preconcentration factor), alleviating reproducibility problems [73]. 

It is often difficult to compare the sonochemical results reported from different laboratories (the reproducibility problem in sonochemistry). The sonochemical power irradiated into the reaction system can be different for different instruments. Several methods are available to estimate the amount of ultrasonic power entered into a sonochemical reaction, the most common being calorimetry. This experiment involves measurement of the initial rate of a temperature rise produced when a system is irradiated by power ultrasound. It has been shown that calorimetric methods combined with the Weissler reaction can be used to standardize the ultrasonic power of individual ultrasonic devices. ... 

The significance of paper and thin-layer chromatography lies in the possibility of pre-separating single elements by simple means and also in the separation of element mixtures from different materials in connection with detection and determination methods. Disadvantages for quantitative trace analysis are reproducibility problems... 

Triethylborane-induced stereoselective radical addition of RsSiH to carbon-carbon triple bonds was investigated by Oshima and Utimoto [5]. Even though the use of triethylborane frequently proved to cause reproducibility problems, the reaction represents a simple, convenient route to alkenylsilanes (Eq. 1), including bulky tris(trimethylsilyl)vinylsilanes (3a, 3b). 

Another problem with the filament pyrolysers is the possibility that the filament may be non-uniformly heated over its length. This may determine different Teq s in different points of the filament. If the sample is not always placed in the same point of the filament in repeated experiments, this may introduce a rather drastic reproducibility problem. In spite of these disadvantages, the resistively heated filament pyrolysers are among the most common ones, and very good reproducibility has been reported frequently [12]. 

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