Lead sampling procedures reproducibility

Pre-treatment of the sample is especially important if lead particles are present in the sample. To obtain representative results, prescription of representative, reproducible, precise and accurate sampling, pre-treatment and methods of analysis are essential in the sampling procedure. 

For PMMA/additive dissolutions, it was not possible to identify any additive characteristic mass peaks, either by direct laser desorption or with matrix-assistance (dithranol, DHBA or sinapinic acid, 4-hydroxy-3,5-dimethoxy-cinnamic acid). This has again been ascribed to very strong interaction between PMMA and additives, which suppresses desorption of additive molecules. Also, partial depolymerisation of pho-tolytically labile PMMA by laser irradiation may play a role, which leads to saturation of the detector by PMMA fragment-ions and disappearance of additive mass peaks below noise level. Meyer-Dulheuer [55] has also reported MALDI-TOFMS analysis of a coating/2-ethylhexyldiphenylphosphate sample. Quantitative determination of the additives by means of MALDI-ToFMS proved impossible. Possibly the development of reproducible (automated) sample handling procedures or thin films might overcome this problem. 

A problem with monofunctional reactions, e.g., cracking, alkylation, etc. is that they have a tendency to quickly deactivate because of coke deposition. This problem is usually not of concern with bifunctional reactions, e.g., those that employ a metal function in addition to the acid sites. However, we avoided the use of metal function because of the possible unknown modifications that could be introduced to a given sample by the metal deposition procedure. This is especially important when dealing with samples like VPI-5. Thus, to minimize the rate of deactivation, the alkylation experiments were conducted at 463 K. This low temperature introduces another problem, namely, the adsorption of reactants and products. At the experimental conditions employed here, the catalyst bed becomes saturated at time of 10 minutes or less (depending on sample). From this point onward, deactivation is clearly observable via the decrease in conversion with time. The data reported here were obtained at 11-13 minutes on-line. Since meta-diisopropylbenzene proceeds through several reaction pathways that lead to a number of products, it is most appropriate to compare the catalytic data at the constant level of conversion. Here we report selectivities at approximately 25 % conversion. For each catalyst, the results near 25 % conversion were repeated three times to ensure reproducibility. 

The use of a styrene comonomer procedure overcomes the problems (Table VII) and leads to reproducible, uniform copolymerization. Samples from these reaction mixtures can be extracted easily from the vessel at the completion of the irradiation, even for the materials where the... 

The reason for chilling the sample is to slow down the reaction rate so that a negligible amount of reactant will decompose between the time you chill the sample and the time you make the absorbance measurement. Since it is impossible to eliminate completely errors due to reaction after withdrawal and coohng, it is important to try to perform the sample removal and chilling procedure in as reproducible a fashion as possible. This will lead to a partial cancellation of such errors. ... 

Recalibration of the instrument response function reduces or eliminates most of the instrumental factors that lead to relative intensity variations over time. For example, a luminescent standard could be used at the beginning of each session as described in Section 10.3.3. Use of the same standard and correction procedure during qualification could establish the true value of one or more peak ratios for future reference. Table 10.9 shows results for this approach applied to the example of calcium ascorbate. The ratio of the 767- and 1587 cm" peak intensities was monitored after calibration of the response function with a luminescent standard. The standard deviations listed in Table 10.9 for the 767/1582 peak height ratio provide indications of the reproducibility of the response correction and sample spectra. 

As discussed in the previous chapter a preparation procedure leads to several sets of samples, often produced in batches. It is necessary to assess that no difference exists within each of the sets, between sets and between batches. Consequently, homogeneity testing will try to measure differences between sub-samples within or between vials of materials. As it is not possible to measure all samples produced (unless a nondestructive technique is available) a strategy for the selection of representative samples is necessary. To demonstrate the absence or the existence of differences between samples, it is necessary that the analytical procedure is fully reproducible. If differences between measurements are too large due to the measurement method, inhomogeneity cannot be detected. In order to reveal presence of spot contamination, the measurement must be done on the substance of interest or any other substance known to present exactly the same properties and showing the same behaviour or distribution pattern (tracer). 

Electron ionization is the most commonly method used for the analysis of volatile compounds. It is the case for organic molecules. This is a ionization in the gas phase that occurs in the ion source by the collision of the neutral molecules of the sample and electrons emitted from a filament by a thermoionic process (Figure 16.17). The ejection of the most weakly held electron leads to positive ions. This reproducible procedure facilitates the identification of a compound by comparing its spectrum with those collected in a spectral library, assuming the compound is registered within. 

There has also been a move from slow manual sample preparation techniques to faster automated techniques. Automated sample preparation can be carried out on-line (with sample preparation connected directly to the analysis system) or off-line (sample preparation is automated, but the sample has to be manually transferred to the analysis system). Automated sample preparation offers the potential of performing sample clean-up, concentration, and analyte separation in a closed system. This reduces the sample preparation time, and the whole sample becomes available for analysis, leading to improved limits of detection. It also removes some of the human element from a procedure, thereby improving precision and reproducibility. Eurther-more, automated sample preparation reduces cost by using... 

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