Preparing the Sample

This section describes some of the most important aspects of sample preparation for NMR analysis, a topic that is all too frequently given insufficient consideration in the research laboratory. Even for routine applications of NMR it is wise for the chemist to adopt a systematic strategy for sample preparation that will give consistently good results, so saving instrument time and eliminating frustration. 

---

Atomic emission is used for the analysis of the same types of samples that may be analyzed by atomic absorption. The development of a quantitative atomic emission method requires several considerations, including choosing a source for atomization and excitation, selecting a wavelength and slit width, preparing the sample for analysis, minimizing spectral and chemical interferences, and selecting a method of standardization. 

Preparing the Sample Flame and plasma sources are best suited for the analysis of samples in solution and liquid form. Although solids can be analyzed by direct insertion into the flame or plasma, they usually are first brought into solution by digestion or extraction. 

Faradaic currents due to impurities can usually be minimized by carefully preparing the sample. For example, one important impurity is dissolved O2, which is reduced first to H2O2 and then to H2O. Dissolved O2 is removed by bubbling an inert gas such as N2 through the sample before the analysis. 

In preparing the sample for analysis the initial solution is filtered. Why is it not necessary to collect the entire filtrate before proceeding ... 

The mechanical properties of polystyrene depend to some extent on the nature of the polymer (e.g. its molecular weight), on the method of preparing the sample for testing and on the method of test, as is the case with all plastics materials. 

Prepare the sample solution in a similar manner to give a fluorescence value falling within the range of the calibration curve, and hence obtain the original calcium concentration in the sample. 

In those cases where there are any doubts about the feasibility of producing a sufficiently homogeneous and stable reference material, a feasibility study might be needed. For this study, an extra amount of material is needed. Questions regarding the best way of preparing the sample, the stability of the material, or the fitness for purpose might justify the inclusion of a feasibility study in the project. In the BCR projects, it is common practice to have a feasibility study, which usually has as the sole purpose of assessing the performance of the laboratories in the collaborative study in relation to the certification of the reference material. The feasibility study allows the participants to fine-tune their equipment, their methods, and their procedures in view of the characterization measurements. In each of these cases, a considerable extra number of samples is needed. 

The 1970 s demonstrated a trend chemistry is going out of analytical chemistry . However, what was not used anymore up-ffont for analysis came bade in the form of sample preparation techniques. For example, lUPAC devoted as much attention as ever before, but now to the chemistry needed to prepare the sample for measurement and to avoid losses and contamination. 

The concentration of analyte in the unknown sample is extrapolated from the calibration curve. To obtain an accurate and precise quantitative value, the optical density (OD) for the sample solutions must fall on the linear portion of the calibration curve. If the sample OD is too high, the sample solution must be diluted until the OD falls within the quantitative range of the assay. The concentration of the analyte in the original sample is calculated by correcting for any dilution factor that was introduced in preparing the sample for application to the microplate. 

Alkylenebis(dithiocarbamates) are not stable and decompose on contact with acidic fruit and vegetable juice generated during the sample preparation. The sample preparation is a critical step in their determination. The homogenization should be made with frozen samples as rapidly as possible followed immediately by the analysis. 

How much of a crystallizable material X can I blend uniformly into a polymer until it starts to form crystals A series of blends with increasing amount of X is prepared. The samples are studied by WAXS (cf. Sect. 8.2) using laboratory equipment. Crystalline reflections of X are observed, as X starts to crystallize. Peak areas can be plotted vs. the known concentration in order to determine the saturation limit. Think of X being Ibuprofen and Y a polystyrene-(7 )-polyisoprene copolymer, and you have an anti-rheumatism plaster. 

Zhan [61] reported the use of an oscillopolarographic method for the determination of primaquine phosphate and other drugs in pure form and in pharmaceutical preparations. The sample solution was mixed with potassium bromide and 6 M hydrochloric acid and the mixture was titrated with 0.1 M sodium nitrite. A micro platinum electrode and a platinum electrode were used as indicators and reference electrodes, respectively. The mean recoveries were 96.88-99.88%. Results agreed well with those obtained by the Chinese Pharmacopoeia method. 

The accuracy of an analytical method is given by the extent by which the value obtained deviates from the true value. One estimation of the accuracy of a method entails analyzing a sample with known concentration and then comparing the results between the measured and the true value. The second approach is to compare test results obtained from the new method to the results obtained from an existing method known to be accurate. Other approaches are based on determinations of the per cent recovery of known analyte spiked into blank matrices or products (i.e., the standard addition method). For samples spiked into blank matrices, it is recommended to prepare the sample at five different concentration levels, ranging over 80-120%, or 75-125%, of the target concentration. These preparations used for accuracy studies usually called synthetic mixtures or laboratory-made preparations . 

Prepare the samples dissolved in water or non-amine-containing buffer at an expected concentration level that falls within the standard curve range. The assay can tolerate the presence of most buffer salts, denaturants, and detergents. However, the standard curve should be run in the same buffer environment as the samples to obtain consistent response. 

There may also be cases where the maximum amount of analyte permitted is fixed by law, i.e. statutory limits. For these, there may be laid down standard procedures for sampling. The Codex maximum residue limit for the pesticide cypermethrin is 2 mg kg-1 (at the time of writing) in both citrus fruits and peaches [13]. When preparing the sample for analysis you need to know that in the case of the citrus fruit you take the whole fruit - skin, pith, pips, flesh and juice - whereas in the case of peaches it is the fruit after removal of stems and stones, but the residue is calculated and expressed in terms of the whole commodity (including the stone) without stems [14]. In some instances, it may be necessary to take a certain number of samples and that these must be taken in the presence of a witness. 

The Fe3(C0)- 2"Y adducts are prepared under argon atmosphere with the HY and NaY zeolites previously heated under vacuum at 350°C. The impregnation of the support is performed either from pentane solution at 25°C or from dry mixing of the carbonyl and the zeolite to avoid any complication from the solvent. In this last preparation the sample stands in vacuo for 2kh at 60°C in order to favour the sublimation of the carbonyl into the pores of the zeolite. 

---