Measurement sample preparation

Matrix effects play an important role in the accuracy and precision of a measurement. Sample preparation steps are often sensitive to the matrix. Matrix spikes are used to determine their effect on sample preparation and analysis. Matrix spiking is done by adding a known quantity of a component that is similar to the analyte but not present in the sample originally. The sample is then analyzed for the presence of the spiked material to evaluate the matrix effects. It is important to be certain that the extraction recovers most of the analytes, and spike recovery is usually required to be at least 70%. The matrix spike can be used to accept or reject a method. 

The database is critical to the accuracy of match predictions and batch corrections. Measuring samples prepared with colorant, resin, and known mixtures of each creates the database. Once created, the database can be edited to meet changes in colorant and resin inventories. 

Infrared Spectral Measurements. Sample preparation techniques and apparatus have been described previously (3, 4). The spectra were recorded on disks of powdered material with a Cary-White Model 90 infrared spectrophotometer. Spectra were obtained from 4000 to 1200 cm" at a spectral slit width of 4 cm" and a scan speed of 3 cm Vsec. The infrared cell was so constructed as to permit calcinations to be done... 

Accurate sample preparation prior to chemical measurement is often a limiting step not only is it an important source of uncertainty in final results consideration but is also lengthy and labour-intensive. The preparation step is intermediate between the correct sampling procedure of the bulk matrix and actual measurement. Sample preparation for modern chemical analysis involves a considerable amount of analytical time (see Figure 3.1) and is considered as the main bulk in costing. 

Figure 36.5c shows the results of sorption measurements. Samples prepared in the absence of PTMOS show the highest porosities after extraction the surface area of 1356 m g and the pore volume of 1.00 cm g measured for the sample synthesized at 110 °C correspond to optimal MCM l samples. When PTMOS is integrated into the synthesis, the jump in the isotherms, which is characteristic of mesoporosity, vanishes and the characteristics of a type-fV isotherm are lost. Similar observations were made by Bambrough et al. for a phenyl-modified silica. Whereas the 110 °C sample still retains high values of surface area and pore volume, the one synthesized at 25 °C shows a partial collapse of the pore structure, which is in line with the low quality of mesostructural order reflected by the PXRD experiments. 

As with any physical measurement, sample preparation is of utmost importance. This is particularly true for particle size determinations. If the measurement is performed in suspension, dispersing aids may have to be added and ultrasound may have to be apphed to ensure homogeneous dispersion. This may disrupt aggregates, which can be desired or undesired. In any case, it is highly advisable to observe the effects of sample preparation by additional methods such as microscopy. Guidelines for method development, instrument calibration, and acceptance criteria can be found in regulatory documents. ... 

The optimization of the AMD-HPTLC method was investigated (134f). Thin layers (100 pm) enhance the sensitivity of UV detection by a factor of 2 to 3. No effect was observed in the visible spectrum or in fluorescence measurements. Sample preparation is of great importance. Various phthalate esters can interfere with several pesticides, such as parathion, trifluralin, vinclozolin, and pendimethalin. The determination of polar substances is hindered by humins if present in high concentration. In this case, other AMD gradients should be used. Despite these limitations and drawbacks, the AMD-HPTLC method offers very sensitive quantitative determination of numerous pesticide residues in water samples (134g) (Table 11). 

For IR measurements, sample preparation has been a labor-intensive operation requiring some measure of skill and experience. Conventional IR spectroscopy is mainly based on transmission measurements except for samples for which the preparation of a thin layer is problematic, inadequate, or prohibited. In these cases other sampling techniques are required. 

The following experiments introduce students to the importance of sample preparation and methods for extracting analytes from their matrix. Each experiment includes a brief description of the sample and analyte, as well as the method of analysis used to measure the analyte s concentration. 

Extended x-ray absorption fine stmcture measurements (EXAFS) have been performed to iavestigate the short-range stmcture of TbFe films (46). It is observed that there is an excess number of Fe—Fe and Tb—Tb pairs ia the plane of the amorphous film and an excess number of Tb—Fe pairs perpendicular to film. The iacrease of K with the substrate temperature for samples prepared by evaporation is explained by a rearrangement of local absorbed atom configurations duting the growth of the film (surface-iaduced textuting) (47). 

Powder diffraction patterns have three main features that can be measured t5 -spacings, peak intensities, and peak shapes. Because these patterns ate a characteristic fingerprint for each crystalline phase, a computer can quickly compare the measured pattern with a standard pattern from its database and recommend the best match. Whereas the measurement of t5 -spacings is quite straightforward, the determination of peak intensities can be influenced by sample preparation. Any preferred orientation, or presence of several larger crystals in the sample, makes the interpretation of the intensity data difficult. 

Replicate Analyses. Confidence in the test result is improved by reducing the measurement variabihty. This variabihty in repeat analyses is known as precision. One method to improve the precision of the measurement is to perform complete rephcate analyses of the same sample beginning with the sample preparation (26). This is appropriate when the sample is known to be representative of the material sampled. When this is not the case, multiple samples should be taken for analysis. 

As-polymerized PVDC does not have a well-defined glass-transition temperature because of its high crystallinity. However, a sample can be melted at 210°C and quenched rapidly to an amorphous state at <—20°C. The amorphous polymer has a glass-transition temperature of — 17°C as shown by dilatometry (70). Glass-transition temperature values of —19 to — 11°C, depending on both method of measurement and sample preparation, have been determined. 

Bulk density is easily measured from the volume occupied by the bulk solid and is a strong func tion of sample preparation. True density is measured by standard techniques using liquid or gas picnometry Apparent (agglomerate) density is difficult to measure directly. Hink-ley et al. [Int. ]. Min. Proc., 41, 53-69 (1994)] describe a method for measuring the apparent density of wet granules by kerosene displacement. Agglomerate density may also be inferred from direcl measurement of true density and porosity using Eq. (20-42). 

PL measurements are generally nondestructive, and can be obtained in just about any configuration that allows some optically transparent access within several centimeters of the sample. This makes it adaptable as an in situ measurement tool. Little sample preparation is necessary other than to eliminate any contamination that may contribute its own luminescence. The sample may be in air, vacuum, or in any transparent, nonfluorescing medium. 

Raman spectroscopy is a very convenient technique for the identification of crystalline or molecular phases, for obtaining structural information on noncrystalline solids, for identifying molecular species in aqueous solutions, and for characterizing solid—liquid interfaces. Backscattering geometries, especially with microfocus instruments, allow films, coatings, and surfaces to be easily measured. Ambient atmospheres can be used and no special sample preparation is needed. 

Sample preparation requirements in solid state NMR are strikingly simple because the measurement is carried out at ambient temperature and pressure. Wide-line NMR experiments can be carried out on solid samples in any form, as far as the sample dimensions fit those of the coil in the NMR probe. MAS experiments require the material to be uniformly distributed within the rotor. 

Most of the transition elements that are of primary interest in the semiconductor industry such as Fe, Cr, Mn, Co, and Ni, can be analyzed with very low detection limits. Second to its sensitivity, the most important advantage of NAA is the minimal sample preparation that is required, eliminating the likelihood of contamination due to handling. Quantitative values can be obtained and a precision of 1-5% relative is regularly achieved. Since the technique measures many elements simultaneously, NAA is used to scan for impurities conveniently. 

Even though the mechanical profiler provides somewhat limited two dimensional information, no sample preparation is necessary, and results can be obtained in seconds. Also, no restriction is imposed by the need to measure craters through several layers of different composition or material type. 

---