Examples sample matrix

This section gives information on the technical limitations of the standard method caused by, for example, sample matrix effects (coloured samples can interfere with the photometric detection or element specific spectral interferences (AAS, ICP-OES) or chemical interferences (precipitation reactions, formation of reaction byproducts). These details are validated experimentally in laboratories participating actively in the standardization work. The documentation of the interferences m help potential users of a specific standard to decide whether the standard method could be applicable for the requirements of their analytical businesses. 

Particulate interferents can be separated from dissolved analytes by filtration, using a filter whose pore size retains the interferent. This separation technique is important in the analysis of many natural waters, for which the presence of suspended solids may interfere in the analysis. Filtration also can be used to isolate analytes present as solid particulates from dissolved ions in the sample matrix. For example, this is a necessary step in gravimetry, in which the analyte is isolated as a precipitate. A more detailed description of the types of available filters is found in the discussion of precipitation gravimetry and particulate gravimetry in Chapter 8. 

Finally, in particulate gravimetry the analyte is determined following its removal from the sample matrix by filtration or extraction. The determination of suspended solids is one example of particulate gravimetry. 

The analysis of clinical samples is often complicated by the complexity of the sample matrix, which may contribute a significant background absorption at the desired wavelength. The determination of serum barbiturates provides one example of how this problem is overcome. The barbiturates are extracted from a sample of serum with CHCI3, and extracted from the CHCI3 into 0.45 M NaOH (pH 13). The absorbance of the aqueous extract is measured at 260 nm and includes contributions from the barbiturates as well as other components extracted from the serum sample. The pH of the sample is then lowered to approximately 10 by adding NH4CI, and the absorbance remeasured. Since the barbiturates do not absorb at this pH, the absorbance at pH 10 is used to correct the absorbance at pH 13 thus... 

Sensitivity Sensitivity in flame atomic emission is strongly influenced by the temperature of the excitation source and the composition of the sample matrix. Normally, sensitivity is optimized by aspirating a standard solution and adjusting the flame s composition and the height from which emission is monitored until the emission intensity is maximized. Chemical interferences, when present, decrease the sensitivity of the analysis. With plasma emission, sensitivity is less influenced by the sample matrix. In some cases, for example, a plasma calibration curve prepared using standards in a matrix of distilled water can be used for samples with more complex matrices. 

Quantitative Analysis Using the Method of Standard Additions Because of the difficulty of maintaining a constant matrix for samples and standards, many quantitative potentiometric methods use the method of standard additions. A sample of volume, Vx) and analyte concentration, Cx, is transferred to a sample cell, and the potential, (ficell)x) measured. A standard addition is made by adding a small volume, Vs) of a standard containing a known concentration of analyte, Cs, to the sample, and the potential, (ficell)s) measured. Provided that Vs is significantly smaller than Vx, the change in sample matrix is ignored, and the analyte s activity coefficient remains constant. Example 11.7 shows how a one-point standard addition can be used to determine the concentration of an analyte. 

Figure 8.9 is an example of the use of solvent extraction to isolate polychlorinated biphenyls from a fat sample [6]. In this example the matrix is chemically modified to improve the selectivity of the extraction. The fat is first hydrolyzed by refluxing in 1 N ethanolic potassium hydroi prior to the... 

The example shown in Fig. 3.4g uses off-line column switching to combine exclusion and reverse phase chromatography for the separation of pesticides from a complex sample matrix. 

There are many potential sources of uncertainty. Examples include sampling the nature of the sample matrix sample storage conditions ... 

One of the requirements of this approach is that the analytes must be stable at the boiling point of the solvent, since the analytes collect in the flask. The solvent must show high solubility for the analyte and none for the sample matrix. Since this is one of the oldest methods of sample preparation, there are hundreds of published methods for all kinds of analytes in as many matrices. For example, XAD-2 resin (sty-rene-divinylbenzene) that was used to collect air samples to monitor current usage of pesticides in Iowa was Soxhlet-extracted for 24 h with hexane/acetone [22], This is common in environmental sample analysis, and one will see rows and rows of these systems in environmental laboratories. One disadvantage of Soxhlet extraction is the amount of solvent consumed, though modern approaches to solid sample extraction have tried to reduce the amount of solvent used. 

Chemical interferences are the result of problems with the sample matrix. For example, viscosity and surface tension affect the aspiration rate and the nebulized droplet size, which, in turn, affect the measured absorbance. The most useful solution to the problem is matrix matching, matching the matrix... 

One notable chemical interference occurs when atomization is hindered due to an unusually strong ionic bond between the ions in the ionic formula unit. A well-known example occurs in the analysis of a sample for calcium. The presence of sulfate or phosphate in the sample matrix along with the calcium suppresses the reading for calcium because of limited atomization due to the strong ionic bond between calcium and the sulfate and phosphate ions. This results in a low reading for the calcium in the sample in which this interference exists. The usual solution to this problem is to add a substance to the sample that would chemically free the element being analyzed, calcium in our example, from the interference. 

Proteins have, to date, only rarely been purified by SMB. The first attempt was made by Huang et al. in 1986 [42]. They isolated trypsin from porcine pancreas extracts using an SMB made of only six columns. In addition, this example also demonstrates that SMB systems with a very limited number of columns can be efficient. Another example for a successful protein-separation by SMB is the purification of human serum albumin (HSA) using two SMB-systems connected in series [43]. The first SMB was used for removing the less strongly retained components and the second one for removing the more strongly retained components of the sample matrix. 

Fig. 7.14 Example for standard matrix sample. Matrix was obtained by incubation of mGATl-membrane preparation in Tris-NaCI buffer ( 10 ig protein content), subsequent filtration and elution with methanol. This empty matrix was then spiked with NO 711 and [ Hio]NO 711 to obtain standard matrix samples for LC-MS method validation purposes, (a) Trace for 0.015 nM NO 711 (mass transition 381 180, m/z). (b) Trace for 1 nM [ Hio]NO 711 (391 190, m/z) in the same sample.

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