Sample matrix additives

Two factors are said to interact when the effect of one of them is different at different levels of the other. In general, when factors operate independently of each other, they do not exhibit interaction. As an example, consider that in a hypothetical reaction intended to improve the extraction of some metal from a sample matrix addition of 10 mg of a catalyst at a temperature of 25 °C increases the yield. We can try to predict what happens at 45 °C. If the yield decreases, it is a proof that an interaction between the factors catalyst and temperature exists. Why Well, answering this question is not an experimental design task rather, it is simply a chemical problem and it may be a challenge for chemists to explain it. 

A variety of other techniques eg. voltammetry, flame atomic absorption spectrometry, flame emission spectrometry. ICP-atomic emission, ICP-fluorescence and ICP-mass spectrometry are almost invariably applicable to aqueous solutions, though solid sampling using ETA technique with the ICP-MS, is well established by now (Baumann, 1992 Voell-kopf et al., 1992). If the analytical technique employed is amenable only to a sub sample in solution, or is sensitive to the sample matrix, additional steps for dissolution and removal (partial or complete) of the biological matrix become unavoidable. We shall be mainly concerned in this chapter with these steps. 

Whilst these standards are mostly prepared simply within the solvent matrix used, variations on this theme may result in inclusion of these standards actually within the sample matrix. These procedures are often referred to as internal standardization, standard addition, or standard recovery, depending on the actual implementation of how the known analyte addition is used within the spectrophoto-metric method. These procedures either provide spectral calibration or method quality assurance data. For example, spectroscopic methods will often require pretreatment to remove the interfering sample matrix. Addition of a known amount of the test analyte spike (at the expected level) to a sample is... 

Attenuation of radiation as it passes through the sample leads to a transmittance of less than 1. As described, equation 10.1 does not distinguish between the different ways in which the attenuation of radiation occurs. Besides absorption by the analyte, several additional phenomena contribute to the net attenuation of radiation, including reflection and absorption by the sample container, absorption by components of the sample matrix other than the analyte, and the scattering of radiation. To compensate for this loss of the electromagnetic radiation s power, we use a method blank (Figure 10.20b). The radiation s power exiting from the method blank is taken to be Pq. 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

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. 

Spike recoveries for samples are used to detect systematic errors due to the sample matrix or the stability of the sample after its collection. Ideally, samples should be spiked in the field at a concentration between 1 and 10 times the expected concentration of the analyte or 5 to 50 times the method s detection limit, whichever is larger. If the recovery for a field spike is unacceptable, then a sample is spiked in the laboratory and analyzed immediately. If the recovery for the laboratory spike is acceptable, then the poor recovery for the field spike may be due to the sample s deterioration during storage. When the recovery for the laboratory spike also is unacceptable, the most probable cause is a matrix-dependent relationship between the analytical signal and the concentration of the analyte. In this case the samples should be analyzed by the method of standard additions. Typical limits for acceptable spike recoveries for the analysis of waters and wastewaters are shown in Table 15.1. ... 

Standards used to constmct a cahbration curve must be prepared such that the matrix of the standard is identical to the sample s matrix because the values of the parameters k and b associated with a linear cahbration curve are matrix dependent. Many areas of chemical analysis are plagued by matrix effects, and it is often difficult to duphcate the sample matrix when preparing external standards. Because it is desirable to eliminate matrix effects, cahbration in the sample matrix itself can be performed. This approach is called the standard addition method (SAM) (14). In this method, the standards are added to the sample matrix and the response of the analyte plus the standard is monitored as a function of the added amount of the standard. The initial response is assumed to be Rq, and the relationship between the response and the concentration of the analyte is... 

Although rum ammonia levels are not routinely measured, it is a useful indicator of Reye s syndrome and should be monitored in newborns at risk of developing hyperammonemia Ammonia is produced in many analytically useful enzyme reactions and the ammonium ISE has been used as the base sensor in several enzyme electrodes (see next section). In addition to valinomycin, other antibiotics such as the nonactin homalogs and gramicidins also behave as ionophores. The nonactin homolo were originally studied for their ability to selectively bind potassiiun ions It was then discovered that ammonium ions were preferred over potassium ions, and the selectivity coefficient Knh+ = 0.12 was reported. Since ammonia is present at fairly low levels in serum, this selectivity is not sufficient to to accurately measure NH4 in the presence of K. An extra measure of selectivity can be gained by using a gas permeable membrane to separate the ammonia gas from the sample matrix... 

For each group, one representative sample matrix has to be used for method validation. If the intended use is restricted to one of the crop groups, the method must be validated only for this group. On the other hand, the method has to be validated for all groups if the use is intended for a variety of crops that belong to two or more different groups. In addition, specific crops which are difficult to analyze due to matrix interference require individual method validation (e.g., hops, brassica varieties, bulb vegetables, herbs, tea). 

Specifically for triazines in water, multi-residue methods incorporating SPE and LC/MS/MS will soon be available that are capable of measuring numerous parent compounds and all their relevant degradates (including the hydroxytriazines) in one analysis. Continued increases in liquid chromatography/atmospheric pressure ionization tandem mass spectrometry (LC/API-MS/MS) sensitivity will lead to methods requiring no aqueous sample preparation at all, and portions of water samples will be injected directly into the LC column. The use of SPE and GC or LC coupled with MS and MS/MS systems will also be applied routinely to the analysis of more complex sample matrices such as soil and crop and animal tissues. However, the analyte(s) must first be removed from the sample matrix, and additional research is needed to develop more efficient extraction procedures. Increased selectivity during extraction also simplifies the sample purification requirements prior to injection. Certainly, miniaturization of all aspects of the analysis (sample extraction, purification, and instrumentation) will continue, and some of this may involve SEE, subcritical and microwave extraction, sonication, others or even combinations of these techniques for the initial isolation of the analyte(s) from the bulk of the sample matrix. 

Once method validation has been completed, the treated samples may be analyzed. The method should be under control so that no additional changes will be necessary. Analysis of laboratory-fortified samples and control samples will be used to monitor the quality of the study. The purpose of laboratory-fortified samples is confirmation of the recovery efficiency of residues from the sample matrix. A minimum of two laboratory recovery samples need to run with each set. Recoveries should average 70-120%. 

HPLC/MS and HPLC/MS/MS analyses are susceptible to matrix effects, either signal enhancement or suppression, and are often encountered when the cleanup process is not sufficient. To assess whether matrix effects influence the recovery of analytes, a post-extraction fortified sample (fortified extract of control sample that is purified and prepared in the same manner as with the other samples) should be included in each analytical set. The response of the post-extraction fortified sample is assessed against that of standards and samples. Matrix effects can be reduced or corrected for by dilution of samples, additional cleanup, or using calibration standards in the sample matrix for quantitation. 

Solubilizing all or part of a sample matrix by contacting with liquids is one of the most widely used sample preparation techniques for gases, vapors, liquids or solids. Additional selectivity is possible by distributing the sample between pairs of immiscible liquids in which the analyte and its matrix have different solubilities. Equipment requirements are generally very simple for solvent extraction techniques. Table 8.2 [4,10], and solutions are easy to manipulate, convenient to inject into chromatographic instruments, and even small volumes of liquids can be measured accurately. Solids can be recovered from volatile solvents by evaporation. Since relatively large solvent volumes are used in most extraction procedures, solvent impurities, contaminants, etc., are always a common cause for concern [65,66]. 

For solid samples forming homogeneous solutions the model system may be used if pure sample matrix materials are available otherwise, the standard additions method is used. 

For the purpose of the identification and quantification of additives (broadly defined) in polymeric materials extraction and dissolution methods are favoured (Sections 3.3-3.7). However, additives are also made accessible analytically by digestion of the sample matrix (cf. Section 8.2). Such wet chemical techniques, that remove the sample matrix first, are often limited to mg amounts because of pressure build-up in destruction vessels. Another reactive extraction approach to facilitate additive analysis is depolymerisation by acid hydrolysis or saponification, sometimes under pressure. This is then frequently followed by chemical methods such as titrimetry or photometry for final identification and quantification. 

Recovery procedures have traditionally involved some form of solvent, gas or heat extraction from the bulk sample matrix. Some of these lend themselves to precolumn hyphenation (e.g. SFE, TD, Py, HS), as opposed to others (e.g. Soxhlet, ultrasonics). Extraction of additives should not be considered as an isolated step, because it may strongly influence the subsequent chromatographic separation. The success of an analysis may very often depend more on the extraction procedure than on the chromatographic separation. In hyphenation there should be compatibility between the sample preparation and subsequent chromatographic analysis. 

In an ideal case, the signal y A = f(zA), as shown in Fig. 3.6, is determined only by the analyte A (or the phenomenon of interest), namely both the position, zA = /(A), and intensity, yA = f(xA). But in real samples, matrix constituents are present which can principally interfere with the analyte signal. In structure analysis the same holds for the neighboring relationships (the environment of the species A of interest). Therefore, signal parameters are additionally influenced by the matrix (or the neighborhood , respectively), namely the species B,C,...,N, and follow then the complex relationships zA = /(A N), yA = /(xa xb,Xc,...,xN). Additionally, influencing factors a,b,...,m, background, y0, and noise (random deviations eA) may become relevant and have to be considered. 

From the chemical point of view, in cases where matrix effects appear and no suitable certified reference materials are available, the calibration may be performed in the sample matrix itself by means of standard addition. 

SIMS is one of the only mass spectrometric techniques that allow solid samples to be analysed without any extraction of compounds or matrix addition. Generally, no specific preparation technique is required, and solid samples can directly be analysed if they are small enough to be fixed on the sample holder. In most cases, this means that the sample size must not be more than 1 cm. 

In addition to the analyte, the matrix will contain many other compounds. The method chosen must discriminate between the analyte of interest and other compounds also present in the sample. The test portion may have to pass through many analytical stages before the analyte is obtained in a form suitable for final measurement. First, the analyte may need to be separated from the bulk of the sample matrix. Further treatment may then be required to obtain an aliquot that is sufficiently clean (i.e. free from potential interferences) for the end-measurement technique. A general scheme of analysis is presented in Table 4.4 to illustrate the different approaches used depending on the nature of the analyte and of the matrix. 

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