Errors and Interferences

Instrumentation. Sample Preparation. Qualitative and Quantitative Analysis. Interferences and Errors Associated with the Excitation Process. Applications of Arc/Spark Emission Spectrometry. 

Interferences and Errors Associated with the Excitation Process... 

INTERFERENCES AND ERRORS ASSOCIATED WITH THE EXCITATION PROCESS... 

Instrumentation. Sample preparation. Qualitative and quantitative analysis. Interferences and errors associated with the excitation process. Applications of arc/spark emission spectrometry. 

Designing a good analytical method requires knowing how to obtain a representative sample of the material to be analyzed, how to store or preserve the sample until analysis, and how to prepare the sample for analysis. The analyst must also know how to evaluate possible interferences and errors in the analysis and how to assess the accuracy and precision of the analysis. These topics will be discussed subsequently and specific interferences for given instmmental methods are discussed in the following chapters. 

Although the most sensitive line for cadmium in the arc or spark spectmm is at 228.8 nm, the line at 326.1 nm is more convenient to use for spectroscopic detection. The limit of detection at this wavelength amounts to 0.001% cadmium with ordinary techniques and 0.00001% using specialized methods. Determination in concentrations up to 10% is accompHshed by solubilization of the sample followed by atomic absorption measurement. The range can be extended to still higher cadmium levels provided that a relative error of 0.5% is acceptable. Another quantitative analysis method is by titration at pH 10 with a standard solution of ethylenediarninetetraacetic acid (EDTA) and Eriochrome Black T indicator. Zinc interferes and therefore must first be removed. 

More efficient estimation methods exist than the simple method described here [17]. The generalized standard addition method (GSAM) shares the strong points (e.g correction for interferences) and weak points (e.g. error amplification because of the extrapolation involved) of the simple standard addition method [18]. 

Systematic error is also known as bias. The bias is the constant value difference between a measured value (or set of values) and a consensus value (or true value if known). Specificity is the analytical property of a method or technique to be insensitive to interferences and to yield a signal relative to the analyte of interest only. Limit of reliable measurement predates the use of minimum detection limit (MDL). The MDL... 

Isaeva [181] described a phosphomolybdate method for the determination of phosphate in turbid seawater. Molybdenum titration methods are subject to extensive interferences and are not considered to be reliable when compared with more recently developed methods based on solvent extraction [182-187], such as solvent-extraction spectrophotometric determination of phosphate using molybdate and malachite green [188]. In this method the ion pair formed between malachite green and phosphomolybdate is extracted from the seawater sample with an organic solvent. This extraction achieves a useful 20-fold increase in the concentration of the phosphate in the extract. The detection limit is about 0.1 ig/l, standard deviation 0.05 ng-1 (4.3 xg/l in tap water), and relative standard deviation 1.1%. Most cations and anions found in non-saline waters do not interfere, but arsenic (V) causes large positive errors. 

When the second derivative of (5.32) is calculated and set equal to zero, the inflection point of the titration curve is obtained [23, 24, 133, 134). It has been found that the theoretical titration error generally increases with decreasing sample concentration, with increasing value of the solubility product or of the dissociation constant, with increasing value of the dilution factor and with increasing concentration of the interferents. Larger errors are obtained with unsymmetrical titration reactions. The overall error is a combination of these factors the greatest effect is exerted by the sample concentration, a smaller one by the equilibrium constant and the interferents, and the smallest by dilution. To obtain errors below 1%, it must approximately hold that eg, > 10 2 i,K< 10 , < 10 to 10" and r < 0.3. 

For ultraviolet irradiations care is needed in choosing a solvent— water, alkanes and acetonitrile are transparent and often unreactive towards electronically excited substrates alcohols and ethers are transparent but are more likely to be reactive acetone and benzene are sometimes used even though they are not transparent to all relevant wavelengths. It may be necessary to find out by trial and error whether or not an otherwise suitable solvent interferes with the photochemical process. A similar approach can be taken to decide about the need to purge continuously with an inert gas to remove dissolved oxygen. 

Sources of Error. Several common causes nl measurement problems are electrode interferences and/or fouling of the pH sensor, sample matrix effects, reference electrode instability, and improper calibration of the measurement system. 

It has become a standard procedure to test how much the washing step removes the interferents and how little of the analyte is removed during washing. This test is useful to support the method development, but often it is based merely on a trial and error approach, so that any changes in the method or the sample matrix would require a reoptimization. 

Interferences in chromatography can generally be overcome by finding the right conditions to give separation. However, this might be costly, since development of separations is largely a trial-and-error process. 

Accuracy can also be demonstrated through participation in properly conducted interlaboratory studies, which are also useful to detect systematic errors (Gtinzler 1996) related to, e.g. sample pretreatment (e.g. extraction, clean-up), final measurement (e.g. calibration error, spectral interference) and laboratory competence. As described below, interlaboratory studies are organised in such a way that several laboratories analyse a common material which is distributed by a central laboratory responsible for the data collection and evaluation. 

SPE. However, LLE may reduce the sample loss, experiment procedures and errors, and save time in comparison to SPE [8, 25, 28], In contrast, if a larger amount of sample is available, e.g., 2-10 mL of urine or 1-10 L of environmental water, SPE is a better choice, because it concentrates the sample and minimizes the interferences from other materials, leading to a higher sensitivity and selectivity of the method or test [34, 37, 39], The sample extraction throughput can be significantly enhanced by using automated 96-well SPE plates [31,49],... 

Most ultramicrobiosensors use differential measurement to overcome the problems of interferences and electrode fouling. The practical use of these biosensors for direct measurement is limited by interferents, such as ascorbic acid, acetaminophen (paracetamol), uric acid, etc., which are present in complex matricies such as serum. The specificity of the biochemical system is compromised by the partial selectivity of the electrode. The electrode not only oxidizes the desired product (e.g., H2O2 formed in the enzymatic oxidation of glucose by glucose oxidase), but also any other species oxidizable at the working potential. This produces a larger current response and a positive error. 

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