Sampling, automation errors

Unless a comparison is made with a standard, which is identical in composition to the sample, systematic errors in XRF may be large. Particle size and shape are important and determine the degree to which incident beam is absorbed or scattered. Hence, standards and samples must be powdered to the same mesh size, preferably finer than 200 mesh. Simultaneous determination of several elements is possible by XRF with automated equipment. 

Overview Sample Handling Glucose Sarcosine, Creatine, and Creatinine Inborn Errors of Metabolism. Polycyclic Aromatic Hydrocarbons Environmental Applications. Sample Handling Comminution of Samples Automated Sample Preparation Robotics. Sampling Theory. 

Separate sample blanking requires an additional analytical channel, and is therefore wasteflil of both reagents and hardware. An alternative approach that is used on several automated systems, eg, Du Pont ACA, BM-Hitachi 704, Technicon RA-1000, is that of bichromatic analysis (5) where absorbance measurements are taken at two, rather than one, wavelength. When the spectral curves for the interference material and the chromogen of the species measured differ sufficiently, this can be an effective technique for reducing blank contributions to assay error. Bichromatic analysis is effective for blanks of both the first and second type. 

Modern transducers and microprocessors have been used successfully to automate particulate sampling trains in order to eliminate the operating curves and manual adjustments (7). The automated samplers adjust continuously to maintain isokinetic conditions. In addition, the microprocessor continuously calculates and displays both instantaneous sampling conditions and the total sample volume collected at any given moment. The use of the automated system with the microprocessor, therefore, eliminates both operator and calculation errors. 

By automation one can remove the variation of the analysis time or shorten the analysis time. Although the variation of the analysis time causes half of the delay, a reduction of the analysis time is more important. This is also true if, by reducing the analysis time, the utilization factor would remain the same (and thus q) because more samples are submitted. Since p = AT / lAT, any measure to shorten the analysis time will have a quadratic effect on the absolute delay (because vv = AT / (LAT - AT)). As a consequence the benefit of duplicate analyses (detection of gross errors) and frequent recalibration should be balanced against the negative effect on the delay. 

Spencer and Brewer [144] have reviewed methods for the determination of nitrite in seawater. Workers at WRc, UK [ 145] have described an automated procedure for the determination of oxidised nitrogen and nitrite in estuarine waters. The procedure determines nitrite by reaction with N-1 naphthyl-ethylene diamine hydrochloride under acidic conditions to form an azo dye which is measured spectrophotometrically. The reliability and precision of the procedure were tested and found to be satisfactory for routine analyses, provided that standards are prepared using water of an appropriate salinity. Samples taken at the mouth of an estuary require standards prepared in synthetic seawater, while samples taken at the tidal limit of the estuary require standards prepared using deionised water. At sampling points between these two extremes there will be an error of up to 10% unless the salinity of the standards is adjusted accordingly. In a modification of the method, nitrate is reduced to nitrite in a micro cadmium/copper reduction column and total nitrite estimated. The nitrate content is then obtained by difference. 

Particularly in autoanalyser methods this wide variation in chloride content of the sample can lead to serious salt errors and, indeed, in the extreme case, can lead to negative peaks in samples that are known to contain ammonia. Salt errors originate because of the changes of pH, ionic strength and optical properties with salinity. This phenomenon is not limited to ammonia determination by autoanalyser methods it has, as will be discussed later, also been observed in the automated determination of phosphate in estuarine samples by molybdenum blue methods. 

Part—I has three chapters that exclusively deal with General Aspects of pharmaceutical analysis. Chapter 1 focuses on the pharmaceutical chemicals and their respective purity and management. Critical information with regard to description of the finished product, sampling procedures, bioavailability, identification tests, physical constants and miscellaneous characteristics, such as ash values, loss on drying, clarity and color of solution, specific tests, limit tests of metallic and non-metallic impurities, limits of moisture content, volatile and non-volatile matter and lastly residue on ignition have also been dealt with. Each section provides adequate procedural details supported by ample typical examples from the Official Compendia. Chapter 2 embraces the theory and technique of quantitative analysis with specific emphasis on volumetric analysis, volumetric apparatus, their specifications, standardization and utility. It also includes biomedical analytical chemistry, colorimetric assays, theory and assay of biochemicals, such as urea, bilirubin, cholesterol and enzymatic assays, such as alkaline phosphatase, lactate dehydrogenase, salient features of radioimmunoassay and automated methods of chemical analysis. Chapter 3 provides special emphasis on errors in pharmaceutical analysis and their statistical validation. The first aspect is related to errors in pharmaceutical analysis and embodies classification of errors, accuracy, precision and makes... 

When performing dissolution testing, there are many ways that the test may generate erroneous results. The testing equipment and its environment, handling of the sample, formulation, in situ reactions, automation and analytical techniques can all be the cause of errors and variability. The physical dissolution of the dosage form should be unencumbered at all times. Certain aspects of the equipment calibration process may show these errors as well as close visual observation of the test. The essentials of the test are accuracy of results and robustness of the method. Aberrant and unexpected results do occur, however, and the analyst should be well trained to examine all aspects of the dissolution test and observe the equipment in operation. 

While automation of dissolution sampling is very convenient and laborsaving, errors often occur with these devices because the analysts tend to overlook problem areas. Sample lines are often a source of error for a variety of reasons unequal lengths, crimping, wear beyond limits, disconnection, carryover, mix-ups or crossing, and inadequate cleaning. 

Automated programmable instruments that can carry out the repeated thermal cycles necessary for PCR and that can accommodate multiple samples simultaneously are now widely available. The procedure is usually performed with thermostable DNA polymerases. PCR is widely used to facilitate detection of minute amounts of viral DNA. The technique can also be used to detect specific point mutations, provided the approximate site of mutation is known. One limiting feature of this approach arises from the fact that the bacterial polymerases frequently make errors when synthesizing new strands and so can introduce mutations that are not present in the original sample. 

Bohdan Automation Inc. The Balance Automator offers a cost-effeetive alternative to manual weighing, reduces errors and operator tedium, and proeesses up to 120 samples per hour. This system can accommodate various container sizes and balanees with the help of interchangeable parts. 

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