Stress instrumentation, detection

Viscometric measurements were made with a Haake Rotovisco MV 1 system with a rotor of diameter 40.08 mm, and length of 60 mm and a cup of 42.00 mm inside diameter. The shear stress was detected using a Head 50. Before measurements, calibrations were made with standard oils to obtain the instrument constant to convert stress readings to shear stress. Shear rate could be varied over the range of 50.74 to... 

Although the need for complete decomposition is often stressed (see also Table 8.3), not all detection techniques demand the same degree of mineralisation. Table 8.6 classifies analytical techniques according to the amount of mineralisation that they need [4]. Ideally, a purely instrumental approach is the only way to prevent losses and contamination due to decomposition. Choosing a decomposition mode simply to be able to meet the requirements of the detection technique is an incomplete approach. The choice of decomposition should primarily be directed by both the matrix and element of interest. 

For non-compendial procedures, the performance parameters that should be determined in validation studies include specificity/selectivity, linearity, accuracy, precision (repeatability and intermediate precision), detection limit (DL), quantitation limit (QL), range, ruggedness, and robustness [6]. Other method validation information, such as the stability of analytical sample preparations, degradation/ stress studies, legible reproductions of representative instrumental output, identification and characterization of possible impurities, should be included [7], The parameters that are required to be validated depend on the type of analyses, so therefore different test methods require different validation schemes. 

From those techniques given in Table 1 my personal preference is for failure mode, effects, and criticality analysis (FMECA). This technique can be applied to both equipment and facilities and can be used to methodically break down the analysis of a complex process into a series of manageable steps. It is a powerful tool for summarizing the important modes of failure, the factors that may cause these failures, and their likely effects. It also incorporates the degree of severity of the consequences, their respective probabilities of occurrence, and their detectability. It must be stressed, however, that the outcome of the risk assessment process should be independent of the tool used and must be able to address all of the risks associated with the instrument that is being assessed. 

We successfully applied an AChE inhibition assay to the detection of dichlorvos in durum wheat samples using a simplified extraction procedure. The total assay time, including the extraction step, was 30 min. Considering that several extractions and assay steps can be run simultaneously, the throughput for one operator is 12 determinations per hour. It is also important to stress that the choline oxidase biosensor used in this work showed an excellent functioning stability after 20 days from preparation, the blank measurement lost only 10% of the signal intensity. The method allowed the accurate analysis of dichlorvos in wheat samples at the MRL, 2 mg/kg, and below that value. The mean recovery was 75%, and neither false nor positive samples were detected. Finally, the portable electrochemical instrumentation combined with the simple extraction procedure was quite well suited for in situ analysis of dichlorvos in durum wheat. 

Clarifying some oonoepts of US-based detection techniques may help increase the appeal of the applioations discussed below. Thus, these techniques require no cavitation as the power levels of US-based instruments are up to millions of times iowerthan those of US baths and probes. For example, US velocity measurements are usuaiiy made at very low power levels, so the analysed material is normally left intact. In addition, the use of low US power levels (e.g. below ca. 10 kW/m in water at room temperature) results in elastic displacements ( .e. strain and stress are linearly related). 

The application of ultraviolet and visible spectroscopy to the identification and measurement of carbenium ions derived from aromatic and dienic monomer has already been discussed (see Sect. II-G-2). The use of this technique to monitor stable carbenium salts is also well known. We have finally stressed in a preceding section that the fate of certain anions could be followed spectrophotometrically during a cationic polymerisation. The limits of detection allowed by the values of the extinction coefficients of all these species and by the sensitivity of present-day instruments is 10 to 10 M. 

In thermomechanical analysis (TMA) the deformation of the sample under stress is monitored against time or temperature while the temperature increases or decreases proportionally to time. Changes are detected by mechanical, optical, or electrical transducers. The stress may be a compression, penetration, tension, flexure, or torsion. Generally the instruments are also able to measure the sample dimensions, a technique called thermodilatometry. The stress F/A) expressed in N/m or Pa may be a normal tensile stress cr, a tangential shearing stress x, or a pressure change Ap the force applied is F and A is the area. 

The Du Pont Model 943 TMA module is shown in Figure 11.4. The apparatus uses a LVDT to sense linear displacements of the sample probe. A thermocouple in direct contact with or in close proximity of the sample is used to detect the sample temperature. The sample and probe are surrounded by a temperature-controlled cylindrical heater and Dewar assembly. Various probe configurations allow the apparatus to be used in the expansion, compression, penetration, tension, stress relaxation, parallel plate rheometry, and fiber tension. The temperature range of the instrument is — 180-800°C an optional furnace can be used to extend the range to 1200 C. 

---