Instrumentation error detection elements

Figure 9-17 is a schematic diagram of a typical closed control loop. If the error detection is done by an experienced human operator, then the process is controlled manually. Manual control is used only where the required instrumentation either is not available or is too expensive from either a first cost or maintenance standpoint. Mechanical and electrical error detecting elements or controllerB are almost always used in closed-loop control to give a fully automatic process control system. 

X-ray fluorescence spectrometry is based upon the excitation of a sample by X-rays. A primary X-ray beam excites secondary X-rays (X-ray fluorescence) which have wavelengths characteristic of the elements present in the sample. The intensity of the secondary X-rays is used to determine the concentrations of the elements present by reference to calibration standards, with appropriate corrections being made for instrumental errors and the effects the composition of the sample has on its X-ray emission intensities. Alternatively, the X-rays may be detected without... 

Analytical Procedure. The cold-trap gas phase mercury detection system was designed and used for both laboratory and shipboard measurements of mercury in seawater. The Coleman Instruments mercury analyzer (MAS-50) was incorporated into the analytical system because of its portable and convenient design. However, the effective use of this simple one-element atomic absorption unit requires scrupulous attention to blank determinations for each seawater sample. For example, the undetected presence of either naturally occurring or sampling induced volatile organics which may absorb at the mercury wavelength in the seawater sample can be a serious error. Such artifacts were observed when acidifled seawater samples were stored in low density polyethylene bottles (21), Therefore, the analytical procedure used to determine the mercury concentration in a seawater sample consists of the following steps ... 

Table 3.4 compares detection limits with secondary fluorescers to the results with the RMF method and 15-kV broadband excitation [16,17]. Four different fluorescence analyzers were tested (units A, B, C, and D), and the results were corrected for differences in performance for the energy-dispersive spectrometers employed on each unit. Unit A used a chromium anode tube, and unit B used a tungsten anode tube. Unit A was a commercial, general-purpose instrument. Unit B was specifically designed for atmospheric aerosol analysis, where closer coupling between the tube, fluorescer, sample, and detector could be employed with some sacrifice of insensitivity to specimen-positioning errors. Table 3.5 lists the x-ray tube operating conditions required for Table 3.4. For medium- to high-atomic-number elements, the secondary fluorescer method provides detection limits equivalent to the RMF element, but requires much higher x-ray tube power. For light elements.

The Idaho Chemical Processing Plant (ICPP) soluble poison meter was developed as an instrument to monitor continuously the amount of nuclear poison entering non-geometrically safe process vessels for dissolution of spent nuclear fuel elements. The nuclear poison (gadolinium or boron) is added to the dissolver with the dissolving acid. The meter detects the poison concentration with an accuracy of il%, including errors resulting from calibration, analytical techniques, drift, and statistical... 

Nowadays, atomic absorption spectrometry (AAS) systems are comparatively inexpensive element selective detectors, and some of the instruments also show multi(few)-element capability. There are flame (F AAS), cold vapor (CV AAS), hydride-generating (HG AAS), and graphite furnace (GF-AAS) systems. However, the use of AAS-based detectors for on-line speciation analysis is problematic. F AAS is usually not sensitive enough for speciation analysis at "normal" environmental or physiological concentrations and sample intake is high (4—5 ml/min), which complicates on-line hyphenations with LC an auxiliary flow is necessary. CV AAS and HG AAS use selective derivatization for matrix separation and increased sensitivity for the derivatized species. But, the detector response is species dependent and interferences can be a problem. GF AAS requires only a few microliters of sample and provides low detection limits, between 0.1 and 5 gg/1. Matrix interferences are widely eliminated by Zeeman correction and matrix modifiers nevertheless, quantification errors were reported as atomization temperature does not exceed 2900°C. The most critical problem in respect to speciation analysis is the discontinuous measiuement because of the temperature program operation employed, which takes a few minutes. Therefore, GF AAS is unsuitable for on-line hyphenations as chromatograms carmot be monitored with sufficient resolution. 

There are many situations in which the X-ray absorption spectrum is most easily measured (indirectly) by monitoring the fluorescence produced following absorption of X-rays. One measures variations in the fluorescence intensity of a particular atomic species as the energy of incident photons is varied over an absorption edge of a selected element. This fluorescence excitation spectrum , distinct from the fluorescence spectrum, provides an indirect measurement of the X-ray absorption coefficient, albeit one that is subject to several well-known instrumental effects. If care is taken in sample preparation, systematic errors can be minimized. Fluorescence detection is the method of choice for dilute systems, because it provides an improved signal-to-noise ratio. 

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