Controlled reference method

The basic technique is only applicable to dilute suspensions where multiple scattering does not occur and this technique is sometimes referred to as through dynamic light scattering. The introduction of the controlled reference method has extended it to more concentrated systems [275]. 

Lecotrac LTU-150 operates in the 0.003 to 6.5 pm size range using the controlled reference method. 

Leeds and Northrup Microtrac Ultrafme Particle Analyzer (VPA) uses the controlled reference method, using a sapphire tipped waveguide that collects back-scattered light within 100 pm of probe tip, to cover the size range 0.003 to 6.5 pm. 

P. E. Plantz. Ultrafine particle size measurement in the range 0.003 to 6.5 micrometers using the controlled reference method. In T. Provder, ed.. Particle Size Distribution III Assessment and Characterization, ACS Symp. Ser. 693, 1998, pp. 103-129. 

New Techniques in Sub-micron Particle Size Analysis The Controlled Reference Method... 

Because the controlled reference method utilizes a constant reflected beam it has two major advantages over other methods using self beating. 

Plantz, P. E., Ultrafine Particle Size Measurement in the Range 0.003 to 6.5 micrometers Using the Controlled Reference Method, in Particle Size Distribution III, Ed. Provder, T., ACS Symp. Series 693, American Chemical Society, Washington D.C., 1998, Chpt.9, pp. 103-129. 

Sampling of Particulates. Ambient air suspended particulate concentration was traditionaUy measured gravimetricaUy over a 24-h period with a "Hi-Vol" sampler. However, in 1987 the EPA changed ambient particulate control to the PM q reference method (24). In the PM q method, a... 

The primary reference method used for measuring carbon monoxide in the United States is based on nondispersive infrared (NDIR) photometry (1, 2). The principle involved is the preferential absorption of infrared radiation by carbon monoxide. Figure 14-1 is a schematic representation of an NDIR analyzer. The analyzer has a hot filament source of infrared radiation, a chopper, a sample cell, reference cell, and a detector. The reference cell is filled with a non-infrared-absorbing gas, and the sample cell is continuously flushed with ambient air containing an unknown amount of CO. The detector cell is divided into two compartments by a flexible membrane, with each compartment filled with CO. Movement of the membrane causes a change in electrical capacitance in a control circuit whose signal is processed and fed to a recorder. 

Ihnat M (1998a) Reference materials for data quality control. In Kalra YP, ed. Handbook of Reference Methods for Plant Analysis, pp 209-220. CRC Press, Boca Raton. 

Applications Specific applications of the direct spectrometric analysis methods of solid samples of Table 8.36 are given under the specific headings. One investigation that is practically only possible by direct solids analysis is checking the homogeneity of polymers [136,137] this is of significance for reference materials and for quality control. A method for the assessment of microhomogeneity should meet various requirements [223] ... 

Control limit, n - for validation tests, the maximum difference allowed between a valid analytical result, and a reference method result for the same sample. A measured value that exceeds a control limit requires that action be taken to correct the process. Control limits are statistically determined. 

Validated result, n - a result produced by the spectroscopic (or instmmental) method that is equivalent, within control limits, to the result expected from the reference method so that the result can be used in lieu of the direct measurement of the sample by the reference method. 

Software sensors and related methods - This last group is considered because of the complexity of wastewater composition and of treatment process control. As all relevant parameters are not directly measurable, as will be seen hereafter, the use of more or less complex mathematical models for the calculation (estimation) of some of them is sometimes proposed. Software sensing is thus based on methods that allow calculation of the value of a parameter from the measurement of one or more other parameters, the measurement principle of which is completely different from an existing standard/reference method, or has no direct relation. Statistical correlative methods can also be considered in this group. Some examples will be presented in the following section. 

Ideally both the control materials and those used to create the calibration should be traceable to appropriate certified reference materials or a recognised empirical reference method. When this is not possible, control materials should be traceable at least to a material of guaranteed purity or other well characterised material. However, the two paths of traceability must not become coincident at too late a stage in the analytical process. For instance, if control materials and calibration standards were prepared from a single stock solution of analyte, IQC would not detect any inaccuracy stemming from the incorrect preparation of the stock solution. 

Quality control refers to an internal scheme that will give a warning when unforeseen factors cause a reduction in the analytical performance of the method. This allows an immediate decision to be made on whether the test results are acceptable or must be rejected. This is usually done by the analysis of a control sample with each batch of tests. 

Kansiz et al. has published a paper wherein they used MIR and sequential injections to monitor an acetone-butanol fermentation process.17 In this work, acetone, acetate, n-butanol, butyrate, and glucose were analyzed automatically, using computer-controlled sampling techniques. In this case, gas chromatography was the reference method. The SEPs for the components were acetone, 0.077 acetate, 0.063 butyrate, 0.058 -butanol, 0.301 and glucose, 0.493 g/1. The authors state that the precision and accuracy of the MIR methods were as good as the reference method. 

Near infrared spectroscopy (NIRS), a technique based on absorption and reflectance of monochromatographic radiation by samples over a wavelength range of 400-2500 run, has been successfully applied for food composition analysis, for food quality assessment, and in pharmaceutical production control. NIRS can be used to differentiate various samples via pattern recognitions. The technique is fast and nondestructive method that does not require sample preparation and is very simple to use compared too many other analytical methods such as HPLC. The drawback of NIRS, however, is that the instrument has to be calibrated using a set of samples typically 20-50 with known analyte concentrations obtained by suitable reference methods such as FIPLC in order to be used for quantitative analyses. Simultaneous quantification of the... 

Several method performance indicators are tracked, monitored, and recorded, including the date of analysis, identification of equipment, identification of the analyst, number and type of samples analyzed, the system precision, the critical resolution or tailing factor, the recovery at the reporting threshold level, the recovery of a second reference weighing, the recovery for the control references (repeated reference injections for evaluation of system drift), the separation quality, blank issues, out of spec issues, carry over issues, and other nonconformances. The quantitative indicators are additionally visualized by plotting on control charts (Figure 23). 

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