The controlled reference method

For a single particle size the power function takes the form of a Lorenzian function. The (Uq term depends inversely on size so the power spectrum plots for different sizes show a shift to higher frequencies as the particle size decreases. In terms of the Brownian motion, smaller particles move more rapidly than large ones. An assembly of particles will have a 

T] = viscosity A = wavelength in fluid T = absolute temperature and a = particle radius. 

One advantage of this system over conventional PCS is that since the light is reflected back rather than transmitted through the suspension, higher concentrations can be monitored. 

Measurement of bimodality for mixtures of sizes ranging from less than 0.1 p.m to sizes greater than 0.1 p m is difficult because of the rapid decrease in scattering efficiency as the size decreases. Broad distributions can be measured accurately. 

A laboratory made fiber optic dynamic light scattering instrument has been described together with a description of its use to study the kinetics of aging processes in emulsions [290]. The method is particularly useful for this purpose since it permits measurements in concentrated emulsions. 

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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. 

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. 

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). 

Use of hormonal-type substances for growth promotion and fattening purposes in food-producing animals is prohibited or restricted in many countries. Administration of diethylstilbestrol, in particular, has been totally banned worldwide. This implies that adequate analytical methods should be available for regulatory control. Due to its sensitivity, radioimmunoassay has have become most important in diethylstilbestrol analysis and represents the final detection step in the EU reference method for stilbene residues analysis (100). 

Sampling of Particulates. Ambient air suspended particulate concentration was traditionally measured gravimetiically over a 24-h period with a "Hi-Vol" sampler. However, in 1987 the EPA changed ambient particulate control to the PAL reference method (24). In the PA110 method, a... 

On the other hand, the electrode potential is kept constant against a reference electrode in the controlled potential method. The fact that potential is constant throughout the reaction often leads to better tegio- and chemo-selectivity than in die constant current method. 

Much less data are available on the adsorption of elements onto surfaces from blood or urine samples. Stoeppler (1980) did not detect any loss of added Ni from urine samples onto polyethene container walls. Concentrations of nickel or chromium in urine samples, spiked with small concentrations of the metals and stored for 6 months at 4°C did not show a decrease (Kiilunen et al.. 1987). The lUPAC reference method for nickel in urine calls for acidification of urine quality control samples with nitric acid and storage in polypropene tubes with a screw-cap at -20°C (Brown et al., 1981). No adsorption of cadmium onto container walls (type not specified) was seen from urine acidified to pH < 2 (Stoeppler and Brandt, 1980). 

Recalibration is necessary whenever (a) the instrument changes, (b) samples are believed to be part of the original calibration population but on analysis exhibit many large H values, or (c) during monitoring of NIRS analyses using the primary reference method, the test results continue to fall outside the control limits. 

One of the main requirements for implementation of a new analytical method to a routine analysis is the reduction of cost of one determination in relation to the actual/ reference method. In the studies described above, the replacement of an expensive enzyme by a commercially available material in a flow injection system resulted in a cheaper and faster strategy that could be used in routine analytical laboratories to provide quality control and monitoring of LA in food and beverages. 

These specifications and characteristics are defined with references to standard test methods which the different parties to a contract should conduct for quality control. The tables that follow show specifically the standards that are applicable in France, but a more general table in Appendix 2 shows the main test methods commonly referenced in specifications. 

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