Powder mixtures internal standard method

Rao et al. reported a high performance liquid chromatographic method to determine diloxanide furoate and metronidazole in single and in combined dosage forms [41]. A 30 mg equivalent of diloxanide furoate and 25 mg of metronidazole (either as the bulk drug substances or in powdered tablets) was dissolved in methanol, amidopyrine added as the internal standard, and the mixture analyzed by HPLC at room temperature. The analytical column (30 cm x 3.9 mm) consisted of p-Bondapak Cig, with 9 9 1 1 methanol water 0.05 M KH2PO4 0.05 M NaH2P04 as the mobile phase. The flow rate was 1 mL/min), and detection was performed at 254 nm. 

Quantification of the relative abundance of crystalline phases in a multiphase mixture is an everyday problem in a wide range of applications. Common examples are evaluation of the yield in inorganic synthesis and catalytic processes, characterization of raw mineral materials for industrial processes, quality check of fired ceramic products, and many more. While in most cases the required accuracy level of the analysis is a few percent at best, in particular cases such as in the quantification of phase contaminants in technologically important materials, or of hazardous and toxic phases in environmentally dispersed aerosols, the required level of accuracy must be substantially lower than 1 wt% relative abundance. Accuracy levels of 2-3 wt% are commonly reached if standard procedures of quantitative phase analysis by diffraction data are properly performed. Generally employed analytical methods include the internal or external standard method, the matrix flushing method, and the reference intensity ratio method. Very recently, the availability of analysis techniques of powder diffraction data based on full-profile (Rietveld method), originally developed... 

The absorption effect has already been referred to in Section II. Clearly, in a multiphase mixture, different phases will absorb the diffracted photons by different amounts. As an example, the mass absorption coefficient for CuKa, radiation is 308 cm/g for iron, but only 61 cm/g for silicon. Thus iron atoms are five times more efficient than silicon atoms in absorbing CuKo, photons. There is a variety of standard procedures for correcting for the absorption problem, of which by far the most common is the use of the internal standard. In this method a standard phase is chosen that has about the same mass absorption coefficient as the analyte phase, and a weighed amount of this material is added to the unknown sample. The relative intensities of lines from the analyte phase and the internal standard phase are then used to estimate the relative concentrations of internal standard and analyte phases. The relative sensitivity of the diffractometer for these two phases is determined by a separate experiment. Other procedures are available for the analysis of complex mixtures, but these are beyond the scope of this particular work. For further information the reader is referred to specific fexts dealing with the X-ray powder method. 

The direct determination of trace elements (Al, Ba, Cu, I, Mn, Mo, Pb, Rb, Se, Sr, and Zn) by ICP-MS in powdered milk was reported [14]. Samples were diluted with a 5 or 10 percent (v/v) water-soluble, mixed tertiary amine reagent at pH 8. This reagent mixture dissociated casein micelles and stabilized liquid phase cations. Mass intensity losses were not observed. The quantitative ICP-MS procedure was applied the standard additions method with a Y internal reference. This direct technique is as fast as the slurry approach without particle size effects or sensitivity losses. 

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