New reference material for analysis of elements in glutinous rice produced at the National Institute of Metrology (Thailand)

Reference materials play an important role for evaluating the accuracy of analytical results, and are essential parts of good laboratory practice. They represent a key tool for quality control of chemical analyses. In Thailand, the demand of food and environmental reference materials is constantly increasing, and the National Institute of Metrology (NIMT, Thailand) is responding to the urgent needs for affordable materials, which require collaborative efforts at the national level. This paper describes the preparation of a new glutinous rice reference material, along with homogeneity and stability studies and the analytical work carried out for the certification of the contents of inorganic elements. The incurred material was collected from an actual rice paddy field. Material preparation along with homogeneity and stability testing were carried out at the Environmental Research Training Centre (ERTC). The homogeneity study was designed to have three experimental conditions; (A) 10 bottles of candidate materials being analyzed each with 2 replicates, (B) 20 bottles with 2 replicates, (C) 10 bottles with 7 replicates, in order to study the suitable treatments for homogeneity testing in the reference material production. It was shown that a minimum number of 10 bottles with duplicate analyses are enough to demonstrate the homogeneity of candidate reference material. Certification of a candidate reference material in a single laboratory using reference method was confirmed with an interlaboratory comparison participated by a certain number of well recognized testing laboratories in Thailand. Further elaborative results will be discussed.     

On the temperature dependence of the dielectric membrane properties of human red blood cells

Electrorotation (ER) spectra of human red blood cells (HRBCs) have been recorded in the frequency range from 10 kHz to 250 MHz in a 4-electrode microchip chamber. The cells were suspended at conductivities in the range from 0.02 to 3.00 S/m (corresponding to an ionic strength range from 1.6 to 343 mM) at temperatures between 10 degrees C and 35 degrees C. Generally, the characteristic frequencies as well as the rotation speeds of the first (membrane-dispersion) and second ER peaks increased with temperature. The rotation speed increase was largely correlated to the temperature dependence of the medium viscosity. Standard temperature dependencies were assumed for the conductivities and permittivities of cytoplasm, membrane, and external solution to explain the frequency shifts, starting from the cell parameters of Gimsa et al. [Gimsa et al., 1996, Biophys. J. 71: 495-506.]. The membrane capacitance was assumed to be temperature independent, based on the permittivity of alkyl-chains. Under these assumptions, the spectra could be well fitted only in a narrow temperature range around 20 degrees C. The temperature dependence of the first characteristic frequency was much stronger than predicted. In addition, around 15 degrees C, an anomalously high rotation speed was observed for the first peak at low external conductivities. Interestingly, this finding corresponds to the change in the chloride transport rate described by Brahm [Brahm, 1977, J. Gen. Physiol. 70: 283-306.].