Passaging technique, reference cell

Caco-2 cells are heterogeneous cell lines. Cultures from different laboratories may generate different Papp values. These differences may be partially due to different techniques, media, or passages of the cells used in different laboratories. Thus, individual laboratories should routinely calibrate their results with a set of reference compounds (Food and Drug Administration, 2000). 

The differential technique described under (a) has an advantage in removal of the liquid-junction potential and of mechanical faults often encountered in work with reference electrodes of the second kind. The procedure described under (b) suppresses the potential fluctuations, but difficulties can arise from the very high resistance of a cell containing two ISEs. A differential amplifier was designed for this prupose [15]. The two ISEs used can also exhibit different slopes electrode membranes were therefore prepared by cutting a single crystal into two halves, where each half contains a chaimel for passage of the solution and functions as an ISE [163]. 

Fundamentals. A beam of monochromatic light is polarized and split into two beams. One beam passes an optical medium, e.g. the electrolyte solution in an electrochemical cell close to the solution/electrode interface. After passage it interferes with the other beam for a review of two-beam interferometry see [915, 916]. This technique was limited both with respect to spatial resolution and sensitivity for further details, see [917]. Temporal or spatial modulation of the reference beam enabled the use of very thin cells (i.e. the sensitivity was increased) and the two-dimensional reconstruction of concentration maps. 

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