Semi-permanent cells

Sealed and semi-permanent cells are most common, but variable pathlength cells are also available. A simple version of a liquid sample cell is the demountable Nujol cell which is suitable for qualitative examination of pastes or viscous liquids. The use of spacers in these devices may or may not be necessary, depending on the viscosity of the sample under investigation. NaCl, KBr, Csl, and polyethylene window materials are convenient if the sample does not contain water. A drop of liquid or paste is placed on one of the windows, and the two windows are clamped with front and back metal plates. Semi-permanent cells are suitable for quantitative work and can be cleaned easily after use (Fig. 5.2.1). 

Fig. 5.2.1. Liquid sampling in the transmission mode. Construction of a semi-permanent liquid cell...

Figure 2.9 Schanatic of a typical semi-permanent liquid cell. From Stuart, B., Biological Applications of Infrared Spectroscopy, ACOL Series, Wiley, Chichesta, UK, 1997. University of Greenwich, and reproduced by permission of the Univasity of Greenwich.

The ATM cell has two parts the header and the payload. The header has 5 bytes and the payload has 48 bytes. As mentioned earher, the major function of the ATM layer is to provide fast multiplexing and routing for data transfer based on information included in the 5-byte header. This 5-byte header includes information not only for routing (i.e., VPI and VCI fields), but also fields used to (1) indicate the type of information contained in the cell payload (e.g., user data or network operations information), (2) assist in controUing the flow of traffic at the UNI, (3) establish priority for the cell, and (4) facilitate header error control and cell delineation functions. One key feature of this ATM layer is that ATM cells can be independently labeled and transmitted on demand. This allows facility bandwidth to be allocated as needed, without fixed hierarchical channel rates required for STM networks. Since ATM cells may be sent either periodically or randomly (i.e., in bursts), both constant and variable bit-rate services are supported at a broad range of bit rates. The connections supported at the VP layer are either permanent or semi- atm cell ... 

A particularly useful system for studies of cell differentiation was found to be murine erythroleukemia cells (Marks and Rifkind, 1978). These proerythroblasts, transformed by the Friend virus complex, can be permanently grown in a cell culture. Exposure to dimethylsulphoxide or a variety of other inducers stimulates MEL cells to make a transition to the more advanced stages of erythroid maturation. An analysis of the differentiated state of individual cells, based on cloning cells in a semi-solid medium, has revealed (Housman et al., 1978) that an irreversible commitment to differentiation occurs as a discrete event at a given time after the application of the inducer. Removal of the inducer prior to this event returns a cell to its initial state, whereas its removal after this event leaves it in the differentiated state. 

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