Salt windows

For microbore HPLC, with a flow of less than lOOpLmin-1, off-line LC-FT1R has been developed using matrix isolation techniques. The solutes are deposited on a moving IR salt window [504] or on a rotating plated disc [486], and are measured afterwards with the aid of a FITR microscope or a reflectance accessory. FTIR detection was first applied to the analysis of microbore HPLC eluent by Teramae and Tanaka [505]. In microbore HPLC-FTIR the amount of mobile phase required for separation is much less than for conventional scale HPLC. This simplifies both flow-cell and mobile-phase elimination interfaces. Flow-cell... 

The fact that soil always contains water, or more precisely an aqueous solution, is extremely important to keep in mind when carrying out an analytical procedure because water can adversely affect analytical procedures and instrumentation. This can result in an over- or under-determination of the concentrations of components of interest. Deactivation of chromatographic adsorbents and columns and the destruction of sampling tools such as salt windows used in infrared spectroscopy are examples of the potential deleterious effects of water. This can also result in absorbance or overlap of essential analytical bands in various regions of the spectrum. 

The cavity assembly is mounted inside a large box filled with 3 atm of helium gas (at room temperature) and the laser beam, split into two parts of about 2 J each (because of the breakdown threshold of the helium gas), enters the box through the Nal-salt windows shown on the left side of the figure. Each beam stays in the cavity for 32 reflections and then exits the outcoming radiation beams are then used for monitoring and controlling purposes. 

Application of IR-SEC to biomolecules brings additional challenges and these are mostly related to the availability of material, its maximum concentration, interference from other vibrational modes of the macromolecule, and the need to work in highly absorbing solvents such as water. The water solubility of most salt window materials means that studies must be conducted using insoluble... 

A new approach to collecting transmission spectra of solids is the use of a diamond anvil cell. Diamond is transparent through most of the mid-IR region, with the exception of a broad absorption around 2000 cm . A solid sample is pressed between two small parallel diamond anvils or windows to create a thin film of sample. A beam condenser is required because of the small cell size. Very high pressures can be used to compress solid samples because diamonds are very hard materials. As a result, the diamond anvil cell permits transmission IR spectra to be collected of thin films of very hard materials. Hard materials cannot be compressed between salt windows because the salt crystals are brittle and crack easily. 

Mapes and I did not have an in situ sample cell. Our sample was placed in a tube to which salt windows were attached with sealing wax after the sample had been dried and ammonia chemisorbed. Despite this primitive technique, with its danger of converting Lewis sites to Bronsted sites by inadvertent exposure to water vapor, the infrared spectrum indicated that most of the ammonia was in the Lewis configuration (1). 

On depositing simultaneously OF2 diluted in Ar and a beam of Li, Na, K, or Mg atoms onto a salt window at 15 K, absorption bands due to the appropriate metal fluorides and weakly bonded MF --OF dimers were observed. In the case of Li the formation of LiOF is also assumed [3]. 

The following are words of caution about the preparation of evaporated films from solvent-based solutions If KBr or NaCl windows are used, ensure that any solvents or solutions used are water free. When the film is cast, allow the solvent to evaporate slowly (to prevent moisture condensation) and finish evaporation under a heat lamp to remove any residual solvent. If a solid is cast from solution, make sure that crystallization has not occurred. Note that for some crystalline solids a preferred orientation can occur (such as along the crystal lattice lines of the salt window), causing intensity variations as a function of position of the evaporated film. Generally, in the preparation of any film—cast, smear, or capillary—ensure that the film is regular and free from voids, air bubbles, or pinholes. Failure to do this will result in photometric errors in the recording of the spectrum. 

When the operator is satisfied that the mull has been properly prepared, it is transferred to a salt window with either a microspatula or a rubber policeman (the former is more likely to scratch the window while the latter is more likely to contaminate the mull), and spread in a narrow line down the center of the long axis of the window. A second window is placed on top of the mull, and the two windows are slowly rotated in opposite directions under finger pressure to spread the mull in a thin, even layer between the two windows. The window-mull sandwich is then placed in a demountable cell which is mounted on the spectrophotometer, and the sample is ready to scan. 

Pressed Samples. Some samples are quite soft and can be pressed into a thin film between salt windows. This technique is especially useful for semisolids. Other materials can be pressed into thin films in a press. In this case, the sample is a free film and will show an interference-fringe pattern. 

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