Sample compartment purging

Figure 20-29 Fourier transform infrared spectrum of polystyrene film. The Fourier transform of the background interferogram gives a spectrum determined by the source intensity, beamsplitter efficiency, detector response, and absorption by traces of H20 and C02 in the atmosphere. The sample compartment is purged with dry N2 to reduce the levels of H20 and C02. The transform of the sample interferogram is a measure of all the instrumental factors, plus absorption by the sample. The transmission spectrum is obtained by dividing the sample transform by the background transform.

Self-supporting disks were used for the IR measurements. Before pressing the catalyst into disks, the catalyst was first reduced in flowing H2 for 3 hours at 400°C. The disks were placed in an IR cell, that could be evacuated to 10 mbar. The IR cell was placed in a sample compartment of a Fourier infrared spectrometer. (Galaxy 2020 from Mattson) This compartment, however, was not purged with an inert gas. As a result, no iirformation about the possible formation of products like CO2 could be extracted. Thirty-two scans were taken with a resolution of 4 cm for one spectrum. The catalysts were all reduced or oxidised in-situ, with 100 mbar H2 or O2, followed by an evacuation at lO" mbar for 30 min. Background spectra were taken at several temperatures at 10 mbar. Sample spectra were taken at several temperatures after the addition of 5 mbar CO or NO or a mixture of 10 mbar CO and NO. 

The films of polymer blends used for the measurements of FT-IR were prepared by casting the polymer solution on the surface of a silicon wafer and dried under vacuum condition for 2 days. The film used in this study was thin enough to obey the Lambert-Beer law (<0.6 absorbance units). FT-IR spectra were recorded on a Perkin-Elmer Spectrum 2000 spectrometer using a minimum of 64 co-added scans at a resolution of 4cm-i. Nitrogen was used to purge CO2 and gaseous water in the detector and sample compartments prior to and during the scans. 

Sample preparation was greatly simplified in the late 1990s by the introduction of commercial diamond attenuated total internal reflection (ATR) systems. These use a small diamond as a single-pass ATR element held in a metal plate. The sample is placed on the diamond, a plate is then brought down onto it and clamped in position. This ensures excellent contact between the ATR element and the sample, so immediately eliminates the problem of poor contact that made ATR so problematical for use with solids, particularly powders. The accessory can be permanently installed in the purged sample compartment of the spectrometer, so there is only a very short pathlength that is in the open air. This means that there is no need to wait for the sample compartment to purge to reduce the atmospheric water and carbon dioxide absorption. The result is that the sample can be placed in the accessory and scanned immediately. 

TUm on the instrument at least half an hofur before making measurements to allow the Dj lamp to warm-up to minimize instrument drift Purge the sample compartment with nitrogen at least 20 min before a melting curve is recorded. Place buffer in one cuvette and zero the absorbance reading. 

Instrumentation. IR spectra of monolayers were determined with a Perkin-Elmer 1725 X instrument fitted with an MCT detector and a grazing angle accessory (Spectra-Tech Inc.). Freshly cleaned (cone. HNO3, 10 min) bare gold slides were used as background. The sample compartment of the spectrometer was purged with nitrogen. 

Figure 6.1 Dependence of water vapour absorption features in single channel mid-IR spectra with (left panel) and without (right panel) purging of the microscope sample compartment by dry air. Spectra were taken in transmission mode from real-world samples (cryo-sections of colon tissue mounted on Cap2 windows). Note the sharp and intense water vapour absorption bands in the spectral regions 1350-1950 and 3600-3900cm , with nearly total absorption between 1500 and 1800cm (no purge box,...

Consequently, the reduction of water vapour by purging and/or the use of desiccants along the complete optical path of IR light within IR instruments, including the sample compartment, and a stable concentration of carbon dioxide are considered to be essential prerequisites for the successful computational removal of water vapour residues in the IR spectra. It is thus recommended that adequate drying and/or purging of the instrumentation is ensured and that software algorithms for water vapour and carbon dioxide correction are applied for reproducible spectra acquisition." ... 

Water vapor in the beam of FT-IR spectrometers is a major source of interference. Thus, the optical bench should ideally either be purged with dry air or evacuated completely. Some low-resolution spectrometers are hermetically sealed and desiccated, so that the only humid air is in the sample compartment. These instruments are adequate if the spectra of conventional films, KBr disks, and solutions... 

One distinct advantage of a horizontal ATR accessory is that the accessory can be mounted in the sample compartment so that the entire beam path is always purged. The accessory can be sealed to the walls of the sample compartment so that the atmosphere inside the ATR accessory is identical to that inside the spectrometer. If the spectrometer is simply desiccated rather than purged continuously, at least the concentration of water vapor and CO2 in the atmosphere will remain relatively constant, and interference due to these species will be minimal (Figure 15.8). 

As the absorption bands caused by water vapor (its rotational spectrum) are intense throughout the far-infrared region, it is important to purge efficiently the inside of the spectrometer with dried air or nitrogen. Some commercial spectrometers can be evacuated, but usually their sample compartment needs to be purged with dried air or nitrogen. This is commonly needed also in terahertz time-domain spectrometry, which is described in the following section. 

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