Evaluación de las condiciones de calidad de los programas. La institución de educación superior debe presentar información que permita verificar:

The importance of characterising VOC oxidation mechanisms has already been highlighted in Chapter 1. Environmental chambers provide a suitable environment for the study of these processes, and the availability of a Fourier transform Infra-Red (FTIR) absorption spectrometer is one of the most essential detection systems for chamber studies. IR absorption is an almost universal detection method with high selectivity.

FTIR is based on measuring absorption of IR radiation by a sample. An interferometer enables the measurement of all IR frequencies simultaneously. The only requirement for a molecule to be detectable is that it must have a vibrational mode that changes its dipole moment. Contrary to the HIRAC GC-FID, the HIRAC FTIR system is non-destructive since the sample remains intact during measurements.

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An advantage of the FTIR system over GC-FID is the measurements are done in-situ compared to the direct sampling of GC-FID, which is beneficial as it does not dilute the chamber and also reduces the uncertainties associated with sampling and gas samples making it through a sample loop. Being such a versatile tool for analytical purposes, it is a useful quantitative and continuous technique that makes it a universal detection method for most trace gases in the atmosphere.

A Bruker IFS/66 FTIR spectrometer (CaF2 beam splitter, Si diode detector, measurement range 4,000 – 400 cm^-1), is coupled to HIRAC and the multipass optical arrangement on the interior of the chamber via a series of transfer optics housed in a sealed purged box. This multipass optics system was developed by Glowacki and Goddard (Glowacki et al., 2007b), and a diagram of the arrangement is shown in Figure 2.20.

Figure 2.20: Modified multipass Chernin cell optics in HIRAC (Glowacki et al., 2007b).

This optical system is known as a modified multipass Chernin type cell, and is a modified multipass matrix system that features three objective mirrors with input and output apertures placed on opposite sides of the small field mirror(Chernin, 2001).

The latter publications give a description of the modified multipass system which differs from the original system in that it separates input and output beams to opposite sides of the field mirror, thus making it a more convenient arrangement. Figure 2.20 shows a schematic of the 72 pass arrangement for this modified multipass Chernin type cell optics developed by Glowacki and Goddard (Glowacki et al., 2007b). The above figure also shows the image pattern on the field mirror and points the centre of

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curvature for each objective mirror (F1 located at midpoint of O1 and O2, while F2 located at midpoint of centres of O1 and O3). Some temperature effects due to expansion of mirrors may lead to loss in alignment of the FTIR optical system; this can be minimised by selecting mirrors with materials that have low thermal expansion coefficients.

Under normal operation, IR beams pass in and out of HIRAC via two 6 mrad wedged potassium bromide (KBr) windows, in order to avoid etalon effects (Glowacki et al., 2007b). The transfer optics are housed in a sealed purged Perspex box flushed with nitrogen to avoid absorption by laboratory air. Alignment of the FTIR system is done using a near-infrared (NIR) output from the Bruker FTIR spectrometer, which is visible on the mirrors surfaces and field mirror spots may be easily counted this way. The colinearity of the NIR and MIR beams ensured the aligned NIR beam almost exactly positioned as the aligned MIR beam (Glowacki et al., 2007b). A sample volume or approximately 0.083 m³ of HIRAC is illuminated by this FTIR optical system, corresponding to around 3.6% of the volume of the chamber.

The importance of the mirror systems to be robust and able to handle ranges of temperatures and pressures is an important consideration that needed to be taken into account during design of the HIRAC FTIR system. This would ensure the system developed would maintain alignment through pressure and temperature variations.

The HIRAC mixing fans and pumping systems described previously result in vibration that could also have an effect on the FTIR alignment. The mirrors used in the HIRAC FTIR system are made of zerodur glass which has a very small thermal expansion coefficient.

The sensitivity of the FTIR instrument is related to absorbance, (Io/I), via the Beer-Lambert law:

A = ln (^Io_I) = σ c l (Eq 2.9)

Where A is the absorbance, Io and I are the initial and detected intensities respectively, σ the absorption cross-section, c the concentration and l the path length.

According to Eq 2.9, a long path length clearly enhances the sensitivity of the system and this is achievable in HIRAC with a multipass system by-passing the chamber 72 times. This means that long path lengths (up to 140 m) achieved in the HIRAC-FTIR

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increases the sensitivity of the instrument, explaining why a multipass system is used in HIRAC. The robustness of the system developed in HIRAC also influences the capabilities of this system to withstand various conditions of temperature and pressure, and all the time and effort put into the design of this system has ensured it can withstand such conditions. HIRAC is mounted on a stainless steel frame that rests on neoprene and cork pads to damp vibrations that otherwise affect the performance of the optical system (Glowacki et al., 2007a).

The FTIR has been a useful technique in the work reported in this work. The unavailability of the HIRAC FTIR system at the start of these studies meant relative rate experiments in chapter 4 could not make use of this facility. Experiments described in chapter 5 reports both GC-FID and FTIR measurements of the product branching ratios from the reaction of Cl atoms with butanes and chapter 6 will also involve use of FTIR to investigate HCl/DCl ratios from the reaction of Cl atoms with ethanol and its deuterated isotopologues.