Variation of Sampling Time

Sampling time modification provided another source of optimisation for this technique. There are two targets to these experiments. Firstly, an optimal sample time needs to be established. While a longer experiment would be expected to generate a greater signal, there are limitations to this approach, with a balance between sampling time and limit of detection required to make best use of sampling time. However, with this sampling technique, the impact of products being blown away from the support will also need to be considered. Secondly, the minimum time required for sample to be detected was essential to establish. This is key with regards to establishing limits of temporal resolution for this trapping technique under the conditions currently being tested.

5.3.1. Optimisation of Sampling Time

Experiments have shown that longer sampling times provide an increased opportunity for the radical trap to be blown away from the sampling site – for example overnight experiments (ca. 16 hours) detected almost no compound after the experiment. While sample being blown away will still be a feature of experiments run at low exposure times, the overall percentage of sample blown away will reduce as exposure decreases. Therefore, as different exposure times were tested, an initial increase in signal intensity was expected, which will peak and begin to decrease as exposures get longer.

A second issue with longer sampling times is the potential for all of 2.09 to have reacted, hence resulting in no further trapped products beyond this point. However, unless the trapping molecule forms a perfect monolayer on the silica (which is highly unlikely), the material will effectively be saturated once surface material has reacted – bringing the time at which no new products will be caught considerably earlier. Radical diffusion into the bulk trapping material is unlikely: studies on radical rates of diffusion have found that these are generally lower than those of the equivalent closed shell species.270 _{This is believed to}

164 interactions, and as such lower diffusion rates. However, relative radical concentrations are likely to be such that sample saturation from gas phase sampling is highly unlikely.

The optimal length of sampling experiments was probed by a series of experiments at exposure times from 4 to 90 minutes. The trend from these experiments, in Figure 174, shows an increase in signal with exposure time, suggesting that the trapping material is not fully saturated over these time periods, or suffering from excessive loss due to being blown away. Therefore, experimental exposures of up to 90 minutes would be suitable for trapping. Signals from 90 minute experiments are on the same order of magnitude as experiments with a 5 minute sampling time, suggesting that 5 minute exposures can be utilised. However, as these experiments were conducted in a very simple system (i.e. only one source of peroxyl radicals) it is likely that sample times greater than 5 min will be required when this methodology is applied to more diverse systems.

Figure 174: Change in signal for 4.11 as sampling time is modified from 4-90 minutes

5.3.2. Minimum Sampling Time

The minimum time required for signal detection within this system was probed in the same manner. If this methodology were to be applied to ‘real world’ scenarios, the ability to detect radicals with a temporal resolution of minutes, or below, would be of great use,

165 hence it was useful to test whether this is possible within this system prior to ‘real world’ tests (Chapter 7). As an example this would enable the variation in radicals present from reactions of emissions from rush hour traffic to be easily monitored. Results from short exposure times are presented below in Figure 175.

Figure 175: Change in signal for 4.11 as sampling time is modified between 20-120 seconds

It appears that signal can be detected with sampling times as low as 20 s. This is a promising result, suggesting that detection limits (for 4.11) are reasonably low, and that temporal resolutions of under a minute are possible within this system. This compares well to literature methods of radical detection. LIF based techniques have typical temporal resolutions up to 100 s, while resolutions of up to 500 s have been recorded for DOAS experiments.271 _{Meanwhile, MI-ESR based measurements have significantly higher}

temporal resolutions of ca. 30 min, but CIMS based techniques are capable of resolutions of a few seconds, significantly better than that achieved here.19,272 _{However, the}

difference in signal between exposures of 1 and 90 minutes is considerable between Figure 174 and Figure 175, so at this stage in the research longer sampling times up to 90 minutes will be much more useful to pursue during future experiments.

It should however be mentioned that the low detection times of this system are unlikely to be reproducible in sampling of real outdoor/indoor sampling systems. This system

166 utilises a single reaction in order to trap a range of radicals – while several radicals are indeed detected, the concentrations of these species are significantly higher than predicted at atmospheric levels. The nature of this trapping technique means that in a mixture of radicals, the amount of each radical captured is likely to decrease as the total number of different radical species increases. Hence it is unlikely that temporal resolutions from the radical trapping technique will be able to reach levels of detection from field measurements via LIF, DOAS or CIMS.19,271,272

A final area of note for these experiments regards the stability of the non-radical species formed upon reaction of 2.09 with a radical. One of the key aspects of this technique is for the reaction products to be analysed offline, which requires the trapped species to survive for an extended period of time. Here, products of radical reactions with 2.09 (e.g. 4.11, 4.02) can be identified from reaction mixtures that have been stored at 253 K for up to four weeks. Therefore, it appears that the trapping technique fulfils the key requirement of products being sufficiently stable for off-line analysis.