The authors are grateful for the financial support from the ERC, under grant number 259619 PHOTO EM and grant number 620298 PHOTO AIR (Proof of Concept).

1. Makeup of VOC permeation tubes, and determination of its diffusion rate
<ol>
	<li>Makeup of 2-propanol permeation tubes 
	Note: To avoid contamination, wear gloves during this process. 
	Caution: 2-propanol is flammable and an irritant. Carry out this procedure away from any open flames. Wear gloves when handling 2-propanol. Consult MSDS of 2-propanol for further information.
	<ol>
		<li>Measure out and cut a 14 cm length of PTFE tubing.</li>
		<li>Seal and crimp one end of the tube by inserting a 2 cm length of PTFE rod into the end of the PTFE tubing, and then covering with a 2 cm metallic crimp</li>
		<li>Place the PTFE tubing, rod and crimp into the crimping tool, and then place this into a vice. Turn the vice, tightening as much as possible to seal the PTFE tube with the crimp.</li>
		<li>Pipette into the open-end of the PTFE tube an amount of 2-propanol, such that the PTFE tubing is around 1/3 full (approximately 3 - 4 mL).</li>
		<li>Repeat 1.1.2 - 1.1.3 to seal and crimp the open end of the permeation tube; the permeation source is then complete.</li>
	</ol>
	</li>
	<li>Determination of diffusion rate of VOC in the permeation tube
	<ol>
		<li>Weigh the permeation tube, using a calibrated balance, to at least 4 decimal places, noting both the weight and time.</li>
		<li>From a compressed air supply (ideally medical grade compressed air or equivalent), connect tubing (PTFE tubing, diameter 1/8 in, internal diameter 0.063 in) to an in line pressure regulator. From the regulator, connect, using the same diameter PTFE tubing, to one of the ports of a GL45 4 port connector, screwed to a 250 mL GL45 glass bottle. Block off two of the ports, and connect a length of PTFE tubing to the final port and guide this outlet to a fume hood.</li>
		<li>Position the permeation tube into the GL45 glass bottle, and ensure there is a constant steam of compressed air at a flow rate of 2.5 L min-1. Alternatively, position the tube in the dilution chamber of the system as illustrated in Figure 1, and described in section 2.1.</li>
		<li>At specific time intervals (e.g. daily) repeat the weight measurement (1.2.1) and place back into the system (1.2.2). If the decrease in weight is undetectable using the balance, increase the time interval between weighing the permeation tube (e.g. weekly, bi-weekly). Note that this calibration process, depending on the diffusion rate, could take a time period of a few months.</li>
		<li>Graph the diffusion rate with time in minutes on the x axis and the mass loss in nanograms (ng) on the y axis. Draw a straight line between points; using the straight-line equation (y = mx + c), determine the slope (m) of the line. This is the permeation rate in ng min-1.</li>
	</ol>
	</li>
</ol>
2. Photo-oxidation Reaction
<ol>
	<li>Set-up of equipment for use in the blank and photo-oxidation reaction (Figure 1)
	<ol>
		<li>Connect tubing (PTFE tubing, diameter 1/8 in, internal diameter 0.063 in) from a compressed air supply to an in line pressure regulator. From this, connect a moisture trap, to ensure a consistent low level of moisture enters the setup. From here, connect the PTFE tubing to a scrubber to further clean the compressed air.</li>
		<li>From the moisture trap or scrubber, connect, using the same diameter PTFE tubing, to a glass bottle, which will be the dilution chamber that will be used to hold the permeation tubes (GL45, 500 mL). To ensure a gas tight connection, use a screw cap HPLC, GL45 4 port connector, complete with silicone seals: block off two of the ports, and connect the tubing from the scrubber or moister trap to one of the other two ports, ensuring the connection is tight. Screw HPLC GL 45 screw cap onto the 500 mL glass bottle.</li>
		<li>Connect PTFE tubing to the final port or the HPCL GL45 screw cap, and then connect this to a second HPLC GL45 4 port connector. As with 2.1.2, block off two of the ports. Screw this HPLC FG45 screw cap on to a glass bottle (GL 45, 250 mL), which will be used as the reaction chamber.</li>
		<li>Connect PTFE tubing to the final port on the HPLC GL45 screw cap, and from this, connect the tubing to the FAIMS gas analyzer, using Swagelok 1/8 gas tight fittings. Ensure the external port of the gas analyzer is guided to a fume hood to ensure no contamination enters the laboratory work area.</li>
		<li>Position the reaction chamber so that the center of the chamber is 15 cm from a UV lamp (e.g. a UV lamp, consisting of 2 x 8 W tube lamps, with a peak photon emission wavelength of 356 nm). 
		Caution: UV light is dangerous to eyes; ensure the lamp and reactor is surrounded by a metallic shield to avoid exposure to the light.</li>
	</ol>
	</li>
	<li>Photo-oxidation of 2-propanol
	<ol>
		<li>Place two 2-propanol permeation tubes assembled previously (1.1) in the dilution chamber of the setup described above. Place the catalyst (e.g., a titanium dioxide based felt, dimensions 55 mm x 25 mm x 1 mm) in the reaction chamber, and ensure the catalyst is facing the UV lamp. Turn on the flow of compressed air, and adjust so the flow is 2.5 L min-1, and the pressure is 1 bar.</li>
		<li>Turn on the FAIMS instrument, and set up the instrument so that the ion current of the 2-propanol can be seen. Using software configured for the FAIMS device, increase the RF waveform, so that distinct ion peaks can be seen on the spectrum being produced by the FAIMS instrument.</li>
		<li>Using the software configured for the FAIMS device, monitor and record the ion current that is emanating from the distinct ion peaks seen on the spectrum produced by the FAIMS for a period of time, with the catalyst in the dark. The peaks will be 2-propanol, and water. At a set point (e.g. after leaving overnight), turn on the UV lamp, and monitor the FAIMS spectrum for the 2-propanol and water ion currents, plus additional signals from intermediate VOCs such as acetone. Using system software, increase or decrease the RF waveform to determine new signals emanating from the intermediate ions. 
		Caution: Ensure both the UV light and reactor are covered with a metallic shield before the lamp is illuminated, and that the shield is present throughout the entire UV light reaction.</li>
		<li>At a set point (e.g. after 4 hours), turn off the UV lamp, and continue to monitor the FAIMS spectrum for 2-propanol and additional peaks.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Wang, S. B., Ang, H. M., Tade, M. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Volatile+organic+compounds+in+indoor+environment+and+photocatalytic+oxidation:+State+of+the+art.">Volatile organic compounds in indoor environment and photocatalytic oxidation: State of the art.</a> Environ. Int. 33 (5), 694-705 (2007).</li><li>Shah, J. J., Singh, H. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Distribution+of+Volatile+Organic-Chemicals+in+Outdoor+and+Indoor+Air+-+a+National+Vocs+Data-Base.">Distribution of Volatile Organic-Chemicals in Outdoor and Indoor Air - a National Vocs Data-Base.</a> Environ. Sci. Technol. 22 (12), 1381-1388 (1988).</li><li>Jones, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Indoor+air+quality+and+health.">Indoor air quality and health.</a> Atmos. Environ. 33 (28), 4535-4564 (1999).</li><li>Hoffmann, M. R., Martin, S. T., Choi, W. Y., Bahnemann, D. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Environmental+Applications+of+Semiconductor+Photocatalysis.">Environmental Applications of Semiconductor Photocatalysis.</a> Chem. Rev. 95 (1), 69-96 (1995).</li><li>Mills, A., LeHunte, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+overview+of+semiconductor+photocatalysis.">An overview of semiconductor photocatalysis.</a> J. Photochem. Photobiol., A. 108 (1), 1-35 (1997).</li><li>Osterloh, F. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inorganic+materials+as+catalysts+for+photochemical+splitting+of+water.">Inorganic materials as catalysts for photochemical splitting of water.</a> Chem. Mater. 20 (1), 35-54 (2008).</li><li>Osterloh, F. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inorganic+nanostructures+for+photoelectrochemical+and+photocatalytic+water+splitting.">Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting.</a> Chem. Soc. Rev. 42 (6), 2294-2320 (2013).</li><li>Paz, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Application+of+TiO2+photocatalysis+for+air+treatment:+Patents'+overview.">Application of TiO2 photocatalysis for air treatment: Patents' overview.</a> Appl. Catal., B. 99 (3-4), 448-460 (2010).</li><li>Herrmann, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photocatalysis+fundamentals+revisited+to+avoid+several+misconceptions.">Photocatalysis fundamentals revisited to avoid several misconceptions.</a> Appl. Catal., B. 99 (3-4), 461-468 (2010).</li><li>Guevremont, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-field+asymmetric+waveform+ion+mobility+spectrometry:+A+new+tool+for+mass+spectrometry.">High-field asymmetric waveform ion mobility spectrometry: A new tool for mass spectrometry.</a> J. Chromatogr. A. 1058 (1-2), 3-19 (2004).</li><li>Kolakowski, B. M., Mester, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Review+of+applications+of+high-field+asymmetric+waveform+ion+mobility+spectrometry+(FAIMS)+and+differential+mobility+spectrometry+(DMS).">Review of applications of high-field asymmetric waveform ion mobility spectrometry (FAIMS) and differential mobility spectrometry (DMS).</a> Analyst. 132 (9), 842-864 (2007).</li><li>Kanu, A. B., Dwivedi, P., Tam, M., Matz, L., Hill, H. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ion+mobility-mass+spectrometry.">Ion mobility-mass spectrometry.</a> J. Mass Spectrom. 43 (1), 1-22 (2008).</li><li>Ireland, C. P., Ducati, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Investigating+the+photo-oxidation+of+model+indoor+air+pollutants+using+field+asymmetric+ion+mobility+spectrometry.">Investigating the photo-oxidation of model indoor air pollutants using field asymmetric ion mobility spectrometry.</a> J. Photochem. Photobiol., A. 312, 1-7 (2015).</li><li>Vildozo, D., Ferronato, C., Sleiman, M., Chovelon, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photocatalytic+treatment+of+indoor+air:+Optimization+of+2-propanol+removal+using+a+response+surface+methodology+(RSM).">Photocatalytic treatment of indoor air: Optimization of 2-propanol removal using a response surface methodology (RSM).</a> Appl. Catal., B. 94 (3-4), 303-310 (2010).</li><li>Vildozo, D., Portela, R., Ferronato, C., Chovelon, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photocatalytic+oxidation+of+2-propanol/toluene+binary+mixtures+at+indoor+air+concentration+levels.">Photocatalytic oxidation of 2-propanol/toluene binary mixtures at indoor air concentration levels.</a> Appl. Catal., B. 107 (3-4), 347-354 (2011).</li></ol>

The authors Lauren Brown and Russell Paris are employees of Owlstone Nanotechnology, the company that manufactures the FAIMS analysis instrument that is used in this article.

The protocol describes an effective way of determining the effectiveness of the titanium oxide based catalyst, by determining its behavior in degrading a model VOC, 2-propanol, under UV illumination. Using FAIMS, the amount of 2-propanol can be monitored continuously throughout the reaction, in addition to any other VOC products that could be produced in the reaction, at concentrations comparable to indoor air. This continuous nature differs significantly from gas chromatography, traditionally used to monitor photocatalytic indoor air purification, which uses a batch process. An expensive, sensitive GC/MS system is generally required to determine the concentration of VOCs at such low concentrations, and detailed analysis of the photo-oxidation products generally requires further processing of the photo-oxidation products, such as adsorbing products onto activated carbon, and then desorbing them into the mass spectrometer. Whilst mass spectrometry is able to detect all products, a limitation of FAIMS is that only products with a high proton affinity can be detected. FAIMS is excellent at determining low concentration VOCs, but can be saturated at higher concentrations, which limits the system to indoor air level concentration applications. The advantages of FAIMS makes the system described here an effective, simple tool that can provide insights into photocatalytic reactions that gas chromatography is limited in achieving.
With the FAIMS system described here, medical grade air is used as the flow gas. With the FAIMS system being so sensitive, a high quality grade of air is critical in allowing the photo-oxidation to be analyzed. This ensures that any products detected are from the photo-oxidation process. Similarly, it is critical to ensure there is no leakage into the system, as laboratory air generally contains VOCs at concentrations the FAIMS is capable of detecting. The consumables listed for the setup of the system provide a reliable system, and continuous monitoring over a period of days has indicated no detectable VOCs when no catalyst or permeation tube is present.
Whilst the system is simple, it is also very flexible - alternative VOCs can be tested in this way, by simply making a permeation tub containing the alternative VOC, such as ethanol, acetone, or toluene, and following the protocol. Photocatalytic reactions are often affected by humidity. The system developed here operates under low humidity; however testing can be carried out at higher humidities buy introducing a humidifier into the system. Depending on the VOC used, it can result in the sensitivity of the FAIMS being reduced, but effective testing can be carried out.16
The continuous nature of FAIMS highlights an advantage over gas chromatography, which is traditionally used to determine the photocatalyst effectiveness in purifying air.16,17 Gas chromatography uses a batch process to collect and analyze air samples; FAIMS, with its continuous nature, allows a more detailed look at the kinetics of the photocatalytic reaction, which can be challenging to interpret with the batch gas chromatography technique. The simplicity of FAIMS is another advantage. In order to carry out the complex analysis of multiple VOCs FAIMS is capable of, the gas chromatograph will need to be linked to a mass spectrometer, which can be expensive and require additional processing. Additionally, to carry out long term reactions with a gas chromatograph, an expensive automated system would be required, or labor intensive sampling; this is not the case with FAIMS.
The continuous nature of FAIMS offers significant advantages over gas chromatography that can be utilized to gain a greater understanding of the photocatalysis process at these ppb concentrations. Moreover, the simple setup illustrated here is flexible, allowing alternative photocatalysts and VOCs to be tested under comparable conditions, further improving the understanding of the photocatalytic process.

The quality of indoor air has come to the forefront recently. Perhaps surprisingly, indoor air contains a greater number of volatile organic carbons (VOCs), and in higher concentrations than outdoor air.1 With people spending over 80% of their time indoors, in places such as residential homes, workplaces, and transport including cars, trains, and aircraft, air quality can be a real issue. Many of the VOCs common in indoor air are mutagenic or carcinogenic,2,3 and so the removal of these is a key priority, especially as the phenomena of &#39;sick building syndrome&#39; can lead to ill health and lost production through time off work.1 Air purification devices can include a photocatalyst, where a semiconductor, invariably titanium dioxide (TiO2), activated with UV light, degrades the VOC through a photo-oxidation process. Photocatalysis is a growing area of research, with applications in water splitting for hydrogen production and pollutant degradation4,5,6,7; air purification is a particularly active area due to the commercial viability of this application8. However, detecting VOCs at concentrations that are present in indoor air (typically ppb) is challenging. With the kinetics of the photocatalytic reaction following Langmuir Hinshelwood kinetics9, the effectiveness of the photocatalyst at degrading VOCs at high concentrations is not representative of its effectiveness at low concentrations. Here we describe a versatile system and protocol for determining the effectiveness of photocatalysts at degrading VOCs at such low concentrations using field asymmetric ion mobility spectroscopy (FAIMS), illustrating this with a TiO2 based photocatalyst, and the model VOC 2-propanol.
Ionizing a gas flow, FAIMS separates and identifies chemical ions based on their mobility under a varying electric field at atmospheric pressure10,11,12. Molecules with a high proton affinity, such as VOCs are well suited to be separated and detected by FAIMS, with parts per billion (ppb) resolution, and at ppb concentrations13. Capable of continuously monitoring multiple VOCs simultaneously, it is an ideal analysis to use in photocatalytic air purification testing, as in addition to monitoring the VOC used as pollutant. FAIMS can also detect intermediates or other VOC products with a high proton affinity from the photocatalytic reaction, a key requirement in proving that the photocatalyst is effective, as if the degradation is incomplete, some of the VOCs produced may be as toxic or more toxic than the VOC being degraded.
FAIMS has only recently been used for the first time in photocatalytic air purification applications14, and although not suggesting FAIMS is superior to gas chromatography, it clearly offers a versatile alternative, which has the potential to be a potent tool in studying air purification. Here we illustrate this technique with a protocol involving the photo-oxidation of 2-propanol with a titanium dioxide based photocatalyst. To generate 2-propanol at the indoor air level concentrations permeation tubes are used15. Consisting of a PTFE tube containing the liquid VOC, that is sealed and crimped at both ends, under a constant flow, the VOC contained within the sealed PTFE permeation tube diffuses out at a constant rate, at concentrations comparable to indoor air. This flow is then passed into a reaction chamber containing the felt, and then into the FAIMS analyzer, where the identity and quantification of the VOC can be determined. FAIMS allows the concentration of 2-propanol to be determined, and via a library of spectra of know VOCs, the identity of additional VOCs produced during the photo reaction such as acetone determined through comparison of their spectra with the library. A key advantage of this technique is its flexibility: by simply changing the permeation tube or catalyst, alternative VOCs and catalysts can be tested.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>PTFE Tubing</td>
			<td>Sigma-Aldrich</td>
			<td>58699 SUPELCO&#160;</td>
			<td>L x OD x ID 50 ft x 1/8 in x 0063 in</td>
		</tr>
		<tr>
			<td>In-line pressure regulator</td>
			<td>Sigma-Aldrich</td>
			<td>23882 SUPELCO</td>
			<td>High purity version (outlet pressure 0-100 psi, 1/8 in stainless steel fittings</td>
		</tr>
		<tr>
			<td>Moisture trap</td>
			<td>Sigma-Aldrich</td>
			<td>N9301193</td>
			<td>70 ml 1/8 fittings</td>
		</tr>
		<tr>
			<td>Screw Cap HPLC, GL 45</td>
			<td>VWR</td>
			<td>554-3002</td>
			<td>4 ports complete with silicone seals</td>
		</tr>
		<tr>
			<td>Duran GL 45 Glass Bottle</td>
			<td>Scientific Laboratory Supplies</td>
			<td>BOT5206</td>
			<td>250 ml&#160;</td>
		</tr>
		<tr>
			<td>Duran GL 45 Glass Bottle</td>
			<td>Scientific Laboratory Supplies</td>
			<td>BOT5208</td>
			<td>500 ml</td>
		</tr>
		<tr>
			<td>Permeation tube making kit</td>
			<td>Owlstone Nanotechnology</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>2-propanol</td>
			<td>Fisher Scientific</td>
			<td>10477070</td>
			<td>&#160;Isopropanol, extra pure, SLR</td>
		</tr>
		<tr>
			<td>Quartzel PCO Felt</td>
			<td>Saint Gobain</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>UVIlite&#160; Lamp</td>
			<td>UVItec Limited</td>
			<td>LI-208BL</td>
			<td></td>
		</tr>
		<tr>
			<td>Swage Fittings</td>
			<td>Swagelok</td>
			<td>SS-202-1 / SS-200-SET</td>
			<td></td>
		</tr>
		<tr>
			<td>Lonestar Portable Analyzer</td>
			<td>Owlstone Nanotechnology</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

analyzing the photo-oxidation of 2-propanol at indoor air level concentrations using field asymmetric ion mobility spectrometry

We demonstrate a versatile protocol to be used for determining the effectiveness of photocatalysts in degrading indoor air concentration (ppb) volatile organic carbons (VOCs), illustrating this with a titanium dioxide based catalyst, and the VOC 2-propanol. The protocol takes advantage of field asymmetric ion mobility spectroscopy (FAIMS), an analysis tool that is capable of continuously identifying and monitoring the concentration of VOCs such as 2-propanol and acetone at the ppb level. The continuous nature of FAIMS allows detailed kinetic analysis, and long-term reactions, offering a significant advantage over gas chromatography, a batch process traditionally used in air purification characterization. The use of FAIMS in photocatalytic air purification has only recently been used for the first time, and with the protocol illustrated here, the flexibility in allowing alternative VOCs and photocatalysts to be tested using comparable protocols offers a unique system to elucidate photocatalytic air purification reactions at low concentrations.

The FAIMS gas analyzer continuously produces spectra of ion current vs. compensation voltage during the course of the photo oxidation reaction described in 2.2, utilizing two 2-propanol permeation tubes in the dilution chamber, and a titanium dioxide based felt photocatalyst in the reaction chamber. Spectra typically produced by the FAIMS analyzer when the felt is in the dark, and when the felt is illuminated are illustrated in Figure 2a. To obtain the spectra with the FAIMS instrument, the RF waveform on the instrument is set to 64% of the maximum. At this RF waveform value, hydronium ions (water clusters), acetone monomers, and 2-propanol monomers which can be formed from the FAIMS instrument ionization process reach the detector in the FAIMS at distinct compensation voltages (cv), and so are separated on the spectra. Flowing individual gasses exclusively through the FAIMS system can be used to determine the spectra and compensation values for each gas16. On the spectrum, the peak at a compensation voltage of -2.15 V is the hydronium ion, a water cluster ion formed when moisture in the air is ionized. The peak at a cv of -0.14 V is that of 2-propanol14. The ion current is directly proportional to that of the 2-propanol concentration, and so using the diffusion rate (1.2), the concentration of 2-propanol entering the FAIMS can be determined. Similarly with acetone, occurring at a cv of -1.44 V. Figure 2b shows the ion current measured at the specific peaks identified as 2-propanol and acetone in the spectra with the RF waveform at 64% of maximum, as a function of time throughout the photo-oxidation protocol described in section 2.2. As subtle changes in flow and humidity can have an effect of shifting the ion current peak cv value positively or negatively, the peak height at a CV value of &#177; 0.2 V is used.
The amount of 2-propanol entering the FAIMS analyzer, with the reaction chamber in the dark increases over time. As 2-propanol enters the dilution chamber, 2-propanol is adsorbed onto the surface of the catalyst, which accounts for the initial low amount of 2-propanol entering the FAIMS. As time continues a higher ion current is recorded, indicating that a higher amount of 2-propanol enters the FAIMS. This suggests that the surface of the felt is being covered with 2-propanol, hence adsorption onto the catalyst is decreasing.
When the reactor chamber is illuminated, there is an immediate increase in 2-propanol entering the FAIMS. This implies that an amount of 2-propanol desorbs from the surface of the felt, and enters the FAIMS analyzer. Concurrently, there is an increase in ion current from the peak at cv -1.44 V, which has previously been identified as acetone, indicating the felt under illumination has photo-oxidized 2-propanol to acetone. As time continues, the amount of 2-propanol decreases to a level significantly below the level at the initial point of illumination, and acetone continues to be detected, with both ion currents consistent over a period of around 3 hours. This implies that 2-propanol is consistently being photo-oxidized either to acetone, or to carbon dioxide and water. 2-propanol adsorbs onto the surface, is photo-oxidized, and the products desorb and enter the FAIMS, where acetone is recorded. After the light is turned off, the 2-propanol ion current increases, whilst the acetone ion current decreases implying the photo-oxidation has ceased.
The results are representative of the concentration of 2-propanol and acetone, continuously monitored at ppb concentrations. By comparing the steady state 2-propanol current under illumination with that of 2-propanol current entering the FAIMS before illumination, the effectiveness of the catalyst can be seen, with a greater decrease in 2-propanol entering the FAIMS indicative of a superior photocatalyst. The monitoring of additional VOCs also allows a better assessment of the effectiveness of the photocatalyst. In air purification applications, ideally the VOC should be degraded to carbon dioxide and water. Additional compounds detected demonstrate an ineffective catalyst or poor air purification strategy (flow rates, light intensity, humidity levels). FAIMS can monitor the photo-reaction, and so demonstrate the effectiveness of catalyst and air purification setup.
<img alt="Figure 1" src="/files/ftp_upload/54209/54209fig1.jpg" /> 
Figure 1. The Reactor Setup. Diagram illustrating the photocatalysis setup developed for use with the FAIMS gas analyzer (see 2.1). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54209/54209fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/54209/54209fig2.jpg" /> 
Figure 2. Typical Results. (a) Typical spectra produced by the FAIMS when the RF waveform is 64% of maximum when the reaction containing the felt is in the dark (grey line) and when it is illuminated (green line). (b) Graph showing the ion current at peaks from the compensation voltage vs ion current spectra produced during the 2-propanol photo-oxidation reaction when the RF waveform is at 64% of maximum; 2-propanol (red line) and acetone (blue line) shown, with the illuminated reaction highlighted. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54209/54209fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Analyzing the Photo-oxidation of 2-propanol at Indoor Air Level Concentrations Using Field Asymmetric Ion Mobility Spectrometry at JoVE.com

Analyzing the Photo-oxidation of 2-propanol at Indoor Air Level Concentrations Using Field Asymmetric Ion Mobility Spectrometry

A protocol for determining the effectiveness of photocatalysts in degrading indoor air concentration (ppb) model volatile organic carbons such as 2-propanol is described.

We demonstrate a versatile protocol to be used for determining the effectiveness of photocatalysts in degrading indoor air concentration (ppb) volatile ...

analyzing-photo-oxidation-2-propanol-at-indoor-air-level

Nanyang Technological University

Research

JoVE Journal

Chemistry

8.4K Views.  University of Cambridge. A protocol for determining the effectiveness of photocatalysts in degrading indoor air concentration (ppb) model volatile organic carbons such as 2-propanol is described.

Video: Analyzing the Photo-oxidation of 2-propanol at Indoor Air Level Concentrations Using Field Asymmetric Ion Mobility Spectrometry

Analyzing Protein Dynamics Using Hydrogen Exchange Mass Spectrometry

All cellular processes depend on the functionality of proteins. Although the functionality of a given protein is the direct consequence of its unique amino acid sequence, it is only realized by the folding of the polypeptide chain into a single defined three-dimensional arrangement or more commonly into an ensemble of interconverting conformations. Investigating the connection between protein conformation and its function is therefore essential for a complete understanding of how proteins are able to fulfill their great variety of tasks. One possibility to study conformational changes a protein undergoes while progressing through its functional cycle is hydrogen-1H/2H-exchange in combination with high-resolution mass spectrometry (HX-MS). HX-MS is a versatile and robust method that adds a new dimension to structural information obtained by e.g. crystallography. It is used to study protein folding and unfolding, binding of small molecule ligands, protein-protein interactions, conformational changes linked to enzyme catalysis, and allostery. In addition, HX-MS is often used when the amount of protein is very limited or crystallization of the protein is not feasible. Here we provide a general protocol for studying protein dynamics with HX-MS and describe as an example how to reveal the interaction interface of two proteins in a complex. &#160;&#160;

Protein conformation and dynamics are key to understanding the relationship between protein structure and function. Hydrogen exchange coupled with high-resolution mass spectrometry is a versatile method to study the conformational dynamics of proteins as well as characterizing protein-ligand and protein-protein interactions, including contact interfaces and allosteric effects.

All cellular processes depend on the functionality of proteins. Although the functionality of a given protein is the direct consequence of its unique ...

A Study of the Complexation of Mercury(II) with Dicysteinyl Tetrapeptides by Electrospray Ionization Mass Spectrometry

In this study we evaluated a method for the characterization of complexes, formed in different relative ratios of mercury(II) to dicysteinyl tetrapeptide, by electrospray ionization orbitrap mass spectrometry. This strategy is based on previous successful characterization of mercury-dicysteinyl complexes involving tripeptides by utilizing mass spectrometry among other techniques. Mercury(II) chloride and a dicysteinyl tetrapeptide were incubated in a degassed buffered medium at varying stoichiometric ratios. The complexes formed were subsequently analyzed on an electrospray mass spectrometer consisting of a hybrid linear ion- and orbi- trap mass analyzer. The electrospray ionization mass spectrometry (ESI-MS) spectra were acquired in the positive mode and the observed peaks were then analyzed for distinct mercury isotopic distribution patterns and associated monoisotopic peak. This work demonstrates that an accurate stoichiometry of mercury and peptide in the complexes formed under specified electrospray ionization conditions can be determined by using high resolution ESI MS based on distinct mercury isotopic distribution patterns.

A Study of the Complexation of MercuryII with Dicysteinyl Tetrapeptides by Electrospray Ionization Mass Spectrometry

The characterization of complexes formed in different relative ratios of mercury(II) to dicysteinyl tetrapeptides by electrospray ionization orbitrap mass spectrometry is presented.

In this study we evaluated a method for the characterization of complexes, formed in different relative ratios of mercury(II) to dicysteinyl tetrapeptide, ...

Rapid High-throughput Species Identification of Botanical Material Using Direct Analysis in Real Time High Resolution Mass Spectrometry

We demonstrate that direct analysis in real time-high resolution mass spectrometry can be used to produce mass spectral profiles of botanical material, and that these chemical fingerprints can be used for plant species identification. The mass spectral data can be acquired rapidly and in a high throughput manner without the need for sample extraction, derivatization or pH adjustment steps. The use of this technique bypasses challenges presented by more conventional techniques including lengthy chromatography analysis times and resource intensive methods. The high throughput capabilities of the direct analysis in real time-high resolution mass spectrometry protocol, coupled with multivariate statistical analysis processing of the data, provide not only class characterization of plants, but also yield species and varietal information. Here, the technique is demonstrated with two psychoactive plant products, Mitragyna speciosa (Kratom) and Datura (Jimsonweed), which were subjected to direct analysis in real time-high resolution mass spectrometry followed by statistical analysis processing of the mass spectral data. The application of these tools in tandem enabled the plant materials to be rapidly identified at the level of variety and species.

A method for species identification of botanical material by direct analysis in real time-high resolution mass spectrometry and multivariate statistical analysis is presented.

We demonstrate that direct analysis in real time-high resolution mass spectrometry can be used to produce mass spectral profiles of botanical material, ...

Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level

We demonstrate a method for the synthesis of cyclic polymers and a protocol for characterizing their diffusive motion in a melt state at the single molecule level. An electrostatic self-assembly and covalent fixation (ESA-CF) process is used for the synthesis of the cyclic poly(tetrahydrofuran) (poly(THF)). The diffusive motion of individual cyclic polymer chains in a melt state is visualized using single molecule fluorescence imaging by incorporating a fluorophore unit in the cyclic chains. The diffusive motion of the chains is quantitatively characterized by means of a combination of mean-squared displacement (MSD) analysis and a cumulative distribution function (CDF) analysis. The cyclic polymer exhibits multiple-mode diffusion which is distinct from its linear counterpart. The results demonstrate that the diffusional heterogeneity of polymers that is often hidden behind ensemble averaging can be revealed by the efficient synthesis of the cyclic polymers using the ESA-CF process and the quantitative analysis of the diffusive motion at the single molecule level using the MSD and CDF analyses.

A protocol for the synthesis and characterization of diffusive motion of cyclic polymers at the single molecule level is presented.

We demonstrate a method for the synthesis of cyclic polymers and a protocol for characterizing their diffusive motion in a melt state at the single ...

An HS-MRM Assay for the Quantification of Host-cell Proteins in Protein Biopharmaceuticals by Liquid Chromatography Ion Mobility QTOF Mass Spectrometry

The analysis of low-level (1-100 ppm) protein impurities (e.g., host-cell proteins (HCPs)) in protein biotherapeutics is a challenging assay requiring high sensitivity and a wide dynamic range. Mass spectrometry-based quantification assays for proteins typically involve protein digestion followed by the selective reaction monitoring/multiple reaction monitoring (SRM/MRM) quantification of peptides using a low-resolution (Rs ~1,000) tandem quadrupole mass spectrometer. One of the limitations of this approach is the interference phenomenon observed when the peptide of interest has the "same" precursor and fragment mass (in terms of m/z values) as other co-eluting peptides present in the sample (within a 1-Da window). To avoid this phenomenon, we propose an alternative mass spectrometric approach, a high selectivity (HS) MRM assay that combines the ion mobility separation of peptide precursors with the high-resolution (Rs ~30,000) MS detection of peptide fragments. We explored the capabilities of this approach to quantify low-abundance peptide standards spiked in a monoclonal antibody (mAb) digest and demonstrated that it has the sensitivity and dynamic range (at least 3 orders of magnitude) typically achieved in HCP analysis. All six peptide standards were detected at concentrations as low as 0.1 nM (1 femtomole loaded on a 2.1-mm ID chromatographic column) in the presence of a high-abundance peptide background (2 &#181;g of a mAb digest loaded on-column). When considering the MW of rabbit phosphorylase (97.2 kDa), from which the spiked peptides were derived, the LOQ of this assay is lower than 50 ppm. Relative standard deviations (RSD) of peak areas (n = 4 replicates) were less than 15% across the entire concentration range investigated (0.1-100 nM or 1-1,000 ppm) in this study.

Here, we describe a chromatographic assay coupled with the ion mobility separation of peptide precursors followed by the high-resolution (~30,000) MS-detection of peptide fragments for the quantification of spiked peptide standards in a monoclonal antibody digest.

The analysis of low-level (1-100 ppm) protein impurities (e.g., host-cell proteins (HCPs)) in protein biotherapeutics is a challenging assay requiring ...

Real-time Breath Analysis by Using Secondary Nanoelectrospray Ionization Coupled to High Resolution Mass Spectrometry

Exhaled volatile organic compounds (VOCs) have aroused considerable interest, since they can serve as biomarkers for disease diagnosis and environmental exposure in a non-invasive manner. In this work, we present a protocol to characterize the exhaled VOCs in real time by using secondary nanoelectrospray ionization coupled to high resolution mass spectrometry (Sec-nanoESI-HRMS). The homemade Sec-nanoESI source was readily set up based on a commercial nanoESI source. Hundreds of peaks were observed in the background-subtracted mass spectra of exhaled breath, and the mass accuracy values are -4.0-13.5 ppm and -20.3-1.3 ppm in the positive and negative ion detection modes, respectively. The peaks were assigned with accurate elemental composition according to the accurate mass and isotopic pattern. Less than 30 s is used for one exhalation measurement, and it takes approximately 7 min for six replicated measurements.

A protocol for characterizing chemical composition of exhaled breath in real time by using secondary nanoelectrospray ionization coupled to high resolution mass spectrometry is demonstrated.

Exhaled volatile organic compounds (VOCs) have aroused considerable interest, since they can serve as biomarkers for disease diagnosis and environmental ...

Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry

Corrosion developed at the paint and aluminum (Al) metal-paint interface of an aluminum alloy is analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS), illustrating that SIMS is a suitable technique to study the chemical distribution at a metal-paint interface. The painted Al alloy coupons are immersed in a salt solution or exposed to air only. SIMS provides chemical mapping and 2D molecular imaging of the interface, allowing direct visualization of the morphology of the corrosion products formed at the metal-paint interface and mapping of the chemical after corrosion occurs. The experimental procedure of this method is presented to provide technical details to facilitate similar research and highlight pitfalls that may be encountered during such experiments.

Time-of-flight secondary ion mass spectrometry is applied to demonstrate the chemical mapping and corrosion morphology at the metal-paint interface of an aluminum alloy after being exposed to a salt solution compared with a specimen exposed to air.

Corrosion developed at the paint and aluminum (Al) metal-paint interface of an aluminum alloy is analyzed using time-of-flight secondary ion mass ...

Ion Mobility-Mass Spectrometry Techniques for Determining the Structure and Mechanisms of Metal Ion Recognition and Redox Activity of Metal Binding Oligopeptides

Electrospray ionization (ESI) can transfer an aqueous-phase peptide or peptide complex to the gas-phase while conserving its mass, overall charge, metal-binding interactions, and conformational shape. Coupling ESI with ion mobility-mass spectrometry (IM-MS) provides an instrumental technique that allows for simultaneous measurement of a peptide&#8217;s mass-to-charge (m/z) and collision cross section (CCS) that relate to its stoichiometry, protonation state, and conformational shape. The overall charge of a peptide complex is controlled by the protonation of 1) the peptide&#8217;s acidic and basic sites and 2) the oxidation state of the metal ion(s). Therefore, the overall charge state of a complex is a function of the pH of the solution that affects the peptides metal ion binding affinity. For ESI-IM-MS analyses, peptide and metal ions solutions are prepared from aqueous-only solutions, with the pH adjusted with dilute aqueous acetic acid or ammonium hydroxide. This allows for pH dependence and metal ion selectivity to be determined for a specific peptide. Furthermore, the m/z and CCS of a peptide complex can be used with B3LYP/LanL2DZ molecular modeling to discern binding sites of the metal ion coordination and tertiary structure of the complex. The results show how ESI-IM-MS can characterize the selective chelating performance of a set of alternative methanobactin peptides and compare them to the copper-binding peptide methanobactin.

Ion mobility-mass spectrometry and molecular modeling techniques can characterize the selective metal chelating performance of designed metal-binding peptides and the copper-binding peptide methanobactin. Developing new classes of metal chelating peptides will help lead to therapeutics for diseases associated with metal ion misbalance.

Electrospray ionization (ESI) can transfer an aqueous-phase peptide or peptide complex to the gas-phase while conserving its mass, overall charge, ...

Self-Assembly of Hybrid Lipid Membranes Doped with Hydrophobic Organic Molecules at the Water/Air Interface

Because of their unique properties, including an ultrathin thickness (3-4 nm), ultrahigh resistivity, fluidity and self-assembly ability, lipid bilayers can be readily functionalized and have been used in various applications such as bio-sensors and bio-devices. In this study, we introduced a planar organic molecule: copper (II) 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (CuPc) to dope lipid membranes. The CuPc/lipid hybrid membrane forms at the water/air interface by self-assembly. In this membrane, the hydrophobic CuPc molecules are located between the hydrophobic tails of lipid molecules, forming a lipid/CuPc/lipid sandwich structure. Interestingly, an air-stable hybrid lipid bilayer can be readily formed by transferring the hybrid membrane onto a Si substrate. We report a straightforward method for incorporating nanomaterials into a lipid bilayer system, which represents a new methodology for the fabrication of biosensors and biodevices.

We report a protocol for producing a hybrid lipid membrane at the water/air interface by doping the lipid bilayer with copper (II) 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (CuPc) molecules. The resulting hybrid lipid membrane has a lipid/CuPc/lipid sandwich structure. This protocol can also be applied to the formation of other functional nanomaterials.

Because of their unique properties, including an ultrathin thickness (3-4 nm), ultrahigh resistivity, fluidity and self-assembly ability, lipid bilayers ...

3D Depth Profile Reconstruction of Segregated Impurities Using Secondary Ion Mass Spectrometry

The presented protocol combines excellent detection limits (1 ppm to 1 ppb) using secondary ion mass spectrometry (SIMS) with reasonable spatial resolution (~1 &#181;m). Furthermore, it describes how to obtain realistic three-dimensional (3D) distributions of segregated impurities/dopants in solid state materials. Direct 3D depth profile reconstruction is often difficult to achieve due to SIMS-related measurement artifacts. Presented here is a method to identify and solve this challenge. Three major issues are discussed, including the i) nonuniformity of the detector being compensated by flat-field correction; ii) vacuum background contribution (parasitic oxygen counts from residual gases present in the analysis chamber) being estimated and subtracted; and iii)&#160;performance of all steps within a stable timespan of the primary ion source. Wet chemical etching is used to reveal the position and types of dislocation in a material, then the SIMS result is superimposed on images obtained via scanning electron microscopy (SEM). Thus, the position of agglomerated impurities can be related to the position of certain defects. The method is fast and does not require sophisticated sample preparation stage; however, it requires a high-quality, stable ion source, and the entire measurement must be performed quickly to avoid deterioration of the primary beam parameters.

The presented method describes how to identify and solve measurement artifacts related to secondary ion mass spectrometry as well as obtain realistic 3D distributions of impurities/dopants in solid state materials.