The overall goal of this procedure is to demonstrate the use of Field Asymmetric Ion Mobility Spectrometry or FAIMS to continuously monitor the photo-oxidation of model air pollutants here 2-propanol under ultraviolet light using a photocatalyst. This method can help answer key questions in the photocatalysis field helping to understand the processes involved in photo-oxidation of indoor air concentration level pollutants. The main advantage to this technique is that it allows continuous monitoring of 2-propanol and reaction intermediates in parts per billion concentrations.
To make up the 2-propanol permeation tubes, first measure and cut a 14 centimeter length of PTFE tubing. Seal and crimp one end of the tube by inserting a two centimeter length of PTFE rod into the end of the PTFE tubing and then cover with a two centimeter metallic crimp. Place the PTFE tubing, rod, and crimp into the crimping tool.
Then place this into a vice. Turn the vice, tightening as much as possible, to seal the PTFE tube with the crimp. Into the open end of the PTFE tube, pipette approximately three to four milliliters of 2-propanol such that the PTFE tubing is around 1/3 full.
Seal and crimp the open end of the permeation tube as before. This completes the permeation source. To determine the diffusion rate of VOC in the permeation tube, first use a calibrated balance to weigh the permeation tube to at least four decimal places noting both the weight and time.
From a compressed air supply, connect tubing to an inline pressure regulator. From the regulator, connect one of the ports to a GL45 four-port connector screwed to a 250 milliliter GL45 glass bottle. Lock off two of the ports.
Then connect the length of PTFE tubing to the final port. If the apparatus is not in a fume hood, guide tubing from this outlet to a fume hood. Position the permeation tube into the GL45 glass bottle and ensure there is a constant stream of compressed air at a flow rate of 2.5 liters per minute.
At specific time intervals, repeat the weight measurement of the permeation tube and then place it back into the system. If the decrease in weight is undetectable using the balance, increase the time interval between weighing the permeation tube. Note that this calibration process could take a time period of a few months depending on the diffusion rate.
Graph the diffusion rate with time in minutes on the x-axis and the mass loss in nanograms on the y-axis. Draw a straight line between the points. Using the straight line equation, determine the slope of the line.
This is the permeation rate in nanograms per minute. To set up the equipment for the photo-oxidation reaction, first connect the tubing from a compressed air supply to an inline pressure regulator. Connect a moisture trap to ensure that a consistent low level of moisture enters the setup.
From here, connect the PTFE tubing to a scrubber to further clean the compressed air. From the moisture trap or scrubber, connect to a glass bottle which will be the dilution chamber that will hold the permeation tubes. To ensure a gas tight connection, use a screw cap HPLC GL45 four-port connector complete with silicone seals.
Lock off two of the ports and connect the tubing from the scrubber or moisture trap to one of the other two ports ensuring the connection is tight. Screw the HPLC GL45 screw cap onto the 500 milliliter glass bottle. Connect the PTFE tubing to the final port of the HPLC GL45 screw cap and then connect this to a second HPLC GL45 four-port connector.
Block off two of the ports as before and screw this screw cap onto a glass bottle which will be used as the reaction chamber. Next, connect the PTFE tubing to the final port on the second screw cap. Then connect the tubing to the FAIMS gas analyzer using 1/8 gas tight fittings.
If the apparatus is not in a fume hood, ensure the external port of the gas analyzer is guided to a fume hood to ensure no contamination enters the laboratory work area. Position the reaction chamber so that the center of the chamber is 15 centimeters from an ultraviolet lamp. To perform photo-oxidation of 2-propanol, place two 2-propanol permeation tubes in the dilution chamber.
Place the catalyst in the reaction chamber and ensure the catalyst is facing the UV lamp. Turn on the flow of compressed air and adjust the flow to 2.5 liters per minute and the pressure to one bar. Turn on the FAIMS instrument and set up the instrument so that the ion current of the 2-propanol is seen.
Using the 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. Monitor and record the ion current that is emanating from the distinct ion peaks 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, 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. After a set amount of time, turn off the UV lamp and continue to monitor the FAIMS spectrum for 2-propanol and additional peaks.
Representative results of the photo-oxidation of 2-propanol are illustrated here showing spectra produced by the FAIMS when the RF waveform is 64%of maximum. The gray line represents the reaction containing the felt in the dark and the green line represents the illuminated reaction. The decrease in size of the 2-propanol peak indicates photo-oxidation is taking place under illumination.
The development of the acetone peak is indicative of the 2-propanol being photo-oxidized into acetone. Representative results of the 2-propanol photo-oxidation reaction are illustrated here showing the ion current of 2-propanol and acetone peaks throughout the reaction. The decrease in 2-propanol as the reaction is illuminated can clearly be seen as can the increase in the acetone concentration upon illumination.
Finally, as the light is turned off, the acetone decreases and 2-propanol increases as the reaction stops. Once mastered, this technique can be adapted using other volatile organic carbons that can be detected by FAIMS including ethanol, toluene, and benzine. The continuous method and simplicity of the technique presented here offers a flexible addition to other techniques such as GCMS and have some potential to be a protein tool in indoor air purification studies.
