VOLATILE ORGANIC COMPOUNDS
Volatile organic compounds (VOCs) are organic chemicals that have a high vapour pressure at room temperature. This is due to their low boiling point which means they typically vaporize easily (transition from liquid or solid to a gas) - and this can make them somewhat tricky to analyse.
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Most smells, scents or odours are the result of one or more VOCs detected by the human nose and VOCs are all around us – from perfumes to paints and cosmetics to cleansers. Applications for VOC analysis are far-reaching, from the creation of the perfect sensory profile for coffee to analysis of VOCs from exhaled breath for disease diagnosis.
HOW DOES IT WORK?
One of the most popular methods for analysing VOCs is mass spectrometry which is described below.
However, new gas sensors are being developed and there is a particular interest in those which are small and portable as this opens up the field for development of breath analysis devices and devices which can detect harmful VOCs in the environment. Cellix has experience in the development of devices for analysis of VOCs from exhaled breath - click here to learn more about SniffPhone, a device for detection of gastric cancer.
Mass Spectrometry
Step 1:
Water vapour molecules are injected into the ion source region, where hydronium ions (H3O+) are generated by a hollow cathode discharge.
Step 2:
Hydronium ions (blue) react with analyses (yellow) injected directly into the drift tube. As proton transfer reaction occurs, analytes are ionised (green).
Step 3:
Ions are separated in a mass analyser according to their mass to charge ratio (m/z), and transferred to a detector for identification.
WHAT DO I NEED TO GET STARTED?
​We can provide you with the necessary set-up to connect to your mass spectrometer or gas sensor. Click here to see an example experimental set-up.
In general, as a minimum, you will need the following to conduct analysis of VOCs:
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1 x Microfluidic Pressure Pump (with compressor): this is essential for flow control of the VOC sample. We recommend pressure pumps as they enable analysis of a large volume of VOC sample over a long period of time. Pressure pumps pressurise the reservoir in which the sample is held; this effectively pushes the fluid into the tubing which is connected to your mass spectrometer or gas sensor. Your choice of pressure pump depends on the number of samples you wish to analyse:
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​4U Pressure Pump will enable you to analyse up to 4 separate VOC samples at one time.
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UniGo pressure pump enables analysis of 1 VOC sample at a time but up to 4 UniGo pumps can be racked together to increase your throughput. This is a more budget-friendly option but allows you to expand in the future if your VOC sample analysis increases.
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1 x Flow sensor: required for active feedback of the flow control of the VOC sample. Fluidic capacitance in microfluidic devices can often introduce instabilities when trying to vary flow rates. Therefore, it is extremely important to have accurate microfluidic pumps with flow sensors to limit such instabilities. Check our Cellix's Top Tips below for calibrating flow sensors for different types of VOC.
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1 x Microfluidic Syringe Pump for flow sensor calibration: We recommend the ExiGo syringe pump.​
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Tubing: to connect from your pump to sample reservoirs, flow sensors and mass spectrometer / gas sensor.
EXAMPLE EXPERIMENTAL SET-UP:
Analysis of 3 VOC samples pre-dissolved in water
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1 x 4U Pressure Pump (with compressor)
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3 x Sample reservoirs
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3 x Flow Sensors
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1 x Vaporizer (with air supply)
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1 x Mass Spectrometery or Gas Sensor.
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Tubing & Connections.
Mass Spectrometry Results
APPLICATIONS OF VOC ANALYSIS
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Environmental research, e.g. air quality.
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Food / Flavour design - e.g. creating the perfect sensory profile for foods.
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Household products: paints, varnishes, cleaning detergents, cosmetics etc.
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Exhaled breath VOC analysis - "breath biopsy"
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Biological research
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Security: detection of harmful VOCs
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Cleanroom monitoring
CELLIX TOP TIPS - Resolve common challenges with VOC Analysis
1. Flow Sensor Calibration - dependent on VOC type: Flow sensors are usually calibrated for aqueous solutions. You must ensure that the flow sensor is calibrated for your sample type.
Cellix's top tips:
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Use ExiGo pump to perfuse sample from glass syringe (with stainless steel plunger; inert) through the flow sensor you want to calibrate.
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Use SmartFlo program, using the sensor calibration step: for example for the 7uL/min sensor, the pump will perfuse in steps of 1uL/min and record the corresponding curve produced by the sensor.
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The SmartFlo program will save the calibration coefficients for that specific VOC sample and that specific flow sensor. Make sure you mark or label your flow sensor so you don't mix it up with another one when doing your experiments.
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You can use this calibration coefficient again for another flow sensor if you are using the same VOC sample type and the same flow sensor type / range.
Flow sensor calibration
with ExiGo syringe pump.
2. Use inert tubing: It is possible that the VOC may react with the tubing in your experimental set-up; the solvent in which the VOC is dissolved can degrade the tubing. Examples include PVC tubing (commonly known as Tygon tubing) which can react with many VOCs such as chlorine or ammonia; or tubing that contains silicone where silicone vapours may be off-gassed from the tubing and react with the sensor.
Cellix's top tips:
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Our top recommendation for tubing is PEEK: this is a highly chemically-resistant tubing which is also biocompatible and easily withstands high pressures.
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Use high-quality tubing suitable for the VOC you are measuring. A good choice for reactive VOCs is a Teflon-based (PTFE) or Teflon-lined material.
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Polyurethane-based tubing is more suitable for some reactive gases but be careful as it can attract moisture in some humid environments. This can reduce the concentration of the VOC, thus negatively impacting your readings.
Selecting the correct tubing
will ensure accurate
VOC measurements.
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