Producing Reliable Trace VOC Analysis

Being able to accurately determine the levels of trace Volatile Organic Compounds (VOCs) is important for environmental and air quality research. This article will cover VOC research carried out by the Key-VOCs project.

This article will talk about:

• The correct test method for VOC analysis
• How the results of VOC analysis can be affected by surface reactivity
• The measurement methods developed by the VOC team to determine the level of interaction between VOCs and surfaces
• Using inert coatings to provide more accurate measurements (when compared to conventional methods)

The Key VOCs team, with funding from the DWD of the German Metrological Service (Meteorological Observatory Hohenpeissenberg), EURAMET and the European Metrology Research Program, conducted a comprehensive study on VOCs. The resulting report “Metrology for VOC Indicators in Air Pollution and Climate Change” has had a large impact on the field of trace VOC measurement.

The research team worked on novel test methods for the quantification of environmental VOC interactions and losses. The team also determined suitable materials, which can be used in measurement devices, to minimize VOC absorption and make measurements more accurate. This allowed them to build new sensor-based systems for the determination of key environmental VOCs.

The Key VOCs team decided to use coatings from SilcoTek in their research because they provided the inertness needed to prevent VOC absorption in the sample path, and for the support and knowledge they can provide.

Why is the Measurement of Trace VOCs Important?

VOCs are strongly associated with polluted air, free radical and ozone related pollution, health problems and greenhouse gas generation. The World Health Organization states that in 2012 alone approximately 7 million people died as result of air pollution, most of which was from fossil fuel combustion, which results in VOC emission.

VOCs, which evaporate from household sources and are also emitted by industry during production, also have a negative impact on the quality of indoor air.

For these reasons the accurate determination of environmental VOC levels is paramount as this will help to understand their effects better and to reduce their environmental impact. As knowledge of the harm of VOCs has increased, the allowable inside limit of VOCs has become increasingly lower, making the accurate detection of trace limits more important.

A new regulation, EPA 325, which is on fenceline monitoring of refineries, means that analytical systems of high accuracy and reliability are required to safeguard both the environment and businesses. This means that the systems must have highly inert analytical and sample transport areas to ensure accurate results.

VOCs can show reactivity towards and/or be absorbed by most sample pathway surfaces, including aluminum and steel. In instances where analytes traveling down the flow path are absorbed the trace analysis results will not be reliable. Problems that can be encountered include:

• Exposed metals, such as stainless steel, in connectors and valves reacting with VOCs/analytes
• Absorption and desorption of VOCs by the system resulting in false results
• Absorption of VOCs by fritted filters
• Retention and degradation of VOC standards in gas calibration cylinders resulting in changes in calibration gas reliability

The research concluded that different material surfaces in analytical equipment can react with and transform VOCs. This is significant for the transport, storage and analysis of VOC samples and calibration standards.

The research showed that conventional aluminum passivation methods only provide small benefits at trace VOC levels and that the material chosen for transport and tubing has a significant effect on the loss or retention of VOC analytes.

Comparing Passivized Cylinders for the Storage and Calibration of VOCs

The research team focused on the adsorption and loss of VOCs in gas cylinders. They studied four different passivated aluminum cylinders from market leaders such as Air Liquide, Linde, Air Products and Takachiho; to remove bias these cylinders were randomized and labeled A, B, C and D.

An additional cylinder, made of stainless steel coated with SilcoNert^®, was labeled SW.

The different cylinders are listed below. It should be noted that the SilcoNert 2000 cylinder (here named Sulfinert^®) is significantly smaller than the other cylinders. This means the reaction surface area is impacted as the smaller SilcoNert cylinder has a greater volume to wall surface area ratio, meaning (in this test) it is at a disadvantage.

The study noted the following test parameters: "The first part of the research consisted of quantitatively studying the adsorption of VOC at trace levels for a selection of different aluminum and stainless steel pressurized cylinders having proprietary passivation treatments.”

“A batch of gas mixtures containing several OVOCs, including methanol, ethanol and acetone, at 100 nmol/mol, a batch containing formaldehyde (F) at 1 µmol/mol and a batch containing several monoterpenes (MT) at 2 nmol/mol have been prepared using state-of-the-art gravimetric preparation methods (ISO 6142-1)."

The report diagrammed the decanting and analysis below.*

The study went on to clarify the decanting result summary as follows, "When the decanting effect (deviation of the concentrations of the daughter cylinder/mixture from the mother cylinder/mixture) is within 5%, the symbol “+” is used. This symbol indicates that the type of cylinder is suitable for the component; when the decanting effect is between 5% and 10%, the symbol “-” is used.”

“This symbol indicates that the type of cylinder is less suitable. When the decanting effect is above 10%, the symbol “▬” is used. This symbol indicates that the type of cylinder is not suitable for the tested component. The symbol “(-)” means that the effect is close to “less suitable”, and the symbol “(+)” close to “suitable”.*”

The results given show that the SilcoNert 2000 coating performed better than most passivation treatments. This is in spite of the fact that the smaller size of the cylinder put it at an initial disadvantage.

Comparing Performance with Methanol

The research team then compared the performance of uncoated stainless steel tubing (10 m) with the same tubing coated with SilcoNert 2000 (results below). To do this they passed a methanol mixture (180 umol/mol) through a bypass (the grey region) into the test tubing. The team measured the time taken for a baseline concentration to be reached in each system.

The ‘naked’ stainless steel tube (A) took over 40 minutes to stabilize whereas the SilcoNert coated tube took less than one minute (B). This research shows the strong adsorption of trace methanol by uncoated stainless steel and demonstrates the response delays which can result in erroneous data for trace VOC analysis.

The research found that in terms of low methanol absorption polymer lines (e.g. FEP and PTFE) were the most effective; however, they have limited use in VOC analysis as they have issues with durability, permeability and a low temperature stability.

It was demonstrated that at temperatures higher than 50 °C other trace compounds leached from the polymer resulting in false peaks. This is in comparison to SilcoNert which is stable to 450 °C with no leeching during heating.

The study stated: "The adsorption by most polymers such as PTFE and FEP is extremely low, while for uncoated metals it is very high. Coated stainless steel with Silconert-2000 and Sulfinert are the preferred choice, as they also have the benefit of being robust non-permeating materials."

Choosing the Correct Surface for VOC Analysis

The selection of the right materials, of high durability and inertness, for the internal flow path of a surface is important for accurate analyses. Research conducted by Stefan Persijn from the Dutch Metrology Institute (alongside the German DWD) compared the absorptive properties of different materials used for VOC analysis.

This research involved exposing different tube surfaces to methanol at 50 °C and 100 °C, and comparing the different adsorption rates. The research demonstrated that using an inert coating (such as SilcoNert^®) on stainless steel stopped the absorption and surface reactions of VOCs. The table below demonstrates these results.

It was found that ‘naked’ stainless steel will absorb active compounds and completely absorbs methanol; this effect is reduced with electropolishing (which reduces the surface area) though the level of adsorption is still high. It was also found that PEEK and PFA surfaces must be carefully managed to provide adequate results.

Stainless steel with a surface coated with Sulfinert^® has been shown to not absorb any active compounds. This is because the silicon coating bonds with active sites in the steel, diffusing into the metal and preventing further interactions. The sputter depth profile shown below shows the diffusion profile of a typical SilcoTek coating developed with chemical vapor deposition.

The Important Factor for VOC Analysis Systems – An Inert Surface

All of the flow path should be coated to ensure that the selective adsorption of analytes does not occur. The failure to coat fritted filters can have a large detrimental impact on the results of analysis, requires more system maintenance, the need for retesting, false negatives and positives, issues with compliance to regulations and failed calibrations.  

Flow path surfaces that should be coated include:

• Tubing
• Fittings
• Flow control and regulators
• Valves
• Sample cylinders
• GC, FT-IR and other instrument parts
• Liners – the coating of glass is possible
• Sample containers

This information has been sourced, reviewed and adapted from materials provided by SilcoTek.

For more information on this source, please visit SilcoTek.