Continuous Monitoring of Vinyl Chloride and Ethylene Dichloride in the Air

Vinyl chloride (VCM) and ethylene dichloride (EDC) are critical industrial chemicals, primarily used in the production of polyvinyl chloride (PVC). Due to their carcinogenic properties and stringent regulatory exposure limits, continuous monitoring using mass spectrometry is essential to help ensure workplace safety and compliance. This technology provides real-time detection and differentiation of VCM and EDC, safeguarding workers from harmful exposure.

Introduction

Vinyl chloride, also known as vinyl chloride monomer (VCM) or chloroethene, is a crucial industrial chemical primarily used in the production of polyvinyl chloride (PVC). With an annual global consumption of approximately 34 million tons, PVC is the third-most widely produced synthetic plastic polymer. Vinyl chloride is a highly flammable and potentially explosive gas, produced by reacting chlorine and ethylene to initially form ethylene dichloride (EDC), also known as 1,2-dichloroethane (DCE). The EDC is then cracked at high temperatures and pressures to produce vinyl chloride and hydrogen chloride.

Workplace Monitoring of Vinyl Chloride and Ethylene Dichloride

Early laboratory studies identified vinyl chloride as a carcinogen, capable of causing angiosarcoma, a cancer of the liver’s blood vessels. The first fatalities linked to VCM exposure were reported in the 1970s, prompting significant changes in production processes to protect workers. Governments worldwide began mandating maximum allowable industrial exposure levels to safeguard workers’ health.

The US Occupational Safety and Health Administration (OSHA) established a short-term exposure limit for vinyl chloride at 5 ppm (12.8 mg/m³) for any 15-minute period. Additionally, OSHA specifies that no employee may be exposed to vinyl chloride concentrations greater than 1 ppm (2.56 mg/m³) averaged over any 8-hour period. An “Action Level” of 0.5 ppm as an 8-hour time-weighted average indicates the VCM level requiring medical surveillance, increased industrial hygiene monitoring, or biological monitoring. The US National Institute for Occupational Safety and Health (NIOSH) classifies VCM as a ‘Potential Occupational Carcinogen’ and recommends minimizing exposure limits to the lowest feasible level.

Challenges in Monitoring

Monitoring systems for VCM must differentiate between VCM and EDC to avoid false alarms caused by EDC releases, which, while potentially high, may still be below the permitted exposure level. Effective monitoring is crucial to ensure workplace safety and compliance with regulatory standards.

Mass Spectrometry for Monitoring

Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of ions to identify and quantify molecules in a sample. It involves ionizing the sample, separating the ions based on their mass-to-charge ratio using a mass analyzer, and detecting them to generate a mass spectrum. This technique is highly sensitive and widely used in various fields for precise molecular analysis.

Mass spectrometers monitor vinyl chloride (VCM) and ethylene dichloride (EDC) by leveraging their ability to detect and quantify chemical compounds based on their mass-to-charge ratio (m/z). Here’s a concise explanation of the process:

1. Sample Introduction: Air samples from the environment are continuously drawn into the mass spectrometer. This can be done through a Rapid Multistream Sampler (RMS) which allows for the rapid sequential monitoring of multiple air streams.
2. Ionization: The air sample containing VCM and EDC is introduced into the ionization chamber of the mass spectrometer. Here, the molecules are ionized, typically using an electron impact ionization method. This process involves bombarding the molecules with high-energy electrons, causing them to lose electrons and form positively charged ions.
3. Mass Analysis: The ionized molecules are then directed into the mass analyzer, which separates the ions based on their mass-to-charge ratio. The most reliable and precise type of mass analyzer is the scanning magnetic sector, applying a variable magnetic field to the ions, causing them to follow curved paths. The radius of these paths depends on the mass-to-charge ratio of the ions.
4. Detection: As the ions are separated, they reach the detector at different times. The detector measures the intensity of the ions, which is proportional to the concentration of the respective compounds in the sample. The mass spectrometer generates a mass spectrum, which is a plot of ion intensity versus mass-to-charge ratio.
5. Data Analysis: The mass spectrum is analyzed to identify and quantify the specific compounds present in the air sample. Each compound, including VCM and EDC, has a unique mass spectrum, allowing for precise identification. The concentrations of VCM and EDC are determined by comparing the detected ion intensities to calibration curves obtained from precise standard gases.
6. Real-Time Monitoring: Continuous, real-time monitoring enables immediate detection of any deviations from safe exposure levels, allowing for prompt corrective actions to mitigate risks.

Summary

By using mass spectrometry, industries can help ensure compliance with regulatory standards and protect workers from the harmful effects of VCM and EDC exposure. The high sensitivity and specificity of mass spectrometers make them ideal for detecting even trace amounts of these volatile organic compounds in the workplace environment.

You can read additional information about continuous monitoring of vinyl chloride and ethylene dichloride in air in our Application note Sentinel PRO Process Mass Spectrometer Continuous monitoring of vinyl chloride and ethylene dichloride in air. It includes exposure limits, composite mass spectrum of ethylene dichloride and vinyl chloride (NIST), stability charts, analysis specifications, instruments used, analysis configuration for VCM, ECF and VA, and more.

Resources and References

• Application note: Sentinel PRO Process Mass Spectrometer Continuous monitoring of vinyl chloride and ethylene dichloride in air. https://documents.thermofisher.com/TFS-Assets/LSG/Application-Notes/epm-an-sentinel.pdf
• Thermo Scientific™ Sentinel PRO 710 Mass Spectrometer
• Frequently asked questions Prima PRO 700 & 710, Sentinel PRO 700 & 710
  

• US Occupational Safety and Health Administration (OSHA). “Permissible Exposure Limits (PELs).”  https://www.osha.gov/
• US National Institute for Occupational Safety and Health (NIOSH). “Recommended Exposure Limits (RELs).”  https://www.cdc.gov/niosh