Carry-over effects of the membrane interface probe.

Area of Science:

• Environmental Science
• Geochemistry
• Analytical Chemistry

Background:

• Membrane Interface Probes (MIP) are standard for mapping subsurface volatile organic compounds (VOCs).
• A significant challenge with MIP is the 'carry-over effect,' where signals persist after leaving a contaminated zone, leading to inaccurate depth profiling.
• This effect is primarily due to compound retention times in unheated transfer lines.

Purpose of the Study:

• To conduct a field evaluation of the carry-over effect in conventional MIP systems.
• To assess the impact of unheated transfer lines on MIP signal accuracy.
• To explore methods for improving the interpretation of MIP data.

Main Methods:

• A conventional MIP system was coupled with a mobile mass spectrometer for real-time analysis.
• Field data from the coupled MIP-mass spectrometer system were compared with results from a laser-induced fluorescence (LIF) system.
• Multidirectional probing (forward and backward) was employed alongside conventional MIP probing.

Main Results:

• The study confirmed that carry-over effects cause complex signal superpositions, blurring MIP data beneath VOC source zones.
• MIP signals were found to be not representative of specific depths in the saturated zone due to this superposition.
• Multidirectional probing successfully identified the upper and lower boundaries of VOC source zones.
• MIP results using this advanced method showed excellent correlation with LIF data.

Conclusions:

• Coupling a mobile mass spectrometer to a MIP system significantly enhances the interpretation of MIP signals.
• The carry-over effect can be managed, allowing for more accurate characterization of subsurface VOC contamination.
• Advanced MIP techniques, including multidirectional probing, provide reliable data for delineating contaminant source zones.