Direct Push Analytical’s Membrane Interface Probe (MIP) Capabilities:
general, the Membrane
Interface Probe (MIP) system has the ability to detect concentrations
of Volatile Organic Constituents in the soil down to concentrations
of roughly 200-500 ppb, depending on the compound and the detector
used for the analysis. The results obtained from the MIP system
are only semi-quantitative as each chemical has a different transfer
rate across the MIP membrane and each compound reacts differently
in different sub-surface mediums. This transfer rate simplistically
- the volatility of the compound
- the solubility of the compound
- it’s concentration in the surrounding lithology
- the permeability of the material in contact with the probe
- the gas flow in the MIP system
- the status of the membrane
One of the options available for MIP analysis is to use two detectors in series to extend the number of compounds that can potentially be identified in the subsurface soil/water zones. By combining a Photo-Ionization Detector (PID) in series with an Electron Capture Detector (ECD) or a Flame Ionization Detector (FID), the working range of the system can be increased and a greater number of compounds can be detected in the subsurface lithology.
A proven technology available through Direct Push Analytical that greatly enhances the capability of the MIP system is to couple the MIP instrumentation with the Mobile Laboratory GC/MS for compound identification. This technique provides valuable information when degradation products or metabolic compounds are suspected. This is invaluable for identifying unknown contamination sources. The gas stream from the MIP system is first passed through a detector to determine the presence of potential target compounds at specific depths. The gas that exits the detector is then passed through a trap designed to retain volatile compounds. Method specific surrogates and internal standards can be manually added to the trap just prior to analysis and the compounds on the trap are then desorbed into the GC/MS system by heating the trap to a specified temperature for a specific time. From this point, the analysis proceeds just like a normal GC/MS analysis for volatile compounds.
Results using the GC/MS system are semi-quantitative and relate only to the amount of material transferred to the analytical system, therefore a variety of options are available to increase the overall sensitivity of the combined system. Each option can add a degree of complexity to the overall project, but has the added benefit of providing a greater degree of detail to the results achieved. Typical “on-column” sensitivities of 2-5ng per compound are common for the GC/MS system employed. Three common techniques for increasing the sensitivity of the system are the use of SIM for specific compound identification, collecting a larger sample for analysis from the MIP system by varying the speed of the MIP system probe advancement, or using larger collection intervals for the analysis.
Lastly, QA/QC for this type of analysis is inherently limited. There are no expeditious means of performing LCS, sample duplicate, Matrix Spike, and Matrix Spike Duplicate analysis using the GC/MS in conjunction with the MIP system. Each QC sample would have to be pushed through an identical contamination zone with the assumption that all analytes were present in the zone at comparable concentrations and that the transfer rate for compounds was exactly the same for each compound on each push of the probe. If QA/QC samples are critical, then the mobile lab is the next best alternative.