Improving the Confidence of Unknown Compound Identification by First Responder Mobile GC–MS Laboratories...

Sep 01, 2008
Volume 26, Issue 9, pg 938–945

Improving the Confidence of Unknown Compound Identification by First Responder Mobile GC-MS Laboratories in Time-Critical Environmental and Homeland Security Incidents



The Connecticut Department of Environmental Protection (CT DEP) located in Windsor, Connecticut, utilizes a unique mobile laboratory equipped with state-of-the-art analytical instrumentation, including gas chromatography–mass spectrometry (GC–MS) systems charged with the on-site, rapid identification of unknown chemical compounds in time-critical environmental and homeland security incidents. The CT DEP emergency responder team is always on call and typically is called in by firefighters and local or federal law enforcement officials in situations where unknown and possibly dangerous chemicals are encountered. Their charge is to provide rapid identification of these substances to allow incident commanders and law enforcement agents to respond appropriately. Rapid and accurate identification is critical in these situations, as this quickly can determine the difference between the need for simple controlled disposal of nontoxic substances to the potential evacuation of entire neighborhoods.

GC–MS is the tool of choice for the rapid identification of volatile and semivolatile organic compounds. Standard EPA methods cover the identification of over 200 compounds of environmental concern. These methods provide extremely sensitive and accurate identifications of these target compounds based upon their retention time and GC–MS library search match. However, for compounds that are not characterized by these methods, a significant uncertainty can exist for these "unknown" compounds when using GClibrary search routines alone for identification. Misidentification can lead to an expensive and potentially dangerous or fatal incorrect response by first responders and law enforcement. Uncertainties or delays in confident identification also can contribute to high costs and consume valuable public resources.

To increase the likelihood of a correct unknown identification, many analytical methods require a second confirmatory analysis, for example, GC–MS library matching followed by formula confirmation by accurate mass measurement resulting in the assignment of an unambiguous elemental composition. For first responder chemical identification, the challenge is further complicated by the fact that the analysis must have a high degree of confidence, it must be done rapidly (typically within 1–3 h), and it must rely on the analytical equipment available in the mobile laboratory. For these applications, a tool for an extra level of confirmation is needed on the GC–MS instrument.

Recent advances in MS data processing have shown the potential for obtaining accurate formula identification of unknowns by unit resolution MS (1). The application is based upon a novel method for accurate mass and accurate line-shape calibration for unit resolution mass spectrometers that allows both the accurate mass and accurate isotope pattern matching (spectral accuracy) to be used for formula identification, competitive to results obtainable by high-resolution accurate mass instrumentation.

This article presents a new method for increasing the speed and accuracy of unknown identification by GC–MS by utilizing the standard GC–MS library search, combined with confirmation of the result by formula identification on a standard unit resolution single-quadrupole GC–MS instrument. The method is based upon the standard EPA method for volatile organic compound (VOC) analysis (2) (EPA 8260B) and will be designated here as EVOCA (extended VOC analysis).

Experimental

A test solution for evaluating EVOCA was prepared using the standard EPA 8260B protocol utilizing seven surrogate compounds and internal standards. A test solution was created from 73 target compounds of environmental concern and an additional 17 compounds in separate runs, for a total of 90 compounds to evaluate the method.


Figure 1
The 73 target compound test mixture was prepared at approximately 20 μg/L for each component in 10 mL water and introduced through a purge-and-trap system with approximately 4 ng per component introduced into the GC–MS instrument through a 50:1 GC split. The mixture of 17 additional compounds was prepared at approximately 2000 μg/mL and introduced by a 1-μL injection through the same splitter allowing about 40 ng per component into the mass spectrometer.


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