Report
Direct Sampling MS for Environmental Screening DSMS is a simple technique that provides real-time response, high sample throughput, and ppb detection limits at low cost
D
ecades of improper storage and disposal of hazardous materials by government and industry have resulted in ~ 40,000 contaminated sites that will require cleanup by state and federal agencies. Almost 1,300 of these sites are classified as especially serious threats and are listed on the Superfund National Priorities List (NPL) (I) for accelerated remediation; by 2020, it is estimated that the number of NPL sites will exceed 3,000. Considering that ~ 73 million people now live within 4 miles of one or more Superfund sites, the potential for detrimental effects on the public and the economies of nearby communities is profound. Given that an average of 10 years is required to clean up a Superfund site (2), remediation of known hazardous sites will continue well into the next century. Cost projections for environmental restoration range from nearly $200 billion to more than $2 trillion (3). Because budget constraints threaten to slow progress in reducing the public's exposure to environmental hazards, the Departments of Energy (DOE) and Defense (DOD) and the Environmental Protection Agency (EPA) have established programs for developing and demonstrating innovative technologies for faster and less expensive approaches to environmental restoration. This Report discusses screening methods and direct sampling MS (DSMS) for more cost-effective and efficient environmental characterization.
Marcus B. Wise Michael R. Guerin Oak Ridge National Laboratory 26 A
Why screen? Compared with conventional laboratory methods that provide detailed qualitative and quantitative information for a sample,
Analytical Chemistry News & Features, January 1, 1997
screening methods simply indicate whether targeted analytes are present above or below a preset concentration threshold. Field screening is most effective in situations where the immediate availability of results will affect the cost, progress, or safety of site operations (4). For example, sampling crews can use screening to quickly differentiate clean areas from "hot spots" of contamination. More detailed sampling and analysis can then be focused on the contaminated areas. Lengthy sample turnaround time can also cause errors. Up to 80% of the errors in the data for a sample may be attributed to the delay between collection and analysis caused by analyte losses from microbial degradation, absorption, and volatilization (5). Cost is also a factor: Depending on the required method, analysis of a single environmental sample can cost $200-$2,000, and the customer is charged for the analysis whether or not contaminants are present With prudent use of screening, the cost of environmental testing can be reduced by 50% or more (3). MS has been slow to emerge as an environmental screening tool for several reasons. Compared with screening tools such as organic vapor analyzers, field GCs, and immunoassay kits, MS requires a sizeable capital investment In addition, there is a lack of approved screening methods and a perception that MS instruments are too large, complex, and unreliable for practical use in the field. These obstacles have diminished in recent years with improvements in instrumentation and wider acceptance of screening data by regulators. New analyzer technologies, such as quadrupole ion traps (6), have 0003-2700/97/0369-26A/$14.00/0 © 1996 American Chemical Society
also become available that are small and simEliminating the chromatographic sepaple yet provide advanced analytical capabiliration means that the mass spectrum will ties such as MS/MS (7) and selective lowbe a composite spectrum of all the volatile pressure chemical ionization (CI) (8). components in a sample. For screening, this is usually not a problem. Some samples are contaminated with only a few What is DSMS? Direct sampling MS refers to the introduc- compounds, allowing characteristic ions tion of analytes from a sample directly into for each analyte to be used for identificaa mass spectrometer using a simple inter- tion and quantitation. For samples conface with minimal sample preparation and taminated with 10-20 compounds, specno prior chromatographic separation. This tral subtraction routines can be used to deconvolute the spectra and accurately translates into simplicity, real-time response, and high sample throughput capa- quantitate the compounds. At worst, even bility. Multiple inlet configurations permit if a complex contaminant such as gasoline is encountered, DSMS can still provide the screening of most types of environuseful information—non-detects can be mental samples for volatile and semivolareliably identified, hot spots of contaminatileorganics (9). tion can be quickly pinpointed, and speCompared with traditional GC/MS analytical methods such as EPA Methods cific compounds such as alkyl aromatic hydrocarbons can be selectively detected 624 and 8260, which require 30 min or and quantitated if CI and/or MS/MS techmore for analysis, total analysis times usniques are used. ing DSMS range from real-time to only a few minutes for all analytes in a sample. Depending on the inlet and mass analyzer used, detection limits in the low part-perbillion range or better are possible.
Instrumentation
Analyzers and inlets. The most common mass analyzers used for DSMS in-
clude single and triple quadrupoles, ion traps, small single magnetic sector instruments, and smalltime-of-flightinstruments (Table 1). For screening volatile organic compounds (VOCs), a mass range of at least 200 Da is required; for semivolatile organic compounds (SVOCs), the mass range must extend to ~ 600 Da. Multiple ionization options such as electron ionization (EI) and CI provide greater versatility with regard to the range of analytes that can be detected, and MS/MS can dramatically improve specificity for trace components in a complex background. Despite differences in hardware, all DSMS inlet systems share several common functions—they extract analytes directly without sample preparation, function as a transfer system, and act as a protective barrier between atmospheric pressure, in which sampling is performed, and the high vacuum inside the instrument. This function is critical to ensure that the internal spectrometer components are not exposed to excessive oxygen and water, which accelerate their de-
Analytical Chemistry News & Features, January 1, 1997 27 A
Report
T a b l e 1 . C o m m o n l y used i n s t r u m e n t s for D S M S . 3
Size (ft ) Weight (lbs.) Power consumption (W) Ionization modes Mass range (Da) Detection limits Benefits
Limitations
Linear quadrupole
Triple quadrupole
Quadrupole ion trap
Time of flight
Magnetic sector
3 60 150-500
20 300 and up > 500
8 100-150 300-500
1.5 and up 40 and up > 50
1.5 and up 40 and up > 50
El, CI, API, GD
El, CI, API, GD
El, low-pressure CI, El, laser
1O—1000
1 o—1000
pptr-ppm pptr-ppm Rugged, transportMS/MS capable able, commercially available as a field instrument Less selective than Field use requires MS/MS large vehicle, complicated to operate
terioration. Detection limits using these inlets are generally at the part-ber-billion level or lower. Sample turnaround times are generally 2-5 min. There are four major types of inlets for direct sampling. Capillary restrictors, a good choice for sampling polar and nonpolar compounds, consist of a 10- -o 20cm length oo narrow-bore (50-150 um) deactivated fusedsilica capillary that extends from the atmosphere into the ion source. The capillary limits the gas flow into the instrument to 0.1 to 1.0 mL/min, which ii scmpatible with instruments equipped with conventional EI or CI sources. Modular sampling devices and a gaa flow splitter allow the inlet configuration to be quickly changed for sampling different media (9) The primary disadvantage of capillary restrictors is that air and water vapor enter the mass spectrometer during sample analysis Membrane inlets use a thin synthetic membrane to extract analytes from a sample and directly introduce them into the spectrometer while blocking the flow of air and liquids (10), which reduces the pumping requirements of the instrument and can increase the lifetime of the ion source and detector. Membrane inlets can be used for screening organics in air, water, and soil samples. Their major disadvantages include slower response than capillary inlets, temperature-dependent performance and selectivity between polar and nonpolar compounds Sudden rupture of a membrane also produce a catastrophic failure of the spectrometer 28 A
1 O — ObO
1 b —
