Measuring and testing – Gas analysis – Solid content of gas
Reexamination Certificate
2002-01-25
2004-05-11
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Solid content of gas
C073S028010, C073S023200
Reexamination Certificate
active
06732569
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to chemical analysis of the elemental composition of atmospheric aerosol particles and, more particularly, to a system and method for collecting sub-hourly ambient aerosol for elemental and chemical analysis.
The present invention also relates to the automated sampling of ambient aerosol particles in a manner suitable for multi-element analysis of collections made at sub-hourly intervals with commonly available laboratory analysis instruments, wherein the samples are collected with sufficient efficiency to be regarded as quantitative.
Even more particularly, the present invention relates to a system and method which allows removal of aerosol particles from ambient air introduction of them into an aqueous slurry at a concentration suitable for analysis by a broad range of off-the-shelf analytical instrumentation for elemental and chemical compound or chemical ion analysis, as well as for toxicological testing.
The present invention further relates to a monitoring system and method for determination of trace elements and heavy metals in ambient aerosol particles where the collected samples may be stored either in a series of vials for off-line analysis and testing without interfacing problems, or can be introduced directly into analytical instruments for near-real-time on-line analysis.
BACKGROUND OF THE INVENTION
Measurements of airborne concentrations of major, minor, and trace elements are clearly needed for assessments of human health risk, environmental contamination, compliance with National Air Quality Standards (NAAQS), and source attribution by receptor modeling techniques. Important issues include improvements in the characterization of the temporal history of aerosol exposure and in the linkage of the levels of aerosol particles and their toxic constituents to their sources, i.e., improved source-receptor relationships. Some states, including Maryland are considering continuous source monitoring for toxic air pollutants.
With regard to health risks, residental, workplace, and ambient outdoor environments are of interest. Most elements of environmental interest reside on aerosol particles, usually those with diameters smaller than 10 &mgr;m. Elements of interest include Pb (a criteria pollutant); elements listed as “air toxins”, e.g., Cd, Cr, Cu, Ni, As, Se, and Hg; essential nutrients, e.g., Fe; and those toxic to marine organisms, e.g., Al. Elements of interest to the receptor modeling community include all of the aforementioned and much of the rest of the periodic table. Since many elements must be determined simultaneously, requirements for receptor modeling are perhaps the most demanding. Average concentrations of these elements in the urban and rural areas usually are of the order of 0.1 ng/m
3
for Cd, Co, and Ag; 1 to 5 ng/m
3
for As, Cr, Mn, Pb, Se, and V, about 10 ng/m
3
for Ti and Zn; and 100 ng/m
3
for Al and Fe.
At the above-mentioned concentrations, continuous or even near-continuous monitors are virtually nonexistent. Instead, air sampling for elemental analysis typically involves preconcentration of the aerosol by filtration or impaction, followed by off-line analysis by highly sensitive analytical techniques such as X-ray fluorescence (XRF), instrumental neutron activation analysis (INAA), atomic spectroscopy techniques (atomic absorption and atomic emission), and mass spectrometry (e.g., inductively-coupled plasma-mass spectrometry, ICPMS). This is generally performed with dichotomous and impactor samplers which size-fractionate aerosol particles (thereby increasing required analytical sensitivity) and can accommodate low-mass, low-blank substrates, e.g., Gelman “Teflo” PTFE filters. Standard samplers use 47- or 37-mm substrates and typically operate at from 17 to 40 L/min, although a flow rate of 80 L/min is achieved with 47-mm “Teflo” filters in the UMCP dichotomous sampler (Wu, et al. “Chesapeake Bay Atmospheric Deposition Study, Year 1: Sources and Deposition of Selected Elements in Aerosol Particles”, Atmos. Environ, 28:1471-1486, 1994). As a result, sample collection times, as long as 6-24 hours, are much longer than time scales for changes in source strengths and important meteorological parameters, e.g., wind direction, mixing height, temperature, and relative humidity (RH).
Multi-elemental analysis is often performed first by a conventional energy-dispersive (ED) XRF technique, followed by INAA. The two techniques are complimentary and can provide concentrations for more than 40 elements in typical air samples. The XRF technique provides good sensitivity for Pb, Cu, Ni, S, and Si, i.e., elements which are not readily detected by INAA while INAA provides many more elements for which XRF is insensitive, e.g., V, As, Se, Sb, and rare earths. However, neither technique routinely provides Cd data in typical air samples. In Maryland (where the average airborne Cd concentration is only about 0.1 ng m
3
), the far superior performance of Graphite-Furnace atomic absorption (with Zeeman background correction) is routinely used for Cd analyses. This is accomplished subsequent to sample dissolution and removal of silica and carbon using nitric, perchloric, and hydrofluoric acids. With this type of methodology, multi-elemental analyses are greatly time consuming and, thus expensive, which makes high temporal resolution often impractical, simply from the standpoint of analytical costs. Additionally, when samples are dissolved prior to analysis by GFAA, only a small fraction of the sample (typically 5 to 20 &mgr;L) is introduced into the furnace. To overcome this deficiency, techniques have been devised for introducing atmospheric particulate matter directly without dissolution, thus reducing air sampling requirements substantially.
As described in Sneddon (“Direct collection of lead in the atmosphere by impaction for determination by electrothermal atomization atomic absorption spectrometry,” Anal. Chem., 56:1982-1986, 1984; “Use of an impaction-electrothermal atomization atomic absorption spectrometric system for the direct determination of cadmium, copper, and manganese in the laboratory atmosphere,” Anal. Letters, 18:1261-1280, 1985; “Direct and near real time determination of metallic compounds in the atmosphere by AA”, American Laboratory, 18 (3): 43-50 1986; “Direct and near real-time determination of metals in the atmosphere by atomic spectroscopic techniques,” Trends in Anal. Chem., 7:222-226, 1988; “Collection efficiency of an impaction-electrothermal atomization atomic absorption spectrometric system for the direct and near-real-time determination of metals in aerosols: some preliminary results,” Appl. Spectrosc., 43:1100-1102, 1988; and “Multielement atomic absorption spectrometry, a historical perspective,” Microchem J., 48:318-325, 1993) a single, tantalum-jet impactor was designed to fit in the sample introduction port of a graphite furnace platform. An air-tight container was used to house the platform during sample collection. Sampling rates were limited by the small size of the 8-mm-id platform thus limiting the ability to aspirate ambient aerosol at rates only up to 15 L/min. After collection, the platform was removed from the sample collection housing and placed in the furnace for single-element analysis of the sample.
Sneddon reported detection limits in ng/m
3
for 17 elements including Cr, Co, Fe, Ni, Pb, Se, Si, Sn, and Zn based on direct, sequential, single-element analysis of aerosol particles collected for 5 min at an air flow rate of 10 L/min using an Instrumentation laboratory 457 AA with model 655 graphite furnace. Sneddon's detection limits are the concentration giving a signal-to-noise ratio of 3. Except for Zn and Fe, these detection limits exceed ambient concentrations by from 10- to 20-fold, indicating that samples would need to be collected for from about 50 to 100 minutes to achieve analyses at the detection limit for elements occurring in the atmosphere at their average concentrations; while concentrations less than the average would not be detected. To achie
Kidwell Christopher B.
Ondov John M.
Tuch Thomas M.
Frank Rodney T.
Rosenberg , Klein & Lee
University of Maryland
Williams Hezron
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