Steven A. Hendrix
Associate Professor of Chemistry, Analytical Chemistry and Instrumental Chemistry
Director, Laboratory for Atmospheric and Marine Analysis
- 1984 University of South Florida, B.A. Chemistry
- 1990 University of South Florida, Ph.D. Chemistry; Research adviser: Robert S. Braman; "Development of New Analytical Techniques for NOx Speciation in the Environment
Nitrogen oxides are important participants in the chemistry of both clean and polluted atmospheres, and ways to analyze these compounds with speed and low limits of detection are constantly being sought. The major hurdles in improving analysis and interpretation are adequate speciation, interferences and achieving lower limits of detection.
At UT, we use a unique method of analysis utilizing hollow glass tubes that are interior-coated with chemical compounds, which will collect individual NOx gases from air. These pre-concentration tubes are heated for analysis, with the collected nitrogen oxide gases thermally desorbing as NO, which is then measured with a chemiluminescence analyzer. This technique was invented in the research lab of Robert S. Braman at the University of South Florida in the early 1980’s, and is still used today by NASA to measure HNO3 and NH3 in the stratosphere. Additional hollow tubes were subsequently developed that collect HNO2, NO2, NO, and particulate NO3- and NH4+.
In addition to studying the chemistry of NOx gases, the ions nitrate (NO3-) and nitrite (NO2-) are also important species of interest as they pertain to both freshwater and saltwater ecosystems. Again utilizing chemiluminescence detection, these ions can be analyzed quickly and at very low concentrations via a unique transition metal reduction method. The use of vanadium(III) has been shown to be applicable to a wide variety of sample types, including seawater, human fluids, food products and plant material.
In the 1990's, UT supplied correlation data to the U.S. Navy using this method as part of an autonomous underwater vehicle development program in collaboration with USF, which would, when complete, provide in situ nutrient analyses in real time for use in both tactical and oceanographic applications.
- We are developing a computer controlled, automatic, semi-continuous NOx analysis system utilizing the hollow tube technology that will provide speciation and concentration information for the NOx gases and particulate species in outdoor locations over long periods of time. This kind of data is useful in understanding the sources and fates of these primary and secondary pollutants, particularly here in an urban environment. This type of system requires knowledge of digital devices and their application to chemical instrumental analysis design.
- We are still working with marine scientists at USF continuing to use the UT chemiluminescence analysis system to both ground truth other methods of nutrient analysis currently used by oceanographers, and to study trends in nitrate and nitrite distributions in seawater. In 2009, a real-time nitrate/salinity/temperature monitoring station was installed on the UT campus along the banks of the Hillsborough River. This satellite-based station is part of a network of monitoring sites established by the USF College of Marine Science along the Florida coastline. We are cross-analyzing samples in order to correlate results obtained by the in situ sensor installed underwater at the monitoring site near the UT docks.
Articles of Interest
Hendrix, Steven A. ;Braman, Robert S., "NOx Speciation in the Southeastern Gulf of Mexico", Florida Scientist, 1995, 58, 292.
Braman, Robert S.; Hendrix, Steven A. "Nanogram Nitrite and Nitrate Determination in Environmental and Biological Materials by Vanadium(III) Reduction with Chemiluminescence Detection," Anal. Chem. 1989, 61, 2715.
Braman, R. S.; Shelley, T. J.; McClenny, W. A., "Tungstic Acid Preconcentration and Determination of Gaseous and Particulate Ammonia and Nitric Acid in Ambient Air", Anal. Chem. 1982, 54, 358.
Braman, R. S.; de la Cantera, M. A., "Sequential Selective Hollow Tube Preconcentration and Chemiluminescence Analysis System for Nitrogen Oxide Compounds in Air", Anal. Chem. 1986, 58, 1537.
Hendrix, S.A.,; Masserini, R.T.*, "Application of a Simplified Chemiluminescence Technique for Nitrate and Nitrite Analysis of Oligotrophic Seawater Samples". Manuscript in preparation. *College of Marine Science, University of South Florida, St. Petersburg, FL.