Speciation of chromium in environmental media using capillary electrophoresis with  multiple wavelength UV/visible detection

     The new Beckman-Coulter capillary electrophoresis instrument was delivered in January 2002, and we now have eight months of experience with its use.  It had been our expectation that multiwavelength detection would be especially useful in the region between 300 and 600 nm, since this is the wavelength range employed most extensively by inorganic spectroscopists.  Our experience is that molar absorptivities of Cr^III complexes are typically not high enough in this range to be analytically useful.  On the other hand, we have found that multiwavelength detection between 190 and 300 nm is enormously useful.  Our current practice is to collect a full spectrum on each electromigrating peak.  Calibration curves can then be generated using the wavelength of maximum absorbance (l_max).  Using this approach, our detection limits for dissolved Cr^III complexes have improved more than 10-fold. 
     An illustrative electropherogram using detection at 229 nm is shown in Figure 1.  This wavelength is close to l_max for most Cr^III-containing compounds, but not close to l_max for Cr^VI species.  (Switching to a different wavelength would dramatically increase the area of the peak corresponding to CrO₄^2-).  Complexes with four different synthetic chelating agents (nta, ida, hedta, and edta) are effectively distinguished from one another.  Figure 1 also shows that a 2:2 dimer and 1:1 monomer of Cr^III-nta can be effectively distinguished.  
     Calibration curves (Figure 2) yield detection limits of 1 mM (50 mgCr^.L^-1) for HCrO₄^- at 214 nm and 3 mM (160 mgCr^.L^-1) for Cr^III(OH₂)₃(ida)⁺ at 229 nm.  It should be noted that the analyses shown in Figure 1 and for the HCrO₄^- calibration curve were performed using anion mode, while the calibration curve for Cr^III(OH₂)₃(ida)⁺ was performed using cation mode.

1. Carbonaro, R.F.; Stone, A.T. "Capillary Electrophoresis Analysis of Cr(III) and Cr(VI) Aqueous Speciation", Symposium on Emerging Technologies in Hazardous Waste Management: Current EPA Focus Areas", Division of Industrial & Engineering Chemistry, American Chemical Society 224th National Meeting, Boston, MA (August, 2002).
2. Carbonaro, R.F.; Stone, A.T. "Monitoring Environmental Transformations of Chromium Using Capillary Electrophoresis" [Poster], Environmental Sciences: Water Gordon Conference, Plymouth, NH (June, 2002).
3. Stone, A.T. "Exploring Transition Metal Ion Reactions In Aquatic Environments - New Insights From Capillary Electrophoresis", Department of Chemistry, Ohio State University, Columbus, OH (March, 2002).

1. Preparation and analysis of Cr^III complexes with oxalate and citrate.  These two naturally-occurring chelating agents are released by the roots of most vascular plants.  Plants growing on chromium-contaminated soils may potentially generate Cr^III-oxalate and Cr^III-citrate complexes
2. Cr^III complexes with relatively weak chelating agents (e.g. oxalate) will be added to natural organic matter-containing solutions from the Great Dismal Swamp (Virginia).  Using CE, we will then monitor loss of the Cr^III-chelating agent peak and appearance of new Cr^III-natural organic matter peaks
3. Cr^VI (chromate ion) will also be added to natural organic matter-containing solutions.  In this case, loss of Cr^VI and the appearance of new Cr^III-natural organic matter complexes will be monitored by CE.