Cr(VI) Measurements: Feasibility, Fate, and Stability Ruth E. Wolf, Suzette A. Morman, and Geoffrey S. Plumlee USGS Denver CO 80225 USGS, Denver, CO 80225 U.S. Department of the Interior U.S. Geological Survey
Recent Issues Concerning Cr(VI) M Measurement t • State of California proposal for Cr(VI) State of California proposal for Cr(VI) regulatory limit from 0.05 µg/L to 0.02 µg/L in drinking water in drinking water – Requires MDLs in 0.002 – 0.004 µg/L • Recent study finds Cr(VI) in numerous Recent study finds Cr(VI) in numerous municipal water sources • No recent studies on Cr(VI) stability C ( ) – Particularly in “naturally occurring matrices”
Analytical Method † y HPLC – reversed phase, ion-pairing Column: Brownlee C8 Column Oven 35 ºC Column Oven 35 C Mobile Phase: 2mM tetrabutylammonium hydroxide (TBAOH) + 0.5mM K 2 EDTA, pH = 7.4 – 7.6 EDTA converts Cr(III) to anionic EDTA-Cr(III) EDTA converts Cr(III) to anionic EDTA Cr(III) 5% MeOH added on-line Autosampler 10 ºC Samples: Diluted minimally 1:1 in mobile- phase, 50 µL injection The use of company, trade, and/or DRC-ICP-MS product names is for identification purposes only and does not imply Reaction gas: N 2 or NH 3 endorsement by the United States Government. S Sample Introduction: l I t d ti Baffled quartz cyclonic spray chamber Meinhard TQ-30-A3 nebulizer † Complete description in J. Anal. At. Spectrom., 2007, 22, 1051-1060 * Stated column working range to pH=7; however, have run up to pH=8 without significant column life issues
Analytical Figures of Merit: NH 3 Reaction Gas N 2 Reaction Gas Flow 0.75, RPq=0.65 Flow 1.0, RPq=0.50 Parameter Cr(III) Cr(VI) Cr(III) Cr(VI) Instrument Detection Limit (IDL), µg/L 0.09 0.06 0.1 0.1 Practical Quantitation Limit (PQL), µg/L 0.2 0.2 0.2 0.2 Chromatogram showing low calibration standard at 0.5 µg/L and standard at PQL of 0.2 µg/L Using 50 µL injection volume
Method Modifications to Improve IDLs Method Modifications to Improve IDLs • Use NH 3 Reaction Gas 3 Cr 52 200 ppt spex: Cr 52: 1 100 ppt spex: Cr 52: 1 50 ppt spex: Cr 52: 1 5500 Cr VI • Investigate use of larger 5000 injection volume Cr III 4500 ntensity (cps) 4000 • Careful optimization of Careful optimization of 3500 3500 I instrument parameters 3000 2500 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)
50 ppt at different injection volumes 50 ppt at different injection volumes Cr 52 50 ppt spex: Cr 52: 1 50 ppt spex 100: Cr 52: 1 50 ppt spex 150: Cr 52: 1 50 ppt spex 200: Cr 52: 1 7000 Inj. Vol. Area 6500 ( L) (µL) cps cps 6000 50 5249 5500 100 7394 ps) 5000 Intensity (c 150 10755 4500 Cr VI 200 16559 4000 3500 3500 3000 2500 2000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)
IDL Determination – 200µL injections IDL Determination 200µL injections Cr 52 200 ppt: Cr 52: 1 100 ppt: Cr 52: 1 50 ppt: Cr 52: 1 25 ppt: Cr 52: 1 12000 12000 Cr VI • Larger injection 11000 Cr(III) ppb Cr(VI) ppb volume did not 10000 0.106 0.055 drastically improve drastically improve 0.071 0.069 IDL for Cr(VI) 9000 0.086 0.091 0.069 0.078 • More improvement 8000 (cps) 0.066 0.064 seen for Cr(III) 0.08 0.072 7000 Intensity 0.104 0.05 Cr III 0.076 0.039 6000 MEAN 0.082 0.065 SD 0.015 0.017 5000 IDL=3*SD IDL=3*SD 0 046 0.046 0.050 0 050 ppb ppb 4000 3000 2000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)
Stability of Cr(III) and Cr(VI) y ( ) ( ) • Cr(VI) standards stable in mobile phase at 10 °C • Storage at -21 °C detrimental if other species are present St t 21 °C d t i t l if th i t – As(III), As(V), Se(IV), Se(VI) – Reduction of Cr(VI) to Cr(III) ( ) ( )
Stability of Cr(III) and Cr(VI) • Standards spiked into USGS reference water M-172 showed varying stability • • Elapsed time in hours from t=0 to t=120 Elapsed time in hours from t=0 to t=120 • M-172 preserved with NaOCl – causes oxidation of Cr(VI)! • Addition of 10 mM K 2 EDTA preservative (E-172) showed limited improvement if As and Se species present p p p – Eliminates oxidation of Cr(III) to Cr(VI) – More info in Analytical & Bioanalytical Chemistry, in press • http://dx.doi.org/10.1007/s00216-011-5275-x Cr(III) oxidized to Cr(VI)! ( )
In Vitro Bioaccessibility Methods - C (VI) S Cr(VI) Stability in the Human Body bili i h H B d • Ingestion Pathway g y – Simulated gastric leach fluid (based on Drexler and Brattin, 2007) • pH 1-2 glycine/HCl solution to simulate stomach environment • 1 part solid sample to 100 parts leachate fluid, placed in orbital shaker incubator at 37 ºC for 1 hour, filtered and analyzed for pH and metals – Simulated intestinal fluid (modified from Basta, 2007) • Gastric leach fluid titrated to a pH of 5.5 ± 0.1 with Na 2 CO 3 prior to addition of porcine pancreatin and bile f i ti d bil • Inhalation Pathway – Simulated lung fluid (SLF) (based on Mattson, 1994) • Simple proxy for a near neutral pH solution encountered along an inhalation pathway • 1 part solid sample to 100 parts SLF for 24 hours at 37 ºC
Gastric Fluid: Converts Cr(VI) to Cr(III) ≤ 200 ppb Cr VI Time = 0 35000 5 ppb standard pH = 1.5 30000 Cr III ps) 25000 ensity (c 200 ppb Cr(VI) – Gastric 1:50 20000 100 ppb Cr(VI) – Gastric 1:50 Inte 50 ppb Cr(VI) – Gastric 1:50 15000 100 ppb Cr(III) – Gastric 1:50 10000 10000 Blank Gastric 1:50 5000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)
Gastric: Partial conversion to Cr(III) ≥ 500 ppb 55000 50000 Time = 0 1000 ppb Cr(VI) – Gastric 1:50 1000 ppb Cr(VI) Gastric 1:50 pH 1.5 pH = 1 5 45000 % Conversion ~ 80% 40000 Cr VI 5 ppb standard ps) 35000 35000 ensity (cp C Cr III 30000 500 ppb Cr(VI) – Gastric 1:50 % Conversion ~ 93% 25000 25000 Inte 20000 100 ppb Cr(III) – Gastric 1:50 15000 15000 Blank Gastric 1:50 Blank Gastric 1:50 10000 5000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)
Intestinal: No immediate conversion Time = 0 120K pH = 4 8 pH 4.8 100K 1000 ppb Cr(VI) – Intestinal 1:50 s) nsity (cps 500 ppb Cr(VI) – Intestinal 1:50 80K 200 ppb Cr(VI) – Intestinal 1:50 60K 60K Inten 5 ppb standard 100 ppb Cr(VI) – Intestinal 1:50 40K Cr VI 50 ppb Cr(VI) – Intestinal 1:50 pp ( ) II C Cr I 100 ppb Cr(III) – Intestinal 1:50 20K 0 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)
Lung Fluid: No immediate conversion 140K Time = 0 120K pH = 7 4 pH 7.4 100K ps) 1000 ppb Cr(VI) – Intestinal 1:50 ensity (cp 500 ppb Cr(VI) – Intestinal 1:50 80K 200 ppb Cr(VI) – Intestinal 1:50 60K 60K Inte 5 ppb standard 100 ppb Cr(VI) – Intestinal 1:50 40K Cr VI 50 ppb Cr(VI) – Intestinal 1:50 pp ( ) I C Cr II 20K 100 ppb Cr(III) – Intestinal 1:50 0 0 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)
Cr(VI) Conversion vs. Time ) (
Removal of Cr(VI) in T ap Water Cr(VI) – 100 ppb (tap water) • Addition of lemon juice to tap water 70000 spiked with 100 ppb Cr(VI) causes immediate reduction to Cr(III) immediate reduction to Cr(III) 60000 • 200µL to 10 mL (equivalent of 1 tsp in 8 oz glass) causes complete conversion to Cr(III) conversion to Cr(III) 50000 50000 ensity (cps) 40000 Inte + 200 µL lemon juice pH = 3.2 30000 + 100 µL lemon juice 87% Cr(VI) converted to Cr(III) to Cr(III) 20000 20000 100 ppb Cr(III) + 100 µL lemon juice 100 b C (III) 100 L l j i pH = 3.5 10000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)
Lemon Juice Converts Cr(VI) to Cr(III) Lemon Juice Converts Cr(VI) to Cr(III)
Effect of Acids on Cr(VI) Stability ( ) y 100 ppb Cr(VI) in tap water Added 100 µL of 2% acid solution to 10 mL Cr VI 100 ppb Cr(VI) 70000 + 100 µL Citric acid (pH=4 1) + 100 µL Citric acid (pH 4.1) 60000 + 100 µL Nitric acid (pH=2.7) 50000 cps) + 100 µL Hydrochloric acid (pH=3.0) Intensity (c 40000 + 100 µL Ascorbic acid (pH=4.7) 30000 20000 10000 10000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)
NaOCl can cause oxidation of Cr(III) Added 100 µL 5% NaOCl to 100 ppb Cr(III) in tap water Cr VI 100 ppb Cr(VI) 70000 60000 50000 cps) Intensity ( 100 ppb Cr(III) + 100 µL NaOCl 40000 pH = 9.3 30000 ~10% conversion 100 ppb Cr(III) of Cr(III) to Cr(VI) 20000 10000 10000 0.0 0.5 1.0 1.5 2.0 2.5 Time (min)
Conclusions Larger injection volumes did not dramatically improve IDL for Cr(VI) in our method Studies on standards shows Cr(VI) stability highly variable and Studies on standards shows Cr(VI) stability highly variable and can be affected by other elements in the sample Sample preservation and holding times need to address this Stability studies with just Cr(VI) present may not be valid Stability studies with just Cr(VI) present may not be valid Studies done on effects of stomach acid on Cr(VI) show immediate conversion to Cr(III) up to 200 µg/L Potential impact on Drinking Water limit for Cr(VI) Potential impact on Drinking Water limit for Cr(VI) Study shows addition of lemon juice or ascorbic acid can convert > 90% of Cr(VI) to Cr(III) “in the glass ” for concentrations concentrations up to 200 µg/L Cr(VI) p to 200 g/L Cr(VI) Addition of NaOCl can oxidize Cr(III) to Cr(VI) Agrees with studies that water chlorination can oxidize Cr(III)
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