Improved Extraction and Analysis of Hexavalent Chromium from Soil and Water Richard F. Jack, 1 Jinshui Che, 2 Lipika Basumallick, 1 and Jeffrey Rohrer 1 1 Thermo Fisher Scientific, Sunnyvale, CA, USA; 2 Thermo Fisher Scientific, Shanghai, People s Republic of China
Overview Purpose: Hexavalent chromium (CrVI) refers to chemical compounds that contain the element chromium in the +6 oxidation state. All CrVI compounds are strong oxidizing agents, and are considered toxic and potentially carcinogenic. Hence, chromates are regulated in drinking water and the environment in the U.S. and many countries around the world. For example, the state of California established a public health goal (PHG) of 0.2 µg/l (ppb) for CrVI and 2.5 µg/l for total chromium in 1999. The PHG is based on an estimated one-in-a-million lifetime cancer risk level. However, U.S. EPA Method 3060A does not allow sufficient sensitivity for routine analysis at the recently proposed California PHG level of 0.02 µg/l. Modifications to this EPA method presented here result in a 0.003 µg/l limit of quantitation (LOQ), which is more than sufficient for analysis at the proposed California PHG level. Water is not the only exposure source for CrVI, as it is also prevalent in soils and many other matrices. Therefore, as an alternative to the labor-intensive EPA Method 3060A, a faster, easier procedure for extracting CrVI from soil matrices by using a Thermo Scientific Dionex ASE 350 Accelerated Solvent Extractor is shown here. Introduction Improvements described here to both analysis and extraction of CrVI are based on the conditions detailed in EPA Method 218.6. This includes use of the column in the 2 mm format and a smaller reaction coil to increase method sensitivity. By using Thermo Scientific Dionex IonPac AG7 Guard (2 50 mm) and Dionex IonPac AS7 Analytical (2 250 mm) columns, an eluent of 250 mm ammonium sulfate/100 mm ammonium hydroxide at a flow rate of 0.36 ml/min, a 1000 µl injection volume, and postcolumn reaction with 2 mm diphenylcarbazide/10% methanol/1 N sulfuric acid (using a 125 µl reaction coil) followed by visible absorbance detection at 530 nm, a 0.001 µg/l, minimum detection limit (MDL) for chromate is achieved. Although the efficacy of an automated method to extract CrVI from soil using accelerated solvent extraction (as an alternative to EPA Method 3060A) has previously been demonstrated, 1 the removal recovery was limited. This was due to the lower ph of the sample extraction buffer than that specified in the EPA method. This lower ph was required in earlier experiments due to the stainless steel (SST) accelerated solvent extraction cells, which are less resistant to high ph buffers. But the Dionium (Zr) cells in the Dionex ASE 350 overcome this limitation. Reported here is the use of the same ph with cells able to withstand higher ph values. Methods Ion Chromatography Thermo Scientific Dionex ICS-2100, ICS-1600, ICS-1100, or ICS-5000 system including: SP Single Pump or DP Dual Pump module DC Detector/Chromatography module Injection loop, 1000 µl Reaction coil, 125 µl Sample syringe, 5 ml ICS Series VWD Variable Wavelength Absorbance Detector Postcolumn Reagent (PCR) Delivery: PC-10 pneumatic delivery or AXP pump if not using the DP AS Autosampler Thermo Scientific Dionex Chromeleon Chromatography Data System (CDS) software Reagents and Standards 2 Improved Extraction and Analysis of Hexavalent Chromium from Soil and Water
Reagents and Standards Prepare all solutions from analytical reagent-grade chemicals (when commercially available). Note, there is a possibility of the presence of trace levels of chromate in some commercially available chemicals. Deionized (DI) water, 18 MΩ-cm or better Ammonium sulfate (Mallinckrodt General P/N AR 7725) Ammonium hydroxide (Sigma P/N A6899) Sulfuric acid, 95-98% (JT Baker Instra-Analyzed P/N 9673) Methanol, HPLC grade (Fisher Optima P/N A454-4) Potassium dichromate (JT Baker P/N 4765-01) Sodium and potassium salts, ACS reagent-grade, for preparing the anion standards 1,5-diphenylcarbazide (JT Baker K620-03) Method Columns: Dionex IonPac AG7 Guard 2 50 mm (PN 063099), Dionex IonPac AS7 Analytical 2 250 mm (PN 063097) Eluent: 250 mm Ammonium sulfate and 100 mm ammonium hydroxide Flow Rate: 0.36 ml/min Inj. Volume: 1000 µl (Full loop) Temperature: 30 C Back Pressure: 1700 2000 psi PCR: 2 mm diphenylcarbazide, 10% methanol, 1 N sulfuric acid Flow Rate: 0.12 ml/min Detection: Visible absorbance, 530 nm Noise: 6 8 µau Run Time: 25 min Eluent 250 mm Ammonium sulfate 100 mm Ammonium hydroxide Dissolve 66 g of ammonium sulfate in ~1 L of DI water and add 13 ml of 29% ammonium hydroxide solution. Dilute to 2.0 L with DI water. PCR 2 mm Diphenylcarbazide 10% Methanol 1 N Sulfuric acid Sample Adjustment Buffer 250 mm Ammonium sulfate 1000 mm Ammonium hydroxide Dissolve 3.3 g of ammonium sulfate in ~75 ml of DI water and add 6.5 ml of 29% ammonium hydroxide. Dilute to 100 ml with DI water. Accelerated Solvent Extraction Conditions Solvent: 10 mm NaOH, 4 g/l NaCl (ph > 11.5) Temperature: 100 C Preheat Time: 5 min Static Time: 5 min Cycle Time: 2 Flush Volume: 60% Purge Time: 90 s Cell: 66 ml Zr cell Total Time: Approximately 20 min Sample Pretreatment: Dry soil samples at 50 C and pulverize. Place 10 g of soil sample mixed with quartz sand into a 66 ml Zr extraction cell, and insert into the Dionex ASE 350 system with an extract constant volume of 100 ml. Filter through a 0.45 µm membrane and inject into the Dionex ICS system. Results Thermo Scientific Poster Note PN70094_e 05/12S 3
Results Blank of SST Cell and Zr Cell By filling the 66 ml blank sample in both SST and Zr cells with quartz sand, five parallel extracts yielded a background of approximately 19.1 µg/l. The background of the blank in the 66 ml Zr cell was approximately 0.9 µg/l, lower than the LOQ of 1.6 µg/l. The limit of detection (LOD) was 0.5 µg/l. 2.5 mau -0.5 0.1 2 4 6 8 10 12 14 16 18 20 22 24 25 Minutes FIGURE 1. Chromatograms of blank extractions using either an SST cell (2:66 ml) or a Zr cell (1:66 ml) Recovery and Reproducibility The recovery experiment was carried out by filling the 66 ml Zr cell with the quartz sand, adding the standard solution of CrVI, then performing the accelerated solvent extraction. The results are shown in Table 1. The average recoveries were 105.2% and 105.1%, with the relative standard deviation (RSD) 2.2% and 0.4% (n = 3) for the different added levels. TABLE 1. Recoveries and their RSDs (n = 3) at different added amounts of CrVI Amount Added (µg/l) Recovery % RSD 10 105.2 2.2% 100 105.1 0.4% Sample Detection Only one soil sample was used to evaluate this method. The chromatogram below in Figure 2 shows a CrVI content of 39.4 ng/g..9 Cr(VI) mau 0 -.1 0.1 2 4 6 8 10 12 14 16 18 20 22 24 25 Minutes FIGURE 2. Chromatogram of CrVI from a soil sample known to be contaminated. 4 Improved Extraction and Analysis of Hexavalent Chromium from Soil and Water
Conclusion The preliminary results shown here for the extraction of CrVI from soil using accelerated solvent extraction demonstrates the potential for a faster extraction that is easy to use with minimal background contamination. The use of the Dionex ASE 350 is potentially a viable alternative to the labor-intensive EPA Method 3060A for compliance monitoring. Data showing improvements to the detection limits have been previously reported. 2,3 References 1.Giuriati, C.; Abballe, F.; Cristoforia. M.C.; and Gorni, A. Accelerated and Automated Extraction of Hexavalent Chromium from Solid Waste Materials Coupled with Ion Chromatography Determination. LC/GC Eur. 2005, 18 (4) 220 224. 2.Dionex (now part of Thermo Scientific) Application Note 144: Determination of Perchlorate in High Ionic Strength Fertilizer Extracts by Ion Chromatography, 2002 [Online]. www.dionex.com/en-us/webdocs/4096-an144_lpn1425.pdf (accessed Apr. 9, 2012). 3.Dionex (now part of Thermo Scientific) Application Note 179: Carbohydrate and Amino Acid Analysis Using 3-D Amperometry, 2007 [Online]. www.dionex.com/en-us/ webdocs/56256-an179_released032707.pdf (accessed Apr. 9, 2012). 2016 Thermo Fisher Scientific Inc. All rights reserved. All trademarks are the property of Thermo Fisher Scientific Inc. and its subsidiaries. Specifications, terms and pricing are subject to change. Not all products are available in all countries. Please consult your local sales representative for details. U.S./Canada (847) 295 7500 Brazil (55) 11 3731 5140 Austria (43) 1 616 51 25 Benelux (31) 20 683 9768 (32) 3 353 42 94 Denmark (45) 36 36 90 90 France (33) 1 39 30 01 10 Germany (49) 6126 991 0 Ireland (353) 1 644 0064 Italy (39) 02 51 62 1267 Sweden (46) 8 473 3380 Switzerland (41) 62 205 9966 United Kingdom (44) 1276 691722 Australia (61) 2 9420 5233 China (852) 2428 3282 India (91) 22 2764 2735 Japan (81) 6 6885 1213 Korea (82) 2 3420 8600 Singapore (65) 6289 1190 Taiwan (886) 2 8751 6655 PN70094_E 08/16S