Seawater Extraction Procedure for Trace Metals

Transcription

1 Title: Seawater Extraction Page 1 of 10 Seawater Extraction Procedure for Trace Metals References: Danielson, L., B. Magnusson, and S. Westerlund An improved metal extraction procedure for the determination of trace metals in seawater by atomic absorption spectrometry with electrothermal atomization. Anal. Chimica Acta, 98: Magnusson, B. and S. Westerlund Solvent extraction procedures combined with back-extraction for trace metal determinations by atomic absorption spectrometry. Anal. Chimica Acta, 131: Wallace, G.T., N. Dudek, R. Dulmage, and O. Mahoney Trace element distributions in the Gulf Stream adjacent to the southeastern Atlantic continental shelf influence of atmospheric and shelf water inputs. Canadian Jour. of Fisheries and Aquatic Sci., 40 (supplement 2): Scope and Application Matrices: Seawater and brackish water samples Definitions: Refer to Alpha Analytical Quality Manual. This chelation-extraction procedure is used to extract total trace metals from seawater and brackish water samples for subsequent analysis by inductively coupled plasma mass spectrometry (ICP MS), inductively coupled plasma spectrometry (ICP) or graphite furnace atomic absorption spectroscopy (GFAA). The extraction is applicable to dissolved (0.45µm filtered) and total (unfiltered) metals in seawater. The standard laboratory reporting limits for each total trace metal in seawater are listed in Table 1. Lower reporting limits can be achieved by altering the initial and final volumes in the extraction procedure (see Section 10.3) The data report packages present the documentation of any method modification related to the samples tested. Depending upon the nature of the modification and the extent of intended use, the laboratory may be required to demonstrate that the modifications will produce equivalent results for the matrix. Approval of all method modifications is by one or more of the following laboratory personnel before performing the modification: Area Supervisor, Department Supervisor, Laboratory Director, or Quality Assurance Officer. This method is restricted to use by or under the supervision of trained analysts. Each analyst must demonstrate the ability to generate acceptable results with this method by performing an initial demonstration of capability, analyzing a proficiency test sample and completing the record of training. After initial demonstration, ongoing demonstration is based on acceptable laboratory performance of at least a quarterly laboratory control sample or acceptable performance from an annual proficiency test sample. A major modification to this procedure requires demonstration of performance. The identification of major method modification requiring performance demonstration is directed by the Quality Assurance Officer and/or Laboratory Director on a case-by-case basis.

2 Title: Seawater Extraction Page 2 of 10 Table 1. Standard Laboratory Reporting Limits for Trace Metals in Seawater 100 ml seawater aliquot Trace Metal Reporting Limit ICP-MS (µg/l) Reporting Limit ICP-AES (µg/l) Reporting Limit GFAA (µg/l) Cadmium Cobalt Copper Iron Lead Molybdenum Nickel Silver Vanadium NA Zinc Summary of Method This chelation-extraction method is used to quantitatively extract trace metals from seawater samples. Using this procedure, metals are extracted into a clean acid matrix without the sodium interference present in the original seawater sample. Approximately 100 to 200 ml of unpreserved seawater is filtered (for dissolved analysis) and acidified. The metals are chelated with a combination of 1-pyrolidine carbodithioic acid and the diethyl ammonium salt of diethyldithiocarbamic acid and then extracted into chloroform or freon. In order to achieve acceptable extraction efficiency, samples are extracted with the solvent three times. Then, the solvent extract that contains the trace metals in solution is back-extracted with acid (either nitric or a combination of nitric and hydrochloric acids) to a volume of 2 ml. Following extraction, the sample is analyzed by stabilized temperature platform graphite furnace atomic absorption spectroscopy (GFAA) with Zeeman background correction, simultaneous inductively coupled plasma atomic emission spectroscopy (ICP-AES) or inductively coupled plasma mass spectrometry (ICP MS). 2.1 Method Modifications from Reference Extraction Modification for Silver (Ag) If silver is one of the analytes to be extracted, then the back extraction in acid, Section , must be changed from 0.2 ml HNO 3 to 0.2 ml of a 1:1 HNO 3 :HCl solution. Prepare a small volume of 1:1 HNO 3 :HCl by adding equal amounts of each acid to a vial. Mix and allow it to stand for 1 to 2 minutes until the solution turns a yellow-orange color. Then proceed with steps through Extraction Modifications for Lower Level Detection To obtain even lower trace metal detection limits with the seawater extraction procedure, larger volumes of seawater can be extracted. Make sure that the buffer added adjusts the sample to the correct ph range (perform buffer check using large volume seawater sample).

3 Title: Seawater Extraction Page 3 of Another way to lower the detection limits is to decrease the final volume of the extract. The final volume can be lowered by altering steps and to 0.1 ml conc. HNO 3 and 0.9 ml DI for a total final volume of 1.0 ml instead of 2.0 ml Analysis by GFAA obtains a lower detection level by an order of magnitude or more compared to ICP or ICP MS for most trace metals. See Table 1 for detection levels for the two analytical methods using the seawater chelationextraction procedure. 3. Reporting Limits Refer to Table 1 in Section 1 for typical Reporting Limits per determinative method. 4. Interferences Highly contaminated seawaters may show emulsions during extraction that are not easily separated. Consult with the Inorganics Manager if an emulsion forms during extraction. 5. Health and Safety The toxicity or carcinogenicity of each reagent and standard used in this method is not fully established; however, each chemical compound must be treated as a potential health hazard. From this viewpoint, exposure to these chemicals must be reduced to the lowest possible level by whatever means available. A reference file of material data handling sheets is available to all personnel involved in the chemical analysis. Additional references to laboratory safety are available in the Chemical Hygiene Plan. All personnel handling environmental samples known to contain or to have been in contact with municipal waste must follow safety practices for handling known disease causative agents. Many metal compounds encountered in environmental samples are highly toxic if swallowed, inhaled, or absorbed through the skin. Analyses are conducted in a laboratory exhaust hood and exhausts from the instrument are properly ventilated. Standard laboratory safety practices should be observed including safety glasses and appropriate laboratory gloves when handling samples and reagents. 6. Sample Collection, Preservation, Shipping and Handling 6.1 Sample Collection Seawater for total metals analysis is collected in 500mL or 1 L polypropylene bottles. 6.2 Sample Preservation Samples are preserved with HNO 3 to a ph <2. Samples for dissolved metals are not preserved until after filtration at the laboratory. Following filtration, samples are acidified with HNO 3 to ph<2 prior to extraction. 6.3 Sample Shipping No special sample shipping requirements. Typical shipping procedures may be found in the Alpha Sample Management SOP (G-005)

4 Title: Seawater Extraction Page 4 of Sample Handling Samples are refrigerated to 4 C (±2 C). All samples must be analyzed within 6 months (180 days) of sample collection for all total metals. Unpreserved samples collected for dissolved metals are filtered (0.45 µm) upon receipt in the laboratory, under clean conditions. The filtrate is then acidified with HNO 3 to a ph <2 prior to extraction. Refer to the Sample Management SOP (G-005) for Sample Receipt, Login and internal Sample Custody information. 7. Equipment and Supplies 7.1 Teflon separatory funnels: 500mL capacity. Teflon separatory funnels must be ultraclean for low level seawater analysis. To clean, add 10mL of 25% HNO3 (for cleaning only) shake and pour off while rotating the stopcock. Then add 10mL DI water, shake and pour off while rotating the stopcock. Rinse thoroughly with DI water. 7.2 Automatic pipettors: Appropriate volumes to make up reagents, spike solutions, and add reagents to seawater during the extraction procedure ml Teflon Vials 7.4 Transfer pipettes (plastic) 7.5 Filtration Apparatus: Nitric Acid and DI rinsed µm Filters: Used to perform filtration of seawater samples for dissolved metals. 7.7 Polyethylene or Polystyrene Cups with Caps: 2mL volume 7.8 ph Meter 8. Reagents and Standards All reagents are assayed by the chemical manufacturer for the metals being analyzed and meet ACS specifications. Method blank (Section 9) analysis must show levels of metals < laboratory reporting limit to confirm the purity of the reagents used. 8.1 Concentrated nitric acid, HNO 3 : Trace metal grade or better 8.2 Acetic Acid: Trace metal grade or better 8.3 Metal-free laboratory deionized (DI) water: ASTM Type II water or equivalent. 8.4 Chloroform: Optima; or Freon 8.5 Ammonium Hydroxide: Trace metal grade or better

5 Title: Seawater Extraction Page 5 of APDC (1-pyrolidine carbodithioic acid ammonium salt): Vendor: Aldrich Chemical Company, Cat. # 14, DDDC (the diethyl ammonium salt of diethyldithiocarbamic acid): Vendor: Aldrich Chemical Company, Cat # 31, :1 HNO 3 and 1:1 HCl: For back-extraction when extracting for silver. 8.9 Chelate Solution: Add 2 grams each of APDC and DDDC to 200 ml of DI water in a clean Teflon separatory funnel or clean teflon or poly bottle. Add 1 ml ammonium hydroxide to preserve the chelate solution so that it does not degrade. (If left unpreserved, the chelate solution must be made up fresh each day of extraction as it will degrade over time.) 8.10 Buffer Solution: Add 50 ml acetic acid and 40 ml ammonium hydroxide to 200 ml DI water in a clean Teflon separatory funnel or clean teflon or poly bottle. Test the buffer solution to make sure it will bring the seawater sample to the correct ph of 4.0 to 4.5 using the following procedure. Note: the ph of the seawater, after buffer addition, is critical to the accuracy of the extraction - if the ph is not within the range of 4.0 to 4.5, the extraction may not work to quantitatively extract metals from seawater. Test procedure for buffer solution on seawater sample prior to extraction: 1. Pour a 100 ml of DI into a beaker. Water must be acidified to ph 2 (with HNO3) prior to performing this test. 2. Add 3 ml buffer solution to sample in the beaker. 3. Add 1.5 ml of chelate solution. 4. Test ph of this water solution with a ph meter. Remember to calibrate meter prior to use with at least two buffer solutions. Refer to laboratory SOP/W-003 ph in Water. 5. If the ph is between 4.0 and 4.5 then the buffer is ready for use in the seawater extraction procedure. 6. If ph is NOT between 4.0 and 4.5, adjust buffer solution with the addition of more acid (acetic acid) or base (ammonium hydroxide), as necessary using the following guidance below. If ph is < 4.0: Add several mls of ammonium hydroxide to buffer solution and repeat steps 1 through 3. If ph still is <4.0 then keep adding ammonium hydroxide until the buffer brings the ph of the seawater solution to ph If the ph is > 4.0: Add several mls of acetic acid to buffer solution and repeat steps 1 through 3. If ph is still >4.0 then keep adding acetic acid until the buffer brings the ph of the seawater solution to ph Matrix Spike Solutions: Preparation of the matrix spike solution must be documented in the metals lab logbook. All appropriate information must be included, such as standard source, lot #, dilutions, date, analyst. The spike solution is placed in a clean poly container with a clear label of the contents, including expiration date Spike levels must be an appropriate level for sample concentrations expected and the analytical method used (ICP MS, ICP or GFAA). If extracts are to be analyzed by ICP, a higher concentration spike is added than when performing analysis by GFAA. Typically, a spike level of 100 ng is used for GFAA analysis, 1000 ng is used for ICP MS analysis and 10,000 ng for ICP AES.

6 Title: Seawater Extraction Page 6 of The spike solution must be made in 0.1% HNO 3 such that only a 0.1 ml aliquot of the spike solution is added to the sample. This will avoid excess addition of acid which would interfere with the extraction efficiency As an example for GFAA: prepare a metal spike solution at a concentration of 1.0 mg/l. Spike 0.1 ml of this spike solution into a sample. The resultant matrix spike level will be 100 ng LCS : Add 100mL DI to separatory funnel, add 0.100mL conc. HNO3 to bring acidity within sample acidity range. 9. Quality Control The laboratory must maintain records to document the quality of data that is generated. Ongoing data quality checks are compared with established performance criteria to determine if the results of analyses meet the performance characteristics of the method. 9.1 Blank(s) The Method Blank (MB) is DI water, (filtered if the samples were filtered) that is acidified in same manner as the seawater samples and taken through the entire extraction procedure. One MB must be extracted with each batch of samples, at a minimum frequency of 1 per 20 field samples. The MB data is used to assess contamination from the laboratory environment and to characterize spectral background from the reagents used in sample processing. If the concentration of the required metal exceeds the reporting limit in the MB, then laboratory or reagent contamination is suspected. Corrective Action: Any determined source of contamination must be eliminated and the affected samples in the batch re-extracted and re-analyzed. If the contamination is greater than the laboratory reporting limit but less than the client required reporting limit, the metal reporting limit may be raised to a level above the blank contamination. This requires consultation with the project manager and the client for approval of the raised reporting limit. Additionally, samples whose concentration exceeds the blank concentration by ten fold do not require re-work as the blank level would not be a significant percent of the result. Any such blank actions or changes in reporting levels must be included in the project narrative for reporting to the client. 9.2 Laboratory Control Sample (LCS) Extract at least one LCS or LFB (see Section 9.7) with each batch of samples at a minimum frequency of 1 per 20 field samples. The LCS is carried through the entire extraction and analysis procedures the same as a field sample. Calculate the percent recovery of the metal and compare it to established control limits supplied by the vendor of the LCS. Corrective action: If recovery is outside the acceptance range, identify and correct the source of the problem. Then re-extract and re-analyze all associated seawater samples in the batch. 9.3 Initial Calibration Verification (ICV) Not applicable to this method. 9.4 Continuing Calibration Verification (CCV) Not applicable to this method.

7 Title: Seawater Extraction Page 7 of Matrix Spike Spike and extract a minimum of 5% (1 per 20) of seawater samples. Calculate the percent recovery as a measure of accuracy, corrected for background concentrations measured in the unspiked sample, and compare to control limits of %. Quality control criteria do not apply if the sample concentration is greater than 4x the spike added. Corrective Action: If the recovery of the matrix spike falls outside the designated range, and the laboratory performance is shown to be in control based on the recovery of the LFB and LCS, then the recovery problem encountered with the MS is judged to be matrix related, not system related. Report the results with a project narrative for the client that explains that evidence of matrix interference on the accuracy of the results was seen based upon poor MS recovery. At the client s request, analysis by the method of standard additions may be performed to more accurately quantify the sample concentration in cases of matrix interferences as shown by poor MS recovery. 9.6 Laboratory Duplicate Extract a seawater sample in duplicate at a minimum of 5% (1 per 20) of field samples. Calculate the Relative Percent Difference (RPD) of the duplicate measurements as a measure of precision and evaluate against the acceptance limit of 20%. Corrective Action: If the RPD exceeds 20%, the data is reported to the client in a project narrative stating that evidence of matrix effects on the precision of the results was observed based upon poor duplicate precision. 9.7 Method-specific Quality Control Samples Laboratory Fortified Blank - Extract at least one LCS (Section 9.2) or LFB with each batch of samples at a minimum frequency of 1 per 20 field samples. An LFB is only required if an appropriate seawater LCS is not available. The laboratory fortified blank (LFB) is prepared by fortifying a sample-size volume of generic seawater with the matrix spike solution to obtain a suitable concentration of the metals of interest (typically 40 µg/l). The LFB is carried through the entire sample extraction and analysis scheme. Calculate the percent recovery of the metal and compare it to established control limits of 80%-120%. Corrective action: If recovery is outside the acceptance range, identify and correct the source of the problem. Then re-extract and re-analyze all associated seawater samples in the batch. 9.8 Method Sequence Method Blank LCS Samples 1 18 Laboratory Duplicate Matrix Spike

8 Title: Seawater Extraction Page 8 of Procedure 10.1 Equipment Set-up Pre-extraction preparation of seawater samples. Examine seawater sample bottles. Wipe bottles clean of any debris. If any samples contain sediment, and/or if performing dissolved metals analysis, filter entire batch of samples through a 0.45 µm filter. Add a filter blank by filtering DI water through the same apparatus using the same procedure as for the filtered seawater samples. This will be the method extraction blank (MB). Rinse the original sample bottle with DI and a small portion of the filtered seawater. Place the filtered seawater sample back into the original sample bottle. After filtration, acidify the filtrate with nitric acid to ph < 2. Let acidified filtrate sit overnight Initial Calibration Not applicable to this method Equipment Operation and Sample Processing Extraction Procedure Note: Timing of this extraction is critical to the efficiency of the extraction procedure. Time all steps accurately as listed above. Times listed are minimums. The times may be extended during the extraction procedure but not shortened.\ Aliquot 100 to 200 ml of filtered seawater sample into a clean Teflon separatory funnel Add 3.0 ml buffer solution to sample in separatory funnel Add 1.5 ml chelate solution to sample in separatory funnel Add 5 ml chloroform or freon and shake for 1 minute Allow to stand for at least 5 minutes to separate phases Transfer the chloroform or freon phase (bottom phase) into a clean 30 ml Teflon screw cap vial Repeat steps through twice more; each time adding 5 ml chloroform or freon and transferring the chloroform or freon phase into the 30 ml Teflon vial Back extract the metals into acid by adding 0.2 ml concentrated HNO 3 to the chloroform or freon in the Teflon vial Cap vial and shake for 1 minute Allow to stand for at least 20 minutes.

9 Title: Seawater Extraction Page 9 of Add 1.8 ml DI water to the vial Cap vial and shake for 1 minute Allow to stand for at least 10 minutes to separate the phases Transfer the 2.0 ml aqueous phase (top phase) to a 2 ml pre-cleaned polyethylene sample cup using a clean plastic transfer pipette Analysis of Seawater Extracts See the appropriate analytical SOPs for details on sample analysis Continuing Calibration Not applicable to this method Preventive Maintenance Inspect separatory funnels for debris and residue. If contamination is suspected, shake funnels with chloroform, disassemble and clean stopcocks. 11. Data Evaluation, Calculations and Reporting See the appropriate analytical SOPs for details on data evaluation, calculations and data reporting. 12. Contingencies for Handling Out-of-Control Data or Unacceptable Data See the Section 9 and the appropriate analytical SOPs for details on handling out-of-control data or unacceptable data. 13. Method Performance 13.1 Method Detection Limit Study (MDL) / Limit of Detection Study (LOD) / Limit of Quantitation (LOQ) The laboratory follows the procedure to determine the MDL, LOD, and/or LOQ as outlined in Alpha SOP/ These studies performed by the laboratory are maintained on file for review Demonstration of Capability Studies Refer to Alpha SOP/08-12 for further information regarding IDC/DOC Generation Initial (IDC) The analyst must make an initial, one-time, demonstration of the ability to generate acceptable accuracy and precision with this method, prior to the processing of any samples.

10 Title: Seawater Extraction Page 10 of Continuing (DOC) The analyst must make a continuing, annual, demonstration of the ability to generate acceptable accuracy and precision with this method. 14. Pollution Prevention and Waste Management Refer to Alpha s Chemical Hygiene Plan and the Hazardous Waste and Sample Disposal SOP (G- 006) for further pollution prevention and waste management information. 15. Referenced Documents Sample Management SOP (G-005) Chemical Hygiene Plan Hazardous Waste and Sample Disposal SOP (G-006) SOP/08-05 MDL Generation SOP/08-12 IDC Generation 16. Attachments None.