Clean Sampling and Analysis of River and Estuarine Waters for Trace Metal Studies

The overall goal of this clean sampling and analysis protocol is to provide adequate techniques for obtaining reliable data for trace metal studies, especially when low detection limits are required. This method can help answer key questions in environmental chemistry field, in aquatic environments with dynamic hydrological and geochemical conditions. The advantage of this technique is that under the suggested guidelines, reliable data can be obtained and related to variable key environmental parameters.

So this method can provide techniques to obtain information in rivers and estuaries. It can also be applied to other systems, such as lakes, coastal zones, and oceanic environments. Generally, individuals new to this method will struggle due to a lack of knowledge on contamination controls and the inability to recognize proper means to eliminate contamination during trace metal analysis.

Demonstrating the procedure will be Chih-Ping Lee and Pei-Yu Lin, students from our labs. To assemble the sampler, connect four meter long fluorinated ethylene propylene tubing to a 1.5 meter chemically-resistant silicone pumping tube. Insert a polypropylene Y-connector into the pumping tube.

Connect a 50 centimeter pumping tube to one outlet, and a 0.45 micrometer capsule filter to the other. Assemble the tubings in a clean room after they are cleaned as described in the text protocol and store the assemblage in two layers of polyethylene bags. For sample containers, use polyethylene and FEP bottles for trace metal determination containers.

Clean the bottles by soaking them first in 1%detergent solution for 24 hours. After rinsing the bottles with the ionized water, soak them in 50%nitric acid solution for 48 hours. Rinse the bottles a second time and then soak the bottles in 10%hydrochloric acid, or HCl solution for 24 hours.

After the final HCl soaking, rinse the bottles thoroughly with the ionized water, abbreviated as DIW. Dry the bottles with the caps sealed in a clean room or class 100 clean bench. Seal the cleaned bottles in polyethylene zippered bags and double bag them in polyethylene bags for transport.

At the riverbank or on a boat, have one person open the bag with the sampler and attach the four meter FEP tubing to a cleaned polypropylene pole. Extend the pole as far from the bank as possible and keep the inlet of the FEP tubing approximately 30 centimeters below the surface of the running river water before the pump is turned on. Have a second person attach the pumping tube to the pump-head of a peristaltic pump.

Start the pump and drain the water on the downstream side at least three times of the total sampler volume. Have a third person be in charge of opening the inner bags and sample caps, and holding the sampling tube that drains the sample into the bottle. First, collect an unfiltered sample through the outlet without the filter into a 125 milliliter plastic bottle for measurements of conductivity, temperature, and pH in the field.

Then, collect a filtered water sample by closing the outlet with a plastic clamp to force the water to go through the capsule filter. Collect filtered samples in 40 milliliter amber glass bottles for DOC and EDTA measurements. For each type of sample, collect extra samples, as well as field blanks at selected locations to serve as quality control aliquots.

Place the 40 milliliter glass bottles on ice and store them in an ice chest, and place the polyethylene bottles in ice chests. Finally, collect the suspended particulate matter, or SPM, on 0.4 micrometer polycarbonate membrane filters by vacuum filtration using a plastic filtration funnel and vessel. For dissolved trace metal determination, add 2 milliliters of concentrated sub-boiled nitric acid into the one liter sample.

UV irradiate the acidified samples in the FEP bottles for 24 hours. Next, weigh 2 grams of cation exchange resin into a small plastic cup and add a small amount of two normal nitric acid solution into the cup. Pour the resin into a 10 milliliter capacity chromatography column.

Clean the resin by washing the column with 5 milliliters of two normal nitric acid twice. Then, rinse with high purity water, abbreviated as HPW, three times. Finally, add 10 milliliters of one molar ammonium hydroxide into the column to convert the resin to the ammonium form.

Adjust the pH of the acidified UV-irradiated samples to 5.5 plus or minus 0.3, by adding 30 milliliters of one molar ammonium acetate buffer solution and some ammonium hydroxide into the samples. Then, place the sample bottle on a rack approximately 30 centimeters above the column packed with cationic exchange resin. Connect the sample bottle and preconcentration column with a 60 centimeter FEP tube, chromatography cap, and connector.

Control the flow rate at three to five milliliters per minute using a two-way stopcock connected above the column. Allow the samples to pass through the preconcentration columns. After the samples have passed through the column, disconnect the tubes and caps from the columns.

To separate major cations from other trace metals, treat the columns with 5 milliliters of HPW twice. Then, add 5 milliliters of one molar ammonium acetate, pH 5.5, four times. Finally, treat with an additional 5 milliliters of HPW twice.

Place a 30 milliliter acid-washed PE bottle just below the column and wash the column with seven 1 milliliter aliquots of two normal nitric acid into the PE bottle. To digest the suspended particulate matter, freeze-dry the polycarbonate filters with SPM samples and weigh them after drying. Next, place the SPM samples with filters into pre-weighed PFA vessels and add 3 milliliters of concentrated nitric acid into the vessels.

Tighten the vessels to a constant torque of 2.5 killogram-meters. Digest the samples in a conventional oven at 130 degrees Celsius for 12 hours. After cooling, open the vessels and add 2 milliliters of hydrogen fluoride.

Then, tighten the vessels and digest in a conventional oven, as before. After cooling, open the vessels and add 16 milliliters of 4.5%boric acid solution. Once again, tighten the vessels and digest in a conventional oven in the same manner.

Weigh each vessel and determine the specific gravity of each digested solution to yield the final digest volume. Using the presented techniques for low contamination sampling, sample treatments, and analysis, resulted in low detection limits, good recoveries for our referenced standard material, and low field blank concentrations. This indicates the effectiveness of this method.

Very good agreement was found between trace metal concentrations determined using two independent methods and separate aliquots of the same sample. One sample was analyzed directly by inductively-coupled plasma atomic emission spectrometry and the other was preconcentrated by ion exchange techniques following determination by inductively-coupled plasma mass spectrometry. The large concentration range implies that these techniques are suitable for trace metal studies and distinctively different environments where trace metal concentrations show significant differences.

Obtaining reliable trace metal data in natural waters requires great care during sample collections, processing, pretreatment, and analysis that aim to reduce contamination. The procedure to prepare sampling gear, sample containers, and material used to process and analyze samples are all critical steps toward obtaining high data quality for trace metal in aquatic environments. The techniques and methods demonstrated here can be easily applied to different types of aquatic systems.

In addition to river and estuary waters, such as oceans, lakes, and groundwater. After watching this video, you should have a good understanding of how to properly plan and conduct trace metal studies that can be applied to environmental monitoring and geochemical investigation.