Metals Analysis

By: Cayla Zenie

The Lehigh Gap Nature Center is founded on the grounds of the New Jersey Zinc Company which was established in Palmerton in 1898. The company heavily polluted the water and environment with zinc and other toxic metals. In an effort to avoid approaching a toxic level of heavy metal concentrations, it is important to monitor the amounts of metals existing in the Lehigh Gap Nature Center Water. Certain metals such as Zinc, Calcium, and Magnesium can have cascading effects on the environment when found either in very low or very high concentrations.

Atomic Absorption Spectroscopy

In order to test for the concentrations of these metals, an analytical technique called Atomic Absorption Spectroscopy (AAS) was used. The atomic absorption spectrometer uses the principle that atoms absorb radiation at a specific wavelength corresponding to the energy of an electron transitioning to a higher energy atomic orbital. When this specific wavelength of light is provided, it is absorbed by the atom. The absorbance value increases linearly with concentration, which allows us to plot a calibration curve (absorbance as a function of concentration). 

Figure 1: A picture of the Varian 240FS (Fast Sequential) Atomic Absorption Spectrometer (AAS).

	Calcium	Magnesium	Zinc
Lamp Position	2	3	4
Lamp Current	5.000 mA	5.000 mA	8.000mA
Wavelength	285.2 nm	285.2 nm	217.0nm
Slit Width	0.5 nm	0.5 nm	1.00 nm

To generate calibration curves, multiple standards of magnesium, calcium and zinc were made using different known concentrations. Before the standards were prepared, an acid washing technique was used to clean the glassware of any debris that might be present. The procedure called for 1 M hydrochloric acid rinse followed by deionized water rinse, 1 M nitric acid rinse followed by another deionized water rinse and one final Millipore water rinse¹⁶.

The tables below list the standard concentrations that were used:

	Calcium	Magnesium
Standard 0	0.00 ppm	0.00 ppm
Standard 1	0.250 ppm	0.0500 ppm
Standard 2	0.500 ppm	0.100  ppm
Standard 3	1.00 ppm	0.200 ppm
Standard 4	2.00 ppm	0.400 ppm
Standard 5	3.00 ppm	0.600 ppm
Standard 6	5.00 ppm	1.00 ppm

The calcium and magnesium standards were created together using 1000 ppm calcium stock solution that was diluted to 100 ppm with Millipore water, and 1000 ppm magnesium stock solution that was diluted down to 25 ppm also using Millipore water. 2.5 mL of lanthanum solution was also added. Millipore water was then used to fill the standards up to the 25 mL line on each flask.                

	Zinc Concentrations
Standard 0	0.000 ppm
Standard 1	0.0025 ppm
Standard 2	0.005 ppm
Standard 3	0.025 ppm
Standard 4	0.100 ppm
Standard 5	0.250 ppm
Standard 6	0.500 ppm
Standard 7	0.800 ppm

The Zinc standards were created using the following dilution method: Standards 4-7 were prepared in 100 mL flasks using 100 ppm of zinc stock solution and diluted to the fill line with Millipore water. Standards 1-3 were prepared in 50 mL flasks using 0.500 ppm of the zinc stock solution which was made by diluting the 100 ppm with Millipore water.

Calibration

After preparing the standards, they were analyzed by AAS, along with the water samples from each site. The absorbance measured for the standards were used to plot the following calibration curves:

Figure 2: Calibration curves for calcium, magnesium, and zinc that were used to calculate the concentrations in the water samples

The equation of each line was used to calculate the concentration (in ppm) of calcium, magnesium, and zinc found in each sample water from the different sample sites.

Based on the graphs shown below there was generally an increase in calcium, magnesium, and zinc concentration across the seven different sites. Comparing the summer 2023 summer results to the spring 2022 results, a decrease in S1, S2, and S3 can be seen for calcium, while S4, R1, R2, and P1 increased. Looking at the 2023 spring results compared to the 2023 summer results for calcium there was an increase in S1, S2, S4, R1, R2, and P1, and a decrease in S4. For magnesium, the 2023 summer results showed an increase in concentration at S1, S3, S4, R1, R2, and P1 compared to 2022 results and 2023 spring results, while S2 summer 2023 results decreased compared to previous results. Zinc showed an increase in the 2023 summer results compared to previous results at S1, R1, R2, and P1.

The river sites had the highest concentrations for magnesium and calcium, while the stream sites were more concentrated for zinc. The concentrations for zinc were found to be higher than the normal range found in freshwater; however, these levels are not considered harmful or toxic to aquatic habitats. Magnesium and calcium levels were within the normal range of freshwater.