Analysis of Five Different Soil and Wastewater Samples

This article examines the analysis of five different wastewater and soil samples, which are listed below. Note that samples 1 to 4 contained dilute HNO₃.

• Sample 1: Distilled water
• Sample 2: Wastewater after treatment in wastewater plant (exit)
• Sample 3: Wastewater before treatment in the waste plant
• Sample 4: Industrial wastewater
• Sample 5: Dried soil from agriculture waste, digested with HNO₃ and HClO₄

The abovementioned samples were initially analyzed using a semi-quantitative method in order to identify elements and their particular concentrations per sample. Profiles of each element were taken from all samples. They were then compared to show peaks and relative background levels. After this, a quantitative analysis was performed using calibration curves set at appropriate levels. Finally, the Standard Addition Method was utilized to obtain results from Sample 5. This sample is suspected to have a more significant matrix effect in comparison to the other four samples.

Principle

An elemental analysis of samples was made using an Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). After the samples have been nebulized, they were transferred to an argon plasma where it was decomposed, atomized, and ionized, fully exciting the atoms and ions. Light was emitted when atoms or ions returned to lower levels of energy. The intensity of this process was measured. Every element emits light at a particular wavelength. This enables users to quantitatively analyze the acquired data after calibration.

Instrument Specification

The primary device used in analysis was the ULTIMA 2. The specifications of such instrument are presented in the tables below.

Table 1. Specification of spectrometer

Table 2. Specification of RF Generator

Semi-Quantitative Analysis

The method of analysis used in this article aimed to utilize a semi-quantitative approach to facilitate the detection of present elements and their concentration estimates, ultimately performing quantitative analysis to obtain accurate and precise results.

A semi-quantitative method is integrated into the Analyst software for the ICP. It allows for rapid identification of elements in the sample from a qualitative and quantitative point of view.

The current method focuses on 34 elements. Wavelengths have been selected to cover a large variety of samples, wherein the most sensitive lines were assigned to most elements (with the exception of Ca and Mg) as well as appropriate background correction positions. A background correction position is placed on both side of the Pb and Al peaks, incased of high AI or Ca concentrations, respectively. A calibration is performed with two points (0 and 5 mg/L in deionized water), however, both calibration and analyses are measured with one replicate. The acquisition time is 0.1 s per data point, with 7 data points measured to fit a gaussian curve.

With these conditions, a semi-quantitative analysis is undertaken in about 3-4 minutes for the 34 elements and allows the identification of different kinds of samples.

It should be noted, however, that the matrix for the calibration standards may be adjusted (NaCl 100 g/L, 20 % H₂SO₄ ...) according to the samples that need to be analyzed. This method provides more accurate results.

The following plasma conditions were applied. A key finding to note is the slight increase of power from 1200 W from the 1000 W. This would be typically used for clean water. The increase of power was undertaken in order to minimize matrix effects that may have an influence on signal quantity, since the standards are in deionized water and the samples are waste water and soil and acids.

Table 3. Operating conditions

A K3- or C1-type Meinhard nebulizer and cyclonic spray chamber may be used to gain sensitivity, if required. The results can be printed or exported from the ICP software. To validate results, a certified sample was analyzed prior to sample measurement.

Table 4. Results for sample “Reference 1643-d”

n.c. means non-certified

Table 5. Results for various samples

Quantitative Analysis

The results obtained from the semi-quantitative method yielded several standards that were prepared in the appropriate concentration range for each element.

Table 6. Standard concentration

Meanwhile, plasma parameters are presented in the following table.

Table 7. Operating conditions

The combination of a Meinhard C1 nebulizer and cyclonic Spray Chamber for sample introduction was used for optimum sensitivity and detection limits. It should be noted that sample 5 was analyzed using the Standard Addition Method. This is due to the hypothesis that a more significant matrix effect could be discovered in such sample due to the presence of major elements (650 mg/l of Ca and acids).

Profiles of several samples for Cd are presented in the below figure. This demonstrates that the matrix is different because the spectral background is raised in sample 5.

Cd spectrum in sample 5

The methodology of the Standard Addition Method (S.A.M.) is as follows:

1. Using a rapid semi-quantitative method, estimate the approximate concentration for each element of interest.
2. Prepare a stock solution with all the elements of interest and with appropriate concentrations. Ensure that the solution is prepared 2 to 5 times for each element concentration.
3. Prepare at least 3 standards. Spike the sample with increasing concentrations. There should be an unknown sample to be declared as a blank (standard 0) as well as 3 standards in the same matrix. In total, 4 standards are recommended.
4. Prepare the method in the software using background correction for baseline correction. When creating the method, it can be set up as a Standard Addition Method.
5. Run the calibration. The results will be presented under the Standard Addition Tab. Alternatively, if the method was not set up as a Standard Addition, the intercept (or BEC) of each curve corresponds to the concentration of the element in the sample.

From this calibration, other samples with similar matrices could be analyzed.

The global report of the quantitative analyses is shown below the following tables. Note the abbreviations that were used:

Conc.: from quantitative method;
Conc. (S.Q.): from semi-quantitative method;
Conc. SAM: from standard addition method.

Table 8. Results for Sample 1

Table 9. Results for Sample 2

Table 10. Results for Sample 3

Table 11. Results for Sample 4

Table 12. Results for Sample 5

Table 13. Results for Standard 0.1

The standard at 0.1 mg/l was analyzed to check and validate the calibration at the end of the analyses.

Table 14. Detection limits

Conclusion

The study presented the use of quantitative and semi-quantitative methods in yielding good results in a wide range of concentrations (µg/L levels to hundreds of mg/L), with radial viewing allowing the user to minimize matrix effects. The fast and precise semi-quantitative method could be used routinely.

This information has been sourced, reviewed and adapted from materials provided by HORIBA.

For more information on this source, please visit HORIBA.