The Ultimate Guide to Soil Heavy Metal Analysis

In environmental science, the closely related soil composition and heavy metal analysis are becoming increasingly important.

Soil is a crucial resource, providing the basis for food production and preserving ecosystems, but human activities, particularly metal releases into the environment, are influencing its quality more.

Heavy metals are natural elements, essential for organisms in low concentrations but toxic in higher quantities. They enter the soil via various sources, including industrial emissions, wastewater, and agricultural fertilizers, and their accumulation can severely affect human health and the environment.

Heavy metal analysis is key to determining soil concentrations and identifying potential hazards.

Modern analytical techniques, such as atomic absorption spectrometry, enable the accurate detection of even traces of heavy metals. This data is vital for environmentalists, farmers, and governments to adopt suitable measures to remediate soil and prevent more contamination.

This article discusses the quantification of cadmium, chromium, cobalt, copper, lead, manganese, nickel, and zinc in aqua regia extracts using the flame HR-CS-AAS contrAA 800 F following DIN ISO 11047.

Aqua regia extracts of soil, sediment, and sewage sludge samples must be performed according to ISO 11466, and the AS-FD autosampler can be utilized for automated sample introduction and dilution.

The high-resolution flame atomic absorption spectrometer contrAA 800 F with a xenon short arc lamp continuum source offers the possibility of measuring the absorption of the eight metals mentioned above in a rapid, sequential order in one aspiration process.

It is possible to measure 23 absorption lines precisely in a single step from a sample volume of 50 mL. The special AA spectrometer provides the novel ability to detect spectral overlaps by observing the spectral vicinity of the analyte absorption line, allowing for compensation if necessary.

As well as the special wavelength-independent background correction, the novel design of the contrAA 800 removes the requirement to wait for light source output stabilization and the element-specific settings of slit groups due to factors such as secondary lines.

The best light throughput is always available, and the chemical-physical properties of the analytes do not influence the element-independent light source.

The lowest detection limits can be achieved by accurately measuring analytes, even in the smallest amounts.

Utilizing the AS-FD autosampler, fully automatic dilutions can be applied before measurement and if the highest calibration standard is exceeded.

The autosampler can also be employed to automatically prepare solutions necessary for the calibration function from a single stock standard. It features a fully automated solution preparation procedure for the standard addition procedure.

Materials and Methods

Reference Material

• GBW07408 (NCS DC 73326), soil (Institute of Geophysical and Geochemical Exploration, Langfang, China)
• GBW07306, river sediment (Institute of Geophysical and Geochemical Exploration, Langfang, China)
• BCR-143R sewage sludge with enriched soil (European Commission, Institute for Reference Materials and Measurements)
• BCR-146R sewage sludge of industrial origin (European Commission, Institute for Reference Materials and Measurements)
• BAM-U110 contaminated soil (BAM, Bundesanstalt für Materialforschung und -prüfung, 2006)
• PACS-2 marine sediment (National Research Council of Canada)

Reagents

• Concentrated HNO₃ (65 %, p.a.)
• Concentrated HCl (37 %, p.a.)
• Cesium chloride-lanthanum chloride buffer solution (Cs/La) according to Schinkel (10 g L^-1 CsCl, 100 g L^-1 LaCl₃)
• Certified single element standards for Cr, Mn, Co, Ni, Cu, Zn, Cd, and Pb (concentration of the analytes 1,000 mg L^-1)

Sample Preparation

The samples were digested using aqua regia, according to DIN ISO 11466. The sample material was weighed at aliquots of approximately 0.3 g with a filling volume of 50 mL.

Flame measurement sample preparation followed the DIN ISO 11407 norm. Stable measuring solutions for standards and sample dilution were produced with lower acid contents than those described in the norm.

Samples were diluted with a solution containing 21 % (v/v) concentrated HCl and 7 % (v/v) concentrated HNO₃ for measurements via the flame technique. An additional 10 % (v/v) of Cs/La buffer solution was incorporated for chromium and manganese.

Calibration

Calibration standards to be used as the blank value for the calibration were prepared in a solution with 21 % (v/v) HCl and 7 % (v/v) HNO₃, under the DIN ISO 11047 standard. As mentioned previously, stable measurement solutions were achieved with lower acid contents for standards than described in the norm.

For measuring chromium and manganese using an air-acetylene flame, an additional 10 % (v/v) of the Cs/La buffer solution was added. In these cases, adding the Cs/La solution can be omitted.

Table 1. Concentrations used for calibration according to DIN ISO 11047. Source: Analytik Jena US

Table 2. Typical calibration functions according to DIN ISO 11047. Source: Analytik Jena US

Element	Flame type	Correlation R2 (adj.)	Graphic plot
Cr	Air-acetylene	0.9992 non-linear
Cr	Nitrous oxide-acetylene	0.9997 non-linear
Mn	Air-acetylene	0.9993 non-linear
Mn	Nitrous oxide-acetylene	0.99990 non-linear
Co	Air-acetylene	0.9993 non-linear
Ni	Air-acetylene	0.9997 non-linear
Cu	Air-acetylene	0.9996 non-linear
Zn	Air-acetylene	0.9994 non-linear
Cd	Air-acetylene	0.9997 non-linear
Pb	Air-acetylene	0.9997 non-linear
Pb	Air-acetylene	0.9990 linear

Instrument Settings

The contrAA 800 F is a high-resolution flame atomic absorption spectrometer with continuum emitter as the light source. This spectrometer was employed to determine soil extracts, per DIN ISO 11466.

The 50 mm burner head is equipped with a scraper, enabling automatic cleaning during measurement using the nitrous oxideacetylene flame. The alternative 100 mm burner head provides improved measurement sensitivity with the air-acetylene flame.

An AS-F or AS-FD autosampler with the dilution function can be used to automate the measurement. The AS-FD autosampler can automatically prepare the standard addition procedure and adjust sample dilutions.

Table 3 lists the instrument specifications and measurement parameters used in this study. Table 4 displays the instrument settings and measurement parameters of the method employed. For background correction, the iterative baseline correction was employed.

According to DIN ISO 11047, the absorption wavelength for lead is specified at 217 nm, but the absorption band at 283 nm can also be used.

The 217 nm wavelength exhibits a lower signal-to-noise ratio for line sources, such as hollow cathode lamps, and can be used without significant limitations for HR-CS-AAS. In this investigation, both lines were studied.

Table 3. General instrument parameters. Source: Analytik Jena US

Table 4. Applied method parameters. Source: Analytik Jena US

Results and Discussion

Table 5 displays the detection and quantification limits for the instrument type and measurement settings. The limits were ascertained using the blank value method with an 11-fold blank value measurement and the 3σ or 9σ standard deviation criterion.

Cadmium, chromium, cobalt, copper, lead, manganese, nickel, and zinc were measured in the soil and sediment samples per DIN ISO 11047. Table 6 displays the measurements' results and the expected values of the reference materials.

For chromium and manganese, the air-acetylene flame can give false lower results, even when excess La is added, but such interferences are almost nonexistent in the nitrous oxide-acetylene flame.

Concerning the ionization potential of the nitrous oxide flame, excess K or Cs should be added as KCl or CsCl solutions (Cs/K concentration 0.1-0.2 % [m/m]).

The HR-CS-AA spectrometer contrAA 800 allows the spectral vicinity of analytical lines to be recorded, allowing for spectral overlays to be detected and compensated if required.

Table 7 displays example spectra from the sample BAM U 110.

Iron absorption bands can be seen in the spectra that do not overlap with the analyte lines for some analysis lines. A direct overlap occurs for the analysis line of zinc at 213 nm, originating from NO bands and leading to a false higher result if left uncompensated.

The least-squares background correction (LSBC) method can compensate for overlays, making undisturbed detection of the analysis line possible.

Table 5. Achievable limits of detection (LOD) and limits of quantification (LOQ) of the presented method according to the 3σ or 9σ criterion. Source: Analytik Jena US

Table 6. Measurement results of analyte content determination in soil, sediment and sewage sludge samples. Source: Analytik Jena US

LOQ: Limit of quantification

Table 7. Spectral vicinity of the analysis lines. Source: Analytik Jena US

Element	Spectral vicinity	Remarks
Cr (sample BAM U110)
Mn (sample BAM U110)
Co (sample BAM U110)
Ni (sample BAM U110)
Cu (sample BAM U110)
Zn (sample BAM U110)		*LSBC applied
Cd (sample BAM U110)
Pb (sample BAM U110)
Pb (sample BAM U110)		*LSBC applied

LSBC: Least-squares background correction

Correction Spectra

Spectral overlap of the analyte wavelength caused by molecular or other atomic absorption bands can result in a false analysis. Spectral correction can be employed to compensate for overlaps.

The ASpect CS software can record a suitable correction spectrum using the LSBC algorithm to reduce or remove overlaps of the analysis line.

LSBC was employed for the NO molecule bands adjacent to the zinc analysis line in the series of measurements presented here. The reagent blank value (no detectable Zn contamination present) was employed to generate the correction spectrum. Table 8 displays the effect of LSBC on the 213.9 nm analyte absorption line of zinc.

Table 8. Spectral vicinity of the analytic lines used. Source: Analytik Jena US

Sample	Spectral vicinity
Blank
Std. 1 without correction model
Std. 1 using correction model
sample BAM U110 without correction model
sample BAM U110 using correction model

Figure 1. contrAA 800 with AS-FD. Image Credit: Analytik Jena US

Conclusions

The contrAA 800 F provides a rapid, user-friendly, and cost-effective means of analyzing cadmium, chromium, cobalt, copper, lead, manganese, nickel, and zinc in soil and sewage sludge extracts under DIN ISO 11047.

The method discussed in this article demonstrates the procedure from sample preparation of acid extracts to analysis with a flame atomic absorption spectrometer.

The contrAA 800 F facilitates fast sequential analysis, allowing analysis lines to be detected in a single aspiration process without changing lamps and automatic and intelligent dilution of the AS-FD autosampler. These capabilities make sample analysis with the contrAA 800 F rapid and simple.

The analytical performance capability offered by the pioneering contrAA 800 F is unmatched, taking AA analysis to a new level.

Table 9. Overview of devices, accessories, and consumables. Source: Analytik Jena US

References and Further Reading

1. ISO (1998) Soil quality — Determination of cadmium, chromium, cobalt, copper, lead, manganese, nickel and zinc in aqua regia extracts of soil — Flame and electrothermal atomic absorption spectrometric methods, ISO 11047:1998. https://cdn.standards.iteh.ai/samples/24010/7d67c069009f4c999844d77b20e00e38/ISO-11047-1998.pdf.
2. Slovenski inštitut za standardizacijo and ISO (1995) Soil quality - Extraction of trace elements soluble in aqua regia. https://cdn.standards.iteh.ai/samples/19418/6d98cb74c4a24a81b80a9a6f56642248/SIST-ISO-11466-1996.pdf.

This information has been sourced, reviewed, and adapted from materials provided by Analytik Jena US.

For more information on this source, please visit Analytik Jena US.