Analysis of carbon and nitrogen in soils

This application note illustrates the use of the Ultimate edition of the Zetium XRF spectrometer for the analysis of carbon and nitrogen.

Soil is a vitally important resource that supports approximately 90 % of all human food, livestock feed, fiber and fuel requirements. Furthermore, soil is increasingly recognized as an important sink for atmospheric CO2 that can help offset greenhouse gas emissions due to the burning of fossil fuels. Unfortunately, soil is not renewable and readily suffers from erosion, contamination, acidification and degradation below that required to support profitable cultivation. For these reasons there is increasing pressure on governments to pass protective legislation and promote best practices for the sustainable exploitation of soil. 

Zetium Ultimate edition

Introduction

Soil is a vitally important resource that supports approximately 90 % of all human food, livestock feed, fiber and fuel requirements. Furthermore, soil is increasingly recognized as an important sink for atmospheric CO2 that can help offset greenhouse gas emissions due to the burning of fossil fuels. Unfortunately, soil is not renewable and readily suffers from erosion, contamination, acidification and degradation below that required to support profitable cultivation. For these reasons there is increasing pressure on governments to pass protective legislation and promote best practices for the sustainable exploitation of soil.

Soil chemistry is an important parameter for gauging the fertility of soils, particularly the carbon, nitrogen, phosphorus and trace heavy metal content. In natural, uncultivated soils, the concentration of these elements depends on biological activity and the composition of the underlying geology. However, in cultivated soils tilling, heavy cropping and fertilization can radically change soil composition. In Europe the use of organic slurries and inorganic fertilizers is regulated to protect soils and the surrounding environment. The EU Directive 91/676/EEC concerns the protection of waters against pollution caused by nitrates from agricultural sources and aims to reduce and prevent pollution of surface waters and groundwater by nitrates from agricultural sources, particularly livestock manure and chemical fertilizers.

Similarly, EU Directive 86/278/EEC seeks to restrict the maximum permissible concentrations of potentially toxic heavy metals in soils and sewage sludges used on the land. X-ray fluorescence spectrometry is used extensively for geochemical analysis. The main reasons for this are simple sample preparation, high accuracy and precision, wide dynamic range (% to ppm) and good to excellent detection limits across large parts of the periodic table (Na-U). These benefits also apply to soil samples, for which average detection limits for many elements of agricultural and environmental importance are well below 1 ppm. This application note illustrates the use of the Ultimate edition of the Zetium XRF spectrometer for the analysis of carbon and nitrogen.

Preparation of standards and samples 

The application was calibrated using the GBW-series (GBW07401 to GBW07408) of geochemical reference materials (Chinese Institute of Geophysical and Geochemical Prospecting, PRC).

The preparation of both standards and routine samples is easily mastered, safe and inexpensive. The oven-dried (105 °C) sample is pulverized for 30 seconds in a swing mill. This material is then pressed at 200 kN for 30 s into 40 mm diameter pellets, using 4 g boric acid as backing. Carbon- and/or nitrogen-containing binder compounds cannot be used, however, boric acid can be used as a backing to the pellet to improve its durability. For very sandy soils with a high quartz content, adding 10 % boric acid as a binder also helps pellet formation. The surface finish of the pressed pellets is important for reproducible analysis of carbon and nitrogen and care should be taken to control its quality.

The Ultimate edition of the Zetium XRF spectrometer

The Ultimate edition of the Zetium XRF spectrometer meets the requirements for the most demanding XRF analysis - from high-precision analysis of majors to fast and accurate determination of traces and light elements. The standard configuration includes an extensive amount of features, including the SST R-mAX50 X-ray tube with CHI-BLUE coating, duplex detector, the Hi-Per scint detector with a linear count rate range up to 3.5 Mcps and Omnian standardless software.

Accuracy

The accuracies of the calibrations for carbon and nitrogen are presented in Table 1. The calibration RMS value is a statistical comparison (1 sigma) of the certified chemical concentrations of the standards with the concentrations calculated by the regression of the calibration procedure. Figures 1 and 2 show calibration plots for carbon and nitrogen giving graphic illustrations of the accuracy of the method.

Table 1. Calibration quality 

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Figure 1. Calibration plot for carbon (wt%) 

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Figure 2. Calibration plot for nitrogen (wt%) 

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Method precision

The stability of the Ultimate edition of the Zetium XRF spectrometer is excellent. Repeatability data on stable media measured over a ten-day period, typically have relative standard deviations better than two times the error inherent in X-ray counting statistics. To demonstrate the precision of the analytical method, including sample preparation, 10 preparations of a soil sample were made and measured against the calibration. The mean concentration and RMS (absolute and relative) of these measurements are presented in Table 2 and illustrated graphically in Figures 3 and 4. The total measurement time for one sampl was 232 seconds. For comparison, the counting statistical error (CSE), which represents the minimum possible error at a given measurement time, is also given in Table 2.

Table 2. Analytical precision: based on twelve separated preparations of a soil sample 

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Figure 3. Analytical repeatability: carbon data for twelve separated preparations of a soil sample (2σ errors indicated) 

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Figure 4. Analytical repeatability: nitrogen data for twelve separated preparations of a soil sample (2σ errors indicated) 

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Total soil analysis by XRF

The Ultimate edition of the Zetium XRF spectrometer is capable of measuring all the major and trace element constituents of soil. Important macro-nutrients like Mg, P, S, K and Ca, can be analyzed with a very high degree of accuracy and precision.

For trace element, or micro- nutrient, analysis the Pro-Trace option of the SuperQ software is ideal. Developed by highly experienced analysts, Pro-Trace is a unique solution for trace element analysis, with unrivaled accuracy and precision.

Pro-Trace is founded on advanced algorithms that address the specific challenges  associated with determining low elemental concentrations. Expert knowledge embedded within the software covers the ‘what to measure‘ and ‘how to measure’ and high-purity, multi-element setup standards complete the package.

Refer to PANalytical Pro-Trace application notes for detailed information and application examples of trace elemental analysis using Pro-Trace.

Detection limits

The detection limits for carbon and nitrogen in typical soil samples is given in Table 3. The lower limit of detection (LLD) is calculated from:

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Table 3. Detection limits 

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Components typeset in bold were present in the spectrometer used to obtain the data in this note. 

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Summary and conclusion

The results clearly demonstrate that the Ultimate edition of the Zetium spectrometer is capable of analyzing carbon and nitrogen at the levels commonly encountered in soils (0.5 - 2.5 % C and 0.02 - 0.15 % N). Measurements are accurate and precise and the method benefits from simple, essentially hazard-free sample preparation.

The stability of the Ultimate edition of the Zetium spectrometer is such    that individual calibrations can be used for many months. The data presented here are comparable to data that can be obtained on dedicated, combustion-based carbon and nitrogen analyzers that are available on the market. However, with the addition of light-element (carbon and nitrogen) specific PX-crystals to the configuration, the Ultimate edition of the Zetium spectrometer offers a full range of elemental analysis for soils from carbon to uranium. This is a distinct value proposition compared to the purchase of a dedicated instrument for light-element analysis.

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