Analyzing soil samples with HDXRF significantly differs from the use of wet chemistry. The following application note outlines the differences between HDXRF and wet chemistry methods in both theory and practice, and illustrates that substantial variability exists in wet chemistry results, motivating an increase in sampling to ensure statistical significance.
AN OVERVIEW OF SOIL ANALYSIS METHODS
HIGH DEFINITION X-RAY FLUORESCENCE (HDXRF):
X-ray fluorescence (XRF) is an elemental analysis technique that uses the unique X-ray signatures of individual elements to identify the atomic composition of a sample exposed to incident X-rays. HDXRF extends traditional XRF using XOS’ patented Doubly Curved Crystal (DCC) optics to enhance measurement sensitivity, precision, and accuracy. Multiple optics capture X-rays from a divergent X-ray beam. The optics filter and focus multiple monochromatic incident beams to the sample. By using focused monochromatic excitation beams in three different energy regions, HDXRF can eliminate scattering background and reduce interferences that hinder measurement sensitivity, repeatability, and speed. HDXRF technology can analyze heavy metal content in soil directly, without the need for sample digestion, and can be applied both in the lab and on-site.
Analytical methods such as ICP-AES, ICP-MS, AAS, or AFS are generally referred to as “wet chemistry” because they require a liquid sample for proper measurement. For soil analysis, where the sample of interest starts in a solid state, extensive preparation is required to ensure accurate results. The sample must be chemically dried, ground and mixed into a homogeneous consistency, then a 1–2g subsample is heated with chemical reagents (aqua regia or other mixed acids) in a digestion process that results in a solution. The digestion and analysis by wet chemistry are typically performed in a laboratory, and can take several hours using local facilities, or several days if the task is outsourced to a third-party lab. The quality and consistency of the results are often determined by the calibration of the instrument (which must be performed daily) and any interferences from the sample matrix itself. Samples which do not fall within the calibration curve will need to be remade or diluted and then analyzed again which can also contribute to longer overall testing time. It requires a highly trained chemist or operator to interpret the results and the hazardous chemical waste must be handled and disposed of appropriately.
HETEROGENEITY BETWEEN SAMPLES AND WITHIN A SAMPLE:
Depending on the source of contamination and site conditions, soil samples can be extremely heterogenous, requiring careful sampling practices to ensure that measurements accurately reflect the distribution and concentration of heavy metals. This variation can occur both within an individual sample, and across a survey region of interest. Best practices have a common root in making multiple measurements to provide a sufficient population to calculate the statistical characteristics of the site. Even within an individual sample, the mixing and subdividing of a sample can provide an estimate for the localized variance of heavy metal contamination.
“XRF DATA CANNOT MATCH ICP DATA ANY BETTER THAN ICP DATA CAN MATCH ITSELF!”
- Deana Crumbling, US EPA
This quote originated from a presentation titled “Incremental-Composite Sampling (ICS) and XRF: Tools for Improved Soil Data” (3/5/2013), given by Deana Crumbling. ICP data shows substantial variation for individual samples, and inter-lab comparisons can reveal calibration biases and trends. It is important to recognize that variations between ICP and XRF results can come from both methods, along with components due to sample inhomogeneity itself.
A DIRECT COMPARISON OF HDXRF AND WET CHEMISTRY
To compare HDXRF with wet chemistry, there are three different methods typically employed:
1. CRM (CERTIFIED REFERENCE MATERIALS) SAMPLE ANALYSIS:
Using Certified Reference Materials is a common way of testing both HDXRF and wet chemistry methods for accuracy. In this case, the HDXRF results can be compared directly against the certified composition of the soil which is determined by wet chemistry. These samples are homogenous, and using CRM standards, HDXRF has demonstrated very high accuracy, with typical deviation of less than 15 – 20% of the certified value. With no costs associated with repeat measurements, it is easy to build statistical confidence in the result in a very short time.
2. WELL PREPARED SAMPLES:
In the absence of a CRM, preparing a homogenous sample for independent measurement is suitable for comparison of HDXRF with wet chemistry. For example, taking a substantial sample (50 – 100g) and grinding it to pass through a number 50 sieve or higher (< 275 um particle size), then thoroughly blending the sample to make a homogenous distribution makes a sample that is anticipated to have little variance in elemental concentration due to sample heterogeneity. Figure 1 shows a comparison of HDXRF with ICP for Arsenic (As) using this approach:
3. RAW SAMPLES:
With raw samples, homogeneity cannot be guaranteed, especially for different measurement locations, and the separation of true sample variation from analytical deviation becomes difficult. Below shows a comparison of 3 independent wet chemistry results for 7 samples across 4 different heavy metals with results measured by HDXRF. Though there is substantial variation between the wet chemistry tests, the HDXRF results are within the range of variation for each of the resulting sets.
Under a variety of sample and measurement conditions, HDXRF shows strong correlation with results achieved with wet chemistry. ASTM D8064-16 provides a standard method for using HDXRF to measure soil samples for heavy metal contamination, and documents the excellent agreement of the technique with NIST standard reference materials.