New Workflow for X-ray Photoelectron Spectroscopy Allows for Soil Analysis

The Environmental Molecular Sciences Laboratory (EMSL) recently developed a new high-throughput workflow for a widely used surface analysis technique, allowing scientists to analyze 50 soil samples per day. 

This technique—X-ray photoelectron spectroscopy (XPS)—is generally used to analyze the chemical state of elements on mineral surfaces, but the traditional sample preparation and measurement process—especially for soil samples—is so slow that it lacks efficiency. 

Responding to the need to analyze thousands of soil samples from various locations for Molecular Observation Network projects, EMSL scientists created a new workflow applying a sample preparation method originally used for protein and nucleic acid analysis to multimodal physical and chemical soil analysis. 

“What’s really exciting about this new workflow is that it provides an entirely new way to rapidly analyze the chemistry of soil organic matter associated with mineral surfaces, which is incredibly difficult to measure through ‘conventional’ extraction methods,” said John Bargar, the leader of EMSL’s Environmental Transformations and Interactions science area. “The XPS method does an ‘end-run’ around conventional methods by skipping the extractions and directly observing the mineral surfaces to determine the chemical form and oxidation states of mineral-associated organic carbon.” 

The new method utilizes a dot blot apparatus that will allow researchers to gather quantitative information on carbon speciation and associated minerals in the soil’s near-surface region. 

This resource isn’t limited to MONet projects. Users can access the XPS soil analyzer through EMSL’s upcoming Exploratory Research proposal call. 

How the XPS soil analyzer works 

Previously, soil samples for XPS analysis were prepared in small batches of about five samples mounted onto a sample holder. This method presented two challenges—data collection is time-consuming, and the soil samples can’t be used for other analyses such as scanning electron spectroscopy (SEM) or Raman spectroscopy, explained Shuttha Shutthanandan, an EMSL materials scientist. 

The sample preparation, sample handling, and the sample holder constraints limit the seamless transfer of samples between instruments. As a result, samples are prepared separately for each technique for a routine analysis. This method is very inefficient because it is very slow, Bargar said. Additionally, it often leads to uncorrelated data analysis, especially if the sample is as heterogenous as a soil sample. 

With this innovation, soil samples are prepared on filter paper using the dot blot apparatus, a molecular biology technique for detecting proteins. Once prepared, the filter paper is transferred between instruments, such as those for XPS and SEM, for analysis. 

“The new method developed for the high-throughput analysis of soil samples allows researchers to analyze the same sample prepared on filter paper using multiple techniques, resulting in better correlation among the data and analysis,” Shutthanandan said. 

Studies using the XPS soil analyzer 

Nutrient dynamics, microbial interactions, and water retention all have profound environmental and agronomic impacts and are often influenced by the surface properties of soil minerals. 

With this new high-throughput workflow, researchers can learn more about carbon and other nutrients at mineral surfaces, Shutthanandan said. For MONet, surface carbon composition, both organic and inorganic, is of particular interest. While the MONet database already includes data on total carbon and water-extractable (loosely bonded) carbon, information about organic carbon that is tightly bonded to mineral surfaces is lacking. 

Beyond MONet, the high-throughput correlative analysis will appeal to the materials and chemistry research community involved in the design and synthesis of novel materials and understanding their interactions, Shutthanandan offered. 

“This will include materials in catalysis, biomedical, nanomaterials, separations, and electronics applications,” he said. “It will also appeal to the topical area of predictive materials synthesis, which relies on a large amount of correlative analysis data to train artificial intelligence and machine learning models. The high-throughput characterization workflow could enable the generation of high-quality data from multiple samples from multiple instruments in a very short time.” 

The XPS soil analyzer can answer quantitative questions about carbon speciation and the associated minerals in the soil’s near-surface region. Additionally, scientists can correlate these findings with topographic images obtained from SEM, as XPS and SEM analyses are conducted in almost the same location. The high-throughput method provides comprehensive surface chemical, molecular, and topographical information of soil samples. 

The XPS soil analyzer will be the focus of a May 7 webinar. At the free EMSL LEARN webinar, Shutthanandan will provide an overview of XPS and will explain how EMSL’s new XPS high-throughput soil analyzer can accelerate elemental soil interrogation. He will be joined by Catherine Pettinger, an EMSL user from the University of Wisconsin-Madison, who will demonstrate how she used the high-throughput soil analyzer for her research. Ajay Karakoti, a Pacific Northwest National Laboratory materials scientist, will discuss how to leverage artificial intelligence/machine learning tools for XPS surface analysis capabilities.