Matching soil on a killer’s shoes to link them to an outdoors crime scene or disposal site can be up to 100 percent accurate with a combination of two lab analyses, according to a new study in the journal Forensic Science International.

The use of high-performance liquid chromatography (HPLC) and gas chromatography (GC) together produced reproducible results that were always accurate in challenging terrain at four mock crime scene sites, according to the study.

“The 100 percent accuracy achieved in this study using this combined approach suggest that it would be beneficial to measure both HPLC and GC markers where additional exclusionary discrimination is required, providing there is sufficient sample quantity of questioned sample and available resources,” write the scientists, led by the University College London Centre for the Forensic Sciences.

Three of the four sites were from parks in the United Kingdom: Brockwell Park in London, Lochen Park in Edinburgh, and Craigiebuckler Estate in Aberdeen. The fourth was Central Park in Manhattan.

Each site had four “forensically relevant” sampling sites, including potential alibi sites, crime scene sites near water, and concealed locations on paths in wooded areas, according to the paper.

The samples were collected in January 2014, using stainless-steel spatulas, with five grams of topsoil collected at the corners and center point of a 1 meter grid, the scientists explain.

The HPLC sample was 250 mg that had had been centrifuged—a portion of which was injected into an Agilent 1100 HPLC system.

The GC sample was focused on the plant wax markers, the traces left behind by the plants’ leaves and roots and stems in the area. The samples were prepared by crushing with mortar and pestle, and then dried gently in an oven. Prepared subsequently with alcohol and other chemicals, the hydrocarbons were dissolved prior to the GC analyses to determine the kinds of waxes.

The accuracy of each method varied.

For instance, HPLC was perfect in one part of the park in Edinburgh, but not in another. In Central Park, HPLC produced accurate results—but the gas chromatography of the wax markers was 89.9 percent correct in assigning samples to their groups.

But when taken together, and cross-referenced, they were 100 percent accurate.

“In all of the cases where less than 100 percent accuracy was achieved using only one set of markers, 100 percent accuracy was achieved using the combination of the two types of organic marker sets,” they write.

But while GS accuracy rates were slightly lower, and sample preparation is more complicated, the HPLC demands much more material (for instance, approximately 250 mg in comparison with 10 mg for GC), they write.