Technique takes chemical analysis to femtogram scale

The technique is so precise it will allow scientists to study in detail the different parts of a nanostructure. It will also be useful to minimise the cost of taking a sample from valuable materials. "Some new materials cost many thousands of dollars per gram. One would try to minimise the material consumed for various chemical analytical techniques​," says William King from the University of Illinois Urbana-Champaign, who recently published the work in Analytical Chemistry​. Previously, the smallest possible samples to be analysed weighed around 10 femtograms, whereas King's technique can determine the chemical composition and structure of samples just one tenth of this mass. These systems were also very costly, whereas King's technique uses equipment readily available in many laboratories. "Previous work used 'boutique' chemical spectroscopy techniques which exist in only a few laboratories. In our paper, we report femtogram chemical analysis which uses instrumentation available in many thousands of laboratories​," he explained. King's system uses atomic force microscopy to image the material as the sample is being taken from the material using a silicon cantilever probe. The AFM microscopy is precise enough to allow the scientists to extract the sample from a specific part of the material. The composition of many structures varies at the nanoscale, but this precision allows scientists to analyse and identify each part in turn, something other techniques had failed to do. "Our technique is useful for analysing samples that have chemical or morphological heterogeneity at the nanometer scale​," he says. "This is particularly useful for understanding the composition of new polymers, pharmaceutical candidates, and tissue samples​." The extracted sample changes the way the cantilever resonates depending on its mass. By measuring this change, scientists can calculate how much the sample weighs. Once the sample has been taken, the cantilever is heated to a precision of 0.1 degrees until the sample melts, allowing the scientists to determine its precise melting point. The scientists then analyse the melted sample using Raman or Fourier Transform infrared spectroscopic imaging to determine the composition and molecular structure of the material. "Fourier transform infrared and Raman spectroscopic imaging have become commonplace in the last five to 10 years​," said Rohit Bhargava, a professor of bioengineering at the University of Illinois Urbana-Champaign and a co-author of the paper. "Our method combines atomic force microscopy with spectroscopic imaging to provide data that can be rapidly used for spectral analyses for exceptionally small sample sizes​." To clean the cantilever for reuse, the tip is heated to well above the decomposition temperature of the sample. "Since the tip can be heated to 1,000 degrees Celsius, most organic materials can be readily vaporised and removed in this manner​," King said.