PADI dives into the chemical analysis of solids
a mass spectrometer (MS) that could provide benefits for
researchers working in drug discovery and quality control (QC)
The new MS technique is known as plasma-assisted desorption/ionisation (PADI) and initial results show that it could provide benefits over existing atmospheric sampling techniques when trying to analyse sensitive samples. The research, published as an early access article in the journal Analytical Chemistry was conducted in the UK by a team of scientists from the University of Nottingham, Keele University, Heriot-Watt University, the Forensic Science Service and Hiden Analytical. The researchers, led by Dr David Barrett of Nottingham University, showed that the source could be used in similar applications to other ionisation sources like desorption electrospray ionisation (DESI) and Jeol's DART (direct analysis in real time) for quickly analysing molecules present on surfaces. DART and DESI have been used to detect molecules on the surfaces of TLC (thin layer chromatography) plates used in organic chemistry laboratories to allow researchers to more efficiently check the progress of their reactions. DESI was developed by researchers from Professor Graham Cooks' group at Purdue University in the US, and recently used to detect and accurately identify bacteria in minutes rather than hours, removing the need for lengthy and laborious pre-treatment steps. The researchers write that: "this initial study has shown that PADI has considerable potential as a valuable and versatile tool for forensic, pharmaceutical and biological applications." The analysis of powders and tablets without the need for dissolution could find use in drug discovery and pharmaceutical manufacturing QC laboratories, as well as in a myriad of forensic applications including checking the authenticity of drugs. "PADI is inherently a very rapid technique which is very easy to use, there are lots of different possibilities and this study has only just started to scratch the surface of where this technique could be applied," said Dr Frank Rutten, of Keele University and one of the authors of the study. He was keen to stress that the majority of the work had been conducted by Dr Lucy Ratcliffe during a postdoctoral fellowship in Dr Barrett's laboratory as well as the vital input from Hiden Analytical who supplied the plasma needle for the source. The PADI technique ionises samples by generating a non-thermal plasma which interacts directly with the surface of interest to create the ions which can then be sampled by a mass spectrometer. This radio frequency-driven atmospheric plasma is cold to the touch and does not heat the sample allowing direct allows the analysis of heat-sensitive samples with no requirement for the removal of any highly energetic species as when using DART. The researchers compared the technique with a DESI source for detecting active pharmaceutical ingredients (APIs) in a range of over-the-counter and prescription pharmaceutical formulations. They coupled the source with two different MS, a single quadrupole Waters LCZ MS and a Hiden Analytical HPR-60 molecular beam MS which consists of a single quadrupole MS with a differentially pumped three stage inlet system. They found that not only was the PADI source faster and easier to set up, but it also appeared to be a 'softer' ionisation technique, causing fewer fragment ions with some analytes. "PADI seems to be at least on a par with the DESI technique and has some practical advantages because it doesn't require solvent and is much less geometry dependant," said Dr Rutten. He was keen to stress that this research was still a work in progress but that he could envisage that the source could, in theory, be combined with very compact MS systems such as the Mini 10 developed by Dr Cooks at Purdue University.