Reliable and repeatable measurements are essential for the strong uptake of any analytical technique. Ambient surface mass spectrometries have demonstrated exciting and revolutionary measurement capability but for wider acceptance in industry they need a secure metrology foundation. Plasma sources are increasingly being used since the development of DART (direct analysis in real time) in 20051 and subsequent imaging methods such as PADI (plasma assisted desorption ionisation)2 and LTP (low temperature plasma)3. Here, we commence establishing the essential metrology for PADI, which has already been shown to be successful at analysing pharmaceuticals2 and personal care products4. A detailed study of the instrumental contributions to the spectral repeatability is conducted. Firstly, we identify and separate out the contributions to the repeatability (relative standard deviation) of the peak intensities. We show that the Thermo LTQ-Orbitrap mass spectrometer used in the study has a noise distribution approximately as expected for random noise. However, the standard deviation of the peak intensity is proportional to the peak intensity which is clearly not random but related to systematic effects in the source operation. Optimisation of the plasma source is essential for ensuring robustness and reliability. We show how the signal varies with geometry, helium flow rate and plasma power. Thermal imaging of the sample surface shows that the temperature rises approximately linearly with plasma power and at 19 W is 70oC. To reduce the effects of damage for surface sampling, we recommend keeping the surface temperature below 40 °C by operating at less than 15 W. General guidance is given for practical analysis. Importantly, PADI can analyse a wide variety polymers. Analysis of polymers gives mass spectra with repeating series containing different fragments and adducts. Preliminary studies of sample surfaces after PADI analysis show that the chemical damage, quantified by XPS, is less than 10%. With the recommended operating parameters, the plasma erodes the polymer sample at a rate of 0.87 nm s-1 (for PMMA). This shows the potential for polymer depth profiling.

1. RB Cody, JA Laramee, HD Durst, Analytical Chemistry 2005, 77, 2297.

2. LV Ratcliffe, FJM Rutten, DA Barrett, T Whitmore, D Seymour, C Greenwood, Y Aranda-Gonzalvo, S Robinson, M McCoustra, M. Analytical Chemistry 2007, 79, 6094.

3. JD Harper, NA Charipar, CC Mulligan, XR Zhang, RG Cooks, Z Ouyang, Analytical Chemistry 2008, 80, 9097.

4. TL Salter, FM Green, N Faruqui, IS Gilmore, Analyst, 2011, 136, 3274.