Molecular Surface Sampling and Chemical Imaging using Proximal Probe Thermal Desorption/Secondary Ionization Mass Spectrometry

Abstract

Proximal probe thermal desorption/secondary ionization mass spectrometry was studied and applied to molecular surface sampling and chemical imaging using printed patterns on photopaper as test substrates. With the use of a circular cross section proximal probe with a tip diameter of 50 μm and fixed temperature (350 °C), the influence of probe-to-surface distance, lane scan spacing, and surface scan speed on signal quality and spatial resolution were studied and optimized. As a compromise between signal amplitude, signal reproducibility, and data acquisition time, a surface scan speed of 100 μm/s, probe-to-paper surface distance of 5 μm, and lane spacing of 10 μm were used for imaging. Under those conditions the proximal probe thermal desorption/secondary ionization mass spectrometry method was able to achieve a spatial resolution of about 50 μm as determined by the ability to distinguish surface patterns of known dimensions that were printed on the paper substrate. It is expected that spatial resolution and chemical image quality could be further improved by using probes of smaller cross section size and by incorporating a means to maintain a fixed optimal probe-to-surface distance real time, continuously adapting to the changing topography of the surface during a lane scan.

Mass spectrometry imaging under ambient conditions

“…In a recent report, resolution of 50μm was demonstrated with a heated probe using printed patters of ink on paper as test materials. This technique, termed thermal desorption proximal probe with secondary ionization (TD/SI-MS) (Ovchinnikova et al, 2011), is related to PESI considering that both techniques require an initial contact between the probe and the material on the surface to produce images. However, TD/SI-MS promotes thermal desorption of the analytes in a similar way to DAPPI (see section F) in contrast to PESI in which the probe is used as an ESI emitter (Figure 8a).…”

“…In this latter case, transmission geometry laser ablation is used to desorb the sample into an ablation plume which is captured by the liquid meniscus of the probe (Figure 9d). (Ovchinnikova et al, 2011) A methanol/water solvent is normally used and the flow rate (10–20 μL/min) adjusted depending on the system details. Other examples of solvent probe methods include the scanning mass spectrometry (SMS) probe (Kottke et al, 2010), nano-DESI (Roach et al, 2010), and LESA (Eikel et al, 2011).…”

“…The imaging resolution was limited by the laser ablation area (~210 μm) and although it can be reduced to ~20 μm or less since laser ablation spots of this size had been reported in other transmission geometry laser ablation/mass spectrometry experiments,(Richards et al, 2011) the increased resolution found in the non-contact mode(100–150 μm) compared to the direct contact (0.67–1mm) mode represents an exciting advance for LMJ-SSP MSI. (Ovchinnikova, et al, 2011)…”

“…The blue circles show the regions of interest with an apparent resolution of 100μm. Panels a and c are reproduced from (Van Berkel, et al, 2008), panel d and e are reproduced from (Ovchinnikova, et al, 2011), and panel b is reproduced from (Van Berkel et al, 2009) with permission.…”

Imaging Mass Spectrometry for Drugs and Metabolites

“…In this study, they used an automated DESI source connected to a linear ion trap mass spectrometer and a spray solvent of 70:30 methanol : water. Acquired image resolution varied, depending on the experimental objective, with the highest being 245 × 245 μm, although more recent experiments have demonstrated the capacity to achieve ∼50 × 50 μm [60]. An example image of clozapine in brain is shown in Figure 11.4.…”