Electrospray/Photochemical Ionisation Mass Spectrometry Imaging Source (DESI/PI) for in situ imaging of polar and non-polar substances

Areas of application: neuroscience; pathology; plant metabolomics;

‌Introduction

The electrospray/photochemical ionisation mass spectrometry imaging source (DESI/PI) employed in this experiment utilises secondary ionisation from a krypton vacuum ultraviolet lamp to achieve simultaneous imaging of polar and non-polar biomolecules in tissue sections under ambient pressure, thereby addressing the issues of low ionisation efficiency for non-polar compounds and significant ion suppression effects associated with conventional DESI. This detection method requires no matrix and no complex sample preparation; it enhances the signal intensity of neutral molecules by two orders of magnitude and is suitable for two types of samples: animal brain tissue and fresh plant leaves, providing a novel characterisation tool for research in neuroscience, clinical pathology and plant bioactive compounds.

‌Materials and Methods

Instrumentation: Electrospray/Photochemical Ionisation Mass Spectrometry Imaging Source (DESI/PI), Agilent 6224 high-resolution time-of-flight mass spectrometer;

Samples: Mouse brain tissue sections, fresh tea plant leaf sections;

Spray solvent: Methanol/toluene/formic acid (volume ratio 70:30:1)

 Figure 1  Schematic diagram of the DESI/PI experimental setup

 Results and Conclusions

1. Imaging of mouse brain tissue sections

In cation mode, standard DESI shows prominent signals only for choline and phosphatidylcholine, whilst neurotransmitters and neutral lipids show almost no response; DESI/PI can simultaneously detect small neuro-molecular species such as GABA, glutamine, glutamate, creatine and adenosine; the creatine signal is amplified 200-fold, whilst non-polar lipids such as cholesterol and galactosylneuraminide are clearly visualised, with their distribution primarily in the cerebellar white matter and corpus callosum.

  Figure 2: Average mass spectra of whole-brain tissue sections (background-subtracted) obtained using DESI/PI with methanol/toluene/FA (v:v:v = 70:30:1) as the solvent, and DESI with methanol/FA (100:1) as the solvent, along with imaging results for some representative species.

In negative ion mode, DESI can detect PE, PE-O and PS lipids, whilst DESI/PI can independently visualise glutamine, glutamic acid and hexosylneuraminide; the ion yield for PE and PE-O is increased several-fold, whilst the signals for PI and sulpholipids are slightly reduced, with the two detection modes providing complementary information.

 Figure 3  Average mass spectra (background subtracted) of two brain tissue sections acquired using DESI/PI and DESI in negative ion mode (A). Methanol/toluene (v:v = 7:3) was used as the solvent. Mass spectrometry imaging of brain tissue (B).

2. Imaging of fresh tea leaf tissue

Tea polyphenols are highly non-polar; traditional DESI has poor ionisation efficiency, with the main peak in the spectrum being only phosphocoline and the caffeine signal being very weak; In DESI/PI mode, the caffeine signal is amplified 200-fold, enabling in situ imaging of neutral flavanols such as epicatechin, ECG and EGCG, whilst characteristic fragments of thermally unstable tannins can be reliably detected at m/z 272.07 and 289.07.

Figure 4: Average mass spectra (background subtracted) of two consecutive fresh tea plant leaf-stem cross-sections obtained in positive ion mode using DESI/PI with a methanol/toluene/FA (v:v:v = 70:30:1) solvent system and DESI with a methanol/FA (100:1) solvent system, respectively.

Conclusions

The electrospray/photochemical ionisation mass spectrometry imaging source (DESI/PI) allows for seamless switching between DESI and DESI/PI operating modes, enabling matrix-free detection at atmospheric pressure. It eliminates the need for tissue rinsing or chemical derivatisation, thereby preventing the loss of endogenous metabolites and the displacement of molecular spatial positions; It utilises a low-cost krypton lamp in place of a laser ionisation source, features a compact design, and is compatible with mainstream commercial mass spectrometry instruments. This technology serves as a complementary approach to MALDI mass spectrometry imaging and holds significant application value in the fields of basic neuroscience research, pharmaceutical analysis, the delineation of tumour surgical margins, and the detection of the spatial distribution of plant secondary metabolites.

References

[1] Chengyuan Liu, Keke Qi, Lei Yao, Ying Xiong, Xuan Zhang, Jianye Zang, Changlin Tian, Minggao Xu, Jiuzhong Yang, Zhenkun Lin, Yongmei Lv, Wei Xiong*, and Yang Pan*, Imaging of polar and nonpolar species using compact desorption electrospray ionisation/post-photoionisation mass spectrometry, Analytical Chemistry, 2019, 91, 110, 6616–6623