How can existing time-of-flight mass spectrometers be upgraded to support MSI? A detailed guide to retrofitting mass spectrometry imaging sources

Laboratories that have acquired a time-of-flight mass spectrometer (TOF-MS) often face a practical challenge: whilst the instrument itself offers the advantages of high acquisition speed and full mass range, it is limited to ‘point sampling’-style qualitative and quantitative analysis, and cannot reveal the spatial distribution of molecules. To extend the capabilities of existing equipment to include mass spectrometry imaging (MSI), installing a mass spectrometry imaging source whilst retaining the original main unit is currently the most practical approach. This article clearly explains the technical rationale, compatibility limitations and operational considerations for such an upgrade.

Why TOF-MS is suitable as an imaging platform

The core characteristics of a time-of-flight (TOF) mass spectrometer are fast acquisition speed, a wide mass range, and high resolution at the high-mass end. Imaging experiments essentially involve raster scanning of the sample surface, with each pixel representing a mass spectrum, which places high demands on acquisition throughput. TOF-MS is naturally well-suited to this, which is why it has long been adopted in MSI applications. The challenge lies not in the mass analyser, but in ‘how to transform the molecules on the sample surface into a beam of ions that can be detected by the TOF’—a task that relies on the imaging ion source.

Retrofit path: Upgrading from point sampling to area imaging

By retrofitting existing TOF-MS front-ends with imaging sources such as MSI DPI (20–200 μm spatial resolution) or MSI LDPI (2–3 μm spatial resolution), the instrument’s function shifts from ‘conventional mass spectrometry’ to ‘imaging mass spectrometry’. Taking the MSI DPI as an example, it utilises a dual-mode DESI/PI approach: desorption electrospray ionisation (DESI) brings surface molecules into the gas phase, followed by photochemical ionisation (PI) for a second round of soft ionisation. The entire process is matrix-free and requires no pre-treatment. This means that sectioned samples, which previously required matrix preparation, freeze-drying and coating, can now be analysed directly.

For applications requiring higher resolution, MSI LDPI utilises a 355 nm laser scanning at a frequency of 1,000 Hz, with a minimum detectable m/z of approximately 70, achieving spatial resolution in the order of 2–3 μm. This makes it suitable for observing fine distributions and molecular gradients in the vicinity of subcellular structures.

Compatibility: Check first, then proceed

Neo Source INSTRUMENT’s MSI DPI and MSI LDPI have now been adapted for compatibility with the three major mass spectrometry platforms: Agilent, AB SCIEX and Thermo. However, ‘completed compatibility’ does not mean ‘plug-and-play’—for each individual existing instrument, three aspects must still be verified on a case-by-case basis: mechanical interface dimensions, vacuum port specifications and acquisition software protocols. No manufacturer will omit this step on the user’s behalf; verifying these details in advance can significantly shorten the commissioning time.

Five steps for installation

First, submit the instrument model and configuration list for the source supplier to confirm the interfaces and vacuum configuration; second, complete the mechanical and electrical connections; for ambient-pressure sources (such as DESI/PI dual-mode), major modifications to the sample inlet are usually not required; third, calibrate the spray or laser parameters to establish a stable desorption/ionisation window; Fourth, integrate the data acquisition software with the imaging scan control; fifth, run through the entire process using standard samples (such as plant sections with known distribution) to confirm that pixel intensity and spatial registration are correct.

Situations where retrofitting is not recommended

Not all existing instruments are suitable for retrofitting. If the instrument is nearing the end of its service life, the vacuum system is severely degraded, or the software protocol is completely closed and cannot be integrated with the scanning control system, forcing a retrofit may prove counterproductive. In such cases, it is more prudent to evaluate a complete instrument replacement solution. The core value of retrofitting lies in extracting further value from an ‘instrument that is still functioning normally’, rather than forcibly extending the lifespan of obsolete equipment.

Summary

For existing time-of-flight mass spectrometers seeking to expand their capabilities to include MSI, retrofitting an imaging source is a clear and well-established approach. The key technical consideration is selecting the right source solution (DPI’s matrix-free surface imaging, or LDPI’s 2–3 μm high resolution), whilst the key procedural consideration is to verify compatibility at the outset. Once these two points are properly addressed, existing equipment can produce imaging results that may well surprise you.