# An Absolute Quantitative Approach to Study the Desorption Step in Plasma-Based Ambient MS Methods

## TL;DR

This paper introduces a method to precisely measure how much sample is lifted into the gas phase during plasma-based ambient mass spectrometry experiments.

## Contribution

A protocol for absolute quantification of the desorption step in plasma-based ambient MS is developed and validated.

## Key findings

- Desorption efficiency was measured with high precision (RSD ≤ 7%) using low-temperature plasma.
- Subtle changes in desorption were observed based on variables like discharge gas and exposure time.
- Fluorescence microscopy showed uneven analyte deposition on the substrate.

## Abstract

The desorption step in ambient mass spectrometry, concerted
or
decoupled with ionization, triggers the transfer of a sample (analytes)
from the condensed phase or surface to the gas phase. Depending on
the type of method, the desorption is caused by momentum transfer,
ultrasound, thermal energy, or laser pulses, among other means. In
the case of plasma-based methods, thermally assisted desorption is
the most commonly discussed route for analyte desorption, and although
often postulated, there is no clear evidence of other mechanisms related
to high-energy species created in the discharge. This study addresses
the assessment of a protocol to allow absolute quantification of the
desorption step during plasma-based ambient MS experiments. As a proof
of principle, we measured the desorption efficiency of low-temperature
plasma (LTP), which is the more widespread DBD-based ambient MS method.
Model analytes such as arginine, cocaine, rhodamine G, or imazalil
have been selected to quantify the desorption efficiency using 20
ng in each experiment. Two microliters of analyte solution were deposited
on a glass substrate (18 mm × 18 mm) and allowed to dry. Then,
they were exposed to different LTP plasma conditions (discharge gas,
probe position, and desorption time). After the sample substrate was
redissolved with an appropriate solvent, quantitative data were obtained
using liquid chromatography/tandem mass spectrometry. Selected experiments
have been completed, demonstrating the ability to quantitatively measure
the amount of analyte desorbed with high precision (RSD ≤ 7%),
finding subtle changes (in the absolute picomole range) when different
variables such as discharge gas nature or exposure time were evaluated.
Through the use of spatially resolved fluorescence microscopy measurements,
we also noticed that analyte deposition is not evenly distributed
on the substrate. This evidence, together with the experimental quantitative
data, confirms that the conditions used to quantify the desorption
are also rugged to experimental aspects such as sample deposition,
since the analyte spot size interrogated (1 mm diameter) is distinctly
smaller than the LTP probe diameter (4 mm i.d.) and the plasmajet
area that impinges the entire sample surface. Analyte–analyte
interactions and sample thickness may also be relevant in explaining
the desorption in plasma-based ambient MS methods.