Solvent-Assisted Laser Desorption Flexible Microtube Plasma Mass Spectrometry for Direct Analysis of Dried Samples on Paper

Abstract

The present study investigated the potential for solvent-assisted laser desorption coupled with flexible microtube plasma ionization mass spectrometry (SALD-FμTP-MS) as a rapid analytical technique for direct analysis of surface-deposited samples. Paper was used as the demonstrative substrate, and an infrared hand-held laser was employed for sample desorption, aiming to explore cost-effective sampling and analysis methods. SALD-FμTP-MS offers several advantages, particularly for biofluid analysis, including affordability, the ability to analyze low sample volumes (<10 μL), expanded chemical coverage, sample and substrate stability, and in situ analysis and high throughput potential. The optimization process involved exploring the use of viscous solvents with high boiling points as liquid matrices. This approach aimed to enhance desorption and ionization efficiencies. Ethylene glycol (EG) was identified as a suitable solvent, which not only improved sensitivity but also ensured substrate stability during analysis. Furthermore, the addition of cosolvents such as acetonitrile/water (1:1) and ethyl acetate further enhanced sensitivity and reproducibility for a standard solution containing amphetamine, imazalil, and cholesterol. Optimized conditions for reproducible and sensitive analysis were determined as 1000 ms of laser exposure time using a 1 μL solvent mixture of 60% EG and 40% acetonitrile (ACN)/water (1:1). A mixture of 60% EG and 40% ACN/water (1:1) resulted in signal enhancements and relative standard deviations of 12, 20, and 13% for the evaluated standards, respectively. The applicability of SALD-FμTP-MS was further evaluated by successfully analyzing food, water, and biological samples, highlighting the potential of SALD-FμTP-MS analysis, particularly for thermolabile and polarity diverse compounds.

Figure 1

(a) Schematic representation of the SALD-FμTP-MS final setup. (b) Photograph displaying the setup with the optimized distances utilized for the analysis. The distance used were d1 = 12 mm, d2 = 2.5 mm, and d3 = 1 mm.

Figure 2

Comprehensive analysis and evaluation of the results obtained under different conditions using LD-FμTP-MS for the analysis of 250 picomoles of amphetamine, imazalil, and cholesterol deposited on patterned paper. (a) Mass spectrum obtained from LD-FμTP-MS analysis of a sample (250 picomoles) deposited on white paper without graphite. (b) Mass spectrum obtained from LD-FμTP-MS analysis of the same sample deposited on white paper with graphite applied on the backside. (c) Extractive ion chromatogram (EIC) of three replicates in three different spots for imazalil (blue trace), cholesterol (pink trace), and amphetamine (black trace); in the figure, the desorption order of the compounds. (d) Mass spectrum obtained from SALD-FμTP-MS analysis of the sample deposited on white paper with graphite on the backside and EG as the desorbing solvent.

Figure 3

Optimization of SALD-FμTP-MS parameters for the analysis of 250 picomoles of amphetamine, imazalil, and cholesterol deposited on patterned paper by investigating the (a) solvent effect, (b) cosolvent effect, (c) MALDI-like matrix influence, (d) laser exposition time, and (e) solvent volume. The bars corresponded to amphetamine (gray), imazalil (blue), and cholesterol (pink).

Figure 4

Mass spectra of an energy drink analyzed in (a) positive ion mode and (b) negative ion mode.

Figure 5

Mass spectra of an energy drink analyzed in (a) positive ion mode and (b) negative ion mode.

Figure 6

Mass spectra of (a) oral fluid spiked with 2 ng/μL of codeine, (b) plasma spiked with 2 ng/μL of cocaine, and (c) blood. The insets show the tandem MS spectra for (a) codeine and (b) cocaine and (c) the zoom-in of m/z range 700–850.