LASER DESORPTION/ABLATION POSTIONIZATION MASS SPECTROMETRY: RECENT PROGRESS IN BIOANALYTICAL APPLICATIONS

Hierarchical superstructure aerogels for in situ biofluid metabolomics

High-throughput biofluid metabolomics analysis for screening life-threatening diseases is urgently needed. However, the high salt content of biofluid samples, which introduces severe interference, can greatly limit the analysis throughput. Here, a new 3-D interconnected hierarchical superstructure, namely a "plasmonic gold-on-silica (Au/SiO2) double-layered aerogel", integrating distinctive features of an upper plasmonic gold aerogel with a lower inert silica aerogel was successfully developed to achieve in situ separation and storage of inorganic salts in the silica aerogel, parallel enrichment of metabolites on the surface of the functionalized gold aerogel, and direct desorption/ionization of enriched metabolites by the photo-excited gold aerogel for rapid, sensitive, and comprehensive metabolomics analysis of human serum/urine samples. By integrating all these unique advantages into the hierarchical aerogel, multifunctional properties were introduced in the SALDI substrate to enable its effective utilization in clinical metabolomics for the discovery of reliable metabolic biomarkers to achieve unambiguous differentiation of early and advanced-stage lung cancer patients from healthy individuals. This study provides insight into the design and application of superstructured nanomaterials for in situ separation, storage, and photoexcitation of multi-components in complex biofluid samples for sensitive analysis.

Development of Transmission Ambient Pressure Laser Desorption Ionization/Postphotoionization Mass Spectrometry Imaging

Laser-based high-resolution mass spectrometry imaging at ambient conditions has promising applications in life science. However, the ion yield during laser desorption/ablation is poor. Here, transmission atmospheric pressure laser desorption ionization combined with a compact postphotoionization (t-AP-LDI/PI) assembly with a krypton discharge lamp was developed for the untargeted imaging of various biomolecules. The spatial distributions of numerous lipid classes, fatty acids, neurotransmitters, and amino acids in the subregions of mouse cerebellum tissue were obtained. Compared with single laser ablation, the sensitivities for most analytes were increased by 1 to 3 orders of magnitude by dopant-assisted postphotoionization. After careful optimization, a spatial resolution of 4 μm could be achieved for the metabolites in mouse hippocampus tissue. Finally, the melanoma tissue slices were analyzed using t-AP-LDI/PI MSI, which revealed the metabolic heterogeneity of the melanoma microenvironment and exhibited the phenomenon of abnormal proliferation and invasion trends in tumor cells.

All-aqueous microfluidic printing of multifunctional bioactive microfibers promote whole-stage wound healing

Wound healing involves multi-stages of physiological responses, including hemostasis, inflammation, cell proliferation, and tissue remodeling. Satisfying all demands throughout different stages remains a rarely addressed challenge. Here we introduce an innovative all-aqueous microfluidic printing technique for fabricating multifunctional bioactive microfibers, effectively contributing to all four phases of the healing process. The distinctive feature of the developed microfibers lies in their capacity to be printed in a free-form manner in the aqueous-two phase system (ATPS). This is achieved through interfacial coacervation between alkyl-chitosan and alginate, with enhanced structural integrity facilitated by simultaneous crosslinking with calcium ions and alginate. The all-aqueous printed microfibers exhibit exceptional performance in terms of cell recruitment, blood cell coagulation, and hemostasis. The inclusion of a dodecyl carbon chain and amino groups in alkyl-chitosan imparts remarkable antimicrobial properties by anchoring to bacteria, complemented by potent antibacterial effects of encapsulated silver nanoparticles. Moreover, microfibers can load bioactive drugs like epidermal growth factor (EGF), preserving their activity and enhancing therapeutic effects during cell proliferation and tissue remodeling. With these sequential functions to guide the whole-stage wound healing, this work offers a versatile and robust paradigm for comprehensive wound treatment, holding great potential for optimal healing outcomes.

Laser-Pulse-Length Effects in Ultrafast Laser Desorption

Shortening the laser pulse length opens up new opportunities for laser desorption (LD) of molecules, with benefits for mass spectrometry (MS) sampling and ionization. The capability to ablate any material without the need for an absorbing matrix and the decrease of thermal damage and molecular fragmentation has promoted various applications with very different parameters and postionization techniques. However, the key issues of the optimum laser pulse length and intensity to achieve efficient and gentle desorption of molecules for postionization in MS are not resolved, although these parameters determine the costs and complexity of the required laser system. Here, we address this research gap with a systematic study on the effect of the pulse length on the LD of molecules. Keeping all other optical and ionization parameters constant, we directly compared the pulses in the femtosecond, picosecond, and nanosecond range with respect to LD-induced fragmentation and desorption efficiency. To represent real-world applications, we investigated the LD of over-the-counter medicaments naproxen and ibuprofen directly from tablets as well as the LD of retene and ship emission aerosols from a quartz filter. With our study design, we excluded interfering effects on fragmentation and LD efficiency from, for example, collisional cooling or postionization by performing the experiments in vacuum with resonance-enhanced multiphoton ionization as the postionization technique. Regarding LD-induced fragmentation, we already found benefits for the picosecond pulses. However, the efficiency of LD was found to continuously increase with decreasing pulse length, pointing to the application potential of ultrashort pulses in trace analytics. Because many interfering effects beyond the LD pulse length could be excluded in the experiment, our results may be directly transferable to the LD applied in other techniques.

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

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.

Single‐Photon‐Induced Post‐Ionization to Boost Ion Yields in MALDI Mass Spectrometry Imaging**

Matrix‐assisted laser desorption/ionization mass spectrometry imaging (MALDI‐MSI) is a rapidly growing method in the life sciences. However, for many analyte classes, its sensitivity is limited due to poor ionization efficiencies. To mitigate this problem, we here introduce a novel post‐ionization scheme based on single‐photon induced chemical ionization using pulsed RF‐Kr lamps. The fine‐vacuum conditions of a dual ion‐funnel ion source effectively thermalize the evolving MALDI plume and enable ample gas‐phase reactions. Injected chemical dopants crucially support fragment‐less ionization to [M+H]⁺/[M−H]⁻ species. Based on this interplay, numerous glycerophospho‐, sphingo‐, and further lipids, registered from mammalian tissue sections, were boosted by up to three orders of magnitude, similar to results obtained with laser‐based post‐ionization (MALDI‐2). Experiments with deuterated matrix and dopant, however, indicated complex chemical ionization pathways different from MALDI‐2.

Mass Spectrometry Imaging for Spatial Chemical Profiling of Vegetative Parts of Plants

The detection of chemical species and understanding their respective localisations in tissues have important implications in plant science. The conventional methods for imaging spatial localisation of chemical species are often restricted by the number of species that can be identified and is mostly done in a targeted manner. Mass spectrometry imaging combines the ability of traditional mass spectrometry to detect numerous chemical species in a sample with their spatial localisation information by analysing the specimen in a 2D manner. This article details the popular mass spectrometry imaging methodologies which are widely pursued along with their respective sample preparation and the data analysis methods that are commonly used. We also review the advancements through the years in the usage of the technique for the spatial profiling of endogenous metabolites, detection of xenobiotic agrochemicals and disease detection in plants. As an actively pursued area of research, we also address the hurdles in the analysis of plant tissues, the future scopes and an integrated approach to analyse samples combining different mass spectrometry imaging methods to obtain the most information from a sample of interest.

Effect of the Laser Pulse Width in MALDI-2: A Comparative Study of Picosecond versus Nanosecond Wide Pulses for Laser Postionization

MALDI-2 is a recently introduced technique for postionization (PI) in matrix-assisted laser desorption/ionization (MALDI). It is based on an initial photoionization of neutrally desorbed matrix molecules and subsequent charge-transfer reactions in a fine vacuum or atmospheric pressure ion source. MALDI-2 significantly increases the ion yields for numerous classes of analytes, including lipids, glycans, and a range of pharmaceuticals. To obtain insights into the ionization mechanisms underlying the primary step of PI in MALDI-2, we here conducted a set of experiments with two lasers at 266 nm wavelength and pulse durations of 28 ps and 6 ns, respectively, on a modified orthogonal-extracting time-of-flight mass spectrometer (QTOF, Synapt). 2,5-Dihydroxybenzoic acid (DHB) and 2,5-dihydroxyacetophenone (DHAP) were investigated as MALDI matrices in the positive-ion mode with standardized lipid samples. Analyte- and matrix-derived ion signals were recorded as a function of PI laser pulse energies. The ion signal intensity displays a quadratic dependency on PI-laser pulse energy for low to moderate intensities of up to ∼10⁷ W/cm². This behavior suggests the involvement of resonance enhanced two-photon ionization (REMPI) of neutral matrix molecules in the ionization pathways. Comparing nanosecond and picosecond pulses at the same PI laser pulse energy, higher photon density produced by the shorter pulses generally produced sizably higher ion signal intensities, also corroborating an involvement of REMPI-like processes. Based on a theoretical description of the MALDI-2 process derived from prevalent REMPI theory, comparative measurements allow us to determine the lifetime of the excited states of the employed matrices. Resulting values for both matrices are in good agreement with the literature and corroborate the REMPI-based approach.

Development of laser ablation dielectric barrier discharge optical emission spectrometry (LA-DBD-OES) for direct determination of sulphur and chloride in the condensed phase and its application in pharmaceutical analysis

An infrared laser (808 nm) has been coupled with dielectric barrier discharge (DBD) for optical emission spectrometric determination of S and Cl in organic compounds. The use of a continuous wave IR laser with an output power of 1-2 W allows volatilization of analytes from condensed surfaces. Analytes thermally delivered to the gas phase are excited and atomized by the DBD plasma triggered by an alternating voltage of 10 kV at 25 kHz under atmospheric pressure. Direct analysis of S- and Cl-containing organics in manufactured tablets by measuring the S and Cl emissions resulted in a dynamic range of 0.5%-20% with linearities (R²) above 0.93 and limits of detection (LODs) in the μg g^-1 range. The detection precision was examined by measuring inter-day and intra-day reproducibilities, leading to relative standard deviations (RSDs) ranging from 4.6% to 15.0%. The feasibility of LA-DBD-OES was further demonstrated with commercial pharmaceutical tablets of sulfadiazine (SDZ) and chloramphenicol (CAP). There is the potential for probing the tablet uniformity by monitoring the elemental emissions of S and Cl. Quantitative results of the commercial tablets were consistent with the indication amounts and were verified by HPLC measurements. All these results suggest the proposed methodology as a promising tool for online analysis of solids and pharmaceutical tablets with minimal sample treatments and rapid detection response.

Hand-Held Diode Laser for On-Site Analysis Using Transportable Mass Spectrometry

A hand-held laser diode thermal desorption electrospray ionization (LDTD-ESI) mass spectrometry (MS) method was developed for rapid screening of illegal substances in solid samples. To achieve that, a simple, inexpensive, battery-powered surgical laser diode at 940 nm was employed to ablate the solid samples. The potential of using a black polytetrafluoroethylene substrate to enhance the analytes’ desorption to the gas phase was investigated and demonstrated. Among the optimized ESI parameters, the solvent (methanol/water, 50:50, v/v) and the flow rate (50 μL h^–1) were critical to obtain the best sensitivity. The applicability was demonstrated for the rapid identification of selective androgen receptor modulators (SARMs) in pills and powders based on accurate mass measurements by time-of-flight MS. Also, the hand-held LDTD-ESI was combined with a transportable single quadrupole MS. The same SARMs samples were analyzed, and identifications were based on in-source cone voltage fragmentation patterns observed. These initial results demonstrate the applicability of the developed simplified LDTD-ESI MS method for future on-site testing of organic compounds in solid samples.