Real-time analysis of living animals by electrospray ionization mass spectrometry

Automated sample preparation for electrospray ionization mass spectrometry based on CLOCK-controlled autonomous centrifugal microfluidics

We report a centrifugal microfluidic device that automatically performs sample preparation under steady-state rotation for clinical applications using mass spectrometry. The autonomous microfluidic device was designed for the control of liquid operation on centrifugal hydrokinetics (CLOCK) paradigm. The reported device was highly stable, with less than 7% variation with respect to the time of each unit operation (sample extraction, mixing, and supernatant extraction) in the preparation process. An agitation mechanism with bubbling was used to mix the sample and organic solvent in this device. We confirmed that the device effectively removed the protein aggregates from the sample, and the performance was comparable to those of conventional manual sample preparation procedures that use high-speed centrifugation. In addition, probe electrospray ionization mass spectrometry (PESI-MS) was performed to compare the device-treated and manually treated samples. The obtained PESI-MS spectra were analyzed by partial least squares discriminant analysis, and the preparation capability of the device was found to be equivalent to that of the conventional method.

μ-PESI-based MS profiling combined with untargeted metabolomics analysis for rapid identification of red wine geographical origin

BACKGROUND

The commercial value of red wine is strongly linked to its geographical origin. Given the large global market, there is great demand for high-throughput screening methods to authenticate the geographical source of red wine. However, only limited techniques have been established up to now.

RESULTS

Herein, a sensitive and robust method, namely probe electrospray ionization mass spectrometry (μ-PESI-MS), was established to achieve rapid analysis at approximately 1.2 min per sample without any pretreatment. A scotch near the needle tip provides a fixed micro-volume for each analysis to achieve satisfactory ion signal reproducibility (RSD < 26.7%). In combination with a machine learning algorithm, 16 characteristic ions were discovered from thousands of detected ions and were utilized for differentiating red wine origin. Among them, the relative abundances of two characteristic metabolites (trigonelline and proline) correlated with geographical conditions (sun exposure and water stress) were identified, providing the rationale for differentiation of the geographical origin.

CONCLUSION

The proposed μ-PESI-MS-based method demonstrates a promising high-throughput determination capability in red wine traceability.

Solid-phase microextraction- probe electrospray ionization devices for screening and quantitating drugs of abuse in small amounts of biofluids

Probe electrospray ionization (PESI) is an ambient ionization mass spectrometry technique (AIMS) that is primarily used in qualitative studies, though researchers have recently combined it with sample preparation for the quantitative analysis of various analytes in biological matrices. This study presents a method that integrates solid-phase microextraction with PESI for direct coupling to a triple quadrupole mass spectrometer, and examines its ability to quantitate drugs of abuse. Intra- and inter-probe reproducibility experiments were conducted to assess the stability and reproducibility of the extraction-phase-coated PESI probes (coating length: 2 mm; coating thickness: 6.5 μm). This research is the first documented instance wherein highly sensitive determinations were successfully attained using these microextraction and micro-desorption techniques in conjunction with small volumes of sample and extraction phase. A mixture consisting of IPA/H₂O (1/1 v/v) + 0.1% FA was determined to be the optimal desorption solvent for SPME-PESI-MS/MS, as it facilitated high analyte enrichment in a picolitre of the solvent, which acted at the same time as efficient electrospray media. Furthermore, a method of quantifying drugs of abuse in 30 μL of plasma without matrix modification was also developed. This method had an intra-day accuracy within the 80-120% range for all eight drugs of abuse at concentrations of 3, 30, and 90 pg μL^-1; the exception to this result was lorazepam at 30 pg μL^-1, which had an intra-day accuracy of 122%. The lower limit of quantification (LLOQ) for fentanyl and nordiazepam was pg μL^-1; the LLOQ for buprenorphine, codeine, diazepam, lorazepam, and propranolol was 5 pg μL^-1; and the LLOQ of oxazepam was 10 pg μL^-1.

Applying Probe Electrospray Ionization Mass Spectrometry to Cytological Diagnosis: A Preliminary Study by Using Cultured Lung Cancer Cells

OBJECTIVES

Cytology and histology are 2 indispensable diagnostic tools for cancer diagnosis, which are rapidly increasing in importance with aging populations. We applied mass spectrometry (MS) as a rapid approach for swiftly acquiring nonmorphological information of interested cells. Conventional MS, which primarily rely on promoting ionization by pre-applying a matrix to cells, has the drawback of time-consuming both on data acquisition and analysis. As an emerging method, probe electrospray ionization-MS (PESI-MS) with a dedicated probe is capable to pierce sample and measure specimen in small amounts, either liquid or solid, without the requirement for sample pretreatment. Furthermore, PESI-MS is timesaving compared to the conventional MS. Herein, we investigated the capability of PESI-MS to characterize the cell types derived from the respiratory tract of human tissues.

STUDY DESIGN

PESI-MS analyses with DPiMS-2020 were performed on various type of cultured cells including 5 lung squamous cell carcinomas, 5 lung adenocarcinomas, 5 small-cell carcinomas, 4 malignant mesotheliomas, and 2 normal controls.

RESULTS

Several characteristic peaks were detected at around m/z 200 and 800 that were common in all samples. As expected, partial least squares-discriminant analysis of PESI-MS data distinguished the cancer cell types from normal control cells. Moreover, distinct clusters divided squamous cell carcinoma from adenocarcinoma.

CONCLUSION

PESI-MS presented a promising potential as a novel diagnostic modality for swiftly acquiring specific cytological information.

Probe Electrospray Ionization (PESI) and Its Modified Versions: Dipping PESI (dPESI), Sheath-Flow PESI (sfPESI) and Adjustable sfPESI (ad-sfPESI)

In 2007, probe electrospray ionization/mass spectrometry (PESI/MS) was developed. In this technique, the needle is moved down along a vertical axis and the tip of the needle touched to the sample. After capturing the sample at the needle tip, the needle is then moved up and a high voltage is applied to the needle at the highest position to generate electrospray. Due to the discontinuous sampling followed by the generation of spontaneous electrospray, sequential and exhaustive electrospray takes place depending on the surface activity of the analytes. As modified versions of PESI, dipping PESI (dPESI), sheath-flow PESI (sfPESI) and adjustable sfPESI (ad-sfPESI) have been developed. These methods are complementary to each other and they can be applicable to surface and bulk analysis of various biological samples. In this article, the characteristics of these methods and their applications to real samples will be reviewed.

Polypyrrole-coated needle as an electrospray emitter for ambient mass spectrometry

Polypyrrole (PPy) is a polymer widely used as an extraction phase due to its ability to perform intermolecular interactions with the analyte, such as acid-base, π-π, dipole-dipole, hydrophobic, and hydrogen bonding. In this manuscript, we report the coating of a stainless steel needle with a PPy film for analyte extraction and subsequent analysis by electrospray ionization mass spectrometry (ESI-MS) under ambient and open-air conditions. The method, named PPy-ESI-MS, was optimized for analysis of 3,4-methylenedioxyamphetamine (MDA) and 3,4-methylenedioxymethamphetamine (MDMA) in synthetic urine. Seven cycles of electrodeposition of the PPy film onto the needle surface, sample at pH 8, and 40 min of extraction of analytes were determined as the best analysis conditions. The analytical performance of PPy-ESI-MS was evaluated for MDA and MDMA compounds. Analytical curves were obtained with R2 > 0.98. Limits of detection (LODs) and limits of quantification (LOQs) were determined as 20 μg L-1 and 70 μg L-1 for MDA and as 25 μg L-1 and 80 μg L-1 for MDMA, respectively. Values of precision were below 17%, and values of accuracy below 5%. The apparent recoveries ranged between 84.5% and 111.3%. In addition, the PPy-ESI-MS method was applied for the analysis of sarcosine in synthetic urine in order to evaluate the performance of the method for another class of compounds. The calibration curve was obtained with R2 > 0.98, along with LOD and LOQ of 30 μg L-1 and 100 μg L-1, respectively. The precision and accuracy values were below 5% and 8%, respectively, and the apparent recoveries close to 100%. This work demonstrates the usefulness of combining an extraction phase with ESI-MS analysis under ambient conditions to determine different classes of small molecules in a complex sample.

Time-resolved in situ monitoring of photocatalytic reactions by probe electrospray ionization mass spectrometry

Probe electrospray ionization mass spectrometry (PESI-MS) has been demonstrated to be a useful in situ and online analytical technique for monitoring of various reactions. In this work, PESI-MS with a surface-modified probe was adopted and applied to in situ monitoring of photocatalytic reactions. Typical reactions of semiconductor photocatalysts, namely TiO₂, SnO₂, WO₃, SiC and ZnS catalyzed methylene blue (MB) and brilliant green (BG) degradation, were selected to demonstrate the potential of PESI-MS to monitor heterogeneous photocatalytic reactions occurring in suspensions. Surface modification of the probe ensures increased wettability during the whole monitoring process. PESI-MS could provide continuous sampling and real-time MS results without time-consuming and cumbersome sample pretreatments. This method has other merits including good reproducibility and stability (time scale > 60 min), convenience of operation, low sample consumption, high time resolution and high tolerance to suspended photocatalyst particles. Time-resolved mass spectra and ion chromatograms of every chemical species e.g. the substrate and reactive intermediates could be obtained, which is helpful for a better understanding of the photocatalytic reaction process. Thus, PESI-MS could be a versatile analytical technique for in situ photocatalytic reaction analysis and could be an alternative means for the evaluation of photocatalyst performance.

Lipidome-based rapid diagnosis with machine learning for detection of TGF-β signalling activated area in head and neck cancer

Background

Several pro-oncogenic signals, including transforming growth factor beta (TGF-β) signalling from tumour microenvironment, generate intratumoural phenotypic heterogeneity and result in tumour progression and treatment failure. However, the precise diagnosis for tumour areas containing subclones with cytokine-induced malignant properties remains clinically challenging.

Methods

We established a rapid diagnostic system based on the combination of probe electrospray ionisation-mass spectrometry (PESI-MS) and machine learning without the aid of immunohistological and biochemical procedures to identify tumour areas with heterogeneous TGF-β signalling status in head and neck squamous cell carcinoma (HNSCC). A total of 240 and 90 mass spectra were obtained from TGF-β-unstimulated and -stimulated HNSCC cells, respectively, by PESI-MS and were used for the construction of a diagnostic system based on lipidome.

Results

This discriminant algorithm achieved 98.79% accuracy in discrimination of TGF-β1-stimulated cells from untreated cells. In clinical human HNSCC tissues, this approach achieved determination of tumour areas with activated TGF-β signalling as efficiently as a conventional histopathological assessment using phosphorylated-SMAD2 staining. Furthermore, several altered peaks on mass spectra were identified as phosphatidylcholine species in TGF-β-stimulated HNSCC cells.

Conclusions

This diagnostic system combined with PESI-MS and machine learning encourages us to clinically diagnose intratumoural phenotypic heterogeneity induced by TGF-β.

High-resolution imaging mass spectrometry combined with transcriptomic analysis identified a link between fatty acid composition of phosphatidylinositols and the immune checkpoint pathway at the primary tumour site of breast cancer

BACKGROUND

The fatty acid (FA) composition of phosphatidylinositols (PIs) is tightly regulated in mammalian tissue since its disruption impairs normal cellular functions. We previously found its significant alteration in breast cancer by using matrix-assisted laser desorption and ionisation imaging mass spectrometry (MALDI-IMS).

METHODS

We visualised the histological distribution of PIs containing different FAs in 65 primary breast cancer tissues using MALDI-IMS and investigated its association with clinicopathological features and gene expression profiles.

RESULTS

Normal ductal cells (n = 7) predominantly accumulated a PI containing polyunsaturated FA (PI-PUFA), PI(18:0/20:4). PI(18:0/20:4) was replaced by PIs containing monounsaturated FA (PIs-MUFA) in all non-invasive cancer cells (n = 12). While 54% of invasive cancer cells (n = 27) also accumulated PIs-MUFA, 46% of invasive cancer cells (n = 23) accumulated the PIs-PUFA, PI(18:0/20:3) and PI(18:0/20:4). The accumulation of PI(18:0/20:3) was associated with higher incidence of lymph node metastasis and activation of the PD-1-related immune checkpoint pathway. Fatty acid-binding protein 7 was identified as a putative molecule controlling PI composition.

CONCLUSIONS

MALDI-IMS identified PI composition associated with invasion and nodal metastasis of breast cancer. The accumulation of PI(18:0/20:3) could affect the PD-1-related immune checkpoint pathway, although its precise mechanism should be further validated.

Mapping Lipid C=C Location Isomers in Organ Tissues by Coupling Photochemical Derivatization and Rapid Extractive Mass Spectrometry

Lipid desaturation plays important roles in biological processes and the disease states. Here, we report a simple but efficient method for mapping unsaturated phospholipids including the spatial distribution of lipid C=C location isomers in animal organs by coupling the C=C specific derivatization with direct analysis mass spectrometry (MS). Lipids are sampled directly by a stainless-steel wire from rat brain or kidney, extracted, and derivatized via the Paternò-Büchi reaction in a glass emitter of the nanoelectrospray ionization (nanoESI) source. Subsequent analysis by nanoESI-tandem mass spectrometry reveals C=C locations and relative quantities of lipid C=C location isomers. Unsaturated lipids, such as phospholipids and free fatty acids, have been identified with ion intensities spanning two orders of magnitude in rat brain. Typical sample consumption is less than 10 μg/measurement and the time for each analysis is about 3 min. This method should serve as a complementary method to high spatial resolution mass spectrometry imaging techniques, because it offers a streamlined experimental workflow for rapid profiling of lipids with C=C specificity to enable such applications as point-of-care disease diagnostics.

Real-time analysis of living animals and rapid screening of human fluid samples using remote sampling electrospray ionization mass spectrometry

Real-time and in-situ mass-spectrometry analyses of living animal and biological sample were performed using a novel remote sampling electrospray ionization (RS-ESI) probe. Unlike conventional ESI, in which injection or syringe loading is required for sample introduction, the RS-ESI probe ionizes the samples when the sampling capillary is in contact with the sample. As the sampling capillary is electrically held at ground potential, the safety of the animal and operator is assured. The liquid sample is aspirated to the ESI emitter at the other end of the capillary by the Venturi effect. Subsequently, the electrospray is generated when a high voltage is applied to the counter electrode placed inside the ion source chamber. The probe unit is attached to the mass spectrometer with a long flexible tube and its position can be freely manipulated during the analysis. In this report, we demonstrate a real-time analysis of a living mouse liver and an automatic analysis of 138 serum samples using this new technique.

Direct detection of the psychoactive substance MT-45 in human tissue samples by probe electrospray ionization-tandem mass spectrometry

Analyses of drugs and poisons in tissue samples are essential in forensic toxicology and pharmacology. However, current procedures for tissue analysis are laborious and time-consuming. Therefore, we assessed the utility of a newly devised probe electrospray ionization (PESI) technique with tandem mass spectrometry (MS/MS) for easy, ultra-rapid drug detection in human tissue samples. Using this system, typical pretreatment procedures, such as solid-phase extraction, liquid-liquid extraction, deproteinization, or homogenization, can be avoided. Briefly, a tissue sample of 1-2 mm³ was supplemented with a solution of ethanol and 10 mmol/L ammonium formate, and measurements were obtained. We demonstrated the successful application of this method in a forensic case by detecting an opioid analgesic, MT-45, in all tissue samples (liver, kidney, lung, brain, and heart). We also detected oxidized metabolites of MT-45 in the liver. Since the analysis required only 0.5 minutes per sample, PESI-MS/MS is an ultra-rapid detection method. Furthermore, for a quantitative approach, the total analysis time for the combination of PESI-MS/MS with the quick, easy, cheap, effective, rugged, and safe (QuEChERS) extraction method (from instrument start-up to extraction and PESI-analysis) was within 8 minutes. MT-45 concentrations obtained by QuEChERS-PESI-MS/MS and liquid chromatography (LC) -MS/MS were similar for all tissue samples. PESI-MS/MS cannot be used to separate isobars/isomers (ie, compounds with the same m/z value), similar to other direct introduction techniques. Further studies are needed to validate the quantitation method. However, our results indicate that PESI-MS/MS is a potentially easy and rapid technique for the analysis of drugs and poisons in human tissue samples.

Construction of mass spectra database and diagnosis algorithm for head and neck squamous cell carcinoma

OBJECTIVES

Intraoperative identification of tumor margins is essential to achieving complete tumor resection. However, the process of intraoperative pathological diagnosis involves cumbersome procedures, such as preparation of cryosections and microscopic examination, thus requiring more than 30 min. Moreover, intraoperative diagnoses made by examining cryosections are occasionally inconsistent with postoperative diagnoses made by examining paraffin-embedded sections because the former are of poorer quality. We sought to establish a more rapid accurate method of intraoperative assessment.

MATERIALS AND METHODS

A diagnostic algorithm of head and neck squamous cell carcinoma (HNSCC) using machine learning was constructed by mass spectra obtained from 15 non-cancerous and 19 HNSCC specimens by probe electrospray ionization mass spectrometry (PESI-MS). The clinical validity of this system was evaluated using intraoperative specimens of HNSCC and normal mucosa.

RESULTS

A total of 114 and 141 mass spectra were acquired from non-cancerous and cancerous specimens, respectively, using both positive- and negative-ion modes of PESI-MS. These data were fed into partial least squares-logistic regression (PLS-LR) to discriminate tumor-specific spectral patterns. Leave-one-patient-out cross validation of this algorithm in positive- and negative-ion modes showed accuracies in HNSCC diagnosis of 90.48% and 95.35%, respectively. In intraoperative specimens of HNSCC, this algorithm precisely defined the borders of the cancerous regions; these corresponded with those determined by examining histologic sections. The procedure took approximately 5 min.

CONCLUSION

This diagnostic system, based on machine learning, enables accurate discrimination of cancerous regions and has the potential to provide rapid intraoperative assessment of HNSCC margins.

Ambient mass spectrometry in metabolomics

Since the introduction of desorption electrospray ionization (DESI) mass spectrometry (MS), ambient MS methods have seen increased use in a variety of fields from health to food science. Increasing its popularity in metabolomics, ambient MS offers limited sample preparation, rapid and direct analysis of liquids, solids, and gases, in situ and in vivo analysis, and imaging. The metabolome consists of a constantly changing collection of small (<1.5 kDa) molecules. These include endogenous molecules that are part of primary metabolism pathways, secondary metabolites with specific functions such as signaling, chemicals incorporated in the diet or resulting from environmental exposures, and metabolites associated with the microbiome. Characterization of the responsive changes of this molecule cohort is the principal goal of any metabolomics study. With adjustments to experimental parameters, metabolites with a range of chemical and physical properties can be selectively desorbed and ionized and subsequently analyzed with increased speed and sensitivity. This review covers the broad applications of a variety of ambient MS techniques in four primary fields in which metabolomics is commonly employed.

In vivo endoscopic mass spectrometry using a moving string sampling probe

A novel moving string sampling probe and sample transportation system for performing in situ and in vivo endoscopic MS.

Ambient ionization and miniature mass spectrometry system for chemical and biological analysis

Ambien ionization and miniaturization of mass spectrometers are two fields in mass spectrometry that have advanced significantly in the last decade. The integration of the techniques developed in these two fields is leading to the development of complete miniature analytical systems that can be used for on-site or point-of-care analysis by non-expert users. In this review, we report the current status of development in ambient ionization and miniature mass spectrometers, with an emphasis on those techniques with potential impact on the point-of-care (POC) diagnostics. The challenges in the future development of the integrated systems are discussed with possible solutions presented.

Tissue spray ionization mass spectrometry for rapid recognition of human lung squamous cell carcinoma

Tissue spray ionization mass spectrometry (TSI-MS) directly on small tissue samples has been shown to provide highly specific molecular information. In this study, we apply this method to the analysis of 38 pairs of human lung squamous cell carcinoma tissue (cancer) and adjacent normal lung tissue (normal). The main components of pulmonary surfactants, dipalmitoyl phosphatidylcholine (DPPC, m/z 757.47), phosphatidylcholine (POPC, m/z 782.52), oleoyl phosphatidylcholine (DOPC, m/z 808.49), and arachidonic acid stearoyl phosphatidylcholine (SAPC, m/z 832.43), were identified using high-resolution tandem mass spectrometry. Monte Carlo sampling partial least squares linear discriminant analysis (PLS-LDA) was used to distinguish full-mass-range mass spectra of cancer samples from the mass spectra of normal tissues. With 5 principal components and 30-40 Monte Carlo samplings, the accuracy of cancer identification in matched tissue samples reached 94.42%. Classification of a tissue sample required less than 1 min, which is much faster than the analysis of frozen sections. The rapid, in situ diagnosis with minimal sample consumption provided by TSI-MS is advantageous for surgeons. TSI-MS allows them to make more informed decisions during surgery.

Development of Non-proximate Probe Electrospray Ionization for Real-Time Analysis of Living Animal

Ambient ionization mass spectrometry is one of the most challenging analytical tools in the field of biomedical research. We previously demonstrated that probe electrospray ionization mass spectrometry (PESI-MS) could potentially be used in the rapid diagnosis of cancer. Although this technique does not require a tedious sample pretreatment process, it was not possible for our previously reported setup to be applied to cases involving the direct sampling of tissues from living animal and large animal subjects, because there would not be enough room to accommodate the larger bodies juxtaposed to the ion inlet. To make PESI-MS more applicable for the real-time analysis of living animals, a long auxiliary ion sampling tube has been connected to the ion inlet of the mass spectrometer to allow for the collection of ions and charged droplets from the PESI source (hereafter, referred to as non-proximate PESI). Furthermore, an additional ion sampling tube was connected to a small diaphragm pump to increase the uptake rate of air carrying the ions and charged droplets to the ion inlet. This modification allows for the extended ion sampling orifice to be positioned closer to the specimens, even when they are too large to be placed inside the ionization chamber. In this study, we have demonstrated the use of non-proximate PESI-MS for the real-time analysis for biological molecules and pharmacokinetic parameters from living animals.

Electrospray Ionization on Solid Substrates

Development of electrospray ionization on solid substrates (solid-substrate ESI) avoids the clogging problem encountered in conventional capillary-based ESI, allows more convenient sampling and permits new applications. So far, solid-substrate ESI with various materials, e.g., metals, paper, wood, fibers and biological tissue, has been developed, and applications ranging from analysis of pure compounds to complex mixtures as well as in vivo study were demonstrated. Particularly, the capability of solid-substrate ESI in direct analysis of complex samples, e.g., biological fluids and foods, has significantly facilitated mass spectrometric analysis in real-life applications and led to increasingly important roles of these techniques nowadays. In this review, various solid-substrate ESI techniques and their applications are summarized and the prospects in this field are discussed.

Mass spectrometry: towards in vivo analysis of biological systems

In vivo analysis is of paramount importance in monitoring physiological processes that take place in living organisms. Mass spectrometry, an analytical technique with high speed, sensitivity and specificity, is indispensable in biochemical studies nowadays. However, traditional mass spectrometric techniques are of limited applicability in direct analysis of living organisms due to various constraints, e.g., the necessity of ionization of analytes under vacuum and perturbation of physiological functions of living organisms during analysis. Recent development of mass spectrometry, particularly the development of ambient ionization techniques, has opened the door for direct analysis of living organisms. These new mass spectrometric techniques have the features that the ionization processes take place under atmospheric pressure and no or only little sample preparation is required, thus are well suited for analysis of living specimens without significantly perturbing their physiological states. The role of these mass spectrometric techniques in in vivo analysis has been increasingly important in recent years and is expected to be further expanded in the future. In this review, the use of various mass spectrometric techniques in in vivo analysis of biological systems is summarized and the prospects are discussed.

Real-time diagnosis of chemically induced hepatocellular carcinoma using a novel mass spectrometry-based technique

Real-time analyses of hepatocellular carcinoma were performed in living mice to assess the applicability of probe electrospray ionization-mass spectrometry (PESI-MS) in medical diagnosis. The number of peaks and the abundance of ions corresponding to triacylglycerols (TAGs) were higher in cancerous tissues than in noncancerous tissues. Multiple sequential scans of the specimens were performed along a predetermined line extending over the noncancerous region to detect the boundary of the cancerous region. Our system successfully discriminated the noncancerous and cancerous tissues based on the intensities of the TAG ions. These results highlight the potential application of PESI-MS for clinical diagnosis in cancer.

Surface analysis of lipids by mass spectrometry: more than just imaging

Mass spectrometry is now an indispensable tool for lipid analysis and is arguably the driving force in the renaissance of lipid research. In its various forms, mass spectrometry is uniquely capable of resolving the extensive compositional and structural diversity of lipids in biological systems. Furthermore, it provides the ability to accurately quantify molecular-level changes in lipid populations associated with changes in metabolism and environment; bringing lipid science to the "omics" age. The recent explosion of mass spectrometry-based surface analysis techniques is fuelling further expansion of the lipidomics field. This is evidenced by the numerous papers published on the subject of mass spectrometric imaging of lipids in recent years. While imaging mass spectrometry provides new and exciting possibilities, it is but one of the many opportunities direct surface analysis offers the lipid researcher. In this review we describe the current state-of-the-art in the direct surface analysis of lipids with a focus on tissue sections, intact cells and thin-layer chromatography substrates. The suitability of these different approaches towards analysis of the major lipid classes along with their current and potential applications in the field of lipid analysis are evaluated.

Generation of picoliter droplets of liquid for electrospray ionization with piezoelectric inkjet

We report the association of inkjet and electrospray ionization MS to detect picoliter droplet, where the liquid volume and its position onto the tip can be precisely controlled to form ultrafine droplets for successive ionization of the analyte. Single rectangle pulse was applied to piezoelectric device on inkjet microchip for the ejection of each picoliter droplet, and it was controlled by a computer. The voltage and width of driving pulse for the inkjet were optimized to make reproducible ejection of the solvent with low viscosity. The volume of each droplet was about 600 pl, and a trigger of 10 droplets was selected as the best inlet mode taking relative standard derivation of the droplets into consideration. The target substrate used with high voltage to form ionization was graphite, after several attempts with some materials. High-speed camera was used to capture the breaking-up process of a droplet. The distance between the inkjet nozzle and the tip was set at 2 cm to avoid short circuit. The influences on the mass intensity of the diameter of the tip, the volume and the concentration of the sample were examined. The tip with a small diameter performed greater intensity, and the limit of detection decreased, whereas the small volume of liquid played high ionization efficiency. Linear regression in the range between 1 and 200 ppm for caffeine was conducted, where internal standard theobromine was used. Some real samples were also detected with the instrument.

Development of high-pressure probe electrospray ionization for aqueous solution

RATIONALE

Probe electrospray ionization (PESI) is a recently developed method that uses a sharp solid needle as electrospray emitter and the sample is loaded to the needle tip by repetitive movement of the needle probe. This method has been previously used for the analysis of sample with high salt concentration and real-world samples without sample pretreatment. Although PESI is also applicable to aqueous solution, the ion signal stability and reproducibility were not satisfactory due to the spontaneous occurrence of corona discharge taking place on the metallic needle tip.

METHODS

In the previous version of PESI, the ionization was performed under an open environment, thus ambient condition such as humidity and the presence of environmental contaminant also affected the performance of PESI. In this paper, we report a modified version of PESI in which the electrospray and the desolvation of charged droplets are performed inside an enclosed chamber which is pressurized to >1 atm with dry air.

RESULTS

Under a super atmospheric condition, the dielectric strength of the ambient gas was enhanced and stable electrospray could to be initiated without the occurrence of corona discharge even for liquid sample with high surface tension such as pure water. A comparative study of atmospheric-pressure PESI and high-pressure PESI (HP-PESI) had been performed to demonstrate the improvement of PESI in sensitivity and signal stability for the detection analytes in aqueous solution.

CONCLUSIONS

HP-PESI offers a higher sensitivity and signal stability over PESI due to the absence of gaseous breakdown, better desolvation, and higher ion sampling rate by the mass spectrometer.

Application of probe electrospray ionization mass spectrometry (PESI-MS) to clinical diagnosis: solvent effect on lipid analysis

We have examined several combinations of solvents with the aim of optimizing the ionization conditions for molecular diagnosis of malignant tumours by PESI-MS. Although the best conditions may depend on the actual species in the sample, the optimal conditions for renal cell carcinoma (RCC) were achieved by using alcohols. PESI-MS successfully delineated the differential expression of phospholipids (PCs) and triacylglycerols (TAGs) in noncancerous and RCC tissues by using these solvent systems. This study paves the way for the application of PESI-MS in medical samples.

Analysis of renal cell carcinoma as a first step for developing mass spectrometry-based diagnostics

Immediate diagnosis of human specimen is an essential prerequisites in medical routines. This study aimed to establish a novel cancer diagnostics system based on probe electrospray ionization-mass spectrometry (PESI-MS) combined with statistical data processing. PESI-MS uses a very fine acupuncture needle as a probe for sampling as well as for ionization. To demonstrate the applicability of PESI-MS for cancer diagnosis, we analyzed nine cases of clear cell renal cell carcinoma (ccRCC) by PESI-MS and processed the data by principal components analysis (PCA). Our system successfully delineated the differences in lipid composition between non-cancerous and cancerous regions. In this case, triacylglycerol (TAG) was reproducibly detected in the cancerous tissue of nine different individuals, the result being consistent with well-known profiles of ccRCC. Moreover, this system enabled us to detect the boundaries of cancerous regions based on the expression of TAG. These results strongly suggest that PESI-MS will be applicable to cancer diagnosis, especially when the number of data is augmented.