Applicability of low-flow atmospheric pressure chemical ionization and photoionization mass spectrometry with a microfabricated nebulizer for neutral lipids

Structural characterization of wax esters using ultraviolet photodissociation mass spectrometry

Wax esters play critical roles in biological systems, serving functions from energy storage to chemical signaling. Their diversity is attributed to variations in alcohol and acyl chains, including their length, branching, and the stereochemistry of double bonds. Traditional analysis by mass spectrometry with collisional activations (CID, HCD) offers insights into acyl chain lengths and unsaturation level. Still, it falls short in pinpointing more nuanced structural features like the position of double bonds. As a solution, this study explores the application of 213-nm ultraviolet photodissociation (UVPD) for the detailed structural analysis of wax esters. It is shown that lithium adducts provide unique fragments as a result of Norrish and Norrish-Yang reactions at the ester moieties and photoinduced cleavages of double bonds. The product ions are useful for determining chain lengths and localizing double bonds. UVPD spectra of various wax esters are presented systematically, and the effect of activation time is discussed. The applicability of tandem mass spectrometry with UVPD is demonstrated for wax esters from natural sources. The UHPLC analysis of jojoba oil proves the compatibility of MS2 UVPD with the chromatography time scale, and a direct infusion is used to analyze wax esters from vernix caseosa. Data shows the potential of UVPD and its combination with CID or HCD in advancing our understanding of wax ester structures.

Liquid phase detection in the miniature scale. Microfluidic and capillary scale measurement and separation systems. A tutorial review

Microfluidic and capillary devices are increasingly being used in analytical applications while their overall size keeps decreasing. Detection sensitivity for these microdevices gains more importance as device sizes and consequently, sample volumes, decrease. This paper reviews optical, electrochemical, electrical, and mass spectrometric detection methods that are applicable to capillary scale and microfluidic devices, with brief introduction to the principles in each case. Much of this is considered in the context of separations. We do consider theoretical aspects of separations by open tubular liquid chromatography, arguably the most potentially fertile area of separations that has been left fallow largely because of lack of scale-appropriate detection methods. We also examine the theoretical basis of zone electrophoretic separations. Optical detection methods discussed include UV/Vis absorbance, fluorescence, chemiluminescence and refractometry. Amperometry is essentially the only electrochemical detection method used in microsystems. Suppressed conductance and especially contactless conductivity (admittance) detection are in wide use for the detection of ionic analytes. Microfluidic devices, integrated to various mass spectrometers, including ESI-MS, APCI-MS, and MALDI-MS are discussed. We consider the advantages and disadvantages of each detection method and compare the best reported limits of detection in as uniform a format as the available information allows. While this review pays more attention to recent developments, our primary focus has been on the novelty and ingenuity of the approach, regardless of when it was first proposed, as long as it can be potentially relevant to miniature platforms.

Open tubular liquid chromatography: Recent advances and future trends

Nano-liquid chromatography (nanoLC) is gaining significant attention as a primary analytical technique across various scientific domains. Unlike conventional high-performance LC, nanoLC utilizes columns with inner diameters (i.ds.) usually ranging from 10 to 150 μm and operates at mobile phase flow rates between 10 and 1000 nl/min, offering improved chromatographic performance and detectability. Currently, most exploration of nanoLC has focused on particle-packed columns. Although open tubular LC (OTLC) can provide superior performance, optimized OTLC columns require very narrow i.ds. (< 10 μm) and demand challenging instrumentation. At the moment, these challenges have limited the success of OTLC. Nevertheless, remarkable progress has been made in developing and utilizing OTLC systems featuring narrow columns (< 2 μm). Additionally, significant efforts have been made to explore larger columns (10-75 μm i.d), demonstrating practical applicability in many situations. Due to their perceived advantages, interest in OTLC has resurged in the last two decades. This review provides an updated outlook on the latest developments in OTLC, focusing on instrumental challenges, achievements, and advancements in column technology. Moreover, it outlines selected applications that illustrate the potential of OTLC for performing targeted and untargeted studies.

Positive Effect of Acetylation on Proteomic Analysis Based on Liquid Chromatography with Atmospheric Pressure Chemical Ionization and Photoionization Mass Spectrometry

A typical bottom-up proteomic workflow comprises sample digestion with trypsin, separation of the hydrolysate using reversed-phase HPLC, and detection of peptides via electrospray ionization (ESI) tandem mass spectrometry. Despite the advantages and wide usage of protein identification and quantification, the procedure has limitations. Some domains or parts of the proteins may remain inadequately described due to inefficient detection of certain peptides. This study presents an alternative approach based on sample acetylation and mass spectrometry with atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI). These ionizations allowed for improved detection of acetylated peptides obtained via chymotrypsin or glutamyl peptidase I (Glu-C) digestion. APCI and APPI spectra of acetylated peptides often provided sequence information already at the full scan level, while fragmentation spectra of protonated molecules and sodium adducts were easy to interpret. As demonstrated for bovine serum albumin, acetylation improved proteomic analysis. Compared to ESI, gas-phase ionizations APCI and APPI made it possible to detect more peptides and provide better sequence coverages in most cases. Importantly, APCI and APPI detected many peptides which passed unnoticed in the ESI source. Therefore, analytical methods based on chymotrypsin or Glu-C digestion, acetylation, and APPI or APCI provide data complementary to classical bottom-up proteomics.

Gas Dynamic Virtual Nozzle Sprayer for an Introduction of Liquid Samples in Atmospheric Pressure Ionization Mass Spectrometry

Electrospray may exhibit inadequate ionization efficiency in some applications. In such cases, atmospheric-pressure chemical ionization (APCI) and photoionization (APPI) can be used. Despite a wide application potential, no APCI and APPI sources dedicated to very low sample flow rates exist on the market. Since the ion source performance depends on the transfer of analytes from the liquid to the gas phase, a nebulizer is a critical component of an ion source. Here, we report on the nebulizer with a gas dynamic virtual nozzle (GDVN) and its applicability in APCI at microliter-per-minute flow rates. Nebulizers differing by geometrical parameters were fabricated and characterized regarding the jet breakup regime, droplet size, droplet velocity, and spray angle for liquid flow rates of 0.75-15.0 μL/min. A micro-APCI source with the GDVN nebulizer behaved as a mass-flow-sensitive detector and provided stable and intense analyte signals. Compared to a classical APCI source, an order of magnitude lower detection limit for verapamil was achieved. Mass spectra recorded with the nebulizer in dripping and jetting modes were almost identical and did not differ from normal APCI spectra. Clogging never occurred during the experiments, indicating the high robustness of the nebulizer. Low-flow-rate APCI and APPI sources with a GDVN sprayer promise new applications for low- and medium-polar analytes.

Nebulization Swab Assisted Photoionization Tandem Miniaturized Ion Trap Mass Spectrometry for On-Site Analysis of Nonvolatile Compounds

Nonvolatile compounds usually have a high molecular weight and exhibit a high boiling point, which poses great challenges to the ionization method of MS. Ambient ionization sources can efficiently analyze the nonvolatile compounds without complex pretreatment, but they generally require special media such as heating devices, laser optical devices, or corona needles. Acoustic nebulization assisted photoionization (ANPI) is a potential method for the analysis of nonvolatile compounds that uses nebulization as a prerequisite for photoionization and introduces many advantages of PI, including excellent ionization efficiency, a high yield of molecular ions, and simplified spectrum interpretation. However, the ANPI source can be limited in on-site applications by the complexity of the analytical devices and the high cost of the nebulization chip. To address this issue, in this paper, we explored cheap and commercially piezoelectric materials used in a mist sprayer and fabricated a nebulization swab assisted photoionization (NSAP) as an ambient ionization source. Some useful results are presented: numerical simulation was introduced successfully for optimizing the aerosol distribution in the NSAP source; nonvolatile muscle relaxants, drugs of abuse, antibiotics, phthalates, and cholesterol were detected mostly as their protonated molecular ions while some special acetone/water cluster ions were detected. In addition, the LOD for most of the target analytes ranged from 10.0 to 50.0 pg with RSD ≤ 9%. Finally, this method is implemented for Chinese baijiu spiked with phthalates. The experimental data shows the capability of a NSAP source in high sensitivity and on-site analysis of the nonvolatile compounds.

Utility of Atmospheric-Pressure Chemical Ionization and Photoionization Mass Spectrometry in Bottom-Up Proteomics

In a typical bottom-up proteomics workflow, proteins are enzymatically cleaved, and the resulting peptides are analyzed by HPLC with electrospray ionization (ESI) tandem mass spectrometry. This approach is practical and widely applied. It has, however, limitations mostly related to less efficient or even inefficient ionization of some peptides in ESI sources. Gas-phase ionization methods like atmospheric-pressure chemical ionization (APCI) or atmospheric-pressure photoionization (APPI) offer alternative ways of detecting various analytes. This work is a systematic study of the ionization efficiencies of peptides in ESI, APCI, and APPI and the applicability of the mentioned ionizations in proteomics. A set of peptide standards and bovine serum albumin digests were examined using a high-resolution mass spectrometer coupled to an ultra HPLC system. Since the ionization efficiency in APCI and APPI depends strongly on experimental conditions, the ion source settings and mobile phase compositions were optimized for each ionization technique. As expected, tryptic peptides were best detected using ESI. The numbers of chymotrypsin peptides successfully detected by ESI, APPI, and APCI were comparable. In the case of Glu-C digest, APPI detected the highest number of peptides. The results suggest that gas-phase ionization techniques, particularly APPI, are an interesting alternative for detecting peptides and delivering complementary data in proteomics.

An overview of open tubular liquid chromatography with a focus on the coupling with mass spectrometry for the analysis of small molecules

Open tubular liquid chromatography (OT-LC) can provide superior chromatographic performance and more favorable mass spectrometry (MS) detection conditions. These features could provide enhanced sensitivity when coupled with electrospray ionization sources (ESI-) and lead to unprecedented detection capabilities if interfaced with a highly structural informative electron ionization (EI) source. In the past, the exploitation of OT columns in liquid chromatography evolved slowly. However, the recent instrumental developments in capillary/nanoLC-MS created new opportunities in developing and applying OT-LC-MS. Currently, the analytical advantages of OT-LC-MS are mainly exploited in the fields of proteomics and biosciences analysis. Nevertheless, under the right conditions, OT-LC-MS can also offer superior chromatographic performance and enhanced sensitivity in analyzing small molecules. This review will provide an overview of the latest developments in OT-LC-MS, focusing on the wide variety of employed separation mechanisms, innovative stationary phases, emerging column fabrication technologies, and new OT formats. In the same way, the OT-LC's opportunities and shortcomings coupled to both ESI and EI will be discussed, highlighting the complementary character of those two ionization modes to expand the LC's detection boundaries in the performance of targeted and untargeted studies.

Atmospheric pressure chemical ionization mass spectrometry at low flow rates: Importance of ion source housing

RATIONALE

Hyphenation of atmospheric pressure chemical ionization (APCI) mass spectrometry with capillary and micro high-performance liquid chromatography (HPLC) is attractive for many applications, but reliable ion sources dedicated to these conditions are still missing. There are a number of aspects to consider when designing such an ion source, including the susceptibility of the ionization processes to ambient conditions. Here we discuss the importance of ion source housing for APCI at low flow rates.

METHODS

Selected compounds dissolved in various solvents were used to study ionization reactions at 10 μL/min flow rate. APCI spectra were generated using the Ion Max-S source (Thermo Fisher Scientific) operated with or without the ion source housing.

RESULTS

The APCI spectra of most compounds measured in the open and enclosed ion sources were markedly different. The differences were explained by water and oxygen molecules that entered the plasma region of the open ion source. Water tended to suppress charge transfer processes while oxygen diminished electron capture reactions and prevented the formation of acetonitrile-related radical cations useful for localizing double bonds in lipids. The effects associated with the ion source housing were significantly less important for compounds that are easy to protonate or deprotonate.

CONCLUSIONS

The use of ion source housing prevented alternative ionization channels leading to unwanted or unexpected ions. Compared with the conventional flow rate mode (1 mL/min), the effects of ambient air components were significantly higher at 10 μL/min, emphasizing the need for ion source housing in APCI sources dedicated to low flow rates.

Temperature-programmed capillary high-performance liquid chromatography with atmospheric pressure chemical ionization mass spectrometry for analysis of fatty acid methyl esters

A new capillary high-performance liquid chromatography method with atmospheric pressure chemical ionization mass spectrometry was developed for the analysis of fatty acid methyl esters and long-chain alcohols. The chromatographic separation was achieved using a Zorbax SB-C18 HPLC column (0.3 × 150 mm, 3.5 μm) with a mobile phase composed of acetonitrile and formic acid and delivered isocratically at a flow rate of 10 μL/min. The column temperature was programmed simply, using a common column oven. Good reproducibility of the temperature profile and retention times were achieved. The temperature programming during the isocratic high-performance liquid chromatography run had a similar effect as a solvent gradient; it reduced retention times of later eluting analytes and improved their detection limits. Two atmospheric pressure chemical ionization sources of the mass spectrometry detector were compared: an enclosed conventional ion source and an in-house made ion source with a glass microchip nebulizer. The enclosed source provided better detectability of saturated fatty acid methyl esters and made it possible to determine the double bond positions using acetonitrile-related adducts, while the open chip-based source provided better analytical figures of merit for unsaturated fatty acid methyl esters. Temperature-programmed capillary high-performance liquid chromatography is a promising method for analyzing neutral lipids in lipidomics and other applications.

Capillary photoionization: interface for low flow rate liquid chromatography-mass spectrometry

The first report on capillary photoionization interfacing a liquid chromatograph and mass spectrometer.