New Method for Improving LC/Time-of-Flight Mass Spectrometry Detection Limits Using Simultaneous Ion Counting and Waveform Averaging

A Method for High Throughput Free Fatty Acids Determination in a Small Section of Bovine Liver Tissue Using Supercritical Fluid Extraction Combined with Supercritical Fluid Chromatography-Medium Vacuum Chemical Ionization Mass Spectrometry

A novel ionization technique named medium vacuum chemical ionization (MVCI) mass spectrometry (MS), which is a chemical ionization using oxonium (H3O+) and hydroxide (OH-) formed from water, has excellent compatibility with the supercritical fluid extraction (SFE)/supercritical fluid chromatography (SFC). We have studied a method to determine free fatty acids (FFAs) in a small section of bovine liver tissue using SFE/SFC-MVCI MS analysis without further sample preparation. A series of FFA molecules interact with the C18 stationary phase, exhibiting broad chromatographic peaks when using a non-modified CO2 as the mobile phase. It can be optimized by adding a small content of methanol to the mobile phase as a modifier; however, it may dampen the ionization efficiency of MVCI since the proton affinity of methanol is slightly higher than water. We have carefully evaluated the modifier content on the ion detection and column efficiencies. The obtained result showed that an optimized performance was in the range of 1 to 2% methanol-modified CO2 mobile phase for both column efficiency and peak intensity. Higher methanol content than 2% degrades both peak intensity and column efficiency. Using optimized SFC conditions, a section of bovine liver tissue sliced for 14 µm thickness by 1 mm square, which is roughly estimated as about 3300 hepatocytes, was applied to determine 18 FFAs amounts for carbon chains of C12-C24. An amount of each tested FFA was estimated as in the range of 0.07 to 2.6 fmol per cell.

Microscale supercritical fluid extraction combined with supercritical fluid chromatography and proton-transfer-reaction ionization time-of-flight mass spectrometry for a magnitude lower limit of quantitation of lipophilic compounds

The application of proton transfer ionization reaction mass spectrometry (PTR MS) combined with microscale supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC) aiming to quantitate single-cell fatty acid analysis levels was investigated. Using a microscale extraction vessel, the obtained low limits of quantitation (LLOQs) of arachidonic acid and arachidic acid were 1.2 and 2.7 fmol, respectively, by using less than 1 µL of sample on stainless steel frit. A series of phthalate, vitamin K₁, and α-tocopherol were also tested, and the LLOQ was less than one femtomole for phthalate and 35 and 13 fmol for vitamin K₁ and α-tocopherol, respectively. A microliter portion of SFE extracts was introduced into the SFC column by split injection, improving the reproducibility of the chromatography and separation efficiency. The method in the present study has great potential to quantitate lipophilic molecules on the nanogram scale of a sample without complex preparation procedures.

Rapid Analysis of α-Tocopherol and Its Oxidation Products Using Supercritical Carbon Dioxide and Proton Transfer Reaction Ionization Mass Spectrometry

We have developed a rapid and sensitive analytical method for α-tocopherol and its oxidative products by combining online hyphenation of supercritical fluid extraction-supercritical fluid chromatography (SFC) with proton transfer reaction (PTR) ionization mass spectrometry (MS). α-Tocopherol is a well-known antioxidant that plays a vital role in the antioxidant defense system in plant cells. However, studies on the cellular mechanisms of α-tocopherol have been limited owing to the lack of a rapid analytical method, which limits the comparison of plant cells incubated in various conditions. Additionally, complex sample preparation and long chromatography separation times are required. Moreover, the majority of the involved molecules are a combination of isomers, which must be separated before applying tandem MS. α-Tocopherol produces the α-tocopheroxyl radical in the first step of its antioxidant function; another ion with the same mass may also be generated from the source. SFC separation effectively distinguished the observed ions from their oxidative products in the sample and those produced during the ionization reaction process. This method enabled the measurement of α-tocopherol and its oxidative products such as α-tocopheroxyl radical and α-tocopheryl quinone in approximately 3 min per sample, including the time required for sample preparation.

Attempts to Detect Lipid Metabolites from a Single Cell Using Proton-Transfer-Reaction Mass Spectrometry Coupled with Micro-Scale Supercritical Fluid Extraction: A Preliminary Study

Proton-transfer-reaction (PTR) mass spectrometry (MS), a widely used method for detecting trace-levels of volatile organic compounds in gaseous samples, can also be used for the analysis of small non-volatile molecules by using supercritical fluid as a transporter for the molecules. Supercritical fluid extraction (SFE) is a method that permits lipophilic compounds to be rapidly and selectively extracted from complex matrices. The combination of the high sensitivity of PTR MS with the SFE is a potentially novel method for analyzing small molecules in a single cell, particularly for the analysis of lipophilic compounds. We preliminarily evaluated this method for analyzing the components of a single HeLa cell that is fixed on a stainless steel frit and is then directly introduces the SFE extracts into the PTR MS. A total of 200/91 ions were observed in positive/negative ion mode time-of-flight mass spectra, and the masses of 11/10 ions could be matched to chemical formulae obtained from the LipidMaps lipids structure database. Using various authentic lipophilic samples, the method could be used to detect free fatty acids in the sub-femtomole to femtomole order in the negative ion mode, the femtomole to sub-picomole order for fat-soluble vitamins, and the picomole order for poly aromatic hydrocarbons in both the positive and negative ion mode.

Analysis of Nonvolatile Molecules in Supercritical Carbon Dioxide Using Proton-Transfer-Reaction Ionization Time-of-Flight Mass Spectrometry

Proton-transfer-reaction (PTR) mass spectrometry (MS) is capable of detecting trace-level volatile organic compounds (VOCs) in gaseous samples in real time. Therefore, PTR-MS has become a popular method in many different study areas. Most of the currently reported PTR-MS applications are designed to determine volatile compounds. However, the method might be applicable for nonvolatile organic compound detection. Supercritical fluid chromatography (SFC) has been studied in the last 5 decades. This approach has high separation efficiency and predictable retention behavior, making separation optimization easy. Atmospheric ionization techniques, such as atmospheric chemical ionization (APCI) and electrospray ionization (ESI), are the most studied SFC-MS interfaces. These processes require the addition of makeup solvents to prevent precipitation or crystallization of the solute while depressurizing the mobile phase. In contrast, the PTR process is carried out in a vacuum; supercritical carbon dioxide may release solute into the PTR flow tube without a phase transition as long as it is maintained above a critical temperature. Therefore, this might constitute yet another use for the SFC-MS interface. Caffeine and a few other nonpolar compounds in supercritical carbon dioxide were successfully detected with time-of-flight MS without adding solvent by using preliminarily assembled supercritical flow injection and supercritical fluid extraction (SFE)-PTR interfaces.

Salt-resistant nanosensor for fast sulfadimethoxine tracing based on oxygen-doped g-C3N4 nanoplates

A novel oxygen-doped g-C₃N₄ nanoplate (OCNP) structure that can serve as an efficient sulfadimethoxine (SDM) sensing platform has been developed. Taking advantage of its inherent oxygen-containing functional groups and 2D layered structure with π-conjugated system, OCNP exhibits effective radiative recombination of surface-confined electron-hole pairs and efficient π-π interaction with SDM. This causes rapid fluorescence response and thus ensures the fast and continuous monitoring of SDM. Based on the fluorescence experiments and band structure calculation, the mechanism of the SDM-induced quenching phenomenon was mainly elucidated as the photoinduced electron transfer process under a dynamic quenching mode. Under optimized conditions, the as-proposed nanosensor, which emitted strong fluorescence at 375 nm with an excitation wavelength at 255 nm, presents an excellent analytical performance toward SDM with a wide linear range from 3 to 60 μmol L^-1 and a detection limit of 0.85 μmol L^-1 (S/N = 3). In addition, this strategy exhibits satisfactory recovery varied from 94 to 103% with relative standard derivations (RSD) in the range 0.9 to 6.8% in real water samples. It also shows marked tolerability to a series of high concentrations of metals and inorganic salts. This strategy not only broadens the application of oxygen-doped g-C₃N₄ nanomaterial in antibiotic sensing field but also presents a promising potential for on-line contaminant tracing in complex environments.