Machine Learning for Characterizing Biofuels Based on Femtosecond Laser Ionization Mass Spectrometry

A sample mixture of fatty acid methyl esters (FAMEs) was measured by femtosecond laser ionization mass spectrometry (fsLIMS) using the fifth (206 nm) and fourth (257 nm) harmonic emissions of an ytterbium (Yb) laser (1030 nm). Molecular ions were observed as the major signals in this technique, providing valuable information concerning the molecular weight and the number of double bonds in the molecule. The mass spectral data were then used as explanatory variables in machine learning based on artificial intelligence (AI) to correlate with objective variables such as the cetane number, kinematic viscosity, specific gravity, a higher heating value, an iodine value, flash point, oxidative stability index, and a cloud point measured for reference biofuel samples containing various FAMEs. The properties of biofuels, i.e., the objective variables, were evaluated from the mass spectral data obtained for unknown samples. The errors in the evaluation were a few percent when the distribution of the FAMEs in the unknown biofuel sample was similar to those of the biofuels used for machine learning. As demonstrated herein, the present approach, involving a combination of fsLIMS and AI, has the potential for use in evaluating the properties of a biofuel and then in solving of environmental issues associated with global warming.

Determination of Barbiturates by Femtosecond Ionization Mass Spectrometry

Barbiturates are highly susceptible to dissociation in mass spectrometry (MS) because of their long side chains combined with a nonaromatic ring consisting of several carbonyl and amine groups. As a result, they exhibit extensive α-cleavage and subsequent rearrangement, making the identification of these compounds difficult. Although a library of electron ionization MS (EIMS) is available, most barbiturates have very similar fragment patterns. Accordingly, it would be desirable to develop a technique for soft ionization, providing a molecular ion and large fragment ions as well. In this study, a molecular ion was clearly observed, in addition to large fragment ions, for a variety of barbiturates based on multiphoton ionization MS (MPIMS) using a tunable ultraviolet femtosecond laser as the ionization source (fs-LIMS). This favorable result was achieved when the optimal laser wavelength for minimizing the excess energy remaining in the ionic state was used. An examination of the photofragmentation pathways suggested that an H atom in the side chain was abstracted by an oxygen atom in the carbonyl group in the ring structure thus initiating fragmentation and subsequent rearrangement. Barbiturates that are substituted with alkyl groups (amobarbital and pentobarbital) had narrower spectral regions for optimal ionization than the other barbiturates with alkyl and alkenyl groups (butalbital and secobarbital) and more with alkyl and phenyl groups (phenobarbital). All of the barbiturates studied provided unique mass spectral patterns in fs-LIMS, which was useful for the reliable identification of these compounds in practical trace analysis.

Comprehensive Analysis of Analogues of Amine-Related Psychoactive Substances Using Femtosecond Laser Ionization Mass Spectrometry

Amine-related psychoactive molecules contain N-C_α and C_α-C_β bonds, which easily dissociate to form various fragment ions in electron ionization mass spectrometry (EIMS). Therefore, observing a molecular ion and then determining the molecular weight of the analyte is difficult. In this study, we examined phenethylamine, 3,4-methylenedioxyphenethylamine, tryptamine, N-methylephedrine, and nicotine as well as analogues of amine-related psychoactive substances using EIMS and femtosecond laser ionization mass spectrometry (fs-LIMS) combined with gas chromatography for comprehensive analysis. A molecular ion was clearly observed in fs-LIMS for all of these compounds, which was in contrast to EIMS providing fragment ions dominantly (no molecular ion was observed for N-methylephedrine). This favorable result was obtained by adjusting the laser wavelength to the optimal value for two-photon ionization to minimize the excess energy remaining in the molecular ion. It therefore appears that fs-LIMS is superior to EIMS in terms of observing a molecular ion and would be potentially useful for identifying a variety of amine-related psychoactive substances, some of which are illegal and are of interest in the field of forensic science.

Femtosecond ionization mass spectrometry for chromatographic detection

Mass spectrometry is now in widespread use for the detection of the analytes separated by chromatography. Electron ionization is the most frequently used method in mass spectrometry. However, this ionization technique sometimes suffers from extensive fragmentation of analytes, which makes identification difficult. A photoionization technique has been developed for suppressing this fragmentation and for subsequently observing a molecular ion. A variety of lasers have been employed for the sensitive and selective ionization of organic compounds. A femtosecond laser has a high peak power and is preferential for efficient ionization as well as for suppressing fragmentation, providing valuable information concerning molecular weight and chemical structure as well. In this review, we report on applications of femtosecond ionization mass spectrometry combined with gas chromatography.

Single-Photon Ionization Mass Spectrometry Using a Vacuum Ultraviolet Femtosecond Laser

The wavelength of a femtosecond Ti:sapphire laser (TS, 800 nm) was converted into the ultraviolet (UV, 200 nm) using three β-barium borate crystals (β-BaB2O4) for frequency doubling and subsequent mixing. The UV pulse was further converted into the vacuum ultraviolet (VUV, 185 nm) based on four-wave Raman mixing, in which a two-color pump beam consisting of the fundamental beam (800 nm) of the TS and the signal beam of an optical parametric amplifier (1200 nm) pumped by the TS was focused onto a capillary waveguide filled with hydrogen gas for molecular phase modulation and the single-color UV probe beam (200 nm) was then focused onto the waveguide for frequency modulation to generate anti-Stokes and high-order Stokes Raman sidebands at wavelengths of 185 and 218-267 nm, respectively. The efficiency of conversion from the UV (200 nm) to the VUV (185 nm) was 6%. The ionization energy was calculated for 13 amino polycyclic aromatic hydrocarbons using density functional theory, since they are associated with the development of occupational bladder cancers. The values calculated by the B3LYP/cc-pVDZ and ωB97Xd/cc-pVTZ methods were 6.24-7.14 eV (199-174 nm) and 6.41-7.35 eV (194-169 nm), respectively. A sample containing a mixture of 9-aminoanthracene, 3-aminofluoranthene, and 1-aminopyrene was separated by gas chromatography (GC), and the eluents were ionized with the VUV pulse (0.015 μJ) in mass spectrometry (MS). The analytes were observed on a two-dimensional display of GC/MS, and the detection limit obtained by single-photon ionization of 3-aminofluoranthene was 1 ng/μL.

Time-correlated Single Ion Counting Mass Spectrometer with Long and Short Time-of-Flight Tubes and an Evaluation of Its Performance for Use in Trace Analysis of Allergenic Substances

An analyte molecule was ionized using a femtosecond laser as the ionization source and was measured by a twin-type time-of-flight mass spectrometer with long (42 cm) and short (6.4 cm) flight tubes. The signal was measured using an analog signal digitizer and a time-correlated single ion counting system, and performance was evaluated by comparing data obtained from both instruments. The short mass spectrometer had a mass resolution of 450 and was used in the trace analysis of allergenic substances in a fragrance.

Resonant and non-resonant femtosecond ionization mass spectrometry of organochlorine pesticides

The ionization mechanism was studied based on resonance-enhanced two-photon ionization and non-resonant two- and three-photon ionizations.

Laser Desorption Combined with Laser Postionization for Mass Spectrometry

Lasers with pulse lengths from nanoseconds to femtoseconds and wavelengths from the mid-infrared to extreme ultraviolet (UV) have been used for desorption or ablation in mass spectrometry. Such laser sampling can often benefit from the addition of a second laser for postionization of neutrals. The advantages offered by laser postionization include the ability to forego matrix application, high lateral resolution, decoupling of ionization from desorption, improved analysis of electrically insulating samples, and potential for high sensitivity and depth profiling while minimizing differential detection. A description of postionization by vacuum UV radiation is followed by a consideration of multiphoton, short pulse, and other postionization strategies. The impacts of laser pulse length and wavelength are considered for laser desorption or laser ablation at low pressures. Atomic and molecular analysis via direct laser desorption/ionization using near-infrared ultrashort pulses is described. Finally, the postionization of clusters, the role of gaseous collisions, sampling at ambient pressure, atmospheric pressure photoionization, and the addition of UV postionization to MALDI are considered.

Determination of Pesticides by Gas Chromatography Combined with Mass Spectrometry Using Femtosecond Lasers Emitting at 267, 400, and 800 nm as the Ionization Source

A standard sample mixture containing 51 pesticides was separated by gas chromatography (GC), and the constituents were identified by mass spectrometry (MS) using femtosecond lasers emitting at 267, 400, and 800 nm as the ionization source. A two-dimensional display of the GC/MS was successfully used for the determination of these compounds. A molecular ion was observed for 38 of the compounds at 267 nm and for 30 of the compounds at 800 nm, in contrast to 27 among 50 compounds when electron ionization was used. These results suggest that the ultraviolet laser is superior to the near-infrared laser for molecular weight determinations and for a more reliable analysis of these compounds. In order to study the conditions for optimal ionization, the experimental data were examined using the spectral properties (i.e., the excitation and ionization energies and absorption spectra for the neutral and ionized species) obtained by quantum chemical calculations. A few molecules remained unexplained by the currently reported rules, requiring additional rules for developing a full understanding of the femtosecond ionization process. The pesticides in the homogenized matrix obtained from kabosu ( citrus sphaerocarpa) were measured using lasers emitting at 267 and 800 nm. The pesticides were clearly separated and measured on the two-dimensional display, especially for the data measured at 267 nm, suggesting that this technique would have potential for use in the practical trace analysis of the pesticides in the environment.

Determination of Hexachlorocyclohexane by Gas Chromatography Combined with Femtosecond Laser Ionization Mass Spectrometry

Structural isomers and enantiomers of hexachlorocyclohexane (HCH) were separated using a chiral column by gas chromatography and quantitatively determined by multiphoton ionization mass spectrometry using an ultraviolet femtosecond laser (200 and 267 nm) as the ionization source. The order of elution of the enantiomers (i.e., (+)-α-HCH and (-)-α-HCH) was predicted from stabilization energies calculated for the complexes using permethylated γ-cyclodextrin as the stationary phase of the column, and the results were compared with the experimental data. The molecular ions observed for HCH were weak, even though they can be ionized through a process of resonance enhanced two-photon ionization at 200 nm. This unfavorable result can be attributed to the dissociation of the molecular ion, as predicted from quantum chemical calculations. Graphical Abstract ᅟ.