Mass spectrometry imaging (MSI) is an innovative and powerful tool in biomedical research. It is well-known that folic acid (FA) has a high affinity for folic acid receptor (FR), which is overexpressing in epithelial cancer. Herein, we propose a novel method to diagnose cancer through direct mapping of the label-free FA spatial distribution in tissue sections by state-of-the-art laser desorption postionization-mass spectrometry imaging (LDPI-MSI). Compared with other tumor imaging methods, such as fluorescence imaging, photoacoustic imaging (PAI), magnetic resonance imaging (MRI), and micro-SPECT/CT, complicated synthesis and labeling processes are not required. The LDPI-MSI was performed on 30 μm thick sections from a murine model of breast cancer (inoculation of 4T1 cells) that were predosed with 20 mg/kg of FA. The image obtained from the characteristic mass spectrometric signature of FA at m/z 265 illustrated that FA was concentrated primarily in tumor tissue and displayed somewhat lower retention in adjacent normal controls. The results suggest that the proposed method could be used potentially in cancer diagnosis.

•Measured vibrational spectra in a specific mass channel reveal structures of the species in the gas phase without thermal effects and perturbations.•Gas-phase tetrahydrofurfuryl alcohol and its clusters were detected by the VUV single photon ionization and time of flight mass spectrometry.•The dominant conformer in the gas-phase for THFA monomer was assigned.•The conformational flexibility of the dimer in the gas phase was discussed.Tetrahydrofurfuryl alcohol (THFA, C5H10O2) is a close chemical analog of the sugar rings present in the phosphate-deoxyribose backbone structure of the nucleic acids. In present report, the infrared (IR) spectra of the size-selected THFA monomer and its dimer have been investigated in a pulsed supersonic jet using infrared-vacuum ultraviolet (VUV) ionization. Herein, the laser light at 118 nm wavelength served as the source of “soft” ionization in a time-of-flight mass spectrometer. The IR features for the monomers located at 3622 cm− 1 can be assigned to the intramolecular hydrogen bonding stretch vibrations mainly referring to A and C conformers. Compared with the monomer, however, characteristic peaks for the dimer centered at 3415 and 3453 cm− 1, red shifted 207 and 169 cm− 1, respectively, were associated with the intermolecular hydrogen bonding stretch vibrations. Combined with the quantum-chemical calculations, the dimer in the gas phase preferred cyclic AC conformer stabled by forming two strong intermolecular hydrogen bonds, which shown the high hydrogen bond selectivity in the cluster. The conclusions drawn from the role played in the conformational flexibility by the hydroxyl and ether groups may be extended to other biomolecules.Download high-res image (102KB)Download full-size image

•It was imperative to develop sensitive and selective methods for the detection of PSA.•The proposed sensor overcame the imperfection of the conventional immunological techniques.•It was based on the MNPs core- AuNPs satellite assemblies.•Bio-modified MNPs were used as supporting materials instead of the conventional planar solid.The accurate and highly sensitive detection of prostate specific antigen (PSA) is particularly important, especially for obese men and patients. In this report, we present a novel aptamer-based surface-enhanced Raman scattering (SERS) sensor that employs magnetic nanoparticles (MNPs) core-Au nanoparticles (AuNPs) satellite assemblies to detect PSA. The high specific biorecognition between aptamer and PSA caused the dissolution of the core-satellite assemblies, thus the concentration of functionalized AuNPs (signal probes) existing in the supernatant was on the rise with the continual addition of PSA. The aptamer-modified MNPs were used as supporting materials and separation tools in the present sensor. With the assistance of magnet, the mixture was removed from the supernatant for the concentration effects. It was found that the corresponding SERS signals from the supernatant were in direct correlation to PSA concentrations over a wide range and the limit of detection (LOD) was as low as 5.0 pg/mL. Excellent recovery was also obtained to assess the feasibility of this method for human serum samples detection. All of these results show a promising application of this method. And this novel sensor can be used for the accurate and highly sensitive detection of PSA in clinic samples in the future.

Glycoaldehyde (GA, HOCH2CHO) is the simplest sugar unit of the carbohydrates and the only sugar to have been detected in interstellar space to date. In the present report, the conformation of GA and its flexible hydrated complexes have been investigated in the gas phase by using mass-selected infrared (IR) spectroscopy based on vacuum-ultraviolet single photon ionization (118 nm). With the aid of theoretical calculations, the neutral GA bearing a ring-type intramolecular hydrogen bonding interaction was confirmed to be the dominant isomer in the gas phase. Moreover, the water molecules in the monohydrated complexes preferentially broke the intramolecular hydrogen bond and bridged the carbonyl oxygen and hydroxyl hydrogen of GA with two additional intermolecular H-bonds, revealing the “working rules” governing preferred binding. The theoretical results confirmed that the existence probability of the two lowest energy conformations stabilized by two intermolecular hydrogen bonds would be larger than that of the next two isomers with one intramolecular plus one intermolecular hydrogen bond. Structural investigation of hydrated GA conformers has revealed that the water molecules play the role of a bridge through intermolecular H-bonds, achieving selective population of specific GA molecular conformations. These results suggest that these hydrogen-bonded bridge structures in the hydrated complexes may provide good models for recognition in larger systems.

In recent years, the illegal abuse of β2-adrenergic agonists in livestock for growth promotion has been reported frequently. For conventional immunoassay methods, the development of quantitative and simultaneous detection is a great challenge. In this report, a highly sensitive multiplexed immunoassay based on competitive surface-enhanced Raman scattering (SERS) spectroscopy has been developed for the simultaneous detection of clenbuterol and ractopamine. The SERS nanoprobes based on gold nanoparticles (AuNPs) were attached by 4,4′-dipyridyl (DP), clenbuterol antibody, and 2,2′-dipyridyl (BP), ractopamine antibody respectively. The mode of detection of multiple antibodies was implemented by competitive binding between free clenbuterol, free ractopamine and clenbuterol–BSA, ractopamine–BSA immobilized on the substrates with their antibodies labeled on SERS nanoprobes. The results indicated that compared to conventional methods, the proposed method can detect clenbuterol and ractopamine simultaneously over a wide concentration range (1–1000 pg mL−1) with a lower limit of ca. 1.0 pg mL−1, which holds great potential for applications in biochemical analysis and food safety.

Localization and quantification of the target drug in tissues is a key indicator of efficacy in drug discovery. In contrast to established methods that require matrices and complex sample pretreatment steps, matrix-free and low cost in situ analysis of small molecule drugs by mass spectrometry (MS) remains challenging. Here, we present a novel approach, laser desorption postionization (LDPI), which is coupled to a linear time-of-flight (TOF) MS and used to image the distribution of acriflavine (ACF) directly from a histological section of mouse kidney without any matrix or sample pretreatment. The identification of the mass peaks assigned to ACF was further confirmed by DESI-MS/MS. Moreover, the matrix effect from the tissue section was explored, showing minimal desorption and ionization suppression in the LDPI-MS process. LDPI-MS imaging (LDPI-MSI) was performed on 30 μm kidney sections from mice 15 min postdose that were dosed with 30 mg kg−1 of ACF by monitoring the fragment ion at m/z 209. The LDPI-MS image revealed a global view of the distribution of ACF in the kidney compartments (pelvis, medulla, and cortex). Estimated concentrations of ACF residue in mouse kidney were obtained by LDPI-MSI and LC-MS/MS and a 12.1% difference in measured tissue concentration was found. These results suggest that the use of LDPI-MS in small molecule drug localization and quantification directly from biological tissue at the same time is favorable.

Morpholine (NH(CH2CH2)2O) is a typical six-membered aliphatic heterocyclic compound. Herein, infrared plus vacuum ultraviolet (IR/VUV) single photon “soft” ionization spectroscopy was employed to study the structures of neutral morpholine and its monohydrated clusters. Theoretical calculations revealed that the structures containing equatorial-chair and axial-chair conformations were the most stable conformers in the gas phase, and this was confirmed by IR spectral analysis. Analysis of the observed and calculated spectra of the monohydrated clusters suggested that multiple conformers may co-exist in the molecular beam, and that the water molecule acts as a hydrogen donor. In the most stable structure, the hydrogen atom of the water molecule is bound to the NH group of the equatorial-chair conformer of morpholine. Moreover, the water molecules simultaneously serve as hydrogen bond donors for the NH group and interact with the CH group weakly. It is suggested that the weak intermolecular CH⋯O interaction is also responsible for the molecular stability.

A novel pretreatment-free method involving laser desorption postionization (LDPI) coupled with time-of-flight mass spectrometry (MS) was developed for the monitoring of proflavine level in rat whole blood. It comprises a protocol for dosing via intravenous administration and collection of whole blood, followed by direct LDPI-MS analysis without any sample pretreatment. An intense ion signal at m/z 209 was observed from whole blood without any interference signals, except some background signals below m/z 100. The calibration curve was established with use of 9-phenylacridine as the internal standard for proflavine determination from the plotting of the peak ratios of proflavine to the internal standard, with a correlation coefficient (R2) greater than 0.99. The limit of detection was estimated to be 0.48 pmol/mm2 and the quantification range was 0.5-16.5 μg/mL for proflavine. In addition, only a minimal matrix effect was observed, as expected from considerations of the desorption and ionization mechanism. Interday and intraday accuracy and precision were calculated to be within 13% and 82–114%, respectively. Estimated concentrations of proflavine residue in whole blood were also successfully obtained at selected time points after dosing. The proposed method is simple, low cost, and sensitive, and should be seen as a complementary method for monitoring drug levels in blood.

Photoionization and dissociation of the 1-propanol dimer and subsequent fragmentations have been investigated by synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry and theoretical calculations. Besides the protonated monomer cation (C3H7OH)·H+ (m/z = 61) and Cα–Cβ bond cleavage fragment CH2O·(C3H7OH)H+ (m/z = 91), the measured mass spectrum at an incident photon energy of 13 eV suggests a new dissociation channel resulting in the formation of the (C3H7OH)·H+·(C2H5OH) (m/z = 107) fragment. The appearance energies of the fragments (C3H7OH)·H+, CH2O·(C3H7OH)H+ and (C3H7OH)·H+·(C2H5OH) are measured at 10.05 ± 0.05 eV, 9.48 ± 0.05 eV, and 12.8 ± 0.1 eV, respectively, by scanning photoionization efficiency (PIE) spectra. The 1-propanol ion fragments as a function of VUV photon energy were interpreted with the aid of theoretical calculations. In addition to O–H and Cα–Cβ bond cleavage, a new dissociation channel related to Cβ–Cγ bond cleavage opens. In this channel, molecular rearrangement (proton transfer and hydrogen transfer after surmounting an energy barrier) gives rise to the generated complex, which then dissociates to produce the mixed propanol/ethanol proton bound cation (C3H7OH)·H+·(C2H5OH). This new dissociation channel has not been reported in previous studies of ethanol and acetic acid dimers. The photoionization and dissociation processes of the 1-propanol dimer are described in the photon energy range of 9–15 eV.

•We used aptamer-modified SERS nanosensor to detect melamine in milk.•Poly thymine aptamer was used for melamine recognition.•Oligonucleotide chip was used as capture substrate.•The proposed sensor has a low limit of detection of 1.0 pg/mL.•The method is low-cost and the sample pretreatment is simplified.Herein, we developed a novel nanosenor to detect trace amount of melamine (M) in the milk using aptamer-modified surface-enhanced Raman scattering (SERS) nanosensor and oligonucleotide chip. The SERS nanosensor was prepared by attaching Raman tag and ploy-thymine (T) aptamer to the gold nanoparticles, respectively. SERS nanosensor can be absorbed to the oligonucleotide chip with a high specificity where a “T–M–T” structure via multi-hydrogen-bond could be formed between the melamine molecule and the thymine molecule. In the measurement, the Raman signal intensities obtained from the chip increase when the concentrations of melamine in solutions increase. The proposed method allows us to detect melamine with a limit of 1.0 ppt (1.0 pg mL−1), which is fall far below the strictest safety limit. Compared with the previous methods, the sensitivity of this sensor shows about 100-fold improvement. The advantages of the present sensor are its low detection cost and the simplified sample pretreatment. Furthermore, the reliable and enough accurate results have been obtained by the using of the proposed nanosensor in the assay of trace melamine in real milk sample.

Wide and abusive applications of fungicides (such as chlorothalonil) in agricultural production have caused various adverse effects on the environment, especially on soil. Herein, a novel laser desorption VUV single photon post-ionization mass spectrometry (LDPI-MS) method has been applied for the first time for the direct and fast detection of chlorothalonil in soil. In the experiment, three different wavelength lasers were used as the ionization sources (SPI at 118 nm, REMPI at 266 nm and 355 nm) and the results showed that only SPI at 118 nm could achieve expected “soft” ionization. The limit of detection at 118 nm ionization was determined to be 0.5 pmol per spot, ca. 1 mg kg−1 of chlorothalonil in soil. Moreover, no other additives were needed to assist desorption/evaporation of chlorothalonil from soil samples and the detection process could be rapidly completed on the basis of a time-saving sample pretreatment. The results demonstrated that the LDPI-MS method holds great potential for detecting real natural soil contaminated with chlorothalonil.

•The adsorption behavior of 2-thiouracil on gold surface is explored by SERS and DFT.•The 2TU and 2TU2 conformers exist stably in the aqueous solution.•2-Thiouracil is vertically chemisorbed on gold surface through N and S atoms.2-Thiouracil, a thio-derivative of uracil, may appear in various tautomeric forms due to the different positions of protons. In this paper, the adsorption behavior and conformations of 2-thiouracil on the gold substrates are inspected by means of surface-enhanced Raman scattering (SERS) and density functional theory (DFT) calculations. The results indicate that all the enhanced bands are assigned to in-plane vibration modes. Besides, most of the bands related to N and S atoms are significantly enhanced and have obvious shifts in the SERS spectra. Furthermore, the CO stretching band at 1695 cm−1 also appears in the SERS spectra. The theoretical SERS spectra of 2TU-Au4 and 2TU2-Au4 complexes agree well with the experimental SERS spectra of 2-thiouracil at 0.04 mM. Meanwhile, we calculate that the binding energies for 2TU2-Au4 and 2TU-Au4 are ca. 70 and 50 kcal/mol, respectively. Those results above imply that the 2TU and 2TU2 conformers can exist stably in the aqueous solution and both of them are vertically chemisorbed on the gold surfaces. For the 2TU, it is adsorbed on the gold surfaces through N1H position and the sulfur atom. While the 2TU2 adsorbed on the gold substrates through the N1 site and its deprotonated sulfur atom.

Accurate and quantitative analysis of mycotoxin (such as zearalenone) is particularly imperative in the field of food safety and animal husbandry. Here, we develop a sensitive and specific method for zearalenone detection using competitive surface-enhanced Raman scattering (SERS) immunoassay. In this assay, a functional gold nanoparticle was labeled with the Raman reporter and the zearalenone antibody, and a modified substrate was assembled with the zearalenone–bovine serum albumin. With the addition of free zearalenone, the competitive immune reaction between free zearalenone and zearalenone–bovine serum albumin was initiated for binding with zearalenone antibody labeled on gold nanoparticle, resulting in the change of SERS signal intensity. The proposed method exhibits high sensitivity with a detection limit of 1 pg/mL and a wide dynamic range from 1 to 1000 pg/mL. Furthermore, this method can be further applied to analyze the multiple natural feed samples contaminated with zearalenone, holding great potential for real sample detection.

In present report, we explored hyperconjugation effects on the site- and bond-selective dissociation processes of cationic ethanol conformers by the use of theoretical methods (including configuration optimizations, natural bond orbital (NBO) analysis, and density of states (DOS) calculations, etc.) and the tunable synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry. The dissociative mechanism of ethanol cations, in which hyperconjugative interactions and charge-transfer processes were involved, was proposed. The results reveal Cα–H and C–C bonds are selectively weakened, which arise as a result of the hyperconjugative interactions σCα-H → p in the trans-conformer and σC–C → p in gauche-conformer after being ionized. As a result, the selective bond cleavages would occur and different fragments were observed.

The Raman spectral studies of creatinine with pH dependence were performed to explore the effects of pH values on the Raman spectroscopy of creatinine. Firstly, we calculated vibrational spectra by DFT to derive the equilibrium geometries and protonated form of creatinine. Comparing simulated and observed Raman spectra of creatinine in aqueous solution at pH 2, it is found the theoretical predicted spectra agree well with those of the experiment while seven water molecules are aggregated around the creatinine. Additionally, the tautomeric equilibrium of creatinine in aqueous solutions was studied and two tautomers are found to coexist by comparing its experimental and calculated Raman spectra. A water dimer being used to solvate creatinine would make the thermodynamic energy favor convert from the imino tautomer to the amino tautomer. Besides, the molecular electrostatic potential (MEP) analysis of the creatinine further confirms their discrepancies of typical experimental Raman spectra at different pH values.Graphical abstractHighlights► The tautomeric equlibrium and behavior of creatinine in aqueous solutions have been firstly studied by means of Raman spectroscopy and theoretical calculations (DFT). ► As 7 water molecules are gradually aggregated around the creatinine, theoretical results show an excellent accordance with the experimental spectrum. ► Analysis of molecular electrostatic potential (MEP) for creatinine (two tautomers and one protonated form) could explain why typical experimental Raman spectra with different pH values have obvious discrepancies at the electrical level.

Benzoic acid (BA) and phthalic acid (PTA) are the simplest aromatic carboxylic acids, and they can be regarded as typical model compounds in investigating the interaction of aromatic carboxylic acids with metal surfaces by use of SERS spectroscopy. In this work, we have investigated the structure and adsorption behavior of benzoic acid and phthalic acid on the gold surface with combination of SERS and DFT calculation methods. The experimental results show that both BA and PTA may be adsorbed on the Au surface with a bidentate bridging structure, namely, the carboxylate group(s) being bound to gold via two oxygen atoms in the carboxylate group(s). Comparison of the observed SERS and predicted spectra of the complexes of these two substances with Au atoms indicates that BA is favorable to adsorb on the gold surface with a vertical orientation rather than a flat one, and PTA could “stand up” on the Au surface as a slight tilt with a two-legged geometry, i.e. all four oxygen atoms in two carboxylate groups interact on the metal surface. Apart from that, we compare the discrepancy of SERS spectra between those two molecules, which could be taken as a potential analysis technique in food safety field.Graphical abstractHighlights► Adsorption of benzoic acid (BA), phthalic acid (PTA) on Au surface was studied. ► The study relies on Raman spectroscopy and theoretical calculations (DFT) methods. ► BA is vertically adsorbed on Au surface via two oxygen of the carboxylate group. ► PTA could “stand up” on Au surface with a minor angle through a bridging structure. ► Discrepancy in BA, PTA SERS spectra shed some light on food safety analysis field.

In this report, we present a novel approach to detect clenbuterol based on competitive surface-enhanced Raman scattering (SERS) immunoassay. Herein, a SERS nanoprobe that relies on gold nanoparticle (GNP) is labeled by 4,4′-dipyridyl (DP) and clenbuterol antibody, respectively. The detection of clenbuterol is carried out by competitive binding between free clenbuterol and clenbuterol-BSA fastened on the substrate with their antibody labeled on SERS nanoprobes. The present method allows us to detect clenbuterol over a much wider concentration range (0.1–100 pg mL−1) with a lower limit of detection (ca. 0.1 pg mL−1) than the conventional methods. Furthermore, by the use of this new competitive SERS immunoassay, the clenbuterol-BSA (antigen) is chosen to fasten on the substrate instead of the clenbuterol antibody, which could reduce the cost of the assay. Results demonstrate that the proposed method has the wide potential applications in food safety and agonist control.Graphical abstractSchemes of SERS nanoprobes preparation (a) and competitive SERS immunoassay for clenbuterol (b).Highlights► A new method for clenbuterol detection by the use of a competitive SERS immunoassay has been developed. ► 4,4′-Dipyridyl is chosen as the Raman reporter due to its fast-labeled, nontoxic and bifunctional properties. ► The present method could detect clenbuterol over a wide dynamic concentration range and exhibit significant specificity in real samples. ► The technique is more sensitive and simpler than the conventional method ELISA.

As a particular ionization technique, laser photoionization has been widely used in mass spectroscopy. Matrix-assisted laser desorption ionization (MALDI), in which only one laser was used, has become the standard method in the biological mass spectrometry. In this review, a novel mass spectrometry is introduced, i.e., two-step laser desorption/laser ionization mass spectrometry (L2MS). In comparison with MALDI, no matrix is needed to add on the substrate to form a good cocrystallization with sample molecules. The signal can be optimized by changing the power and wavelength of two lasers independently. In this article, we review the recent research progress on L2MS and its applications on the analysis of amino acids and peptides, environmental pollutants such as polycyclic aromatic hydrocarbons, chemical additives, mineral composition, metabolic products, and so on. Future prospect in this field is provided in the end of this article.

Morpholine is a typical six-membered saturated heterocycle with the molecular formula HN(CH2CH2)2O. In this work, the conformations of Morpholine in liquid and adsorbed on the surface of gold nanoparticles were studied by means of Raman spectroscopy and theoretical calculations. Ab initio calculations indicate that the energy of the chair conformers of Morpholine is ca. 7.5 kcal/mol lower than the skew-boat conformers, which implies that the chair conformers would be favorable in liquid Morpholine. Comparison of the observed Raman spectra of liquid Morpholine, its solution, and the predicted spectra of the chair conformers (equatorial and axial) revealed that both of the chair conformers coexist in its liquid, and the content of the equatorial-chair conformer may reduce in the solution. Considering the concentration-dependent Surface-Enhanced Raman Scattering (SERS) spectral profile, the surface selection rule, and the theoretical calculations, it has been inferred that at higher concentrations Morpholine is vertically chemisorbed on gold nanoparticles through the N atom of the ring, and the dominant conformation adsorbed is the axial-chair conformer. However, at the dilute concentrations, Morpholine is gradually flatly chemisorbed on the gold nanoparticles through the N atom, and the Morpholine may be deprotonated. Furthermore, the predicted spectra agree with the experimental ones very well, which confirms the results above.

2-Phenylethylamine (PEA) is the simplest aromatic amine neurotransmitter, as well as one of the most important. In this work, the conformational equilibrium and hydrogen bonding in liquid PEA were studied by means of Raman spectroscopy and theoretical calculations (DFT/MP2). By changing the orientation of the ethyl and the NH2 group, nine possible conformers of PEA were found, including four degenerate conformers. Comparison of the experimental Raman spectra of liquid PEA and the calculated Raman spectra of the five typical conformers in selected regions (550−800 and 1250−1500 cm−1) revealed that the five conformers can coexist in conformational equilibrium in the liquid. The NH2 stretching mode of the liquid is red-shifted by ca. 30 cm−1 relative to that of an isolated PEA molecule (measured previously), implying that intermolecular N−H···N hydrogen bonds play an important role in liquid PEA. The relative intensity of the Raman band at 762 cm−1 was found to increase with increasing temperature, indicating that the anti conformer might be favorable in liquid PEA at room temperature. The blue shift of the band for the bonded N−H stretch with increasing temperature also provides evidence of the existence of intermolecular N−H···N hydrogen bonds.

Aniline−methanol mixed clusters are ionized by single photon vacuum ultraviolet (VUV, 118 nm) radiation with which absorption to an excited intermediate S1 state is not required. Aniline ion (An+), a series of (An)n+−(CH3OH)m (n = 1, 2) cluster ions, and their hydrogenated cluster ions, (An)n+−(CH3OH)mH (n = 1, 2) are observed by mass spectrometry. Infrared (IR) absorption spectra of aniline−methanol cluster cations and neutrals are measured through IR and VUV (118 nm) “ion dip” spectroscopy in the range 2500−4000 cm−1. The observed mid-IR spectrum of the An+−CH3OH has two sharp absorption bands, at 3438 and 3668 cm−1, which are assigned to the free NH stretch vibration of the aniline cation and the free OH stretch vibration of methanol, respectively. Calculations demonstrated that a change in the charges on the nitrogen atom of the amine group upon ionization of the neutral to the cluster cation alters the role of aniline from hydrogen acceptor to hydrogen donor in its interaction with methanol. Theoretical and experimental results suggest that a hydrogen bond forms between one of the H atoms of the aniline amine group and the lone pair of electrons of the methanol oxygen atom in the aniline−methanol cluster cation. Measured IR spectra and theoretical results for neutral clusters suggest that the H atom of the methanol OH moiety is bonded to the aniline amine group lone pair electrons for the neutral ground state aniline−methanol cluster.

We report the first observation of infrared (IR) and mass spectra of neutral, aliphatic amino acid clusters: the example presented herein is for (valine)n, n = 2−5. The clusters are generated in a supersonic expansion and their IR spectra are recorded in various fragment and (valine)n−1H+ (n = 2−5) mass channels. The ions are created by single photon ionization with a VUV laser at 118 nm (10.5 eV/photon) following IR absorption in the single photon energy range 2500−4000 cm−1. Mass channels obviously associated with valine clusters lose intensity and mass channels associated with valine monomers gain intensity under IR irradiation. No free OH modes are identified in any of these spectra suggesting that for (valine)n, n = 1−5, all the OH groups are hydrogen bonded. The infrared transition for the hydrogen bonded OH moiety appears as a very broad, shifted feature ca. 3000 cm−1 (∼2800 to ∼3200 cm−1). Free and perturbed NH2 modes can also be identified in the cluster spectra. CH modes ca. 3000 cm−1 can be identified and appear to be coupled to the shifted and broadened OH modes of the clusters. Fragmentation pathways for three (valine)2 isomers under 118 nm ionization are proposed and discussed.

Wide and abusive applications of fungicides (such as chlorothalonil) in agricultural production have caused various adverse effects on the environment, especially on soil. Herein, a novel laser desorption VUV single photon post-ionization mass spectrometry (LDPI-MS) method has been applied for the first time for the direct and fast detection of chlorothalonil in soil. In the experiment, three different wavelength lasers were used as the ionization sources (SPI at 118 nm, REMPI at 266 nm and 355 nm) and the results showed that only SPI at 118 nm could achieve expected “soft” ionization. The limit of detection at 118 nm ionization was determined to be 0.5 pmol per spot, ca. 1 mg kg−1 of chlorothalonil in soil. Moreover, no other additives were needed to assist desorption/evaporation of chlorothalonil from soil samples and the detection process could be rapidly completed on the basis of a time-saving sample pretreatment. The results demonstrated that the LDPI-MS method holds great potential for detecting real natural soil contaminated with chlorothalonil.

Photoionization and dissociation of the 1-propanol dimer and subsequent fragmentations have been investigated by synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry and theoretical calculations. Besides the protonated monomer cation (C3H7OH)·H+ (m/z = 61) and Cα–Cβ bond cleavage fragment CH2O·(C3H7OH)H+ (m/z = 91), the measured mass spectrum at an incident photon energy of 13 eV suggests a new dissociation channel resulting in the formation of the (C3H7OH)·H+·(C2H5OH) (m/z = 107) fragment. The appearance energies of the fragments (C3H7OH)·H+, CH2O·(C3H7OH)H+ and (C3H7OH)·H+·(C2H5OH) are measured at 10.05 ± 0.05 eV, 9.48 ± 0.05 eV, and 12.8 ± 0.1 eV, respectively, by scanning photoionization efficiency (PIE) spectra. The 1-propanol ion fragments as a function of VUV photon energy were interpreted with the aid of theoretical calculations. In addition to O–H and Cα–Cβ bond cleavage, a new dissociation channel related to Cβ–Cγ bond cleavage opens. In this channel, molecular rearrangement (proton transfer and hydrogen transfer after surmounting an energy barrier) gives rise to the generated complex, which then dissociates to produce the mixed propanol/ethanol proton bound cation (C3H7OH)·H+·(C2H5OH). This new dissociation channel has not been reported in previous studies of ethanol and acetic acid dimers. The photoionization and dissociation processes of the 1-propanol dimer are described in the photon energy range of 9–15 eV.