Photolysis Research Articles

Generation of Reactive Nitrogen Species in UV Photolysis of Dichloramine and Their Incorporation into Nitrogenous Byproducts.

Dichloramine (NHCl2) often coexists with monochloramine (NH2Cl) in reverse osmosis (RO) permeate in potable reuse scenarios when NH2Cl is added upstream of RO for membrane fouling control such that UV photolysis of NHCl2 occurs during the downstream UV/chloramine process. However, the formation of reactive nitrogen species (RNS) and their incorporation into byproducts during the UV/NHCl2 process are largely unknown. This study quantitatively evaluated the generation of RNS in the UV/NHCl2 process and investigated the role of RNS in micropollutant transformation. UV photolysis of NHCl2 produced comparable RNS concentration to that of NH2Cl at the same oxidant dosage (100 μM) at pH 5.5. Under the experimental conditions, the RNS contributed greatly (40.6%) to N,N-diethyl-3-methylbenzamide (DEET) degradation. By using 15N-labeling and mass spectrometry methods, seven nitrogenous byproducts of DEET degradation with the incorporation of nitrogen originating from the RNS were detected. Among these seven byproducts, six were identified to contain a nitro group (-NO2). While the UV/NHCl2 process formed comparable intensities of -NO-containing products to those in the UV/NH2Cl process, the later process formed 3-91% higher intensities of -NO2-containing products. These findings are essential in furthering our understanding of the contribution of the UV/NHCl2 process in potable reuse scenarios.

Comparison of Medium-Pressure UV/Peracetic Acid to Remove Three Typical Refractory Contaminants of Textile Wastewater

In this work, the performance of medium-pressure UV/peracetic acid (MPUV/PAA/H2O2) was explored on removing reactive black 5 (RB5), aniline (ANL), and polyvinyl alcohol (PVA), three typical refractory contaminants in printing and dyeing wastewater, compared with MPUV/H2O2. MPUV/PAA/H2O2 showed 75.0, 44.9, and 57.7% removals of RB5, ANL, and PVA, respectively, within 5 min. The removal of RB5 increased from 68.98 to 91.2%, with pH increasing from 6 to 9, while the removals of ANL and PVA were much less pH-dependent. Quenching experiment results indicated that UV photolysis and radical (i.e., •OH and R-C•) oxidation contributed to RB5 removal, while PAA showed high activity in the oxidation of ANL. For PVA, •OH oxidation and UV photolysis were likely the main mechanisms. The coexisting natural organic matter had a negative effect on the degradation of RB5 and PVA. In addition, MPUV/PAA/H2O2 could effectively degrade those pollutants without increasing the toxicity. This work provides a theoretical reference for the utilization of MPUV/PAA/H2O2 in removing structurally diverse refractory contaminants from printing and dyeing wastewater.

Dissociative photoionisation of O2 with tunable 10, 20 and 30 eV photons

Atomic oxygen ions, O+, created by photoexcitation of a cold supersonic beam of O2 by the output of a free-electron laser operating in the 128–122 nm region are detected by sliced velocity map imaging. Analysis of the richly structured high resolution kinetic energy and angular distributions obtained show that O+ production is due to the second and third harmonics present in the laser output. Excellent slicing and optimal velocity mapping allows separation of overlapping signal from dissociative photoionisation processes driven by 20 eV versus 30 eV photons. A two-colour experiment combining the free electron laser and a nanosecond dye laser output at 225.6 nm allowed detection of direct photodissociation of the metastable O2 + b 4Σg - (v = 0–4) state is observed and assigned to a parallel b 4Σg - – (c 4Σu -)4s Rydberg transition followed by dissociation to O+ (4S) + O(3P2).

A new view on the mechanism of UV photodegradation of the tricyclic antidepressant carbamazepine in aqueous solutions

Mechanism of direct UV photolysis of the tricyclic antidepressant carbamazepine (CBZ) at neutral pH was revealed by a combination of nanosecond laser flash photolysis, steady-state photolysis combined with high resolution LC-MS and DFT quantum-chemical calculations. The detection of short-lived intermediates and the detailed identification of final products were performed for the first time. The quantum yield of CBZ photodegradation (282 nm) is about 0.1% and 0.18% in air-equilibrated and argon-saturated solutions. The primary stage is photoionization with the formation of CBZ cation radical followed by a rapid nucleophilic attack by a solvent molecule. The primary photoproducts are 10-oxo-9-hydro-carbamazepine, 9-formylacridine-10(9H)-carboxamide (a result of ring contraction) and various isomers of hydroxylated CBZ. Prolonged irradiation results to accumulation of acridine derivatives, which should lead to an increase of the toxicity of photolyzed CBZ solutions. The obtained results may be important for understanding the fate of tricyclic antidepressants in processes of UVC disinfection and in natural waters under action of sunlight.

Investigating the Rydberg states and photodissociation dynamics of Kr2using velocity map imaging

Although the krypton dimer, Kr2, is a prototypical weakly bound van der Waals complex in its ground electronic state, the excited states of Kr2exhibit many and various interactions. Resonance-enhanced multiphoton ionization and velocity map imaging provide an avenue to explore the electronic structure and photofragmentation dynamics of Kr2in the vacuum ultraviolet region. By monitoring photofragmentation product channels and recoil anisotropy distributions, one can gain insights into excited state symmetries and lifetimes, which can aid in interpreting spectroscopy. Here, we study the spectroscopy and photodissociation dynamics of Kr2in the 91 000–94 500 cm−1region. We record and assign photofragment product channels for 32 vibronic bands corresponding to seven electronic transitions. We also identify two previously unobserved vibronic band systems, which we assign as the Kr2 5p [5/2]3 [Formula: see text] and 5p [3/2]2[Formula: see text] transitions. Velocity map images show photofragments arising from predissociation of the neutral Rydberg states and from dissociative photoionization at the n > 2 photon level. We also observe variation in branching ratios between the predissociative and dissociative photoionization channels depending on the lifetime of the intermediate; longer lived states favour higher dissociative photoionization yields and exhibit β6anisotropy parameters for predissociation product channels, indicating “rotational smearing” of the product angular distributions.

Photoelectron spectroscopy and dissociative photoionization of fulminic acid, HCNO.

We report a joint experimental and computational study of the photoelectron spectroscopy and the dissociative photoionization of fulminic acid, HCNO. The molecule is of interest to astrochemistry and astrobiology as a potential precursor of prebiotic molecules. Synchrotron radiation was used as the photon source. Dispersive photoelectron spectra were recorded from 10 to 22eV, covering four band systems in the HCNO cation, and an ionization energy of 10.83eV was determined. Transitions into the Renner-Teller distorted X+2Π state of the cation were simulated using wavepacket dynamics based on a vibronic coupling Hamiltonian. Very good agreement between experiment and theory is obtained. While the first excited state of the cation shows only a broad and unstructured spectrum, the next two higher states exhibit a well-resolved vibrational progression. Transitions into the excited electronic states of HCNO+ were not simulated due to the large number of electronic states that contribute to these transitions. Nevertheless, a qualitative assignment is given, based on the character of the orbitals involved in the transitions. The dissociative photoionization was investigated by photoelectron-photoion coincidence spectroscopy. The breakdown diagram shows evidence for isomerization from HCNO+ to HNCO+ on the cationic potential energy surface. Zero Kelvin appearance energies for the daughter ions HCO+ and NCO+ have been derived.

A Metastable Ketenyl Radical-Water Complex from UV Photolysis of the Carboxymethyl Radical.

The unpaired electron impacts the binding between radicals and ordinary closed-shell molecules in noncovalent complexes. Conversely, the complexation partner can enhance, decrease, or even control the reactivity of the interacting radical. Previously, such radical-molecule (and especially radical-water) complexes were studied by controlled assembly of the interacting partners which mostly leads to formation of the thermodynamically most stable species. Here, we show that UV photolysis of the resonance-stabilized carboxymethyl radical isolated in a cryogenic argon matrix at 4 K leads to the intermediary formation of a metastable, noncovalent complex of the ketenyl radical with a water molecule. In this complex, the ketenyl radical binds water at its terminal carbon atom, although a more stable isomer exists in which water interacts with the C-H bond of the radical. Rigorous W1 theory computations confirm that the ketenyl radical is a stronger donor in C-H···O interactions than ketene itself, while it performs comparably well as an acceptor. We propose that complex formation proceeds via an initial excited-state C-O bond breaking reaction in carboxymethyl under release of an OH radical, which is supported by multireference QD-NEVPT2 computations.

Application of modified graphene oxide-chitosan composite for the removal of 2-methylpyridine using fixed bed adsorption and subsequent regeneration of the adsorbent by UV photolysis

This article focuses on the development of a photolytically regenerable and practically applicable graphene oxide nanomaterial (GON) based composite adsorbent for the fixed-bed column treatment of 2-methylpyridine (2Mp). The transformation of nano-sized GON into a macro-sized adsorbent was achieved by crosslinking it with a biocompatible crosslinker, chitosan, and then coating it onto sand (abbreviated as GCS). FTIR revealed the nature of the crosslinking as amide bonding. GCS was found to interact with 2Mp via hydrogen-bonded ionic pair mechanism. Column studies of GCS with 2Mp followed Thomas kinetics at variable flow rates, bed depths and initial 2Mp concentrations. The adsorption capacity of GCS was 7.8 mg/g. The hydraulic character of flow was determined as laminar (Re < 1). Doehlert Design of experiment was used in modeling/optimization of experimental data. The polynomial model predicted an optimal column breakthrough time of 645.9 ± 10.63 min, with a regression coefficient of 0.99. Linear terms of bed depth and flow rate had the most impact on prediction. Thereafter, the regeneration of exhausted GCS was performed using direct UV photolysis, based on the SALDI mechanism. No photocatalysts/chemicals were necessary. Regeneration efficiency of 96.5 % was obtained using irradiation time of 30 min and power input of 0.96 W/cm2. GCS was reusable in 10 cycles of adsorption-regeneration, without losing significant adsorption capacity. The spent liquor obtained afterwards was remediated using batch photolysis prior to its discharge into the eco-system. Therein, the mechanism for photo-degradation of 2Mp was found, where succinic acid was detected as the eco-friendly end product. This study overcomes the practical difficulty in application of photolysis to large influent volumes by first isolating 2Mp on the GCS surface and then subjecting it to photolytic treatment in a 95.56 % smaller solution volume.

Dissociative photoionization studies of ethyl iodide using synchrotron radiation photoionization mass spectrometry and photoelectron imaging

The stepwise dissociative photoionization of ethyl iodide(C2H5I) has been investigated via synchrotron radiation photoionization mass spectrometry and photoelectron imaging spectroscopy. By using VUV synchrotron radiation as excitation source, the photoionization and stepwise dissociative photoionization of C2H5I were observed by tuning the photon energy from 8.5 to 14.0 eV. The ionization energy of C2H5I was measured to be 9.33 ± 0.05 eV, and the appearance energies of C2H5+, C2H4+, C2H3+ were also determined according to the photoionization efficiency spectra. The possible pathways of the stepwise dissociation of C2H5I were determined by combining the experiments with theoretical calculation based on DFT. Finally, the multi-photon ionization process of C2H5I molecule was further studied via intensity-dependent photoelectron imaging spectroscopy.

Comparison of photocatalysis and photolysis of 2,2,4,4-tetrabromodiphenyl ether (BDE-47): Operational parameters, kinetic studies, and data validation using three modern machine learning models

Polybrominated diphenyl ethers (PBDEs) are halogenated organic compounds that are among the major pollutants of water, and there is an urgent need for their removal. This work compared the application of two techniques, i.e., photocatalytic reaction (PCR) and photolysis (PL), for 2,2,4,4- tetrabromodiphenyl ether (BDE-47) degradation. Although a limited degradation of BDE-47 was observed by photolysis (LED/N2), photocatalytic oxidation by using TiO2/LED/N2 proved to be effective in the degradation of BDE-47. The use of a photocatalyst enhanced the extent of BDE-47 degradation by around 10% at optimum conditions in anaerobic systems. Experimental results were systematically validated through modeling with three new and powerful Machine Learning (ML) approaches, including Gradient Boosted Decision Tree (GBDT), Artificial Neural Network (ANN), and Symbolic Regression (SBR). Four statistical criteria (Coefficient of Determination (R2), Root Mean Square Error (RMSE), Average Relative Error (ARER), and Absolute Error (ABER)) were calculated for model validation. Among the applied models, the developed GBDT was the desirable model for predicting the remaining concentration (Ce) of BDE-47 for both processes. Total Organic Carbon (TOC) and Chemical Oxygen Demand (COD) results confirmed that BDE-47 mineralization required additional time than its degradation in both PCR and PL systems. The kinetic study demonstrated that BDE-47 degradation for both processes followed the pseudo-first-order form of the Langmuir-Hinshelwood (L-H) model. More importantly, the calculated electrical energy consumption of photolysis was shown to be ten percent higher than that for photocatalysis, possibly due to the higher irradiation time required in direct photolysis, which in turn increases electricity consumption. This study is useful in proposing a feasible and promising treatment process for the degradation of BDE-47.

Ultrafast Photo-Ion Probing of the Relaxation Dynamics in 2-Thiouracil

In this work, we investigate the relaxation processes of 2-thiouracil after UV photoexcitation to the S2 state through the use of ultrafast, single-colour, pump-probe UV/UV spectroscopy. We place focus on investigating the appearance and subsequent decay signals of ionized fragments. We complement this with VUV-induced dissociative photoionisation studies collected at a synchrotron, allowing us to better understand and assign the ionisation channels involved in the appearance of the fragments. We find that all fragments appear when single photons with energy > 11 eV are used in the VUV experiments and hence appear through 3+ photon-order processes when 266 nm light is used. We also observe three major decays for the fragment ions: a sub-autocorrelation decay (i.e., sub-370 fs), a secondary ultrafast decay on the order of 300-400 fs, and a long decay on the order of 220 to 400 ps (all fragment dependent). These decays agree well with the previously established S2 → S1 → Triplet → Ground decay process. Results from the VUV study also suggest that some of the fragments may be created by dynamics occurring in the excited cationic state.

Impact of Polarization Reversal during Photoelectrocatalytic Treatment of WWTP Effluents

Photoelectrocatalysis (PEC) has been already proposed as a polishing treatment for wastewater treatment plants (WWTPs) effluents. In this work, the impact of polarization reversal during PEC process has been studied and evaluated on the basis of the removal of organic substance and color, biodegradability of the matrix, and inactivation of the catalyst. Effluents were sampled from a full-scale WWTP and alternatively treated by electrochemical oxidation (EC), photolysis (PL), photocatalysis (PC), photoelectrocatalysis, and photoelectrocatalysis with reverse polarization (PECr). The efficiency and the kinetics of the process, in terms of removal of organic substance and color, were not affected by reverse polarization and very similar results were obtained by PEC and PECr. The biodegradability of the effluents strongly increased both by PECr (RSBR: 0.84 ± 0.07), and by PEC and PL (0.89 ± 0.11, and 0.78 ± 0.02, respectively). In the selected polarization reversal mode (100 s at −0.1 V every 500 s at 4 V, cell voltage), a similar photocurrent loss after PEC and PECr was observed, suggesting no effect on the activity of the TiO2 mesh. This study can serve as a base for future research on polarization reversal to optimize operation parameters and exploit the procedure to preventing fouling and inactivation of the catalyst.

Combining Constructed Wetlands and UV Photolysis for the Advanced Removal of Organic Matter, Nitrogen, and Emerging Pollutants from Wastewater

This study reports the performance of a three-step lab-scale system including a hybrid digester (HD), a vertical flow (VF) constructed wetland, and a photodegradation (PD) lamp, with two different arrangements regarding the position of the recirculation point. In addition to total suspended solids (TSS), chemical oxygen demand (COD), and nitrogen compounds, removal of the following pollutants was investigated: paracetamol (ACE), ofloxacin (OFL), caffeine (CAF), ketoprofen (KET), ibuprofen (IBU), clofibric acid (ACB), bisphenol A (BPA), and sotalol (SOT). An excellent performance of HD was achieved on the elimination of TSS (82.2 ± 18.5% on average) and COD (63.9 ± 4.1%). TSS and COD removal increased to 91.2 ± 0.4% and 83.4 ± 2.9%, respectively, for the combined HD–VF system. Ammonia removal was 57.0 ± 7.8% in the VF unit while significant denitrification occurred in the HD. The overall HD–VF–PD system achieved mean removals of 100% for OFL, KET, SOT, and IBU, 98 ± 2% for ACE, 87 ± 8% for CAF, 81 ± 38% for ACB and 26 ± 9% for BPA. The removal of ACE, OFL, CAF, and IBU was mostly by biodegradation in the HD and VF units while the PD unit was responsible for the removal of KET, ACB, and SOT.

Degradation of Antibiotic Vancomycin by UV Photolysis and Pulsed Corona Discharge Combined with Extrinsic Oxidants

Antibiotics are the most frequently detected pharmaceuticals in the environment creating conditions for the development of resistant genes in bacteria. Degradation and mineralization of glycopeptide antibiotic vancomycin (VMN) were examined by UV photolysis, pulsed corona discharge (PCD), and their combinations with extrinsic oxidants, hydrogen peroxide (HP), peroxydisulfate (PDS), and peroxymonosulfate (PMS). Both combinations were effective in VMN degradation and faster at pH 11 than in acidic or neutral media. Combined with the UV photolysis, HP showed a higher oxidation rate than other oxidants, whereas PMS and PDS proved to be more efficient in combinations with PCD. In contrast to low-to-moderate mineralization of VMN in the UV/oxidant combinations, PCD and PCD/oxidant combinations appeared to be more effective, reaching up to 90% of TOC removal in acidic/neutral solutions. Application of extrinsic oxidants resulted in an energy efficiency of VMN 90% oxidation improved from 36 to 61 g kW−1 h−1 in HP-assisted photolysis, and from 195 to 250 g kW−1 h−1 in PCD with additions of HP and PDS, thus showing the promising character of the combined treatment.

Occurrence and fate of an emerging drug pollutant and its by-products during conventional and advanced wastewater treatment: Case study of furosemide

Conventional wastewater treatment systems are not designed to remove pharmaceutical compounds from wastewater. These compounds can be degraded into many other transformation products which are hardly, if at all, studied. In this context, we studied the occurrence and degradation of furosemide, a very frequently detected diuretic, along with its known degradation products in several types of wastewater. Influent and effluent from the Seine-Centre Wastewater Treatment Plant (WWTP) (Paris, France) as well as outlet of residential care homes (Dordogne, France) were analyzed by Ultra-Performance Liquid Chromatography-tandem Mass Spectrometry (UPLC-MS/MS) to quantify furosemide and its known degradation products, saluamine and pyridinium of furosemide. Oxidation experiments (chlorination, ozonation and UV photolysis with hydrogen peroxide) were then performed on furosemide solutions and on water from residential care facilities to study the degradation of furosemide by potential advanced processes, and also to identify unknown oxidation products by high-resolution mass spectrometry. Furosemide was well degraded in Seine-Centre WWTP (>75%) but did not increase the concentrations of its main degradation products. Saluamine and pyridinium of furosemide were already present at similar concentrations to furosemide in the raw wastewater (∼2.5–3.5 μg.L−1), and their removal in the WWTPs were very high (>80%). Despite their removal, the three compounds remained present in treated wastewater effluents at concentrations of hundreds of nanograms per liter. Chlorination degraded furosemide without pyridinium production unlike the other two processes. Chlorination and ozonation were also effective for the removal of furosemide and pyridinium in residential care home water, but they resulted in the production of saluamine. To our knowledge this is the first evidence of saluamine and pyridinium of furosemide in real water samples in either the particulate or dissolved phase.

Scaling analysis parameters in 3D-focusing velocity map imaging

Velocity map imaging spectrometer is a powerful tool for measuring the two-dimensional momenta of charged particles in dissociative photoionization processes. By combining a time-position sensitive detector with a three-dimensional (3D) source focusing mode, 3D momentum can be measured at a much higher resolution. However, due to the side effects of the non-uniform electric field in this mode, the retrieval of the 3D momentum becomes complicated. Here, we describe a method to retrieve the 3D momentum from the time of flight and the position at a numerical accuracy better than 0.1%, much below the best achievable relative energy resolution of 1% in the reported experiments. The method is consistently tested on simulated data, including ions with different masses and charges. Moreover, a scaling relationship is established among them.

High-resolution vibrational predissociation spectroscopy of I− · H2O by single-mode CW infrared excitation in a 3D cryogenic ion trap

We describe the integration of a tunable, single-mode, continuous wave infrared laser into a cryogenic ion spectroscopy experiment to measure the rovibrational spectrum of the complex in the OH stretching region. These upper levels lie about 300 cm−1 above the dissociation threshold. The measurements are carried out by loading the ions in a radiofrequency ion trap at 10 Hz and cooling them to 5 K with pulsed He buffer gas. IR photodissociation (PD) of the complex is monitored by recording the product yield by time-of-flight mass spectrometry as a function of laser wavelength. Very narrow ( v∼75 MHz) rotational lines are observed throughout the spectrum, indicating long (ca. 2 ns) lifetimes for the excited metastable rovibrational levels. Rotational analysis of the band arising from the K” = 1 to K’ = 2 transition of the free OH stretching fundamental yields the structure of the complex for the first time. Over 50% of the trapped ion ensemble in the trap can be photodissociated upon excitation of a single rotational line. This enables very high signal-to-noise in the PD spectrum, and is traced to a mechanism in which the ground state rotational levels are rapidly equilibrated by collisions with the buffer gas.

Degradation of odorous 2,4,6-trichloroanisole in chlorinated water by UV-LED/chlorination: kinetics and influence factors.

2,4,6-Trichloroanisole (2,4,6-TCA) has aroused a special concern for their odor problem and potential threats. In this study, the degradation of 2,4,6-TCA by UV/chlorination with different UV sources was compared, including low-pressure mercury lamp (LPUV, 254nm) and ultraviolet light-emitting diode (UV-LED, 275 and 285nm). The maximum removal of 2,4,6-TCA can be achieved by 275-nm UV-LED/chlorination in neutral and alkaline conditions which was 80.0%. The reaction, kinetics, and water matrix parameters on 2,4,6-TCA degradation were also evaluated. During UV-LED (275nm)/chlorination, 2,4,6-TCA degradation was mainly caused by direct UV photolysis and indirect hydroxyl radical (HO·) oxidation, while reactive chlorine radicals (RCSs) had a negligible contribution. The second-order rate constant between HO· and 2,4,6-TCA was determined as 3.1 × 109M-1s-1. Increasing initial chlorine dosage and decreasing 2,4,6-TCA concentration or pH value significantly promoted 2,4,6-TCA degradation during UV/chlorination process. The presence of natural organic matter (NOM) and bicarbonate (HCO3-) can inhibit 2,4,6-TCA degradation, while chloride ion (Cl-) had a negligible effect. The kinetic model for 2,4,6-TCA degradation was established and validated, and the degradation pathways were proposed based on the identified intermediates. Furthermore, UV-LED (275nm)/chlorination also exhibited a promising effect on 2,4,6-TCA removal in real water, which can be used to control 2,4,6-TCA pollution and odor problems.

Enhanced formation of haloacetonitriles during chlorination with bromide: Unveiling the important roles of organic bromamines

The formation of brominated disinfection byproducts (Br-DBPs) is an emerging issue in drinking water disinfection because its toxicity is tens to hundreds of times higher than that of chlorinated analogues and because of the widespread presence of bromide in source water. However, the mechanism and pathways of Br-DBPs formation remain unclear. In this study, we used glycine, alanine, and serine as model precursors and observed that brominated haloacetonitriles (Br-HANs) were more likely to be formed than brominated trihalomethanes. The results showed that there is not only one important way to HAN formation in the presence of bromide. We propose that organic bromamines, similar to organic chloramines, play a significant role in the formation of Br-HANs. Both the experimental and theoretical results confirmed that the decay of organic bromamines was faster than that of organic chloramines, which verified our assumption. The effect of the pH was investigated to further confirm the role of organic bromamines. In addition, we found that the formation of Br-HANs was significantly inhibited when monochloramine was used as a disinfectant, because the formation of organic bromamines was blocked. However, the formation of Br-HANs was promoted during the UV/chlorine process because of the faster decay of organic bromamines under UV photolysis. Overall, our study reveals the formation mechanism of Br-HANs and provides an alternative method to prevent Br-HAN formation.

Isomer-Dependent Threshold Photoelectron Spectroscopy and Dissociative Photoionization Mechanism of Anisaldehyde.

We studied the threshold photoionization and dissociative ionization of para-, meta-, and ortho-anisaldehyde by photoelectron photoion coincidence spectroscopy in the 8.20-19.00 eV photon energy range. Vertical ionization energies by equation of motion-ionization potential-coupled cluster singles and doubles (EOM-IP-CCSD) calculations reproduce the photoelectron spectral features in all three isomers. The dissociative photoionization (DPI) pathways of para- and meta-anisaldehyde are similar and differ markedly from those of ortho-anisaldehyde. In the para and meta isomers, the lowest-energy DPI channel corresponds to hydrogen atom loss to form the C8H7O2+ fragment at m/z 135, which undergoes sequential dissociation processes at higher energies, such as carbon monoxide loss to C7H7O+ (m/z 107) and further, sequential CH3, CH2O, and CH2CO losses to produce C6H4O+ (m/z 92), C6H5+ (m/z 77), and C5H5+ (m/z 65), respectively. Carbon monoxide loss from the parent ions, yielding C7H8O+ (m/z 108), is a subordinate dissociation channel parallel to H atom loss. At higher energies, it also gives rise to sequential formaldehyde (CH2O) loss to produce C6H6+ (m/z 78). In the ortho-anisaldehyde cation, the vicinity of the aldehyde and methoxy groups opens up low-energy hydrogen-transfer processes, which allow for seven fragmentation channels to compete effectively with the H- and CO-loss channels. Thus, the fragmentation mechanism changes considerably, thanks to the steric interaction of the substituents. Functional group interactions, in particular H transfer pathways, must therefore be considered when predicting the isomer-specific unimolecular decomposition mechanism of cationic and neutral species, as well as mass spectra for isomers.