Photoionization and photofragmentation in mass spectrometry with visible and UV lasers.

Ever since the introduction of laser technology to the field of mass spectrometry, several disciplines evolved providing solutions to challenging scientific and analytical tasks in research and industry. Among these are techniques involving multiphoton ionization such as Resonance-Enhanced Multiphoton Ionization (REMPI, R2PI) and Mass-Analyzed Threshold Ionization (MATI) spectroscopy, a variant of Zero Kinetic Energy (ZEKE) spectroscopy, that possess the ability to selectively ionize certain preselected compounds out of complex mixtures, for example, environmental matrices, with a high level of efficiency. Another key feature of multiphoton ionization techniques is the ability to control the degree of fragmentation, whereas soft ionization is most highly appreciated in most applications. In cases where rich fragmentation patterns are desired for diagnostic purposes, Photodissociation mass spectrometry (PD-MS) is applied successfully. PD-MS allows for the cleavage of selected chemical bonds. With the introduction of chromophoric labels in PD-MS, it became possible to target certain molecules or groups within a molecule. In this review article, an overview of the basic principles and experimental requirements of REMPI and MATI spectroscopy and PD mass spectrometry are given. By means of selected examples, the latest developments and application possibilities in this field over the past decade with special focus on the German research landscape are pointed out. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 38: 202-217, 2019.

Detailed analysis of the vibronic structure of phenetole in its first excited state and ionic ground state.

The vibronic structure of the first electronically excited state S1 and ionic ground state D0 of phenetole has been investigated by means of resonance enhanced multi photon ionization (REMPI) and mass analyzed threshold ionization (MATI) spectroscopy. The vibronic levels were assigned with the aid of quantum chemical calculations at the (TD)DFT level of theory and a multidimensional Franck-Condon approach. The S1 excitation energy of phenetole has been determined to be 36370 ± 4 cm-1 (4.5093 ± 0.0005 eV). The adiabatic ionization energy was determined to be 65665 ± 7 cm-1 (8.1415 ± 0.0008 eV). The vibronic structure has been analyzed whereby the in-plane bending vibration νbend shows high activity in the first excited state but is more pronounced in the ionic ground state. Moreover, a strong Duschinsky rotation effect can be observed for several D0←S1 transitions that causes violations of the Δv = 0 propensity rule.

Fragmentation studies on metastable diethylaniline derivatives using mass-analyzed ion kinetic energy spectrometry.

It is well known that small substituted aromatic amines lose their substituents after electron ionization in a multistep fragmentation mechanism. In this contribution, the fragmentation reactions of N,N-diethyl-aniline and different methyl-substituted N,N-diethyl-methyl-aniline isomers are investigated with respect to the position of the methyl group attached to the aromatic ring system. Metastable ion decay reveals a complicated fragmentation mechanism leading to an interplay of the aromatic methyl group and the ethyl substituents at the amine function. A small change in the substitution leads to a significant change of the observed fragments indicating a participation of o-methyl groups in the fragmentation mechanism of the diethyl amino side chain. Because the underlying mechanisms are not fully understood, the presented investigations deliver through 13C isotopic labeling a deep insight into the hidden rearrangement and fragmentation mechanisms in these compounds.

Indocyanine green MS/MS investigations using femtosecond laser-pulse photodissociation and collision-induced dissociation.

The dissociation reactions of indocyanine green (also known as cardiogreen) have been studied in a Fourier-transform ion-cyclotron resonance mass spectrometer by the application of unimolecular, collision-induced dissociation and photodissociation using femtosecond laser pulses. Ions were prepared by electrospray ionization (ESI) in various solvents. Depending on the properties of the solvent mixture, the mono sodiated molecule could be measured through cation exchange in various compositions. Using a mixture of methanol/water/formic acid as solvent, protonated ions, formed by exchange of sodium, are predominantly observed. A mixture of isopropanol/formic acid leads to the addition of a further sodium cation to the molecule yielding an intense bi-sodiated ion signal. The photodissociation of the stored ions was achieved using pulses at a wavelength of 790 nm and approximately 150 fs pulse duration. The results show that only fs-photodissociation leads to the fragmentation of the different molecular structures, while in case of sodiated indocyanine green collision induced dissociation fails completely to observe any fragments. In all other investigated ion structures, the collision-induced-dissociation spectra show unspecified and little intense spectra. It is shown that fs-photodissociation mass spectrometry is the only method that leads to substance specific fragmentation for this sample. Furthermore, indocyanine green is known to form aggregation products. In the ion-cyclotron resonance experiment, the reactions of a dimeric cluster were also investigated. The necessary condition and the interference of the fragmentations with those of the monomer are discussed.

Measuring the effects of Coulomb repulsion via signal decay in an atmospheric pressure laser ionization ion mobility spectrometer.

Using lasers in ion mobility spectrometry offers a lot of advantages compared to standard ionization sources. Especially, the ion yield can be drastically increased. It can, however, reach levels where the Coulomb repulsion leads to unwanted side effects. Here, we investigate how the Coulomb repulsion can be detected apart from the typical signal broadening by measuring effects created already in the reaction region and comparing them with corresponding finite element method simulations.

Subsequent radical fragmentation reactions of N, N-diethylamino-substituted azobenzene derivatives in a Fourier transform ion cyclotron resonance mass spectrometer using collision-induced dissociation and photodissociation.

The fragmentation behavior of N, N-diethylamino-substituted azobenzene derivatives is investigated by high-resolving mass spectrometry using a Fourier transform ion cyclotron resonance mass spectrometer. Former investigations by photodissociation as well as collision-induced dissociation experiments used to induce a loss of C3H8 from the diethylamino group. The position of the additional proton in [M + H]+ ions is important due to the sequences of radical fragmentation reactions. Two possibilities arise. First, a charge is located at the azo group leading to a methyl radical loss. The second possibility is that the charge has been located on the aniline nitrogen of the molecule resulting in an ethyl radical loss. Only o-ethyl red has shown the overall loss of C3H8 in a two-step radical reaction mechanism. Nevertheless, p-ethyl red and ethyl yellow have shown systematic fragmentation reactions as well. Loss of C3H8 has not been likely regarding both these molecules. All experimental findings together with quantum chemical calculations as well as kinetic calculations support the proposed fragmentation mechanisms of the three azo dyes.

Analysis of the (1)A' S1 ← (1)A' S0 and (2)A' D0 ← (1)A' S1 band systems in 1,2-dichloro-4-fluorobenzene by means of resonance-enhanced-multi-photon-ionization (REMPI) and mass-analyzed-threshold-ionization (MATI) spectroscopy.

Resonance enhanced multiphoton ionization (REMPI) and mass analyzed threshold ionization (MATI) spectroscopy have been applied in order to investigate the vibrational structure of 1,2-dichloro-4-fluorobenzene (1,2,4-DCFB) in its first excited state (S1) and the cationic ground state (D0). The selection of the state prior to ionization resulted in MATI spectra with different intensity distributions thus giving access to many vibrational levels. To support the experimental findings, geometry optimizations and frequency analyses at DFT (density functional) and TDDFT (time-dependent density functional) levels of theory have been applied. Additionally, a multidimensional Franck-Condon approach has been used to calculate the vibrational intensities from the DFT calculations. An excellent agreement between simulated and measured REMPI and MATI spectra allowed for a confident assignment of vibrational levels and mechanisms active during excitation and ionization. In order to avoid any ambiguity regarding the assignment of the vibrational bands to normal modes, Duschinsky normal mode analysis has been performed to correlate the ground state (S0) normal modes of 1,2,4-DCFB with the benzene derived Wilson nomenclature. From the REMPI spectra the electronic excitation energy (EE) of 1,2-dichloro-4-fluorobenzene could be determined to be 35 714 ± 2 cm(-1) while the MATI spectra yielded the adiabatic ionization energy (IE) of 1,2-dichloro-4-fluorobenzene which could be determined to be 73 332 ± 7 cm(-1).

A comparative study of APLI and APCI in IMS at atmospheric pressure to reveal and explain peak broadening effects by the use of APLI.

The details of the ionization mechanism in atmospheric pressure are still not completely known. In order to obtain further insight into the occurring processes in atmospheric pressure laser ionization (APLI) a comparative study of atmospheric pressure chemical ionization (APCI) and APLI is presented in this paper. This study is carried out using similar experimental condition at atmospheric pressure employing a commercial ion mobility spectrometer (IMS). Two different peak broadening mechanisms can then be assigned, one related to a range of different species generated and detected, and furthermore for the first time a power broadening effect on the signals can be identified.

Fragmentation of deuterated rhodamine B derivates by laser and collisional activation in an FT-ICR mass spectrometer.

Xanthene dyes such as rhodamine B undergo an interesting mass-spectrometric fragmentation reaction that eliminates small neutral alkanes such as propane. This fragmentation reaction has been investigated in a Fourier transform ion cyclotron mass spectrometer by means of laser photodissociation with visible light as well as by collision-induced dissociation. Different isotopically labeled decarboxyrhodamine B compounds were used to investigate the fragmentation mechanism. The results support a concerted mechanism for the formation of the alkanes instead of a two-step radical mechanism.

Fragmentation reactions of labeled and untabeled Rhodamine B in a high-resolution Fourier transform ion cyclotron resonance mass spectrometer.

The fragmentation reactions of Rhodamine B have been investigated by the use of electrospray ionization mass spectra in a high mass resolving ion cyclotron resonance mass spectrometer. Using high resolution, it could be shown that the loss of 44 mass units from the molecular ion is due to propane; the measured masses were inconsistent with loss of carbon dioxide. These conclusions are supported using deuterium-labeled Rhodamine B. This sample again only shows the loss of fully-deuterated propane verifying the high-resolution data. These findings illustrate very clearly that the conclusions based solely on low resolution spectra were false. The general implication on fragmentations of aromatic acids is discussed.

Fragmentation of xanthene dyes by laser activation and collision-induced dissociation on a high-resolution Fourier transform ion cyclotron resonance mass spectrometer.

Surprising fragmentation reactions in different xanthene dyes have been investigated by means of photodissociation and collision-induced dissociation in a 9.4 T FT-ICR mass spectrometer. It is shown that extensive rearrangement reactions lead to the formation of unexpected fragments which are identified for the first time by the use of high resolving mass power. The observed reactions are an example of the fragmentation of a quinoidal even-electron cation.

Photodissociation mass spectra and mass-selected resonant (1+1)-photodissociation spectroscopy of some alkyl iodide radical cations.

Photodissociation (PD) mass spectra and mass selected (1+1)-photodissociation spectra of C(2)H(5)I(+•), C(2)D(5)I(+•),1- C(3)H(7)I(+•), 2-C(3)H(7)I(+•), 1-C(4)H(9)I(+•) and 2- C(4)H(9)I(+•) radical cations were studied within the à ← X~ absorption band. The photodissociation mass spectra within the range 13,600-15,900 cm(-1) (1.68-1.97 eV) evidence only a simple cleavage of the C-I bond and formation of the corresponding alkyl ions. The resonant (1+1)-photodissociation spectra of C(2)H(5)I(+•) and C(2)D(5)I(+•) show intense vibrational structure in the excited à state. The thresholds for formation of the states of C(2)H(5)I(+•) and C(2)D(5)I(+•) were estimated to be (13,278 ± 12) cm(-1) (1.6462 ± 0.0014 eV)and (13,363 ± 12) cm(-1) (1.6586 ± 0.0014 eV), respectively. Whereas a few resonant vibronic excitations could be identified with 1-C(3)H(7)I(+•) and 1- C(4)H(7)I(+), no vibrational features were observable with 2- C(3)H(7)I(+•) and 2-C(4)H(9)I(+•). It is concluded that 1- and 2-iodoalkane radical cations do not rearrange, even under the conditions of electron ionisation used to generate the molecular ions.

Elucidating the Fragmentation Mechanism of Protonated Lewis A Trisaccharide using MSn CID.

Lewis blood group antigens are a prominent example of isomeric oligosaccharides with biological activity. Understanding the fragmentation mechanism in the gas phase is essential for their identification and assignment by mass spectrometric methods such as ESI-MS. In this work, the [M + H]+ species of Lewis A trisaccharide and Lewis A trisaccharide methyl glycoside were studied by ESI-MS with FT-ICR as mass analyzer with respect to their fragmentation mechanism. The comparison between the underivatized and the methylated species has shown that the reducing end plays a key role in this mechanism. The results of this study question the existence of Z-type fragment ions after activation of the protonated species. The main product of the fragmentation are Y-type fragment ions and a combination of Y-type fragmentation and the loss of water at the reducing end instead of Z-type fragmentation. C-type fragment ions could not be detected. MS3 measurements also reveal that each fragment ion only occurs with the participation of a mobile proton and the possibility of glycosidic bond cleavage after fragmentation has already occurred at the reducing end as B2 fragment ion.