Secondary Ion Spectra Research Articles

Evaluation of Molecular Fragmentation in Polycyclic Aromatic Hydrocarbons by Time-of-Flight Secondary Ion Mass Spectrometry

In time-of-flight secondary ion mass spectrometry (TOF-SIMS), ionized molecules and molecular fragments (secondary ions) are generated in collisions of high-energy ions (primary ions) with a solid sample surface. Mass spectra of the emitted secondary ions are typically used to identify molecular species and to determine their spatial distribution on the sample surface. Here, we extend this application in a TOF-SIMS study of a series of polycyclic aromatic hydrocarbons (PAHs) where we focus on the fragmentation of these molecules, with the purpose of better understanding the fragmentation patterns of heavy aromatic molecules in petroleum. For all PAHs, the collision process generated (i) a series of smaller cation fragments and (ii) cations close in size to the original PAH (molecular cations). Stark differences are measured for various PAHs regarding the abundance of smaller fragments versus molecular cations. Observation of hydrogen-deficient (H-deficient) cation fragments indicates the formation of polyynes and allenes. For PAHs producing higher fractions of small cation fragments, these ions are surprisingly hydrogen rich (H-rich). The H/C ratio of fragments does not scale with the fraction of Clar sextet carbon, nor with energies of low-lying electronic transitions. Free radical cation fragments tend to be suppressed. For sufficiently large fragments, aromatic cations appear to be formed and include some free radical aromatics. There is ample production of molecular ions with loss of a single carbon atom or a methine group, which corresponds to the reduction of a 6-membered aromatic ring to a 5-membered ring. There is some enhancement of free radical molecular cations due to the corresponding formation of neutral polyynes. Fragment anions are also produced with a strong preference for very H-deficient carbon clusters, in some cases being the same as carbon cluster anions observed in space. Comparisons of PAH TOF-SIMS spectra with those of asphaltenes are discussed in detail.

Investigation of nanoporous silicon on the frontal surface of solar cells by mass spectroscopy

When using nanoporous silicon (nano-PSi) as a texture and anti-reflective coating, the nanoporous layer is formed directly on the silicon surface of the front layer of solar cells (SC). Along with blooming, it must also provide passivation of the SC surface. Research in this area has shown that the passivating properties of PSi are insufficient to minimize surface recombination. Partial improvement of the passivating properties of nano-PSi was achieved by thermal and anodic oxidation, as well as by plasma spraying methods. However, none of these methods were allowed to achieve the required level of surface passivation and its stabilization over time. A more effective method of improving the passivating properties of PSi is electrochemical hydrogenation in hydrogen-enriched electrolytes. Here we used mass spectroscopy to study the frontal surface of the SC. We also studied the surface appearance of multicrystalline silicon samples before and after hydrogenation in hydrogen and in an electrolyte based on hydrofluoric acid, both in the spectra of secondary ions and in the image of the distribution of elements on the surface.

Detection of Label-Free Drugs within Brain Tissue Using Orbitrap Secondary Ion Mass Spectrometry as a Complement to Neuro-Oncological Drug Delivery.

Historically, pre-clinical neuro-oncological drug delivery studies have exhaustively relied upon overall animal survival as an exclusive measure of efficacy. However, with no adopted methodology to both image and quantitate brain parenchyma penetration of label-free drugs, an absence of efficacy typically hampers clinical translational potential, rather than encourage re-formulation of drug compounds using nanocarriers to achieve greater tissue penetration. OrbiSIMS, a next-generation analytical instrument for label-free imaging, combines the high resolving power of an OrbiTrapTM mass spectrometer with the relatively high spatial resolution of secondary ion mass spectrometry. Here, we develop an ex vivo pipeline using OrbiSIMS to accurately detect brain penetration of drug compounds. Secondary ion spectra were acquired for a panel of drugs (etoposide, olaparib, gemcitabine, vorinostat and dasatinib) under preclinical consideration for the treatment of isocitrate dehydrogenase-1 wild-type glioblastoma. Each drug demonstrated diagnostic secondary ions (all present molecular ions [M-H]− which could be discriminated from brain analytes when spiked at >20 µg/mg tissue. Olaparib/dasatinib and olaparib/etoposide dual combinations are shown as exemplars for the capability of OrbiSIMS to discriminate distinct drug ions simultaneously. Furthermore, we demonstrate the imaging capability of OrbiSIMS to simultaneously illustrate label-free drug location and brain chemistry. Our work encourages the neuro-oncology community to consider mass spectrometry imaging modalities to complement in vivo efficacy studies, as an analytical tool to assess brain distribution of systemically administered drugs, or localised brain penetration of drugs released from micro- or nano-scale biomaterials.

SIMS Analysis of Thin EUV Photoresist Films.

This study reports on the application of secondary ion mass spectrometry (SIMS) for examining thin (20-50 nm) chemically amplified resist films on silicon. SIMS depth profiling was carried out using a gas cluster ion beam to ensure minimal sputter-induced damage to the organic constituents of interest. Specific attention concerned the distribution of the photo acid generator (PAG) molecule within these films, along with the photo-induced fragmentation occurring on extreme ultra-violet photo exposure. Positive secondary ion spectra were collected using a traditional time of flight (ToF)-SIMS and the latest generation IONTOF Hybrid SIMS instrumentation equipped with an OrbitrapTM mass analyzer. Tandem mass spectrometry (MS/MS) capability within the OrbitrapTM secondary ion column was utilized to verify that the C19H17S+ secondary ion did indeed have the molecular structure consistent with the PAG structure. The superior mass resolving power of the OrbitrapTM mass analyzer (∼20× of the ToF mass analyzer) along with improved mass accuracy (a few ppm) proved pivotal in the mass spectral and depth profile analysis of these films. This was not the case for the ToF-SIMS experiments, as the mass spectra, as well as the associated depth profiles, exhibited severe molecular (isobaric) interferences.

ЕЛЕКТРОХІМІЧНА ГІДРОГЕНІЗАЦІЯ ФРОНТАЛЬНОЇ ПОВЕРХНІ СОНЯЧНИХ ЕЛЕМЕНТІВ ТА ЇХ ДОСЛІДЖЕННЯ МЕТОДАМИ МАСС-СПЕКТРОСКОПІЇ

Purpose. Texturing of the silicon surface, both by chemical and electrochemical etching, is an integral part of the technology of modern high-performance silicon solar cells. The texture on the front surface of the solar cells not only reduces reflection losses, but also helps to capture long-wave light in the SC structure, thereby expanding its operating spectral range and increasing the short-circuit current. In this regard, the study of the PSi layer in a nanotexture by mass-spectroscopy on the functional properties of the frontal silicon surface, which was pre-textured, is of particular interest. Methodology. The study of all stages of formation of hydrogenated porous structure and identification of technological patterns that affect the characteristics of the obtained nano-, meso-, macropores are very important, because most of the porous parameters are laid at the stage of formation of nucleation (seed) centers. In order to improve the passivating properties of PSi layers used in solar cells, the process of electrochemical hydrogenation of PSi as on p - type silicon substrates with a resistivity of 0.1 ÷ 10 Ohmsm also on substrates with a formed emitter junction n+- p of conductivity was studied. Various solutions were used as the electrolyte, such as (HF: C2H5OH=1: 1), (HF: C2H5OH=1:2) and so on. The current density and anodizing time varied over a wide range of values, which allowed the formation of PSi layers with different porosity and thickness. A solution of HF: C2H5OH = 1: 1 was used as the electrolyte. The anodizing current and time varied over a wide range of values to obtain PSi layers with different porosity and thickness. To determine the conditions of the process of electrochemical hydrogenation of PSi during its cathodic polarization, potentiostatic current-voltage curves of the system Pt (anode)-electrolyte-porous silicon/ silicon (cathode) for electrolytes with different chemical composition were taken. Finding. Comparison of the surface appearance of Baysix type multicrystalline silicon samples before and after hydrogen hydrogenation in an electrolyte based on hydrofluoric acid ((HF: C2H5OH=10:1), (HF:C2H5OH=10:2) and so on), both in the secondary ion Spectra and in the image of the distribution of elements over the surface (mass spectral ion microscope and Ion microzond mode). Originality. We get a 2D image (size 200x200 microns) of the surface of the current of secondary ions H+ hydrogen and 2D image of the secondary ion current of molecular SiH2+ the surface of a Baysix type multicrystalline substrate passivated with hydrogen by hydrogenation, which is comparable to the optical image of a pure silicon surface that has been hydrogenated. From the form of 2D-ion images, as well as their individual parts, it can be seen that all the difference in the current intensity of the secondary ions is due to the topography of the surface of the samples. Practical value. The prospects of creating efficient solar cells using multifunctional multi-textures of porous silicon are shown. References 24, figures 6.

Time-of-flight secondary ion mass spectrometry fragment regularity in gallium-doped zinc oxide thin films

Time-of-flight secondary ion mass spectrometry fragment analysis remains a challenging task. The fragment appearance regularity (FAR) rule is particularly useful for two-element compounds such as ZnO. Ion fragments appearing in the form of ZnxOy obey the rule 2x ge 2y + 1 in the positive secondary ion spectrum and 2x le 2y + 1 in the negative spectrum where the valence of Zn is + 2 and that of O is − 2. Fragment analysis in gallium-doped ZnO (GZO) films can give insights into the bonding of the elements in this important semiconductor. Fragment analysis of 1 and 7 wt% GZO films shows that only the negative ion fragments obey the FAR rule where ZnO‒, 66ZnO‒, 68ZnO‒ and ZnO2‒ ion fragments appear. In the positive polarity, subdued peaks from out-of-the-rule ZnO+, 66ZnO+ and 68ZnO+ ion fragments are observed. The Ga ion peaks are present in both the positive and negative spectra. The secondary ion spectra of undoped ZnO also shows consistency with the FAR rule. This implies that Ga doping even in amounts that exceed the ZnO lattice limit of solubility does not affect the compliance with the FAR rule.

Enhanced sputter and secondary ion yields using MeV gold nanoparticle beams delivered by the Andromede facility

Andromede is a new instrument for mass spectrometry analysis of nanodomains and nano-objects present on a surface. The molecular sample information (mass and structure) is obtained from time-of-flight spectra of secondary ions emitted under the impact of nanoparticles (NPs), as Aunq+ with n/q up to 200 accelerated by a 4 MV single stage electrostatic Pelletron® accelerator. Such projectiles, providing huge emission rates per impact, permit the topographic analysis of complex samples. The energy deposition of these massive gold NPs induces surprisingly large holes of about 100 nm diameter in very thin graphene foils and average sputtered volume per impact of about 106 nm3 in a thick polymer material. The use of such massive clusters associated with multianode detectors, allowing the simultaneous detection of several ions of a given mass, permits correlation studies between the emitted ions within a single impact, shedding additional light on the chemical composition and structure of the analysed sample as illustrated here for various samples from metallic surfaces to biologic molecules deposits.

Анализ углеродсодержащих материалов методом вторично-ионной масс-спектрометрии: содержание атомов углерода в sp-=SUP=-2-=/SUP=-- и sp-=SUP=-3-=/SUP=--гибридных состояниях

A new approach to the analysis of carbon-containing materials by the method of secondary ion mass spectrometry is studied, which allows one to determine the concentration of carbon atoms in the states of sp2 and sp3 hybridization. It is proposed to use the ratio of the intensities of cluster secondary ions C8/C7 as the main parameter of the mass spectra of secondary ions characterizing the concentration of N(sp3). From measurements of several test structures, a calibration dependence of N (sp3) on the C8/C7 ratio was obtained. The N(sp3) profiles of diamond-like carbon samples grown on diamond and silicon substrates were measured, showing an N(sp3) concentration of 0.3 to 0.6 for different growth modes and an inhomogeneous distribution of the N(sp3) concentration over the thickness of the samples.

Experimental Observation of the Confinement of Atomic Collision Cascades during Ion Sputtering of Porous Silicon

Results of investigation of the mass spectra of secondary ions sputtered from bulk (continuous) and porous silicon targets with various characteristic core particle dimensions are presented. It has been established that the sputtering of nanostructured Si samples leads to a significant increase in the relative yield of massive cluster ions. This effect is especially pronounced when the characteristic nanoparticle size approaches the range of bombarding ions in the Si target. The obtained results are explained taking into account the confinement of atomic collision cascades.

Экспериментальное наблюдение эффекта ограничения каскада столкновений при распылении пористого кремния

AbstractResults of investigation of the mass spectra of secondary ions sputtered from bulk (continuous) and porous silicon targets with various characteristic core particle dimensions are presented. It has been established that the sputtering of nanostructured Si samples leads to a significant increase in the relative yield of massive cluster ions. This effect is especially pronounced when the characteristic nanoparticle size approaches the range of bombarding ions in the Si target. The obtained results are explained taking into account the confinement of atomic collision cascades.

Analysis of ToF‐SIMS spectra of poly(2‐vinylpyridine) and poly(4‐vinylpyridine) with density functional theory calculations

The time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) positive and negative ion spectra of poly(2‐vinylpyridine) (P2VP) and poly(4‐vinylpyridine) (P4VP) were analyzed using density functional theory calculations. Most of the ions from these structural isomers shared the same accurate mass, but had different relative abundance. This could be attributed to the fact that from a thermodynamics perspective, the disparity in the molecular structures can affect the ion stability if we assume that they shared the same mechanistic pathway of formation with similar reaction kinetics. The molecular structures of these ions were assigned, and their stability was evaluated based on calculations using the Kohn‐Sham density functional theory with Becke's 3‐parameter Lee‐Yang‐Parr exchange‐correlation functional and a correlation‐consistent, polarized, valence, double‐zeta basis set for cations and the same basis set with a triple‐zeta for anions. The computational results agreed with the experimental observations that the nitrogen‐containing cations such as C5H4N+ (m/z = 78), C8H7N+· (m/z = 117), C8H8N+ (m/z = 118), C9H8N+ (m/z = 130), C13H11N2+ (m/z = 195), C14H13N2+ (m/z = 209), C15H15N2+ (m/z = 223), and C21H22N3+ (m/z = 316) ions were more favorably formed in P2VP than in P4VP due to higher ion stability because the calculated total energies of these cations were more negative when the nitrogen was situated at the ortho position. Nevertheless, our assumption was invalid in the formation of positive ions such as C6H7N+˙ (m/z = 93) and C8H10N+ (m/z = 120). Their formation did not necessarily depend on the ion stability. Instead, the transition state chemistry and the matrix effect both played a role. In the negative ion spectra, we found that nitrogen‐containing anions such as C5H4N− (m/z = 78), C6H6N− (m/z = 92), C7H6N− (m/z = 104), C8H6N− (m/z = 116), C9H10N− (m/z = 132), C13H11N2− (m/z = 195), and C14H13N2− (m/z = 209) ions were more favorably formed in P4VP, which is in line with our computational results without exception. We speculate that whether anions would form from P2VP and P4VP is more dependent on the stability of the ions.

Sputtering of sodium and potassium from nepheline: Secondary ion yields and velocity spectra

Silicates are the dominant surface material of many Solar System objects, which are exposed to ion bombardment by solar wind ions and cosmic rays. Induced physico-chemical processes include sputtering which can contribute to the formation of an exosphere. We have measured sputtering yields and velocity spectra of secondary ions ejected from nepheline, an aluminosilicate thought to be a good analogue for Mercury's surface, as a laboratory approach to understand the evolution of silicate surfaces and the presence of Na and K vapor in the exosphere. Experiments were performed with highly charged ion beams (keV/u–MeV/u) delivered by GANIL using an imaging XY-TOF-SIMS device under UHV conditions. The fluence dependence of sputtering yields gives information about the evolution of surface stoichiometry during irradiation. From the energy distributions N(E) of sputtered particles, the fraction of particles which could escape from the gravitational field of Mercury, and of those falling back and possibly contributing to populate the exosphere can be roughly estimated.

Secondary Ion Mass Spectrometry Analysis of Renal Cell Carcinoma with Electrospray Droplet Ion Beams.

Tissue samples from renal cell carcinoma patients were analyzed by electrospray droplet ion beam-induced secondary ion mass spectrometry (EDI/SIMS). Positively- and negatively-charged secondary ions were measured for the cancerous and noncancerous regions of the tissue samples. Although specific cancerous species could not be found in both the positive and negative secondary ion spectra, the spectra of the cancerous and noncancerous tissues presented different trends. For instance, in the m/z range of 500–800 of the positive secondary ion spectra for the cancerous tissues, the intensities for several m/z values were lower than those of the m/z+2 peaks (indicating one double bond loss for the species), whereas, for the noncancerous tissues, the inverse trend was obtained. The tandem mass spectrometry (MS/MS) was also performed on the tissue samples using probe electrospray ionization (PESI), and some molecular ions produced by PESI were found to be fragmented into the ions observed in EDI/SIMS analysis. When the positive secondary ion spectra produced by EDI/SIMS were analyzed by principal component analysis, the results for cancerous and noncancerous tissues were separated. The EDI/SIMS method can be applied to distinguish between a cancerous and a noncancerous area with high probability.

ToF-SIMS Parallel Imaging MS/MS of Lipid Species in Thin Tissue Sections.

Unambiguous identification of detected species is essential in complex biomedical samples. To date, there are not many mass spectrometry imaging techniques that can provide both high spatial resolution and identification capabilities. A new and patented imaging tandem mass spectrometer, exploiting the unique characteristics of the nanoTOF II (Physical Electronics, USA) TOF-SIMS TRIFT instrument, was developed to address this.Tandem mass spectrometry is based on the selection of precursor ions from the full secondary ion spectrum (MS1), followed by energetic activation and fragmentation, and collection of the fragment ions to obtain a tandem MS spectrum (MS2). The PHI NanoTOF II mass spectrometer is equipped with a high-energy collision induced dissociation (CID) fragmentation cell as well as a second time-of-flight analyzer developed for simultaneous ToF-SIMS and tandem MS imaging experiments.We describe here the results of a ToF-SIMS imaging experiment on a thin tissue section of an infected zebrafish as a model organism for tuberculosis. The focus is on the obtained ion distribution plot of a fatty acid as well as its identification by tandem mass spectrometry.

Challenges Determining the Correct Deposition Order of Different Intersecting Black Inks by Time-of-Flight Secondary Ion Mass Spectrometry.

The distinction of different inks and determination of their deposition order are important forensic tasks when evaluating questioned documents. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has received attention for these tasks due to the technique's nondestructive nature, rich mass spectral information, ability to provide chemical images, and excellent surface sensitivity. Prior literature results demonstrate the ability of ToF-SIMS to differentiate between many varieties of blue ballpoint pens and to determine the correct deposition order for selected ink intersections. The current work therefore sought to further study the intersections of different black inks. Three black pens were initially tested and were successfully distinguished. However, interesting phenomena were observed where certain inks dominated over others, causing distinctly incorrect conclusions to be made regarding ink deposition order. To explore whether these incorrect results could be corrected, different primary ions (Bi3+, Bi3+2, and Ar1000+) were used and positive and negative secondary ions were evaluated. In data analysis, restriction of the mass range was considered and different multivariate analysis techniques, principal component analysis (PCA) and multivariate curve resolution (MCR), were performed on the secondary ion images and spectra from selected regions of interest. Issues regarding incorrect apparent order of deposition persisted through the variation of all these parameters. In addition to ink differentiation, and deposition order, evidence of ink aging was also observed. These results raise several questions to be answered regarding the widespread use of ToF-SIMS for forensic ink analysis.

Secondary ion yields for vacuum-type electrospray droplet beams measured with a triple focus time-of-flight analyzer.

We previously developed a massive cluster ion beam gun for secondary ion mass spectrometry (SIMS) in which the primary beam source is a vacuum electrospray. The secondary ion yields produced by this method had not yet been measured with a commercial time-of-flight (TOF) secondary ion mass spectrometer, and the ionization performance was unknown. A vacuum-type electrospray droplet ion gun was connected to a triple-focus TOF analyzer. The flight time of the secondary ions was measured using a sample-bias pulsing method, because a short pulse of the electrospray droplet beam could not be obtained. The secondary ion yields of an amino acid sample produced by the electrospray droplet beams and atomic Ga ion beams were compared. TOF secondary ion spectra were measured for the amino acid and peptide samples with a mass resolution of ~500 using the sample-bias pulsing method. The secondary ion yield of the amino acid sample produced with the 10 kV vacuum-type electrospray droplet beams was much higher than that produced by 10 kV Ga ion beams. In addition, the secondary ion yields for the peptide sample and amino acid samples were almost similar. This is the first report on secondary ion yields produced with vacuum-type electrospray droplet ion beams and measured with a semi-commercial TOF analyzer. The enhancement of secondary ion yields, in particular for relatively high-mass molecules, would be very useful in the SIMS analysis of a wide variety of biological samples. Copyright © 2016 John Wiley & Sons, Ltd.

Systematic Temperature Effects in the Argon Cluster Ion Sputter Depth Profiling of Organic Materials Using Secondary Ion Mass Spectrometry.

A study is presented of the effects of sample temperature on the sputter depth profiling of two organic materials, NPB (N,N'-Di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine) and Irganox 1010, using a 5keV Ar2000 (+) cluster ion beam and analysis by secondary ion mass spectrometry. It is shown that at low temperatures, the yields increase slowly with temperature in accordance with the Universal Sputtering Yield equation where the energy term is now modified by Trouton's rule. This occurs up to a transition temperature, T T, which is, in turn, approximately 0.8T M, where T M is the sample melting temperature in Kelvin. For NPB and Irganox 1010, these transition temperatures are close to 15 °C and 0 °C, respectively. Above this temperature, the rate of increase of the sputtering yield rises by an order of magnitude. During sputtering, the depth resolution also changes with temperature with a very small change occurring below T T. At higher temperatures, the depth resolution improves but then rapidly degrades, possibly as a result first of local crater surface diffusion and then of bulk inter-diffusion. The secondary ion spectra also change with temperature with the intensities of the molecular entities increasing least. This agrees with a model in which the molecular entities arise near the crater rim. It is recommended that for consistent results, measurements for organic materials are always made at temperatures significantly below T T or 0.8 T M, and this is generally below room temperature. Graphical Abstract ᅟ.

Secondary ion emission from ethanol microdroplets induced by fast heavy ions

We have developed a new experimental setup that allowed us to study collision interactions between fast ions and liquid microdroplets under a high vacuum condition. Microdroplets of ethanol are irradiated with 1.0-MeV H+ and 2.0-MeV C2+ ions. The size distribution of the droplets is evaluated from energy-loss measurements of projectile ions penetrating through the microdroplets. We obtain time-of-flight mass spectra of secondary ions from ethanol droplets. It is demonstrated that coincidence measurements with secondary electrons can distinguish specific ions produced in collisions with the droplets. Production mechanisms of H3O+, C4H9O+, (C2H5)2OH+ in the liquid ethanol are discussed.

Time-of-flight secondary neutral & ion mass spectrometry using swift heavy ions

We report on a new time-of-flight (TOF) spectrometer designed to investigate sputtering phenomena induced by swift heavy ions in the electronic stopping regime. In this experiment, particular emphasis is put on the detection of secondary ions along with their emitted neutral counterparts in order to examine the ionization efficiency of the sputtered material. For the detection of neutral species, the system is equipped with a pulsed VUV laser for post-ionization of sputtered neutral atoms and molecules via single photon ionization at a wavelength of 157nm (corresponding to 7.9eV photon energy). For alignment purposes and in order to facilitate comparison to nuclear sputtering conditions, the system also includes a 5keV Ar+ ion beam directed to the same sample area. The instrument has been added to the M1-branch beam line at the German accelerator facility in Darmstadt (GSI) and was tested with 4.8MeV/u Au26+ ions impinging onto various samples including metals, salts and organic films. It is found that secondary ion and neutral spectra obtained under both bombardment conditions can be acquired in an interleaved manner throughout a single accelerator pulse cycle, thus making efficient use of valuable beam time. In addition, the keV ion beam can be intermittently switched to dc mode between subsequent data acquisition windows and accelerator pulses in order to ensure reproducible surface conditions. For the case of a dynamically sputter cleaned metal surface, comparison of secondary ion and neutral signals obtained under otherwise identical instrumental conditions reveals a nearly identical ionization probability of atoms emitted under electronic and nuclear sputtering conditions.

Positive and negative ion emission from microdroplets by MeV energy ions

SynopsisWe have developed a new experimental setup that allowed us to study collision interactions between fast ion beams and liquid droplets under a vacuum condition. Droplets of water and ethanol are irradiated with 0.4-1.5 MeV H+ and 2.0 MeV C2+ ions. The droplet diameter is estimated from energy loss measurements of projectile ions penetrating through droplets. Time-of-flight mass spectra of positive and negative secondary ions exhibit a series of cluster ions generated via protonation and deprotonation.