Biogenic volatile organic compounds (BVOCs) are key precursors of secondary organic aerosol (SOA) , a major component of atmospheric particulate matter with profound implications for climate and human health . New particle formation (NPF), where molecules form thermodynamically stable clusters through nucleation and continue to grow to larger sizes , is the dominant source in terms of aerosol particle number concentration across most of the troposphere and contributes significantly to the particle and cloud condensation nuclei (CCN) budget in the continental boundary layer . Monoterpenes and particularly α-pinene (C10H16) emitted by trees contribute significantly to global BVOC emissions and have therefore become central to studies of SOA and NPF.

Upon reaction with atmospheric ozone, α-pinene ozonolysis is initiated, leading to the formation of peroxy radicals (RO2) which can undergo autoxidation. In this process, consecutive intramolecular H-shifts followed by rapid O2 additions produce increasingly oxygenated organic molecules (OOMs), often containing hydroperoxide [-OOH] or peroxy acid [-(C=O)OOH] functionalities. Their volatility can decrease to the regime of (extremely) low volatility organic compounds ((E)LVOC), enabling condensation onto existing molecular clusters or sufficiently large particles, or even direct participation in nucleation driven solely by ELVOC interactions in the case of highly oxygenated organic molecules (HOMs) .

OOMs govern key stages of continental boundary layer NPF: covalently bound dimers serve as the main nucleators driving cluster formation , while OOM monomers drive early particle growth due to their higher concentrations and low saturation vapor pressures .

The primary technique for measuring OOMs is chemical ionization (CI) mass spectrometry using nitrate (NO3-) as reagent ion, with the CI-APi-TOF being an especially prominent instrument . However, nitrate chemical ionization mass spectrometers (CIMS) are less sensitive towards less oxygenated analytes , which are still decisive for particle growth, especially at lower temperatures . Orbitrap mass spectrometry coupled with the multischeme chemical ionization inlet (MION) is an emerging and promising technique . The ultra-high resolution (> 100 000), fast polarity and reagent switching, and multi-pressure scheme of the MION-Orbitrap enable the measurement of the full distribution of precursor molecules and their progressively oxygenated reaction products in atmospheric environments .

Two key challenges remain. First, any ionization reagent is inherently selective, making complete capture of all molecular species unattainable. Continued development of reagents with broader coverage is therefore essential. Second, authentic monomeric OOM standards, particularly those featuring peroxy acid functionalities, are not commercially available. The absence of suitable OOM standards critically constrains both the study of their detection by CIMS and the development of robust and precise calibration methods . Because their ionization efficiencies remain unknown, quantitative interpretation of OOM concentrations in the atmosphere, and thus accurate model representation, remains severely limited. While the synthesis of OOM dimers has been demonstrated , only a few studies have attempted OOM monomer synthesis, successfully producing α-pinene-derived monoperoxypinic acid and hydroxy hydroperoxides .

In this study, we synthesized two OOM monomers derived from α-pinene ozonolysis with molecular formulas C9H14O4 and C10H16O4, referred to as peroxy norpinonic acid (PNPA) and peroxy pinonic acid (PPA), respectively (structures depicted in Fig. ). They represent the corresponding peroxy acids of norpinonic acid (NPA; C9H14O3) and pinonic acid (PA; C10H16O3), which are major oxidation products of α-pinene . Both standards contain four oxygen atoms and are therefore classified as moderately oxygenated molecules (MOMs). Their peroxy acid functionality makes them useful model compounds for probing the detection of peroxy-acid-containing α-pinene-derived oxidation products, including HOMs, but they should not be treated as quantitative proxies for the broader and more highly functionalized HOM population. We confirmed their structures via 1H and 13C NMR (nuclear magnetic resonance) spectroscopy and subsequently investigated their detection with a MION-Orbitrap operated with nitrate (NO3-) and uronium (CH5N2O+) . The gas-phase ionization was also compared to heated electrospray ionization (H-ESI). We show that these two peroxy acids exhibit strong variability in instrument response across different ionization schemes, indicating that their abundance may be substantially underestimated in atmospheric studies. Whether comparable response behavior occurs for more highly oxygenated atmospheric peroxy acids remains to be established using additional authentic standards.

Due to the synthesis procedure, each sample of synthesized standards contained a specific ratio of peroxy acids to their corresponding carboxylic acids. Figure  displays these ratios for both OOM standards measured by NMR and Orbitrap immediately after synthesis, as described in the previous section. Orbitrap ratios were computed from normalized peak intensities and averaged across repeated experiments; the plotted values represent the mean and the standard uncertainty derived from all measurements. NMR yielded the highest ratios (4.4 for PNPA and 4.7 for PPA), indicating that the freshly prepared standards were dominated by peroxy acids. In contrast, Orbitrap measurements produced significantly lower ratios. The only cases approaching or exceeding unity were nitrate adduct ionization for PNPA (0.87 ± 0.22; consistent with unity within uncertainty) and uronium adduct ionization for PPA (1.77), both indicating a higher abundance of peroxy acids than carboxylic acids, yet still yielding ratios substantially lower than those determined by NMR. Deprotonation and protonation yielded considerably lower ratios (0.03 to 0.39). These discrepancies reflect fundamental differences between techniques: NMR reports bulk composition in a fully non-perturbative, sample-preserving manner, whereas mass spectrometry reports only the fraction of molecules that successfully transfer, ionize, and survive to detection – processes that are inherently sensitive to evaporation and ionization efficiency, in-source chemistry, and matrix effects such as ion suppression. Consequently, the measured mass spectrometric signal response represents an interplay between (i) evaporation and transport kinetics from the liquid sample solution into the gas phase and onward to the Orbitrap, (ii) intrinsic molecular properties such as carbon number, functional group arrangement, and O-O bond lability, and (iii) the ionization pathway, including its efficiency, softness or hardness (adduct formation vs. (de)protonation), clustering energetics, and reagent selectivity.

This framework also clarifies the role of our gas-phase experimental approach (pathway (a) in Fig. ), which relies on evaporation of the liquid sample in the evaporator followed by transport of the volatilized molecules in an airstream toward the MION-Orbitrap. Under these conditions, any observed ratio between peroxy acids and carboxylic acids may be influenced by differences in their inherent volatilities. Table  lists the saturation mass concentrations and liquid vapor pressures of the synthesized OOM standards, mCPBA, and their corresponding carboxylic acids, calculated using the SIMPOL.1 group contribution method . Their saturation mass concentrations, which are on the order of 102 to 105 µgm-3, classify these molecules as intermediate volatility organic compounds (IVOCs) .

These calculations predict that, at 20 °C, peroxy acids exhibit higher liquid vapor pressures and saturation concentrations, and therefore greater volatility, than the corresponding carboxylic acids (Table ). Under these conditions, we would therefore expect preferential evaporation and more efficient transfer of peroxy acids into the gas stream and to the MION2, resulting in an even higher ratio of peroxy to carboxylic acids reaching MION2, being ionized, and ultimately detected in the Orbitrap than measured by NMR.

Figure  shows a representative time trace of the nitrate adduct signals for mCPBA and mCBA, along with the temporal evolution of the peroxy-to-carboxylic-acid ratio. The initial ∼ 25 min correspond to blank measurements, while the pronounced increase in signal intensity at ∼ 30 min indicates the start of sample evaporation following introduction into the evaporator.

Consistent with the SIMPOL.1 group contribution predictions at 20 °C, where the peroxy acid functional group yields a higher vapor pressure and therefore a higher saturation concentration than the corresponding carboxylic acid group, the mCPBA control experiment exhibits the expected volatility contrast: mCPBA produces a strong, early-rising nitrate adduct signal, whereas the mCBA signal appears more slowly and at substantially lower intensity. This difference in evaporation kinetics is reflected in the peroxy-to-carboxylic-acid ratio, which increases gradually over ∼ 12 min before approaching a steady plateau, indicating the time required for the less volatile mCBA to accumulate in the gas phase. The peroxy acid signal is roughly two orders of magnitude higher than that of the carboxylic acid, consistent with both its greater volatility and its larger fraction in the technical mCPBA mixture. Together, these observations reflect the behavior predicted by SIMPOL.1 and demonstrate the characteristic signature of two species with distinctly different volatilities evaporating independently and being transferred to the MION2 and ultimately detected in the Orbitrap.

Figure  shows representative time traces for the nitrate and uronium adduct signals of PNPA / NPA and PPA / PA, respectively, together with the time evolution of the peroxy-to-carboxylic-acid ratio. The first ∼ 25 min capture blank measurements in both ionization modes. The persistent signals at mass-to-charge ratios (m/z) 247 and 231 during the initial uronium blank are attributed to trace carryover retained on inlet or instrument surfaces and detected because of the high sensitivity of uronium; all reported sample signals nevertheless exceeded the corresponding blank-derived 3σ threshold. The onset of sample introduction into the evaporator and subsequent evaporation is evident from the sharp intensity increases in uronium mode at ∼ 27 min in Fig. a and ∼ 30 min in Fig. b. The nitrate adduct signal is already stabilized when that mode is engaged. In contrast to both the SIMPOL.1 predictions and the behavior observed for the mCPBA control, both synthesized OOM standards exhibit a distinctly different pattern: the peroxy acid and carboxylic acid signals rise simultaneously after sample introduction and follow nearly identical temporal profiles. This is reflected directly in the peroxy-to-carboxylic-acid ratio, which, after a brief stabilization period during the onset of evaporation, remains constant over time and reaches this steady level within only 1 to 2 min. Moreover, the signal intensities deviate strongly from expectations based on NMR measurements and SIMPOL-derived volatility differences: instead of the peroxy acids dominating and producing ratios higher than those measured by NMR, the carboxylic acid signals exceed the peroxy acid signals in most cases, with all measured ratios indicating a relative enrichment of carboxylic acids compared to the NMR values. Taken together, these observations support a consistent interpretation: the same parent species (predominantly the peroxy acids) evaporate into the gas phase, but a substantial fraction undergoes decomposition, yielding two detectable signals – one corresponding to the surviving peroxy acids and the other to the carboxylic acids formed upon decomposition.

Decomposition of peroxy acids could in principle occur during transport, ionization, or another step prior to detection. However, several lines of evidence indicate that structural degradation most likely occurs during ionization. The O-O bond in the peroxy acid group is relatively weak and susceptible to cleavage. In liquid-phase electrospray (H-ESI), high electric fields and electrochemical processes at the emitter can promote such bond rupture, particularly during (de)protonation pathways that impart considerable internal energy. Similarly, in gas-phase chemical ionization (MION2), proton-transfer reactions during collisions with reagent ions can destabilize the O-O bond, and electrostatic stress in the inlet may amplify this effect. By contrast, reagent ion adduct formation ionization (e.g., nitrate or uronium) is softer, relying on non-covalent clustering rather than covalent activation, and therefore better preserves fragile functionalities such as peroxy acid groups. This interpretation is consistent with the measured peroxy-to-carboxylic-acid ratios (Fig. ), which show strong differences across ionization techniques (H-ESI vs. MION2), reagent ions (nitrate vs. uronium), and ionization mechanisms ((de)protonation vs. adduct formation), with higher ratios for adduct peaks compared to (de)protonated peaks. These systematic differences across ionization modes support the interpretation that decomposition occurs during ionization, rather than elsewhere in the mass spectrometer; if fragmentation were occurring inside the instrument, the peroxy-to-carboxylic-acid ratios would not depend so sensitively on the ionization pathway. This interpretation is further reinforced by the operating conditions of the Orbitrap Exploris MX: during full-MS¹ acquisition, MS/MS fragmentation is not engaged, and ions are transmitted to the mass analyzer without deliberate collisional activation, making decomposition in the instrument unlikely. Furthermore, our observations align with earlier findings by , who reported rapid decay of synthesized monoperoxypinic acid on filters – about 60 % lost within 5 h and measurable changes also in liquid samples over 22 h, underscoring the intrinsic instability of peroxy acids related to atmospheric BVOC autoxidation.

Taken together, only the mCPBA control reflects the SIMPOL.1 volatility expectation that peroxy acids evaporate more readily than their corresponding carboxylic acids, leading to a significantly greater fraction of peroxy acids orbiting around the mass analyzer and, consequently, a much higher detected signal. This can plausibly be attributed to aromatic stabilization from the benzene ring, which renders mCPBA more persistent than typical aliphatic peroxy acids.

A pure pinonic acid (PA) control experiment showed a markedly different temporal response from the PA signal in the PPA standard (Fig. S1 in the Supplement). Pure PA continued to increase throughout the approximately 25 min uronium interval and retained a slightly increasing trend during the subsequent approximately 20 min nitrate interval, suggesting retention of the less volatile PA during evaporation and transport. In contrast, the PPA and PA signals in Fig. b rose rapidly together and approached a plateau within approximately 4 min. Because pure PA was measured using the same experimental setup and conditions, this difference argues against the PA signal in the synthesized sample arising predominantly from independently evaporating PA already present in the mixture. Although mixture composition may influence absolute evaporation rates, it does not readily explain the rapid, nearly identical profiles of the PPA and PA channels or the absence of the PA signal retention observed in the pure PA experiment. If the reduced PPA / PA ratio resulted primarily from a stronger compound-specific response to PA, the PA signal would still be expected to show the slower temporal response of independently evaporated PA. The comparison therefore argues against independent evaporation and compound-specific response factors as the main causes of the observed ratios and supports conversion from a common PPA-derived parent after evaporation and transport.

We further tested the influence of the ion transfer tube temperature. For the PPA standard measured in nitrate mode, increasing the temperature from 100 to 200 °C increased the PPA / PA nitrate adduct ratio from approximately 0.8 to approximately 1.0 (Fig. S2), which remained below the NMR ratio of 4.7. For commercial mCPBA measured with H-ESI, the deprotonated mCPBA / mCBA ratio increased from 0.02 at 85 °C to 0.03 at 110 °C and 0.04 at 320 °C (Fig. S3). In both experiments, higher transfer tube temperatures increased rather than decreased the relative peroxy acid signal. These results therefore exclude thermal degradation of peroxy acids in the ion transfer tube as a cause of the reduced ratios under the tested conditions.

Considering all experiments and controls, the systematically reduced peroxy-to-carboxylic-acid Orbitrap intensity ratios relative to NMR, the paired temporal profiles of the synthesized standards, and the strong dependence on ionization chemistry are consistent with ionization-associated loss of peroxy acid functionality. The pure PA control argues against independent evaporation of pre-existing PA and compound-specific response factors as the main explanations, and the temperature controls exclude thermal degradation in the ion transfer tube under the tested conditions. However, surface-catalyzed conversion at stainless steel inlet surfaces cannot be ruled out as a contributor to the reduced ratios.

Figure  compares intensities normalized to primary ions for nitrate and uronium adduct peaks of PNPA, PPA, and their corresponding carboxylic acids NPA and PA. The plotted values represent means with standard uncertainties from all measurements. Uronium adduct ionization consistently produced substantially higher intensities than nitrate adduct ionization, exceeding it by 1 to 2 orders of magnitude (factors ranging from 4 × 10¹ to 6 × 10²). This trend is also reflected in the absolute intensities (see Fig. ), indicating that the difference between the two ionization schemes is robust and not an artifact of normalization. Overall, these reagent-ion adduct intensity trends align well with chemical expectations. Nitrate is highly selective for HOMs and exhibits limited sensitivity to moderately oxygenated species such as PNPA, PPA, NPA, and PA. In contrast, uronium (protonated urea), a recently introduced positive-mode reagent, shows strong sensitivity for moderately oxygenated compounds , as evidenced by its consistently higher intensities than nitrate – also observable in blank measurements (Fig. ) – confirming its suitability for this chemical space. On average, nitrate and uronium adduct signals for the peroxy acids were factors of 34 and 18 above their respective 3σ blank thresholds, indicating that both channels operated well beyond their detection limits and that their behavior was unlikely to be governed by near-limit effects.

As a limited validation of the nitrate response at low signal intensity, four mCPBA solution concentrations were measured. The deprotonated mCPBA signal was averaged over 9 min and normalized to the nitrate primary ions (Fig. S4). The response was described by y = 1.213 × 10-4x + 6.440 × 10⁻⁴ (R2 = 0.982; n = 4). This result demonstrates a systematic concentration response for mCPBA within the relevant low-intensity regime. It does not determine the relative molar response factors of PNPA / NPA or PPA / PA; however, the pure PA temporal response shows that compound-specific response factors cannot account for the observed time profiles and are therefore unlikely to be the principal cause of the reduced ratios.

As seen in Fig. , the extent of peroxy acid loss depends on both the reagent-ion chemistry and the molecular structure of the analyte. Although uronium generally yields higher intensities than nitrate for both peroxy acids, it appears to induce greater in-source decomposition of PNPA while better preserving PPA; nonetheless, in both cases, the resulting peroxy-to-carboxylic-acid ratios remain substantially lower than expected from NMR and SIMPOL.1. This compound-specific behavior aligns with the uronium-CI mechanism: show that uronium ionization is fundamentally controlled by analyte-specific cluster geometries, binding enthalpies, and stabilization efficiencies, implying intrinsically molecule-dependent sensitivities. Taken together, these observations add a final layer to our decomposition hypothesis: peroxy acid loss is dependent on both the ionization mechanism and the analyte.

In this work, we synthesized two monomeric OOM standards from α-pinene ozonolysis containing a peroxy acid functionality. With four oxygen atoms, they are moderately oxygenated molecules (MOMs). Because peroxy acid functionalities often occur in OOMs, including highly oxygenated organic molecules (HOMs), these standards can provide useful qualitative insight into their analytical behavior. However, they should not be regarded as quantitative proxies for the broader population of HOMs. We characterized their detectability with a nitrate- and uronium-based MION-Orbitrap and compared these two gas-phase ionization schemes with liquid H-ESI.

Across all techniques, NMR confirmed that the freshly synthesized standards were dominated by peroxy acids, whereas Orbitrap measurements consistently yielded substantially lower peroxy-to-carboxylic-acid ratios, contradicting the SIMPOL.1 expectation that the higher volatility of peroxy acids should produce ratios even greater than those measured by NMR. Crucially, the time evolution of these ratios supports peroxy acid decomposition: after a brief stabilization period, the peroxy-to-carboxylic-acid ratios reach a constant level rapidly, and the peroxy and carboxylic acid signals exhibit nearly identical temporal profiles. In contrast, pure pinonic acid (PA), measured using the same experimental conditions, continued to increase over approximately 45 min and did not show the rapid plateau observed for the peroxy pinonic acid (PPA) standard, suggesting retention of the less volatile PA during evaporation and transport. This argues against independent evaporation and compound-specific response factors as the main causes of the reduced ratios and instead supports the interpretation that both signals originate from a common PPA-derived parent, with conversion occurring after evaporation. Increasing the ion transfer tube temperature did not decrease the relative peroxy acid response for either the PPA standard or mCPBA, excluding thermal degradation in the ion transfer tube under the tested conditions. Together with the divergence between sample-preserving NMR and sample-modifying mass spectrometric measurements, and with the pronounced variability across ionization modes, these observations are consistent with ionization-associated loss of peroxy-acid functionality. Such loss appears more pronounced under harder ionization pathways such as (de)protonation. Softer ionization routes such as nitrate and uronium adduct formation appear to preserve the peroxy acid functionality more effectively; nevertheless, even these channels yield ratios substantially below the bulk values, and the relative responses vary with reagent-ion chemistry and the molecular structure of the analyte, as indicated by the pronounced shifts in the peroxy-to-carboxylic-acid ratios for the two standards under uronium ionization. However, surface-catalyzed conversion at stainless steel inlet surfaces cannot be excluded as a possible contributor to the reduced ratios, highlighting the need for dedicated experiments assessing the influence of inlet material. Finally, our findings highlight that uronium adduct ionization captures this class of moderately oxygenated compounds far more efficiently than nitrate, underscoring its value as a complementary reagent for expanding molecular coverage beyond the HOM-biased selectivity of nitrate-CIMS.

Our results suggest that peroxy acids formed via autoxidation of α-pinene – and potentially other biogenic VOCs – may often be inaccurately quantified across commonly used mass spectrometric techniques. Because nitrate-CIMS is the most prevalent method for gas-phase HOM detection in new particle formation studies, analogous response biases could translate into substantial uncertainties when deriving atmospheric process rates, including nucleation and growth rates of freshly formed particles. For example, in volatility-basis-set derivations from atmospheric measurements, misidentifying peroxy acids as carboxylic acids would artificially lower their assigned oxygen number, thereby underestimating O : C ratios for OOMs and propagating systematic errors in inferred volatilities. The magnitude and broader atmospheric relevance of these effects cannot be inferred directly from the two moderately oxygenated standards studied here and will require investigation using additional authentic standards. These findings underscore the need for calibration procedures capable of capturing such compound-specific effects, together with continued exploration of novel reagent-ion chemistries that provide broader and more uniform coverage across the full range of molecular oxygenation states present in the atmosphere.