Analysis of Volatile Esters and Alkanoic Acids by an Atmospheric Pressure Corona Discharge Ionization Collision-Induced Dissociation Mass Spectrometry in Positive-Ion Mode: Ionization, Fragmentation Patterns, and Discrimination between Isomeric Compounds

Abstract

The ionization and fragmentation patterns of 24 compounds with elemental composition of C_mH_2mO₂, including isomeric esters and alkanoic acids, were investigated by atmospheric pressure corona discharge ionization collision-induced dissociation (CID) mass spectrometry in the positive-ion mode. All compounds were ionized as protonated molecules and NH₄⁺ adducts. In addition, fragment ions were observed in mass spectra of esters other than methyl esters, which are owing to the dissociation of the alkyl group in the alcohol side from the protonated molecules. In CID spectra, protonated alkanoic acids/methyl esters split off H₂O/CH₃OH and CO or the alkyl group in the acid side, depending on the carbon chain length. Protonated esters (other than methyl esters) mainly fragmented the alkyl group in the alcohol side. These general rules on ionization and fragmentation patterns can provide relevant information on the discrimination of isomers.

1. INTRODUCTION

Fruits biosynthesize and emit a large variety of aroma volatile compounds, and terpenoids and fatty-acid derivatives are the dominant classes. Fruit aroma is an important indicator of the quality of the fruit flavor. For examples, esters are the most abundant volatile compounds emitted by apple, and their use has been proposed for cultivar classification.^1,2) Furthermore, it has been reported that watercored (WC) apples have a stronger aroma than non-WC apples because of their relatively high methyl and ethyl esters content; thus, they are widely popular among Japanese consumers.³⁾

As an important trait of fruit quality, fruit aroma volatile research has gained increasing attention in recent years.^4–10) The main instrumentation for aroma research used to date are (i) gas chromatography/mass spectrometry (GC/MS)^5–8) and (ii) proton-transfer reaction mass spectrometry (PTR-MS).^9,10) Comprehensive two-dimensional GC has been used to improve the separation of different compounds, allowing the separate measurements of hundreds of compounds, including structural and steric isomers. However, the GC technique is time and labor intensive, which limits the number of samples that can be analyzed in practice. Conversely, the PTR-MS, one of the chemical ionization mass spectrometry techniques using H₃O⁺ reagent ions, allows the real-time and online measurements of various volatile organic compounds with a high sensitivity and fast response.^11–13) However, the PTR-MS methodology cannot measure isomers separately with the same molecular formulas/mass. Fruit aroma is a complex mixture of a large number of volatile compounds, including isomers. Therefore, the development of real-time measurement techniques that can simultaneously detect various volatile compounds and distinguish isomers would be useful for aroma research in the future.

To achieve this purpose, as a preliminary study, various ester isomers with elemental composition of C_mH_2mO₂ and related compounds (e.g., alkanoic acids) are positively ionized by the atmospheric pressure corona discharge ionization technique (APCDI), which is an atmospheric pressure chemical ionization (APCI)-like ambient ionization technique.^14,15) Thereafter, collision-induced dissociation (CID) is performed for protonated molecules. Resultantly, it is found that those mass spectral data allow distinguishing the isomers. Herein, we discuss general rules on the ionization and fragmentation patterns of alkanoic acid, methyl esters, and other esters. Additionally, we show how the isomers can be distinguished using those rules.

2. EXPERIMENTAL

2.1. Analytes

The analytes used in this study were seven alkanoic acids, seven methyl esters, and ten other esters, as summarized in Tables 1–3. All the analytes were purchased from the Tokyo Chemical Industry (Tokyo, Japan) and had over 98% purity. Hereafter, we use the acronym “R₁COOR₂” to denote different compounds according to the length of the carbon chain in the acid (R₁) or alcohol side (R₂).

Table 1. Summary of seven alkanoic acids (R₂=H): the molecular properties and results of positive-ion APCDI mass spectra obtained by the Q-orbitrap MS.

Molecular properties					Results of positive-ion APCDI mass spectra: m/z, chemical formula, and relative intensities (%) of major ions
Analyte (M)	Molecular mass (nominal Da)	Formula	R1	R2	Protonated molecule, [M+H]+		NH4+ adduct, [M+NH4]+		H2O loss from protonated molecule, R1CO+
Acetic acid	60	C2H4O2	CH3	H	61.0283	100	78.0548	71.2	N.D.a
					C2H5O+		C2H8O2N+
Propanoic acid	74	C3H6O2	C2H5	H	75.0439	100	92.0703	24.2	57.0333	3
					C3H7O2+		C3H10O2N+		C3H5O+
Butanoic acid	88	C4H8O2	C3H7	H	89.0595	100	106.086	31.7	71.049	5
					C4H9O2+		C4H12O2N+		C4H7O+
Pentanoic acid	102	C5H10O2	C4H9	H	103.0751	100	120.1016	18.6	85.0646	3.3
					C5H11O2+		C5H14O2N+		C5H9O+
Hexanoic acid	116	C6H12O2	C5H11	H	117.0907	100	134.1171	37.6	99.0802	4.1
					C6H13O2+		C6H16O2N+		C6H11O+
Heptanoic acid	130	C7H14O2	C6H13	H	131.1058	100	148.1321	11.3	113.0953	2.5
					C7H15O2+		C7H18O2N+		C7H13O+
Octanoic acid	144	C8H16O2	C7H15	H	145.1221	100	162.1486	19.4	127.1116	3.4
					C8H17O2+		C8H20O2N+		C8H15O+

^aNot detected due to detection limit of m/z range.

APCDI, atmospheric pressure corona discharge ionization technique; MS, mass spectrometry.

Table 2. Summary of seven methyl esters (R₂=CH₃): the molecular properties and results of positive-ion APCDI mass spectra obtained by the Q-orbitrap MS.

Molecular properties					Results of positive-ion APCDI mass spectra: m/z, chemical formula, and relative intensities (%) of major ions
Analyte (M)	Molecular mass (nominal Da)	Formula	R1	R2	Protonated molecule, [M+H]+		NH4+ adduct, [M+NH4]+		CH3OH loss from protonated molecule, R1CO+
Methyl acetate	74	C3H6O2	CH3	CH3	75.0439	100	92.0703	29.3	N.D.a
					C3H7O2+		C3H10O2N+
Methyl propionate	89	C4H8O2	C2H5	CH3	89.0595	100	106.086	16.8	57.0334	<1
					C4H9O2+		C4H12O2N+		C3H5O+
Methyl butyrate	102	C5H10O2	C3H7	CH3	103.075	100	120.102	47.5	71.049	1
					C5H11O2+		C5H14O2N+		C4H7O+
Methyl valerate	116	C6H12O2	C4H9	CH3	117.091	100	134.117	13.5	85.0648	<1
					C6H13O2+		C6H16O2N+		C5H9O+
Methyl hexanoate	130	C7H14O2	C5H11	CH3	131.106	100	148.132	27.9	99.0804	<1
					C7H15O2+		C7H18O2N+		C6H11O+
Methyl heptanoate	144	C8H16O2	C6H13	CH3	145.121	100	162.148	11.6	113.096	<1
					C8H17O2+		C8H20O2N+		C7H13O+
Methyl octanoate	158	C9H18O2	C7H15	CH3	159.138	100	176.164	76.7	127.112	<1
					C9H19O2+		C9H22O2N+		C8H15O+

^aNot detected due to detection limit of m/z range.

APCDI, atmospheric pressure corona discharge ionization technique; MS, mass spectrometry.

Table 3. Summary of 10 esters other than methyl ester (R₂≠CH₃): molecular properties and results of positive-ion APCDI mass spectra obtained by the Q-orbitrap MS.

Molecular properties					Results of positive-ion APCDI mass spectra: m/z, chemical formula, and relative intensities (%) of major ions
Analyte (M)	Molecular mass (nominal Da)	Formula	R1	R2	Protonated molecule, [M+H]+		NH4+ adduct, [M+NH4]+		[R2–H] loss from protonated molecule, [R1COOH+H]+		R1-COOH loss from protonated molecule, R2+
Propyl propanoate	116	C6H12O2	C2H5	C3H7	117.0909	100	134.117	64.3	75.0441	29.8	N.D.a
					C6H13O2+		C6H16O2N+		C3H7O2+
Isopropyl propionate	116	C6H12O2	C2H5	C3H7	117.0909	100	134.117	70.5	75.0441	60.3	N.D.a
					C6H13O2+		C6H16O2N+		C3H7O2+
Butyl propionate	130	C7H14O2	C2H5	C4H9	131.1066	90	148.133	100	75.044	28.5	57.0698	<1
					C7H15O2+		C7H18O2N+		C3H7O2+		C4H9+
Isobutyl propionate	130	C7H14O2	C2H5	C4H9	131.1066	84.4	148.133	100	75.044	15.4	57.0698	17.5
					C7H15O2+		C7H18O2N+		C3H7O2+		C4H9+
Hexyl acetate	144	C8H16O2	CH3	C6H13	145.1218	100	162.148	55.2	61.0282	17.1	85.1009	12.8
					C8H17O2+		C8H20O2N+		C2H5O2+		C6H13+
Ethyl hexanoate	144	C8H16O2	C5H11	C2H5	145.1218	100	162.148	33.1	117.091	14.1	N.D.a
					C8H17O2+		C8H20O2N+		C6H13O2+
Propyl hexanoate	158	C9H18O2	C5H11	C3H7	159.1374	100	176.164	67.5	117.091	48.6	N.D.a
					C9H19O2+		C9H22O2N+		C6H13O2+
Butyl hexanoate	172	C10H20O2	C5H11	C4H9	173.153	100	190.179	37.7	117.091	66.5	57.0698	<1
					C10H21O2+		C10H24O2N+		C6H13O2+		C4H9+
Amyl hexanoate	186	C11H22O2	C5H11	C5H11	187.1686	100	204.1951	61.3	117.0906	51.4	71.0853	<1
					C11H23O2+		C11H26O2N+		C6H13O2+		C5H11+
Isoamyl hexanoate	186	C11H22O2	C5H11	C5H11	187.1686	100	204.1951	67.6	117.0906	20.6	71.0853	13
					C11H23O2+		C11H26O2N+		C6H13O2+		C5H11+

^aNot detected due to detection limit of m/z range.

APCDI, atmospheric pressure corona discharge ionization technique; MS, mass spectrometry.

2.2. Atmospheric pressure corona discharge ionization mass spectrometry (APCDI-MS)

Most of the mass spectra were acquired using a Q Exactive Focus quadrupole-orbitrap mass spectrometer (Thermo Fisher Scientific, Bremen, Germany). The mass resolving power for this instrument was 70,000 at m/z 200 at full width at half maximum, and the measurable m/z was above 50. A TSQ Quantum Discovery MAX triple-quadrupole mass spectrometer (Thermo Fisher Scientific) was used to observe ions below m/z 50. Herein, the two mass spectrometers are abbreviated as “Q-orbitrap MS” and “TQMS.”

The analytes were positively ionized by the well-established APCDI in ambient air with a relative humidity of 50% at 298K, which were controlled using an air conditioner.^14,15) The corona needle used as the point electrode was a headless stainless steel pin (Shiga, Tokyo, Japan) with a diameter of 200 μm, 20 mm in length. The needle tip had a radius of curvature of ca. 1 μm, and the shape of the needle tip surface was adequately approximated by a hyperboloid of revolution. The opposite electrode was the stainless steel orifice plate of the mass spectrometers. A positive corona discharge was generated by applying +3.7 kV between the point-to-plane electrodes with 3 mm gap. An Eppendorf microtube (1.5 mL) with each analyte was placed between the needle and the orifice plate. Analytes were diffused into the discharge region and were protonated with hydronium ion H₃O⁺ and its water clusters H₃O⁺(H₂O)_n, which were produced as the major atmospheric background ions in APCDI. This protonation mechanism was analogous to the APCI-like ambient ionization technique, such as direct analysis in real time.¹⁶⁾

In the CID experiments, the width for the selected precursor ions for the Q-orbitrap MS and TQMS was ca. ±0.4 Da and ±1 Da, respectively. The target gas was N₂ for the Q-Orbitrap MS and Ar for the TQMS. The collision energies in the laboratory were 10–20 eV for both the instruments. In use of 10 eV, ion dissociation was observed at approximately the minimum energies.

3. RESULTS AND DISCUSSION

3.1. Ionization and fragmentation patterns

3.1.1. Alkanoic acids

Table 1 summarizes the major ions and corresponding intensities detected in the positive-ion APCDI mass spectra of seven alkanoic acids (R₁COOH; R₁=C_1–7H_3–15). Individual analytes were detected dominantly as protonated molecules [R₁COOH+H]⁺ and NH₄⁺ adducts [R₁COOH+NH₄]⁺. In using the APCI-like (i.e., plasma-based) ambient ionization technique, ammonium ion, NH₄⁺, can be easily formed as the background ion from air constituents and can attach to heteroatoms in analyte molecules.¹⁷⁾ Notably, H₂O loss from the protonated molecules, [R₁COOH+H–H₂O]⁺, was observed at a significantly low intensity (≤5% relative to the protonated molecules, as shown in Table 1). For the PTR-MS, relatively high intensities of H₂O loss were observed (92%, 47%, 32%, 34%, and 60% from the protonated molecules of the alkanoic acids with R₁=CH₃, C₂H₅, C₃H₇, C₄H₉, and C₅H₁₁, respectively).¹⁸⁾ This indicates that the APCDI-MS is a softer ionization technique than the PTR-MS.

The CID experiments were performed for the protonated molecules [R₁COOH+H]⁺. The CID spectra obtained at a collision energy of 10 eV are shown in Fig. 1. The fragmentation patterns varied depending on the carbon chain length of R₁. When the carbon chain was short (R₁=C_1–3H_3–7), H₂O loss was dominant to form acylium ion, R₁CO⁺ (product ions in red color in Fig. 1a–1c). Furthermore, CO was lost from the R₁CO⁺ ion with R₁ larger than C₃H₇ (product ions in green color in Fig. S1a in the Supplementary Information and Fig. 1d and 1e). For the relatively long carbon chain (R₁=C_4–7H_9–15), the loss of C_nH_2n (n≥2), originating from a part of the alkyl group in R₁, became more dominant than the loss of H₂O+CO. The resulting product ions, [R₁COOH+H–C_nH_2n]⁺ (blue color in Fig. 1d–1g) have the same chemical structure as the protonated molecules of relatively small alkanoic acids. For example, the product ions at m/z 89 (assigned as C₄H₉O₂⁺) observed in the CID spectra for [C_4–7H_9–15COOH+H]⁺ (Fig. 1d–1g) corresponded with protonated butanoic acid. This is evidenced by the similarity of the CID spectral patterns between the C₄H₉O₂⁺ at m/z 89 originating from [C_5–7H_11–15COOH+H]⁺ (which were obtained in combination with in-source CID, as shown in Fig. S2a) and the protonated butanoic acid (Fig. 1c). Among the product ions [R₁COOH+H–C_nH_2n]⁺ observed from various precursor ions, C₄H₉O₂⁺ at m/z 89 (i.e., protonated butanoic acid) was dominant, thereby indicating its significant stability.

Fig. 1. CID spectra of protonated molecules for seven alkanoic acids (R₁COOH) obtained at a collision energy of 10 eV; (a) acetic acid, (b) propanoic acid, (c) butanoic acid, (d) pentanoic acid, (e) hexanoic acid, (f) heptanoic acid, and (g) octanoic acid. All the spectra other than those of acetic acid (A) were acquired by the Q-orbitrap MS, whereas acetic acid spectra were obtained by the triple-quadrupole mass spectrometer. Mass spectral data for ion species with relative intensities higher than 1.5% are summarized in Table S1. CID, collision-induced dissociation; MS, mass spectrometry. The spectrum data files are available in J-STAGE Data. https://doi.org/10.50893/data.massspectrometry.23591499

With increasing collision energies, fragmentation proceeded, resulting in higher intensities of product ions formed from secondary dissociation. As an example, CID spectra of protonated pentanoic acid [C₄H₉COOH+H]⁺ obtained at collision energies of 15 and 20 eV are shown in Fig. S3a.

3.1.2. Methyl esters

The ionization and fragmentation patterns for methyl esters were considerably similar to those for the alkanoic acids described earlier. The methyl esters (R₁COOCH₃) were ionized as protonated molecules, [R₁COOCH₃+H]⁺, and NH₄⁺ adducts, [R₁COOCH₃+NH₄]⁺ (Table 2). The dominant fragmentation pattern of the [R₁COOCH₃+H]⁺ ions with relatively short carbon chains of R₁ (R₁=C_1–4H_3–9) was the loss of CH₃OH to form the acylium ion, R₁CO⁺ (red color in Fig. 2a–2d). Furthermore, as the carbon chain became longer, CO loss occurred successively (product ions in green color in Fig. S1b in the Supplementary information and Fig. 2d). For protonated methyl hexanoate (R₁=C₅H₁₁) with middle length of carbon chain, the loss of alkyl fragments, C_nH_2n, from R₁ (n≥2), as well as CH₃OH+CO, was observed (product ions in blue vs. red+green color in Fig. 2e). Protonated methyl esters with relatively long carbon chains (R₁=C_6,7H_13,15) dominantly dissociated C_nH_2n to form product ions [R₁COOCH₃+H–C_nH_2n]⁺, as compared with the loss of CH₃OH+CO (Fig. 2f and 2g). The product ions [R₁COOCH₃+H–C_nH_2n]⁺ were identical to the protonated molecules of relatively small methyl esters, evidenced by the similarity of their CID spectral patterns (Fig. S2b). Higher collision energies progressed fragmentation in which intensities of product ions formed by secondary dissociation were increased (see CID spectra of protonated methyl valerate [C₄H₉COOCH₃+H]⁺ obtained at collision energies of 15 and 20 eV shown in Fig. S3b).

Fig. 2. CID spectra of protonated molecules for seven methyl esters (R₁COOCH₃) obtained at a collision energy of 10 eV; (a) methyl acetate, (b) methyl propionate, (c) methyl butyrate, (d) methyl valerate, (e) methyl hexanoate, (f) methyl heptanoate, and (g) methyl octanoate. All the spectra other than those of methyl acetate (A) were acquired by the Q-orbitrap MS, whereas the methyl acetate spectra were obtained by the triple-quadrupole mass spectrometer. Mass spectral data for ion species with relative intensities higher than 1.5% are summarized in Table S1. CID, collision-induced dissociation; MS, mass spectrometry. The spectrum data files are available in J-STAGE Data. https://doi.org/10.50893/data.massspectrometry.23591574

3.1.3. Esters other than methyl ester

Esters other than methyl esters (R₁COOR₂; R₂≠H, CH₃) were ionized as protonated molecules [R₁COOR₂+H]⁺ and NH₄⁺ adducts [R₁COOR₂+NH₄]⁺ as with the alkanoic acids and methyl esters (Table 3). Additionally, fragment ions due to the dissociation of [R₂–H] or R₁COOH from the protonated molecules, i.e., [R₁COOH+H]⁺ or R₂⁺, were observed with relatively high intensities (Table 3). Those fragment ions originated from McLafferty rearrangement¹⁹⁾ and are dominant product ions in the CID spectra of the protonated molecules (red and blue colors in Fig. 3). Furthermore, secondary dissociation from the product ions [R₁COOH+H]⁺ was observed (product ions in yellow color in Fig. 3), with similar CID patterns of the protonated alkanoic acids, as shown in Fig. 1. If the proton transfers to the fragment [R₂–H], the product ion R₂⁺ is detected. Higher collision energies resulted in progress of secondary dissociation (see CID spectra of protonated propyl propionate [C₂H₅COOC₃H₇+H]⁺ obtained at collision energies of 15 and 20 eV shown in Fig. S3c).

Fig. 3. CID spectra of protonated molecules for ten esters other than methyl esters (R₁COOR₂; R₂≠H, CH₃) obtained at a collision energy of 10 eV; (a) propyl propionate, (b) isopropyl propionate, (c) butyl propionate, (d) isobutyl propionate, (e) hexyl acetate, (f) ethyl hexanoate, (g) propyl hexanoate, (h) butyl hexanoate, (i) amyl hexanoate, and (j) isoamyl hexanoate. The secondary dissociation from product ions C_xH_2x+1O₂⁺ (in red color) was confirmed by CID with in-source CID (Fig. S2c). Mass spectral data for ion species with relative intensities higher than 1.5% are summarized in Table S1. CID, collision-induced dissociation; MS, mass spectrometry. The spectrum data files are available in J-STAGE Data. https://doi.org/10.50893/data.massspectrometry.23591583

Notably, in the CID spectra obtained here, the intensities of the precursor ions at the lowest collision energy of 10 eV were considerably low (approximately zero) for all the esters. These results suggest that the protonated esters (R₂≠H, CH₃) can easily dissociate the R₂ part to form the protonated alkanoic acids including the R₁ part, which is independent of the carbon chain length of R₂.

3.2. Discrimination of isomeric compounds

The systematic ionization and fragmentation patterns described earlier allow us to distinguish the isomeric compounds of alkanoic acids, methyl esters, and other esters. First, suppose that a protonated molecule and NH₄⁺ adduct of an unknown C_mH_2mO₂ with an m number less than or equal to 3 (m≤3) is detected in a mass spectrum. If the CID of the protonated molecule [C_mH_2mO₂+H]⁺ leads to the detection of a product ion resulting from the loss of H₂O or CH₃OH, C_mH_2mO₂ can be identified as an alkanoic acid or methyl ester; if neither H₂O nor CH₃OH is lost, C_mH_2mO₂ should be esters other than methyl ester.

Next, suppose that an unknown C_mH_2mO₂ has an m number greater than 4 (m≥4). If the CID of its protonated molecule generates product ions resulting from CH₃OH loss, the C_mH_2mO₂ is determined as a methyl ester; no loss of CH₃OH indicates other ester or alkanoic acid. C_mH_2mO₂ can be distinguished as an ester or an alkanoic acid by determining the intensity of the precursor ion [C_mH_2mO₂+H]⁺ in the CID spectrum. If the relative intensity of [C_mH_2mO₂+H]⁺ is considerably low (<5%), C_mH_2mO₂ should be an ester. Differences in precursor ion intensity are clearly seen when the collision energy is low such as 10 eV. Therefore, it is preferable to use low collision energy for discrimination of isomers focused in this study.

If there is only the protonated molecule of C_mH_2mO₂ (not NH₄⁺ adduct), the [C_mH_2mO₂+H]⁺ should be a fragment from the protonated molecule of esters other than methyl esters. In this case, we should check whether [C_mH_2mO₂+H]⁺ and [C_mH_2mO₂+NH₄]⁺ originating from esters are in high m/z regions.

4. CONCLUSIONS

This study provided some basic data on isomeric esters and alkanoic acids with elemental composition of C_mH_2mO₂ measured by APCDI-CID-MS in the positive-ion mode. We explained that there are general rules for the ionization and fragmentation patterns of protonated molecules for those compounds, which makes it possible to distinguish the isomers. These rules should enable the development of real-time measurement techniques that can detect various compounds simultaneously but distinguish isomers; furthermore, they will contribute to aroma research in the future.

ACKNOWLEDGMENT

This work was supported by JSPS KAKENHI grant numbers JP21K19111, JP21K12223, and JP23H04969.

Notes

Mass Spectrom (Tokyo) 2023; 12(1): A0127

REFERENCES

• 1)  D. Holland, O. Larkov, I. Bar-Yaákov, E. Bar, A. Zax, E. Brandeis, U. Ravid, E. Lewinsohn. Developmental and varietal differences in volatile ester formation and acetyl-CoA: Alcohol acetyl transferase activities in apple (Malus domestica Borkh.) fruit. J. Agric. Food Chem. 53: 7198–7203, 2005.
• 2)  R. G. Berger. Flavours and Fragrances – Chemistry, Bioprocessing and Sustainability; Springer-Verlag, Berlin, Germany, 2007.
• 3)  F. Tanaka, K. Okazaki, T. Kashimura, Y. Ohwaki, M. Tatsuki, A. Sawada, T. Ito, T. Miyazawa. Profiles and physiological mechanisms of sensory attributes and flavor components in watercored apple. Nippon Shokuhin Kagaku Kogaku Kaishi 63: 101–116, 2016 (in Japanese).
• 4)  M. A. M. El Hadi, F.-J. Zhang, F.-F. Wu, C.-H. Zhou, J. Tao. Advances in fruit aroma volatile research. Molecules 18: 8200–8229, 2013.
• 5)  C. B. Steingass, R. Carle, H.-G. Schmarr. Ripening-dependent metabolic changes in the volatiles of pineapple (Ananas comosus (L.) Merr.) fruit: I. Characterization of pineapple aroma compounds by comprehensive two-dimensional gas chromatography-mass spectrometry. Anal. Bioanal. Chem. 407: 2591–2608, 2015.
• 6)  C. B. Steingass, M. Jutzi, J. Muller, R. Carle, H.-G. Schmarr. Ripening-dependent metabolic changes in the volatiles of pineapple (Ananas comosus (L.) Merr.) fruit: II. Multivariate statistical profiling of pineapple aroma compounds based on comprehensive two-dimensional gas chromatography-mass spectrometry. Anal. Bioanal. Chem. 407: 2609–2624, 2015.
• 7)  A. Muto, C. T. Muller, L. Bruno, L. McGregor, A. Ferrante, A. A. C. Chiappetta, M. B. Bitonti, H. J. Rogers, N. D. Spadafora. Fruit volatilome profiling through GC × GC-TOF-MS and gene expression analyses reveal differences amongst peach cultivars in their response to cold storage. Sci. Rep. 10: 18333, 2020.
• 8)  X. Song, L. Zhu, X. Geng, Q. Li, F. Zheng, Q. Zhao, J. Ji, J. Sun, H. Li, J. Wu, M. Zhao, B. Sun. Analysis, occurrence, and potential sensory significance of tropical fruit aroma thiols, 3-mercaptohexanol and 4-methyl-4-mercapto-2-pentanone, in Chinese Baijiu. Food Chem. 363: 130232, 2021.
• 9)  F. Ciesa, I. Höller, W. Guerra, J. Berger, J. Dalla Via, M. Oberhuber. Chemodiversity in the fingerprint analysis of volatile organic compounds (VOCs) of 35 old and 7 modern apple cultivars determined by proton-transfer-reaction mass spectrometry (PTR-MS) in two different seasons. Chem. Biodivers. 12: 800–812, 2015.
• 10)  T. Bianchi, Y. Weesepoel, A. Koot, I. Iglesias, I. Eduardo, M. Gratacos-Cubarsi, L. Guerrero, M. Hortos, S. van Ruth. Investigation of the aroma of commercial peach (Prunus persica L. Batsch) types by proton transfer reaction-mass spectrometry (PTR-MS) and sensory analysis. Food Res. Int. 99: 133–146, 2017.
• 11)  W. Lindinger, A. Hansel, A. Jordan. On-line monitoring of volatile organic compounds at PPTV levels by means of proton-transfer-reaction mass spectrometry (PTR-MS) medical applications, food control and environmental research. Int. J. Mass Spectrom. Ion Process. 173: 191–241, 1998.
• 12)  B. Yuan, A. R. Koss, C. Warneke, M. Coggon, K. Sekimoto, J. de Gouw. Proton-transfer-reaction mass spectrometry: Applications in atmospheric sciences. Chem. Rev. 117: 13187–13229, 2017.
• 13)  K. Sekimoto, A. R. Koss. Modern mass spectrometry in atmospheric sciences: Measurement of volatile organic compounds in the troposphere using proton-transfer-reaction mass spectrometry. J. Mass Spectrom. 56: e4619, 2021.
• 14)  K. Sekimoto, M. Takayama. Negative ion formation and evolution in atmospheric pressure corona discharges between point-to-plane electrodes with arbitrary needle angle. Eur. Phys. J. D 60: 589–599, 2010.
• 15)  K. Sekimoto, M. Sakai, M. Takayama. Specific interaction between negative atmospheric ions and organic compounds in atmospheric pressure corona discharge ionization mass spectrometry. J. Am. Soc. Mass Spectrom. 23: 1109–1119, 2012.
• 16)  R. B. Cody, J. A. Laramee, H. D. Durst. Versatile new ion source for the analysis of materials in open air under ambient conditions. Anal. Chem. 77: 2297–2302, 2005.
• 17)  T. Zhang, W. Zhou, W. Jin, J. Zhou, E. Handberg, Z. Zhu, H. Chen, Q. Jin. Direct desorption/ionization of analytes by microwave plasma torch for ambient mass spectrometric analysis. J. Mass Spectrom. 48: 669–676, 2013.
• 18)  E. von Hartungen, A. Wisthaler, T. Mikoviny, D. Jaksch, E. Boscaini, P. J. Dunphy, T. D. Märk. Proton-transfer-reaction mass spectrometry (PTR-MS) of carboxylic acids: Determination of Henry’s law constants and axillary odour investigations. Int. J. Mass Spectrom. 239: 243–248, 2004.
• 19)  E. Aprea, F. Biasioli, T. D. Märk, F. Gasperi. PTR-MS study of esters in water and water/ethanol solutions: Fragmentation patterns and partition coefficients. Int. J. Mass Spectrom. 262: 114–121, 2007.