Detection of several volatile organic compounds through Ar⁺ induced chemical ionisation using inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS)

Abstract

A new analytical technique for detection of organic compounds using inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS) is described. Volatile organic compounds (VOCs) were introduced into the collision/reaction cell (CRC), instead of through the ICP ion source, and the molecules were ionised through an ion reaction, induced by collision with the primary ions (Ar⁺) produced in the ICP. The ionisation characteristics of this new approach were investigated by mass spectrometric analysis of eight VOCs (i.e., benzene, toluene, ethyl acetate, methyl butyrate, ethyl butyrate, pentyl acetate, pyridine, and 2-methylfuran). These molecules were detected as molecular ions (M⁺), protonated ions ([M + H]⁺), or deprotonated ions ([M − H]⁺), demonstrating that soft ionisation was achieved by the present ionisation protocol using ICP-MS/MS. In addition, a volatile selenium-containing organic compound, dimethyl diselenide (Se₂(CH₃)₂), was also analysed to investigate the feasibility of this ionisation protocol to achieve soft and hard ionisation simultaneously. Several Se-related ions such as Se⁺, SeH⁺, Se₂⁺, [SeCH₃]⁺, and [Se₂CH₃]⁺, together with [Se₂(CH₃)₂]⁺, were observed, suggesting that while soft ionisation was possible, ion reaction-induced-fragmentation and hard ionisation also occurred. To demonstrate the analytical capability of the present technique, volatile components released from coffee beans were subjected to the present mass spectrometric analysis. Many ion peaks originating from VOCs were detected from the coffee beans. The data obtained here demonstrated that ICP-MS equipped with a CRC can become an effective tool for analyzing both elements and molecules.

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Introduction

Development of new analytical techniques for rapid and sensitive detection of volatile organic compounds (VOCs) is still a key issue in various research fields such as forensic, environmental, or materials sciences, because some of the VOCs behave as either toxic materials or act as effective medical drugs. In fact, it is recognised that tumor cells generate unique cancer VOC profiles that reflect metabolic conditions in biological systems.^1–3 The analysis of VOC biomarkers from exhaled breath can become a new frontier in cancer diagnostics and health inspections owing to its potential in developing rapid, non-invasive, and inexpensive screening tools.²

Mass spectrometry is emerging as a powerful and user-friendly method for detection of various molecules and elements, including VOCs.^4–7 Mass spectrometry coupled with gas chromatography (GC-MS), utilizing electron ionisation (EI), has been widely used for the detection of VOCs. This technique offers sensitive and quantitative data for individual components from a mixture of VOCs. Owing to the ability of EI to produce consistent and reproducible fragmentation patterns and the extensive mass spectral libraries,^8,9 reliable compound identification can be achieved, especially from a sample mixture of various organic/inorganic compounds. However, this technique is time-consuming, and thus the method is suitable for quantitative analysis rather than qualitative or survey analyses.

Soft ionisation techniques such as matrix assisted laser desorption ionisation (MALDI), electrospray ionisation (ESI) and fast atom bombardment (FAB) are used to overcome the challenges faced by EI.^10–13 However, the ionisation efficiencies of VOCs achieved by these techniques are very low because of the high volatility and limited solubility of VOCs into solvents. Another effective technique for VOC analysis is proton transfer reaction-mass spectrometry (PTR-MS). PTR-MS has been widely used for real time-monitoring of various VOCs whose proton affinity is greater than that of H₃O⁺.^14,15 Sensitive and rapid detection of VOCs can be made by PTR-MS. However, great care must be given to the mass spectrometric interference by fragmented ions produced within the drift tube. Moreover, ion currents of the analytes can vary, reflecting the ion current of the primary ion (H₃O⁺), and thus, careful calibration through correction of humidity dependence is required for quantitative analysis of VOCs.¹⁶ To overcome this, an alternative analytical technique for VOC analysis is highly desired.

Mass spectrometry utilizing ICP as an ion source (ICP-MS) is widely used for sensitive detection of elements. Because of the high excitation and kinetic temperatures of the ion source, organic molecules are decomposed in the plasma (i.e., hard ionisation), making the direct detection of molecules impossible. Recently, we reported a new analytical approach for detection of molecules (e.g., noble gasses, CH₄, CO₂, and NH₃) using ICP-tandem mass spectrometry (ICP-MS/MS).¹⁷ Gaseous molecules were introduced into the collision/reaction cell (CRC) (a device typically used to attenuate polyatomic interference or reactive interference).^18–20 The molecules were ionised through ion reactions with primary ions (e.g., H⁺, Ar₂⁺, and Ar⁺ ions) produced in the ICP, suggesting that the charge-transfer reaction or ion reactions occurring in the CRC is used as the ionisation process for the mass spectrometric analysis of molecules. One of the great advantages of this technique is its flexibility in the choice of primary ions. Depending on the preferences and application the user chooses, the magnitude of hardness/softness of the ionisation of the target molecules can be adjusted by selecting adequate primary ions for ionisation. In the ICP, ions with different ionisation potentials, such as H⁺, Ar₂⁺ (14.52 eV (ref. 21)), Ar⁺ (15.8 eV (ref. 22)) and Ar²⁺ (27.6 eV (ref. 23)), are mainly produced. One of these ions is passed through the first quadrupole mass filter (Q1), to act as the primary ion for the ion reaction within the CRC. The third quadrupole mass filter (Q3) then scans the masses of resulting ions and directs them to the detector for detection.

In this study, to extend the analytical capability of ICP-MS/MS for organic compounds, several volatile organic compounds (VOCs) were subjected to analysis using the CRC for gaseous sample introduction to ICP-MS/MS. The ionisation characteristics and contribution of fragmentation were investigated using nine VOCs with various molecular weights (i.e., benzene, toluene, ethyl acetate, methyl butyrate, ethyl butyrate, pentyl acetate, pyridine, 2-methylfuran, and dimethyl diselenide (DMDSe)), ranging from m/z 80 to 200.

A unique feature of this ionisation technique is the contribution of fragmentation during ionisation. Based on our previous report, although molecules were successfully ionised and detected with their structures intact, dissociation of the C–H bonding occurs during the ion reaction of CH₄ with the Ar⁺ primary ion, implying that fragmentation may occur.¹⁷

Experimental section

Materials and chemical reagents

In this study, the ionization characteristics of nine VOCs (benzene, toluene, ethyl acetate, methyl butyrate, ethyl butyrate, pentyl acetate, pyridine, 2-methylfuran, and DMDSe) were investigated. Benzene and toluene were CICA grade reagents purchased from Kanto Chemicals (Nihonbashi, Chuo-ku, Tokyo, Japan). Deuterated benzene (D-benzene) of NMR grade was used. With D-benzene, production efficiencies of two protonated ions ([M + H]⁺ and [M + D]⁺) can be monitored which can provide a potential source of H through ionisation. Ethyl acetate, methyl butyrate, ethyl butyrate, pentyl acetate, pyridine, and 2-methylfuran were all special grade reagents purchased from FUJIFILM Wako Chemicals (Akasaka, Minato-ku, Tokyo, Japan). Dimethyl diselenide (DMDSe) is a special grade reagent purchased from Thermo Scientific Scientific-Alfa Aesar (Waltham, Massachusetts, USA).

For these nine VOCs, no further purifications were made. Prior to the analysis, 10 μL (ca. 10⁻⁴ mol for each analyte) of VOCs were added into an aluminum-made Tedlar bag filled with 2 L of He gas for dilution, giving resulting concentrations of approximately 50 μmol L⁻¹ for each analyte. All nine VOCs were prepared in separate Tedlar bags.

Concentrations of analytes in He were 50 nmol mL⁻¹ for benzene, toluene, ethyl acetate, methyl butyrate, ethyl butyrate, pentyl acetate, pyridine, and 2-methylfuran, and 30 nmol mL⁻¹ for DMDSe. Sample vapors were introduced into the CRC with a flow rate of 0.1–1 mL for a total of ca. 4 minutes (i.e., 3 minutes for stabilization and 15 seconds for data acquisition). With the concentration values for the analytes and flow rates, total sample sizes used for the analysis ranged from 5 to 50 nmol per single data acquisition sequence.

Kilimanjaro-grown coffee beans were purchased from a local coffee shop (Beans Kobo, Midori-ku, Chiba, Japan). The roasting temperature was about 175–180 °C. The coffee beans were ground with a commercially available handle-type coffee mill.

All gas samples were introduced into the CRC through the gas port (cell gas channel C) originally equipped on the instruments. The gas port and Tedlar bags were connected using a Swagelok VCR connector (SUS union, 1/8 inch–1/4 inch). After connecting the Tedlar bag, the system was left for 3 minutes to achieve stable and plateau signal intensities.

Instrumentation

The ICP-MS/MS used in this study was a PerkinElmer NexION5000. In this study,⁴⁰ Ar⁺ produced from the plasma source was used as the reaction ion (primary ion). Q1 was set to m/z 40 to allow the entry of ⁴⁰Ar⁺ into the CRC. Wet plasma conditions were employed for a higher ion current of Ar⁺. Deionised water (Milli-Q, Merck Millipore, Darmstadt, Germany) was introduced with an uptake rate of 0.2 mL min⁻¹ using a PFA ST3 nebulizer (Elemental Scientific, Nebraska, USA) and a cyclonic spray chamber. The ion currents of the primary ion (⁴⁰Ar⁺) can exceed 5 × 10⁹ cps (i.e., about 1 nA) calculated based on the measured count rates (ca. 2 × 10⁹ cps at the ion detector) and the empirical assumption that the transmission efficiency of Q3 is 50% which was calculated based on the ion transmission through the vacuum interface being 1%.²⁴

Sample gasses were directly introduced into the dynamic reaction cell (DRC™) with flow rates of 0.1–1 mL min⁻¹. DRC is a trade name of the CRC introduced by PerkinElmer. The loading of sample gasses was carried out by natural intake based on the pressure difference (i.e., atmospheric pressure against 10⁻² Pa at the DRC). A schematic diagram of the experimental setup and instrumentation is given in Fig. 1, and the operational settings are summarised in Table 1.

Fig. 1 Schematic diagram of the analytical protocol for VOC analysis using ICP-MS/MS.

Table 1 Instrumental and operational settings

ICP-mass spectrometer	PerkinElmer NexION 5000
Plasma conditions
RF power	1.6 kW
Sampling depth	5.0 mm
Plasma conditions	Wet plasma
Nebulizer	PFA ST3
Spray chamber	Cyclonic operating at room temperature
Gas introduction	via Gas Line for DRC (SUS 1/8 inch tubing)
Lens biases
Hyper skimmer	4 V
Omni ring	−210 V
Quadrupole ion deflector	Optimized by the solution mode
Cell entrance	−0.5 V
Cell exit	−5 V
Cell rod offset	−4 V
Axial field technology (AFT)	150 V
Sample loading
Sample uptaka	Not pumped
Flow rates	0.1 mL min^−1.
	For benzene (C6D6), toluene, ethyl acetate, methyl butyrate, ethyl butyrate, and pentyl acetate
	1 mL min^−1
	DMDSe
	0.1 mL min^−1
	Coffee beans
Sample temperature	Room temperature
Reaction ion (primary ion)	Ar^+ (Q1 = 40 Da)
Data acquisition
Number of points	10 points per peak
Dwell time	5 ms
Settling time	4 ms (for peak jumping)
Scanning range (Q3)	m/z 41–90
	For benzene (C6D6), toluene, ethyl acetate, methyl butyrate, and ethyl butyrate
	m/z 50–200 for dimethyl diselenide
	m/z 41–200 for VOCs in coffee beans

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The helium gas flow rate for introduction of VOCs was 0.1 mL min⁻¹ for the VOCs, except for DMDSe. For DMDSe, a He gas flow rate of 1 mL min⁻¹ was adopted. The analysis sequence begins with gas blank analysis of He carrier gas for 5–15 s, depending upon mass ranges (see Table 1). Gas blank values were acquired by introducing the He gas from the Tedlar bag through the same gas line used for the analysis of VOCs. This is very important to evaluate the memory effect of the analytes. The measured backgrounds were 70 cps for D-benzene (m/z 84), 500 cps for toluene (m/z 91), 25 cps for ethyl acetate (m/z 89), 20 cps for methyl butyrate (m/z 103), 10 cps for ethyl butyrate (m/z 117), <5 cps for pentyl acetate (m/z 131), 150 cps for pyridine (m/z 78), and 30 cps for 2-methylfuran (m/z 82). As for toluene, the background count rate was significantly higher than those for other seven VOCs. The high background of toluene may have reflected the ubiquitous presence of toluene in air or from apparatus. For detection of trace amounts of toluene, reduction of the background is highly desired.

The analysis of the background is followed by data acquisitions of sample gasses. The gas blank values are subtracted from all of the signals obtained by loading of sample gasses. After the data acquisition, He gas containing the VOCs was removed from the sample cell and gas tubing through pumping down using a diaphragm pump (Laboport UN86KTP, KNF, Freiburg, Germany) (Fig. 2).

Fig. 2 Sample introduction system for VOCs released from milled coffee beans.

Results and discussion

Product ions of VOCs

To investigate the ionisation characteristics of the present technique, eight VOCs (benzene, toluene, ethyl acetate, methyl butyrate, ethyl butyrate, pentyl acetate, pyridine, and 2-methylfuran) were subjected to analysis through the CRC (Fig. 1).

The resulting mass spectra of these six VOCs are given in Fig. 3. Fig. 3(a) shows the mass spectrum of deuterated benzene (D-benzene). Our previous study¹⁷ showed that the protonation reaction occurred for several molecules, even though no sources of H⁺ is introduced after the first quadrupole mass filter (Q1). Hence, deuterated benzene was used to investigate whether H⁺ was originated from the analyte, when deprotonation (in deuterated benzene's case, the detachment of deuterium) occurred. When normal benzene (C₆H₆) is used, the source of H⁺ will be difficult to determine, whether it comes from contaminated H₂O or the analyte. If the source of H⁺ is from D-benzene, peaks will appear at m/z 84 and m/z 86; on the other hand, if the source is from elsewhere (i.e., residual moisture inside the ICP instrument), peaks will appear at m/z 84 and m/z 85. Peaks at m/z 80, 82, and 84 were observed, most likely signals of C₆D₄⁺, C₆D₅⁺, and C₆D₆⁺, respectively. A peak at m/z 85 with a lower intensity than m/z 84 was also observed, with a relative peak area of 1 : 0.067 (m/z 84 : m/z 85). Since the theoretical relative isotopic abundance of ¹²C to ¹³C for D-benzene is 1 : 0.066, and trace H⁺ or OH⁺ ions originating from Ar gas could be removed at Q1, the source of H⁺ may be originating from contaminated H₂O in the analyte gas, introduced during the dilution procedure. In fact, the possible H₂O content in the sample gasses, calculated based on the H₂O contents in air (e.g., 10⁻³ mol L⁻¹) and internal volume of the gas line, was 1 μmol L⁻¹, which was about 1/50 level of the VOCs. This implies that contaminated H₂O can become a source of H⁺ through an ion reaction.

Fig. 3 Mass spectra of eight VOCs: (a) D-benzene, (b) toluene, (c) ethyl acetate, (d) methyl butyrate, (e) ethyl butyrate, (f) pentyl acetate, (g) pyridine, and (h) 2-methylfuran, obtained by ICP-MS/MS through ion reaction with Ar⁺.

For toluene (Fig. 3(b)), the base peak was of the deprotonated ion (i.e., [M − H]⁺), and the second peak ion was a molecular ion (M⁺) or an isotopologue of the [M − H]⁺ ion (i.e., [M(¹³C) − H]⁺). The calculated peak area was 1 : 0.086 for m/z 91 to m/z 92, which was similar to the theoretical values of the ¹³C related isotopologue (1 : 0.077). This suggests that the production efficiency of the molecular ion (M⁺) for toluene is much lower than that for benzene. The higher contribution of the deprotonation ion found in toluene can be explained either by easier dissociation of C–H bonding on the –CH₃ group induced by the lower electron-density of the –CH₃ group through hyperconjugation. We do not have any further evidence for this, so this must remain as a possibility.

Based on the results obtained here, the production of the protonated ions for both benzene and toluene is limited, and the major ionisation process for these compounds is deprotonation (and de-electronation for benzene). An important finding from this study is that protonation onto the benzene ring did not proceed for both benzene and toluene. The lower magnitude of protonation can be due to an endothermic process (e.g., 7.7 eV)²⁵ for benzene and alkyl benzene compounds. Both the smaller affinity of H⁺ with a benzene ring and the limited number concentration of H⁺ within the DRC can account for the lower production efficiencies of [M + H]⁺ for benzene and toluene. The ionisation characteristics found in the present technique are similar to that of EI for aromatic compounds.²⁶ This suggests that the electron transfer reaction occurred from analyte molecules to Ar⁺ through collision which can also be achieved by electron scattering through EI.

As for the ester compounds, mass spectra of ethyl acetate, methyl butyrate, ethyl butyrate, and pentyl acetate are given in Fig. 3(c), (d), (e), and (f), respectively. The base peak was the protonated ion ([M + H]⁺) for all ester compounds, and the second highest peaks are the ions at +1 Da. The measured ratios of the base peak and second highest peak are 1 : 0.044 for ethyl acetate, 1 : 0.047 for methyl butyrate, 1 : 0.060 for ethyl butyrate, and 1 : 0.090 for pentyl acetate. These ratios agreed well with the calculated ratios of ¹²C to ¹³C (1 : 0.044 for ethyl acetate, 1 : 0.055 for methyl butyrate, 1 : 0.066 for ethyl butyrate, and 1 : 0.088 for pentyl acetate), suggesting that the second highest peaks are isotopologues related to ¹³C isotopes.

Fig. 3(g) and (h) illustrate the resulting mass spectra of pyridine and 2-methylfuran. The measured [M + H]⁺/M⁺ values for pyridine and 2-methylfuran were significantly higher than the calculated levels of isotopologues (i.e., 0.055 for both pyridine and 2-methylfuran: see Table S1†). These data indicate that the major product ions were the deprotonated ion ([M − H]⁺), molecular ion (M⁺), and protonated ion for pyridine, and M⁺ and [M + H]⁺ for 2-methylfuran. For these compounds, the measured production ratio of protonated ions ([M + H]⁺/M⁺) was higher than those of benzene and toluene (Fig. 3(a) and (b)). The higher production efficiency of protonated ions can be due to higher proton affinity originating from the presence of highly electronegative elements (i.e., O and N) in the heteroaromatic rings.

Based on the measured blank counts and signal intensities of the eight VOCs (Fig. 3), we can estimate the limit of detection (LOD) of the present technique. Hence, the LODs were calculated based on the counting statistics of the background three times for analytes. The calculated LODs were 0.3 fg for D-benzene, 0.56 fg for toluene, 75 fg for ethyl acetate, 34 fg for methyl butyrate, 1.0 fg for ethyl butyrate, 400 fg for pentyl acetate, 46 fg for pyridine, and 1.7 fg for 2-methylfuran, demonstrating that less than picogram level of VOCs can be detected by the present technique. These LOD values were slightly poorer than those achieved by the GC-MS technique. The slightly poor LODs can be due to the contribution of the dissociation reaction through the ion reaction with Ar⁺. We believe that the dissociation reaction can be minimised when optimised ionisation conditions using various primary ions were used, resulting in improved sensitivity.

Contribution of fragmentation

In this section, the contribution of fragmentation on the DMDSe during the ion reaction with Ar⁺ is evaluated. DMDSe is an organoselenium compound where both of its selenium atoms are covalently bonded to methyl groups that is diselane covalently bound to two methyl groups. DMDSe is produced as a bacterial, mammalian, plant, and human xenobiotic metabolite. Selenium is present in aquatic systems in different oxidation states: selenide (−2), selenite (+4), and selenate (+6).²⁷ Selenium compounds are generally toxic, but organoselenium compounds such as selenomethionine and selenocysteine are less toxic than selenite.^28,29 Methylation processes in the body can convert inorganic forms of selenium into less toxic forms, facilitating their excretion. There is evidence for the production of volatile organic selenium species, mainly dimethyl selenide (DMSe) and DMDSe, from inorganic selenide salts as well as from selenocystine and selenomethionine by fungi, plants, and animals in the environment. Several pioneering research studies revealed that the reduction of cancer incidence by dietary supplementation with L-selenomethionine, methylselenocysteine, and other methylated selenium compounds.^30,31 Although Se is vital for animals and humans, excessive exposure to Se can give rise to potential hazards. The augmented prevalence of Se pollution or exposure can be related to various human activities, including mining, coal combustion, oil refining, and agricultural irrigation. To investigate the environmental dynamics and also to understand the biological function of Se, rapid and chemical form-specified detection of Se is highly desired.^32–34

Fig. 4(a) illustrates the mass spectrum of DMDSe for a mass range covering m/z 50 to 200. The base peak was m/z 52, which may be ⁴⁰Ar¹²C⁺, produced through the ion reaction of the primary ion (Ar⁺) and the carbon atom originating from the –CH₃ group in DMDSe. Various fragment ions can be identified in the enlarged scale of the mass spectra: Se⁺ ion and their protonated ions (SeH⁺) (Fig. 4(b)), Se–CH₃⁺ ions (Fig. 4(c)), Se-dimer (Se₂⁺) and Se–Se–CH₃⁺ ions (Fig. 4(d)), and molecular ions (H₃C–Se–Se–CH₃⁺) (Fig. 4(e)). Although molecular ions (M⁺) can be produced through the present ionisation protocol, the major ions (second highest peaks) are a series of Se-related ions combined with the –CH₃⁺ group (e.g., ⁷⁸SeCH₃⁺ or ⁸⁰SeCH₃⁺) and their methylated ions ([SeCH₃ + CH₃]⁺), suggesting the contribution of fragmentation during the ion reaction with Ar⁺.

Fig. 4 Mass spectra of dimethyl diselenide (DMDSe) obtained through ion reaction with Ar⁺: (a) whole mass ranges covering m/z being 50 to 200; (b)–(e) enlarged scale for specific mass regions.

As for the isotopic ions of Se, production efficiencies of SeH⁺ can be calculated using ⁷⁸SeH⁺/⁷⁸Se⁺ and ⁸⁰SeH⁺/⁸⁰Se⁺ values. The resulting ratio for peak areas of m/z 78-to-m/z 79 was 1 : 0.56, which is significantly greater than that calculated for peak areas of m/z 80-to-m/z 81 being 1 : 0.018. Discrepancy in the measured production efficiencies of protonated ions can be explained by mass spectrometric interferents such as ⁷⁷SeH⁺ or Se₂²⁺. The contribution of mass spectrometric interferences by ⁷⁷SeH⁺ can be ruled out because of the lower isotopic abundance of ⁷⁷Se (7.6%) compared to ⁷⁸Se (24%), and thus, the higher signal intensities of the peaks m/z 78 and m/z 79 can be due to the contribution of the mass spectrometric interference by Se₂²⁺. Currently, we do not have a clear explanation for the high-production efficiencies of doubly-charged ions in the ion reactions with Ar⁺, so this must remain a possibility. The presence of doubly-charged dimer ions (M₂²⁺) is also found on Kr and Xe in our previous report.¹⁷ The contribution of ⁷⁸Se⁸⁰Se²⁺ accounts for the higher intensity for the peak of m/z 79, suggesting that various Se ions are produced by the present ionisation protocol using the ion reaction with Ar⁺.

Fragmentation occurring in DMDSe during ionisation can be attributed to the greater ionisation potential of Ar⁺ (15.8 eV: ref. 22) than the typical bond energies for R–H (ca. 0.83–1.0 eV).³⁵ Through the collisions occurring within the DRC, electrons can be transferred from DMDSe to Ar⁺ ions, and the electron deficiency can cause dissociation of the chemical bonding, resulting in the production of various fragment ions. To minimize the contribution of the fragmentation, ionisation through ion reactions utilizing lower ionisation-potential elements such as Kr⁺, Xe⁺, Na⁺, or Cs⁺ would be effective. An alternative choice is to use the protonation reaction. It is widely recognised that protonation is an effective process for soft ionisation to reduce the contribution of fragmentation. In our previous study, H⁺ produced in the ICP operating under wet plasma conditions was introduced into the DRC by setting of Q1 in 1 Da, and protonation onto the NH₃ molecule can be achieved.¹⁷ Although further soft ionisation can be achieved by the ion reaction with H⁺, we were not able to introduce the H⁺ ion into the DRC due to software limitation of mass settings for the quadrupole mass filter in the DRC (Q2). When the DRC operates under wider band-pass conditions (i.e., m/z being 1 to 200), transmission of H⁺ (m/z 1) through the DRC becomes very low, resulting in lower ion currents for the analytes. To achieve soft ionization through the protonation reaction of the VOCs, both the software modifications and optimum setting for the band-pass window, together with careful mass calibration of the first quadrupole mass filter, are required.

The major product ions for the above nine organic compounds (D-benzene, toluene, ethyl acetate, methyl butyrate, ethyl butyrate, pentyl acetate, and DMDSe) are summarised in Table 2. For benzene, the molecular ion, and for toluene, deprotonated ions were the major product ions. For the ester compounds (ethyl acetate, methyl butyrate, ethyl butyrate, and pentyl acetate), protonated ions were the major products. As for pyridine and 2-methylfuran, the major product ions were [M − H]⁺, M⁺ and [M + H]⁺ for pyridine, and M⁺ and [M + H]⁺ for 2-methylfuran. As for DMDSe, although molecular ions (M⁺) were found, the major ions were a series of Se-related product ions combined with the –CH₃⁺ group (e.g., ⁷⁸SeCH₃⁺ or ⁸⁰SeCH₃⁺) and their methylated ions ([SeCH₃ + CH₃]⁺). Based on the data for the VOCs analysed in this study, the major product ions for the VOCs were protonated ions. In contrast, for VOCs with benzene rings including toluene, the deprotonation reaction proceeded. The lower magnitude of protonation can be due to an endothermic process for benzene and alkyl benzene compounds.²⁴

Table 2 Major product ions obtained through ion reaction with Ar⁺

Material	Major product ions^a
a [M + H]^+: protonated ion, [M − H]^+: deprotonated ion.
Benzene	M^+ and [M − H]^+
Toluene	[M − H]^+
Ethyl acetate	[M + H]^+
Ethyl butyrate	[M + H]^+
Methyl butyrate	[M + H]^+
Pentyl acetate	[M + H]^+
Pyridine	[M − H]^+, M^+, and [M + H]^+
2-Methylfuran	M^+ and [M + H]^+
Dimethyldiselenide (DMDSe)	[M − CH3]^+ (and their protonated ions)

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With the present technique, mainly because of the higher contribution of mass spectrometric interferences, peak identification can become difficult for samples containing multiple VOCs. This originates from the unit-mass resolution achieved by the quadrupole mass spectrometer used in ICP-MS/MS. For practical use, peak identification based on an accurate mass obtained with high resolution mass spectrometers is highly desired. Nevertheless, the peak patterns of the ion signals, reflecting both the ionisation characteristics and contribution of the ion fragmentations, can provide important clues to identifying the VOCs.

VOCs in coffee beans

To evaluate the analytical capability of ICP-MS/MS for molecular analysis, VOCs in coffee beans were investigated. The resulting mass spectra are given in Fig. 5. Here, two mass spectra for coffee beans with different mass ranges are shown, in which many peaks were observed, reflecting the variety of organic compounds found in the samples. Overall signals showed that signal intensities decreased as m/z values increased. A decrease in the signal intensities for the ions with higher m/z values can be attributed to either lower abundances of heavier molecules in coffee beans, lower transport efficiency, or lower volatilities of the heavier molecules which cannot be released from the sample. The mass spectra obtained from Kilimanjaro coffee beans were basically consistent with the previous reports on various VOCs released from coffee beans.^36,37 Combinations of VOCs can vary, reflecting the provenance, growing, maturing, and roasting conditions.^38–41 For peak identification, mass analysis using a softer ionisation approach is effective.

Fig. 5 Mass spectra of beans from Kilimanjaro: mass ranges of m/z 70–105 (a) and m/z 105–140 (b). The peaks of several candidate molecules were indicated under the assumption that molecules were detected either as deprotonated ([M − H]⁺), molecular (M⁺), or protonated ([M + H]⁺) ions.

One of the drawbacks of the present technique is the inability to conduct accurate compound identification. This is because of the limited mass resolving power achieved by the quadrupole mass filter adopted in most of the ICP-MS/MS instruments. Only by the mass data with the unit mass resolution, exclusive identification of the constituting molecules is difficult. However, scrutiny of the spectra indicates the presence of several candidate molecules given in the mass spectra (Fig. 5). The potential molecules given in Fig. 5 is based on the simple assumption that the candidate molecules are detected either as deprotonated ([M − H]⁺), molecular (M⁺), or protonated ([M + H]⁺) ions, that is empirically implied by the results obtained in this study (Table 2). With the assumption employed here, the candidate ions found at m/z 80, 83, and 115 can be pyridine, 2-methylfuran, and 2,3-hexanedione, respectively. These three compounds and their derivatives are all well-known ingredients in coffee beans (pyridine,^38,42 furan,³⁶ and hexanedione).⁴³ Among these compounds, the presence of two compounds (pyridine and 2-methylfuran) was supported by the mass spectra obtained in Fig. 3(g) and (h). Despite this, for further detailed peak identifications, a series of mass analyses based on mixtures of multiplex VOCs should be conducted. Moreover, gas component analysis through gas chromatography and/or mass analysis based on exact mass coupled with the softer ionisation protocol is effective.

Conclusion and outlook

In ICP-MS, many pioneering studies have demonstrated that the collision/reaction cell (CRC) technique is an effective approach to reduce mass-spectrometric interference. The contribution of these interferents can be removed by charge-transfer reactions with gas species purged in the CRC. Interfering ions would be neutralised through collision/reaction with ambient gas, suggesting that the gas species introduced into the CRC can be ionised via a charge-transfer reaction. To take full advantage of the ion reaction, mass spectrometric analysis was conducted on volatile organic compounds (VOCs) using ICP-MS/MS. Several important features can be derived in this study, which are as follows:

1. For most VOCs analysed in this study, major product ions were molecular ions (M⁺) for benzene, deprotonated ions ([M − H]⁺) for toluene, and protonated ions for ethyl acetate, methyl butyrate, ethyl butyrate, and pentyl acetate, suggesting that soft ionisation can be achieved through the ion reaction with Ar⁺ produced in the ICP.

2. Based on the mass spectrometric analysis of dimethyl diselenide (DMDSe), fragmentation can occur through the ion reaction with Ar⁺ within the DRC. The resulting fragment ions were Se⁺, SeH⁺, Se₂²⁺, Se–CH₃⁺, Se-dimer (Se₂⁺), and Se–Se–CH₃⁺, and molecular ions (H₃C–Se–Se–CH₃⁺).

3. Various organic compounds were detected in coffee beans from Kilimanjaro. Although peak identification is difficult only by mass analysis based on unit-mass resolutions, three potential molecules can be proposed based on compounds reported from previous studies. With the separated mass spectrometric analysis for three chemical reagents, the peaks at m/z 80 and 83 found in Fig. 5 can be deduced as pyridine and 2-methylfuran, or related ions.

With the present analytical technique, organic compounds of m/z 10 to 150 can be detected. However, caffeine, the most widely known compound in coffee beans, was not detected. This is because caffeine is not a volatile molecule. To detect the molecules with lower volatilities, evaporation of compounds through heating for both samples and gas lines may assist in better detection.

For the present technique, peak identification was conducted using unit-mass resolution achieved using a quadrupole-based mass spectrometer. This can make peak identification difficult due to a larger contribution from the mass spectrometric interference. For practical use, peak identification using high resolution mass spectrometers is highly desired. In addition, libraries can be constructed for a more reliable compound identification using ICP-MS/MS.

One of the great advantages of the present technique using ICP-MS/MS is that the analysis of both molecules and elements can be made. When the sample is introduced into the ICP, elemental analysis can be made by hard ionisation, resulting in a small matrix effect, whereas when the sample gas is introduced into the CRC, detection of molecules can be made by soft ionisation achieved through an ion reaction with primary ions produced in the ICP, suggesting that simultaneous detection of both elements and molecules can be made with a single instrument. The data obtained here demonstrate that the new ionisation protocol using an ion reaction can extend the analytical capability of ICP-MS into molecular analysis.

Author contributions

TH, HHK and HA initiated the research project. TH and KK performed the ICP-MS/MS measurements. KK, HHK, and OS performed the system investigations and optimisation of analytical conditions. All authors have given approval to the final version of the manuscript.

Data availability

All relevant data are summarized in the additional files.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to Drs Shuhei Sakata (ERI, The Univ. Tokyo, Japan), Yoshiki Makino (AIST, Japan), Shuji Yamashita (Doshisha Univ.), and Yuki Tanaka (Chiba Univ., Japan) for technical support with the mass spectrometer. This work was financially supported, in part, by a Grant-in-Aid for Scientific Research (JP21H04511) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4an00996g

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