ANALYTICAL SCIENCES OCTOBER 2017, VOL. 33 1161 2017 © The Japan Society for Analytical Chemistry

Cool Mist Scavenging of Gas-Phase Molecules

Pei-Chi WU, Ewelina P. DUTKIEWICZ, and Pawel L. URBAN†

Department of Applied Chemistry, National Chiao Tung University, 1001 University Rd., Hsinchu, 300, Taiwan

The purpose of analytical extractions is to simplify sample matrix without losing analyte molecules. Here we present a technique of extracting volatile compounds by scavenging gas-phase molecules with tiny liquid droplets (<10 μm). A cool mist of the extracting solvent is generated by an ultrasonic transducer, transferred to the headspace of the sample chamber under atmospheric conditions, and pushed by a small pressure difference toward a condenser. By slowly passing over the sample, the microdroplets extract volatile species present in the sample headspace, and they coalesce in a cooled zone. The condensed liquid is collected for analysis by direct infusion mass spectrometry or chromatography coupled with mass spectrometry. Due to the high surface-to-volume ratio of the microdroplets, the mist depletes a great share of the volatile organics present in the headspace. Other advantages of cool mist scavenging include: selective extraction of gas-phase molecules, the extracting solvent can be miscible with the sample solvent, simplicity, high speed, and no requirement for heating that could potentially decompose the sample. In this study, cool mist scavenging was first tested on artificial samples containing esters. The relationship between the sample concentration and the extract concentration was verified theoretically and experimentally. Some of the possible confounding effects were tested and discussed. The technique was subsequently applied to qualitative analysis of selected complex samples in liquid and solid phase as well as an esterification reaction.

Keywords Chemical analysis, chromatography, extraction, mass spectrometry, microdroplets, sample preparation, volatiles

(Received May 3, 2017; Accepted June 21, 2017; Published October 10, 2017)

mini-scale condenser (Fig. 1A). The microdroplets extract Introduction volatile species present in the sample headspace (Fig. 1B). The principle of this gas–liquid extraction is based on the high S/V Extraction of analyte molecules is the key step in many ratio of the microdroplets that are exposed to the sample analytical methods.1 The purpose of extraction is to simplify headspace vapors. After this exposure, the microdroplets the sample matrix without losing the analyte molecules. coalesce under moderate cooling (~0°C). The liquid extract is Removing unwanted matrix components makes analytes collected for analysis (Fig. 1C). amenable to detection with instrumental techniques (e.g. mass spectrometry (MS)) that are vulnerable to chemical interferences. Various extraction protocols have been developed in the past Experimental three decades, mostly targeting solid and liquid samples. For example, in single-drop microextraction,2–6 analytes are absorbed Typical procedure for liquid samples into a hanging drop of an extracting solvent. In the present Scavenging was performed in a 100-mL three-neck spherical study, we hypothesized that fast extraction of gas-phase flask (extraction chamber). The typical test sample was a molecules to liquid phase could be achieved by increasing the mixture of ethyl octanoate, butyl acetate, ethyl acetate. The test surface area-to-volume (S/V) ratio of the liquid extracting phase. sample volume was: ~10 mL. Unless otherwise noted, the The objective was to verify this hypothesis, and to see if the sample was prepared in the mixture of ethanol and water (1:1, concept could be used to process real samples before mass v/v). In some experiments, the test sample was replaced with a spectrometric and chromatographic analysis. reaction mixture or a real sample. The proposed scavenging system was inspired by the air- Cool mist of the extracting solvent was generated by an purifying property of fog. It was previously shown that organic ultrasonic nebulizer (1 – 2 mL min–1, 1.63 MHz, 45 W; and inorganic substances present in gas and particle phase could Ultramist KUN-868; JingHua, Taipei, Taiwan). The output of be scavenged by environmental fogs, reducing airborne pollutant the extraction chamber was coupled with a mini-scale condenser concentrations.7,8 In the laboratory technique presented here, (D151814; Synthware, Taipei, Taiwan). The junction between a cool mist of the extracting solvent—comprising billions of the condenser and the vial (Fig. 1C) was not sealed, so the air tiny droplets (diameter: <10 μm; volume: <1 pL)—is transferred could freely flow pulling the mist from the sample chamber to to the headspace of the sample chamber under ambient the condenser. The system was equilibrated at room temperature conditions, and pushed by a small pressure difference toward a (22 – 25°C) for at least ~1 min before supplying the cool mist to the headspace of the sample chamber. It was operated under the † To whom correspondence should be addressed. hood with the fans on, so that the flow rate of air below the sash E-mail: [email protected] was ~1.7 m s–1. The extracting solvent was a mixture of ethanol 1162 ANALYTICAL SCIENCES OCTOBER 2017, VOL. 33

GC temperature program: 40°C for 2 min, then increase to 180°C at the rate of 70°C min–1, finally hold at 180°C for 1 min. GC column: SPB-5 capillary column (Supelco Analytical); 30 m × 0.53 mm × 1.5 μm. EIC was recorded at the m/z 60.5 – 61.5 because fragmentation of the test analytes (three esters) in EI source gives rise to certain fragment ions at the same m/z. In yet another performance test, the headspace vapors of a standard sample containing ethyl acetate, butyl acetate and ethyl octanoate (each at 0.01% (v/v)) were aspirated into a 500-μL glass syringe, and the sampled gas was immediately infused to the APCI-Q-MS interface.

Results and Discussion

Principles of scavenging In this early demonstration of cool mist scavenging, the microdroplets of extracting solvent (here, ethanol/water, 7:3 (v/v)) were generated by an ultrasonic nebulizer operating in the low megahertz frequency range (see the Experimental section). They were introduced via a hose to a three-neck spherical flask (sample chamber, Fig. 1C) filled with a liquid sample. Notably, in some analytical studies, microdroplets were generated by Fig. 1 Scavenging molecules by micrometer scale liquid droplets 9 traversing sample headspace: (A) schematic of apparatus; (B) pneumatic spray or electrospray, where the droplets have high 10 absorption (Abs) of gas-phase species at the gas/liquid interface and velocities (a few meters per second). Here, the ultrasound- fast diffusion (Diff ) toward the droplet center; (C) photograph of generated microdroplets moved at relatively low velocities apparatus. The turbid interior of the three-neck flask is due to the (~0.02 m s–1; estimated based on visual observations of the fog presence of a suspension of microdroplets delivered from an ultrasonic front passage across the sample headspace). The low velocity nebulizer. of microdroplet motion lets them be exposed to the sample vapors for a few seconds. Taking into account the small size of every microdroplet generated by ultrasound (<10 μm; based on the device and water (7:3, v/v). The composition of the extracting solvent specifications), and the high diffusivity of small molecules in was selected as a result of a compromise between the desired the extracting solvent (~10–9 m2 s–1), the approximate diffusion polarity and the compatibility with the ultrasonic nebulizer. The time across one microdroplet is in the order of a few milliseconds. condensate (extract) was collected into a glass vial, and Thus, even in the absence of mixing, the contents of every subjected to off-line analysis. microdroplet can be homogenized many times during the passage through the headspace of the sample chamber, which is Extract analysis by mass spectrometry important to prevent formation of concentration gradients (high In some cases, the analyses of the extracts were conducted by concentration of analyte at the microdroplet perimeter). means of a triple quadrupole mass spectrometer (LCMS-8030; An important factor in cool mist scavenging is the S/V ratio of Shimadzu, Tokyo, Japan) equipped with a DUIS ion source the mist microdroplets. It is known from various liquid–liquid operated in the atmospheric pressure chemical ionization and liquid–solid extraction protocols that high S/V values can positive-ion mode. The voltage applied to the atmospheric warrant high speed of extraction. For example, activated pressure chemical ionization emitter was 4.5 kV, while the carbon11 or nanoparticles12 are commonly used for efficient temperature of the inlet duct was set to 400°C. The drying gas concentration or removal of chemical species from liquids by flow rate was 15.2 L min–1 and the nebulizing gas flow rate was adsorption, while emulsion droplets accelerate liquid–liquid 2.0 L min–1. The samples were infused at the flow rate of 20 μL extractions.13 Assuming that the mist droplet diameter is ~5 μm min–1. Mass spectra were recorded in the Q3 positive-ion scan (based on typical specifications of the ultrasonic mist generator): mode for the m/z range 40 – 200. In the subsequent data S = 7.9 × 10–11 m2, V = 6.5 × 10–17 m3, and S/V = 1.2 × 106 m–1. processing, 60-s intervals of the extracted ion current (EIC) During a 15-min extraction experiment, the total volume of the traces were averaged. collected extract is in the order of ~100 μL. Thus, the total surface area of the extracting solvent is ~0.1 m2. This area is Headspace vapor analysis more than 10000 times larger than the one in the single drop In one experiment, sample headspace vapors were analyzed microextraction method (with single 1-μL drop). Due to the before and after 10-min cool mist scavenging. The middle neck large surface area of the gas–liquid interface and the negligible of the three-neck flask (Fig. 1C) was protected with a rubber diffusion times, it is expected that the gas-phase molecules can septum. The gas-phase samples were obtained by inserting a rapidly partition to the mist liquid, and an equilibrium state can needle affixed of a glass syringe to the headspace via the septum. be reached—between the analyte species in the headspace and The needle tip was ~3 cm below the septum. A gas in the microdroplets—when the mist passes through the sample chromatograph (TRACE GC; Thermo Fisher Scientific, chamber. Waltham, MA) was used in conjunction with electron ion source and a single quadrupole mass spectrometer (ISQ; Thermo Fisher Scavenging characteristics Scientific) to analyze the headspace vapors. The volume of the The initial tests were conducted on a solution of three esters collected headspace vapors was 2 μL; split ratio was 40. (ethyl octanoate, butyl acetate, ethyl acetate) loaded to the ANALYTICAL SCIENCES OCTOBER 2017, VOL. 33 1163

Fig. 2 Temporal variations of analyte signal in the microdroplet Fig. 3 Dependency of the apparent extract concentration on the extracts collected during cool mist scavenging. Squares, ethyl preset sample concentration. Squares, ethyl octanoate; circles, butyl octanoate; circles, butyl acetate; triangles, ethyl acetate. Concentrations acetate; triangles, ethyl acetate. Sample matrix: mixture of ethanol of standards in the original sample: 0.01% (v/v). Sample matrix: and water (1:1, v/v). Extracting solvent: mixture of ethanol and water mixture of ethanol and water (1:1, v/v). Extracting solvent: mixture of (7:3, v/v). Extract collection time: 5 min. Analyses of the cool mist ethanol and water (7:3, v/v). Sample volume 10 mL. Extract collection scavenging extracts were done by APCI-Q-MS operated in the intervals: 5 min. Fraction volume: ~60 – 250 μL. Detection: APCI-Q- positive-ion mode. Error bars correspond to standard deviations of MS, positive-ion mode (m/z 89, 117, and 173). Every data point is the three replicate measurements. The anomalous decrease of ethyl average of 60-s EIC trace. acetate signal is due to an isobaric interference with another species recorded at the m/z 89. An unidentified contaminant peak—present in the extract—gives rise to a high (artifactual) ion current at the m/z 89. However, this contaminant signal is suppressed at high concentrations of the analytes. For that reason, a positive correlation between ethyl sample chamber. The mist of microdroplets was generated from acetate concentration in the extract and the sample can only be a mixture of ethanol and water (7:3 (v/v)). The raw ion observed at high concentrations in the sample (triangles, 0.005 M). — –2 –2 –5 currents recorded for the collected extracts by atmospheric CE-ethyl_acetate = (4.65 × 10 ± 0.13 × 10 )CS-ethyl_acetate + (2.37 × 10 ± –5 –1 –1 pressure chemical ionization (APCI)-(triple)quadrupole (Q)- 0.74 × 10 ); CE-butyl_acetate = (1.27 × 10 ± 0.06 × 10 )CS-butyl_acetate + –5 –5 –2 MS—were averaged and converted to concentrations following (–5.90 × 10 ± 1.56 × 10 ); CE-ethyl_octanoate = (2.84 × 10 ± 0.18 × –2 –6 –6 a calibration (Fig. S1, Supporting Information). Figure 2 shows 10 )CS-ethyl_octanoate + (8.01 × 10 ± 3.84 × 10 ). the dynamics of scavenging the sample vapors from the sample headspace. Clearly, the amounts of the extracted species decrease as the scavenging time increases. This observation hm m hs s suggests that cool mist scavenging depletes almost all of the Ki Ci = Ki Ci , (2) gas-phase analytes within a few tens of minutes. Following the Le Chatelier’s principle, when the analyte vapor is removed, it or should be regenerated by evaporation from the solution. Khs However, the transfer of the sample components to the gas m i s Ci = hm Ci , (3) phase (headspace) is too slow to compensate for the depletion of Ki the headspace molecules by cool mist, i.e. the analyte species in s the sample and the headspace are not equilibrated fast enough. where Ci is the quantity of the target compound in the sample, m That is because the surface area of the sample–headspace while Ci is the quantity of the target compound in the mist interface is relatively low (~0.002 m2), and much lower than microdroplets. For convenience, the equilibrium constant ratio hs hm that of the mist–headspace interface. On the assumption that the (Ki /Ki ) can be replaced with a constant (ki): transfer from the sample solution to the headspace is negligible, m s the depletion of the analytes present in the headspace is Ci = KiCi . (4) estimated to be 1.7, 3.1, and 4.1% per minute in case of ethyl acetate, butyl acetate, and ethyl octanoate, respectively (based Figure 3 illustrates such a correlation of the signal with on the scavenging period 10 – 30 min). concentration of the extracted compounds. The constant ki It is of interest to consider the usefulness of cool mist characterizes the efficiency of extraction, and is influenced by scavenging in quantitative determinations of volatile species in partitioning of the analyte species among the phases involved in liquid samples. In most analytical applications, sample solutions the scavenging process. Please note that the above treatment is with very small mole fractions of analytes are extracted — just still simplistic because it assumes that the extracted species are as assumed for the Henry’s Law.14 In this case, partial vapor perfect gases under ideal conditions and negligible equilibration pressure (Pi) of a volatile component (i) depends on its time. concentration (Ci) in the sample: It must be emphasized that the linear dependency in Eq. (4) assumes that the sample–headspace–microdroplet equilibriums

Pi = KiCi, (1) are established fast, while vapor scavenging is non-depleting. However, as discussed above, and confirmed by gas where Ki is a constant. The process illustrated in Figs. 1A and chromatography (GC)-MS analysis of the headspace vapor 1B involves two phase equilibriums: sample–headspace and before and after cool mist scavenging, the headspace molecules sh headspace–microdroplets, characterized with constants Ki and are markedly depleted within a few minutes (Fig. 4). On the hm Ki . Because the sample and microdroplets are exposed to the one hand, the mist microdroplets with high S/V and homogeneous same gaseous environment (headspace), one can establish a distribution of extracted molecules can equilibrate with the proportionality: headspace environment fast. On the other hand, saturation of 1164 ANALYTICAL SCIENCES OCTOBER 2017, VOL. 33

Fig. 4 Gas chromatography–mass spectrometry analysis of headspace vapors before (0 min) and after (10 min) cool mist scavenging. Extracted ion currents were recorded at the m/z 60.5 – 61.5, corresponding to a common fragment of the three esters. Concentrations of the ester standards in the original sample: 0.01% (v/v). Sample matrix: mixture of ethanol and water (1:1, v/v). Extracting solvent: mixture of ethanol and water (7:3, v/v). Original sample volume: 10 mL. Headspace gas aliquots of 2 μL were collected with a glass syringe and immediately injected to gas chromatograph.

Fig. 5 Direct headspace analysis, cool mist extract analysis, and single-drop extract analysis by gas chromatography–mass spectrometry. the headspace by sample vapors is limited by the sample Labels: (EA) ethyl acetate; (BA) butyl acetate; (EO) ethyl octanoate. evaporation kinetics, especially in the case of less volatile Concentrations of standards in the original sample: 0.00001% (v/v). Sample matrix: mixture of ethanol and water (1:1, v/v). Extracting compounds. Thus, when scavenging less volatile compounds, solvent: mixture of ethanol and water (7:3, v/v). The cool mist extract the passage of microdroplet mist through the headspace needs to was collected during 5 min. Volume of single drop: 1 μL. Injection be sufficiently slow to enable re-saturation of the headspace volume in the case of cool mist extract and single drop extract: 1 μL. with sample vapors following an initial depletion. Although Injection volume in the case of headspace gas analysis: 2 μL. less volatile compounds can be “pushed out” from a liquid sample—for example, with the aid of heat, stirring, ultrasound,15,16 gas sparging,16,17 or effervescence18—these actions may also transfer some of the sample matrix to the the extracts from cool mist scavenging could also be analyzed miscible microdroplet liquid. Taking into account the subsequent by GC-electron ionization (EI)-MS to improve the analytical mass spectrometric analysis without chromatographic separation, sensitivity, providing LODs of 2.09 × 10–6, 1.25 × 10–6, and and the focus on volatile solutes, this kind of contamination 3.95 × 10–6 M, in the case of ethyl octanoate, butyl acetate, and should be circumvented. However, heating the sample chamber ethyl acetate, respectively (based on the signal-to-noise ratios of is avoided to prevent thermal degradation of the sample,19 the chromatographic peaks in Fig. 5; S/N = 3 criterion; N unpredictable convective motion of gas-phase molecules, fast calculated as root mean square from 60-s EIC interval near the shrinkage of microdroplets, and delays due to the temperature peak). The LODs obtained with single-drop extraction of the ramp. Despite the gradual depletion of the gas-phase species, same sample and subsequent GC-EI-Q-MS detection are which makes modeling of the cool mist scavenging difficult, it 6.18 × 10–6, 1.06 × 10–5, and 9.68 × 10–6 M. However, 30-s is pleasing to note that a proportionality between the analyte incubation was required to enable extraction of sufficient concentrations in the microdroplet and the sample liquid can amounts of analytes to the 1-μL drop of extracting solvent still be observed. (Fig. 5). The LODs of single-drop extraction could be lowered (to approach those of cool mist scavenging) by increasing the Analytical performance incubation time to 60 s. Unfortunately, during such a long The limits of detection (LODs) of the analytical process incubation, the solvent drop occasionally fell down to the encompassing scavenging and detection by APCI-Q-MS were sample due to gravity. On the other hand, injection of headspace 4.06 × 10–4, 3.69 × 10–4, and 4.76 × 10–4 M, in the case of ethyl vapors (2 μL) from above the sample of the same concentration octanoate, butyl acetate, and ethyl acetate, respectively (based (0.00001%, v/v) to GC-EI-Q-MS did not lead to a satisfactory on the datasets presented in Fig. 3). Notably, the LODs of result (Fig. 5). Nevertheless, it should be noted that the LOD of APCI-Q-MS itself (without prior cool mist scavenging) were ethyl octanoate obtained with the current version of cool mist 1.03 × 10–5, 8.31 × 10–5, and 4.10 × 10–5 M, in the case of ethyl scavenging was higher than the LOD obtained with headspace- octanoate, butyl acetate, and ethyl acetate, respectively (based SPME/GC-EI-Q-MS (~2 × 10–8 M).20 In this case, the better on the datasets presented in Fig. S1, Supporting Information). performance of SPME is attributed to its concentrating effect Dividing the above LODs obtained by APCI-Q-MS without and and high selectivity of the fiber material with the properties with cool mist scavenging leads to the values of 0.03, 0.23, and matching those of the analytes. 0.09, respectively. The numbers lower than one indicate that— Similar to GC-EI-Q-MS, APCI-Q-MS can also be used in in all the three cases—the sample components were “diluted” analysis of both liquid as well as gas-phase samples.18 Thus, in rather than “concentrated”. In fact, the lack of a concentrating another performance test, the headspace vapors of a standard effect is expected, taking into account the similarity of the sample were aspirated into a 500-μL glass syringe, and the sample matrix and the extracting solvent (water/ethanol sampled gas was immediately infused to the APCI-Q-MS mixtures). interface. The result was not satisfactory (Fig. S2 top, The choice of detection technique contributes to the analytical Supporting Information). When the same sample was subjected performance of the entire procedure. Apart from APCI-Q-MS, to cool mist scavenging, and the liquid extract analyzed using a ANALYTICAL SCIENCES OCTOBER 2017, VOL. 33 1165 similar APCI-Q-MS method, clear analyte peaks were recorded (Fig. S2 bottom, Supporting Information). Therefore, despite the apparent lack of concentrating capabilities of cool mist scavenging, the technique could assist sampling gas-phase species (transferring them to liquid phase) for subsequent analysis by APCI-Q-MS. It should also be noted that the water-rich extracting solvent makes cool mist scavenging suitable for extracting volatile compounds that are soluble in water. Using anhydrous organic solvents could decrease the polarity of microdroplet content, and increase partitioning of the gas-phase compounds into the microdroplets. However, such extracting solvents could not be used here because of their low compatibility with the plastic Fig. 6 Volume alterations during cool mist scavenging. parts of the ultrasonic nebulizer used to generate the mist. Apart Concentrations of ester standards in the original sample: 0.01% (v/v). Sample matrix: mixture of ethanol and water (1:1, v/v). Extracting from the solubility of gas-phase species in the extracting solvent, solvent: mixture of ethanol and water (7:3, v/v). Extract collection other experimental parameters may affect scavenging efficiency, intervals: 10 min. Error bars correspond to standard deviations of for example, microdroplet size, mist density and speed of mist three replicate measurements. movement.

Confounding effects Attractive interactions between the solvent and solute species and the ultrasonic transducer may be considered to improve the lead to negative deviations from thermodynamic laws and extraction efficiency and analytical reproducibility. repulsive interactions lead to positive deviations. According to In the current experiment, the volumes of the extract recovered the Sechenov equation,21 the presence of salts and organic varied over time to a slight extent (Fig. 6). This variability is compounds can affect solubility of gases in liquids: attributed to the instability of the ultrasonic transducer: depletion of extracting solvent from the vessel, solvent losses, and heating. C0 In future, alterations to the scavenging system can be made to log = KsCel, (5) C increase the stability of mist flow through the sample chamber, and to minimize the variations of the extract volumes recovered where C0 and C are the gas (here: volatile component) solubilities at different time points. in pure solvent and salt solution with the concentration of One more confounding effect to consider is the loss of a electrolyte Cel, while Ks is a constant. The value of that constant certain number of the mist microdroplets, which hit the sample can be positive or negative,22 indicating “salting-out” or “salting- surface. Using methylene blue solution (10–3 M; in ethanol/ in” effects. The experimental results obtained by supplementing water, 7:3 (v/v)) as extracting solvent and absorption spectroscopy the standard sample with moderate amounts of sodium chloride (for calibration plot, see Fig. S5, Supporting Information), we reveal a salting-in effect (Fig. S3, Supporting Information). were able to determine the volume of the extracting solvent This counter-intuitive observation might be due to a decrease of “contaminating” the sample (~65 μL h–1), and compute the partial vapor pressures of the test compounds (esters) above the approximate number of microdroplets hitting the sample surface sample solution, caused by attractive dipole-ion interactions (per hour): ~2.4 × 1012. One can expect that slowing down the within the multi-component sample matrix (composed of water, movement of cool mist might improve extraction of headspace sodium chloride, ethanol, and alkyl ester solutes). The observed vapors into microdroplets. However, this could also increase salting-in effect is unfavorable because it decreases the the volume of extracting solvent retained in the sample. We concentration of gas-phase molecules to which the mist believe that the retained solvent volume can—in future—be microdroplets are exposed. If the salt concentration considerably minimized by modifying the sample chamber geometry and varied from sample to sample, this could decrease scavenging precisely controlling the speed of droplets getting into and out repeatability. To compensate for such matrix effects, calibrations from the sample chamber although such modifications may should be made using the matrices similar to those of the complicate the scavenging system considerably. analyzed samples. In particular, for quantitative analyses, it would be recommended to spike the analyzed real samples with Tests with real matrices isotopologue standards (same chemical structure as the analyte), Although cool mist scavenging is a fundamental development and use the isotopologue signal for normalization. in analytical chemistry, it is interesting to realize if it can be Apart from the sample composition, a few more factors can applied in the chemical laboratory setting. In one experiment, influence the performance of cool mist scavenging. In fact, we used it to collect headspace “samples” of an esterification temperature is one of the factors that affects liquid–gas phase reaction mixture. The cool mist was passed over reacting equilibriums of volatile solutes.23 The current experimental substrates, and the coalesced mist liquid was collected for setup was operated under the fume hood at ambient conditions. APCI-Q-MS analysis. The signal of a volatile product (butyl When the ambient (laboratory) temperature was 23.5°C, the acetate) was detected in the cool mist extract collected 5 min hood temperature was 23.1°C, and the headspace temperature after the start of the reaction; then, it stabilized and declined in was almost the same (23.5°C). However, the headspace the following 1 h of monitoring (Fig. 7). It should be noted that temperature increased slightly in the course of 10 min (up to this way of reaction monitoring is only qualitative or semi- 24.3°C; Fig. S4, Supporting Information) due to the heat quantitative because several phenomena occur simultaneously dissipation from the ultrasonic transducer coupled with the (chemical reaction, transfer of the product from the liquid to the sample headspace. In fact, it was shown previously, that cold gas phase, depletion from the gas phase). solvent can extract more volatiles than hot solvent in single- To further exemplify potential usefulness of cool mist drop microextraction.6 Thus, thermostating the sample chamber scavenging in chemical analysis, we used it to extract volatile 1166 ANALYTICAL SCIENCES OCTOBER 2017, VOL. 33

Fig. 7 Probing a chemical reaction (esterification) by cool mist scavenging followed by APCI-Q-MS analysis in the positive-ion mode: square–butanol at the m/z 75; triangle–acetic acid at the m/z 61; circle–butyl acetate at the m/z 117. Cool mist scavenging was performed directly in the headspace of the reaction chamber. The extraction chamber was initially filled with 10 mL of butanol; the first fraction was collected during the 5-min interval; then acetic acid was added to initiate the reaction at the time “0.” Extracting solvent: mixture of ethanol and water (7:3, v/v). Extract collection intervals: 5 min. The data points represent the end of extract collection. For example, the first fraction was collected during the 5-min interval preceding addition of acetic acid to initiate the reaction. Error bars correspond to standard deviations of the extracted ion currents recorded Fig. 9 Gas chromatography–mass spectrometry analysis of solid during 1 min. sample vapor extract obtained by cool mist scavenging (extracted ion currents at the m/z 130.5 – 131.5): (A) (top) blank (empty flask); (middle) liquid solution of cinnamaldehyde standard (10–4 M) in mixture of ethanol and water (7:3, v/v); (bottom) solid sample: 1.0 g of crushed cinnamon stick (no solvent added); (B) EI mass spectra of the chromatographic peak attributed to cinnamaldehyde based on the chromatogram of the standard solution and the cinnamon sample extract. Extracting solvent: mixture of ethanol and water (7:3, v/v). The extract was collected during 5 min. It was diluted 5 times with ethanol before injection to gas chromatograph. Injection volume: 1 μL.

Fig. 8 Real sample (red wine) was processed by cool mist scavenging, and analyzed by APCI-Q-MS in the positive-ion mode. Conclusions The blank spectrum (extracting solvent) was subtracted from the extract spectrum to reduce the presence of constant (contaminant- Solid materials with large surface areas are widely used to related) signals. Several spectral features (m/z 40 – 200) correspond to remove target compounds (analytes, contaminants) from the volatile molecules present in red wine. Some of them have been liquid phase. Here we showed that a liquid medium with a large identified putatively based on the knowledge of red wine composition surface area—aerosol microdroplets—can be used to trap gas- and matching m/z values: (1) ethanol; (2) ethyl acetate; (3) ethyl phase molecules into the liquid phase. butyrate; (4) ethyl lactate; (5) ethyl hexanoate; (6) ethyl octanoate; (7) diethyl succinate. However, each of these signals may correspond to Cool mist scavenging is limited by the volume solubility of more than one compound. To provide a definitive peak assignment, the extracted molecules in the receiving liquid. Another the extract sample would need to be analyzed further on a high- constraint in its implementation is the limited compatibility of resolution mass spectrometer (e.g. APCI-FT-ICR-MS). Extracting the available ultrasonic nebulizers with some organic solvents. solvent: mixture of ethanol and water (7:3, v/v). Extract collection This constraint can be eliminated when appropriate ultrasound time: 5 min. nebulizer chambers are fabricated using chemically inert structural materials. Cool mist mainly depletes analytes from the sample chamber headspace but it perturbs the bulk of the liquid sample to a slight extent (minor contamination with the compounds emanating from a complex real sample—red wine extracting solvent). Thus, it can be considered a non-destructive (Fig. 8). The APCI mass spectrum of such an extract reveals technique, and further implemented to follow evolution of several signals corresponding to several (putatively identified) volatile compounds in biosynthetic reactions. volatile compounds present in red wine. Apart from liquid This report describes an early version of cool mist scavenging matrices, the technique is also suitable for scavenging vapors of (mainly focusing on tests with standard solutions and qualitative solids. When a small amount of solid cinnamon particles was analyses of few matrices), which is not yet applicable to trace placed in the sample chamber (without any makeup solvent), the analysis. In the following work, it would be desirable to develop volatile ingredients of the spice could be extracted into the this technique further. Improvements can be made to increase liquid phase with the aid of cool mist. The obtained GC-EI-Q- the extraction efficiency—for example, using different extracting MS result (matching the retention times and patterns of EI mass (mist) solvents and controlling the motion of microdroplets spectra; Fig. 9) suggests the presence of cinnamaldehyde, which within the headspace to a greater extent. The simple mini- is one of the flavor and scent components of cinnamon.24 condenser made of glass can be replaced with a cooled spiral- ANALYTICAL SCIENCES OCTOBER 2017, VOL. 33 1167 shaped metal tubing to improve coalescence efficiency and 7. B. Ervens, P. Herckes, G. Feingold, T. Lee, J. L. Collett Jr., minimize microdroplet losses. and S. M. Kreidenweis, J. Atmos. Chem., 2003, 46, 239. 8. P. Herckes, H. Chang, T. Lee, and J. L. Collett Jr., Water, Air, Soil Pollut., 2007, 181, 65. Acknowledgements 9. S. Banerjee, E. Gnanamani, X. Yan, and R. N. Zare, Analyst, 2017, 142, 1399. We thank Mr. Gurpur Rakesh D. Prabhu for discussions. We 10. X. Chen, J. Cheng, and X. Yin, Proc. SPIE, 2003, 5058, 181. also acknowledge the Ministry of Science and Technology of 11. I. A. Kaipper, L. A. S. Madureira, and H. X. Corseuil, J. Taiwan (Grant No.: MOST 104-2628-M-009-003-MY4) for Braz. Chem. Soc., 2001, 12, 514. financial support for this work. 12. J. González-Sálamo, A. V. Herrera-Herrera, C. Fanali, and J. Hernández-Borges, LCGC Europe, 2016, 29, 180. 13. N. Yanase, H. Naganawa, T. Nagano, and J. Noro, Anal. Supporting Information Sci., 2011, 27, 171. 14. W. Henry, Philos. Trans. R. Soc. London, 1803, 93, 29. Additional experimental details and figures (S1 – S5, calibration 15. L. Wang, Z. Wang, H. Zhang, X. Li, and H. Zhang, Anal. plots, spectra, salting effect, temperature measurements). This Chim. Acta, 2009, 647, 72. material is available free of charge on the Web at http://www. 16. K. Chingin, Y. Cai, J. Liang, and H. Chen, Anal. Chem., jsac.or.jp/analsci/. 2016, 88, 5033. 17. T. Wang and R. Lenahan, Bull. Environ. Contam. Toxicol., 1984, 32, 429. References 18. C.-H. Chang and P. L. Urban, Anal. Chem., 2016, 88, 8735. 19. D. Rood, J. Chromatogr. Sci., 1997, 35, 187. 1. J. Pawliszyn (ed.), “Comprehensive Sampling and Sample 20. J.-B. Hu, S. Gunathilake, Y.-C. Chen, and P. L. Urban, RSC Preparation”, 1st ed., 2012, Academic Press, Cambridge. Adv., 2014, 4, 28865. 2. S. Liu and P. K. Dasgupta, Anal. Chem., 1995, 67, 2042. 21. A. Schumpe and W.-D. Deckwer, Biotechnol. Bioeng., 3. H. Liu and P. K. Dasgupta, TrAC, Trends Anal. Chem., 1979, 21, 1075. 1996, 15, 468. 22. N. Šegatin and C. Klofutar, Monatsh. Chem., 2000, 131, 131. 4. A. Jain and K. K. Verma, Anal. Chim. Acta, 2011, 706, 37. 23. R. P. Schwarzenbach, P. M. Gschwend, and D. M. Imboden, 5. L. Guo, N. Nawi, and H. K. Lee, Anal. Chem., 2016, 88, “Environmental Organic Chemistry”, 1993, Wiley, New 8409. York. 6. S. Jahan, Q. Zhang, A. Pratush, H. Xie, H. Xiao, L. Fan, 24. V. Jakhetia, R. Patel, P. Khatri, N. Pahuja, S. Garg, A. and C. Cao, Anal. Chem., 2016, 88, 10490. Pandey, and S. Sharma, J. Adv. Sci. Res., 2010, 1, 19.