- Email: [email protected]
Progress of Sonic-Spray Ionization Mass Spectrometry and Its Applications
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 47, Issue 1, January 2019 Online English edition of the Chinese language journal
Cite this article as: Chinese J. Anal. Chem., 2019, 47(1): 1–12
REVIEW
Progress of Sonic-Spray Ionization Mass Spectrometry and Its Applications LYU Yue-Guang1,2, BAI Hua1, LI Wen-Tao1, YANG Jing-Kui2, HE Yu-Jian2,*, MA Qiang1,* 1 2
Chinese Academy of Inspection and Quarantine, Beijing 100176, China School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: Sonic-spray ionization (SSI) is a previously developed soft ionization technique which does not need auxiliary methods such as voltage, heating, laser, or corona discharge. Spray ionization can be achieved under normal temperature and atmospheric pressure by inputting high-speed gas coaxial within the capillary. As an atmospheric pressure ion source, this technique was widely used as the interface of liquid chromatography-mass spectrometry at the beginning of its development. Based on the principles of sonic-spray ionization, a variety of easy ambient sonic-spray ionization derived techniques have been developed, which can be used for the in-situ, rapid and real-time analysis of samples with little or without any sample pretreatment. In this paper, the principless and characteristics of sonic-spray ionization were elaborated, and the progress of its application in life sciences, food safety, forensic chemistry, reaction monitoring, and other related fields were summarized. Key Words: Sonic-spray ionization; Easy ambient sonic-spray ionization; Mass spectrometry; Review
1
Introduction
Mass spectrometry (MS) has become one of the most rapidly developing analytical techniques in recent years because of its high sensitivity, rapid analysis, and strong specificity. Essentially, mass spectrometry is a science of preparation, manipulation, and detection of gaseous ions. How to produce gaseous ions conveniently and efficiently is one of the main tasks for the development of mass spectrometry. In the development history of mass spectrometry, every breakthrough in ionization techniques would greatly expand its application. In the last decades, it was believed that gaseous ions could only be obtained from gaseous molecules and could not be transferred and produced directly from solution. The emergence of two kinds of soft ionization techniques, matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI), developed the direct release of gaseous ions from solution, and thus made mass spectrum analysis of proteins, nucleic acids, and other biological macromolecules possible. These methods have
gradually become the two most widely used ionization types in the field of mass spectrometry. Liquid chromatography-mass spectrometry (LC-MS) is an effective method for the separation and analysis of complex organic mixtures, with the separation ability of liquid chromatography and the component identification ability of mass spectrometry. In the beginning of this technique, the atmospheric ion source interface between liquid chromatography and mass spectrometry has been the research focus. Ionization techniques such as atmospheric pressure chemical ionization (APCI)[1], thermo spray (TS)[2], electrospray[3], and atmospheric pressure spray (APS)[4] have been successfully applied to liquid chromatography-mass spectrometry using as ion source interface. Sonic-spray ionization (SSI) is considered to be the simplest ionization, as a type of atmospheric soft ionization technique, which can produce charged droplets at both room temperature and atmospheric pressure without the application of heating or high voltage. Using the action of coaxial gas flow within the capillary tube, the solution in the capillary is sprayed out.
________________________ Received 20 June 2018; accepted 28 October 2018 *Corresponding author. E-mail: [email protected]; [email protected] This work was supported by the National Key Research and Development Program of China (No. 2016YFF0203704). Copyright © 2019, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(18)61132-6
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
When the gas velocity reaches a certain value, charged droplets and gaseous ions are generated in the spray; and the maximum ionization efficiency is achieved when the gas velocity approximates the velocity of sound[5]. Initially, sonic-spray ionization technique was primarily used in peptides[6], proteins[7], and drugs[8] as a new type of atmospheric pressure ion source interface for LC-MS. At the same time, the influence of different ion sources as LC-MS interfaces on the analytical results, such as sonic-spray ionization, atmospheric pressure chemical ionization[9], and electrospray ionization[10,11], was compared and studied. Because the ionization process does not require heating, sonic-spray ionization technique shows unique advantages in the analysis of thermally unstable substances, such as peptides and nerve conduction mediums. Conversely, ambient ionization mass spectrometry (AIMS) has become an important direction of mass spectrometry in recent years. As the name implies, AIMS is a technique for sample ionization, followed by mass spectrometry analysis, in an open atmospheric environment with little or even no sample pretreatment. In earlier stages of AIMS, the technique was often referred to as atmospheric ionization, considering that the ionization environment of both electrospray ionization and atmospheric pressure chemical ionization was at atmospheric pressure. This emphasizes that the ionization environment is not only atmospheric pressure, but also an open environment[12]. In 2004, Takáts et al[13] proposed the first ambient ionization technique, desorption electrospray ionization (DESI), which was considered to be the beginning of ambient ionization mass spectrometry research. In 2005, Cody et al[14] proposed another plasma-based ambient ionization technique called direct analysis in real time (DART). These two kinds of ionization techniques were the earliest invented, and also were the most widely used ambient ionization techniques. Dozens of ionization techniques have been developed since their introduction into the field. In 2006, inspired by desorption electrospray ionization technique, Haddad et al[15] proposed the desorption sonic-spray ionization (DeSSI) technique based on the principles of sonic-spray ionization. Sonic-spray ionization technique became well known and was widely used during this time. Considering its simplicity and convenience, Eberlin et al[16,17] later renamed it as easy ambient sonic-spray ionization (EASI). Among those ambient ionization techniques, the easy ambient sonic-spray ionization does not require high voltage, heating, laser, corona discharge, or any other auxiliary means. It can produce ionization with high-speed airflow, and it is the simplest, most mild, and most easily implemented ionization technique. In this paper, the principles and characteristics of sonic-spray ionization technique and its application as an interface for liquid chromatography-mass spectrometry were described. Simultaneously, the applications of the easy ambient sonic-spray ionization and its derived techniques
based on sonic-spray ionization in relevant fields were also reviewed, and the trends of its development were summarized and prospected.
2
Principles and characteristics of sonic-spray ionization
In 1994, Hirabayashi et al[5] first proposed the new sonic-spray ionization technique. As shown in Fig.1, the methanol-water solution was transported into a fused-silica capillary (0.1-mm i.d., 0.2-mm o.d.) by a syringe pump at flow rate of 30 μL min–1, and the fused-silica capillary was installed in a stainless steel capillary (0.25-mm i.d., 1.7-mm o.d.). The capillary was then inserted into the median hole of an aluminum plate (0.4-mm diameter), with the capillary tip exposed 0.6 mm beyond the orifice of the ion source. The distance between the fused-silica capillary tip of the ion source and the sampling orifice of the mass spectrometer was 5 mm. High-speed nitrogen flowed out from the orifice and the flow rate of nitrogen in the standard state (20 °C, 1 atm) was determined by a mass flow controller. Sonic-spray ionization does not require high voltage or heating. Ionization of sample solutions can be achieved simply by applying a high-speed nitrogen gas coaxial within the capillary, which is considered to be the simplest ionization technique. The mass spectra of lysine, dopamine, and gramicidin S were obtained using this method[18]. Singly and doubly charged protonated molecules and fragment ions with low abundance were detected. It was also found that the response intensity of the gas phase ion signal was directly related to the coaxial nitrogen gas-flow rate. The relationship between the detected intensity of the doubly protonated gramicidin S and the gas-flow rate of nitrogen was tested. As shown in Fig.2, the experimental results showed that: (1) the ion signal of gramicidin S was detected when the gas-flow rate of nitrogen reached 1.3 L min–1; (2) the detected signal intensity increased with the increase of gas-flow rate, reaching the maximum at 3.0 L min–1; and (3) after 3.0 L min–1, the increase of nitrogen mass velocity led to the decrease of ion signal intensity. At the same time, with the increase of gasflow rate, the Maher number of gas flow was also increasing. The Schlieren method was used to create the flow state of
Fig.1 Schematic of sonic-spray ionization device[5]
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
Fig.2
Ion current and mass analyzed ion intensity of doubly pronated gramicidin S molecule as a function of gas flow rate[18]
nitrogen at different gas-flow rates. The flow state was subsonic at 2.2 L min–1 and supersonic at 4.0 L min–1. It was concluded that the method had the highest ionization efficiency in the transonic flow state. The Maher number of the gas flow was approximately 1, which was the sonic velocity, and therefore was named sonic-spray ionization. The ion current in Fig.2 represents the sum of charged droplets and gas phase ions in the experiment. It reached the maximum value when the nitrogen gas-flow rate was 3 L min–1, at which the ion signal intensity reached the maximum simultaneously. However, it is worth noting that when the nitrogen gas-flow rate was 1 L min–1, the charged droplets generated, but no gas phase ions were detected at this time. When the nitrogen mass velocity was below 0.8 L min–1, the droplets generated by the spray had not been charged yet. This result demonstrated that the generation of charged droplets and gas phase ions was directly related to the size of these droplets. In subsonic flow, the droplet diameter continues to decrease as the nitrogen flow rate increases. Hirabayashi et al[18] compared the mass spectra of adrenaline, noradrenaline, dopamine, and other substances obtained by sonic-spray ionization with traditional electrospray ionization, and found that the spectra obtained from the two methods were very similar. However, in the sonic-spray ionization mode, the tested substance showed a tendency to be more prone to be multiply charged[19]. In traditional electrospray ionization method[3], a Taylor cone was formed at the capillary tip under a high electric field action. When the solution of the Taylor cone reached the critical point, or Rayleigh limit, the Coulomb repulsion force of the surface charge was comparable to the surface tension of the solution, and the charged droplets ejected from the tip of the Taylor cone[20]. Under the action of auxiliary nebulizer gas, the solvent evaporates and the Coulomb explosion occurred, which finally formed gaseous ions. It is noteworthy that the flow rate of nebulizer gas for the electrospray process is much lower than the flow rate in sonic-spray ionization. When the flow rate reaches or exceeds
100 m s–1, the stable Taylor cone cannot be formed. As a novel ionization technique, the mechanism research for sonic-spray ionization is particularly important. Hirabayashi et al[19] compared three charging models of friction electrification, electrical double-layer and statistical charging to explore the mechanisms of sonic-spray ionization. For the friction electrification model, different capillary materials theoretically had different electrochemical potentials. The author compared the spray ionization effect of glass capillary and stainless steel capillary with the same geometry, and obtained the same experimental results. Therefore, the charged droplet formation could not be ascribed to friction electrification. Contrarily, in the solution close to the surface of the capillary wall, an electric double layer was formed, in which the ion concentration was not uniform. The wall of the capillary became negatively charged, and the proton concentration near the wall grew, producing charged droplets during the rapid evaporation of the solvent[21]. Based on this model, deactivated fused-silica capillaries could seriously reduce the electric double-layer effect. The authors compared the exposed and deactivated fused-silica capillaries with the same geometric size, and found that the experimental results were the same, so the electric double-layer charging mechanism was denied. The statistical charging model proposed by Dodd[22] is often used to explain the charging process of thermal spray ionization technique[23]. The solution evaporates rapidly into small droplets of equal volume in a very short time. In most droplets, the numbers of positive and negative ions are equal, and thus, these droplets are neutral. However, in some droplets, the number of positive ions is higher than negative ions, which subsequently causes them to be positively charged. Due to the microscopic fluctuation of the ionic concentration in the solution, other droplets are negatively charged. According to this model, the average charge of a droplet <|q|> is proportional to the square root of the ionic concentration N in the solution. (1) On the basis of this model and relationship, the detected ion current should increase with the increasing ion concentration of solution. However, it was found that the detected ion current decreased with the increasing concentration of ammonium acetate in solution. The same results were obtained when using trifluoroacetic acid. Thus, the sonic-spray ionization is not a statistical charging mechanism. Furthermore, Hirabayashi et al[18,19] found that the ion current detected was not only related to the gas-flow rate, but also directly related to the gas medium. At a gas-flow rate of 3 L min–1, the ion currents for nitrogen- and oxygen-based mediums were nearly equal and were three times higher than argon. This was because different gas species led to different surface potentials at the gas-liquid interface. Under the influence of surface potential, the distribution of positive and negative ions on the surface of the solution would become
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
uneven. For example, for an aqueous solution with pH = 5, an electrical double layer about 100 nm thick formed near the surface, and a negatively charged surface layer formed with a positively charged layer underneath[18,19]. In the process of sonic spray, there is an electrical double layer at the interface between the gas and the solution affected by the surface potential. Under the strong impact of high-speed gas flow, tiny droplets will be "peeled off" from the surface of the solution and become charged due to the imbalance of the static charge distribution. When the number of positive ions is higher than that of negative ions, some droplets are positively charged, and others are negatively charged accordingly. This mechanism was also confirmed by experimental results which demonstrated that higher concentrations of ammonium acetate and trifluoroacetic acid in the aforementioned solution resulted in a smaller detected ion current. With an increasing ion concentration in the solution, the concentration of ion pairs gradually increased, and the thickness of the electrical double layer formed on the surface of the solution decreased accordingly. This resulted in the thickness of the diffusion electrical double layer being inversely proportional to the square root of the ion concentration N in the solution[24]. Therefore, with the increase of ion concentration, the changes of positive and negative ion concentration decreased in small droplets. In summary, the most probable mechanism of sonic-spray ionization is the charge separation occurring in the droplets under the action of surface potential. With the sonic gas flow, a droplet undergoes fission and the charge concentration fluctuates, thus producing smaller charged droplets. Subsequently, the charged droplets experience a process similar to traditional electrospray, such as solvent evaporation and Coulomb explosion. Finally, the gaseous ions formed by the charge residue model are detected by mass spectrometry. Based on the sonic-spray ionization physical device and charging mechanism, the main factors affecting the ionization efficiency are gas backpressure, solution velocity, and physical size of the device[25]. The sensitivity and application of sonic-spray ionization can be significantly improved by optimizing these parameters. The mechanism of sonic-spray ionization makes it possible to produce both positive and negative ions at the same time, which is useful for the simultaneous, high-throughput, and rapid mass spectrometry analysis. Table 1 compares the differences and similarities between sonic-spray ionization and other ionization techniques. It can be seen from the table that sonic-spray
ionization is the simplest and most universally employed ionization technique. Additionally, due to its absence of high voltage/current and heating, the gaseous ions produced by sonic-spray ionization have lower internal energy and can retain weak non-covalent interactions between molecules. Therefore, this ionization technique is suitable for the study on weak molecules and inter-molecular forces, providing a new direction for the ionization and analysis of biological molecules. In addition, for biomacromolecule analysis, sonic-spray ionization can provide a wider range of charge distribution than conventional electrospray ionization.
3 3.1
Sonic-spray ionization derived techniques Easy ambient sonic-spray ionization
The desorption electrospray ionization technique developed by Takáts in 2004 opened the door for ambient mass spectrometry analysis[13]. In 2006, Haddad et al[15] proposed that the charged droplets produced by sonic spray could be used in an open environment for direct desorption and ionization of analytes on the surface of tablets for mass spectrometry analysis, which was called desorption sonicspray ionization technique. In the experiment, the spray solvent was 0.01% formic acid-methanol solution (1:1, V/V) at a flow rate of 20 μL min–1. The nebulizer backpressure was 3 MPa, the curtain gas pressure was 0.5 MPa, the decluster voltage was 100 V, both the distances from the spray tip to the tablet and the MS inlet were 2 mm, and the angle between capillary and tablet and MS inlet was about 30°[15]. The experimental installation schematic is shown in Fig.3. Direct analysis of tablets showed that desorption sonic-spray ionization had the same sensitivity as desorption electrospray ionization. The desorption sonic-spray ionization mass spectrum was nevertheless always cleaner, due to the much lower abundances of solvent cluster ions. The amount of the protonated analyte molecule was sufficient for the acquisition of its tandem mass spectrum with adequate signal-to-noise ratio for structural investigation. Furthermore, the higher linear velocity of the spray gas helped to immerse the solvent into the sample matrix, thus providing more homogenous sampling and a more stable and long-lasting ion signal. With these characteristics, desorption sonic-spray ionization technique can be used for analysis of low molecular weight components in complex matrix samples. Compared with desorption
Table 1 Comparison of sonic-spray ionization and other ionization techniques Ion source
Soft ionization
Multiple charges
Polarity
Gas
Voltage/current
Heat
Reference
ESI APCI APS TS SSI
Yes Yes Yes Yes Yes
Yes No No Yes Yes
Moderate/Strong Weak Moderate/Strong Moderate/Strong Moderate/Strong
Yes Yes Yes No Yes
Yes Yes Yes Yes No
Yes Yes Yes Yes No
[3] [1] [4] [2] [5]
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
Fig.3 Schematic of desorption sonic-spray ionization analysis for ingredient in tablets[15]
electrospray ionization, desorption sonic-spray ionization technique does not require high voltage or heating on the spray capillary, providing a more friendly environment for ambient mass spectrometry analysis, which is particularly important for an in-vivo analysis and facilitating in-situ tissue analysis during surgical processes. The desorption sonic-spray ionization is the most simple and convenient ionization technique[26] because it does not need heating, high voltage, laser, ultraviolet light, or special gas assistance. Haddad et al[16] renamed it as easy ambient sonic-spray ionization (EASI) to highlight the simple fabrication and operation of this technique in an ambient environment (pronounced as “easy”). The entire analysis process contains three steps: (1) the solvent is ionized by using a sonic-spray; (2) small bipolar (positive and negative) droplets bombard the sample surface and desorb the analytes; and (3) the charged analytes are analyzed by MS eventually after proton or cation (usually Na+, K+) transfer reactions. 3.2
Venturi-easy ambient sonic-spray ionization
To further simplify the device, Santos et al[27] proposed Venturi easy ambient sonic-spray ionization (V-EASI) in 2011. They incorporated Venturi self-pumping and eliminated electrical pumping. Compared with other techniques, the average electrified capacity of bipolar charged droplets produced by V-EASI was lower, and the ionization selectivity and detection sensitivity were improved. The main body of the device was a simple Swagelok T-element with appropriate
ferrules. The nebulizer gas flowed through a 53-mm long stainless steel tube (400-μm i.d., 728-μm o.d.), and its internal coaxial fused-silica capillary (100-μm i.d., 125-μm o.d.) was used to transport solution and perform tip spray. Under the optimized conditions, the best results were observed for compressed nitrogen at ca. 10 bar and a flow of 3.5 L min–1, a stable solution flow rate of ca. 20 μL min–1 for methanol, and 10–15 μL min–1 for aqueous solution, with the limit of detection of 0.2 ng mL–1 for cocaine in acidified methanol. V-EASI is also a very soft ionization process, so the fragmentation is minimized or, most often completely eliminated. Compared with the electrospray ionization technique, V-EASI benefits from reduced chemical noise and therefore improved signal-to-noise ratios, despite the fact that the absolute intensity of analyte ions seems to be lower than electrospray ionization. The mechanism of sonic-spray ionization enables V-EASI technique to produce positive and negative ions simultaneously. The liquid sample and solid surface can be directly analyzed, and its application fields are broadened. Excellent performance in the detection of biomolecules in pure aqueous solution shows that the technique is suitable for the analysis of proteins and polypeptides in biological solution. 3.3
Spartan-easy ambient sonic-spray ionization
Based on V-EASI, Schwab et al[28] developed the Spartaneasy ambient sonic-spray ionization (S-EASI) to further simplify the device. Further simplification was attained by using a can of compressed air, which was inexpensive and readily available. Portable compressed air replaced traditional nitrogen cylinders and similar gas regulators. Other components were also cheap and easy to be obtained, such as a surgical 2-way catheter and a hypodermic needle, which provided a self-assembling and portable device. Cocaine, glyphosate, polyethylene glycol 600, and polypeptide analyses were carried out to prove that this device retained all the advantages of V-EASI. This simple self-assembled ion source was considered to be the simplest and cheapest ambient ion source for direct analysis of both liquid and solid samples. Figure 4 shows the iterative development process of easy ambient sonic-spray ionization devices.
Fig.4 Schematic of easy ambient sonic-spray ionization (EASI), Venturi-easy ambient sonic-spray ionization (V-EASI) and Spartan-easy ambient sonic-spray ionization (S-EASI)[29]
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
4
4.1
Combination and applications of sonic-spray ionization and its derived methods with other techniques Combination with liquid chromatography
When sonic-spray ionization was first put forward as a new type of atmospheric ionization method, researchers applied it as an ion source interface for liquid chromatography tandem mass spectrometry analysis. Direct coupling of semi-micro liquid chromatography and sonic-spray ionization mass spectrometry was demonstrated by Hirabayashi et al[6] in 1996, and was shown to be a very powerful technique for pesticide analysis. The relationship among the sampling cone temperature, nitrogen flow rate, and MS signal response was experimentally studied. Direct coupling of semi-micro liquid chromatography and sonic-spray ionization mass spectrometry can be realized by increasing the cone temperature to 120 °C, and then controlling the solution flow rate at 100 μL min–1. Good linearity was obtained for simazine in the range of 1 pmol‒1 nmol, and the limit of detection was approximately 300 fmol. Björkman et al[30] optimized the conditions of liquid chromatography in tandem with sonic-spray ionization mass spectrometry (Fig.5). Using tolterodine as a model drug substance, the influence of different parameter settings was evaluated using factorial design. A comparison between sonic-spray ionization and electrospray ionization was made for tolterodine, tolterodine metabolites, and a set of steroids. The results showed slightly poorer repeatability and broader peaks for tolterodine analysis due to the higher velocity of sonic-spray ionization. However, the sensitivity was two times higher than electrospray ionization. The analysis result of pregnenolone showed that sonic-spray ionization had less water loss, which was most likely due to less energy being transferred to the analyte upon ionization. Similarly, Dams et al[31] evaluated the performance of this ion source as an interface for liquid chromatography-mass spectrometry in comparison with the traditional electrospray ionization. The effects of volatile acids, organic modifiers, and buffer systems in the eluent on the ionization efficiency of both interfaces were described. The experimental results showed that organic modifiers could improve the ionization efficiency of electrospray and sonic spray, while volatile acids or buffers gave a significant ion suppression effect. Furthermore, the authors applied the sonic-spray interface for the simultaneous determination of seven prime opium alkaloids in heroin impurity profiling. By using monolithic silica column and gradient elution system at 5 mL min–1 flow rate, the seven substances were well separated in 5 min. By means of a post-column split of ca. 1/20, a coupling between the fast liquid chromatograph system and the mass spectrometer was achieved.
Fig.5 Schematic of sonic-spray ionization interface[30]
4.2
Combination with microfluidic chips
In 2007, Pól et al[32] proposed the first microchip version of sonic-spray ionization as an atmospheric pressure ionization source for mass spectrometry analysis. As shown in Fig.6, mild and highly effective ionization of polar compounds in positive and negative modes were achieved by applying highspeed nebulizer gas to microfluidic chips. The microchip is composed of a silicon wafer with an etched inlet for nebulizer gas, a housing for fused silica capillary, a vaporizing channel, and an exit nozzle. Nebulizer gas enters the vaporizing channel where it mixes with the sample that is introduced via fused silica capillary, and thus produces sonic-spray ionization. The results showed that the position of the chip corresponding to the mass spectrometer inlet, gas, and sample velocity had great influence on the detection results. Tetra-Nbutylammonium, verapamil, testosterone, angiotensin I, and other substances were analyzed, and the MS signals were found to be stable. The dynamic linear range was equivalent to electrospray ionization, with consistent reproducibility and detection limits from 15 nM to 4 μM. In 2016, Yu et al[33] proposed a novel microfluidic self-aspiration sonic-spray ionization chip. The proposed ionization chip was fabricated using a three-layer soft lithography technique without the need to fabricate the spraying tips. The microchip structure was optimized by computer simulation and then verified through experiments. The experimental results indicated that the ionization of a sample solution could be achieved through
Fig.6 Schematic of microchip sonic-spray ionization arrangement. (A) Different angles of the sonic spray versus MS orifice plate were tested: 0°, 45° and 90°; (B) Optimum positions of the microchip sonic spray toward the MS orifice in distance (x) and height (y) both of 4 mm[32]
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
spraying nebulizer gas, which resulted in a much lower pressure. Additionally, a dual-channel self-aspiration sonic-spray ionization chip was developed, and the signal intensity was enhanced through applying the same sample into both channels. When a different solution was in the channel, the ion suppression effect would be reduced. This microfluidic chip also effectively improved the integration of ionization and simplified the overall operational process. 4.3
Combination with thin-layer chromatography
Among many ionization techniques, EASI is the simplest, most refined technique, and is also most easily implemented. Another simple and inexpensive method is thin-layer chromatography (TLC), which is one of the most easily performed chromatographic separation techniques. Haddad et al[16] used the combination technique of EASI and TLC for the analysis. The mass spectra of the separated spots on the TLC plate were directly collected and analyzed by EASI. In the study, as shown in Fig.7, the spray solvent was 0.01% formic acid-methanol solution (1:1, V/V). The flow rate was 20 μL min–1, nebulizer gas backpressure was 3 MPa, and curtain gas pressure was 0.5 MPa. The declustering voltage was 100 V, tip-TLC plate distance was ca. 2 mm, and capillary-TLC plate-entrance angle was ca. 30°. Using this method, nitrogen containing compounds of moderate polarity, drugs, and vegetable oils were analyzed and the reactions were monitored. The combination of these two techniques integrated the advantages of high efficiency of chromatography with high sensitivity of mass spectrometry. Eberlin et al[34] identified the biodiesel (B100) and biodiesel mineral diesel mixture (BX) using the combination of high-performance TLC and EASI mass spectrometry. Oil components were separated on highperformance TLC plates, and the separated spots were directly analyzed by EASI. The results showed characteristic signals of fatty acid methyl ester sodium adducts in both samples of B100 and BX, which could be used to identify biodiesel samples. The mass spectrum of mineral diesel oil contained a series of protonated alkyl pyridines, which were used as the characteristic qualitative identification standards for mineral diesel oil. The spectrum for admixture oil spots was characterized by triglycerides sodium adducts. This method was confirmed to be suitable for qualitative identification and quality control for biodiesel and mineral diesel. 4.4
Combination with other techniques
Haddad et al[17] demonstrated that permeable cellulose dialysis membrane could be used as an interface for the direct analysis of solution constituents via EASI and membrane interface mass spectrometry. This combination promoted droplet intake of the analytes from the external surface of the membrane, where the analytes had selectively permeated for
Fig.7 Schematic of TLC-EASI-MS analysis[16]
proper mass spectrometry characterization and quantitation. The results showed that by using the cellulose dialysis membrane, direct analysis of aqueous solutions of common drugs could be well performed. The interface of the cellulose membrane was composed of a small cylindrical Teflon container connected to two silicon tubes, and the top of the container was sealed with a cellulose membrane. The drug solution then circulated through the membrane system at a flow rate of 20 mL min–1 by a syringe pump. The spray solvent was 0.01% formic acid-methanol solution (1:1, V/V). The angle between the spray tip and the membrane was 30°. The resulting charged droplets bombarded the external membrane surface and the analyte was desorbed and ionized for MS analysis. This method could also be used for continuous in-situ monitoring and analysis of polar and semipolar compounds in environmental fluids. Because permeable membranes could block solid particles, cellulose dialysis membrane could also be used for online monitoring of fermentation and biotransformations. These dialysis membranes were also used to monitor hemodialysis processes or to construct very small probes, which might permit in-vivo analysis. Figueiredo et al[35] developed a molecularly imprinted polymer as an interface for the selection and collection of the analytes, which was applied to EASI-MS analysis. First, molecularly imprinted polymers were used to absorb chlorpromazine, triflupromazine, and other compounds found in urine from an extraction/washing tank. After washing and removing other impurities, the molecularly imprinted polymer probe was placed at a suitable position. The spray solvent was acetic acid-methanol solution (1:9, V/V). During the detection process, the probe area (ca. 40 mm2) was exposed to the spray solvent for 1 min through artificially moving the probe. The quantitative analysis was performed by monitoring the quasi-molecular ions. The experimental results showed that the molecularly imprinted polymer could be used as a chemical selective interface for the detection of chlorpromazine in urine by EASI-MS. The method was proven to be rapid and highly sensitive and selective. It also demonstrated other attractive features such as high molecularly imprinted polymer probe lifetime, direct analysis without pre-separation, and ease of extraction/washing and analysis procedures. These characteristics enabled the method
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
to distinctly reduce or nearly eliminate ion suppression effects, which makes this method highly suitable for complex matrix sample analysis, including biologically and environmentally important fluids.
5
Applications of sonic-spray ionization and its derived techniques
Since its introduction in the 1990s, sonic-spray ionization has been applied in many fields such as life sciences, food safety, forensic chemistry, and reaction monitoring, as a universal atmospheric ionization technique. The research and applications based on sonic-spray ionization technique are listed in Table 2. 5.1
Life sciences
Life sciences is the most active field within the 21st century. Most of the life systems are complex. Therefore, increasingly strict requirements are being raised for analytical techniques. Sonic-spray ionization and its derived techniques have been widely used in the field of life sciences[36–40]. Arao et al[41] continuously identified oleandrin and derivatives deacetylated triglycerides, oleandrigenin, and podophyllogenin in blood using a liquid chromatography tandem mass spectrometry system equipped with a sonic-spray ionization interface. Satisfactory linearity of working curves was obtained for oleandrin, deacetylated triglycerides, and oleandrigenin within the range of 5–100 ng mL–1. The limits of detection for oleagin and deacetyltriglyceride in blood were 2 ng mL–1, and 3 ng mL–1 for podophyllogenin. Mortier et al[8] established an analytical method for the determination of para-methoxyamphetamine, 3,4-methylenedioxyamphetamine, and 3,4-methylenedioxymethamphetamine in biological samples by sonic-spray ionization coupled with liquid chromatographyion trap mass spectrometry. This method was suitable for the analysis of whole blood, urine, and tissue samples. The intra-day and inter-day precision of this method was less than 17.5%, and the accuracy was less than 16.2%. The standard curves of different substrates were also established, and the linear correlation coefficients were all greater than 0.995. Arinobu et al[42] established a rapid identification method of pentazocine in human blood using liquid chromatography-
mass spectrometry equipped with sonic-spray ionization source. Dextromethorphan was used as an internal standard to verify the reliability of the method. The limit of detection was 19.5 ng mL–1 and required only 4 min to detect pentazocine in a human blood sample. Alberici et al[43] applied EASI for direct analysis of lipid complexes and explored the difference of lipid samples between mice with hypertriglyceridemia and normal triglycerin. The MS signals of phosphatidylcholine and triacylglycerol were well observed in the positive ion mode, and the signals of phosphatidylethanolamine and phosphatidylinositol were observed in the negative ion mode. The free fatty acids in lipids were then analyzed. It was found that the phospholipid choline and triacylglycerol complexes in mice with hypertriglyceridemia had higher content of oleic acid, and the abundance of arachidonic acid in phosphatidylinositol was also higher, which revealed that there was a distribution of free fatty acids in mice. 5.2
Food safety
Simas et al[44] applied EASI for a fast and reliable analysis of vegetable oils. The main ingredients of vegetable oil were desorbed and ionized efficiently in an open environment. The proposed method was so simple and easy to operate that only one drop of vegetable oil on the inert surface was needed, without any pretreatment. The detected triacylglycerol was sodium adduct in positive ion mode, and free fatty acid was detected as in the deprotonated form in the negative ionization mode. EASI was proved to be a soft ionization technique in that the triacylglycerol molecular ion did not undertake further fragmentation. Therefore, the contents of both diacylglycerol and monoglyceride were determined directly. Based on these results, certification and quality control of vegetable oil were carried out, and adulteration, acidity, oxidation, and hydrolysis level of vegetable oil could be adequately explored. Porcari et al[45] combined thermal imprinting with EASI technique. Only a very small amount of solvent was needed to transfer triacylglycerol compounds from meat, fat, fish, and other food onto paper. Then, EASI was applied to the rapid identification and analysis of triglyceride. The entire analytical process took only a few minutes and consumed a very small amount of sample and solvent. The experimental results were in good agreement with those obtained by gas chromatography and
Table 2 Application of sonic-spray ionization Sample
Analyte
Method
Blood Biological samples Tablet Diesel oil Vegetable oil Fish and meat Ink Tablet
Oleandrin Amphetamine Diazepam FAME TAG TAG Basic Violet 3 Cocaine
LC-SSI-MS LC-SSI-MS DeSSI-MS TLC-EASI-MS EASI-MS TI-EASI-MS EASI-MS V-EASI-MS
Liner range (ng mL–1)
LOD (ng mL–1)
Reference
5–100 10–1000 / / / / / /
2 5 / / / / / /
[35] [8] [15] [29] [38] [39] [48] [23]
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
matrix-assisted laser desorption ionization-mass spectrometry. Thermally imprinted paper of samples could be collected on-site and sent to the laboratory for testing. This technique was applied for the quality control of Russian caviar[46] and Iberian ham[47]. Sawaya et al[48] collected a series of chemical information from 49 different kinds of propolis alcohol extracts of international origins using EASI technique. Principal component analysis of the data obtained from a plant resin could be used to infer the geographical origin of propolis. 5.3
Forensic chemistry
EASI is widely used in forensic chemistry[49] for sample identification and analysis, due to its characteristics of simplicity of use and fast operation[44,48,50‒53]. Using EASI technique, Lalli et al[54] established a fingerprint identification and writing time inference method for ink on the surface of ballpoint pen writing paper. Mass spectrometric information obtained from ink strokes showed that different brands of ink had different chemical characteristics. Basic Violet 3, Basic Blue 26, Basic Blue 7, Basic Violet 10, nickel phthalocyanine, and 1,3-dimethyl-1,3-ditolylguanidine dyes were found in the inks. Different colors of ink contained different types of dyes, and even different brands of the same color ink had different contents of various dyes. Therefore, the brand of ink could be inferred simply based on the obtained fingerprint information. Additionally, the age of ink could be inferred through chemical analysis. For example, alkaline violet 3 showed a series of degradations took place over time, and the abundance of catabolite changed linearly. Therefore, it was used as a "chemical clock" to indicate the age of ink, providing a powerful technical means for the court to identify whether the signature was forged. In another application, Carbral et al[55] utilized V-EASI technique to identify tree species. Mahogany, an endangered tropical plant, contains a typical phytochemical marker of aloin, which can be directly detected on wood surface or methanol extract from sawdust using the V-EASI method. The experimental results showed that the illegal logging trade of mahogany and other endangered trees could be controlled by comparing the chemical profiles of mahogany and other woods, and certification standards were established through this method. Furthermore, fake perfume or perfume of poor quality was a common international phenomenon, causing huge economic losses in industry, as well as posing potential health risks. Haddad et al[56] proposed an identification method for perfume authentication, which could be completed almost instantaneously. In the experiment, the perfume sample was directly sprayed onto a glass rod or a piece of paper and let dried for a few seconds, which was then followed by the EASI-MS analysis. Most of the polar compounds in perfume were detected within 1 min. Unique chemical characteristics of genuine perfume were provided
with good reproducibility. The proposed method can be used to authenticate commercial perfume samples. 5.4
Reaction monitoring
With the characteristics for in-situ and real-time mass spectral analysis, EASI shows great potential in the field of reaction monitoring[57–64], which is important for optimizing reactant ratios, controlling reaction processes, and inferring reaction mechanisms. Na et al[63] applied V-EASI technique to monitor the self-assembly process of nucleic acid bases. Potassium ions were introduced into the reaction system via a syringe, and the cluster ions were then produced by self-assembly of nucleic acid bases. This process could be observed through V-EASI. The schematic of the device is shown in Fig.8. Many cluster peaks for nucleic acid bases were detected during the monitoring process, which proved that the cations played an important role in the self-assembly process of nucleic acid bases. In addition, the proposed method, as a soft ionization technique, was suitable for studying the non-covalent interactions in the self-assembly process. Jansson et al[65] developed a method for monitoring the surface reaction within a narrow space. Two fused-silica capillaries were immersed into the solution, positioned in close proximity to each other, and the functionalized surface created a laminar flow junction with a resulting reaction volume of ca. 5 pL. The reagents were then delivered to the reaction surface through a fused-silica capillary. The other capillary was connected to a V-EASI source. The sampling velocity was slightly higher than that of the feeding capillary. The combined effects of inflow and outflow maintained a stable chemical microenvironment, where the rate of the advective transport overcame the diffusion. Acetylcholinesterase was fixed on the surface of organo-siloxane polymer by electrostatic interaction. Then, acetylcholinesterase hydrolyzed acetylcholine to form choline, which was monitored in real-time. Different acetylcholine concentrations, fused-silica capillary shapes, and flow rates were thoroughly optimized. The experimental results showed that the conversion rate increased with the increasing acetylcholine concentration. This experiment proved that the proposed method was suitable for real-time reaction monitoring.
6
Conclusions and prospects
Sonic-spray ionization technique has been considered as the simplest and most convenient atmospheric ambient ionization method. It has been successfully applied to many fields since its early application as a novel ion source interface for liquid chromatography tandem mass spectrometry, and later developed easy ambient sonic-spray ionization (EASI). During this period, the ionization device has consistently been optimized and simplified, demonstrating its preserving characteristics
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
[8]
Mortier K A, Dams R, Lambert W E, Letter E A D, Calenbergh S V, Leenheer A P D. Rapid Commun. Mass Spectrom., 2010, 16(9): 865–870
[9]
Arinobu T, Hattori H, Seno H, Suzuki O. J. Am. Soc. Mass Spectr., 2002, 13(3): 204–208
[10] Hirabayashi A. Rapid Commun. Mass Spectrom., 2003, 17(5): 391–394 [11] Kovács Z, Dinya Z, Antus S. J. Chromatogr. Sci., 2004, 42(3): 125–129 [12] Cooks R G, Ouyang Z, Takáts Z, Wiseman J M. Science, 2006, 311(5767): 1566–1570 [13] Takáts Z, Wiseman J M, Gologan B, Cooks R G. Science, 2004, Fig.8
Schematic of Venturi-easy ambient sonic-spray ionizationmass spectrometry setup used for real-time measurement of self-assembly reactions[63]
of simplicity, speed, and portability. Even before commercialized, the sonic-spray ionization had gradually become one of the most popular ambient ionization techniques. Teunissen et al[66] summarized the development of easy ambient sonic-spray ionization in the past ten years. Based on the perspective of ambient ionization technique, the paper focused on the simplification of the device from EASI to V-EASI, and then to S-EASI. Furthermore, a summary was made for its many applications in the fields of medicine, food, oil, fuel, forensic chemistry, and biological analysis. The present paper began with the initially proposed sonic-spray ionization technique and expounded on its basic principles and development process. In terms of device development and applications, the present paper covered both its initial use as an interface for liquid chromatography/capillary electrophoresis tandem mass spectrometry, and its subsequent development as an ambient ionization technique, as well as combinations with microfluidic chips, thin-layer chromatography, and other techniques. The principless and applications were then systematically summarized. Now the sonic-spray ionization technique has been emerging in the field of mass spectrometry imaging[67]. It is estimated that sonic-spray ionization will be expanded to more fields in future.
References
306(5695): 471–473 [14] Cody R B, Laramée J A, Durst H D. Anal. Chem., 2005, 77(8): 2297–2302 [15] Haddad R, Sparrapan R, Eberlin M N. Rapid Commun. Mass Spectrom., 2010, 20(19): 2901–2905 [16] Haddad R, Milagre H M, Catharino R R, Eberlin M N. Anal. Chem., 2008, 80(8): 2744–2750 [17] Haddad R, Sparrapan R, Kotiaho T, Eberlin M N. Anal. Chem., 2008, 80(3): 898–903 [18] Hirabayashi A, Sakairi M, Koizumi H. Anal. Chem., 1995, 67(17): 2878–2882 [19] Hirabayashi A, Hirabayashi Y, Sakairi M, Koizumi H. Rapid Commun. Mass Spectrom., 2015, 10(13): 1703–1705 [20] Dary C B L, Inés A V M, Elisa L C, Dazuly M. J. Chem. Phys., 1968, 49(5): 2240–2249 [21] Kulys L. Bioelectrochem. Bioenerg., 1993, 31(3): 330–331 [22] Dodd E E. J. Appl. Phys., 1953, 24(1): 73–80 [23] Katta V, Rockwood A L, Vestal M L. Int. J. Mass Spectrom. Ion Processes, 1991, 103(2-3): 129–148 [24] Whitehouse D R. Anal. Chim. Acta, 1966, 36(1): 412–412 [25] Ozdemir A, Lin J L, Wang Y, Chen C H. RSC Adv., 2014, 4(106): 61290–61297 [26] Alberici R M, Simas R C, Sanvido G B, Romão W, Lalli P M, Benassi M, Cunha I B S, Eberlin M N. Anal. Bioanal. Chem., 2010, 398(1): 265–271 [27] Santos V G, Regiani T, Dias F F, Romão W, Jara J L, Klitzke C F, Coelho F, Eberlin M N. Anal. Chem., 2011, 83(4): 1375–1380 [28] Schwab N V, Porcari A M, Coelho M B, Schmidt E M, Jara J L, Visentainer J V, Eberlin M N. Analyst, 2012, 137(11):
[1]
Kambara H. Anal. Chem., 1982, 54(1): 187–195
[2]
Blakley C R, Vestal M L. Anal. Chem., 1983, 55(4): 750–754
[3]
Yamashita M, Fenn J B. J. Phys. Chem., 1984, 88(20): 4451–4459
[4]
Sakairi M, Kambara H. Anal. Chem., 1989, 61(10): 1159–1164
[5]
Hirabayashi A, Sakairi M, Koizumi H. Anal. Chem., 1994, 66(24): 4557–4559
[6] [7]
Hirabayashi Y, Takada Y, Hirabayashi A, Sakairi M, Koizumi H.
2537–2540 [29] Schwab N V, Eberlin M N. Drug Test. Anal., 2013, 5(3): 137–144 [30] Björkman H T, Edlund P O, Jacobsson S P. Anal. Chim. Acta, 2002, 468(2): 263–274 [31] Dams R, Benijts T, Günther W, Lambert W, De L A. Anal. Chem., 2002, 74(13): 3206–3212 [32] Pól J, Kauppila T J, Haapala M, Saarela V, Franssila S, Ketola
Rapid Commun. Mass Spectrom., 1996, 10(15): 1891–1893
R A, Kotiaho T, Kostiainen R. Anal. Chem., 2007, 79(9):
Hirabayashi Y, Hirabayashi A, Takada Y, Sakairi M, Koizumi H.
3519–3523
Anal. Chem., 1998, 70(9): 1882–1884
[33] Yu C L, Qian X, Chen Y, Yu Q, Ni K, Wang X H. RSC Adv.,
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
2016, 6(55): 50180–50189 [34] Eberlin L S, Abdelnur P V, Passero A, Sa G F, Daroda R J, Souza V, Eberlin M N. Analyst, 2009, 134(8): 1652–1657 [35] Figueiredo E C, Sanvido G B, Arruda M A, Eberlin M N. Analyst, 2010, 135(4): 726–730 [36] Shinozuka T, Terada M, Tanaka E. Forensic. Sci. Int., 2006, 162(1): 108–112 [37] Gréen H, Vretenbrant K, Norlander B, Peterson C. Rapid Commun. Mass Spectrom., 2010, 20(14): 2183–2189 [38] Huang M, Hirabayashi A, Okumura A, Hirabayashi Y. Japan. Soc. Anal. Chem., 2001, 17(10): 1179–1182 [39] Huang M, Hirabayashi A. Japan. Soc. Anal. Chem., 2002, 18(4): 385–387 [40] Schäfer K C, Balog J, Szaniszló T, Szalay D, Mezey G, Dénes J,
Technol., 2010, 112(4): 434–438 [51] Romão W, Vaz B G, Lalli P M, Bueno M I, Correa D N, Telles V L, de Castro E V, Eberlin M N. J. Forensic Sci., 2012, 57(2): 539–543 [52] Garrett R, Schwab N V, Cabral E C, Henrique B V M, Ifa D R, Eberlin M N, Rezende C M. J. Braz. Chem. Soc., 2014, 25(7): 1172–1177 [53] Schmidt E M, Franco M F, Regino K G, Lehmann E L, Arruda M A, Wf D C R, Borges R, De S W, Eberlin M N, Correa D N. Sci. Justice, 2014, 54(6): 459–464 [54] Lalli P M, Sanvido G B, Garcia J S, Haddad R, Cosso R G, Maia D R J, Zacca J J, Maldaner A O, Eberlin M N. Analyst, 2010, 135(4): 745–750 [55] Cabral E C, Simas R C, Santos V G, Queiroga C L, Da C V, de
Bognár L, Oertel M, Takáts Z. Anal. Chem., 2011, 83(20):
Sá G F, Daroda R J, Eberlin M N. J. Am. Soc. Mass Spectrom.,
7729–7735
2015, 47(1): 1–6
[41] Arao T, Fuke C, Takaesu H, Nakamoto M, Morinaga Y, Miyazaki T. J. Anal. Toxicol., 2002, 26(4): 222–227 [42] Arinobu T, Ishii A, Suzuki O, Hattori H, Kumazawa T, Lee X P, Kojima S, Seno H. Anal. Chim. Acta, 2003, 492(1): 249–252 [43] Alberici L C, Oliveira H C F, Catharino R R, Vercesi A E, Eberlin M N, Alberici R M. Anal. Bioanal. Chem., 2011, 401(5): 1651‒1655 [44] Simas R C, Catharino R R, Cunha I B S, Cabral E C, BarreraArellano D, Eberlin M N, Alberici R M. Analyst, 2010, 135(4): 738–744 [45] Porcari A M, Schwab N V, Alberici R M, Cabral E C, Moraes D R d, Montanher P F, Ferreira C R, Eberlin M N, Visentainer J V. Anal. Methods, 2012, 4(11): 3551–3557
[56] Haddad R, Catharino R R, Marques L A, Eberlin M N. Rapid Commun. Mass Spectrom., 2010, 22(22): 3662–3666 [57] Nørgaard A W, Vaz B G, Lauritsen F R, Eberlin M N. Rapid Commun. Mass Spectrom., 2010, 24(23): 3441–3446 [58] Guo Y L, Wang H Y, Zhao Z X. Curr. Org. Chem., 2011, 15(21): 3734–3749 [59] Amaral P H, Fernandes R, Eberlin M N, Höehr N F. J. Am. Soc. Mass Spectrom, 2015, 46(12): 1269–1273 [60] Simas R C, Barreraarellano D, Eberlin M N, Catharino R R, Souza V, Alberici R M. J. Am. Oil Chem. Soc., 2012, 89(7): 1193–1200 [61] Liu Y Q, Zhang J L, Nie H G, Dong C X, Li Z, Zheng Z G, Bai Y, Liu H W, Zhao J D. Anal. Chem., 2014, 86(14): 7096–7102
[46] Porcari A M, Fernandes G D, Belaz K R A, Schwab N V,
[62] Pereira G G, Alberici R M, Fernandes G D, Cunha I B S,
Santos V G, Alberici R M, Gromova V A, Eberlin M N,
Eberlin M N, Dobarganes M C, Daroda R J, Arellano D. Eur. J.
Lebedev A T, Tata A. Anal. Methods, 2014, 6(8): 2436–2443 [47] Fernandes G D, Moreda W, Barreraarellano D, Ruiz G C N Z, Ferreira P L, Eberlin M N, Alberici R M. J. Braz. Chem. Soc., 2014, 25(9): 1565–1570 [48] Sawaya A C H F, Abdelnur P V, Eberlin M N, Kumazawa S, Mokryeon A, Bang K S, Nagaraja N, Bankova V S, Afrouzan H. Talanta, 2010, 81(1): 100–108 [49] Domin M A, Cody R B. Ambient Ionization Mass Spectrometry, Royal Society Chemistry Press, 2014: 2822–2826 [50] Riccio M F, Saraiva S A, Marques L A, Alberici R, Haddad R, Moller J C, Eberlin M N, Catharino R R. Eur. J. Lipid. Sci.
Lipid Sci. Technol, 2014, 116(8): 952-960 [63] Na N, Shi R X, Long Z, Lu X, Jiang F B, Ouyang J. Talanta, 2014, 128: 366–372 [64] Liu N, Lu X, Yang Y H, Yao C X, Ning B M, He D C, He L, Ouyang J. Talanta, 2015, 143: 240–244 [65] Jansson E T, Dulay M T, Zare R N. Anal. Chem., 2016, 88(12): 6195–6198 [66] Teunissen S F, Fernandes A M A P, Eberlin M N, Alberici R M. TrAC, Trends. Anal. Chem., 2017, 90: 135–141 [67] Alberici R M, Vendramini P H, Eberlin M N. Anal. Methods, 2017, 9(34): 5029–5036
Cite this article as: Chinese J. Anal. Chem., 2019, 47(1): 1–12
REVIEW
Progress of Sonic-Spray Ionization Mass Spectrometry and Its Applications LYU Yue-Guang1,2, BAI Hua1, LI Wen-Tao1, YANG Jing-Kui2, HE Yu-Jian2,*, MA Qiang1,* 1 2
Chinese Academy of Inspection and Quarantine, Beijing 100176, China School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: Sonic-spray ionization (SSI) is a previously developed soft ionization technique which does not need auxiliary methods such as voltage, heating, laser, or corona discharge. Spray ionization can be achieved under normal temperature and atmospheric pressure by inputting high-speed gas coaxial within the capillary. As an atmospheric pressure ion source, this technique was widely used as the interface of liquid chromatography-mass spectrometry at the beginning of its development. Based on the principles of sonic-spray ionization, a variety of easy ambient sonic-spray ionization derived techniques have been developed, which can be used for the in-situ, rapid and real-time analysis of samples with little or without any sample pretreatment. In this paper, the principless and characteristics of sonic-spray ionization were elaborated, and the progress of its application in life sciences, food safety, forensic chemistry, reaction monitoring, and other related fields were summarized. Key Words: Sonic-spray ionization; Easy ambient sonic-spray ionization; Mass spectrometry; Review
1
Introduction
Mass spectrometry (MS) has become one of the most rapidly developing analytical techniques in recent years because of its high sensitivity, rapid analysis, and strong specificity. Essentially, mass spectrometry is a science of preparation, manipulation, and detection of gaseous ions. How to produce gaseous ions conveniently and efficiently is one of the main tasks for the development of mass spectrometry. In the development history of mass spectrometry, every breakthrough in ionization techniques would greatly expand its application. In the last decades, it was believed that gaseous ions could only be obtained from gaseous molecules and could not be transferred and produced directly from solution. The emergence of two kinds of soft ionization techniques, matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI), developed the direct release of gaseous ions from solution, and thus made mass spectrum analysis of proteins, nucleic acids, and other biological macromolecules possible. These methods have
gradually become the two most widely used ionization types in the field of mass spectrometry. Liquid chromatography-mass spectrometry (LC-MS) is an effective method for the separation and analysis of complex organic mixtures, with the separation ability of liquid chromatography and the component identification ability of mass spectrometry. In the beginning of this technique, the atmospheric ion source interface between liquid chromatography and mass spectrometry has been the research focus. Ionization techniques such as atmospheric pressure chemical ionization (APCI)[1], thermo spray (TS)[2], electrospray[3], and atmospheric pressure spray (APS)[4] have been successfully applied to liquid chromatography-mass spectrometry using as ion source interface. Sonic-spray ionization (SSI) is considered to be the simplest ionization, as a type of atmospheric soft ionization technique, which can produce charged droplets at both room temperature and atmospheric pressure without the application of heating or high voltage. Using the action of coaxial gas flow within the capillary tube, the solution in the capillary is sprayed out.
________________________ Received 20 June 2018; accepted 28 October 2018 *Corresponding author. E-mail: [email protected]; [email protected] This work was supported by the National Key Research and Development Program of China (No. 2016YFF0203704). Copyright © 2019, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(18)61132-6
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
When the gas velocity reaches a certain value, charged droplets and gaseous ions are generated in the spray; and the maximum ionization efficiency is achieved when the gas velocity approximates the velocity of sound[5]. Initially, sonic-spray ionization technique was primarily used in peptides[6], proteins[7], and drugs[8] as a new type of atmospheric pressure ion source interface for LC-MS. At the same time, the influence of different ion sources as LC-MS interfaces on the analytical results, such as sonic-spray ionization, atmospheric pressure chemical ionization[9], and electrospray ionization[10,11], was compared and studied. Because the ionization process does not require heating, sonic-spray ionization technique shows unique advantages in the analysis of thermally unstable substances, such as peptides and nerve conduction mediums. Conversely, ambient ionization mass spectrometry (AIMS) has become an important direction of mass spectrometry in recent years. As the name implies, AIMS is a technique for sample ionization, followed by mass spectrometry analysis, in an open atmospheric environment with little or even no sample pretreatment. In earlier stages of AIMS, the technique was often referred to as atmospheric ionization, considering that the ionization environment of both electrospray ionization and atmospheric pressure chemical ionization was at atmospheric pressure. This emphasizes that the ionization environment is not only atmospheric pressure, but also an open environment[12]. In 2004, Takáts et al[13] proposed the first ambient ionization technique, desorption electrospray ionization (DESI), which was considered to be the beginning of ambient ionization mass spectrometry research. In 2005, Cody et al[14] proposed another plasma-based ambient ionization technique called direct analysis in real time (DART). These two kinds of ionization techniques were the earliest invented, and also were the most widely used ambient ionization techniques. Dozens of ionization techniques have been developed since their introduction into the field. In 2006, inspired by desorption electrospray ionization technique, Haddad et al[15] proposed the desorption sonic-spray ionization (DeSSI) technique based on the principles of sonic-spray ionization. Sonic-spray ionization technique became well known and was widely used during this time. Considering its simplicity and convenience, Eberlin et al[16,17] later renamed it as easy ambient sonic-spray ionization (EASI). Among those ambient ionization techniques, the easy ambient sonic-spray ionization does not require high voltage, heating, laser, corona discharge, or any other auxiliary means. It can produce ionization with high-speed airflow, and it is the simplest, most mild, and most easily implemented ionization technique. In this paper, the principles and characteristics of sonic-spray ionization technique and its application as an interface for liquid chromatography-mass spectrometry were described. Simultaneously, the applications of the easy ambient sonic-spray ionization and its derived techniques
based on sonic-spray ionization in relevant fields were also reviewed, and the trends of its development were summarized and prospected.
2
Principles and characteristics of sonic-spray ionization
In 1994, Hirabayashi et al[5] first proposed the new sonic-spray ionization technique. As shown in Fig.1, the methanol-water solution was transported into a fused-silica capillary (0.1-mm i.d., 0.2-mm o.d.) by a syringe pump at flow rate of 30 μL min–1, and the fused-silica capillary was installed in a stainless steel capillary (0.25-mm i.d., 1.7-mm o.d.). The capillary was then inserted into the median hole of an aluminum plate (0.4-mm diameter), with the capillary tip exposed 0.6 mm beyond the orifice of the ion source. The distance between the fused-silica capillary tip of the ion source and the sampling orifice of the mass spectrometer was 5 mm. High-speed nitrogen flowed out from the orifice and the flow rate of nitrogen in the standard state (20 °C, 1 atm) was determined by a mass flow controller. Sonic-spray ionization does not require high voltage or heating. Ionization of sample solutions can be achieved simply by applying a high-speed nitrogen gas coaxial within the capillary, which is considered to be the simplest ionization technique. The mass spectra of lysine, dopamine, and gramicidin S were obtained using this method[18]. Singly and doubly charged protonated molecules and fragment ions with low abundance were detected. It was also found that the response intensity of the gas phase ion signal was directly related to the coaxial nitrogen gas-flow rate. The relationship between the detected intensity of the doubly protonated gramicidin S and the gas-flow rate of nitrogen was tested. As shown in Fig.2, the experimental results showed that: (1) the ion signal of gramicidin S was detected when the gas-flow rate of nitrogen reached 1.3 L min–1; (2) the detected signal intensity increased with the increase of gas-flow rate, reaching the maximum at 3.0 L min–1; and (3) after 3.0 L min–1, the increase of nitrogen mass velocity led to the decrease of ion signal intensity. At the same time, with the increase of gasflow rate, the Maher number of gas flow was also increasing. The Schlieren method was used to create the flow state of
Fig.1 Schematic of sonic-spray ionization device[5]
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
Fig.2
Ion current and mass analyzed ion intensity of doubly pronated gramicidin S molecule as a function of gas flow rate[18]
nitrogen at different gas-flow rates. The flow state was subsonic at 2.2 L min–1 and supersonic at 4.0 L min–1. It was concluded that the method had the highest ionization efficiency in the transonic flow state. The Maher number of the gas flow was approximately 1, which was the sonic velocity, and therefore was named sonic-spray ionization. The ion current in Fig.2 represents the sum of charged droplets and gas phase ions in the experiment. It reached the maximum value when the nitrogen gas-flow rate was 3 L min–1, at which the ion signal intensity reached the maximum simultaneously. However, it is worth noting that when the nitrogen gas-flow rate was 1 L min–1, the charged droplets generated, but no gas phase ions were detected at this time. When the nitrogen mass velocity was below 0.8 L min–1, the droplets generated by the spray had not been charged yet. This result demonstrated that the generation of charged droplets and gas phase ions was directly related to the size of these droplets. In subsonic flow, the droplet diameter continues to decrease as the nitrogen flow rate increases. Hirabayashi et al[18] compared the mass spectra of adrenaline, noradrenaline, dopamine, and other substances obtained by sonic-spray ionization with traditional electrospray ionization, and found that the spectra obtained from the two methods were very similar. However, in the sonic-spray ionization mode, the tested substance showed a tendency to be more prone to be multiply charged[19]. In traditional electrospray ionization method[3], a Taylor cone was formed at the capillary tip under a high electric field action. When the solution of the Taylor cone reached the critical point, or Rayleigh limit, the Coulomb repulsion force of the surface charge was comparable to the surface tension of the solution, and the charged droplets ejected from the tip of the Taylor cone[20]. Under the action of auxiliary nebulizer gas, the solvent evaporates and the Coulomb explosion occurred, which finally formed gaseous ions. It is noteworthy that the flow rate of nebulizer gas for the electrospray process is much lower than the flow rate in sonic-spray ionization. When the flow rate reaches or exceeds
100 m s–1, the stable Taylor cone cannot be formed. As a novel ionization technique, the mechanism research for sonic-spray ionization is particularly important. Hirabayashi et al[19] compared three charging models of friction electrification, electrical double-layer and statistical charging to explore the mechanisms of sonic-spray ionization. For the friction electrification model, different capillary materials theoretically had different electrochemical potentials. The author compared the spray ionization effect of glass capillary and stainless steel capillary with the same geometry, and obtained the same experimental results. Therefore, the charged droplet formation could not be ascribed to friction electrification. Contrarily, in the solution close to the surface of the capillary wall, an electric double layer was formed, in which the ion concentration was not uniform. The wall of the capillary became negatively charged, and the proton concentration near the wall grew, producing charged droplets during the rapid evaporation of the solvent[21]. Based on this model, deactivated fused-silica capillaries could seriously reduce the electric double-layer effect. The authors compared the exposed and deactivated fused-silica capillaries with the same geometric size, and found that the experimental results were the same, so the electric double-layer charging mechanism was denied. The statistical charging model proposed by Dodd[22] is often used to explain the charging process of thermal spray ionization technique[23]. The solution evaporates rapidly into small droplets of equal volume in a very short time. In most droplets, the numbers of positive and negative ions are equal, and thus, these droplets are neutral. However, in some droplets, the number of positive ions is higher than negative ions, which subsequently causes them to be positively charged. Due to the microscopic fluctuation of the ionic concentration in the solution, other droplets are negatively charged. According to this model, the average charge of a droplet <|q|> is proportional to the square root of the ionic concentration N in the solution. (1) On the basis of this model and relationship, the detected ion current should increase with the increasing ion concentration of solution. However, it was found that the detected ion current decreased with the increasing concentration of ammonium acetate in solution. The same results were obtained when using trifluoroacetic acid. Thus, the sonic-spray ionization is not a statistical charging mechanism. Furthermore, Hirabayashi et al[18,19] found that the ion current detected was not only related to the gas-flow rate, but also directly related to the gas medium. At a gas-flow rate of 3 L min–1, the ion currents for nitrogen- and oxygen-based mediums were nearly equal and were three times higher than argon. This was because different gas species led to different surface potentials at the gas-liquid interface. Under the influence of surface potential, the distribution of positive and negative ions on the surface of the solution would become
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
uneven. For example, for an aqueous solution with pH = 5, an electrical double layer about 100 nm thick formed near the surface, and a negatively charged surface layer formed with a positively charged layer underneath[18,19]. In the process of sonic spray, there is an electrical double layer at the interface between the gas and the solution affected by the surface potential. Under the strong impact of high-speed gas flow, tiny droplets will be "peeled off" from the surface of the solution and become charged due to the imbalance of the static charge distribution. When the number of positive ions is higher than that of negative ions, some droplets are positively charged, and others are negatively charged accordingly. This mechanism was also confirmed by experimental results which demonstrated that higher concentrations of ammonium acetate and trifluoroacetic acid in the aforementioned solution resulted in a smaller detected ion current. With an increasing ion concentration in the solution, the concentration of ion pairs gradually increased, and the thickness of the electrical double layer formed on the surface of the solution decreased accordingly. This resulted in the thickness of the diffusion electrical double layer being inversely proportional to the square root of the ion concentration N in the solution[24]. Therefore, with the increase of ion concentration, the changes of positive and negative ion concentration decreased in small droplets. In summary, the most probable mechanism of sonic-spray ionization is the charge separation occurring in the droplets under the action of surface potential. With the sonic gas flow, a droplet undergoes fission and the charge concentration fluctuates, thus producing smaller charged droplets. Subsequently, the charged droplets experience a process similar to traditional electrospray, such as solvent evaporation and Coulomb explosion. Finally, the gaseous ions formed by the charge residue model are detected by mass spectrometry. Based on the sonic-spray ionization physical device and charging mechanism, the main factors affecting the ionization efficiency are gas backpressure, solution velocity, and physical size of the device[25]. The sensitivity and application of sonic-spray ionization can be significantly improved by optimizing these parameters. The mechanism of sonic-spray ionization makes it possible to produce both positive and negative ions at the same time, which is useful for the simultaneous, high-throughput, and rapid mass spectrometry analysis. Table 1 compares the differences and similarities between sonic-spray ionization and other ionization techniques. It can be seen from the table that sonic-spray
ionization is the simplest and most universally employed ionization technique. Additionally, due to its absence of high voltage/current and heating, the gaseous ions produced by sonic-spray ionization have lower internal energy and can retain weak non-covalent interactions between molecules. Therefore, this ionization technique is suitable for the study on weak molecules and inter-molecular forces, providing a new direction for the ionization and analysis of biological molecules. In addition, for biomacromolecule analysis, sonic-spray ionization can provide a wider range of charge distribution than conventional electrospray ionization.
3 3.1
Sonic-spray ionization derived techniques Easy ambient sonic-spray ionization
The desorption electrospray ionization technique developed by Takáts in 2004 opened the door for ambient mass spectrometry analysis[13]. In 2006, Haddad et al[15] proposed that the charged droplets produced by sonic spray could be used in an open environment for direct desorption and ionization of analytes on the surface of tablets for mass spectrometry analysis, which was called desorption sonicspray ionization technique. In the experiment, the spray solvent was 0.01% formic acid-methanol solution (1:1, V/V) at a flow rate of 20 μL min–1. The nebulizer backpressure was 3 MPa, the curtain gas pressure was 0.5 MPa, the decluster voltage was 100 V, both the distances from the spray tip to the tablet and the MS inlet were 2 mm, and the angle between capillary and tablet and MS inlet was about 30°[15]. The experimental installation schematic is shown in Fig.3. Direct analysis of tablets showed that desorption sonic-spray ionization had the same sensitivity as desorption electrospray ionization. The desorption sonic-spray ionization mass spectrum was nevertheless always cleaner, due to the much lower abundances of solvent cluster ions. The amount of the protonated analyte molecule was sufficient for the acquisition of its tandem mass spectrum with adequate signal-to-noise ratio for structural investigation. Furthermore, the higher linear velocity of the spray gas helped to immerse the solvent into the sample matrix, thus providing more homogenous sampling and a more stable and long-lasting ion signal. With these characteristics, desorption sonic-spray ionization technique can be used for analysis of low molecular weight components in complex matrix samples. Compared with desorption
Table 1 Comparison of sonic-spray ionization and other ionization techniques Ion source
Soft ionization
Multiple charges
Polarity
Gas
Voltage/current
Heat
Reference
ESI APCI APS TS SSI
Yes Yes Yes Yes Yes
Yes No No Yes Yes
Moderate/Strong Weak Moderate/Strong Moderate/Strong Moderate/Strong
Yes Yes Yes No Yes
Yes Yes Yes Yes No
Yes Yes Yes Yes No
[3] [1] [4] [2] [5]
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
Fig.3 Schematic of desorption sonic-spray ionization analysis for ingredient in tablets[15]
electrospray ionization, desorption sonic-spray ionization technique does not require high voltage or heating on the spray capillary, providing a more friendly environment for ambient mass spectrometry analysis, which is particularly important for an in-vivo analysis and facilitating in-situ tissue analysis during surgical processes. The desorption sonic-spray ionization is the most simple and convenient ionization technique[26] because it does not need heating, high voltage, laser, ultraviolet light, or special gas assistance. Haddad et al[16] renamed it as easy ambient sonic-spray ionization (EASI) to highlight the simple fabrication and operation of this technique in an ambient environment (pronounced as “easy”). The entire analysis process contains three steps: (1) the solvent is ionized by using a sonic-spray; (2) small bipolar (positive and negative) droplets bombard the sample surface and desorb the analytes; and (3) the charged analytes are analyzed by MS eventually after proton or cation (usually Na+, K+) transfer reactions. 3.2
Venturi-easy ambient sonic-spray ionization
To further simplify the device, Santos et al[27] proposed Venturi easy ambient sonic-spray ionization (V-EASI) in 2011. They incorporated Venturi self-pumping and eliminated electrical pumping. Compared with other techniques, the average electrified capacity of bipolar charged droplets produced by V-EASI was lower, and the ionization selectivity and detection sensitivity were improved. The main body of the device was a simple Swagelok T-element with appropriate
ferrules. The nebulizer gas flowed through a 53-mm long stainless steel tube (400-μm i.d., 728-μm o.d.), and its internal coaxial fused-silica capillary (100-μm i.d., 125-μm o.d.) was used to transport solution and perform tip spray. Under the optimized conditions, the best results were observed for compressed nitrogen at ca. 10 bar and a flow of 3.5 L min–1, a stable solution flow rate of ca. 20 μL min–1 for methanol, and 10–15 μL min–1 for aqueous solution, with the limit of detection of 0.2 ng mL–1 for cocaine in acidified methanol. V-EASI is also a very soft ionization process, so the fragmentation is minimized or, most often completely eliminated. Compared with the electrospray ionization technique, V-EASI benefits from reduced chemical noise and therefore improved signal-to-noise ratios, despite the fact that the absolute intensity of analyte ions seems to be lower than electrospray ionization. The mechanism of sonic-spray ionization enables V-EASI technique to produce positive and negative ions simultaneously. The liquid sample and solid surface can be directly analyzed, and its application fields are broadened. Excellent performance in the detection of biomolecules in pure aqueous solution shows that the technique is suitable for the analysis of proteins and polypeptides in biological solution. 3.3
Spartan-easy ambient sonic-spray ionization
Based on V-EASI, Schwab et al[28] developed the Spartaneasy ambient sonic-spray ionization (S-EASI) to further simplify the device. Further simplification was attained by using a can of compressed air, which was inexpensive and readily available. Portable compressed air replaced traditional nitrogen cylinders and similar gas regulators. Other components were also cheap and easy to be obtained, such as a surgical 2-way catheter and a hypodermic needle, which provided a self-assembling and portable device. Cocaine, glyphosate, polyethylene glycol 600, and polypeptide analyses were carried out to prove that this device retained all the advantages of V-EASI. This simple self-assembled ion source was considered to be the simplest and cheapest ambient ion source for direct analysis of both liquid and solid samples. Figure 4 shows the iterative development process of easy ambient sonic-spray ionization devices.
Fig.4 Schematic of easy ambient sonic-spray ionization (EASI), Venturi-easy ambient sonic-spray ionization (V-EASI) and Spartan-easy ambient sonic-spray ionization (S-EASI)[29]
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
4
4.1
Combination and applications of sonic-spray ionization and its derived methods with other techniques Combination with liquid chromatography
When sonic-spray ionization was first put forward as a new type of atmospheric ionization method, researchers applied it as an ion source interface for liquid chromatography tandem mass spectrometry analysis. Direct coupling of semi-micro liquid chromatography and sonic-spray ionization mass spectrometry was demonstrated by Hirabayashi et al[6] in 1996, and was shown to be a very powerful technique for pesticide analysis. The relationship among the sampling cone temperature, nitrogen flow rate, and MS signal response was experimentally studied. Direct coupling of semi-micro liquid chromatography and sonic-spray ionization mass spectrometry can be realized by increasing the cone temperature to 120 °C, and then controlling the solution flow rate at 100 μL min–1. Good linearity was obtained for simazine in the range of 1 pmol‒1 nmol, and the limit of detection was approximately 300 fmol. Björkman et al[30] optimized the conditions of liquid chromatography in tandem with sonic-spray ionization mass spectrometry (Fig.5). Using tolterodine as a model drug substance, the influence of different parameter settings was evaluated using factorial design. A comparison between sonic-spray ionization and electrospray ionization was made for tolterodine, tolterodine metabolites, and a set of steroids. The results showed slightly poorer repeatability and broader peaks for tolterodine analysis due to the higher velocity of sonic-spray ionization. However, the sensitivity was two times higher than electrospray ionization. The analysis result of pregnenolone showed that sonic-spray ionization had less water loss, which was most likely due to less energy being transferred to the analyte upon ionization. Similarly, Dams et al[31] evaluated the performance of this ion source as an interface for liquid chromatography-mass spectrometry in comparison with the traditional electrospray ionization. The effects of volatile acids, organic modifiers, and buffer systems in the eluent on the ionization efficiency of both interfaces were described. The experimental results showed that organic modifiers could improve the ionization efficiency of electrospray and sonic spray, while volatile acids or buffers gave a significant ion suppression effect. Furthermore, the authors applied the sonic-spray interface for the simultaneous determination of seven prime opium alkaloids in heroin impurity profiling. By using monolithic silica column and gradient elution system at 5 mL min–1 flow rate, the seven substances were well separated in 5 min. By means of a post-column split of ca. 1/20, a coupling between the fast liquid chromatograph system and the mass spectrometer was achieved.
Fig.5 Schematic of sonic-spray ionization interface[30]
4.2
Combination with microfluidic chips
In 2007, Pól et al[32] proposed the first microchip version of sonic-spray ionization as an atmospheric pressure ionization source for mass spectrometry analysis. As shown in Fig.6, mild and highly effective ionization of polar compounds in positive and negative modes were achieved by applying highspeed nebulizer gas to microfluidic chips. The microchip is composed of a silicon wafer with an etched inlet for nebulizer gas, a housing for fused silica capillary, a vaporizing channel, and an exit nozzle. Nebulizer gas enters the vaporizing channel where it mixes with the sample that is introduced via fused silica capillary, and thus produces sonic-spray ionization. The results showed that the position of the chip corresponding to the mass spectrometer inlet, gas, and sample velocity had great influence on the detection results. Tetra-Nbutylammonium, verapamil, testosterone, angiotensin I, and other substances were analyzed, and the MS signals were found to be stable. The dynamic linear range was equivalent to electrospray ionization, with consistent reproducibility and detection limits from 15 nM to 4 μM. In 2016, Yu et al[33] proposed a novel microfluidic self-aspiration sonic-spray ionization chip. The proposed ionization chip was fabricated using a three-layer soft lithography technique without the need to fabricate the spraying tips. The microchip structure was optimized by computer simulation and then verified through experiments. The experimental results indicated that the ionization of a sample solution could be achieved through
Fig.6 Schematic of microchip sonic-spray ionization arrangement. (A) Different angles of the sonic spray versus MS orifice plate were tested: 0°, 45° and 90°; (B) Optimum positions of the microchip sonic spray toward the MS orifice in distance (x) and height (y) both of 4 mm[32]
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
spraying nebulizer gas, which resulted in a much lower pressure. Additionally, a dual-channel self-aspiration sonic-spray ionization chip was developed, and the signal intensity was enhanced through applying the same sample into both channels. When a different solution was in the channel, the ion suppression effect would be reduced. This microfluidic chip also effectively improved the integration of ionization and simplified the overall operational process. 4.3
Combination with thin-layer chromatography
Among many ionization techniques, EASI is the simplest, most refined technique, and is also most easily implemented. Another simple and inexpensive method is thin-layer chromatography (TLC), which is one of the most easily performed chromatographic separation techniques. Haddad et al[16] used the combination technique of EASI and TLC for the analysis. The mass spectra of the separated spots on the TLC plate were directly collected and analyzed by EASI. In the study, as shown in Fig.7, the spray solvent was 0.01% formic acid-methanol solution (1:1, V/V). The flow rate was 20 μL min–1, nebulizer gas backpressure was 3 MPa, and curtain gas pressure was 0.5 MPa. The declustering voltage was 100 V, tip-TLC plate distance was ca. 2 mm, and capillary-TLC plate-entrance angle was ca. 30°. Using this method, nitrogen containing compounds of moderate polarity, drugs, and vegetable oils were analyzed and the reactions were monitored. The combination of these two techniques integrated the advantages of high efficiency of chromatography with high sensitivity of mass spectrometry. Eberlin et al[34] identified the biodiesel (B100) and biodiesel mineral diesel mixture (BX) using the combination of high-performance TLC and EASI mass spectrometry. Oil components were separated on highperformance TLC plates, and the separated spots were directly analyzed by EASI. The results showed characteristic signals of fatty acid methyl ester sodium adducts in both samples of B100 and BX, which could be used to identify biodiesel samples. The mass spectrum of mineral diesel oil contained a series of protonated alkyl pyridines, which were used as the characteristic qualitative identification standards for mineral diesel oil. The spectrum for admixture oil spots was characterized by triglycerides sodium adducts. This method was confirmed to be suitable for qualitative identification and quality control for biodiesel and mineral diesel. 4.4
Combination with other techniques
Haddad et al[17] demonstrated that permeable cellulose dialysis membrane could be used as an interface for the direct analysis of solution constituents via EASI and membrane interface mass spectrometry. This combination promoted droplet intake of the analytes from the external surface of the membrane, where the analytes had selectively permeated for
Fig.7 Schematic of TLC-EASI-MS analysis[16]
proper mass spectrometry characterization and quantitation. The results showed that by using the cellulose dialysis membrane, direct analysis of aqueous solutions of common drugs could be well performed. The interface of the cellulose membrane was composed of a small cylindrical Teflon container connected to two silicon tubes, and the top of the container was sealed with a cellulose membrane. The drug solution then circulated through the membrane system at a flow rate of 20 mL min–1 by a syringe pump. The spray solvent was 0.01% formic acid-methanol solution (1:1, V/V). The angle between the spray tip and the membrane was 30°. The resulting charged droplets bombarded the external membrane surface and the analyte was desorbed and ionized for MS analysis. This method could also be used for continuous in-situ monitoring and analysis of polar and semipolar compounds in environmental fluids. Because permeable membranes could block solid particles, cellulose dialysis membrane could also be used for online monitoring of fermentation and biotransformations. These dialysis membranes were also used to monitor hemodialysis processes or to construct very small probes, which might permit in-vivo analysis. Figueiredo et al[35] developed a molecularly imprinted polymer as an interface for the selection and collection of the analytes, which was applied to EASI-MS analysis. First, molecularly imprinted polymers were used to absorb chlorpromazine, triflupromazine, and other compounds found in urine from an extraction/washing tank. After washing and removing other impurities, the molecularly imprinted polymer probe was placed at a suitable position. The spray solvent was acetic acid-methanol solution (1:9, V/V). During the detection process, the probe area (ca. 40 mm2) was exposed to the spray solvent for 1 min through artificially moving the probe. The quantitative analysis was performed by monitoring the quasi-molecular ions. The experimental results showed that the molecularly imprinted polymer could be used as a chemical selective interface for the detection of chlorpromazine in urine by EASI-MS. The method was proven to be rapid and highly sensitive and selective. It also demonstrated other attractive features such as high molecularly imprinted polymer probe lifetime, direct analysis without pre-separation, and ease of extraction/washing and analysis procedures. These characteristics enabled the method
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
to distinctly reduce or nearly eliminate ion suppression effects, which makes this method highly suitable for complex matrix sample analysis, including biologically and environmentally important fluids.
5
Applications of sonic-spray ionization and its derived techniques
Since its introduction in the 1990s, sonic-spray ionization has been applied in many fields such as life sciences, food safety, forensic chemistry, and reaction monitoring, as a universal atmospheric ionization technique. The research and applications based on sonic-spray ionization technique are listed in Table 2. 5.1
Life sciences
Life sciences is the most active field within the 21st century. Most of the life systems are complex. Therefore, increasingly strict requirements are being raised for analytical techniques. Sonic-spray ionization and its derived techniques have been widely used in the field of life sciences[36–40]. Arao et al[41] continuously identified oleandrin and derivatives deacetylated triglycerides, oleandrigenin, and podophyllogenin in blood using a liquid chromatography tandem mass spectrometry system equipped with a sonic-spray ionization interface. Satisfactory linearity of working curves was obtained for oleandrin, deacetylated triglycerides, and oleandrigenin within the range of 5–100 ng mL–1. The limits of detection for oleagin and deacetyltriglyceride in blood were 2 ng mL–1, and 3 ng mL–1 for podophyllogenin. Mortier et al[8] established an analytical method for the determination of para-methoxyamphetamine, 3,4-methylenedioxyamphetamine, and 3,4-methylenedioxymethamphetamine in biological samples by sonic-spray ionization coupled with liquid chromatographyion trap mass spectrometry. This method was suitable for the analysis of whole blood, urine, and tissue samples. The intra-day and inter-day precision of this method was less than 17.5%, and the accuracy was less than 16.2%. The standard curves of different substrates were also established, and the linear correlation coefficients were all greater than 0.995. Arinobu et al[42] established a rapid identification method of pentazocine in human blood using liquid chromatography-
mass spectrometry equipped with sonic-spray ionization source. Dextromethorphan was used as an internal standard to verify the reliability of the method. The limit of detection was 19.5 ng mL–1 and required only 4 min to detect pentazocine in a human blood sample. Alberici et al[43] applied EASI for direct analysis of lipid complexes and explored the difference of lipid samples between mice with hypertriglyceridemia and normal triglycerin. The MS signals of phosphatidylcholine and triacylglycerol were well observed in the positive ion mode, and the signals of phosphatidylethanolamine and phosphatidylinositol were observed in the negative ion mode. The free fatty acids in lipids were then analyzed. It was found that the phospholipid choline and triacylglycerol complexes in mice with hypertriglyceridemia had higher content of oleic acid, and the abundance of arachidonic acid in phosphatidylinositol was also higher, which revealed that there was a distribution of free fatty acids in mice. 5.2
Food safety
Simas et al[44] applied EASI for a fast and reliable analysis of vegetable oils. The main ingredients of vegetable oil were desorbed and ionized efficiently in an open environment. The proposed method was so simple and easy to operate that only one drop of vegetable oil on the inert surface was needed, without any pretreatment. The detected triacylglycerol was sodium adduct in positive ion mode, and free fatty acid was detected as in the deprotonated form in the negative ionization mode. EASI was proved to be a soft ionization technique in that the triacylglycerol molecular ion did not undertake further fragmentation. Therefore, the contents of both diacylglycerol and monoglyceride were determined directly. Based on these results, certification and quality control of vegetable oil were carried out, and adulteration, acidity, oxidation, and hydrolysis level of vegetable oil could be adequately explored. Porcari et al[45] combined thermal imprinting with EASI technique. Only a very small amount of solvent was needed to transfer triacylglycerol compounds from meat, fat, fish, and other food onto paper. Then, EASI was applied to the rapid identification and analysis of triglyceride. The entire analytical process took only a few minutes and consumed a very small amount of sample and solvent. The experimental results were in good agreement with those obtained by gas chromatography and
Table 2 Application of sonic-spray ionization Sample
Analyte
Method
Blood Biological samples Tablet Diesel oil Vegetable oil Fish and meat Ink Tablet
Oleandrin Amphetamine Diazepam FAME TAG TAG Basic Violet 3 Cocaine
LC-SSI-MS LC-SSI-MS DeSSI-MS TLC-EASI-MS EASI-MS TI-EASI-MS EASI-MS V-EASI-MS
Liner range (ng mL–1)
LOD (ng mL–1)
Reference
5–100 10–1000 / / / / / /
2 5 / / / / / /
[35] [8] [15] [29] [38] [39] [48] [23]
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
matrix-assisted laser desorption ionization-mass spectrometry. Thermally imprinted paper of samples could be collected on-site and sent to the laboratory for testing. This technique was applied for the quality control of Russian caviar[46] and Iberian ham[47]. Sawaya et al[48] collected a series of chemical information from 49 different kinds of propolis alcohol extracts of international origins using EASI technique. Principal component analysis of the data obtained from a plant resin could be used to infer the geographical origin of propolis. 5.3
Forensic chemistry
EASI is widely used in forensic chemistry[49] for sample identification and analysis, due to its characteristics of simplicity of use and fast operation[44,48,50‒53]. Using EASI technique, Lalli et al[54] established a fingerprint identification and writing time inference method for ink on the surface of ballpoint pen writing paper. Mass spectrometric information obtained from ink strokes showed that different brands of ink had different chemical characteristics. Basic Violet 3, Basic Blue 26, Basic Blue 7, Basic Violet 10, nickel phthalocyanine, and 1,3-dimethyl-1,3-ditolylguanidine dyes were found in the inks. Different colors of ink contained different types of dyes, and even different brands of the same color ink had different contents of various dyes. Therefore, the brand of ink could be inferred simply based on the obtained fingerprint information. Additionally, the age of ink could be inferred through chemical analysis. For example, alkaline violet 3 showed a series of degradations took place over time, and the abundance of catabolite changed linearly. Therefore, it was used as a "chemical clock" to indicate the age of ink, providing a powerful technical means for the court to identify whether the signature was forged. In another application, Carbral et al[55] utilized V-EASI technique to identify tree species. Mahogany, an endangered tropical plant, contains a typical phytochemical marker of aloin, which can be directly detected on wood surface or methanol extract from sawdust using the V-EASI method. The experimental results showed that the illegal logging trade of mahogany and other endangered trees could be controlled by comparing the chemical profiles of mahogany and other woods, and certification standards were established through this method. Furthermore, fake perfume or perfume of poor quality was a common international phenomenon, causing huge economic losses in industry, as well as posing potential health risks. Haddad et al[56] proposed an identification method for perfume authentication, which could be completed almost instantaneously. In the experiment, the perfume sample was directly sprayed onto a glass rod or a piece of paper and let dried for a few seconds, which was then followed by the EASI-MS analysis. Most of the polar compounds in perfume were detected within 1 min. Unique chemical characteristics of genuine perfume were provided
with good reproducibility. The proposed method can be used to authenticate commercial perfume samples. 5.4
Reaction monitoring
With the characteristics for in-situ and real-time mass spectral analysis, EASI shows great potential in the field of reaction monitoring[57–64], which is important for optimizing reactant ratios, controlling reaction processes, and inferring reaction mechanisms. Na et al[63] applied V-EASI technique to monitor the self-assembly process of nucleic acid bases. Potassium ions were introduced into the reaction system via a syringe, and the cluster ions were then produced by self-assembly of nucleic acid bases. This process could be observed through V-EASI. The schematic of the device is shown in Fig.8. Many cluster peaks for nucleic acid bases were detected during the monitoring process, which proved that the cations played an important role in the self-assembly process of nucleic acid bases. In addition, the proposed method, as a soft ionization technique, was suitable for studying the non-covalent interactions in the self-assembly process. Jansson et al[65] developed a method for monitoring the surface reaction within a narrow space. Two fused-silica capillaries were immersed into the solution, positioned in close proximity to each other, and the functionalized surface created a laminar flow junction with a resulting reaction volume of ca. 5 pL. The reagents were then delivered to the reaction surface through a fused-silica capillary. The other capillary was connected to a V-EASI source. The sampling velocity was slightly higher than that of the feeding capillary. The combined effects of inflow and outflow maintained a stable chemical microenvironment, where the rate of the advective transport overcame the diffusion. Acetylcholinesterase was fixed on the surface of organo-siloxane polymer by electrostatic interaction. Then, acetylcholinesterase hydrolyzed acetylcholine to form choline, which was monitored in real-time. Different acetylcholine concentrations, fused-silica capillary shapes, and flow rates were thoroughly optimized. The experimental results showed that the conversion rate increased with the increasing acetylcholine concentration. This experiment proved that the proposed method was suitable for real-time reaction monitoring.
6
Conclusions and prospects
Sonic-spray ionization technique has been considered as the simplest and most convenient atmospheric ambient ionization method. It has been successfully applied to many fields since its early application as a novel ion source interface for liquid chromatography tandem mass spectrometry, and later developed easy ambient sonic-spray ionization (EASI). During this period, the ionization device has consistently been optimized and simplified, demonstrating its preserving characteristics
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
[8]
Mortier K A, Dams R, Lambert W E, Letter E A D, Calenbergh S V, Leenheer A P D. Rapid Commun. Mass Spectrom., 2010, 16(9): 865–870
[9]
Arinobu T, Hattori H, Seno H, Suzuki O. J. Am. Soc. Mass Spectr., 2002, 13(3): 204–208
[10] Hirabayashi A. Rapid Commun. Mass Spectrom., 2003, 17(5): 391–394 [11] Kovács Z, Dinya Z, Antus S. J. Chromatogr. Sci., 2004, 42(3): 125–129 [12] Cooks R G, Ouyang Z, Takáts Z, Wiseman J M. Science, 2006, 311(5767): 1566–1570 [13] Takáts Z, Wiseman J M, Gologan B, Cooks R G. Science, 2004, Fig.8
Schematic of Venturi-easy ambient sonic-spray ionizationmass spectrometry setup used for real-time measurement of self-assembly reactions[63]
of simplicity, speed, and portability. Even before commercialized, the sonic-spray ionization had gradually become one of the most popular ambient ionization techniques. Teunissen et al[66] summarized the development of easy ambient sonic-spray ionization in the past ten years. Based on the perspective of ambient ionization technique, the paper focused on the simplification of the device from EASI to V-EASI, and then to S-EASI. Furthermore, a summary was made for its many applications in the fields of medicine, food, oil, fuel, forensic chemistry, and biological analysis. The present paper began with the initially proposed sonic-spray ionization technique and expounded on its basic principles and development process. In terms of device development and applications, the present paper covered both its initial use as an interface for liquid chromatography/capillary electrophoresis tandem mass spectrometry, and its subsequent development as an ambient ionization technique, as well as combinations with microfluidic chips, thin-layer chromatography, and other techniques. The principless and applications were then systematically summarized. Now the sonic-spray ionization technique has been emerging in the field of mass spectrometry imaging[67]. It is estimated that sonic-spray ionization will be expanded to more fields in future.
References
306(5695): 471–473 [14] Cody R B, Laramée J A, Durst H D. Anal. Chem., 2005, 77(8): 2297–2302 [15] Haddad R, Sparrapan R, Eberlin M N. Rapid Commun. Mass Spectrom., 2010, 20(19): 2901–2905 [16] Haddad R, Milagre H M, Catharino R R, Eberlin M N. Anal. Chem., 2008, 80(8): 2744–2750 [17] Haddad R, Sparrapan R, Kotiaho T, Eberlin M N. Anal. Chem., 2008, 80(3): 898–903 [18] Hirabayashi A, Sakairi M, Koizumi H. Anal. Chem., 1995, 67(17): 2878–2882 [19] Hirabayashi A, Hirabayashi Y, Sakairi M, Koizumi H. Rapid Commun. Mass Spectrom., 2015, 10(13): 1703–1705 [20] Dary C B L, Inés A V M, Elisa L C, Dazuly M. J. Chem. Phys., 1968, 49(5): 2240–2249 [21] Kulys L. Bioelectrochem. Bioenerg., 1993, 31(3): 330–331 [22] Dodd E E. J. Appl. Phys., 1953, 24(1): 73–80 [23] Katta V, Rockwood A L, Vestal M L. Int. J. Mass Spectrom. Ion Processes, 1991, 103(2-3): 129–148 [24] Whitehouse D R. Anal. Chim. Acta, 1966, 36(1): 412–412 [25] Ozdemir A, Lin J L, Wang Y, Chen C H. RSC Adv., 2014, 4(106): 61290–61297 [26] Alberici R M, Simas R C, Sanvido G B, Romão W, Lalli P M, Benassi M, Cunha I B S, Eberlin M N. Anal. Bioanal. Chem., 2010, 398(1): 265–271 [27] Santos V G, Regiani T, Dias F F, Romão W, Jara J L, Klitzke C F, Coelho F, Eberlin M N. Anal. Chem., 2011, 83(4): 1375–1380 [28] Schwab N V, Porcari A M, Coelho M B, Schmidt E M, Jara J L, Visentainer J V, Eberlin M N. Analyst, 2012, 137(11):
[1]
Kambara H. Anal. Chem., 1982, 54(1): 187–195
[2]
Blakley C R, Vestal M L. Anal. Chem., 1983, 55(4): 750–754
[3]
Yamashita M, Fenn J B. J. Phys. Chem., 1984, 88(20): 4451–4459
[4]
Sakairi M, Kambara H. Anal. Chem., 1989, 61(10): 1159–1164
[5]
Hirabayashi A, Sakairi M, Koizumi H. Anal. Chem., 1994, 66(24): 4557–4559
[6] [7]
Hirabayashi Y, Takada Y, Hirabayashi A, Sakairi M, Koizumi H.
2537–2540 [29] Schwab N V, Eberlin M N. Drug Test. Anal., 2013, 5(3): 137–144 [30] Björkman H T, Edlund P O, Jacobsson S P. Anal. Chim. Acta, 2002, 468(2): 263–274 [31] Dams R, Benijts T, Günther W, Lambert W, De L A. Anal. Chem., 2002, 74(13): 3206–3212 [32] Pól J, Kauppila T J, Haapala M, Saarela V, Franssila S, Ketola
Rapid Commun. Mass Spectrom., 1996, 10(15): 1891–1893
R A, Kotiaho T, Kostiainen R. Anal. Chem., 2007, 79(9):
Hirabayashi Y, Hirabayashi A, Takada Y, Sakairi M, Koizumi H.
3519–3523
Anal. Chem., 1998, 70(9): 1882–1884
[33] Yu C L, Qian X, Chen Y, Yu Q, Ni K, Wang X H. RSC Adv.,
LYU Yue-Guang et al. / Chinese Journal of Analytical Chemistry, 2019, 47(1): 1–12
2016, 6(55): 50180–50189 [34] Eberlin L S, Abdelnur P V, Passero A, Sa G F, Daroda R J, Souza V, Eberlin M N. Analyst, 2009, 134(8): 1652–1657 [35] Figueiredo E C, Sanvido G B, Arruda M A, Eberlin M N. Analyst, 2010, 135(4): 726–730 [36] Shinozuka T, Terada M, Tanaka E. Forensic. Sci. Int., 2006, 162(1): 108–112 [37] Gréen H, Vretenbrant K, Norlander B, Peterson C. Rapid Commun. Mass Spectrom., 2010, 20(14): 2183–2189 [38] Huang M, Hirabayashi A, Okumura A, Hirabayashi Y. Japan. Soc. Anal. Chem., 2001, 17(10): 1179–1182 [39] Huang M, Hirabayashi A. Japan. Soc. Anal. Chem., 2002, 18(4): 385–387 [40] Schäfer K C, Balog J, Szaniszló T, Szalay D, Mezey G, Dénes J,
Technol., 2010, 112(4): 434–438 [51] Romão W, Vaz B G, Lalli P M, Bueno M I, Correa D N, Telles V L, de Castro E V, Eberlin M N. J. Forensic Sci., 2012, 57(2): 539–543 [52] Garrett R, Schwab N V, Cabral E C, Henrique B V M, Ifa D R, Eberlin M N, Rezende C M. J. Braz. Chem. Soc., 2014, 25(7): 1172–1177 [53] Schmidt E M, Franco M F, Regino K G, Lehmann E L, Arruda M A, Wf D C R, Borges R, De S W, Eberlin M N, Correa D N. Sci. Justice, 2014, 54(6): 459–464 [54] Lalli P M, Sanvido G B, Garcia J S, Haddad R, Cosso R G, Maia D R J, Zacca J J, Maldaner A O, Eberlin M N. Analyst, 2010, 135(4): 745–750 [55] Cabral E C, Simas R C, Santos V G, Queiroga C L, Da C V, de
Bognár L, Oertel M, Takáts Z. Anal. Chem., 2011, 83(20):
Sá G F, Daroda R J, Eberlin M N. J. Am. Soc. Mass Spectrom.,
7729–7735
2015, 47(1): 1–6
[41] Arao T, Fuke C, Takaesu H, Nakamoto M, Morinaga Y, Miyazaki T. J. Anal. Toxicol., 2002, 26(4): 222–227 [42] Arinobu T, Ishii A, Suzuki O, Hattori H, Kumazawa T, Lee X P, Kojima S, Seno H. Anal. Chim. Acta, 2003, 492(1): 249–252 [43] Alberici L C, Oliveira H C F, Catharino R R, Vercesi A E, Eberlin M N, Alberici R M. Anal. Bioanal. Chem., 2011, 401(5): 1651‒1655 [44] Simas R C, Catharino R R, Cunha I B S, Cabral E C, BarreraArellano D, Eberlin M N, Alberici R M. Analyst, 2010, 135(4): 738–744 [45] Porcari A M, Schwab N V, Alberici R M, Cabral E C, Moraes D R d, Montanher P F, Ferreira C R, Eberlin M N, Visentainer J V. Anal. Methods, 2012, 4(11): 3551–3557
[56] Haddad R, Catharino R R, Marques L A, Eberlin M N. Rapid Commun. Mass Spectrom., 2010, 22(22): 3662–3666 [57] Nørgaard A W, Vaz B G, Lauritsen F R, Eberlin M N. Rapid Commun. Mass Spectrom., 2010, 24(23): 3441–3446 [58] Guo Y L, Wang H Y, Zhao Z X. Curr. Org. Chem., 2011, 15(21): 3734–3749 [59] Amaral P H, Fernandes R, Eberlin M N, Höehr N F. J. Am. Soc. Mass Spectrom, 2015, 46(12): 1269–1273 [60] Simas R C, Barreraarellano D, Eberlin M N, Catharino R R, Souza V, Alberici R M. J. Am. Oil Chem. Soc., 2012, 89(7): 1193–1200 [61] Liu Y Q, Zhang J L, Nie H G, Dong C X, Li Z, Zheng Z G, Bai Y, Liu H W, Zhao J D. Anal. Chem., 2014, 86(14): 7096–7102
[46] Porcari A M, Fernandes G D, Belaz K R A, Schwab N V,
[62] Pereira G G, Alberici R M, Fernandes G D, Cunha I B S,
Santos V G, Alberici R M, Gromova V A, Eberlin M N,
Eberlin M N, Dobarganes M C, Daroda R J, Arellano D. Eur. J.
Lebedev A T, Tata A. Anal. Methods, 2014, 6(8): 2436–2443 [47] Fernandes G D, Moreda W, Barreraarellano D, Ruiz G C N Z, Ferreira P L, Eberlin M N, Alberici R M. J. Braz. Chem. Soc., 2014, 25(9): 1565–1570 [48] Sawaya A C H F, Abdelnur P V, Eberlin M N, Kumazawa S, Mokryeon A, Bang K S, Nagaraja N, Bankova V S, Afrouzan H. Talanta, 2010, 81(1): 100–108 [49] Domin M A, Cody R B. Ambient Ionization Mass Spectrometry, Royal Society Chemistry Press, 2014: 2822–2826 [50] Riccio M F, Saraiva S A, Marques L A, Alberici R, Haddad R, Moller J C, Eberlin M N, Catharino R R. Eur. J. Lipid. Sci.
Lipid Sci. Technol, 2014, 116(8): 952-960 [63] Na N, Shi R X, Long Z, Lu X, Jiang F B, Ouyang J. Talanta, 2014, 128: 366–372 [64] Liu N, Lu X, Yang Y H, Yao C X, Ning B M, He D C, He L, Ouyang J. Talanta, 2015, 143: 240–244 [65] Jansson E T, Dulay M T, Zare R N. Anal. Chem., 2016, 88(12): 6195–6198 [66] Teunissen S F, Fernandes A M A P, Eberlin M N, Alberici R M. TrAC, Trends. Anal. Chem., 2017, 90: 135–141 [67] Alberici R M, Vendramini P H, Eberlin M N. Anal. Methods, 2017, 9(34): 5029–5036