Article pubs.acs.org/ac
Microwave-Induced Plasma Desorption/Ionization Source for Ambient Mass Spectrometry Xuefang Zhan, Zhongjun Zhao, Xin Yuan, Qihui Wang, Dandan Li, Hong Xie, Xuemei Li, Meigui Zhou, and Yixiang Duan* Research Center of Analytical Instrumentation, Analytical Testing Center and College of Chemistry, Sichuan University, Chengdu, China ABSTRACT: A new ionization source based on microwave induced plasma was developed for ambient desorption/ionization. The microwave-induced plasma desorption/ionization source (MIPDI) was composed of a copper Surfatron microwave cavity where a fused-silica tube was centered axially. Stable nonlocal thermodynamic equilibrium plasma was generated in the quartz discharge tube when a microwave at a frequency of 2450 MHz was coupled to the microwave cavity. Analytes deposited on the surface of poly(tetrafluoroethylene) (PTFE) or quartz slide after hydrofluoric acid (HF) etching were desorbed and ionized by the plasma. The performance of the MIPDI technique was validated by the analysis of a variety of chemical substances, polymer compounds, and pharmaceutical drugs using argon or helium as the discharge gas. Protonated [M + H]+ or deprotonated [M − H]− ions were observed in the positive or negative mode. MIPDI was also used for the analysis of compounds in a complex matrix without any sample preparation. MIPDI was also capable of analyzing liquid samples. The signal-to-noise ratio was 463 in the analysis of 9.2 ng of phenylalanine, and the limit of detection was 60 pg for phenylalanine. MIPDI could desorb and ionize analytes with a molecular weight of up to 1200, which was demonstrated by the analysis of polyethylene glycol 800 (PEG800). MIPDI has advantages of simple instrumentation, relatively high temperature, stability, and reproducibility.
A
in real-time (DART),10−13 low-temperature plasma probes (LTP), 14−16 flowing atmospheric pressure afterglow (FAPA), 17−21 plasma-assisted desorption-/ionization (PADI),22 dielectric barrier discharge ionization (DBDI),23 and microplasma;24 (3) laser desorption/ablation methods, including laser-assisted desorption electrospray ionization (LDESI),25 matrix-assisted laser desorption electrospray ionization (MALDESI),26 electrospray-assisted laser desorption ionization (ELDI),27 and surface-assisted laser desorption/ ionization (SALDI).11 These ambient pressure ionization sources have been applied to the analysis of a wide variety of compounds, ranging from small molecules to biomolecules, during high-throughput, nondestructive, and reaction-monitoring analyses. Plasma-based techniques that employ atmospheric plasma are DART, DAPCI, FAPA, PADI, LTP, DBDI, and microplasma. These methods involve the generation of a direct current or radiofrequency electrical discharge between a pair of electrodes that contact a flowing gas such as argon or helium, thereby generating nonequilibrium, relatively low temperature plasmas, i.e., chemically active species such as high-energy electrons, radicals, ionized molecules, and metastable neutrals.
mbient mass spectrometry can desorb and ionize analytes from sample surfaces directly in an ambient atmosphere. Ambient ionization techniques have several significant advantages for real-time and in situ chemical analysis, and they are increasingly attractive. First, they can be operated in the open air. Second, they allow direct ionization with little or no sample pretreatment such as preconcentration, extraction, derivatization, dissolution, or chromatographic or electrophoretic separation. Third, they can interface with most types of mass spectrometers without substantial modifications of the ion transfer optics or vacuum interface. Desorption electrospray ionization (DESI)1−4 was the first ambient desorption/ ionization technique, and it overcame the difficulties associated with vacuum devices. In the DESI technique, a pneumatically assisted electrospray jet is directed toward the sample surface to facilitate desorption/ionization and it has been shown to be very sensitive while producing little damage on the surface. In recent decades, more than a dozen ambient desorption/ ionization techniques have been reported.5−7 Various chemical and physical agents and processes have been used for desorbing and ionizing analytical samples, including charged droplets, plasmas, photons, and heated gas. Depending on the agents and processes used, desorption/ionization techniques can be divided into three main categories: (1) spray and solid liquid extraction-based techniques, including probe electrospray ionization (PESI)8 and easy ambient sonic-spray ionization (EASI);9 (2) plasma-based techniques, including direct analysis © 2013 American Chemical Society
Received: January 14, 2013 Accepted: March 28, 2013 Published: March 28, 2013 4512
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Figure 1. MIPDI for ambient mass spectrometry: (a) Schematic of the configuration and (b) photo of the experimental apparatus.
requirements for solvents and electrodes, which means there is no pollution due to the oxidation of electrodes and no threat from high voltage. However, microwave radiation is present, and personal protective equipment, such as microwave protective clothing and safety glasses, is highly recommended. Nonequilibrium microwave-induced plasma31,33 has a relatively high gas temperature, which is distinctly different from most plasma-based ambient desorption/ionization sources. The relatively high plasma temperature enhances thermal desorption, which makes it free from the usual supplementary heating sources, thereby simplifying the instrumentation. The performance of the MIPDI technique was demonstrated by the analysis of a variety of chemical substances, polymer compounds, and pharmaceutical drugs using argon or helium as the discharge gas. Protonated and deprotonated molecular ions of [M + H]+ or [M − H]− and their adduct ion species were observed in the positive or negative mode. MIPDI was also used for the analysis of compounds in a complex matrix, i.e., the direct analysis of pharmaceutical drugs including drug tablets, capsules, and ointments, with no sample preparation. MIPDI was demonstrated to be capable of direct desorption/ionization on liquid sample surfaces.
Some plasma species, such as DART, require secondary heating of the plasma gas stream to enhance desorption. However, plasma ambient sampling/ionization techniques have the advantages of simple instrumentation, rugged construction, and no need for solvents. All of these plasma-based desorption/ ionization sources have good sensitivity and versatility during the determination of various compounds in the pure state or complex samples in gaseous, solution, or condensed phases. These methods are promising tools in many scientific and technological areas, including pharmaceuticals, environmental samples, food, beverages, forensics, and explosives. However, their use is limited mainly to analytes in a relatively low mass range, usually below 800−1200 Da. In this Article, we describe a new plasma-based desorption/ ionization source, i.e., microwave-induced plasma desorption/ ionization (MIPDI). Microwave-induced plasma (MIP) operated in the GHz region was initially described in the mid-1950s. Different structures have been used for microwave-induced plasma including the TM010 resonator reported by Beenakker,28 the TE101 resonant structure introduced by Matusiewics,29 the surface wave propagation known as a Surfatron described by Hubert et al.30 and later refined by Selby and Hieftje,31 and the microwave plasma torch (MPT) introduced by Jin et al.32 Their easy operation using argon and helium as discharge gases, as well as their low electrical power and gas consumption, mean they have been used as excitation sources for optical emission spectrometry (OES)33 and as ionization sources for inorganic mass spectrometry34 with high sensitivity and low detection limits. Microwave-induced plasma has been used as a single or tandem ionization source34−36 for inorganic mass spectrometry. We are the first to combine a MIP Surfatron source with organic mass spectrometry employing the desorption/ionization technique. The microwave-induced plasma source we used was a Surfatron structure. The discharge gas (argon or helium) absorbed microwave energy imported from a microwave generator to generate a high density of chemically active species such as high-energy electrons, metastable neutrals, ionized molecules, and radical ions at atmospheric pressure, which were ignited by a slender metal wire. Charge transfer reaction or heat transmission occurred when a high density of chemically active species impacted the surfaces of analytes. MIPDI has the advantages of simple instrumentation and no
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EXPERIMENTAL SECTION Chemicals and Reagents. HPLC grade methanol was purchased from Thermo Fisher (Thermo Fisher, San Jose, CA). Other chemicals were obtained from Sangon Biotech (Shanghai Sangon Biological Engineering Technology & Services Co. Ltd.) and used directly without any further purification. The discharge gases, argon (99.9%) and helium (99.999%), were provided by QiaoYuan Gas Company. Ultrapurified water (18 MΩ cm−3) was produced using an UP water purification system. Over-the-counter pharmaceuticals including drug tablets, capsules, and ointments were purchased from a local pharmacy. Mass Spectrometer Conditions. All of the measurements were made using a homemade MIPDI ion source, which was coupled to a commercial ion trap mass spectrometer (LCQ Fleet; Thermo Fisher Scientific, San Jose, CA). Data were processed using the instrument software (Xcalibur version 1.4SR1). The mass spectrometer conditions were as follows:
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Figure 2. Background mass spectra using the microwave induced Ar-plasma (a) in positive mode and (b) in negative mode with a power of 40 W and a flow rate of 1.42 L min−1. The insets are MS/MS spectra.
This generator can produce a maximum power of 150 W at a frequency of 2450 MHz. The forward and reflected microwave powers can be read digitally from two small screens on the front panel of the microwave generator. Infrared thermography (Smart Sensor, AR872D), which can characterize temperatures between −55 and 1150 °C, was used to determine the temperature of substrate surface in contact with the sampling microwave-induced plasma. Safety Considerations. Electrical shock may happen on igniting the discharge with a slender metal wire. Extreme care and precaution should be taken to prevent electrical shock through the use of electrically insulating gloves. Personal protective equipment, such as microwave protective clothing and safety glasses, is highly recommended to prevent microwave radiation.
source voltage (applied to the spray capillary), 0 kV; tube lens voltage, 70 V; heated capillary temperature, 150 °C; capillary voltage, 15 V; and multipole rf amplitude (Vp-p), 400 V. The maximum ion injection time was set to 100 ms, and the number of microscans was set to three. Ambient MIPDI-MS System. The microwave-induced plasma source was a Surfatron device,31 which employed surface wave propagation. Figure 1 shows a schematic of the configuration of MIPDI. The microwave coaxial cavity was cylinder-shaped and made of copper with a fused-silica tube (2.0 mm i.d., 6 mm o.d., 200 mm long) centered axially. The rf power (2450 MHz) from the solid-state microwave generator was input into the cavity via a standard N-type connector. Two tuning screws located at one end of the cavity were used to ensure a stable microwave discharge and to minimize the reflected power by adjusting the distances l and g shown in Figure 1. The reflected power can reach near to 0 W. The entire microwave cavity was mounted on a vertical rotating stage, which was mounted on a 3D translation stage (7SVM0160, Beijing 7-Star Optical Instruments CO., LTD) to allow precise alignment of the exit aperture of the source with the mass spectrometer inlet. The rotating stage allowed the selection of impact angles that ranged from 0° to 90°. The discharge gas, i.e., argon or other gases (helium), was fed through the fusedsilica tube to facilitate discharge at a flow rate range of 0.5−2.0 L min−1, which was measured using a mass flow controller (MFC, D07-19B, Beijing Sevenstar Electronics CO., LTD). A slender metal wire was used to ignite the discharge. The analytes on the surface of a substrate mounted on a separate 3D translation stage were desorbed and ionized by the microwaveinduced plasma. The ions produced were introduced into the mass spectrometer for mass analysis. Pure and complex samples can be analyzed directly by MIPDI with no pretreatment requirements, except dilution in a solvent, spotting on the substrate surface, and drying. MS spectra with a good signal-to-noise (S/N) ratio were recorded for the ions desorbed using MIPDI. The exact position, distance, and angle of the MIPDI with respect to the sample surface and the mass spectrometer inlet are not strict. Samples were placed at a horizontal distance of about 0.5−2 cm and a vertical distance of 1−5 mm from the mass spectrometer, typically at an angle of approximately 45°. A solid-state microwave generator was purchased from Nanjing Yanyou Electronic Science and Technology Co. Ltd. (Nanjing, China) and was connected to the microwave cavity by coaxial cable through a standard N-type female connector.
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RESULTS AND DISCUSSION Background Mass Spectra. The MIPDI source was shown to be capable of directly desorbing/ionizing samples in solid, liquid, or gas phases. Figure 2 shows the background mass spectra in both positive and negative mode using microwave-induced Ar-plasma with a power of 40 W and a flow rate of 1.42 L min−1. The peaks at m/z of 249 and 266 were contributed by detergents, while the peak at m/z 249 corresponded to the fragmentation of m/z 266, which were confirmed by collision-induced dissociation (CID). The peak at m/z 279 was identified as protonated dibutylphthalate, a kind of plasticizer, and this is consistent with results observed by Salter.37 The peaks at m/z 149 and 205 corresponded to the fragmentation of the peak at 279, which were confirmed by CID. The plasticisers were probably derived from the degradation of the polyethylene tube and/or the substrate surface when polytetrafluoroethylene (PTFE) was used, or it may have been in the air in the laboratory. The spectrum obtained in negative mode (Figure 2b) was clearer than in positive mode. The peaks at m/z 62 and 125 were identified as NO3− and HNO3NO3−, respectively, and this is consistent with the results observed by Shelley.19 Given these background levels, the detection of low-abundance or unknown species is quite difficult using MIPDI, so efforts must be made to reduce background spectra. Optimization of MIPDI Operational Parameters. The discharge gas,23 power,23 and the relative positions38 of the MIPDI source, the sample surface, and the mass spectrometer inlet are known to have major effects on ion generation and 4514
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Figure 3. Extracted ion chromatography of 4-acetominophen along with time tracing for repeated introduction of the same amount of sample (50 μg) under (a) different gas flow rate (in unit of L min−1) with a power of 40 W and (b) different microwave power with a flow rate of 1.42 L min−1; (c) mass spectra of 4-acetominophen, (d) substrate surface temperature as a function of the microwave generator power. All these experiments are performed with Ar-plasma.
plasma. The temperatures were measured by infrared thermography (Smart Sensor AR872D), which can characterize temperatures between −55 and 1150 °C. With microwaveinduced plasma, the temperature of the discharge center reached as high as 1000 °C. However, the quartz tube was exposed to the open air, which could act as a heat sink as the Ar-plasma traveled through the fused-silica tube, so the temperature was reduced to dozens or hundreds of degrees centigrade. As expected, the temperature of the substrate surface increased with the microwave power. However, high temperatures may damage the analyte surface, particularly during the analysis of thermally unstable substances. At a high power, ablation of the substrate surface was observed, especially when PTFE or glass slides were used. A power supply of 30−50 W corresponded to a surface temperature of 30−100 °C in the experiments, with considerable ionization efficiency and heating effect. The affinity of the sample for the substrate surface and the surface roughness affected the ionization efficiency. A high affinity of the sample for the surface would lead to a serious diffusion problem and a loss of sensitivity. For the most commonly used solution (50/50 methanol/water), PTFE was a generally applicable surface because of its low affinity for most solvents. Quartz slides were used as a substrate before and after hydrofluoric acid (HF) etching. After HF etching, the quartz slides were found to dramatically increase the ion abundance and detection sensitivity, because they reduced the sample solution diffusion. However, glass slides (before or after HF etching) were not appropriate for the MIPDI technique
transmission. Optimization of the ionization parameters was carried out by varying the output power of the microwave source, the type of gas, the gas flow rate, and surface effects. Argon and helium were tested for the generation of microwave-induced plasma for desorbing and ionizing analytes. Both of these gas species can produce microwave-induced plasma and mass spectra, but the ignition of helium plasma is much more difficult and a higher power (at least 100 W) is required. The microwave-induced helium plasma required exact distances of l and g shown in Figure 1, whereas the argon plasma had a wide tolerance for these two distances.31 Given this and its lower cost, argon was chosen as the typical discharge gas in experiments. The flow rate of the discharge gas can affect the efficiency of desorption ionization as well as the transmission of MIPDI-MS. The extracted ion chromatography of protonated 4-acetaminophen at m/z 152 (Figure 3c) recorded along with time tracing for repeated introductions of the same amount of sample (50 μg) under different argon gas flow rate are shown in Figure 3a. The ion current was enhanced with an increased gas flow rate. Further increases in the gas flow rate had no significant effect and they wasted gas, so a gas flow rate of 1.7 L min−1 was selected. Figure 3b shows the extracted ion chromatography of protonated 4-acetaminophen at m/z 152 recorded along with time tracing for repeated introductions of the same amount of sample (50 μg) under different microwave power (Ar-plasma are used). In general, the ion current increased with the power increasing. Figure 3d shows the temperature of the substrate surface, which was in contact with the microwave-induced 4515
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Table 1. Analysis of Chemical Substances with MIPDI-MS Using Argon as Discharge Gas molecular weight
major m/z observed (relative abundance)
assignment
limit of detection (pg)
L-phenylalanine
165
105
[M + H]+ [2M + H]+ [M + H − HCOOH]+ [M + H]+ [M + H − H2O]+ [M + H]+ [M + H − H2O]+ [M + H]+ [M + H]+ [2M + H]+ [2M + H]+ [M+H]+ [2M + H]+ [M + H]+ [2M + H]+ [M + H]+ [M + H]+ [M + H + O]+ [M + H]+ [M + H − CH2CO]+ [M + O + H]+
60b
L-serine
166 (100%)a 331 (40%) 120 (5%) 106 (100%) 88 (4%) 148 (100%) 130 (20%) 132 (100%) 118 (100%) 235 (70%) 121 (100%) 61 (70%) 359 (100%) 180 (30%) 271 (100%) 136 (20%) 146 (100%) 162 (50%) 152 (100%) 110 (10%) 168 (10%)
analytes
L-glutamic
acid
L-leucine L-valine
urea
147 131 117 60
phenacetin
179
acetamidobenzene
135
8-hydroxyquinoline
145
4-acetaminophen
151
98c 151b 59b 13b 195c 155b 81b 14c 106b
a The values in the bracket are normalized to the dominant peak. bMS/MS are used for MIPDI in these cases. cSIM are used for MIPDI in these cases.
because of the relatively high temperature of the microwaveinduced plasma. Thus, PTFE and quartz slides were the two substrates that we generally used. Desorption and Ionization of Chemical Compounds. MIPDI was applied successfully to desorb/ionize both polar and nonpolar chemicals. For the majority of analytes, protonated ions were observed, which showed that the ionization pathways mainly involved proton transfer reactions. Table 1 shows detailed information for a variety of chemical substances that were desorbed and ionized using MIPDI. Mass spectra were collected in the sampling sequence of the blank and then sample by sample. Peaks obtained from the blank served as noise level. The MS/MS spectra were carried out to confirm the fragmentation and dimerization of ions. The detection limits were determined at relatively low levels. MS/ MS and SIM were used to improve the analyte S/N ratios. Compared with reported techniques such as PADI22 and DBDI,23 the fragmentation ions produced by MIPDI such as [M − HCOOH + H]+ and [M − H2O + H]+) only comprised a small proportion (less than 10%), whereas dimerization ions were often observed during the analysis of many compounds. This observation suggests that mechanisms other than thermal effects are responsible and that MIPDI is potentially a soft ionization technique. Analysis of Polymer Compounds. MIPDI has been found to be capable of polymer analysis such as polyethylene glycol (PEG; HO(CH2CH2O)nH), which is a common lubricant, moisturizer, softener, and plastifier used in cosmetics, pharmaceuticals, and chemical fibers. PEG 400 and PEG 800 have different degrees of polymerization, and they were desorbed and ionized using microwave-induced plasma. Figure 4a,b shows the spectra obtained by helium plasma desorption of PEG 400 and PEG 800, respectively. As shown in Figure 4a, a very clear series of ions were identified in the positive-ion mode as the addition of [M + OH + H]+ with an increment of m/z 44 were observed. It is possible to determine that the mass
Figure 4. Analysis of (a) PEG 400 and (b) PEG 800 using microwaveinduced He-plasma.
separations of m/z 44 corresponded to CH2CH2O. Similarly, a clear series of ions [M + H]+ with an increment of m/z 44 were obtain from the analysis of PEG 800, as shown in Figure 4b. 4516
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Figure 5. Analysis of the active ingredient of pharmaceutical drugs by microwave-induced Ar-plasma: (a) difenidol hydrochloride tablet, (b) paracetamol tablet, (c) norfloxacin capsule, (d) ibuprofen capsule, and (e) miconazole nitrate ointment.
analyzed using the MIPDI source, and the resulting spectra are shown in Figure 5b. The protonated and adduct species of the ingredient molecules were observed, whereas few fragmentation ions were observed. The active ingredients of 4-acetominophen were detected at m/z 152, 194, 211, 303, and 345, which corresponded to [M + H]+, [M + CH2CO + H]+, [M + CH2CO + OH + H]+, [2M + H]+, and [2M + CH2CO + H]+, respectively. All of the adduct ions were confirmed by collisioninduced dissociation (CID). The protonated molecule of aspirin was less apparent in the spectra, whereas the peaks at m/z 163 corresponding to a loss of water [M − H2O + H]+ as well as m/z 240 corresponding to an adduct of acetic acid [M + CH3COO + H]+ were observed. The loss of water from aspirin was reported previously using DESI and DART analyses.39 The fragmention ion at m/z 121 due to the loss of acetic acid was detected by PADI analysis,22 but it was not observed using the MIPDI technique. The active ingredient caffeine was observed
These experiments showed that the mass range that can be analyzed using MIPDI can reach up to 1200. Complex Matrixes Analysis. The MIPDI source can be used for the direct analysis of complex matrixes by ambient sampling with little or no sample preparation. This was demonstrated by analyzing a wide range of over-the-counter pharmaceutical drugs. Tablets such as the analgesic−antipyretic paracetamol and the antivertigo treatment difenidol hydrochloride were detected directly by exposing a piece of the drug tablet to the subsurface region. In the analysis of difenidol hydrochloride, a molecule of hydrochloric acid was removed by desorption with the microwave-induced plasma. The active ingredient was detected at m/z 310 and m/z 292, which corresponded to the protonated molecule and the loss of water, respectively, as shown in Figure 5a. The multiple ingredients in paracetamol tablets (500 mg), which contained 4-acetominophen (125 mg), aspirin (230 mg), and caffeine (30 mg), were 4517
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at m/z 195 and 388, which corresponded to [M + H]+ and [2M]+, respectively. The capacity of the MIPDI source for analyzing samples in complex matrixes was further demonstrated by the detection of capsule drugs. Capsules of the anti-inflammatory drug ibuprofen and norfloxacin were analyzed using MIPDI with little pretreatment. In these studies, capsule drugs were stripped of their thin capsules and diluted in solvent. Next, 2 μL aliquots of the drug solutions were spotted onto the substrate surface and dried. In the analysis of norfloxacin, the protonated molecule of norfloxacin was detected at m/z 320, as shown in Figure 5c. The expected protonated molecule of ibuprofen was detected at m/z 207, and it had a low abundance (Figure 5d). However, the peaks detected at m/z 224, which corresponded to [M + OH + H]+, were observed as base peaks. A tandem MS/MS experiment was performed to analyze the precursor ion at m/z 224, which confirmed the identity of this peak. The spectra produced were similar to those generated using a beta electron-assisted direct chemical ionization (BADCI) probe.40 However, the fragmentation ion at m/z 161 detected using the DESI and PADI22 was not observed using the MIPDI technique. This supports the claim that MIPDI is a soft ionization source. MIPDI was also applied to the analysis of ointments and creams. For example, the antifungal agent miconazole nitrate contains the active ingredient miconazole nitrate at 2% w/w. A thin layer of the miconazole nitrate ointment was spread onto the substrate and positioned in the source region of the mass spectrometer where it was analyzed directly using MIPDI with no pretreatment. A molecule of nitric acid was removed from the miconazole nitrate molecule by MIPDI desorption. The spectra contained the protonated molecule and the dimerization molecule of miconazole (Figure 5e). The peak observed at m/z 150 corresponded to the protonated molecule [M + H]+ of triethylolamine, which was one of the main ingredients. The ion fragment observed in the MIPDI spectra at m/z 132 corresponded to the loss of water from triethylolamine. Direct Analysis of Liquid Samples. The MIPDI source was capable of desorbing and ionizing analytes directly from solutions. To demonstrate this function, a glass culture dish or watch glass containing analytes solution was positioned beneath the inlet of the mass spectrometer and the quartz tube of MIPDI to direct the plasma over the liquid surface. Figure 6 shows the spectra of the antihistamine claritin after its analysis in solid and liquid states. It is obvious that the mass spectrum obtained with liquid sample is clearer. The protonated molecule of the active ingredient loratadine was observed at m/z 383 in claritin in the solid and liquid states, as shown in Figure 6. The peak at m/z 385 corresponded to the isotope of the chloride element of loratadine. The peak at m/z 244 was observed with a high abundance in the solid state but a low abundance in the solution state. This was because the corresponding ingredient dissolved poorly in the solvent (50/50 methanol−water) and it precipitated. Only the surface substances were ionized when the plasma was sprayed over the liquid surface, whereas precipitates were poorly ionized. The results showed that the peak at m/z 244 was due to the main ingredient of claritin and not the air background or detergent detected using DESI.1 MIPDI is capable of the direct analysis of liquid sample attribute to the ability of desorption samples of any size or shape from any angle. Thus, the MIPDI source is capable of the direct analysis of bulk solutions, which will make it a rapid and convenient tool for the analysis of flowing streams or bodily fluids.
Figure 6. Analysis of the active ingredient in generic claritin using microwave-induced Ar-plasma in: (a) the solid state and (b) the liquid state.
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CONCLUSION A versatile MIPDI source was developed to desorb and ionize analytes directly from surface in an ambient atmosphere. Experiments have been demonstrated that this method can be used to produce positive and negative ion mass spectra with a wide range of organic compounds. The MIPDI ion source was shown to have good repeatability and sensitivity for the detection of various organic compounds. The relative standard deviation (RSD) was 6.25% based on four replicate analyses of phenylalanine at the amount of 6.4 μg in each spot. The S/N ratio was 463 in the analysis of 9.2 ng of phenylalanine, and the LOD of phenylalanine was 60 pg using MS/MS. Moreover, the LOD should be further improved with a better mass spectrometer. The simple construction, easy operation, stability, reproducibility, the possibility of producing plasmas with alternative gases, and its high efficiency for desorbing/ionizing analytes directly from sample surfaces in an ambient atmosphere mean that the MIPDI technique could be applied in a wide range of applications. Future studies will focus on the ionization mechanism, applications, and improvements of the MIPDI technique.
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AUTHOR INFORMATION
Corresponding Author
*Address: Research Center of Analytical Instrumentation, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China. E-mail:
[email protected].. Phone: (+86)02885418180. Fax: (+86)028-85418180. 4518
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Notes
(27) Shiea, J.; Huang, M. Z.; Hsu, H. J.; Lee, C. Y.; Yuan, C. H.; Beech, I.; Sunner, J. Rapid Commun. Mass Spectrom. 2005, 19, 3701− 3704. (28) Beenakker, C. I. M. Spectrochim. Acta, Part B 1976, 31, 483− 486. (29) Matusiewicz, H. Spectrochim. Acta, Part B 1992, 47, 1221−1227. (30) Hubert, J.; Moisan, M.; Ricard, A. Spectrochim. Acta, Part B 1979, 33, 1−10. (31) Selby, M.; Hieftje, G. M. Spectrochim. Acta, Part B 1987, 42, 285−298. (32) Jin, Q.; Zhu, C.; Borer, W.; Hieftje, G. M. Spectrochim. Acta, Part B 1991, 46, 417−430. (33) Jin, Q.; Duan, Y.; Olivares, J. A. Spectrochim. Acta, Part B 1997, 52, 131−161. (34) Su, Y. X.; Duan, Y. X.; Jin, Z. Anal. Chem. 2000, 72, 2455−2462. (35) Duan, Y. X.; Su, Y. X.; Jin, Z. J. Anal. Atom. Spectrom. 2000, 15, 1289−1291. (36) Su, Y. X.; Duan, Y. X.; Jin, Z. Anal. Chem. 2000, 72, 5600−5605. (37) Salter, T. L.; Gilmore, I. S.; Bowfield, A.; Olabanji, O. T.; Bradley, J. W. Anal. Chem. 2013, 85, 1675−82. (38) Takáts, Z.; Wiseman, J. M.; Cooks, R. G. J. Mass Spectrom. 2005, 40, 1261−1275. (39) Williams, J. P.; Patel, V. J.; Holland, R.; Scrivens, J. H. Rapid Commun. Mass Spectrom. 2006, 20, 1447−1459. (40) Steeb, J.; Galhena, A. S.; Nyadong, L.; Janata, J.; Fernandez, F. M. Chem. Commun. 2009, 2009, 4699−4701.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support from National Recruitment Program of Global Experts (NRPGE), the Hundred Talents Program of Sichuan Province (HTPSP), the National Natural Science Foundation of China (21275105), and the Startup Funding of Sichuan University for setting up the Research Center of Analytical Instrumentation.
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dx.doi.org/10.1021/ac400296v | Anal. Chem. 2013, 85, 4512−4519
