Rapid Characterization of Chemical Compounds in Liquid and Solid

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Rapid Characterization of Chemical Compounds in Liquid and Solid States Using Thermal Desorption Electrospray Ionization Mass Spectrometry Min-Zong Huang, Chi-Chang Zhou, De-Lin Liu, Siou-Sian Jhang, Sy-Chyi Cheng, and Jentaie Shiea Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac401364k • Publication Date (Web): 03 Sep 2013 Downloaded from http://pubs.acs.org on September 7, 2013

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Analytical Chemistry

Rapid Characterization of Chemical Compounds in Liquid and Solid States Using Thermal Desorption Electrospray Ionization Mass Spectrometry

Min-Zong Huang1,*; Chi-Chang Zhou1; De-Lin Liu1; Siou-Sian Jhang1; Sy-Chyi Cheng1; Jentaie Shiea1,2,*

1.

Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan

2.

Department of Medicinal and applied Chemistry, National Medical University, Kaohsiung, Taiwan

*: To whom correspondence should be addressed, Dr. Jentaie Shiea, [email protected]; Dr. Min-Zong Huang, [email protected] Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, 80424 Taiwan. Tel./Fax: +886-7-5253933.

1

ACS Paragon Plus Environment


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Abstract Rapid characterization of thermally stable chemical compounds in solid or liquid states is achieved through thermal desorption electrospray ionization mass spectrometry (TD-ESI/MS). A feature of this technique is that sampling, desorption, ionization and mass spectrometric detection are four separate events with respect to time and location. A metal probe was used to sample analytes in their solid or liquid states. The probe was then inserted in a preheated oven to thermally desorb the analytes on the probe. The desorbed analytes were carried by a nitrogen gas stream into an ESI plume, where analyte ions were formed via interactions with charged solvent species generated in the ESI plume. The analyte ions were subsequently detected by a mass analyzer attached to the TD-ESI source. Quantification of acetaminophen in aqueous solutions using TD-ESI/MS was also performed in which a linear response for acetaminophen was obtained between 25-500 ppb (R2=0.9978). The standard deviation for a reproducibility test for ten liquid samples was 9.6 %. Since sample preparation for TD-ESI/MS is unnecessary, a typical analysis can be completed in less than 10 seconds. Analytes such as the active ingredients in over-the-counter drugs were rapidly characterized regardless of the different physical properties of said drugs, which included liquid eye drops, viscous cold syrup solution, ointment cream, and a drug tablet. This approach was also used to detect trace chemical compounds in illicit drugs and explosives, in which samples were obtained from the surfaces of a cell phone, piece of luggage made from hard plastic, business card, and wooden desk.

Keywords: Ambient ionization, thermal desorption, electrospray ionization, post-ionization

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Introduction The development of ambient ionization techniques has been attributed to a revolutionary improvement in mass spectrometry in recent times. This research area has rapidly evolved, and these techniques have become one of the most versatile and informative analytical tools for investigating analytes in different states of matter. This state-of-the-art technology is usually defined as mass spectrometric analysis that requires little or no sample preparation, where sample ionization is performed under ambient conditions, allowing for rapid, real-time, and high-throughput analysis of a wide range of substances.1-5 Based on differences in their analytical processes, ambient ionization techniques can be classified into two categories. The first category includes techniques that impact the sample surface with small droplets or plasma for both analyte desorption and ionization; desorption electrospray ionization (DESI),6 easy ambient sonic-spray ionization (EASI),7-8 and direct analysis in real time (DART)9 are three techniques that are in this category. The second category includes two-step ionization, where analytes generated by lasers, shockwaves, thermal heat, or nebulizers are post-ionized via interactions with charged species generated by electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI); techniques in this category include fused-droplet electrospray ionization (FD-ESI),10-11 electrospray laser desorption ionization (ELDI),12-14 atmospheric solids analysis probe (ASAP),15 laser diode thermal desorption (LDTD), 16

and other related techniques.17-20 Given the different characteristics of ambient ionization techniques, use of these

techniques depends on the nature of the analyte. For instance, FD-ESI is useful for characterizing both volatile and non-volatile compounds in solution.21-22 In ELDI 3



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and

LIAD-ESI, a

improve production

pulsed of

laser is

used

to irradiate the sample in order to

nonvolatile analytes.23-24 However, for the analysis of

thermally stable compounds, analytes can be more efficiently generated by thermally heating large amounts of samples so the detection sensitivity can be increased. Several examples of techniques using thermal heating together with ESI for post-ionization are given below. First, gas chromatography-electrospray ionization (GC-ESI), where the gaseous analyte eluted from GC column is conducted into an ESI plume for ionization.21 Second, electrospray-assisted pyrolysis ionization mass spectrometry (ESA-Py/MS) is used to characterize pyrolytic products from a pyrolyzer.25 Third, atmospheric pressure-thermal desorption (AP-TD) is coupled with electrospray ionization mass spectrometry to analyze thermally desorbed neutrals, where a heated test tube with sample is placed in close proximity to an enclosed pneumatically-assisted ESI spray plume. This technique is applied to the analysis of volatile compounds in complex matrices like Bacillus spores.26 Fourth, thermal desorption-based ambient mass spectrometry (TDAMS) is used to analyze small organic analytes from complex samples without any sample pretreatment, where thermal desorption is induced by using an NIR diode laser beam to irradiate multiple self-assembled layers of gold nanoparticles, after which the desorbed analytes of small organics were post-ionized via electrospray ionization.27 Fifth, a technique known as atmospheric pressure thermal desorption proximal probe coupled with electrospray post-ionization is used for the direct mass spectrometric analysis of a diverse set of compounds separated on various high-performance thin-layer chromatography (HPTLC) plates. Thermal desorption is accomplished by placing a heated metal probe close to or in contact with the surface of TLC plate. Desorbed species are drawn into the ESI plume for post-ionization and mass spectrometric 4


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detection.28 Finally, a technique was recently developed that couples a nanometer-sized heated probe for thermal desorption with electrospray ionization mass spectrometry for sampling and ionizing analytes on surfaces. Here, a 30 nm diameter nano-thermal analysis (nano-TA) probe tip in an atomic force microscope (AFM) was coupled via a vapor transfer line and ESI interface; the thermally desorbed material was then transferred to the mass spectrometer from the AFM tip area. 29-30 Unfortunately, switching between samples during analyses is time-consuming when using the cramped analytical setup of existing techniques that couples thermal desorption to electrospray ionization. In addition, the sampling apparatuses are always attached to the ion sources so that analyte carry-over poses a serious problem between each analysis. Furthermore, due to the cramped analytical space of the ion sources, it is difficult for these techniques to analyze analytes from oversized or immovable objects. To overcome these problems, a simple, ease-of-operation and cost-effective TD-ESI technique was developed. The efficiency of qualitative and quantitative assays using this thermal desorption-electrospray ionization mass spectrometry (TD-ESI/MS) was evaluated.

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Experimental section Methanol (HPLC-grade) and acetic acid (reagent-grade) were purchased from Merck (Darmstadt, Germany) and J. T. Baker (Phillipsburg, NJ, USA), respectively. Distilled deionized water (purified by a Milli-Q plus apparatus; Millipore, Molsheim, France) was used to prepare the standard sample solutions. Acetaminophen was purchased

from

Sigma

(St

Louis,

MO,

USA),

while

diluted

3,4-methylenedioxy-N-methamphetamine (MDMA), codeine, 2,4,6-trinitrotoluene (TNT), and trinitrohexahydro-1,3,5-triazine (RDX) standard solutions were purchased from Cerilliant Inc. (Round Rock, TX, US). All chemicals were used without further purification. Over-the-counter pharmaceutical samples (i.e., eye drops, cold syrup, Ledernin ointment, and Viagra tablet) were obtained from local stores. Acetaminophen solutions with concentrations from 25 ppb to 500 ppb were prepared in methanol and water (1:1, v/v) solutions for quantification tests. Each data point was the average obtained from triplicate experiments. An acetaminophen solution with a concentration of 500 ppb was used for reproducibility tests. In order to detect trace amounts of chemicals on various object surfaces, standard solutions including MDMA, codeine, TNT and RDX were spread onto different surfaces (such as those of a business card, wooden desk, cell phone, and a piece of hard plastic luggage) and dried in an area of approximately 9 mm2 prior to analysis. The concentrations of those surfaces were approximately 50-100 ng/cm2.

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TD-ESI instrument: Figure 1a shows a photograph of the TD-ESI source developed in this study. The TD-ESI source comprised of a direct probe (DP), thermal desorption unit, and electrospray ionization device. First, the direct probe (DP) was used for sampling analytes in solution or on solid surfaces. For liquid samples, a stainless steel inoculating loop serving as a probe (2 mm in diameter) was used to obtain approximately 2 µL of sample solution for each analysis (Figure 2a). For viscous solutions or solid samples, a straight fine needle (350 µm in diameter) was used to either collect analytes of viscous solutions or sweep five times across solid surfaces to obtain analytes. Second, the function of the thermal desorption unit was to vaporize samples on the DP and introduce desorbed analytes into the ESI plume through a quartz tube with single taper (OD= 6 mm, ID= 4 mm, Length= 80 mm) so that effective sample ionization can take place. In addition, an opening in the thermal desorption unit was created when the direct probe was removed from the unit to sample analytes. The thermal desorption unit was sealed after the direct probe was placed back on the thermal desorption unit for analysis of sample analytes on the probe. Third, an electrospray ionization device was used for post-ionizing desorbed analytes. The high voltage and solvent flow rate required to generate an electrospray plume were 5 kV and 2.5 µL/min, respectively. The ESI solution was a methanol and water mixture (1:1, v/v) containing 0.1% acetic acid, and was delivered through a fused-silica spray capillary (ID= 100 µm, OD= 375 µm, Polymicro, Phoenix, AZ) into the mass inlet of the mass spectrometer. The TD-ESI source was attached to a triple quadruple mass spectrometer (Agilent 6410) to detect the analyte ions. To obtain a stable solvent and analyte ion signal, the electrospray capillary was aligned to the MS inlet and the distance from the exit of the TD tube to the electrospray capillary (d1) was kept at 8 mm, while the distance from the MS inlet to the electrospray capillary 7



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(d2) was 5 mm (Figure 1b). The temperature in the thermal desorption unit was set at 250 ℃ and was measured by a thermistor installed in the source. The flow rate of the nitrogen gas used to deliver desorbed analytes into the ESI plume was 0.8 L/min.

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Results and Discussion The field of ambient mass spectrometry encompasses many different techniques. Examples include ELDI, DESI, and DART. However, to be efficiently analyzed by these ambient ion sources, the sample needs to be placed in the ionization source, which is problematic during analysis of an oversized or irregularly-shaped sample due to the spatial limitations of the setup. Thus, a TD-ESI source was developed to overcome this issue in order to analyze analytes on and from any kind of object regardless of sample size. The operation of the TD-ESI developed in this study is similar to that of a direct insertion probe (DIP) used in an electron impact ionization (EI) source, where the sample (while adsorbed on a probe in its liquid or solid state) is introduced into the EI source so that analytes are thermally desorbed and ionized by electron impact. In TD-ESI, analytes thermally desorbed from the probe are delivered by a hot nitrogen gas stream and enter an electrospray plume, reacting with charged solvent species to form analyte ions. These ions are either formed through ion-molecule reactions between analytes and charged solvent ions (e.g., H+, CH3OH2+, or H3O+), or through analyte fusion with charged solvent droplets so that analyte ions are generated from newly-formed charged droplets after ESI. Since the analytes are desorbed inside a closed heated oven and delivered into ESI plume by a nitrogen gas stream for post-ionization, it is favorable to reduce diffusion effects and concentrate these analytes during desorption and ionization rather than analyzing them in an open space. Furthermore, the ionization efficiency would be enhanced due to an increase in reaction times and collision probabilities in a closed space. In addition, such an ion source design can also prevent environmental inferences. Figure 1c shows the TD-ESI desorption and ionization processes. Five separate analytical steps- sampling, desorption, ionization, ion detection and 9



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probe cleaning- are usually involved in a TD-ESI/MS analysis (Figure 2). To sample a liquid, a stainless steel inoculating loop (diameter: 2 mm) was dipped in and rapidly removed from the sample solution to obtain approximately 2 µL of solution (see the insert in Figure 2a). A straight fine needle was used to sample viscous solutions. By dipping and rapidly removing the probe from the solution, a small amount of the sample solution would remain on the probe. The same needle probe can also be used to sample solids by sweeping the loop across sample surfaces for a couple times. The probe was then inserted into a preheated oven so that analytes would be thermally desorbed and separated from the sample matrix. They were then carried by a nitrogen gas stream into an electrospray plume to be ionized and subsequently detected (see Figures 2b and 2c). Since these analytical processes are performed without sample pretreatment under atmospheric pressure, the time required to complete a TD-ESI/MS analysis is short, requiring less than 10 seconds. In addition, the probe is made of stainless steel so that any residues on the probe can be easily removed by burning the probe with a gas torch for a few seconds (see Figure 2d); the same procedure can be used to remove residues inside the desorption unit. Desorption and ionization performance in a TD-ESI source is affected by several parameters, such as (1) source geometry, i.e. the distance between the end of the thermal desorption tube and the ESI plume (d1), and the distance between the ESI capillary and the entrance of the MS inlet (d2) (Figure 1b); (2) desorption unit settings, i.e. desorption temperature and carrier gas flow rate and (3) electrospray settings, i.e. solvent flow rate and ESI voltage. To obtain optimal operating parameters for TD-ESI, a standard solution of acetaminophen (m/z 152) was used as a testing sample. The dependence of analyte ion signal intensity on d1 and d2 is shown

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in Figures 3a and b. The highest analyte ion signal was obtained while d1 and d2 were set as 8 mm and 5 mm, respectively. The parameters from thermal desorption unit including desorption temperature and carrier gas flow rate affect the degree of analyte molecules formation and efficiency of their transfer from desorption unit to ESI plume. The dependence of the analyte ion signal on the desorption temperature and carrier gas flow rate are shown in Figures 3c and d. Analyte ion signals increased when the desorption temperature ranged from 100 to 350 ℃, while signals remained constant even at 400 ℃. The results suggested acetaminophen was efficiently desorbed between 350 and 400 ℃ without decomposition. However, different trends were found for acetaminophen when the carrier gas flow rate was adjusted. The analyte ion signal intensity rapidly increased when the carrier gas flow rate was increased a maximum of 0.8 L/min but decreased when the flow rate was increased beyond this point. A higher carrier gas flow rate decreases the probability for analyte molecules to interact with charged solvent species in the ESI plume and therefore decreases analyte ion signal intensity. Electrospray parameters also affect the analyte ion intensity. The effects of high voltage and solvent flow rate on the acetaminophen ion signal are shown in Figures 3e and f. The analyte ion signal was obtained as the on-set voltage (i.e., 4 kV) for generating the electrospray was reached. The anaiyte ion signal remained unchanged even after further increase in voltage. The ESI solvent flow rate affects the stability of the electrospray plume as well as the formation of charged solvent species like protons, cluster solvent ions and charged droplets. Higher solvent flow rates may result in unstable and inefficient ionization of samples because the large droplets formed degrade the performance of the mass spectrometer.31 In this study, the electrospray became unstable when the solvent flow rate exceeded 2.5 µL/min,

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resulting in low ionization efficiencies and analyte ion intensities. The optimum operating parameters of the TD-ESI are listed in Table1. Figure 4a shows the extracted ion chromatogram (EIC) for ten consecutive analyses of the acetaminophen solution with a concentration of 500 ppb using the optimum operating parameters for TD-ESI. The analyte ion was detected immediately after the sample probe was inserted in the TD-ESI source. The peak areas from the extracted ion chromatograms for m/z 152 were used to evaluate the reproducibility tests. The standard deviation calculated from the ten analyses was 9.6% (n=10). To determine the detection limit and linearity of quantification, acetaminophen solutions with different concentrations ranging from 25 to 500 ppb were analyzed. The calibration curve (R2=0.9978) is shown in Figure 4b, which indicated that the detection limit of TD-ESI for acetaminophen is below 25 ppb. To demonstrate the usability of TD-ESI/MS for the analysis of different types of samples, the technique was used to characterize the active ingredients in different types of pharmaceutical products formulated as clear liquids, viscous liquid-syrups, ointments, and solid tablets. Figure 5a shows TD-ESI mass spectrum of eye drops analyzed in positive mode. The dominant ion signal in the mass spectrum belonged to the protonated sulfamethoxazle ion (m/z 254), an active ingredient in eye drops. TD-ESI/MS was also used to characterize active ingredients in cold syrup, in which cold

syrup

contained

acetaminophen

(14.94

mg/mL),

dl-methylephedrine

hydrochloride (0.49 mg/mL), caffeine (1.8 mg/mL), and chlorpheniramine maleate (0.124 mg/mL). Since the cold syrup sample was quite viscous and the concentrations of its active ingredients were high, the inoculating loop normally used for the sample probe was replaced with a straight fine needle to avoid sampling too many analytes. As shown in Figure 5b, protonated ion signals for the active ingredients in cold syrup - acetaminophen (m/z 152), methylephedrine hydrochloride (m/z 180), caffeine (m/z 12


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195) and chlorpheniramine maleate (m/z 275) were detected on the TD-ESI mass spectrum. The variations in the ion intensities of the detected analytes may be due to their low ionization efficiencies and matrix effects from complex mixtures. However, the relative intensities of these four compounds obtained by TD-ESI/MS were similar to those obtained by ESI/MS, in which the sample was diluted at least 10 times before analysis in the latter (data not shown). An ointment (Ledernin) containing econazol nitrate as the active ingredient was also examined by TD-ESI/MS using a fine needle probe. The mass spectrum showed predominant signals characteristic of tri-chlorine isotopic signatures, where the peak for protonated econazol is observed at m/z 381 and its isotope peaks appear at m/z 383, m/z 385 and m/z 387 with an abundance ratio of 27:27:9:1. The relative intensities of these isotopes is in good agreement with the expected Cl3 isotopic ratio (Figure 5c.). A tablet containing sildenafil citrate (Viagra) as the active ingredient was analyzed by TD-ESI/MS. The sample was collected by sweeping the tablet surface with a straight fine needle. The protonated sildenafil ion (m/z 475) was detected as the predominant ion peak on the TD-ESI mass spectrum (Figure 5d). There is an increasing need to rapidly identify trace chemical compounds on various surfaces for forensics or homeland security. The use of TD-ESI/MS as a tool for rapidly characterizing trace chemical compounds on various surfaces was conducted using MDMA (50 ng/cm2), codeine (50 ng/cm2), TNT (100 ng/cm2) and RDX (100 ng/cm2); these compounds were applied on the surfaces of a business card, wooden desk, cell phone, and piece of hard plastic luggage, respectively. A fine needle probe was swept across the surface of the object for sampling. Figure 6 displays the mass spectra of the samples analyzed by TD-ESI/MS. The TD-ESI mass spectra of MDMA (on a business card) and codeine (on a wooden desk) are shown in Figures 6a and b. Protonated molecular ion signals for MDMA and codeine were 13



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detected at m/z 194 and 304, respectively. Additionally, a fragment ion signal at m/z 163 indicating the loss of CH3NH2 was observed in the MDMA mass spectra. In short, the mass spectra for MDMA and codeine are similar to those obtained with other ambient ionization techniques.32-33 TD-ESI/MS analysis in the negative ion mode detected TNT and RDX applied on cell phones and pieces of hard plastic luggage, respectively (Figures 6c-6d). As seen in Figure 6c, the major ions for TNT in the mass spectrum are at m/z 227 and m/z 226, which correspond to the negative M˙and [M-H] ions. Fragment ions detected at m/z 210 ([M-OH]) and m/z 197 ([M-NO]) were the thermal decompositional products from the TD-ESI procedure. RDX ions at m/z 268 ([M+NO]) and m/z 221 ([M-H]) were detected; many ions were also detected at m/z 157, 171, 199, 241, 255, 283, 299, 311 and 319. These ions were formed by the reaction of the solvent species in electrospray. These ion signals are in accordance with previous reports that used electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). 34-35

Conclusion TD-ESI/MS provides a simple analytical strategy for characterizing thermally stable chemical compounds through ambient ionization. The principle of the technique involves separate sampling, desorption, ionization and detection stages. The operation protocol of the technique is as follows: (1) use of a direct probe to collect samples in their solid or liquid states, regardless of sample size; (2) thermal desorption for characterizing thermally stable and volatile compounds; and (3) ionization through interactions between desorbed analytes and charged solvent species in the ESI plume. Samples with different types of viscosities (including liquids, syrups, ointments, and solids) were directly analyzed without extraction and separation. Although sampling, desorption, ionization and detection are separate 14


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events, the analytical time required to complete a typical TD-ESI/MS analysis takes only a few seconds. In addition, compared to other ambient ion sources such as DESI, DART and ELDI, the sample is usually placed in the ion source. The use of a direct sampling probe in TD-ESI/MS is suitable for the analysis of chemical compounds present on surfaces of over-sized or immovable objects. A linear response in the range of 25-500 ppb and a standard deviation less than 10 % for liquid samples suggests TD-ESI/MS is also useful for quantitative analysis. These features make the technique particularly useful for screening a large number of samples.

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(22) Shieh, I. F.; Lee, C. Y.; Shiea, J. J. Proteome Res. 2005, 4, 606-612. (23) Huang, M. Z.; Jhang, S. S.; Cheng, C. N.; Cheng, S. C.; Shiea, J. Analyst 2010, 135, 759-766. (24) Huang, M. Z.; Cheng, S. C.; Jhang, S. S.; Chou, C. C.; Cheng, C. N.; Shiea, J.; Popov, I. A.; Nikolaev, E. N. Int. J. Mass Spectrom. 2012, 325, 172-182. (25) Hsu, H. J.; Kuo, T. L.; Wu, S. H.; Oung, J. H.; Shiea, J. Anal. Chem. 2005, 77, 7744-7749. (26) Basile, F.; Zhang, S. F.; Shin, Y. S.; Drolet, B. Analyst 2010, 135, 797-803. (27) Lin, J. Y.; Chen, T. Y.; Chen, J. Y.; Chen, Y. C. Analyst 2010, 135. 2668-2675. (28) Van Berkel, G. J.; Ovchinnikova, O. S. Rapid Commun. Mass Spectrom. 2010, 24, 1721-1729. (29) Van Berkel, G. J.; Ovchinnikova, O. S.; Kertesz, V. Anal. Chem. 2011, 83, 598-603. (30) Ovchinnikova, O. S.; Nikiforov, M. P.; Bradshaw, J. A.; Jesse, S.; Van Berkel, G. J. Acs Nano 2011, 5, 5526-5531. (31) Schmidt, A.; Karas, M.; Dulks, T. J. Am. Soc. Mass Spectrom. 2003, 14, 492– 500. (32) Talaty, N.; Mulligan, C. C.; Justes, D. R.; Jackson, A. U.; Noll, R. J.; Cooks, R. G. Analyst 2008, 133, 1532-1540. 18


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(33) Brewer, T. M.; Verkouteren, J. R. Rapid Commun Mass Spectrom. 2011, 25, 2407-2417. (34) Yinon, J.; McClellan, J. E.; Yost, R. A. Rapid Commun. Mass Spectrom. 1997, 11, 1961-1970. (35) Song, Y. S.; Cooks, R. G. Rapid Commun. Mass Spectrom. 2006, 20, 3130-3138.

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Table

Parameter

Table 1 Summary of optimal parameters in TD-ESI Optimal setting

Distance between quartz tube and ESI capillary Distance between ESI capillary and MS inlet Desorption temperature

8 mm 5 mm 250-350 ℃a

Carrier gas flow rate

0.8 L/min

Electrospray high voltage Electrospray solvent flow rate

5 kV 2.5 µL/min

a

The desorption temperature depends on the target analyte. In this study, the optimal desorption is around 350 ℃ for acetaminophen.

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Figure Legends Figure 1. Photographs of (a) TD-ESI source coupled to a triple quadrupole mass spectrometer, (b) the inside of the TD-ESI source, showing the mixing of desorbed analytes with the electrospray plume, and (c) schematic illustrations of desorption and ionization process in the TD-ESI source. Charged solvent droplets (acidic MeOH solution) are generated by electrospray; analytes in their liquid or solid states on the probe are desorbed via thermal heating, after which the desorbed analytes enter the electrospray plume to react with charged solvent species to form analyte ions in order to enter the MS inlet for detection.

Figure 2. TD-ESI/MS involves four independent analytical steps: (a) sampling with a probe (the right panel shows two types of probes) (b) insertion of the probe in the TD-ESI unit for desorption via thermal heating and post-ionization via electrospray; (c) detecting analytes with a mass spectrometer, and (d) cleaning the probe by burning it with a torch.

Figure 3. Dependence of signal intensity for acetaminophen ions on the (a) distance between quartz tube and ESI capillary (d1 in Fig.1), (b) distance between ESI capillary and the MS inlet (d2 in Fig.1), (c) desorption temperature, (d) carrier gas flow rate, (e) high ESI voltage, and (f) ESI solvent flow rate.

Figure 4.

(a) Extracted ion chromatogram (m/z 152) for ten consecutive

acetaminophen analyses via TD-ESI and (b) A linear response for acetaminophen (R2 =0.9978) in the 25 – 500 ppb range. The error bars represent the standard deviation of samples analyzed in triplicate.

21



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Figure 5. TD-ESI mass spectra of pharmaceutical samples: (a) eye drops, (b) cold syrup, (b) Ledernin ointment, and (c) Viagra tablet.

Figure 6. TD-ESI mass spectra of chemical compounds on different surfaces: (a) MDMA on a business card (50 ng/cm2); (b) codeine on a wooden desk (50 ng/cm2); (c) TNT on a cell phone (100 ng/cm2); and (d) RDX on a piece of hard plastic luggage (100 ng/cm2).

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For TOC only

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Figure1. Photographs of (a) TD-ESI source coupled to a triple quadrupole mass spectrometer, (b) the inside of the TD-ESI source, showing the mixing of desorbed analytes with the electrospray plume, and (c) schematic illustrations of desorption and ionization process in the TD-ESI source. Charged solvent droplets (acidic MeOH solution) are generated by electrospray; analytes in their liquid or solid states on the probe are desorbed via thermal heating, after which the desorbed analytes enter the electrospray plume to react with charged solvent species to form analyte ions in order to enter the MS inlet for detection. 229x260mm (96 x 96 DPI)


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Figure 2. TD-ESI/MS involves four independent analytical steps: (a) sampling with a probe (the right panel shows two types of probes) (b) insertion of the probe in the TD-ESI unit for desorption via thermal heating and post-ionization via electrospray; (c) detecting analytes with a mass spectrometer, and (d) cleaning the probe by burning it with a torch. 218x162mm (150 x 150 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Dependence of signal intensity for acetaminophen ions on the (a) distance between quartz tube and ESI capillary (d1 in Fig.1), (b) distance between ESI capillary and the MS inlet (d2 in Fig.1), (c) desorption temperature, (d) carrier gas flow rate, (e) high ESI voltage, and (f) ESI solvent flow rate. 254x190mm (96 x 96 DPI)


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Figure 4. (a) Extracted ion chromatogram (m/z 152) for ten consecutive acetaminophen analyses via TD-ESI and (b) A linear response for acetaminophen (R2 =0.9978) in the 25 – 500 ppb range. The error bars represent the standard deviation of samples analyzed in triplicate. 195x150mm (150 x 150 DPI)



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Figure 5. TD-ESI mass spectra of pharmaceutical samples: (a) eye drops, (b) cold syrup, (b) Ledernin ointment, and (c) Viagra tablet. 254x190mm (150 x 150 DPI)


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Figure 6. TD-ESI mass spectra of chemical compounds on different surfaces: (a) MDMA on a business card (50 ng/cm2); (b) codeine on a wooden desk (50 ng/cm2); (c) TNT on a cell phone (100 ng/cm2); and (d) RDX on a piece of hard plastic luggage (100 ng/cm2). 254x190mm (96 x 96 DPI)


Rapid Characterization of Chemical Compounds in Liquid and Solid

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