Thermal Diffusion Desorption for the Comprehensive Analysis of

Article pubs.acs.org/ac

Thermal Diffusion Desorption for the Comprehensive Analysis of Organic Compounds Zhibin Yin, Xiaohua Wang, Weifeng Li, Miaohong He, Wei Hang,* and Benli Huang Department of Chemistry and the MOE Key Lab of Spectrochemical Analysis & Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: Comprehensive analysis of organic compounds is crucial yet challenging considering that information on elements, fragments, and molecules is unavailable simultaneously by current analytical techniques. Additionally, many compounds are insoluble or only dissolve in toxic solvents. A solvent- and matrix-free strategy has been developed which allows the organic compound analyzed in its original form. It utilizes thermal diffusion desorption with the solid analyte irradiated with high energy laser. It is capable of providing explicit elemental, fragmental, and molecular information simultaneously for a variety of organic compounds. Thermal diffusion desorption has many advantages compared to the electrospray and MALDI techniques. The protons that form the protonated molecular ions originate from the analyte itself. All the elements and fragments are also derived from the analyte itself, which provides abundant information and expedites the identification of organic compounds.

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Laser desorption/ionization mass spectrometry (LDI-MS), as a predecessor of high irradiance laser ionization mass spectrometry (HILI-MS), has been developed for the analysis of small molecules.12 Yet in some cases, elemental or molecular information can be hardly acquired due to low laser irradiance or lack of suitable matrices. To date, many reports have highlighted the potential of HILI-MS for the elemental analysis of solids.13−15 Of particular interest is that HILI-MS has been applied to the direct multielemental analysis of biological samples, such as tea leaf, pig skin, and single cell.16,17 In our previous investigation, we demonstrated the capability of HILITOFMS to acquire elemental and molecular information on a small molecule (benzoylferrocene).18 Electrons with different mean free paths at different pressures were considered to represent the primary mechanism generating the elemental- or molecular-dominant spectra. As our research progressed, we found that high laser irradiance results in thermal diffusion in the area surrounding the crater, causing the desorption of intact molecules. We call this process thermal diffusion desorption. Many classes of organic compounds can be desorbed and ionized by this method, even those masses exceeding 1000 amu. In addition to the electron ionization mechanism illustrated previously,18 proton attachment was found to be an important and, in many cases, dominant ionization mechanism. In this paper, we present these advances in HILI-TOFMS for the

ith the development of modern chemistry, increasing numbers of elements are being discovered in naturally occurring compounds or synthesized into organic molecules.1,2 Comprehensive analysis of these compounds is a challenging problem, which requires both determining the elemental composition and elucidating the molecular structure. To unambiguously identify unknown compounds, it is imperative that the molecular, fragmental, and elemental information of interest should be obtained concurrently.3,4 Simultaneously deciphering both the elemental composition and the molecular structure of unknown compounds demands mutually exclusive techniques. Conventional elemental analysis techniques require the analyte to be atomized, destroying its structural information, whereas molecular analysis techniques provide structural information but do not give comprehensive elemental information. The solution to this dilemma is to develop a versatile analytical method that provides both structural information and elemental composition. To this end, some plasma-based ionization techniques, such as lowpower or reduced-pressure inductively coupled plasmas (ICPs),5 microwave-induced plasmas,6 and glow discharges,7 have been proposed as potential candidates for the comprehensive analysis. Modulated glow discharge plasmas, which take full advantage of different ionization mechanisms, have been experimented with in an effort to obtain quasisimultaneous elemental and molecular information.8−10 An integrated mass spectrometer combining two ionization sources, ICP and ESI, to detect elements and molecules, respectively, was also introduced and developed.11 © XXXX American Chemical Society

Received: February 13, 2014 Accepted: May 15, 2014

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dx.doi.org/10.1021/ac500602t | Anal. Chem. XXXX, XXX, XXX−XXX

Analytical Chemistry

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analysis of various organic compounds, tremendously broadening the analytical application of traditional laser ablation and ionization mass spectrometry.

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EXPERIMENTAL SECTION Samples and Materials. Organometallic compounds of zinc(II) acetylacetonate (96% purity), lead acetate (95% purity), copper phthalocyanine (CuPc, 95% purity), and cobalt tetramethoxyphenylporphyrin (CoTMPP, >96% purity) were purchased from J&K Scientific Ltd. (Beijing, China). Vanadyl 2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine (95% purity) was purchased from Sigma-Aldrich Company (Ontario, Canada). Phthalocyanine green (98% purity) was obtained from Damas-Beta Company (Basel, Switzerland). Bacitracin A (potency ≥60 U/mg) was purchased from Solarbio Ltd. (Beijing, China). The auxiliary gas of high-purity helium (99.999%) was purchased from Linde Gas Co. Ltd. (Xiamen, China). The detail information on analytes used in the experiments is listed in Table S-1 of the Supporting Information. Sample Preparation. For HILI experiments, all the samples were in the form of powder, which were dried by the vacuum drying oven for 1 h before analysis. Subsequently, each powder sample was pressed into a disc by the hydraulic press machine under a pressure of 1 × 107 Pa for 5 min. The pressed samples, with a thickness of 0.5 mm and a diameter of 6 mm, were fixed on a homemade direct insertion probe by kapton tape. For ESI experiments, stock solutions of CoTMPP were prepared in chloroform at a concentration of 1 × 10−3 mol/L. And diluted solutions were prepared from the stock solutions at a concentration of 1 × 10−5 mol/L before being introduced into the mass spectrometer. For MALDI experiments, the saturated solution of a-cyano-4-hydroxycinnamic acid (CHCA) was prepared using the mixture solvents of water/acetonitrile (v/v 30/70 with 0.5% formic acid). Solution of bacitracin A was also prepared in water/acetonitrile (v/v 30/70 with 0.5% formic acid), and final concentration of 1.4 × 10−5 mol/L was used for MALDI experiments. Besides, the stock solutions of CoTMPP were prepared in chloroform and final concentration of 1.0 × 10−5 mol/L was adopted. The standard “dried droplet” method by mixing 1 μL of sample solution and 1 μL of saturated matrix solution was employed to prepare MALDI targets.19 Two μL of the CoTMPP sample solution at a concentration of 1 × 10−2 mol/L was directly dropped on the MALDI plate, which was dried before LDI experiments. In LDI analysis of peptide, the powder analyte was directly filled onto a hole of target plate, which was dug in advance. Instrumental and Analytical Procedures. As Figure 1 shown, experiments were carried out using a home-built high irradiance laser ionization time-of-flight mass spectrometry (HILI-TOFMS) system that has been described previously with a few modifications.20 In brief, the ion source consists of a pulsed Nd:YAG laser (NL303G, EKSPLA), a laser focusing lens and a direct insertion probe where the analytes are fixed on. The laser beam passes through the quartz viewport and focusing lens with a focal spot of roughly 45 μm in diameter on the analyte surface. To acquire the high mass resolution, highpurity helium gas (99.999%) was indispensable in the ionization chamber.21 On the one hand, the inert buffer gas can be used to cool energetic ions and reduce the multiply charged ions by means of three-body recombination. On the

Figure 1. Schematic diagram of HILI-TOFMS.

other hand, the ionization of the buffer gas can be ignored due to the high ionization potential of helium (24.5 eV), which avoids chemical reactions. For the purpose of focusing the ion beam spatially, a set of Einzel lens is mounted behind the nozzle. The direct current quadrupole (DCQ) is used to guide the ion beam to the repelling region. The “pulse train” repelling mode was employed in the TOF analyzer.22 An in-housecompiled program written in LabVIEW 8.0 (National Instruments, Inc.) is used for collecting the spectra from a digital storage oscilloscope (42Xs, Lecroy, USA). Typical operation parameters can be found in Table S-2 of the Supporting Information. A high resolution ESI-MS (microTOF-QII, Bruker Daltonics, USA) was operated in the positive ion mode. In the source, the capillary voltage and the end plate offset potential were set at −4500 V and −500 V, respectively. The nebulizer gas pressure and flow rate of dry gas with the temperature of 180 °C were 0.8 bar and 2.0 L/min, respectively. Subsequently, collision induced desorption (CID) MS/MS experiments were used for acquiring the ion dissociation products of analytes. Argon was chosen as collision gas with a collision energy range of 0−65 eV. A commercial MALDI time-of-flight mass spectrometer (microFlex, Bruker Daltonics) was used in the experiment. A 337 nm nitrogen laser (5 ns pulse width and 100 μm of the spot diameter) was used and laser irradiance of 8.4 × 107 W/ cm2 was applied. The criterion for selecting the laser irradiance was to achieve the maximum molecular ion intensity with adequate resolution. The experiments were carried out in reflection and positive ion mode with an accelerating potential of 20 kV. Each spectrum was acquired from the accumulation of 100 laser shots. The same MALDI-MS was also used for laser desorption/ ionization (LDI) experiments. The experiments were carried out in reflection and positive ion mode without any matrix assistance. The laser irradiance of 1.4 × 108 W/cm2 was used.

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RESULTS AND DISCUSSION HILI-TOFMS of Organic Salts. To demonstrate the superior ability of the thermal diffusion desorption technique to simultaneously obtain information about metallic/nonmetallic elements, intact molecules, and the corresponding fragments from large organic compounds and organic salts, illustrative samples were selected. Lead acetate, representing organic salts, was analyzed by HILI-MS. A clean mass spectrum was obtained with very little interference (Figure 2). This spectrum reveals that nonmetallic elements (e.g., C, H, O), which are difficult to detect by conventional analytical methods,

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Figure 2. Spectrum including metallic/nonmetallic elements, fragments, and molecules of lead acetate from HILI-TOFMS.

can be acquired with ease by HILI-MS. In addition, the appearance of Pb ions, intact molecular ions, and a series of fragments offers valuable structural information for the identification of organic salts. Through further investigation, we find that the helium buffer gas pressure exerts a dramatic effect on the generation of dominant ion species (see Figure S1 of the Supporting Information). HILI-TOFMS of Organometallic Compounds. To further elucidate the feasibility of this analytical method, four organometallic compounds representing different types of organic compounds were analyzed. As shown in Figure 3a−d, under high pressure, the signals of the metallic ions are strong, while the intensities of the molecular ions and nonmetallic elements are weak. When the pressure is decreased, the intensities of the molecular ions and nonmetallic elements increase, whereas the signals of the metal ions drop. A detailed look at Figure 3a-d reveals that all of the molecules (M) are observed in the form of their own radical cations [M]+• and protonated parent ions [M + H]+, which, as far as we are aware, have seldom been observed in HILI mass spectra. Evidence for the protonated molecular ions [M + H]+ is shown in the insets of Figure 3a−d (with the theoretical isotope distribution in green). It is worth emphasizing that proton attachment is a well-known mechanism for the formation of [M + H]+ in MALDI with a variety of matrices. In HILI, the strong preference for the formation of [M + H]+ is because this matrix-free method provides abundant protons from the atomization of the target analyte itself at high laser irradiance. Evidence for the Origin of Proton. To further validate that the generation of the protons is from the target analyte, phthalocyanine green was analyzed because its hydrogen atoms are substituted by chloride atoms. The isotope distribution of its parent ions is consistent with the calculated isotope distribution as Figure 4 shown, in striking contrast to the insets of Figure 3a−d. It is worth noting that among the four samples analyzed, CuPc and CoTMPP, like most phthalocyanines and porphyrins, are practically insoluble in water or general organic solvents other than some toxic solvents.23,24 Thus, this solvent- and matrix-free analytical strategy should be highlighted. Mechanism of Thermal Diffusion Desorption. The laser-solid interaction, the mechanism of which has not been completely clarified, is an intricate process at low gas pressure. Under high laser irradiation, the solid sample surface of the analyte undergoes rapid vaporization in the irradiated area

Figure 3. Typical pressure-gradient mass spectra of (a) zinc(II) acetylacetonate, (b) CuPc, (c) CoTMPP, and (d) vanadyl 2,11,20,29tetra-tert-butyl-2,3-naphthaloc-yanine without any matrix assistance using HILI-TOFMS. C



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irradiated area, which results in the desorption of a large number of molecules. At high pressure, plasma expansion is remarkably suppressed by the presence of a large quantity of helium atoms, which causes the mean free paths of electrons and protons to be only several micrometres. Because the electrons and protons are “trapped” inside the plasma, they are unable to ionize the molecules desorbed from the thermal diffusion desorption area (Figure 6a). Therefore, almost no molecular information except

Figure 4. HILI-TOFMS spectra of phthalocyanine green acquired under different helium pressures.

(Figure 5, red circle 1 with a diameter of 45 μm). After the phase transition, an opaque plasma is formed within several

Figure 5. Image of the laser ablation of CoTMPP after 5 shots. Region 1 (red) depicts the ablation crater, and region 2 (green) shows the thermal diffusion desorption area. Figure 6. Mechanism diagrams of laser ablation, ionization, and thermal diffusion desorption under (a) high pressure and (b) low pressure.

picoseconds.25 Intense atomization and ionization processes occur in the plasma, which reaches temperatures of 10000− 50000 K, yielding an abundant supply of energetic free electrons.26,27 Similarly, protons will be generated through the atomization and ionization processes if the target analyte contains hydrogen. In such a harsh plasma environment, no molecules and fragments can survive. However, the laser energy absorbed by the solid generates heat that dissipates into the surrounding area of the crater. Under an irradiance of 1010 W/ cm2, thermal diffusion could reach a length of hundreds of micrometres.28,29 Thermal diffusion desorption of the target analyte will occur immediately in the area surrounding the crater (Figure 5, green circle 2 with a diameter of 400 μm). It can be observed from Figure 4 that the thermal diffusion desorption area is much larger (80 times) than the laser-

for the presence of metal ions can be acquired in the mass spectra obtained. Under low pressure (Figure 6b), electrons and protons will have longer mean free paths, which allows abundant collisions between the expanded high-energy electrons and thermal-desorbed molecules and, thus, the generation of parent radicals. Meanwhile, numerous proton attachments occur that produce protonated parent ions [M + H]+. Therefore, the spectra are dominated by the molecular radical cations [M]+•, protonated parent ions [M + H]+, and their fragments. Considering the relatively high ionization potentials and recombination rates,30 nonmetallic ions are easy to generate at low pressure because collisions are less frequent and the electron temperature is high.31 D



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Figure 7. Mass spectra of CoTMPP acquired via (a) HILI-MS, (b) ESI-MS using chloroform as solvent, (c) MALDI-MS with laser irradiance at 8.4 × 107 W/cm2 and CHCA as matrix, and (d) LDI-MS with laser irradiance at 1.4 × 108 W/cm2.

Comparison among HILI-, ESI-, MALDI-, and LDI-MS and Comparison between HILI-MS and CID Tandem MS. A comparison of HILI-, ESI-, MALDI-, and LDI-MS is shown in Figure 7. In addition to the information about molecular ions, the metallic elements of interest, the nonmetallic elements, and the characteristic fragments can be acquired simultaneously by HILI (Figure 7a). This cannot be achieved using other MS methods. Interference induced by the presence of the solvent and the matrix complicate spectral interpretation within the low mass range for ESI and MALDI, respectively (Figure 7b, c). In the case of LDI, relatively high laser irradiance (>1 × 108 W/cm2) is required to observe the molecular ions. The broad kinetic energy range of the ions is unbearable for the on-axis mass analyzer; as a result, the molecular and some fragment peaks were acquired at the expense of spectral resolution (Figure 7d). The comparison of the HILI pressure gradient spectra and the CID tandem MS product spectra is depicted in Figure 8. Different fragmental information can be alternatively acquired under variant helium pressure in HILI-MS. Under low pressure, the spectra are dominant by molecular ions, fragments, and nonmetallic elements, while the signals of the metallic ions are weak. When the pressure is increased, the intensities of molecular ions, fragments, and nonmetallic elements drop, whereas metallic ions are major species. HILI-TOFMS, as a single-stage MS, can generate similar fragments in high mass range as CID MS/MS. It is also able to provide information on metallic and nonmetallic elements as well as characteristic fragments in the low mass range, which cannot be given by CID MS/MS. HILI-TOFMS of Peptide. Because proton attachment has been rarely reported in laser ionization mass spectrometry, the HILI-MS was applied for the analysis of a peptide antibiotic, bacitracin A. An explicit spectrum can be obtained without matrix assistance (Figure 9). In addition to the molecular weight information provided by the protonated molecular ions,

Figure 8. (a) Pressure gradient mass spectra of CoTMPP from HILITOFMS. (b) The CID gradient spectra using ESI/MS/MS, with the CID energy ranges from 0 to 65 eV.

many peaks detected in the low mass region can be interpreted (inset in Figure 9). It should be noted that other “matrix-free” methods, such as porous silicon32−34 and sol−gel film,35 require the substrates to be chemically modified and the analyte to be dissolved. For thermal diffusion desorption, the protons that form the protonated molecular ions originate from the analyte E



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form, and the analysis is rapid, simple, and environmentally friendly. Compared with other MS techniques, the abundant information provided by HILI-TOFMS facilitates the identification of organic compounds.

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ASSOCIATED CONTENT

S Supporting Information *

Additional material as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

Z.Y. and X.W. contributed equally. Notes

The authors declare no competing financial interest.

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Figure 9. Mass spectra of bacitracin A acquired via HILI-TOFMS.

ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Science Foundation of China (No. 21027011). This work has also been supported by NFFTBS (No. J1310024).

itself. The fragments derived from the analyte provide additional structural information. More importantly, the analyte is analyzed in its original form without dissolution, and the substrate does not require any chemical modification. However, it should also be noted that this technique is essentially a thermal desorption method, which is useful for small peptides. When a large peptide of ∼3400 amu was analyzed, no molecular ions were observed. In addition to the buffer gas pressure, the influences of laser energy and laser wavelength were also studied. The results shown in Figure S-2 of the Supporting Information indicate that the laser irradiance level does not play a critical role in the generation of ion species. However, the wavelength plays an important role in thermal diffusion desorption. Evidence for the influence of laser wavelength on the yield of ion species is shown in Figure S-3 of the Supporting Information, where the contrast can be obvious. In compare with the spectrum acquired at a wavelength of 355 nm in Figure S-3a of the Supporting Information, the intensity of molecular ion is far stronger at 1064 nm wavelength (as shown in Figure S-3c of the Supporting Information), which might be ascribed to the fact that long wavelength is capital of strong penetrating power, and its energy can be absorbed effectively by solid analytes.36 Information of the fragments acquired from 1064 nm wavelength is less than that of 532 nm wavelength (as shown in Figure S-3b of the Supporting Information). Therefore, the laser wavelength of 532 nm was chosen in the experiment.

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CONCLUSIONS In summary, we have demonstrated that thermal diffusion desorption via high-irradiance laser ionization is capable of providing elemental, fragmental, and molecular information simultaneously for a variety of organic compounds. When the high-energy laser irradiates the target, thermal diffusion desorption occurs, and the activation area is much larger than the laser-irradiated area, which results in the desorption of a large number of molecules. While the high temperature plasma accounts for the generation of elemental ions, electron ionization and proton attachment are the major mechanisms responsible for the ionization of the desorbed molecules. By simply varying the buffer gas pressure, alternating spectra dominated by molecular ions or elemental ions can be acquired for organic compounds and small peptides. HILI-TOFMS is matrix- and solvent-free. The analyte is analyzed in its original F



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Thermal Diffusion Desorption for the Comprehensive Analysis of

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