Single Photon Ionization Orthogonal Acceleration Time-of-Flight Mass

Anal. Chem. 2009, 81, 6038–6048

Single Photon Ionization Orthogonal Acceleration Time-of-Flight Mass Spectrometry and Resonance Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry for Evolved Gas Analysis in Thermogravimetry: Comparative Analysis of Crude Oils Robert Geissler,† Mohammad R. Saraji-Bozorgzad,† Thomas Gro¨ger,† Alois Fendt,‡ Thorsten Streibel,‡ Martin Sklorz,‡ Bernhard M. Krooss,§ Katrin Fuhrer,| Marc Gonin,| Erwin Kaisersberger,⊥ Thomas Denner,⊥ and Ralf Zimmermann*,†,‡,# Helmholtz Zentrum Mu¨nchen, Institute of Ecological Chemistry, Ingolsta¨dter Landstr. 1, 85764 Neuherberg, Germany, Lehrstuhl fu¨r Analytische Chemie, Abteilung fu¨r Analytische, Technische und Umweltchemie, Institut fu¨r Chemie der Universita¨t Rostock, Dr.-Lorenz-Weg 1, 18059 Rostock, Germany, RWTH Aachen University, Institute of Geology and Geochemistry of Petroleum and Coal, Lochnerstr. 4-20, Haus B, 52056 Aachen, Germany, Tofwerk AG, Uttigenstr. 22, 3600 Thun, Switzerland, Netzsch Gera¨tebau GmbH, Wittelsbacherstr. 42, 95100 Selb, Germany, and bifa Umweltinstitut GmbH, Kompetenzbereich Chemie, Am Mittleren Moos 46, 86167 Augsburg, Germany Coupling thermal analysis (TA) with a subsequent analytical method in order to investigate evolved gaseous products from the thermal analysis is a well established method. A popular practice to analyze the gaseous products evolving from thermal analysis is mass spectrometry using electron impact ionization (EI).1-4 As the kinetic energy of the electrons thereby is typically far beyond the ionization energies of the assayed samples, the electron impact effects fragmentation particularly of organic compounds, hampering the correlation of the ion signals to the gaseous compounds. This applies for complex mixtures in particular. Fragmentation can be reduced using so-called soft ionization techniques. In the course of the presented setup, single photon ionization (SPI) using electron beam pumped excimer lamps (EBEL) emitting vacuum ultraviolet (VUV) light (λ ) 126 nm) is employed. For the instrumentation, a TA system has been coupled to an EBEL-SPI-oaTOFMS (oaTOFMS: orthogonal acceleration time-of-flight mass spectrometry) system using a heated transfer capillary in order to detect semivolatile organic substances from the gas flow of a thermobalance with high temporal resolution. Presented measurements focus on crude oils of different origins. In-depth analysis demonstrates that it is possible to tell apart different crude oil samples on the basis of temperature resolved mass spectra gained from the described setup. TA allows for the assay of crude oils without sample preparation via a * To whom correspondence should be addressed: E-mail: ralf.zimmermann@ helmholtz-muenchen.de. † Helmholtz Zentrum Mu ¨ nchen. ‡ Lehrstuhl fu ¨ r Analytische Chemie. § RWTH Aachen University. | Tofwerk AG. ⊥ Netzsch Gera¨tebau GmbH. # Bifa Umweltinstitut GmbH.

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Analytical Chemistry, Vol. 81, No. 15, August 1, 2009

distillation process which precedes the thermal decomposition of nonvolatile oil components, i.e., resins and asphaltenes. The gases that evolve during thermal analysis are a complex mixture of organic compounds. These can be analyzed without losing molecular information using mass spectrometry with a soft ionization technique, such as SPI. Thermal analysis (TA) is a widespread standard analytical method to investigate temperature-dependent properties such as thermo-physical parameters and thermodynamical conversions. In combination with differential methodssdifferential thermal analysis (DTA) and differential scanning calorimetry (DSC)s thermogravimetry (TG) is particularly helpful when studying thermal decomposition.5-11 If not only thermal properties of the investigated material itself, but also the evolved gas phase products are of interest, coupling to further chemical analysis techniques becomes necessary. Analytical approaches that are employed in this context are gas chromatography,12-14 Fourier transform (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

Chiu, J.; Beattie, A. J. Thermochim. Acta 1980, 40, 251. Chiu, J.; Beattie, A. J. Thermochim. Acta 1981, 50, 49. Raemakers, K. G. H.; Bart, J. C. J. Thermochim. Acta 1997, 295, 1. Maciejewski, M.; Baiker, A. Thermochim. Acta 1997, 295, 95. Fatu, D.; Geambas, G.; Segal, E.; Budrugeac, P.; Ciutacu, S. Thermochim. Acta 1989, 149, 181. Sourour, S.; Kamal, M. R. Thermochim. Acta 1976, 14, 41. Asaletha, R.; Kumaran, M. G.; Thomas, S. Polym. Degrad. Stab. 1998, 61, 431. Flammersheim, H. J.; Opfermann, J. Thermochim. Acta 1999, 337, 141. Covolan, V. L.; Fernandes, E. G.; D’Antone, S.; Chiellini, E. Thermochim. Acta 1999, 342, 97. Li, Y. H.; Fan, Y. G.; Ma, J. B. Polym. Degrad. Stab. 2001, 73, 163. Stack, S.; O’Donoghue, O.; Birkinshaw, C. Polym. Degrad. Stab. 2003, 79, 29. Chiu, J. Anal. Chem. 1968, 40, 1516. Tsuge, S.; Sugimura, Y.; Nagaya, T. J. Anal. Appl. Pyrolysis 1980, 1, 221. 10.1021/ac900216y CCC: $40.75  2009 American Chemical Society Published on Web 06/25/2009

infrared spectrometery1-4 and mass spectrometery (MS). Particularly, the coupling of TA with MS is well established.15 In the latter context, the interface between the thermobalance and the mass spectrometer has to meet two requirements: 1. it has to provide a representative part of the evolving gases to the MS system and 2. it has to reduce the pressure from the thermobalance to high vacuum level.16 The results from the MS are dependent on the performance of the interface with respect to pressure, temperature, and flow rate. Cold spots can lead to condensation and re-evaporation in the interface region which distorts the correlation of the spectra to the respective temperature.17 Dilution with purge gas effects higher detection limits, temperature restrictions in the interface constrain the probability that certain classes are transferred to the MS. The two dominating concepts of hyphenating TA to MS are coupling via a transfer capillary and coupling via a skimmed supersonic expansion.16,18 Among the various ionization techniques, electron impact (EI) ionization is the most common one, readily available with many mass spectrometers. For EI, electrons emitted from a hot cathode are accelerated in a potential difference of usually 70 V. These accelerated electrons can transfer a part of their kinetic energy to the sample molecules, which leads to ionization and fragmentation. The drawback of this method is the high amount of energy available relative to the ionization energy (EI), which results in a deposited energy in the molecular ion of typically several eV.19 This excess energy causes fragmentation of a considerable part of organic analytes, thus molecular information is preponderated by fragment signals or even lost due to vanishing molecular ions. In order to obtain molecular information of gaseous mixtures in mass spectrometry, different soft ionization techniques are currently in use, e.g., chemical ionization (CI),20,21 metastable atom bombardment (MAB),22 resonance enhanced multiphoton ionization (REMPI)23,24 and single photon ionization (SPI).24 With SPI, a single photon of sufficient energy is absorbed by a molecule, thus an electron gains enough energy to reach the continuum state, leading to a positively charged ion. SPI is a semiselective ionization method as only compounds are accessible that have an EI lower than the photon energy. There are different methods to generate VUV light that is suitable for SPI. VUV photons can be produced through third harmonic generation of intense 355 nm Nd:YAG pulses to λ ) 118 nm using a rare gas cell. Since laser based systems usually involve high costs and the effort of maintaining a complex system, continuous VUV lamps such as deuterium (14) (15) (16) (17)

(18) (19) (20) (21) (22) (23) (24)

Costa, L.; Camino, G.; Trossarelli, L. J. Anal. Appl. Pyrolysis 1985, 8, 15. Materazzi, S. Appl. Spectrosc. Rev. 1998, 33, 189. Kaisersberger, E.; Post, E. Thermochim. Acta 1997, 295, 73. Saraji-Bozorgzad, M.; Geiβler, R.; Streibel, T.; Mu ¨ hlberger, F.; Sklorz, M.; Kaisersberger, E.; Denner, T.; Zimmermann, R. Anal. Chem. 2008, 80, 3393. Kaisersberger, E.; Post, E. Thermochim. Acta 1998, 324, 197. Genuit, W.; Nibbering, N. M. M. Int. J. Mass Spectrom. Ion Process. 1986, 73, 61. Lindinger, W.; Hirber, J.; Paretzke, H. Int. J. Mass Spectrom. Ion Process. 1993, 129, 79. Lindinger, W.; Hansel, A.; Jordan, A. Chem. Soc. Rev. 1998, 27, 347. Boutin, M.; Lesage, J.; Ostiguy, C.; Bertrand, M. J. J. Am. Soc. Mass Spectrom. 2004, 15, 1315. Aimoto, M.; Nishifuji, M.; Hayashi, S. I.; Suzuki, T.; Ishiuchi, S. I.; Fujii, M. Bunseki Kagaku 2007, 56, 1133. Cao, L.; Mu ¨ hlberger, F.; Adam, T.; Streibel, T.; Wang, H. Z.; Kettrup, A.; Zimmermann, R. Anal. Chem. 2003, 75, 5639.

or krypton discharge lamps pose an option.25,26 Recently, several applications17,27-33 with electron beam pumped excimer lamps (EBEL) as photon source for mass spectrometry have been reported. Advantages of these continuous or pulsed VUV photon sources are high luminosity and brilliance in particular. This high brilliance allows for the imaging of the narrow band emission spot at a specific point in the ion source, thus reducing noise signals resulting from photo electrons. Several applications for SPI-MS experiments are described in literature,28,31-33 e.g., TA-SPITOFMS26 or TA-SPI-QMS17 for the investigation of thermal decomposition of plastics. Another soft ionization technique is resonance enhanced multiphoton ionization (REMPI), which is much more selective than SPI. With REMPI, via an excited intermediate state, a molecule absorbs two or more photons with an energy each below the EI. The existence of such an intermediate state is precondition for the ionization. The first photon is absorbed, and the molecule is in an excited state. This step is similar to gas-phase UV absorption spectroscopy.34,35 As more photons have to be absorbed simultaneously in a nonlinear process, typically a photon flux density several orders of magnitude higher is necessary than with SPI.36,37 For this reason, REMPI applications are restricted to laser use. In order to detect evolved gas components even at low concentrations, it is suitable to employ time-of-flight (TOF) mass spectrometry, which combines wide dynamic range of m/z with high sensitivity and mass and time resolving power. There are several realizations of TOFMS, with linear or orthogonal extraction, with or without reflectron. The orthogonal extraction TOFMS (oaTOFMS) with reflectron is particularly suited for measurements with a continuous ion source due to its invariance to spatial distribution effects inherent to a continuous ion source.38 Crude oils are usually characterized according to their physical properties, e.g., density, viscosity. An analytical technique often used for the chemical characterization of crude oils is gas chromatography (GC). GC allows, e.g., a separation of hydrocarbons, which indicate certain properties of the scrutinized crude oil such as age or origin.39-41 Other techniques involve laser (25) Arii, T.; Otake, S.; Takata, Y.; Matsuura, S. J. Mass Spectrom. Soc. Jpn. 2006, 54, 243. (26) Arii, T.; Otake, S. J. Therm. Anal. Calorim. 2008, 91, 419. (27) Mu ¨ hlberger, F.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2002, 74, 3790. (28) Mu ¨ hlberger, F.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2002, 74, 3790. (29) Mu ¨ hlberger, F.; Wieser, J.; Morozov, A.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2005, 77, 2218. (30) Mu ¨ hlberger, F.; Streibel, T.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2005, 77, 7408. (31) Mu ¨ hlberger, F.; Wieser, J.; Morozov, A.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2005, 77, 2218. (32) Mu ¨ hlberger, F.; Streibel, T.; Wieser, J.; Ulrich, A.; Zimmermann, R. Anal. Chem. 2005, 77, 7408. (33) Mu ¨ hlberger, F.; Saraji-Bozorgzad, M.; Gonin, M.; Fuhrer, K.; Zimmermann, R. Anal. Chem. 2007, 79, 8118. (34) Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H.; Kettrup, A.; Boesl, U. Anal. Chem. 1999, 71, 46. (35) Boesl, U. J. Mass Spectrom. 2000, 35, 289. (36) Boesl, U.; Neusser, H. J.; Schlag, E. W. Chem. Phys. 1981, 55, 193. (37) Streibel, T.; Hafner, K.; Mu ¨ hlberger, F.; Adam, T.; Zimmermann, R. Appl. Spectrosc. 2006, 60, 72. (38) Guilhaus, D.S. M.; Mlynski, V. Mass Spectrom. Rev. 2000, 19, 65. (39) Stout, S. A.; Liu, B.; Millner, G. C.; Hamlin, D.; Healey, E. Environ. Sci. Technol. 2007, 41, 7242.


6039

Figure 1. Setup: A. Schematic of the TA-SPI-TOFMS-setup with thermobalance, ion source and attached EBEL and TOFMS. B. Photograph of the actual setup.

desorption ionization mass spectrometry (LDI-MS),42 supercritical fluid chromatography (SFC),43 and high performance liquid chromatography (HPLC).44 Despite mass spectrometric techniques usually lacking the ability for separating isomeric compounds, their advantage is the possibility to perform direct realtime analysis. Thus, it will not involve any problems with temporal correlation when coupled to other analytical techniques such as TA. Crude oils consist of a variety of hydrocarbons. In order to analyze the composition of crude oils, an ionization technique for MS is required which is suitable for a large number of organic substances and which as well conserves molecular information. The aforementioned SPI via EBEL is well suited for this application. A major difference between TA-MS and GC/MS regarding vaporable or partly vaporable samples such as crude oil is the separation technique. GC separates via phase interaction according to Nernst’s distribution law with multiple equilibrium steps, whereas TA separates with distilliation, when applicable to the sample. The distillation process in the TA is not to be equated with fractional distillation that is used, e.g., in oil refineries for fractionating crude oil. Fractional distillation usually proceeds at states closer to thermal equilibrium than TA. An advantage of TA over GC is that complicated sample preparation can be omitted in most cases, thus increasing reproducibility and excluding sources of error. The higher temperature range, the possibility to access thermal decomposition (pyrolysis) and the suitability for solid samples are also assets of TA. As nonfractional distillation alone is not sufficient to analyze complex mixtures such as crude oil, MS is employed to distinguish the components of the evolved gas. As EI fragments most of the organic oil compounds, an assignment of the compounds would become undeconvolutably complex. Therefore, for the analysis of natural organic mixtures, SPI-MS is superior to EI-MS in terms of interpretability of the gained spectra. The oils that were measured in the scope of this project differ from the outward appearance from each other. (40) Udoetok, I. A.; Osuji, L. C. Environ. Monit. Assess. 2008, 141, 359. (41) Wang, Z. D.; Stout, S. A.; Fingas, M. Environ. Forensics 2006, 7, 105. (42) Nguyen, H. P.; Ortiz, I. P.; Temiyasathit, C.; Kim, S. B.; Schug, K. A. Rapid Commun. Mass Spectrom. 2008, 22, 2220. (43) Rudzinski, W. E.; Aminabhavi, T. M. Energy Fuels 2000, 14, 464. (44) Aravanabhavan, G. S.; Helferty, A.; Hodson, P. V.; Brown, R. S. J. Chromatogr, A 2007, 1156, 124.

6040


Turkish crude oil is more viscous than Californian crude oil, so a higher content of asphaltenes can be expected. The color of the crude oils is dark brown to black. Principal components of crude oils are alkanes, cycloalkanes and aromatic hydrocarbons in different concentrations. Using TA-SPI-MS, the distribution of these compounds in evolving gas can be assessed at different temperatures, thus characterizing the different crude oils. EXPERIMENTAL SECTION An STA 409 PG Luxx thermobalance (Netzsch Gera¨tebau GmbH, Selb, Germany) has been used for the thermal decomposition studies. The crude oil samples have been heated from 40 °C up to 800 °C with a heating rate of 10 K/min in an aluminum oxide crucible in N2 atmosphere. The nitrogen flow through the sample chamber was set to 60 mL/min. The thermobalance was connected to the turbo pumped ion source of the mass spectrometer using a deactiviated 250 °C heated fused silica capillary with an inner diameter of 150 µm and a length of 2.1 m. An oaTOFMS (Tofwerk AG, Thun, Switzerland) as well as a homemade EBEL were flange-mounted on the ion source. The complete setup is shown in Figure 1A and B. The functionality of a similar EBEL is described in previous publications.17,31 With the here used argon filling for the excimer process, the obtained wavelength is 126 nm at peak emission (photon energy 9.8 eV), the fwhm of the distribution is 9 nm. As the electrons used for the generation of excimers have a kinetic energy of 12 keV, the minimum wavelength of the hereby emitted Bremsstrahlung is 1.0 × 10-10 m. This radiation is sufficiently blocked by the casing of the lamp. The construction of the interface between TA and vacuum chamber is crucial for the quality of the results. An insufficiently high temperature or nonuniform temperature distribution in the coupling region promote the appearance of so-called cold spots. Sample molecules that emerge from the crucible can condense at these cold areas, thus suspending the corresponding ion signal. A later re-evaporation at higher temperatures results in tailing of the ion signal, blurring the correlation of TA and MS signals. In order to prevent cold spots in the capillary coupling, the transfer line is heated to 250 °C. Additionally, a heated aluminum needle tempers the vacuum chamber end of the transfer line. At the TA end, a newly designed heatable aluminum adapter with a tubular

Figure 2. TA results: A. TG and DTG curve of Turkish crude oil and Californian crude oil, respectively. B. 3D contour plots with mass and temperature resolved intensity of the above crude oil samples. The yellow dots indicate the boiling temperatures of n-alkanes at the respective positions. Boiling temperatures taken from NIST Chemistry WebBook.46

copper core encloses the capillary. This configuration has shown its suitability even with tailing-prone substances such as heptadecane as could be demonstrated in a previous study.17 No tailing could be observed comparing the mass loss signal with the ion signal. This also applies for other “sticky” substances like tertoctylphenol or nonyl-phenol. Thus, the temperature of 250 °C for the transfer line is sufficient. A higher temperature of the transfer line, however, reduces capillary lifetime and throughput. Thus, a capillary temperature of 250 °C has been found to be well suited for our applications. The spectra gained from the TOFMS are averaged on an Acqiris AP240 averager card (Agilent Technologies, Plan-lesOuates, Switzerland, 1 to 2 GS/s) which features two channels for an increased dynamic range of the ion signal. Each second, 62 500 TOF extractions are averaged. The averaged spectra, one per second, are stored on the hard disk drive of the controlling computer. The available mass range with this acquisition rate is m/z ) 1 to 410. The mass resolving power with the used parameters is R ) 1000 according to the fwhm method. A lower spectra rate of 50 000 Hz was used for several measurements in order to increase the observable mass range to m/z ) 1 to 691 at the cost of signal-to-noise ratio. The EBEL was operated with an electron energy of 12 keV and an electron current of 4 µA for all measurements. The sensitivity of the system was determined using nitrogen samples containing 10 ppm benzene, 10 ppm toluene, and 10 ppm p-xylene

(BTX) let in through the heated capillary also used for the crude oil measurements. According to Williams et al.,45 the noise level was calculated using the variance σ of the background signal in a mass-to-charge range (m/z ) 46 to 70) where no ion signal is expected. The averaged signal in this region is defined as baseline B. The variance is put into relation to the average signal peak height S of the target molecule toluene. With the concentration c ) 10 ppm of toluene and a signal-to-noise ratio of 2 as condition, the limit of detection (LOD) can be calculated34 as LOD ) 2σc/ (S - B) ) 22 ppb for toluene at an electron current of 5 µA, an electron energy of 12 keV, and 62 500 averaged TOF extractions within 1 s. This poses a considerable decrease of LOD compared to quadrupol mass spectrometry measurements from previous studies: 114 ppb at a scanning rate of 5 (m/z)/s and 61 ppb at a scanning rate of 1 (m/z)/s with the EBEL operated at an electron current of 5 µA and an acceleration voltage of 13 kV.17 Mu¨hlberger et al.33 found an LOD of 35 ppb for an average over 0.65 s with an EBEL-oaTOFMS-system similar in construction. Note that a higher electron current in the EBEL raises the signal-to-noise ratio and thus lowers the LOD. SPI offers the possibility to semiselectively ionize substances with an EI of the respective photon energy or lower. A higher degree of selectivity can be obtained using REMPI, which specifically ionizes aromatic compounds present in samples like (45) Williams, B. A.; Tanada, T. N.; Cool, T. A. Symposium (International) on Combustion, Twenty-Fourth Symposium on Combustion, 1992, 241587.


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Figure 3. Temperature accumulated mass spectra for the different boiling ranges: top: accumulated spectra of Turkish crude oil for T ∈ (50 °C; 191 °C), (191 °C; 327 °C), (327 °C; 375 °C), and (375 °C; 550 °C). Components are tentatively classified according to residue class; black dotted lines mark: residue class 8 (i.a. alkylated benzenes), red: residue class 12 (i.a. bicycloalkanes/alkadienes/cycloalkenes), green: residue class 0 (i.a. cycloalkanes/alkenes), and blue: residue class 2 (i.a. alkanes). Bottom: Same as above for Californian crude oil.

crude oils. On the other hand, REMPI is not suited for the ionization of aliphatic compounds. Therefore, a REMPI-TOF system has also been coupled to the thermobalance in order to perform parallel measurements with both, SPI and REMPI. As natural samples may show inhomogeneities, it is indicated to conduct relevant measurements in one go. As a proof of concept for the parallel sampling of thermally evolved gases with two capillary based analytical methods, a furcation for sidelining gas to a REMPI-TOF has been coupled to the TA. The setup of the mobile REMPI mass spectrometer consists of a laser cabinet containing an Nd:YAG laser with two frequency-doubling units for the generation of λ ) 266 nm, and the mass spectrometer with the ion source, from which the heating hose containing the capillary proceeds to the heated aluminum casing. Furthermore, several electronic devices for instrument control and data acquisition are mounted in 19 in. racks (vacuum and heat control units, DAQ computer, high6042


Table 1. Tentative Assignments of Substance Classes and Residue Classesa class residue r

compound class formula

2 0 8 12

CnH2n+2 CnH2n C6H6 + CnH2n CnH2n-2

compound class name ng8 ng8 ng0 n g 10

alkanes cycloalkanes/alkenes alkylated benzenes bicycloalkanes/ cycloalkenes/alkadienes

a n g n0 conditions derive from the smallest detectable member of the homologous series.

voltage supplies for the mass spectrometer). As with the unbranched hyphenation, attention was turned on the continuous heating of the transfer lines. The branching has been realized using a y-connection in a heatable aluminum casing. As this kind of coupling by design imposes a constriction, it reduces the gas flow to the ionization region and therefore

Table 2. Averaged SPI Cross Sections for Different Substance Classes, Relative to Benzene and Absolute

rel. cross section cross section [mB]

alkylated benzenes49

alkanes50

alkenes49

1.0 22

0.2 4.4

0.4 8.8

raises the limits of detection. This problem could be remedied later with a redesign of the connexion, which allows the insertion of two capillaries into the sampling region over the crucible. This way, the connexion of two capillaries with each other can be circumvented, thus avoiding any constrictions of the gas flow. Results obtained with this redesigned hyphenation will be presented in a future publication. RESULTS AND DISCUSSION Two crude oil samples have been analyzed with the described setup: a Turkish and a Californian oil. These were measured several times with similar settings in order to gain a statistically relevant data set. Figure 2A shows the TA data from the Turkish and Californian crude oils. The mass loss of the more viscous Turkish crude oil is separated into two distinct steps, as can be seen in the two minima of the DTG curve (differential mass loss with respect to time). The first phase is a quasi-continuous evaporation process which ends at a temperature of about 350 °C. The initial mass loss can be assigned to the evaporation of water, as the bend at 100 °C suggests. This assumption has been verified using EI-MS. The second step is the release of the products of the pyrolytic decomposition of nonvolatile compounds. As the components of the Turkish oil are less volatile than those of the Californian oil, the evaporation phase of the Turkish oil is more sharp-cut. In contrast, the evaporation of the Californian crude oil starts immediately, resulting in a less distinct DTG evaporation peak. Also, the pyrolysis peak of the Turkish oil is more accentuated, meaning more material that decomposes at higher temperatures between 375 °C and 550 °C, due to the lower volatility. The contour plot in Figure 2B depicts the color-coded intensity versus temperature T and mass-to-charge-ratio m/z of single measurements of the respective crude oil sample. The results correlate well with the TG- and DTG-curves above. There is an evaporation phase beginning immediately for Californian crude oil and at 70 °C for Turkish crude oil. The evaporation phase features the appearance of substances according to their volatilities. It resembles a boiling point separation of the evolving compounds. With increasing temperature, components with higher molecular masses evaporate, leading to a shift in the molecular pattern to higher m/z. In the pyrolysis temperature interval from 375 °C to 550 °C, fragments of high molecular compounds are released over the complete observable m/z region. This behavior can be observed in all tested crude oils, with different intensities. For statistical analysis, five measurements of the Californian sample and five measurements of the Turkish sample have been averaged and compared. In the literature,47 crude oil boiling fractions during analysis are divided into naphtha (evaporating at 28 °C to 191 °C), distillate (46) NIST Chemistry WebBook; National Institute of Standards and Technology: Gaithersburg, MD, 2005. (47) Gilesh Crude Oil Assay Manual; United States Department of Energy: Washington, DC, 2008.

(191 °C to 327 °C), gas oil (327 °C to 566 °C) and residuum (above 566 °C). Though the distillation process taking place in the TA system is only distantly comparable to a boiling point separation used for the fractional distillation of crude oil in industrial processes, the temperature ranges for the above-mentioned fractions are used as a reference scale. In order to separate evaporation and pyrolytic decomposition, we split the gas-oil class into two temperature ranges, precracking (327 °C to 375 °C) and cracking (375 °C to 550 °C). At temperatures higher than 550 °C, no relevant signal could be detected and no mass change could be observed. In Figure 3, the accumulated spectra for the four boiling regions are shown for a Turkish and a Californian crude oil measurement. The boiling region (327 °C; 375 °C) of the Californian crude oil features distinct peaks at m/z ) 372, 386, and 400. These are the characteristic masses of C27-C29 steranes, cholestane, ergostane, and sigmastane, indicating a relatively young oil, which is only slightly affected by cracking or aromatization reactions.48 Note that SPI, as a generic ionization method, does not suppress specific chemical classes. In the context of crude oil analysis, this allows conservative quantitative estimates of the composition even with unit resolution of the m/z scale as thus gained mass spectra are generally dominated by hydrocarbons. In order to simplify the composition comparison of crude oils we use the “homologous series classification” to classify all hydrocarbons into five classes. Four of the classes are described in Table 1, whereas the fifth class comprises all remaining molecules. We describe the classification scheme as follows: the mass spectrum from each released organic compound is integrated in order to yield the peak area of each compound which is called its ion signal Ic. Each ion signal is multiplied with its corresponding mass mc which can easily be derived from the mass spectrum as SPI only creates singly charged ions (z ) e). These mass weighted ion signals Icmc are classified according to their mass residue r when applying the function “modulo m(CH2)” to the components mass mc: r ) mcmod m(CH2) For example, the compound with mc ) 78 has a residue of r ) 78 mod 14 ) 8. The compound with mass 92 has the same residue and therefore is in the same class. For each class Kr, the mass weighted ion signals of all its compounds are added up. m(Kr) )

∑I m c

c

Kr

In mathematical terms, the components are classified according to the congruency of their mass modulo m(CH2) ) 14. This reflects a mass-weighted grouping of homologous series which consist of substances differing in the number of added CH2 units. Residue classes 0, 2, 8, and 12 have been assigned to be dominated by cycloalkanes, alkanes, alkylated benzenes, and bicycloalkanes, respectively, thus including major components (48) Peters K. E. Moldowan J. M. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments; Prentice Hall: Englewood Cliffs, NJ, 1993.


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Figure 4. A. Molecular mass weighted assigned crude oil components. Assignments are based on the m/z value of the respective mass signals. Left: Temperature resolved mass loss in relation to the detected release of different constituents of Turkish crude oil. Right: Same as left for Californian crude oil. B. Cross section adjusted mass loss curves. Left: Temperature resolved cross section weighted mass loss in relation to the detected release of different constituents of Turkish crude oil. Right: Same as left for Californian crude oil.

of crude oils, see Table 1. With the here used unit resolution of the m/z scale, this poses a simplification. A higher resolution would be necessary in order to separate isobaric compounds. Additionally, a separation technique such as GC would allow to distinguish structural isomers. Within the context of this work, no such separation has been accomplished. Therefore, the class assignments according to the homologous series method (“modulo m(CH2) ) 14”) solely are an approximation and no unambiguous identification. Crude oils consist of various isobaric hetero components sharing the nominal masses with hydrocarbons, which leads to an error in the approximation made for the homologous series method. According to Speight,51 the elemental mass fractions in petroleum are 83.0% to 87.0% for carbon, 10.0% to 14.0% for hydrogen, 0.1% to 2.0% for nitrogen, and 0.05% to 1.5% for oxygen. For nitrogen, this means a molar fraction of 0.03% to 0.83%, for oxygen 0.03% to 0.55%. Only molecules with an even sum of nitrogen and oxygen atoms (leading to even masses) can contribute to even-numbered residue class peaks that are relevant in the context of this analysis. According to Snyder,52 to Aske,53 and to Woods,54 hetero compounds have the highest concentrations within resins and asphaltenes, thus further lowering the contribution to the relevant ion signals during distillation. Snyder52 lists a mass fraction of 3.0% in the relevant boiling fraction 200% to 6044


370 °C for N, O compounds in Californian petroleum. Thus, this does not mean that hetero compounds in crude oils can generally be neglected. If ionization methods like ESI are used which favor the ionization of polar compounds, ion signals are not necesseraly dominated by the group of hydrocarbons despite their abundance. As crude oils chiefly consist of hydrocarbons CxHy, the homologous series method is applicable in combination with generic ionization such as SPI. For more quantitative results, the mass weighted ion signals of each component should be corrected with its SPI cross section σc. In order to simplify this process, we use an average cross section σr for each class:

m(Kr) )

∑ σ1 I m Kr

c

c

c

=

1 σr

∑I m c

c

Kr

For a better understanding, an example of the calculation is presented: The integrated signal at m/z ) 78 (benzene, residue class 8) is muliplied with 78 in order to correlate the signal with mass instead of molecule number. The integrated signal at m/z ) 78 + 14 ) 92 (toluene, consequently also residue class 8) is multiplied with 92 and so on. The mass weighted signals of this homologous series of alkylated benzenes, in this case, are summed

Figure 5. Accumulated relative concentration profiles of the homologous series of alkanes or other substances of the same respective mass for different fractions of Californian (top) and Turkish (bottom) crude oil samples.

up and divided by the average cross section. Adam et al.49 have recently measured relative cross sections of hydrocarbons with SPI using a similar wavelength (λ ) 118 nm) with an Nd:YAG laser-based TOFMS and gas mixtures made with a standard generator, where mixtures were not commercially available. They found that cross sections are relatively uniform within a substance class while differing clearly among classes. An overview over the cross sections is presented in Table 2. According to their results, the components were weighted in order to roughly compensate for the different cross sections. In a recent investigation using EBEL-SPI with alkanes comprising also higher molecular masses, the cross section listing of alkanes has been verified and complemented. Persuant to these later measurements, the average cross section of alkanes has been adjusted, mass discrimination has been omitted. The released mass fractions correlate well with the mass loss, as shown in Figure 4A. Alkane residue class members dominate evaporation and thermal decomposition as well. The two phases differ from each other in the ratio of alkane residue class members to cycloalkane residue class members. During the cracking process, alkanes decompose, e.g., to alkenes, which share the same residue class with cycloalkanes, thus shifting the ratio of alkane residue class signals to cycloalkane/ alkene residue class signals toward the latter. The two oil types (49) Adam, T.; Zimmermann, R. Anal. Bioanal. Chem. 2007, 389, 1941.

differ in the ratios of mass fraction released during evaporation and decomposition as well as the evaporation onset of cycloalkane and alkylated benzenes, or their classes, respectively. Both observations are in line with the higher viscosity of Turkish crude oil. Additionally, the nonweighted “rest”, i.e., the sum of unassigned masses, is high during thermal decomposition when compared to the evaporation phase. The dominant homologous series of the alkanes and substances of the same residue class respectively, is depicted in Figure 5 for the different boiling fractions, the averaged Californian crude oil “alkane residue class” signals are on the top, while the averaged Turkish oil signals are on the bottom. Qualitative similarities in the relative distribution of the alkane residue class signals are easily visible. Figure 5 (upper, left) shows the “naphtha” fraction with similar distributions of the alkane residue class signals. The relatively low signals at m/z ) 240 in the gas(50) Mitschke, S. unpublished results. (51) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999. (52) Snyder, L. R. Acc. Chem. Res. 1970, 3, 290. (53) Aske, N. Characterisation of Crude Oil Components, Asphaltene Aggregation and Emulsion Stability by Means of near Infrared Spectroscopy and Multivariate Analysis; Department of Chemical Engineering, Norwegian University of Science and Technology: Trondheim, Norway, 2002. (54) Woods, J.; Kung, J.; Kingston, D.; Kotlyar, L.; Sparks, B.; McCracken, T. Oil Gas Sci. Technol. 2008, 63, 151.


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Figure 6. Statistical evalution of crude oil samples: A. Pixel based comparison of 4/5 Turkish and 4/5 Californian crude oil sample measurements. B. PCA clustering of the measurements revealing reproducible distinguishability of Turkish and Californian crude oil. C. VIP plot for B. D and E. 3D plot of averaged crude oil samples; signal intensity in logarithmic scale.

oil 1 fraction and at m/z ) 198 in all fractions are remarkable. Most outstanding as a distinct discrimination criterion is the shift of the maximum signal height in the distillation fraction, where Californian crude oil has its maximum at m/z ) 156, whereas the Turkish crude oil has its maximum at m/z ) 212. While it is possible to fingerprint crude oils looking at their isomeric alkane distributions,39 a more comprehensive approach, e.g., statistical evaluation or the analysis of carbon preference indices, is necessary when no separation technique such as GC is employed. The results show that the technique of analyzing crude oils using TA-SPI-TOFMS is viable. It allows the analysis of major components of this complex natural mixture and its decompostion behavior at elevated temperatures. In order to verify the reliability of the differences found between Turkish and Californian crude oils and to exclude coincidence effects on the discrimination, which might occur during measurement, statistical analysis was done on the repeated 6046


measurements of these two oil species. Five measurements of Turkish and five measurements of Californian crude oil were performed and evaluated in order to chart ion signal as a function of temperature and mass-to-charge-ratio, four of each have been included in the further evaluation. The ion signals were normalized to the respective weighted sample. In order to unify the axis resolutions of the different measurements, the m/z scale was recalculated to unit resolution as well as the temperature scale. Also, ion signal vs temperature functions were smoothed in order to reduce the influence of measurement artifacts. So, one data point is a pixel in a matrix with an m/z dimension, a temperature dimension, and a normalized ion signal value. The thus prepared matrix plots were compared using tstatistics.55 This way, the regions were identified which bear significant differences between both crude oil sorts. The level of (55) Gro ¨ger, T.; Welthagen, W.; Mitschke, S.; Scha¨ffer, M.; Zimmermann, R. J. Sep. Sci. 2008, 31, 3366.

Figure 7. Similarly recorded REMPI and SPI measurements of Turkish crude oil. A. TA-REMPI-TOFMS and TA-SPI-TOFMS 3D plots of Turkish crude oil. B. TA-REMPI/SPI-TOFMS difference plot. Red region indicates dominating REMPI signals, blue region indicates dominating SPI signals. C. REMPI and SPI spectrum at 250 °C. D. REMPI and SPI spectra at 450 °C. REMPI data were recorded at a resolution of 0.5 m/z. Analytical Chemistry, Vol. 81, No. 15, August 1, 2009

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significance for the denial of the null hypothesis was set to 95%, with both samples being the same. As can be expected, the evaporation region and the decomposition region are the regions in which the differences are most likely to be found, as shown in Figure 6A. Taking these regions of interest, a partial least-squares discriminant analysis (PLSDA) is done on the relevant pixels, which were found via the t test. Linear combinations of these pixels pose latent variables. These are calculated in a way that the Euklidean distance between the oil sample plots is maximized. Figure 6B depicts the result of the PLSDA with the first latent variable LV1, which is responsible for the highest fraction of the separation, i.e., 85%. The different measurements cluster strictly according to the origin of the oil, Turkish crude oil in the upper region, Californian crude oil in the lower region. The variance in LV1 is higher for Californian crude oil as the measurements are more strongly affected by starting condition variances due to the higher volatility of the oil. Nevertheless, the overall separation of the two oil types along LV1 is not affected. The pixels with a high influence on the LV1 are found via the variable of importance (VIP) plot, which is shown in Figure 6C. Several homologous series can be identified looking at this plot, which thus can be judged as characteristic for one of the crude oils. The alkanes centered around m/z ) 156 are especially distinct in Californian oil, the alkylated benzenes located at the region of m/z ) 190 are prevalent in Turkish crude oil as well as the alkenes or cycloalkanes around m/z ) 98, which occur during the decomposition. The above-mentioned steranes ergostane, cholestane, and sigmastane are not visible as a characteristic component of Californian crude oil. The corresponding mass peaks have a high relative variance due to their low concentrations. Thus, these substances do not qualify as characteristic via the t test. Higher photon fluxes with future EBEL constructions could solve this drawback. Figure 6D and E show the averaged spectra as 3D graphs with logarithmic intensity scales. As shown above at the example of water (explanation of Figure 2A), comparing SPI and EI is useful, if substances with high EI as well as substances that fragment, have to be analyzed simultaneously. If sensitivity and selectivity for aromatic or polyaromatic compounds are required, REMPI is the state of the art ionizing technique. Thus, combining of REMPI and SPI is of particular interest for mixtures of aromatic and aliphatic compounds. The REMPI technique is suitable for the selective, sensitive, and soft ionization of (poly)aromatic compounds. At the operated wavelength of 266 nm, the above-described two-photon (onephoton resonant) ionization process takes place. Then a second photon is absorbed, ionizing the appropriate molecules. Every 2.5 s, a mass spectrum has been obtained, using a laser repetition rate of 20 Hz and an averaging of 50 single spectra. A comparison of the REMPI and the SPI contour plot of the Turkish crude oil is shown in Figure 7. The first plot, Figure 7A, shows a difference spectrum of normalized REMPI and SPI spectra, Figure 7B depicts the REMPI plot and the SPI plot. The blue part in Figure 7A illustrates the regions where SPI provides higher relative intensities, in the red part, REMPI prevails. During evaporation, SPI

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signals are dominant at lower temperatures when considering similar m/z regions. This finding is supported by the fact that aromatic compounds have a higher boiling point than aliphatic compounds in the same m/z range, e.g., naphthalene (m/z ) 128) has a boiling point of 217 °C, whereas n-nonane (m/z ) 128) boils at only 151 °C.46 Therefore, the REMPI domination at higher temperatures can be attributed to the selectivity of REMPI for aromatic compounds. As the boiling point is irrelevant during pyrolysis, no temperature shift can be observed there. This supports the observation that the temperature shift between REMPI and SPI during evaporation is not a tailing artifact. As aliphatic compounds can have lower masses than aromatic ones, these dominate the lower m/z region during pyrolysis. On the REMPI plot, one can primarily see the peaks of alkylated 3-ring PAHs during the evaporation phase (Figure 7C) and alkylated benzenes during the decomposition phase (Figure 7D). On SPI side, several homologous series are visible during both the evaporation and decomposition phases, but with a lower intensity, particularly at elevated m/z. The benefit of REMPI is the selective detection of aromatic and polyaromatic compounds even at low concentrations. CONCLUSION In the scope of this study, the advantages of the combination of thermal analysis and mass spectrometry using soft ionization have been demonstrated at the example of crude oil fingerprinting. For ionization, a continuous VUV photon source (EBEL) has been employed; its suitability for the ionization of the variety of organic components of crude oil has been exhibited. Via statistical analysis, two crude oils of different origins could distinctly be distinguished. Improved versions of the EBEL with higher photon fluxes will be commercially available in the near future, enabling measurements with higher mass resolution and lower detection limits. A higher mass resolution might faciliate separation of isobaric compounds. A major advantage of the SPI method is the potential of use as generic soft ionization method for EGA-MS (EGA, evolved gas analysis). Other soft ionization techniques such as electrospray ionization (ESI) or laser desorption/ionization (LDI) also lead to ions with preserved molecular or quasi-molecular information but, conditional upon their principles, cannot be used as hyphenated online techniques for EGA, e.g., from TA. Additionally, ESI favors the ionization of polar, e.g., heterocyclic compounds, thus suppressing major components of crude oils such as cyclic and acyclic alkanes. ACKNOWLEDGMENT Support from Netzsch Gera¨tebau GmbH and Coherent GmbH and financial support by the BFS (Bayerische Forschungsstiftung) are gratefully acknowledged.

Received for review January 29, 2009. Accepted June 10, 2009. AC900216Y

Single Photon Ionization Orthogonal Acceleration Time-of-Flight Mass

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