Observation of Molecular Ions and Analysis of Nonpolar Compounds

Dec 30, 2008 - Observation of Molecular Ions and Analysis of Nonpolar Compounds with the Direct Analysis in Real Time Ion Source. Robert B. Cody*. JEO...
73 downloads 12 Views 339KB Size
Download PDF
Anal. Chem. 2009, 81, 1101–1107

Observation of Molecular Ions and Analysis of Nonpolar Compounds with the Direct Analysis in Real Time Ion Source Robert B. Cody* JEOL USA, Inc., 11 Dearborn Road, Peabody, Massachusetts 01960 Positive ions in the direct analysis in real time (DART) ion source are commonly formed by proton transfer. However, the DART source is similar to atmospheric pressure photoionization (APPI) in that it can produce molecular ions as well as protonated molecules, although the two sources differ in the initial ion formation process. This report discusses some of the factors that influence molecular ion formation in DART and shows how the DART source can be used to analyze “difficult” or nonpolar compounds such as alkanes and cholesterol. Trace reagent ions including NO+ and O2+ · formed from atmospheric gases are shown to play important roles in DART ionization. The use of the DART source as a gas chromatography/mass spectrometry (GC/MS) interface is demonstrated to show the difference between mass spectra obtained using conditions that favor proton transfer and those that favor molecular ion formation. Almost 5 years after the development of the direct analysis in real time (DART) ion source1 in 2003 and 3 years after its publication and introduction as a commercial product2-5 in early 2005, ambient ion sources are being used in a growing range of applications. DART and desorption electrospray ionization (DESI6) were the first of a new generation of atmospheric pressure ion sources that have been recently reviewed.7 The DART ion source has been applied to a broad range of applications that include drug discovery and reaction monitoring,8 planar chromatography,9,10 * To whom correspondence should be addressed. E-mail: [email protected]. (1) Cody, R. B.; Laramee, J. A. Method for Atmospheric Pressure Ionization. U.S. Patent 6,949,741, September 27, 2005. Priority date: April 14, 2003. (2) Laramee, J. A.; Cody, R. B. Method for Atmospheric Pressure Analyte Ionization. U.S. Patent 7,112,785, September 26, 2006. (3) Cody, R. B.; Larame´e, J. A.; Durst, H. D. Anal. Chem. 2005, 77 (8), 2297– 2302. (4) Cody, R. B.; Larame´e, J. A.; Nilles, J. M.; Durst, H. D. JEOL News 2005, 40 (1), 8–12. (5) Larame´e, J. A.; Cody, R. B. Chemi-ionization and Direct Analysis in Real Time (DART) Mass Spectrometry. In Encyclopedia of Mass Spectrometry, Vol. 6: Ionization Methods; Gross, M. L., Ed.; Elsevier: Amsterdam, The Netherlands, 2007; pp 377-387. (6) Takats, Z.; Wiseman, J. M.; Gologan, B.; Cooks, R. G. Science 2004, 306, 471–473. (7) Venter, A.; Nefliu, M; Cooks, R. G. Trends Anal. Chem. 2008, 27, 284– 290. (8) Petucci, C.; Diffendal, J.; Kaufman, D.; Mekonnen, B.; Terefenko, G.; Musselman, B. D. Anal. Chem. 2007, 79 (13), 5064–5070. (9) Morlock, G.; Ueda, Y. J. Chromatogr., A 2007, 1143, 243–251. 10.1021/ac8022108 CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008

forensic science,11-16 counterfeit drug identification,17,18 bacterial fatty acid profiling,19 food and beverage analysis,20-22 selfassembled monolayer analysis,23 and monitoring pheromones from live insects.24 The commercially available DART ion source consists of a tube containing a chamber through which helium or nitrogen flows at atmospheric pressure. An atmospheric pressure glow discharge is initiated by applying a kilovolt potential between a needle electrode and a grounded counter electrode. It should be noted here that the present implementation of the DART ion source uses an atmospheric pressure glow discharge, not a corona discharge as has been mistakenly reported in the literature.25 The gas exiting the glow discharge chamber in the DART source passes through a tube containing a perforated intermediate electrode, an optional gas heater, and a grid electrode positioned at the exit of the DART behind an insulating cap. Ionization occurs when the DART gas (10) Smith, N. J.; Domin, M. A.; Scott, L. T. Org. Lett. DOI: 10.1021/ol8012759. Web release date: July 16, 2008. (11) Larame´e, J. A.; Cody, R. B.; Nilles, J. M.; Durst, H. D. Forensic Applications of DART (Direct Analysis in Real Time) Mass Spectrometry. In Forensic Analysis on the Cutting Edge; Blackledge, R. D., Ed.; John Wiley and Sons, Inc.: Hoboken, NJ, 2007; pp 175-195. (12) Jones, R. W.; Cody, R. B.; McClelland, J. F. J. Forensic Sci. 2006, 51 (4), 915–918. (13) Coates, C. M.; Coticone, S.; Barreto, P. D.; Cobb, A. E.; Cody, R. B.; Barreto, J. C. J. Forensic Ident. In press. (14) Ropero-Miller, J. D.; Stout, P. R.; Bynum, N. D.; Casale, J. F. Microgram J., 2007, 5, numbers 1-4, http://www.dea.gov/programs/forensicsci/ microgram/journal_v5_num14/pg5.html. (15) Bennett, M. J.; Steiner, R. R. J. Forensic Sci. In press. (16) Steiner, R. R.; Larson, R. L. J. Forensic Sci. In press (expected publication date, May 2009). (17) Ferna´ndez, F. M.; Cody, R. B.; Green, M. D.; Hampton, C. Y.; McGready, R.; Sengaloundeth, S.; White, N. J.; Newton, P. N. ChemMedChem 2006, 1 (7), 702-705. (18) Newton, P. N.; Fernandez, F. M.; Plancon, A.; Mildenhall, D. C.; Green, M. D.; Ziyong, L.; Christophel, E. M.; Phanouvong, S.; Howells, S.; McIntosh, E.; Laurin, P.; Blum, N.; Hampton, C. Y.; Faure, K.; Nyadong, L.; Soong, C. W. R.; Santoso, B.; Zhiguang, W.; Newton, J.; Palmer, K. PLoS Medicine, 2008 5 (2), http://dx.doi.org/10.1371/journal.pmed.0050032. (19) Pierce, C. Y.; Barr, J. R.; Cody, R. B.; Massung, R. F.; Woolfitt, A. R.; Moura, H.; Thompson, H. A.; Fernandez, F. M. Chem. Commun. 2007, 807–809. (20) Haefliger, O. P.; Jeckelmann, N. Rapid Commun. Mass Spectrom. 2007, 21, 1361–1366. (21) Vail, T.; Jones, P. R.; Sparkman, O. D. J. Anal. Toxicol. 2007, 31 (6), 304– 312. (22) Cajka, T.; Vaclavik, L.; Riddellova, K.; Hajslova, J. LC/GC Eur. 2008, 21 (5), 250–256. (23) Kpegba, K.; Spadaro, T.; Cody, R. B.; Nesnas, N.; Olson, J. A. Anal. Chem. 2007, 79 (14), 5479–5483. (24) Yew, J. Y.; Cody, R. B.; Kravitz, E. A. Proc. Natl. Acad. Sci. U.S.A. 2008, 105 (20), 7135–7140. (25) Andrade, F. J.; Shelley, J. T.; Wetzel, W. C.; Webb, M. R.; Gamez, G.; Ray, S. J.; Hieftje, G. M. Anal. Chem. 2008, 80 (8), 2654–2663.

Analytical Chemistry, Vol. 81, No. 3, February 1, 2009

1101

makes contact with a gaseous, liquid, or solid sample in the openair gap between the DART outlet and the mass spectrometer sampling orifice. The sampling orifice is operated near ground potential so that there are no exposed high voltages, solvent sprays, or radiation in the ionizing region. Ionization does not occur in any atmospheric pressure ion source by a single, clearly defined mechanism such as electron ionization (EI). Instead a complex series of competing reactions can occur, and the outcome depends on factors such as the presence or absence of solvents or trace contaminants, temperature, gas flow, pressure, and electrical potentials. The first step in the previously published mechanism for positive ion formation in the DART ion source is Penning ionization26 in which a longlived (“metastable”) excited-state neutral atom or molecule N* transfers energy to an analyte molecule M, resulting in the formation of a molecular ion M+• and an electron e-:

initial ionization step (Penning ionization and photoionization, respectively) produces molecular ions M+• that can undergo further reactions with suitable analytes. The similarity between DART and APPI has also been pointed out for negative-ion formation mechanisms.32 Two different modes of ionization are illustrated herein by directing the output of a gas chromatography column directly into the DART gas stream. Chemical ionization mass spectra are observed that are similar to those reported for a GC/MS interface based on an atmospheric pressure chemical ionization source by choosing parameters that favor proton transfer.33 Parameters that favor Penning ionization and/or charge transfer produce mass spectra that more closely resemble electron ionization (EI) mass spectra. This report describes how ion source parameters and the presence of other reagent ions, including ions formed from atmospheric gases, can be optimized for the formation of molecular ions and the analysis of nonpolar compounds such as alkanes.

N* + M f M+•eThis reaction will occur if the analyte molecule M has an ionization energy that is lower than the internal energy of the excited neutral N*. The long-lived helium 23S state has an internal energy of 19.8 eV, which is higher than the ionization energies of common atmospheric gases and organic molecules. Atmospheric moisture is ionized by helium in the 23S state with extremely high efficiency: He(23S) + nH2O f [(H2O)n-1 + H]++OH• + He(11S) Proton transfer to produce the protonated molecule [M + H]+ will occur if the analyte molecule M has a higher proton affinity than the ionized water clusters. [(H2O)n + H]+ + M f [M + H]+ + nH2O Fragmentation may be observed if the proton affinity difference between the sample and the ionized water clusters is large. Fragmentation or pyrolysis can also be induced if the DART gas heater is set to a sufficiently high temperature.5,11,27 Ammonium adducts [M + NH4]+ may be formed for polar analytes if a source of ammonium is present. M + NH4+ f [M + NH4]+ Analytes for which ammonium attachment is commonly observed include compounds containing carbonyl functional groups such as acids,28 esters, and ketones. Ammonium attachment is also observed for peroxides.4,5,17 The DART ion source has some similarity to the atmospheric pressure photoionization (APPI) source28-31 in the sense that the (26) Penning, F. M. Naturwissenschaften 1927, 15, 818. (27) Cody, R. B.; Laramee, J. A. Poster presented at the 54th Annual ASMS Conference on Mass Spectrometry and Allied Topics, Seattle, WA, May 28-June 1, 2006. (28) Robb, D. B.; Covey, T. R.; Bruins, A. P. Anal. Chem. 2000, 72, 3653–3659. (29) Syage, J. A.; Evans, M. D. Spectroscopy 2001, 16, 14–22. (30) Kauppila, T. J.; Kuuranne, T.; Meurer, E. C.; Eberlin, M. N.; Kotiaho, T.; Kostiainen, R. Anal. Chem. 2002, 74, 5470–5479. (31) Kauppila, T. Atmospheric Pressure Photoionization Mass Spectrometry. Ph.D. Thesis, University of Helsinki, Finland, 2004.

1102

Analytical Chemistry, Vol. 81, No. 3, February 1, 2009

EXPERIMENTAL SECTION The experimental apparatus has been described previously3 but will be briefly reviewed here because of the importance of the various parameters in controlling the DART chemistry. The reader is referred to the initial DART publication3 for a diagram of the experimental setup. The mass spectrometer used for all experiments was an AccuTOF-DART orthogonal acceleration single-reflectron time-of-flight mass spectrometer (JEOL USA, Inc., Peabody, MA) equipped with a direct-analysis-in-real-time (DART) ion source (IonSense, Saugus, MA). The atmospheric pressure interface potentials were typically set to the following values: orifice 1 ) 20 V, ring lens ) orifice 2 ) 3 V. Under these conditions, proton-bound dimers may be observed for some analytes at high analyte concentrations. The rf ion guide potential was set to 100 V to permit the observation of low-m/z background and reagent ions. For some experiments, the ion guide potential was increased to 600 V to increase the sensitivity for analyte measurements above m/z 60. The mass spectrometer sampling orifice (orifice 1) was typically heated to between 80 and 120 °C to minimize contamination. The mass spectrometer was tuned on installation with the electrospray ion source to give a resolving power 6000 or greater (fwhm definition) for protonated reserpine at m/z 609.2812. Tuning remained stable without further operator intervention or adjustment throughout the duration of this study. A mass spectrum of poly(ethylene glycol) (average molecular weight 600) was included in each data file, measured separately from the analyte, as an external reference standard for exact mass measurements. All peak assignments reported herein were confirmed by exact mass measurements. Typical parameters or ranges of ion source parameters will be listed here, and the effects of these parameters will be discussed in more detail later. The commercial DART ion source permits adjustment of the source between 5 and 15 mm from the mass spectrometer sampling orifice. Closer or wider spacing could be achieved by partially unscrewing the ceramic insulator cap to (32) Song, L.; Dykstra, L.; Yao, H.; Bartmess, J. Poster presented at the 56th ASMS Conference on Mass Spectrometry and Allied Topics, Denver, CO, June 1-5, 2008. (33) McEwen, C. N.; McKay, R. G. J. Am. Soc. Mass Spectrom. 2005, 16 (11), 1730–1738.

Figure 1. (a) Mass spectrum showing a typical atmospheric ion background for the DART ion source. (b) Background mass spectrum showing an increase in the relative abundance in O2+• when the DART ion source is positioned closer to the mass spectrometer sampling orifice and the grid potential is increased from 250 to 650 V.

achieve a closer spacing or by unscrewing the DART source and moving it away from the mass spectrometer onto a table. Nitrogen and helium were used as the DART gases. Gas temperature was varied from ambient temperature up to 500 °C, and gas flow rates were varied from a few milliliters per minute to several liters per minute. The DART source was oriented so that the exit of the source points directly toward the mass spectrometer sampling orifice. DART electrode potentials were typically set to needle (glow discharge) electrode ) 3500 V, electrode 1 ) 150 V, electrode 2 (grid) ) 250 V. To interface a gas chromatograph (GC) to the mass spectrometer, a 0.5 m length of a 30 m DB-5MS column (250 µm i.d., 0.25 µm film thickness) (Agilent Technologies) was extended outside of the GC oven through a length of 1/8 in. copper tubing wrapped with heating tape and heated to 250 °C as measured by a thermocouple placed inside the heating tape. The copper tubing was positioned so that approximately 0.5 cm of GC column extended beyond the end of the tubing directly in front of the mass spectrometer orifice. The DART ion source was operated with helium, and the DART gas heater was set to 250 °C. A split injection was made with a split ratio of 50:1 for analysis of both the Grob test mix and the diesel fuel sample. The GC injector was heated to 250 °C, and the oven temperature was ramped from 50 to 300 °C at a rate of 10 °C min-1. Averaged mass spectra acquired were stored at a rate of 1 spectrum s-1 for the m/z acquisition range 50-400. RESULTS AND DISCUSSION A background mass spectrum obtained under typical DART conditions is shown in Figure 1a. This spectrum was obtained with the exit of the DART source positioned approximately 10 mm from the mass spectrometer orifice. The DART source potentials were set to needle ) 3500 V, electrode 1 ) +150 V, electrode 2 (grid) ) 250 V. Protonated water and water clusters [(H2O)n + H]+ are observed at m/z 19.0184, 37.0299, and 55.0395, and protonated ammonium is observed at m/z 18.0341.

Trace solvent vapors in the laboratory environment produce peaks assigned as MH+ for methanol (m/z 33.042), acetonitrile (m/z 42.0342), ethanol (m/z 47.0493), and acetone (m/z 59.0503). These peaks are indicated by asterisks in Figure 1a. The NO+ peak at m/z 29.9983 is significant despite its relatively low abundance. NO+ is highly reactive and can undergo a variety of ion-molecule reactions including addition, hydride abstraction, charge exchange, and oxidation.34 Figure 1b shows the effect of moving the DART source to within 3 mm of the mass spectrometer sampling orifice and increasing the electrode 2 (grid) potential to 650 V. An abundant peak assigned as O2+• is observed at m/z 31.9900; this is hypothesized to be formed by Penning ionization of atmospheric oxygen. The abundance of O2+• is strongly dependent on the grid potential, the proximity of the DART source to the mass spectrometer orifice, and the presence or absence of excess moisture. Excess moisture can be reduced by flaming the ceramic insulator cap on the DART ion source, by increasing the temperature of the DART gas heater, by minimizing the distance between the DART and the mass spectrometer sampling orifice, and/or introducing a small amount of dry air or oxygen into the DART sampling region. If the distance between the DART source and the mass spectrometer orifice is increased or if additional moisture is introduced, the relative abundance of O2+• diminishes and the relative abundance of the protonated water clusters increases. Conversely, the relative abundance of the peak representing O2+• can be increased by decreasing the DART/orifice distance, eliminating excess moisture, and/or bleeding oxygen gas into the DART sample gap. Charge transfer between O2+• and water is thermodynamically unfavorable. The ionization energies of diatomic oxygen and water are 12.0697 ± 0.0002 and 12.621 ± 0.002 eV, respectively.35 However, water has a higher proton affinity (691 kJ/mol) than oxygen (421 kJ/mol).35 The exact reason for the high relative abundance of protonated water clusters is not clear. It is possible that protonated water clusters are readily formed from residual moisture in the sample or the ceramic insulator cap close to the exit of the DART source where little atmospheric oxygen is present. In any case, we have found that moisture should be minimized to avoid competing reactions and lowering of the relative abundance of O2+•. Mass spectra obtained with DART obtained under the conditions associated with Figure 1a are dominated by protonated molecules and occasional even-electron fragment ions. Abundant molecular ions M+• are rarely observed under these conditions. An example is shown in Figure 2a for the mass spectrum obtained with DART for dibenzosuberone sampled with the sealed end of a melting point capillary. The same compound measured under the conditions that produce the background spectrum shown in Figure 1b gives a very different result, as shown in Figure 2b. This spectrum contains both the molecular ion and the protonated molecule and also the odd-electron fragment ion at m/z 180.0928, corresponding to [M - CO]+•. It should be noted that [M - CO]+• is the most abundant fragment observed in the EI mass spectrum of dibenzosuberone. (34) Harrison, A. G. , Chemical Ionization Mass Spectrometry, 2nd ed.; CRC Press: Boca Raton, FL, 1992. (35) http://webbook.nist.gov/chemistry/.

Analytical Chemistry, Vol. 81, No. 3, February 1, 2009

1103

Figure 2. (a) DART mass spectrum of dibenzosuberone obtained under conditions corresponding to those shown in Figure 1a. (b) DART mass spectrum of dibenzosuberone obtained under conditions corresponding to those shown in Figure 1b. Table 1. Proton Affinities and Ionization Energies for Reagent Ions Discussed in This Paper reagent

IE (eV)

proton affinity (kJ/mol)

reagent ion

water oxygen ammonia nitric oxidea fluorobenzene

12.621 12.0697 10.070 9.2642 9.20

691 421 853.6 531.8 755.9

H3O+ O2+ NH4+ NO+ C6H5F+

a

Formed by reactions with atmospheric oxygen and nitrogen.

The mechanisms involved for forming the molecular ions and other odd-electron species in the DART ion source are not fully understood. The abundance of odd-electron ions from given analytes correlates directly with the abundance of O2+• in the background mass spectrum. This suggests that chargeexchange may play a role as shown in the following reaction sequence, where M denotes an analyte molecule having an ionization energy (IE) less than that of diatomic oxygen (12.07 eV): He* + O2 f O2++e- + He O2+• + M f M+•+O2 O2+• + M f fragment+ + O2 + R• (IE of M < 12.07 eV) DART ionization can be modified by introducing other reagents into the ionizing region. Table 1 gives the ionization energies, proton affinities, and reagent ions for several reagents that have been used with the DART source. Fluorobenzene has been used as a dopant for atmospheric pressure photoionization (APPI).36 (36) Robb, D. B.; Smith, D. R.; Blades, M. W. J. Am. Soc. Mass Spectrom. 2008, 19 (7), 955–963.

1104

Analytical Chemistry, Vol. 81, No. 3, February 1, 2009

It has an ionization potential of 9.2 eV and a proton affinity of 775.9 kJ mol-1 (ref ) and acts as a charge-transfer reagent to ionize compounds that have ionization energies lower than 9.2 eV. Introduction of fluorobenzene vapor during DART ionization of cholesterol results in a dramatic change in the mass spectrum. Under “normal” conditions where O2+• is not present in the DART background, cholesterol tends to form a weak hydride abstraction peak [M - H]+ and an abundant dehydration peak [(M + H) - H2O]+ (Figure 3a). However, if fluorobenzene vapor is present, a molecular ion is observed in the mass spectrum (Figure 3b). A molecular ion is also observed if the DART parameters are adjusted to maximize the abundance of O2+• (Figure 3c). The ability to form odd-electron ions is advantageous for the analysis of compounds that do not undergo protonation, such as alkanes, or compounds such as cholesterol that readily fragment on protonation to form fragments such as [(M + H) - H2O]+. Odd-electron ions can fragment to produce mass spectra having peaks in common with electron ionization (EI) mass spectra, for which extensive databases are available.37,38 Because n-alkanes do not undergo proton transfer from protonated water or water clusters, these compounds provide particularly interesting examples. Mass spectra obtained for alkanes by using the DART ion source can be complex, containing peaks that represent [M - H]+ ions formed by hydride abstraction, numerous oxidation products including [(M + O) - H]+, [(M + O) - 3H]+, [(M + 2O) - H]+, [(M + 2O) 3H]+, [(M + 3O) - H]+, and CnH2n+1+, and CnH2n-1+ fragment ions. An extreme example is shown in Figure S-2a in the Supporting Information for hexadecane analyzed with a grid potential of 50 V. Hydride abstraction is commonly observed in the methane chemical ionization mass spectra of alkanes.34 Chemical ionization with NO+ is also known to produce hydride abstraction and oxidation.34 Although the relative abundances of the peaks representing the [M - H]+ ion and the oxidized species in mass spectra obtained with DART correlate with the abundance of NO+ in the background, the exact origin of the oxidized species is not well understood at this time. It may be noted that the relative abundance of these species is dependent on ion source parameters, humidity, and analyte concentration. Parts a and b of Figure 4 show the temperature dependence of mass spectra obtained with DART of n-hexadecane measured under conditions that produce abundant O2+• in the background mass spectrum. Specifically, the DART ion source exit is positioned to within 3 mm of the mass spectrometer sampling orifice (orifice 1), and the DART source grid potential is set to 650 V. An abundant peak representing M+• is observed at nominal m/z 226, together with a peak that represents the [M - H]+ ion and peaks that represent fragment ions dominated by the CnH2n+1+ ions. At a gas temperature of 200 °C (Figure (37) NIST/EPA/NIH Mass Spectral Database (NIST08), National Standard Reference Data System, National Institute of Standards and Technology, Gaithersburg, MD. (38) Wiley Registry of Mass Spectral Data, 8th ed.; John Wiley and Sons: Hoboken, NJ, 2006.

Figure 3. Mass spectra obtained with DART for cholesterol with (a) no dopant, (b) decreased DART/orifice distance and increased grid voltage, and (c) with the addition of fluorobenzene vapor.

Figure 4. Effect of gas temperature on the relative abundance of the molecular ion of n-hexadecane measured under conditions corresponding to those shown in Figure 1b: (a) gas heater set to 200 °C; (b) gas heater set to 300 °C.

4a), the base peak (100% relative abundance) in the mass spectrum represents M+• and the peaks representing the CnH2n+1+ fragments have relative abundances of approximately 20% or less. As the gas temperature is increased to 300 °C (Figure 4b), the base peak is C6H13+ and the peak representing M+• is reduced to a relative abundance of approximately 20%. Direct Penning ionization to form molecular ions cannot be ruled out because these ions have been observed when no significant O2+• is visible in the background mass spectrum and no other charge-exchange reagent has been introduced (Figures S-2c and S-3c in the Supporting Information). The possibility exists that analytes with a higher ionizing energy than oxygen can undergo direct Penning ionization if ionization occurs close to the exit of the DART ion source. However, these analytes can undergo charge transfer to oxygen and may not be detected by the system.

Increasing the potential on the discharge needle results in an increase in total ion current (Figure S-4 in the Supporting Information) but also increases the fractional abundance of the peak representing NO+ (Figure S-5 in the Supporting Information), which correlates with analyte oxidation. Analyte oxidation is diminished at reduced discharge potential, but sensitivity will also be decreased. The minimum needle voltage required to sustain an atmospheric pressure glow discharge is determined by the breakdown potential of the DART gas. For the present DART geometry, this corresponds to approximately 1800 V applied to the discharge needle. The distance between the exit of the DART ion source and the sampling orifice for the mass spectrometer atmospheric pressure interface has a big influence on the observed spectrum, in particular the presence of molecular ions and oxidation of the analyte. These effects are reflected in the low-mass background spectrum. The relative abundance of O2+• diminishes rapidly as the DART/orifice is increased from 5 to 15 mm, whereas the relative abundance of the water dimer [(H2O)2 + H]+ is maximized at a distance of 15 mm (Figure S-6 in the Supporting Information). The relative abundances of NO+ and NH4+ increase with increasing DART/orifice distance. The same observations hold true for analytes ionized by DART. Increasing the DART/ orifice distance results in diminished relative abundance for molecular ions and increases the relative abundance for oxidized species. The maximum abundance for protonated molecules is generally observed at a distance of about 15 mm. Inserting a glass tube between the DART and the mass spectrometer sampling orifice reduces the relative abundance of NO+ and increases the relative abundance of O2+•, reflective of conditions that minimize oxidation and favor molecular ion formation. Figure S-7 in the Supporting Information shows the effect of inserting a 6 mm i.d. glass tube between the DART and the sampling orifice at a DART/orifice distance of 17 mm. The tube contains the gas stream and reduces further interaction of ions with atmospheric gases. NO+ is minimized while the relative Analytical Chemistry, Vol. 81, No. 3, February 1, 2009

1105

abundance of O2+• is increased. A transfer tube with additional pumping capabilities has been reported to improve detection limits and quantitative reproducibility.39,40 The voltage for the DART grid electrode also influences the observed mass spectrum. The grid electrode is hypothesized to influence DART ionization by reducing or eliminating recombination reactions between opposite-charge species such as the molecular ions and electrons produced by Penning ionization. Figures S-2 and S-3 in the Supporting Information show the effect of changing the grid potential on the background mass spectrum (Figure S-3 in the Supporting Information) and on the measured mass spectrum for n-hexadecane (Figure S-2 in the Supporting Information). The exact mechanisms by which the grid electrode produces these changes are not well understood at this time. Sample placement and sample quantity can have an effect on the relative abundances of the molecular ion and the protonated molecule. Samples placed very close to the exit of the DART ion source tend to show more molecular ion formation than those placed close to the mass spectrometer sampling orifice. Figure S8a in the Supporting Information shows a small amount of molecular ion formed when a melting point tube containing 100 ng of chlorpromazine is placed close to the DART. Placing the same sample quantity closer to the mass spectrometer orifice shows minimal molecular ion formation (Figure S8b in the Supporting Information). Minimal molecular ion formation is also evident if 1 ng of chlorpromazine is held close to the DART source (not shown). A possible explanation is when the sample is further away from the DART source, the formation of protonated water clusters is likely to occur while the metastable helium atoms are unlikely to survive. The majority of published DART applications have used helium flowing at relatively high flow rates (several liters per minute), raising concerns about excessive helium consumption. In fact, only minimal gas flows are required to ensure a constant flow of gas through the glow discharge region of the DART ion source. The DART background mass spectrum shows no significant difference between helium flows of several liters per minute and several tens or hundreds of milliliters per second. Higher flow rates only assist in carrying ions entrained in the gas stream into the mass spectrometer atmospheric pressure interface. Preliminary results in our laboratory using a transfer tube with additional pumping39,40 have shown that by improving ion transport from atmospheric pressure into the mass spectrometer vacuum, excellent detection limits can be achieved with minimal DART gas flow. Nitrogen is inexpensive and readily available and may be substituted for helium for many DART applications. Nitrogen can produce high analyte ion currents, but it has some characteristics that must be taken into account. Nitrogen has a higher breakdown potential than helium and a higher electric field is required to initiate an atmospheric pressure glow discharge. It is more difficult to raise the temperature of nitrogen than helium. Therefore, an efficient gas heater is required to achieve comparable gas temperatures. In addition, analyte oxidation is more commonly (39) Crawford, E. A.; Musselman, B. D.; Tice, J. Presented at the 56th ASMS Conference on Mass Spectrometry and Allied Topics, Denver, CO, June 1-5, 2008. (40) Yu, S.; Crawford, E. A.; Tice, J.; Musselman, B. D.; Wu, J.-T. Presented at the 56th ASMS Conference on Mass Spectrometry, Denver, CO, June 15, 2008.

1106

Analytical Chemistry, Vol. 81, No. 3, February 1, 2009

Figure 5. GC/DART MS analysis of a Grob test mix (a) using a 3 mm DART/orifice spacing (with the ceramic cap unscrewed to extend the DART exit an additional 2 mm closer to the orifice) and a 650 V grid potential to produce abundant molecular ions and (b) using a 15 mm DART/orifice spacing and a 250 V grid potential to produce protonated or ammoniated molecules. The maximum signal for part a is approximately 10 times higher than for part b.

observed for some compounds when nitrogen is substituted for helium in DART analysis (Figure S6 in the Supporting Information). This suggests that nitrogen may be better suited for target compound identification by DART than for the identification of complete unknowns. A better understanding of the mechanisms involved in analyte oxidation may lead to techniques for controlling oxidation. An example of the application of the techniques described in this paper is shown in Figure 5 and Figures S-10a, S-11a, and S-12a in the Supporting Information for gas chromatography/DART mass spectrometry analysis of a Grob test mix (chromatograms in Figure 5 and mass spectra in Figures S-10a, S-11a, and S-12a in the Supporting Information) and a sample of diesel fuel (chromatograms in parts a and b of Figure S-13 in the Supporting Information). Figure 5a shows the total ion current chromatogram (TIC) for the Grob test mix measured with grid potential of 650 V and 3 mm spacing between the DART ion source and the mass spectrometer orifice. Under these conditions, all of the components in the test mix are detected including the two alkanes (ndecane and n-undecane). The mass spectra show some resemblance to EI mass spectra (Figures S-10b, S-11b, and S-12b in the Supporting Information). The mass spectra obtained with DART show more abundant molecular ions than the EI mass spectra. Hydride abstraction [M - H]+ or protonation. [M + H]+ may also be present in the mass spectra obtained with DART (Figures S-10a, S-11a, and S-12a in the Supporting Information) but are not observed in EI mass spectra (Figures S-10b, S-11b, and S-12b in the Supporting Information). Figure 5b shows the total ion current chromatogram for the same test mix obtained with a grid potential of 250 V and a DART/orifice spacing of 15 mm. These conditions favor the formation of protonated and/or ammoniated molecules, and the mass spectra are more similar to chemical ionization (CI) mass spectra. Compounds that do not readily form protonated or ammoniated molecules are not detected; these include decane, undecane, and 2-ethylhexanoic acid. The signal-to-noise for Figure 5a is higher than for Figure 5b, attributed to a diminished abundance of background and reagent ions. It should be noted that due to a typing error in entering the GC program, the analysis shown in

Figure 5b used a 20 °C min-1 oven temperature ramp whereas that shown in Figure 5a used a 10 °C min-1 oven temperature ramp. This does not change the elution order but explains the difference in retention times between the two chromatograms. All other GC conditions were the kept constant for both experiments. CONCLUSIONS The relative abundances of reagent ions present in the lowmass region of the background mass spectrum provide a useful diagnostic for the mechanism for positive-ion formation in the DART ion source. A key observation is that it is possible to produce relatively abundant molecular ions for nonpolar species such as saturated alkanes and cholesterol. Molecular ion formation is favored when the DART ion source is close to the mass spectrometer sampling orifice, the grid potential is raised to a potential greater than +250 V, and residual moisture is eliminated by baking the ceramic insulator cap. Molecular ion formation can also be favored by adding a small amount of a charge-exchange reagent such as oxygen or fluorobenzene into the DART sampling region. It should be noted that these results were obtained with a specific orientation of the DART, a specific design of the

atmospheric pressure interface, and a specific mass spectrometer. Different results may be obtained if the DART geometry and/or the atmospheric pressure interface are changed. Further studies will provide a better understanding of the factors that influence ion formation in the DART source and the chemistry that can occur during ion transport in open-air ion sources. Two preliminary examples of GC/MS analysis with the DART source were shown. The use of the DART source for GC/MS is not proposed as a replacement for conventional GC/MS analysis because the DART source cannot produce classical EI mass spectra. However, there is a potential benefit of this approach for experiments where high carrier gas flow rates are incompatible with vacuum-based ion sources or where the lack of an electron filament is desirable for the analysis of corrosive compounds. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review December 11, 2008.

October

17,

2008.

Accepted

AC8022108

Analytical Chemistry, Vol. 81, No. 3, February 1, 2009

1107