Electron Ionization in Superimposed Magnetic and Radio Frequency

Moreover, only a small percentage of ions formed in the small ionization ... Collisional cooling in radio frequency-only (rf-only) ion guides has been...
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Anal. Chem. 2005, 77, 4160-4166

Electron Ionization in Superimposed Magnetic and Radio Frequency Quadrupolar Electric Fields Bingfang Yue,† Edgar D. Lee,† Alan L. Rockwood,‡ and Milton L. Lee*,†

Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602-5700, and ARUP Institute for Clinical and Experimental Pathology, 500 Chipeta Way, Salt Lake City, Utah 84108

An improved design of a novel electron ionization source for orthogonal acceleration time-of-flight mass spectrometry is described, based on the superimposition of an axial magnetic field with cylindrical symmetry around a radio frequency-only quadrupole. A tubular permanent magnet was designed to generate the required strong magnetic field and field profile. An axial electric field along the ion guide for efficient ion extraction was introduced using segmented quadrupole rods. Details of the source design and the effects of various operating parameters are described. The source produces high-quality mass spectra with regard to fragmentation, relative abundances, and isotopic ratios. Preliminary results have shown excellent sensitivity, with limits of detection in the subfemtogram range (octafluoronaphthalene, full spectrum acquisition) in gas chromatography/mass spectrometry operation. Electron ionization (EI) continues to be the most popular ionization method for the detection and identification of volatile compounds by gas chromatography/mass spectrometry (GC/ MS). EI produces molecular and fragment ions, typically by ionization of neutral molecules in the gas phase with 70-eV electrons. Standard EI spectra are often used for identification of unknown compounds by mass spectral library searching. Different mass analyzers impose specific requirements on the design of EI sources. For example, the ion source for a sector instrument must function with potentials of ∼10 kV. Therefore, the vacuum in the source must be very high to avoid electrical arcing. The resolving power, critically dependent on directional and energy focusing, requires an ion beam with well-defined kinetic energy and emittance. Ion extraction efficiency from the source is limited, which affects consequently instrument sensitivity. Several types of EI sources are currently used in analytical MS, and most of them are based on the design of Nier.1 Nier-type ion sources consist of a closed ion cage volume with an electron-emitting filament positioned outside this volume so that the ionizing electrons enter the ion volume through a narrow slit perpendicular to the instrument axis. Features of Nier-type ion sources include limited sample degradation on the hot filament and better control of the source temperature. However, the * To whom correspondence should be addressed. Tel: 801-422-2135. Fax: 801-422-0157. E-mail: [email protected]. † Brigham Young University. ‡ ARUP Institute for Clinical and Experimental Pathology. (1) Nier, A. O.; Schlutter, D. J. Rev. Sci. Instrum. 1985, 56, 214-224.

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ionization volume in a Nier-type ion source is so small (only part of the ion cage volume is covered by the collimated electron beam) that the ionization path length of electrons is short and the highest emission current available for ionization is limited by space charge effects. Moreover, only a small percentage of ions formed in the small ionization volume can be extracted and focused. Furthermore, especially in GC/MS operation, due to the open design of the source and the high gas flow rate from the GC, the residence times of neutrals are short and the sample partial pressure in the ionization volume is low. Space charge problems, due to highdensity background ions from residual or carrier gas, make ion extraction and transmission efficiency even worse. Recent advances have enhanced the capabilities of time-of-flight MS (TOFMS) to the point that it is now considered to be the MS detector of choice for high-speed separations.2-4 Orthogonal acceleration provides a convenient and efficient way to couple a continuous ion source to TOFMS.5-7 Moreover, it decouples the ion source from the TOF mass analyzer and makes independent design and optimization possible. Many TOFMS instruments have been developed with EI sources. However, most of the ion sources were originally designed for quadrupole, magnetic sector or other types of mass analyzers. An EI source specifically designed and optimized for orthogonal acceleration, TOFMS would be expected to significantly improve instrument performance. Collisional cooling in radio frequency-only (rf-only) ion guides has been widely used to guide or trap ions for improved ion transmission from an intermediate-pressure region to a highvacuum region, especially for atmospheric pressure ionization (API) interfaces.8 The effective potential formed by an inhomogeneous rf field confines ions in the radial direction while collisions of ions with neutral particles lead to dissipation of the initial kinetic energy. This collisional cooling process is similar to what occurs when the pressure of helium in a quadrupole ion trap is increased.9 (2) Wollnik, H.; Becker, R.; Gotz, H.; Kraft, A.; Jung, H.; Chen, C. C.; van Ysacker, P. G.; Janssen, H. G.; Snijders, H. M. J.; Leclercq, P. A.; Cramers, C. A. Int. J. Mass Spectrom. 1994, 130, 7-11. (3) van Deursen M. M.; Beens, J.; Hanssen, H. J.; Leclercq, P. A.; Cramers, C. A. J. Chromatogr., A 2000, 878, 205-213. (4) Davis, S. C.; Markarov, A. A.; Hughes, J. D. Rapid Commun. Mass Spectrom. 1999, 13, 237-241. (5) Sin, C. H.; Lee, E. D.; Lee, M. L. Anal. Chem. 1991, 63, 2897-2900. (6) Chernushevich, I. V.; Ens, W.; Standing, K. G. Anal. Chem. 1999, 452A460A. (7) Guilhaus, M.; Selby, D.; Mlynski, V. Mass Spectrom. Rev. 2000, 19, 65107. (8) Douglas, D. J.; French, J. B. J. Am. Soc. Mass Spectrom. 1992, 3, 398-404. (9) Stafford, G. C.; Kelly, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J. Mass Spectrom. Ion Proccesses 1984, 60, 85-96. 10.1021/ac0401927 CCC: $30.25

© 2005 American Chemical Society Published on Web 05/12/2005

Collisionally cooled ions with reduced ion kinetic energy spread acquire reduced amplitudes of radial oscillations in the effective potential well and occupy a region close to the ion guide axis, leading to improved ion transmission. Recently, we combined EI with collisional cooling in an rf ion guide to improve the ionization efficiency for orthogonal acceleration TOFMS.10,11 A key part of this development was the superimposition of an axial magnetic field of cylindrical symmetry generated by an electromagnet around the rf-only ion guide. The magnetic field compressed the electron beam from a filament into a long narrow volume along the ion guide axis. The magnetic field also helped to maintain a narrow energy distribution of electrons that penetrated the full length of the ion guide despite the influence of the radial rf field. Ionization occurred inside the ion guide with enhanced efficiency due to efficient use of electrons, prolonged interaction time, and nontraditionally large ionization volume. At the same time, the rf field substantially removed lowm/z background ions upon their formation, effectively focused the analyte ions in the radial direction, and confined them to the axis of the ion guide by collisional focusing, leading to high ion transmission efficiency. The main problem encountered using an electromagnet was excessive resistive heating by the dc current passing through the magnetic coil. In this study, we report an improved design using a segmented quadrupole and a permanent magnet. The tubular permanent magnet was able to provide the required strong magnetic field and field profile without the detrimental heating problem caused by the electromagnet. The axial magnetic field effectively confined the ionizing electrons within the quadrupole to achieve a longer ionization path length and, thus, higher ionization efficiency. The segmented quadrupole rods introduced an axial electric field for efficient extraction of ions that were formed with near thermal kinetic energy inside the quadrupole. The weak axial field used for efficient ion extraction little affected the kinetic energy of the ionizing electrons. The source design included consideration of vacuum operation to obtain the required pressure conditions in the ionization volume for collisional focusing while ion-molecule reactions and fragmentation due to collision-activated dissociation (CAD) were minimized. The performance of the source was evaluated for GC/MS operation. EXPERIMENTAL SECTION Ion Source. A top view of the construction of the ion source and its vacuum housing is shown in Figure 1. A cylindrical symmetry was adopted for ease in alignment. A circular opening was cut through the vacuum housing block along the instrument axis. A cylindrical permanent magnet with a side hole (Stanford Materials, Aliso Viejo, CA) was inserted into this opening from one end. A ceramic tube (custom-made by Progressive Technology, Auburn, CA) that contained the quadrupole rods and electron gun assembly was supported and thermally insulated from the vacuum housing and the permanent magnet by two Vespel (SP1, Dupont, Newark, DE) flanges. Vespel polyimide is a good thermal and electrical insulator and is able to handle high temperatures up to 260 °C. The permanent magnet was in contact (10) Yue, B.; Rockwood, A. L.; Lee, E. D.; Lee, M. L. Pittcon 2003, FL, March 9-14, 2003. (11) Yue, B.; Zhou, L.; Lee, E. D.; Dearden, D. V.; Lee, M. L. 225th ACS National Meeting, New Orleans, LA, March 23-27, 2003.

Figure 1. Schematic diagram of the new ion source and its vacuum housing.

with, and at the same temperature as, the ion source vacuum housing. Inside the ceramic tube, five sets of quadrupole rods (stainless steel, 6.35 mm diameter and 14.80 mm long) were fixed such that an inscribed radius of 3.13 mm was achieved and the neighboring rod sets were 1.0 mm apart. The electron gun assembly was inserted into one end. The electron gun assembly consisted of a filament coil, an electron repeller, and an entrance orifice plate that allowed electrons to enter the quadrupole. An exit lens was fixed on one Vespel flange and inserted into the ceramic tube on the other end. The volume inscribed by the quadrupole rods (∼4 cm deep into the quadrupole from the entrance lens, simply defined by the length of the permanent magnet) served as the ionization volume. The magnetic field confined electrons in the first three quadrupole rod sets (see Results and Discussion section for magnetic field profile). The conductance limits of the orifices on the entrance (2.0 mm diameter and 1.0 mm thick) and exit lens (1.0 mm diameter), and extra holes on the exit lens (8 × 2.0 mm diameter), were designed to control the pressure inside the ionization volume. The electron gun assembly was designed to have a high conductance so that the vacuum where the filament was located was the same as in the ion source housing. The vacuum in the ion source housing was monitored using a vacuum gauge connected to an opening in the housing, and the pressure in the ionization volume was estimated based on the gas load and its vacuum conductance. A heating block with heater and temperature sensor was fixed around the ceramic tube on the electron gun end to heat the source and control the source temperature. The source vacuum housing provided structure to use a heated GC/MS interface from a commercial quadrupole instrument (HP5973 GC/MSD, Agilent Technologies, Palo Alto, CA). The heated interface assembly designed for EI operation was simply bolted onto the source housing. The GC column or transfer line could pass through the heated interface, an opening in the tubular permanent magnet, a through-hole (0.5 mm i.d. × 9 mm deep) in a custom-made 4-40 screw installed on the ceramic tube, and directly into the ionization volume. Analytical Chemistry, Vol. 77, No. 13, July 1, 2005

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Table 1. Typical Voltage Settings for the New Ion Source electron repeller filament bias entrance lens DC1 DC2 DC3 exit lens rf voltage

Figure 2. Schematic diagram illustrating the electrical connections on the PC board used to apply voltages to the ion source. FL1 and FL2 were used to apply filament heating current, and Bias is the filament bias voltage. The other three dc voltages were for the electron repeller, quad entrance and quad exit. DC1, DC2, and DC3 were used to control the axial electric field. The rf voltage on the rod sets 1 and 2 could be lowered using a capacitance dividing circuit. C represents the capacitor, R the resistor, and L the inductor. L acts as an rf voltage choke so that a dc voltage can be applied to the quadrupole rods.

A four-layer PC board, gold-plated on both sides, was used for electrical connections (no other feed-throughs). A resistor and capacitor circuit on the board was used to provide dc and rf coupling and voltage dividing. Details are given in Figure 2 using a simplified diagram of the ion source. Capacitors were used to apply rf voltage to the segmented rods, and a capacitor divider was also implemented to apply lower rf voltage to the first two sets of rods if needed. A resistor circuit of six (1 MΩ) resistors was used to apply dc voltage to the five sets of quadrupole rods. Inductors (100 mH, 124 Ω at 1 MHz) were used to apply a third dc voltage to rod set 3 and to pass dc current while blocking the rf voltage. Resistance or inductance was utilized here to block the rf voltage and protect the dc power supply. The DC3 was necessary because most of the emission current ended on rod set 3. If no DC3 was applied, electron current falling on rod set 3 had to dissipate through the resistor divider, causing false dc voltage dividing. Vacuum wire and custom-made copper contact electrodes were used to apply voltages to the quadrupole rods. A home-built rf driver module12,13 (1.0 MHz frequency) was used to supply rf voltage to the PC board, and an oscilloscope was used to monitor the rf voltage. All dc voltages to operate the source were software controllable by the TOFMS instrument used. An external power supply was used to provide dc heating current to the filament. The typical voltage settings for the ion source are listed in Table 1. The ion source was set at 230 °C and the transfer line at 250 °C. The filament was electrically heated using ∼1.5 A dc current and biased at -70 V relative to the anode. The optimum voltage on the electron repeller was found to be 3 V more negative (12) O′Connor, P. B.; Costello, C. E.; Earle, W. E. J. Am. Soc. Mass Spectrom. 2002, 1371-1375. (13) Jones, R. M.; Gerlich, D.; Anderson, S. L. Rev. Sci. Instrum. 1997, 68, 33573362.

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-65 V -60 V +15 V +13 V +10 V +7V -20 V 1.0 MHz, 0-400 Vp-p

than the filament. More negative potential on the electron repeller reduced the effective electron current by space charge effects. However, the improvement was only ∼5%; thus, it was more convenient in this study to set the electron repeller at the same voltage as the filament. Some electrons collided with the entrance lens, and only the electrons that entered the quadrupole were responsible for ionization. The effective emission current was measured by subtracting the current on the entrance lens from the emission current on the filament, which were measured using two multimeters simultaneously. Instrumentation. A commercial TOFMS (Jaguar, LECO, St. Joseph, MI) was used to evaluate the new EI source. The instrumental details are described elsewhere.14 The original API interface was removed, and the new source was retrofitted before the einzel lens housing. With the original pumping system, the pressures in the flight tube chamber and source chamber were approximately 3 × 10-6 and 8 × 10-5 Torr, respectively, with gas load of 1.0 mL/min helium. Pressures listed are gauge readings while the vacuum gauge controller was calibrated for nitrogen. An HP 5890 (Agilent) equipped with a split/splitless injector for capillary columns was used in the GC/MS experiments for introducing helium and investigating sensitivity and detection limits. Materials. Hexane and octafluoronaphthalene (OFN, 99.99% purity) were obtained from Sigma (St. Louis, MO). Perfluorotributylamine (PFTBA, commonly known as FC 43, MS grade) was obtained from Agilent. Samples for GC injection were prepared by dissolving compounds in hexane and then diluting several times to the concentration levels desired. RESULTS AND DISCUSSION Permanent Magnet. As shown in our previous studies,10,11 an electromagnet could be used to provide the required magnetic field crucial for the ionization source design. The axial magnetic field is used to confine the ionizing electrons in the ion source without affecting ion focusing. However, it is difficult to use an electromagnet under vacuum conditions since it generates a large amount of heat that must be dissipated. It was easier and more straightforward to use a permanent magnet. There were, however, several requirements placed on the permanent magnet. First, the magnet had to be stable up to 300 °C, depending on the temperature at which the ion source operates. A permanent magnet can be demagnetized at a high operating temperature. Second, the magnet had to provide the required field strength and distribution. By trajectory calculations, (14) Lazar, I. M.; Rockwood, A. L.; Lee, E. D.; Sin, J. C. H.; Lee, M. L. Anal. Chem. 1999, 71, 2578-2581.

Figure 3. (A) Magnetic flux lines of the tubular permanent magnet and (B) flux density along its symmetrical axis simulated by Maxwell 2D. The permanent magnet as shown in the inset in (B) was made from SmCo R26HS with a side hole. The coordinates used in (B) were in inches and tesla (equal to10 000 G), and the permanent magnet was positioned 3 in. from the origin.

it was found that a field strength of >500 G was strong enough to confine energetic electrons in the presence of a radial rf field (rf amplitude up to 200 Vp-p). Only a samarium cobalt (SmCo) magnet was suitable for this study. A grade R26HS (2-17) SmCo magnet offered a maximum operating temperature of 380 °C and a maximum energy product (BHmax) of 27 Mega Gauss Oersteds (MG Oe). A finite element analysis modeling program (Maxwell 3D, Ansoft, Pittsburgh, PA) was used to assist in designing the permanent magnet by modeling the flux density, and its profile. The final design was a sintered SmCo R26HS (2-17) permanent magnet (50.8 mm long, 52.3 mm o.d., and 41.3 mm i.d. with a 16.5 mm diameter side hole, 24.5 mm from one end). It was magnetized in the direction of its length. The simulated flux

density and profile are given in Figure 3. As shown in Figure 3A, the flux lines emerged normal to the surfaces of each pole, rapidly became parallel to the axis, and passed through either inside or outside the magnet to reach the other pole. Thus, the electron gun had to be positioned inside the magnet to avoid electron reflection or total loss to the magnetic poles. A side hole was cut in the magnet to accommodate the heated transfer line. The effect of the side hole on the magnetic field inside the magnet was carefully examined. No difference in flux density was found in the area close to the axis where the ionization volume was located. Figure 3B shows the simulated flux density along the axis of the permanent magnet. The flux density could be as high as 0.1 T (1000 G) at the middle point of the magnet, decreasing with distance from the middle point. The flux density became much Analytical Chemistry, Vol. 77, No. 13, July 1, 2005

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stronger close to the wall as illustrated with lines of higher density in Figure 3A. The flux density outside the magnet decreased rapidly with distance from the magnet; thus, we expected no effect on mass analysis in the TOF mass analyzer used. The magnet was custom-made. Despite the temperature rating for the SmCo permanent magnet, we still decided to thermally isolate it from the heated ion source block and heated transfer line. Since the material of the permanent magnet was brittle, mechanical machining was avoided. It was tightly inserted into a cylindrical hole in the source vacuum housing and held in place by set screws. Pressure Considerations. Typical pressures are ∼1 Torr in conventional chemical ionization sources,15 ∼10-3 Torr in a collision cell for tandem MS,16 and 10-2-10-3 Torr in a collisional focusing ion guide in an API interface.8 These high pressures lead to a large number of collisions. In this study, we tried to take advantage of collisional cooling for efficient extraction. Moreover, a high pressure in the ionization volume created high neutral density, which led to greater ionization efficiency. However, the residence time of sample neutrals in the ion source must be kept sufficiently short for fast separation applications. Therefore, the pressure inside the ionization volume must be carefully controlled. Compromise was made among ionization efficiency, extraction efficiency, and neutral residence time in the ion source. An effective pressure of 10-3 Torr in the ionization volume was chosen for initial source design and evaluation. Since the pressure in the ionization volume could not be measured directly, the gas load and the conductance limits of the ion source were used to control the pressure in the ionization volume. With 1 mL/min helium gas load, which is typical in GC/MS operation, a pressure of 8 × 10-5 Torr (gauge reading) in the source housing was obtained with a turbomolecular pump (60 L s-1 for nitrogen, 55 L s-1 for helium). The gas load could not be higher because the pressure in the source chamber had to be maintained at ∼10-4 Torr to prevent damage to the electron-emitting filament and to prevent ion scattering loss due to ion/molecule collisions before the ions arrived at the high-vacuum region for mass analysis. With a total vacuum conductance of 4.4 L s-1 for nitrogen at 298 K, the effective pressure inside the ionization volume was estimated to be 1.5 × 10-3 Torr at a source temperature of 503 K. Under these conditions, the residence time of neutral gas in the ionization volume was estimated to be less than 100 µs. The residence time of ions in the ionization volume was estimated to be in the microsecond range, much shorter than that encountered in a quadrupole ion trap (typically in the millisecond range). In such a short time, helium is not an efficient cooling gas because of its low mass. A heavy gas such as nitrogen or methane, especially at high pressure, could be used to enhance collisions. Electrostatic Field. Radial electrodynamic rf fields have been studied thoroughly for use in confining and focusing ions under collisional damping conditions.17,18 It is well known that the absence of an axial electric field leads to inefficient ion transmis(15) Munson B. Int. J. Mass Spectrom. 2000, 243-251. (16) Lock, C. M.; Dyer, E. W. Rapid Commun. Mass Spectrom. 1999, 13, 422431. (17) Tolmachev, A. V.; Udseth, H. R.; Smith, R. D. Anal. Chem. 2000, 72, 970978. (18) Tolmachev, A. V.; Udseth, H. R.; Smith, R. D. Rapid Commun. Mass Spectrom. 2000, 14, 1907-1913.

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Figure 4. Electrical potential (A) and field strength (B) on the axis of a segmented quadrupole assembly including an entrance lens, three sets of rods, and an exit lens. The entrance and exit lens were 1.0 mm thick, the segmented rods were 14.8 mm long, and the spacing between the neighboring parts was 1.2 mm. The applied voltages were 30, 25, 20, 15, and 10 V in the order of entrance lens, segmented rods, and exit lens. The potential and electric field were simulated using SIMION 6.0.

sion, and the transit time of ions can be tens of milliseconds.19 Without an axial electric field, it is the ingress of ions (Coloumbic repulsion), gas flow, and diffusion that causes ion ejection. An axial electric field can greatly reduce the transit time of ions through the rf ion guide. A segmented quadrupole was examined in this study to provide the proper axial electric field needed to mobilize ions in the axial direction. Segmented rods have advantages including low rf field distortion and no quadrupole dc field. Theoretically, there is no upper limit for mass range transmitted. Different rf voltages can also be applied to different rod sets. It was found that -70 V on the filament and electron repeller relative to the entrance lens can penetrate into the ionization volume through the electron entry opening on the entrance lens. Ions formed close to the entrance lens in the ionization volume were attracted toward the filament in the penetration field instead of moving toward the exit lens. A fine grid (90% transmission) was used to cover the orifice on the entrance lens to completely remove the penetration field. It also totally removed the effect of the rf voltage on the filament operation. The electrical potential and axial electric field along the axis of the segmented quadrupole assembly are given in Figure 4 as simulated using SIMION. The quadrupole assembly includes three sets of round rods (14.8 mm long, 6.35 mm diameter) arranged to achieve 3.2 mm inscribed radius. A continuous potential (19) Loboda, A.; Krutchinsky, A.; Loboda, O.; McNabb, J.; Spicer, V.; Ens, W.; Standing, K. G. Eur. J. Mass Spectrom. 2000, 6, 531-536.

gradient (Figure 4A) was achieved by applying gradually decreasing voltages on the entrance lens, the segmented rod sets, and the exit lens. However, it is obvious in Figure 4B that the field strength has maximum values at the junctions of the rod segments and minimum values in the middle of the rod sets. The weakest axial electric field strength was as low as 0.4 V/cm, while it was 0.05 V/cm for rods 25 mm long (data not shown). Therefore, it is advantageous to use rods that are as short as possible; however, it is not practical to use extremely short segmented rods. Rods 15 mm long were used in this study. It was observed experimentally that even a weak axial gradient helped ion extraction and transmission, and the axial electric field affected high-mass ions more than low-mass ions. No ion signal was observed when the entrance lens, quadrupole rods, and exit lens were at the same potential, resulting in no axial electric field. The electrostatic field profile achieved using the segmented qaudrupole can be characterized as focusing or defocusing. For example, there is a focusing field between the entrance lens and the first set of quadrupole rods when a more positive voltage is applied to the entrance lens. Between any two neighboring sets of rods, the field focuses and then defocuses. Since ions are formed with thermal energy, the overall effect of the electrostatic field on the ion beam is defocusing and only a small fraction of ions pass through the exit orifice in the absence of collisional focusing. This explains why the axial electric field needed for efficient ion extraction depends on the actual pressure in the ionization source. Few collisions are expected under high-vacuum conditions in the ionization volume; an electrostatic field causes a defocused and diverging ion beam as explained above. A high negative voltage can be applied to the exit lens so that a strong focusing field can extract ions from the source with high efficiency, but with a large divergence angle or radial energy distribution. Under collisional focusing conditions, the effect of the weak electrostatic field becomes insignificant. The negative voltage applied to the exit lens little affected the ion intensity observed when the source pressure was increased, suggesting that ions were well focused close to the axis. Lower axial electric field resulted in higher ion intensity under high-pressure conditions in the ionization volume because the residence time of the ions was increased, resulting in more collisional focusing. Performance Evaluation. The quality of mass spectra generated (including mass resolution, fragmentation pattern, relative abundances, and isotopic ratios) was studied using PFTBA. Three mass spectra of PFTBA are given in Figure 5. The m/z 502 and its isotopic peaks are baseline resolved as shown in the inset in Figure 5A. In the Figure 5B inset, an expanded view of the lowmass range due to residual air is given with the proper ratio of 3.7 to 1 for m/z 28 to 32. The fragmentation patterns were consistent with the NIST/NIH library. It is reasonable to claim that high-m/z ions were favored (Figure 5C inset) since exact mass peaks were wider at high m/z for the TOF instrument used, and a method to generate unit mass spectra was not available. Importantly, the source pressure had little or no effect on the relative intensities of ions, suggesting that, under the conditions used, fragmentation due to CAD and ion-molecule reactions could be neglected. The isotopic ratios for several major fragments

Figure 5. Mass spectra of PFTBA acquired with an rf voltage optimized at (A) ∼40 V0-p for m/z 69, (B) ∼50 V0-p for m/z 131, and (C) ∼80 V0-p for m/z 219. Low ion abundance was obtained using low sample pressure and emission current. Ion detection system saturation was avoided. The inset in (A) shows m/z 502, (B) residual gas spectrum, and (C) high-mass range. In (C), ions of mass less than m/z 40 were completely cut off by the rf field and the percentage is the ratio of m/z 502 to 219 according to peak height.

of PFTBA (m/z 69, 219, and 502) were consistent with theoretical predictions. The mass range observed was greatly affected by the rf voltage used. Experiments with ions from residual gas and PFTBA showed that there was an optimum rf voltage for transmission of ions of different m/z, and high rf voltage favored high-m/z ions. Figure 5 shows three PFTBA mass spectra acquired with rf voltages optimized for (A) m/z 69 at ∼40 V0-p, (B) m/z 131 at ∼50 V0-p, and (C) m/z 219 at ∼80 V0-p. With higher rf voltages than the optimum, the ion signal of a given m/z began to decrease until it was completely cut off. The mass range observed increased with rf voltage. With the hardware used in this study, a mass range from m/z 20 up to m/z 614 could be covered using the optimum rf voltage of 40 Vp-p for m/z 69, and the ion intensity ratio of m/z 502 to 69 was 8.3%. A rf voltage of 100 Vp-p in Figure 5C was needed for the maximum transmission of m/z 502, while all ions smaller than m/z 40 and some of the m/z 69 ions were eliminated. The m/z 219 peak became the base peak and the ratio of m/z 502 to 219 was greater than 10% as shown in Figure 5C. Analytical Chemistry, Vol. 77, No. 13, July 1, 2005

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Figure 7. Summed mass spectrum across the peak width of 0.934 pg of OFN on-column after background subtraction.

Figure 6. Evaluation of the detection limits of the TOFMS instrument with improved EI source. GC/MS analysis of OFN was performed in the full spectrum mode at (A) 25 and (B) 3.125 spectra s-1; the selected-ion profile was reconstructed for ions of m/z 272. GC method: HP5890 GC; DB-5 column, 9.6 m, 0.18 mm i.d., 0.18 µm film thickness; helium carrier, 12.8 psi, constant pressure, 1 mL min-1 at 50 °C; temperature program at 50-120 °C at 35 °C min-1; 250 °C injector, and manual split, 1:130. MS method: Jaguar TOFMS with new EI source, 250 °C transfer line, 230 °C source, 40 s solvent delay, 2500 V detector voltage, 2030 DAQ threshold, and 100 µA emission current.

It was also observed that, at high ion intensity, the mass range observed was much smaller as suggested by the complete disappearance of high-m/z ions. This can be explained by the m/zstratified ion density radial distribution in collisional cooling rf multipole ion guides.17,18 That is, ion clouds consist of cylindrical layers, each of them having a specific m/z, with high-m/z ions occupying larger radii. Combining the charge capacity limit of the rf quadrupole and especially with the small diameter of quad exit orifice (1 mm) used in this study, most of the high-m/z ions could not pass through the source exit orifice. Additionally, for any rf device,20 when low rf voltages are applied, ions of high m/z are not efficiently trapped while ions below a well-defined m/z cutoff value are ejected. This is a result of the relatively weak effective potential well for high-m/z ions and unstable trajectories for low-m/z ions. (20) Dawson, P. H. Mass Spectrom. Rev. 1986, 5, 1-37.

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The instrument performance using the improved ion source was greatly affected by the type of mass analyzer and the actual instrument configuration used. The TOFMS was used in this study without application of a pulsing scheme to improve the sampling duty cycle of the TOF mass analyzer.10,11 Mass accuracy and mass resolution are more dependent on the mass analyzer used and are not reported here. The detection limits were evaluated by coupling the instrument to a gas chromatograph. Figure 6 shows two gas chromatograms for evaluating the detection limits using OFN. The chromatograms shown are reconstructed by extracting m/z 272 from data acquired in the full spectrum mode. As shown in Figure 6A and B, detection limits of 0.9 and 0.33 fg by linear extrapolation using a peak-to-peak noise criteria (3 rms noise) were obtained at 25 and 3.125 spectra s-1, respectively. Figure 7 shows a summed mass spectrum across the peak width shown in Figure 6B for