Anal. Chem. 2000, 72, 5014-5019
Improved Ion Transmission from Atmospheric Pressure to High Vacuum Using a Multicapillary Inlet and Electrodynamic Ion Funnel Interface Taeman Kim, Harold R. Udseth, and Richard D. Smith*
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352
A heated multicapillary inlet and ion funnel interface was developed to couple an electrospray ionization (ESI) source to a high-vacuum stage for obtaining improved sensitivity in mass spectrometric applications. The inlet was constructed from an array of seven thin-wall stainless steel tubes soldered into a central hole of a cylindrical heating block. An electrodynamic ion funnel was used in the interface region to more effectively capture, focus, and transmit ions from the multicapillary inlet. The interface of seven capillary inlets with the ion funnel showed more than 7 times higher transmission efficiency compared to that of a single capillary inlet with the ion funnel and a 23-fold greater transmission efficiency than could be obtained using the standard orifice-skimmer interface of a triple-quadrupole MS. The multiple-capillary inlet and ion funnel interface showed an overall 10% ion transmission efficiency and approximately 3-4% overall detection efficiency of ions from solution based (i.e., prior to electrospray). The improved performance was achieved under conditions where ESI operation is robust and results in a significant increase in dynamic range. Electrospray ion sources (which we broadly consider to include conventional electrospray, as well as related microelectrospray and nebulizing gas-assisted electrospray, etc.) are widely used with mass spectrometry for biological research. For m/z analysis, the ions created at atmospheric pressure must be transported to the high-vacuum region of a mass spectrometer. A differential pumping system involving several stages for stepwise pressure reduction is commonly used to achieve the vacuum conditions required for m/z analysis, and the major design issues are generally related to optimizing overall ion transmission efficiencies. Improved transmission efficiencies in the intermediate-vacuum stages can be achieved by using the recently developed rf ion funnel at higher interface pressures (∼1 to 10 Torr)1,2 and rf multipole ion guides with buffer gas cooling at lower interface pressures.3 However, there is no efficient ion focusing mechanism in atmospheric pressure for the ion currents relevant to electrospray ionization (ESI) where space charge effects dominate, and ion transmission * Corresponding author:
[email protected]. (1) Shaffer, S. A.; Tang, K.; Anderson, G. A.; Prior, D. C.; Udseth, H. R.; Smith, R. D., Rapid Commun. Mass Spectrom. 1997, 11, 1813-1817. (2) Shaffer, S. A.; Prior, D. C.; Anderson, G. A.; Udseth, H. R. and Smith, R. D. Anal. Chem. 1998, 70, 4111-4119. (3) Douglas, D. J.; French, J. B. J. Am. Soc. Mass Spectrom. 1992, 3, 398-408.
5014 Analytical Chemistry, Vol. 72, No. 20, October 15, 2000
between an ion source and the first vacuum stage is primarily dependent upon the proximity and gas conductance of the interface inlet. In this work, we have aimed to improve the ion transmission through the interface between the ESI source and the first vacuum stage. To improve the ion transmission, a larger inlet is clearly desired, but the inlet size is limited by several factors. For a capillary inlet, simply using a larger inner diameter capillary is problematic. First, the desolvation is less effective for a larger inner diameter capillary inlet because of the greater temperature variation across the capillary radius (resulting in a large variation in droplet desolvation efficiency). The second problem is the ion transmission efficiency in the first vacuum stage may be decreased due to greater gas dynamic effects. To address these issues we have developed a new, multicapillary inlet interface designed for operation with an electrodynamic (rf) ion funnel. Instead of using a larger inner diameter capillary inlet, the new design uses an array of inlet capillaries. Therefore, it provides more uniform droplet evaporation conditions than provided by a single capillary having the same gas conductance. It also changes the gas dynamics by splitting the gas flow. It should be noted that multicapillary inlets have been used previously with multielectrospray sources for purposes different from this study. The first of these was a “Y”-type inlet used to study ion-ion reactions at close to atmospheric pressure.4,5 Another was a multi-inlet capillary used to separately introduce ions from both analyte and calibrant sample with a multispray source (to minimize ion-ion interactions).6 In this paper, we report preliminary results for the achievable ion transmission and detection efficiency with a new ESI-multicapillary inlet and ion funnel interface design. EXPERIMENTAL SECTION The rf ion funnel has been demonstrated to provide for effective ion transportation from ESI interface pressures (∼1 Torr) to high vacuum.1,2,7 To achieve high transport efficiency from the atmosphere through the first pumping stage, one may be inclined to select the largest inlet aperture that is compatible with pumping (4) Ogorzalek Loo, R. R.; Smith, R. D. J. Phys. Chem. 1991, 95, 6412. (5) Ogorzalek Loo, R. R.; Smith, R. D. J. Am. Soc. Mass Spectrom. 1994, 5, 207-220. (6) Jiang, L.; Moini, M. Anal. Chem. 2000, 72, 20-24. (7) Kim, T.; Tolmachev, A. V.; Harkewicz, R. H.; Prior, D. C.; Anderson, G.; Udseth, H. R.; Smith, R. D.; Bailey, T. H.; Rakov, S.; Futrell. J. H. Anal. Chem. 2000, 72, 2247-2255. 10.1021/ac0003549 CCC: $19.00
© 2000 American Chemical Society Published on Web 09/07/2000
Table 1. Bias Potentials of the Ion Optical Element Used for Performance Evaluation
Figure 1. Schematic of the API 3000 MS system with a multicapillary inlet and ion funnel interface. IF, ion funnel; L0, lens after ion funnel; Q1 and Q3, analyzing quadrupole; Q2, rf quadrupole collision cell; IQ1, IQ2, and IQ3, interquadrupole lens; Stub1, Stub2, and Stub3, short rf-only quadrupoles.
requirements of the first vacuum stage. However, when we used a 0.76-mm-i.d., 76-mm-long capillary with an ion funnel, the overall ion transmission efficiency of the capillary and ion funnel interface was not improved because of the low transmission efficiency in the ion funnel.7 The low efficiency of the ion funnel transmission with a larger capillary inlet is attributed to the less effective desolvation and increased gas dynamic effects caused by the increased gas flow. In this study, we used a multicapillary inlet instead of a larger inner diameter capillary inlet. The experiments were conducted using an API 3000 triple quadrupole mass spectrometer system (Sciex, Concord, ON, Canada) modified with a custom multicapillary inlet and an rf ion funnel interface (Figure 1). Multicapillary Inlet. A heated multicapillary inlet was designed and fabricated by silver soldering seven 76-mm-long stainless steel tubes (Small Parts Inc., Miami Lakes, FL) into a hole of a cylindrical stainless steel heating block. Two different capillary diameters were evaluated (0.51-mm i.d., 0.71-mm o.d. or 0.43-mm i.d., 0.64-mm o.d.). A seven-capillary inlet is shown in Figure 2. The same diameter was used for all seven tubes, resulting in inlets whose theoretical conductance differs by a factor of 7 compared to a single capillary of the same dimension. To maintain constant temperature on the inner surfaces of the capillaries, the interstitial space was filled with silver solder. A single 0.51-mm i.d., 76-mm capillary inlet of similar design was also constructed and used as a reference inlet. The stainless steel block was heated by a 60-W cartridge heater (Ogden, Arlington Heights, IL) and the temperature monitored by a thermocouple. A controller maintained the temperature of the block at ∼200 °C. Ion Funnel. The ion funnel is conceptually similar to the rf ring electrode ion beam guide8 but incorporates an additional dc potential gradient and uses electrodes of varying diameter (decreasing “down” the funnel). The funnel interface used in this study has two major parts- (a) a front section of the funnel that consists of 55 25.4-mm-i.d. rings and (b) a rear section that has 45 ring electrodes with diameters linearly decreasing from 25.4 to 2.3 mm. Radio frequency voltages of equal but opposite phases were applied between adjacent rings, and (for positive ions) gradually decreasing dc potentials were applied along the ring (8) Bahr, R.; Gerlich, D.; Teloy, E. Verhandl. DPG(V1) 1969, 4, 343.
component
bias (V)
component
bias (V)
capillary inlet front ion funnel bottom ion funnel L0 Q0 IQ1 Stub1
+120 to +360 +120 to +360 +28 +24 +20 +12 +10
Q1 Stub2 IQ2 Q2 IQ3 Stub3 Q3
+15 +10 0 -20 -40 -60 -80
stack. The operational principles and design concepts of the ion funnel were described in a recent publication.7 Pumping System. Operation of the multicapillary inlet required increased pumping speed. The first vacuum stage was pumped by two root blowers providing nominal pumping speeds of 168 (model EH500A system, Edwards, Crawley, West Sussex, England) and 84 L/s (model WSU251 system, Leybold, Koln, Germany). The pressure in the first vacuum stage was monitored by a model CMLA-11-00 capacitance manometer (Varian, Lexington, MA). In some experiments, the pressure of the first vacuum stage was adjusted by partly closing butterfly valves installed between the ion funnel chamber and the root pumps. The ion funnel was generally operated at a pressure similar to that of the first vacuum stage of the standard API 3000 ESI interface (i.e., ∼1 Torr). The multicapillary inlet, however, actually resulted in a greater downstream pressure. Even though conductance of the last ring electrode (2.3-mm i.d.) of ion funnel was smaller than that of the 2.6-mm-i.d. skimmer of the standard interface, it was evident that a more intense gas jet formed by the multicapillary inlet compared to the standard inlet aperture for the API 3000, implying that the effective pressure in the ion funnel is higher than the ∼1 Torr indicated above. Transmitted Current Measurements. The incoming ion current to the ion funnel from the heated capillary inlet was measured by summing the currents to the ion funnel, the dc lens after ion funnel, the collisional cooling quadrupole ion guide (Q0), and a conductance limit after Q0 (IQ1). The ion funnel-transmitted current was measured by measuring the electric current to Q0 and a conductance limit after Q0 (IQ1). During the current measurements, the downstream components were biased to +20 V. To determine the transmission efficiency through the analyzing quadrupole (Q1), the ion current was measured before and after Q1. The ion current before Q1 was evaluated by measuring the current on lens IQ1 with downstream elements biased to +60 V. The ion current after Q1 was similarly measured on IQ2. Typical bias potentials are given in Table 1. Ion Source. The standard ion inlet of the API 3000 mass spectrometry was used for the transmitted current measurements. In experiments with the standard inlet, the electrospray emitter (i.e., ion source) was tilted by 45°, as in the standard operational configuration for the API 3000. In experiments with the heated capillary inlet, the electrospray emitter was evaluated in both 45° tilted and conventionally aligned configurations. The ion transmission was similar in both configurations after optimization, but the aligned configuration was adapted in this study with the capillary inlet due to its greater ease of optimization. The positions of the emitter tip and the nebulizing gas flow rate were adjusted to optimize the ion current after the ion funnel. Analytical Chemistry, Vol. 72, No. 20, October 15, 2000
5015
Figure 2. Photographs showing the multicapillary-ion funnel interface (left) and closeup of a 0.51-mm-i.d. seven-capillary inlet (right).
Figure 3. Transmitted ion currents as functions of rf amplitude: (a) after ion funnel (closed circle), (b) before analyzing quadrupole (open circle), and (c) after analyzing quadrupole (inverted triangle). The 4.0 µM DDTMA solution was infused at a 5.0 µL/min flow rate. The ion funnel chamber pressure was 0.6 Torr, and the ion funnel was operated at 700 kHz.
Dodecyltrimethylammonium bromide (DDTMA, C15H34NBr) in acetonitrile was used to evaluate ion funnel transmission at relatively low m/z, a region where operation was problematic with our original ion funnel design.1,2 The DDTMA was purchased from Sigma (St. Louis, MO) and the acetonitrile was purchased from Aldrich (Milwaukee, WI) and were used without further purification. The potential applied to the electrospray emitter was 45005500 V. RESULTS AND DISCUSSION Ion Transmission. The measurement of ion currents after m/z analysis largely ensures that the transmitted ion current from an ESI source arises from analytically useful charged species, and this gives increased confidence in performance evaluation. Figure 3 gives the ion currents measured through the ion funnel using the 0.51-mm-i.d. seven-capillary inlet design (closed circles), the ion current through a interquadrupole lens (IQ1, located between Q0 and Q1) (open circles), and the ion current after the analyzing quadrupole (reversed triangles) as functions of ion funnel rf amplitude. The inlet ion current was 5.4 ( 0.2 nA. The results show that the ion transmission through ion funnel increases with increasing rf amplitude to a level where over 60% of the inlet 5016
Analytical Chemistry, Vol. 72, No. 20, October 15, 2000
current is transmitted and then decreases with further rf amplitude increases. That observed transmission trend is typical for an rf ion guide; at first the ion transmission increases with increasing rf amplitude due to the increased pseudopotential of the trapping field and is followed at some point by a decrease with further rf amplitude increase due to the unstable trajectories or rf driven fragmentation of lower m/z ions. The results also clearly show that the transmitted ion current at zero rf amplitude is well below that realized at optimal rf amplitudes (i.e., at 60-100 V), demonstrating that the ion transmission through the ion funnel is a result of ion confinement due to the rf electric field. As a result, the ratio of transmitted ion current to the neutral gas transmission is higher than in a conventional (e.g., orifice-skimmer or capillaryskimmer) interface. In the conventional, orifice (or capillary)-dc focusing lens-skimmer interface, the distance between the inlet and the skimmer is a few millimeters and a much larger fraction of the orifice-passed gas can enter to the second chamber through the skimmer. On the other hand, in an ion funnel interface, the distance between the inlet and conductance limit of ion funnel is 10 cm and only small fraction of the inlet gas will diffuse to the second vacuum stage. Thus, in an ion funnel interface, the ion current transmitted without an applied rf field was very low compared to that at optimal rf amplitude applied to the ion funnel. It is of particular importance that the maximum ion transmission efficiency was similar to that obtained with a single same inner diamter capillary inlet, but with a higher ion current. The high transmission efficiency with the multicapillary - ion funnel interface can be explained by two factors. The multiple-capillary design provides droplet desolvation that is similar to that for a single capillary inlet of the same inner diamter . This is in contrast to the poor transmission efficiency observed for a single capillary of larger inner diamter of a given length where the effective heated surface-to-volume ratio is reduced and desolvation is less efficient.7 We also (in part) attribute the improved performance to a reduced gas jet effect. Instead of a larger expanding gas jet from a single larger inner diamter inlet, the multicapillary inlet will produce several jets that might be expected to interact destructively and lead to a reduced gas jet effect. While the latter is speculative at this point, the data clearly show a substantial improvement in the analytically useful ion current transmitted through the ion funnel. Ion Transmission Comparisons with Standard Interface. The ion transmission for various multicapillary configurations was
Table 2. Ion Transmission for the Interface, Configurations Evaluated (for 4.0 µM DDTMA in Acetonitrile Infused at 5 µL/min)
a
interface
Iina (nA)
Itrb (nA)
Iiq1c (nA)
inlet, pressure of ion funnel chamber
API 3000 standard 1-capillary-ion funnel 7-capillary1- ion funnel 7-capillary2- ion funnel
0.27 ( 0.02 0.37 ( 0.03 5.4 ( 0.2 4.8 ( 0.2
0.2 ( 0.01d 0.21 ( 0.02 3.3 ( 0.1 2.9 ( 0.1
0.13 ( 0.01 0.19 ( 0.02 3.0 ( 0.1 2.6 ( 0.1
0.25-mm-i.d. orifice single 0.51-mm-i.d. capillary, 1.0 Torr seven 0.51-mm-i.d. capillaries, 0.78 Torr seven 0.43-mm-i.d. capillaries, 0.67 Torr
Ion current after inlet. b Ion current after ion funnel. c Ion current after rf-only quadrupole (Q0). d Ion current after skimmer.
compared with that for the standard interface of the API 3000 (Table 2). It should be noted that while the present design with a single 0.51-mm-i.d., 76-mm-long capillary-ion funnel interface could transmit ion currents similar to that of the standard API 3000 orifice-skimmer interface, the heated capillary-ion funnel interface provided a greater ion current to IQ1. This can be attributed to the ion funnel’s reduced transmission for low-m/z solvent-related ions. Thus, the ion current after the ion funnel was mostly due to analyte-related ions. On the other hand, the standard orifice-skimmer interface has no significant differences in transmission for these low-mass ions, which have unstable trajectories in the rf-only quadrupole (Q0). Therefore, the present single capillary inlet-ion funnel interface provided ∼2 times higher transmission efficiency than the standard interface for analyterelated ions. The inlet transmitted current with seven 0.51-mmi.d. capillary inlets was more than 7 times larger than that for a 0.51-mm-i.d. capillary inlet. That higher transmission efficiency for the seven-capillary inlet may be explained by the ion distribution and the collective gas dynamic effects at the entrances of closely packed capillaries. The ion distribution at the entrance of the seven-capillary inlet may vary due to space charge effects, and the gas flow at the entrance region of the multicapillary inlet may differ significantly from the single-inlet design. Table 2 also shows that a 0.51-mm-i.d. seven-capillary inlet provides a greater ion transmission efficiency than that of a 0.43-mm-i.d. sevencapillary inlet, but that the transmission efficiency is not proportional to the conductance increase. The gas conductance of a 0.51mm-i.d. capillary is ∼2 times that of a 0.43-mm-i.d. capillary, but the transmitted ion current for the 0.51-mm-i.d. seven-capillary inlet was only 13% higher than that with the 0.43-mm-i.d. sevencapillary inlet. The lower ion transmission gain with the 0.51-mmi.d. seven-capillary inlet compared to the increased gas conductance may also be attributed to gas dynamic effects. We suspect that the ion transmission efficiency through the multicapillary inlet is highly dependent upon the configuration of the capillaries (e.g., the distance between capillaries and the capillary inner diameter). Most importantly, Table 2 also shows that an interface with a multiple-capillary inlet and ion funnel has ∼23 times higher current to high-vacuum stage (after Q0) compared to the standard orificeskimmer interface. Ion Detection Efficiency. Ion detection efficiency was evaluated with a 0.51-mm-i.d. seven-capillary inlet by monitoring ion current after the analyzing quadrupole. The resolution of the analyzing quadrupole was tuned to achieve unit mass resolution. Figure 3 gives the ion currents measured before and after the analyzing quadrupole with the analyzing quadrupole set at m/z 228.3. The ion transmission efficiency through IQ1 was ∼90%. Figure 3 shows ∼30% transmission through the analyzing quad-
Table 3. Ion Transmission Efficiency through an Analyzing Quadrupole with Unit Mass Resolution (Same Sample and Conditions as in Table 2) interface
infusion rate (µL/min)
Imaxa (nA)
Iiq2b (m/z 228.3)
Iiq2totc/ Imax × 100
7-capillary1 7-capillary1 7-capillary2 7-capillary2
5.0 3.0 5.0 3.0
32 19.2 32 19.2
1.0 ( 0.07 0.58 ( 0.03 0.75 ( 0.04 0.46 ( 0.03
3.6 ( 0.2 3.5 ( 0.2 2.7 ( 0.1 2.8 ( 0.2
a Calculated value from infusion rate assuming 100% ionization efficiency. b Ion current after the analyzing quadrupole. c Sum of ion currents at IQ2 with analyzing quadrupole set at two major isotopic peaks (m/z 228.3 and 229.3).
rupole and that the analyzing quadrupole transmitted current is approximately proportional to the ion current measured before the analyzing quadrupole. The analyzing quadrupole transmitted current of the second isotopic peak (m/z 229.3) was also measured as 17% of the major isotopic peak (m/z 228.3) current, as expected. Table 3 gives the results of detection efficiency measurements for different capillary inlets and for different sample infusion rates. If one assumes 100% ionization efficiency (i.e., complete conversion of solution species to gas-phase ions), the present results indicate that the overall detection efficiencies are ∼3% for two different seven-capillary inlets. When we consider that the transmission efficiency of the analyzing quadrupole is ∼30% at the selected resolution, the ion transmission efficiency of the multicapillary inlet and ion funnel interface can be estimated to be ∼10%. Since this estimate is based upon the assumption of 100% ionization efficiency and operation at a relatively large flow rate where this is unlikely, it is apparent that the overall ion transmission efficiency of the interface is considerably better than 10%. Mass spectrometric detection allows us to evaluate the composition of the transmitted ion current and the resolution of the analyzing quadrupole. Figure 4 shows a spectrum for the 4.0 µM DDTMA solution obtained using a 0.51-mm-i.d. seven-capillary inlet with the ion funnel interface. In this experiment, the electrospray emitter was intentionally positioned off axis to protect the MS detector from degradation by a high ion current. The spectrum which shows 1 u resolution is dominated by the isotopic peaks of DDTMA and otherwise shows only very minor peaks due to impurities. This confirms that the current to the analyzing quadrupole (measured on IQ1) was constituted primarily of by analyte-related ions. To study the detection efficiency for lower ion currents, mass spectra using a much more dilute 4.0 nM DDTMA solution with similar condition for the experiments of high concentrated sample were evaluated. To avoid possible contamination from the sample Analytical Chemistry, Vol. 72, No. 20, October 15, 2000
5017
Figure 4. Mass spectrum obtained for a 4.0 µM DDTMA solution with the multiple-capillary inlet and ion funnel interface. The electrospray emitter was intentionally offset from capillary inlet to lower the ion current and prevent saturation of the detector.
Figure 5. (a) Total ion current as a function of time (b) a typical spectrum (single scan). The 4.0 nM DDTMA solution was infused at 3.0 µL/min flow rate.
transfer line and electrospray emitter by the previous 4.0 µM DDTMA sample, all sample handling components (i.e., transfer line and emitter) were replaced for these experiments, and performance was verified using a “blank” sample and by the absences of a peak at m/z 228.3 u. Figure 5 shows the spectrum obtained for a 4.0 nM DDTMA sample using a 3.0 µL/min infusion rate. Based upon the analyte molecular infusion rate (1.2 × 108 5018
Analytical Chemistry, Vol. 72, No. 20, October 15, 2000
molecules/s) and the sum of detected signals (ion count rates) for two isotopic peaks (3.5 × 106 counts/s), the overall detection efficiency was 2.9%. When we consider the extended beam path (Q2 and Q3) in the spectrum measurement with a low-concentration sample, this detection efficiency is in a good agreement with that obtained by ion current measurements using higher concentration samples (3.5 ( 0.2%). These results verify the high
efficiency of the present interface and clearly indicate the direction of efforts for further improvements. It should be noted that high detection efficiencies have been reported previously using very low flow rate electrospray conditions (i.e., “nanospray”). Using a 0.2 µM peptide solution (average molecular mass 1875.1 u) with a 22 nL/min infusion rate, a 0.26% detection efficiency for the doubly and triply charged ions with low resolution analyzing quadrupole (2.5 u fwhm) has been reported Wilm and Mann.9 With a similar concentration peptide and 1 order lower flow rate (0.1 µM, 1.6 nL/min), a 3.8% detection efficiency has been obtained using reduced quadrupole resolution (which also transmits the entire isotopic envelope) by Tempest and co-workers.10 In both reports, the high detection efficiencies were attributed to the use of very low flow rate electrosprays. Direct comparison with the detection efficiencies obtained in this study, however, is not appropriate because of the different ion source sample, infusion rates, and resolution of the analyzing quadrupole. However, the crucial point is that the detection efficiencies demonstrated here with the multicapillary and ion funnel interface with ion spray ion source were of an order similar to the best previously reported results using nanospray sources. The importance of this is that the present results were obtained under conditions where ESI operation is straightforward (i.e., where the difficulties associated with closely spacing the emitter to the inlet and in initiating at stable nanospray are avoided). Additionally, the use of larger flow rates increases the maximum signal intensity obtainable, resulting in a net increase in the dynamic range achievable of between 1 and 2 orders of magnitude. CONCLUSIONS A heated multicapillary inlet interface was developed for use with an rf ion funnel to obtain increased sensitivity and dynamic range. A seven-capillary inlet and ion funnel interface was found to transmit more than 7 times larger ion currents than a single capillary with the ion funnel interface and 23 times greater ion (9) Wilm, M.; Mann, M. Anal. Chem. 1996, 68, 1-8. (10) Geromanos, S.; Freckleton, G.; Tempest, P. Anal. Chem. 2000, 72, 777790.
current than the standard interface of the API 3000 mass spectrometer. Ion spray source, ion current measurements showed that ∼10% of analyte molecules could be ionized and transmitted through the ESI interface, and direct mass spectral measurements with a 1000-fold more dilute sample showed that 2.9% of the analyte molecules in the sample were detected with an analyzing quadrupole operating with unit mass resolution. Thus, the detection efficiency obtained with the new interface is more than 1 order of magnitude greater than that of the standard interface of the API 3000. The present work has demonstrated that a major gain in sensitivity is achieved by the combined use of a multicapillary inlet and the electrodynamic ion funnel. Most significantly, the present gains have been achieved under conditions where ESI source performance is robust. The larger ion currents realized result in a significant (>10-fold) increase in achievable dynamic range. The high transmission efficiency of the interface and the improved detection efficiency are attributed to the desolvation efficiency of the multicapillary inlet and the high transmission efficiency of the ion funnel. We believe that further significant gains will be achieved by the use of lower flow rates and an optimized multicapillary design. Further studies are in progress to optimize the configuration of the multicapillary inlet and its use with less divergent ion sources. ACKNOWLEDGMENT This research was supported by the Office of the Biological and Environmental Research, U.S. Department of Energy. The authors thank Sciex for providing the API 3000 instrument and Dr. Bruce Thomson for helpful discussions. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the United States Department of Energy through Contract DEACO6-76RLO 1830.
Received for review March 27, 2000. Accepted July 30, 2000. AC0003549
Analytical Chemistry, Vol. 72, No. 20, October 15, 2000
5019
