Correspondence Anal. Chem. 1994, 66,4557-4559
Sonic Spray Ionization Method for Atmospheric Pressure Ionization Mass Spectrometry Atsumu Hirabayashi,* Minoru Sakairi, and Hideaki Koizumi Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185, Japan
We have developed a novel spray ionization method for interfaces in capillary electrophoresis/mass spectrometry and liquid chromatography/mass spectrometry. This method is called "sonic spray"ionization. In this method it is not necessaryto apply an electric field to the capillary of the ion source, which operates at room temperature. A solution in methanol and water from a fused-silica capillary is sprayed under atmospheric pressure with nitrogen gas flow coaxial to the capillary. Gaseous ions as well as droplets are produced from the solution by the spray, and the positive ions are analyzed with a doublefocusing mass spectrometer. We found that the detected ion intensity depends on the Mach number or the gas velocity. Ions are detected at a gas velocity higher than a certain value, and the ion intensity has a maximum at a Mach number of about 1, Le., the sonic velocity. Preliminary results for solutions of dopamine, lysine, and gramicidin S are presented.
droplets are produced by heatinge.. solution at the capillary tip, whereas in ES and IS, a high voltage is applied to the capillary tip so that charged droplets are emitted from the solution. Although the details of the ion formation from charged droplets are not yet clearly understood, the ion evaporation model8 and the single ion in droplet theory (SIDq9are often assumed. Charge droplets can also be produced without heating the capillary or applying an electric field. A solution introduced through a capillary is sprayed under atmospheric pressure with gas flow coaxial to the capillary, and when the gas velocity is higher than a certain value, ions as well as droplets are produced by the spray. We discovered this phenomenon by chance. Since then, we have developed a novel spray ionization method for CE/ MS and LC/MS interfaces. This method is called "sonic spray" ionization. Its most striking feature is that it is not necessary to apply an electric field to the capillary of the ion source or to heat the capillary. In this correspondence, we report our first results obtained using sonic spray ionization.
Spray ionization methods such as atmospheric pressure chemical ionization (APCI) thermospray ('IS) ,2 electrospray @S! ) ,3 ion spray US)? and atmospheric pressure spray (APS)5 have been developed for use as an interface in liquid chromatography/mass spectrometry (LC/MS). However, the use of these ionization methods is limited by restrictionsin solvent composition, solution flow rate, and chemical species being analyzed. Capillary electrophoresis (CE) has recently been recognized as a powerful method for solution separation.'j Also, combined CE/MS with ES interface has been de~eloped.~ In these interfaces, ions or molecules in a solution are effectively converted to gaseous ions. Charged droplets are produced by spraying the solution, and the gaseous ions are emitted from the droplets. For example, in TS and APS,charged
EXPERIMENTAL SETUP
(1) Kambara, H. Anal. Chem. 1982,54, 143-146. (2) Blakley, C. R ; Vestal, M. L.Anal. Chem. 1983,55, 750-754. (3) Yamashita, M.; Fenn, J. B. ]. Phys. Chem. 1984,88,4451-4459. Fenn, J. B.; Menn, M.; Meng, C. IC;Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64-71. (4) Bruins, A P.; Covey, T. R ; Henion, J. D. Anal. Chem. 1987, 59, 26422646. (5) Sakairi, M.; Kambara, H. Anal. Chem. 1989, 61, 1159-1164.Hirabayashi, A; Takada, Y.; Kambara, H.; Umemura, Y.; Ohta, H.; Ito, H.; Kuchitsu, IC Int. J. Mass Spectrom. Ion Processes 1992, 120, 207-216. (6) Kuhr, W. G.; Monnig, C. A Anal. Chem. 1992,64, 389-407. (7) Smith, R D.; Wahl, J. H.; Goodlett, D. R;Hofstadler, A Anal. Chem. 1993, 63, 574A-5W
0003-2700/94/0366-4557$04.50/0 Q 1994 American Chemical Society
A cross-sectional view of the sonic spray ion source is shown in Figure 1. A solution in methanol and deionized water was pumped by a syringe pump (Harvard Apparatus, Model 11) through a 0.5-m-long Teflon tube to a fused-silica capillary (0.1mm id., 0.2" 0.d.) at a flow rate of 30 pL/min. Since the fusedsilica capillarywas flexible, it was fixed in a stainless steel capillary (0.25" id., 1.7" 0.d.) to enable it to be accurately positioned in the ion source body. The fused-silicacapillary tip was inserted into a Duralumin orifice (0.4" diameter), and their center axes were aligned. The fused-silica capillary tip was exposed by 0.6 mm beyond the orifice of the ion source. Nitrogen gas was passed through the orifice to the atmosphere. The flow rate of nitrogen gas in the standard state (20 "C, 1 atm) was determined with a mass flow controller (Brooks, 5850E). A spray was thus generated in which droplets and ions were produced. A double-focusing mass spectrometer (Hitachi, M-80) was operated at 1.3 x Pa. The distance between the fused-silica capillary tip of the ion source and the sampling oritice of the mass spectrometer was 5 mm, and their center axes were approximately aligned. There was no potential difference between the capillary (8) Iribarne, J. V.; Thomson, B. A/. Chem. Phys. 1976,64, 2287-2294. (9) Rollgen, F. W.; Bramer-Weger, E.; Biitfering, L.]. Phys. (Paris) 1987,48, C6-253-256. Tang, L;Kebarle, P. Anal. Chem. 1993,65, 3654-3668.
Analytical Chemistry, Vol. 66, No. 24, December 15, 1994 4557
100
Gas
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Fused Silica Capillary
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Figure 1 1. Cross-sectional view (schematic) of the sonic spray ion source. 1st Intermediate Pressure Region
Ceramic
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2nd Intermediate PressureRegion
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.-
Syringe
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5 20 a 0 Orifice 1st Aperture/
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Pump
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60 80 100 120 140 160 180 mlz
Figure 2. 2. Schematic diagram of the experimental setup.
n
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and the sampling orifice. The sampling orifice and the intermediate pressure regions of the mass spectrometer were almost the same as those described in ref 10. As shown in Figure 2, ions produced at atmospheric pressure were passed into the first intermediate pressure region through the sampling orifice, 0.25 mm i.d., 15 mm long. The ions then passed into the second intermediate pressure region through the first aperture (0.8" diameter) in the first electrode. Fially, ions passed into the mass analyzing region through the second aperture (0.2-mm diameter) in the second electrode. The sampling orifice was heated with a ceramic heater (50 W) to about 120 "C. This was to suppress the charged droplets produced by adiabatic expansion when ions were introduced into the first intermediate pressure region.1° Remaining gases in the first intermediate pressure region were pumped through the second intermediate pressure region, which was then evacuated by a 1600 L/min mechanical booster pump and a 670 L/s rotary pump. The pressure in the second intermediate pressure region was 40 Pa. A drift voltage of 30 V was applied between the sampling orifice and the first electrode, and another drift voltage of 20 V was applied between the first and second electrodes. A voltage of 3 kV was applied to the second electrode to accelerate the ions. Mass-analyzed ions were detected with an electron multiplier, to which typically 1.8 kV was applied. The output of the electron multiplier was fed to a data acquisition system (Hitachi, M-003) and analyzed by a computer system (Hitachi, 0101).
0
200
400
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mlz Figure 3. 3. Mass spectra obtained from 5050 methanol-water solutions of (a) lysine, (b) dopamine, and (c) gramicidin S. The solution concentrations are 1 pM.
I. Sonic Spray Mass Spectra. ?srpical mass spectra are shown in Figure 3. Positive ions were produced from 5050 methanol-water solutions of (a) lysine, (b) dopamine, and (c)
gramicidin S at concentrationsof 1pM under the condition that the gadlow rate was 3.0 l/min. In an aqueous solution with pH = 5, the abundanceof protonated lysine molecules to the dissolved molecules was estimated to be more than 99% from the dissocia tion constants.ll Figure 3a shows the mass spectrum for positive ions produced from the lysine solution. Although protonated methanol (m/z = 33) was detected, the hydronium ion (H30+, m/z = 19) or its hydrated clusters were not detected. Besides the protonated lysine molecule (m/Z = 147), its fragments were also detected. The ion with m/z = 129 is the protonated molecule of dehydrated lysiie. The fragments are probably produced by collisional d i d a t i o n in the intermediate pressure regions in the mass spectrometer. On the other hand, in an aqueous solution of dopamine, the abundance of protonated dopamine molecules is expected to be much smaller, and most molecules are neutral in a dilute solution. F i r e 3b shows the results obtained from the dopamine solution. The protonated dopamine molecule (m/z = 154) was strongly detected. Multiplycharged ions can be
(10) Hirabayashi, A;Sakairi, M.;Takada, Y.J. Mass Spectrom. Soc..J'
(11) Iide, D. R
RESULTS AND DISCUSSION
1993,
41.2w-m.
4550 Analytical Chemistry, Vol. 66, No. 24, December 15, 1994
CRC Hundbook ofckemistry und Physics, 71st ed.; CRC Press: Boca Raton, FL, 1990.
has not been elucidated in the present experiment. Thus, we should discuss the Mach number qualitatively. At a g a d o w rate of 6.1 Wmin, the gas pressure inside the ion source (P, is 7 atm. With the followingequation,13the Mach number (Ma) is estimated to be 1.93:
P = [l + 0.5(y - 1)M~2]y'(y-') C 0
-
0.2 0
1 .Q ,
0
1
,
2 3 4 5 Gas-Flow Rate (i/min)
6
Figure 4. 4. Dependence of the detected intensity for doubly protonated gramicidin S molecule ( d z= 571) on gas-flow rate in the standard state (20 "C,1 atm). The ion is produced from a solution of 1 pM gramicidin S in 5050 methanol-water.
produced in sonic spray. An example is given in Figure 3c. The singly and doubly protonated molecules of gramicidin S were detected at m/z = 1141 and 571, respectively. The intensity for the doubly protonated molecule is much higher than that for the singly protonated one. This spectrum pattern itself is similar to that obtained in electrospray (ES) ionization,i2but the ions appear to be formed by different mechanisms. The charged droplets are produced by eledrospray, and their charge densities are expected to be much higher. 11. Ion Formation. Figure 4 shows the intensity of the doubly protonated gramicidin S molecule (M 2W2+as a function of the gas-flow rate. The ions were produced from gramicidin S solution (1pM) in 50:50 methanol-water. From this figure we conclude the following: (1) ions are detected at a gas-flow rate higher than about 1.3 Wmin, (2) the ion intensity increases with increasing gas-flow rate below 3.0 L/min, (3) the ion intensity has a maximum at about 3.0 L/min, and (4) at a higher flow rate the ion intensity decreases with increasing gas-flow rate. When ions are produced from solutions (1 pM) in 2080 and 80:20 methanol-water, results obtained are similar, although the ion intensities are lower, i.e., 80%and 9%,respectively, to that from the solution in 50:50 methanol-water. With increasing gas-flow rate, the gas velocity and therefore the Mach number are expected to increase. However, this relation
+
(12) Whitehouse, C. M.;Dreyer, R N.; Yamashita, M.; Fenn, J. B. Anal. Chem. 1985,57,675-679. (13) Liepmann, H. W.; Roshko, A Elements ofGasdynamics;John Wiley & Sons Inc., New York, 1960.
(1)
where y is the specitic-heat ratio, 1.4 for nitrogen gas. Thus, we conclude that the Mach number is less than 2 in the experiment shown in Figure 4. On the other hand, photographs taken by the Schlieren method13show that subsonic and supersonic flows are generated at 2.2 and 4.0 L/min, respectively. Therefore, we conclude that the ion intensity has a maximum around a Mach number of about 1, Le., the sonic velocity. At the fused-silica capillary tip, the solution is converted into droplets having various diameters by the spray. Among these, the fine droplets are likely to produce gaseous ions? although the relation between droplet diameter and detected ion intensity is not yet clearly understood. In Figure 4, the ion intensity increases with increasing gas velocity below the sonic velocity. This suggests that the number of fine droplets increases with increasing velocity. At a velocity below the threshold, corresponding to a flow rate of 1.3 Wmin, droplets are produced but no ions are detected. This is probably because larger droplets are produced by the spray. Figure 4 also shows that, in the supersonic region, the ion intensity decreases with increasing gas velocity. This is probably due to the shock wave generated in the supersonic jet. Since the gas velocity at the orifice is likely to be related to the pressure gradient around the capillary tip, the formation of fine droplets probably depends on the pressure gradient. When the shock wave is generated, the pressure gradient is believed to become unstable. Therefore, the formation of fine droplets is suppressed, and mainly larger droplets, which are not related to ion formation, are produced. ACKNOWLEDGMENT
The authors are gratefulto Mr. Y. Takada and Mr. M. Kojima for their invaluable assistance in this experiment. They wish to thank Drs. M. Yoshida and S. Kieda for their encouragement throughout this work. They also acknowledge Professor T. Akamatsu and Dr. H. Takahira of Kyoto University for their fruitful discussions. Received for review June 3, 1994. Accepted September
26, 1994.@ @
Abstract published in Advance ACS Abstracts, November 1, 1994.
Analytical Chemistry, Vol. 66, No. 24, December 15, 1994
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