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Anal. Chem. 1989, 61 , 2582-2583
a system would result in a broader (less fragile) CP match condition. It is worth emphasizing that the level of precision indicated above should be expected only if one’s experiments are limited to a specific spinner assembly. Calibrations of the type represented by Figure 1should be carried out for each spinner assembly (assuming reproducibility of spinner assemblies is not perfect) and periodically even for a specific unit (assuming some wearing of a spinner system with repeated usage). For the reasons discussed above, one should expect greater variability, or lower precision, due to variations in rf field strength or homogeneity in CP spin counting experiments than in SP spin counting experiments carried out by this approach. The results presented in this paper offer major hope for a convenient strategy for the measurement of absolute 13C intensities or 13Cspin counting in 13C NMR studies of certain types of systems in which there has been substantial uncertainty in the significance of 13CNMR intensities. Examples include cases in which paramagnetic centers in the sample may
interfere with the observation of 13Cintensity due to excessive broadening and/or paramagnetic shifta, cases in which certain carbon environments may preclude the existence of reasonably efficient 13C spin-lattice relaxation, and cases in which the remoteness of certain carbon atoms from hydrogen may interfere with 13C observation via lH-13C cross polarization. Applications of these types are under way in this laboratory. Registry No. 13C, 14762-74-4.
LITERATURE CITED (1) Zhang. M.; Maciel, G. E. J . Magn. Reson., in press. (2) Sullivan. M.; Maclel, G. E. Anel. chem.1982, 54, 1615. (3) Mehring, M.
[email protected] of H@b ResoMIon NMR h SOWS;Springer-Verlag: New York, 1983; pp 152-154.
RE~EIVED for review May 19,1989. Accepted August 22,1989. The authors gratefully acknowledge partial support of this research by National Science Foundation Grant No. CHE8610151 and Department of Energy Contract No. DE-AC2288PC88813.
TECHNICAL NOTES Evaporation Device for Continuous Flow Liquid Secondary Ion Mass Spectrometry Ming-chuen Shih,* Tao-Chin Lin Wang, and S. P. Markey* Laboratory of Clinical Science, National Institute of Mental Health, Bethesda, Maryland 20892 INTRODUCTION Continuous flow (CF) liquid secondary ion m w spectrometry (LSIMS) or fast atom bombardment (FAB) mass spectrometry has attracted considerable interest (1-4) because it offers several advantages in comparison to static LSIMS. First, new samples can be analyzed conveniently without breaking vacuum or changing instrument focus, facilitating automatic operation and rapid sample throughput. Second, samples dissolved in a variety of organic solvents may be admixed with glycerol to form a liquid matrix compatible with both chromatographic and LSIMS requirements. Third, spectra are characterized by lower chemical noise than that observed with static LSIMS. Finally, since samples elute discretely, chromatographic data processing techniques are applicable. In the operation of a CF-LSIMS or FAB probe, one of the common handicaps for extended continuous stable operation is the accumulation of liquid around the probe tip region. This problem exists whether the liquid is delivered either internally or externally to the probe shaft to the target surface to form a thin film. The results are peak broadening from stagnancy of liquid flow, film instability from the freezing of mobile phase in the capillary, and instability of ion source pressure due to dripping and bubbling of mobile phase. Various efforts have been directed toward minimizing these undesirable effects [e.g., selection of mobile phase, variation of control parameters for mass and heat transfer and the use of fritted surfaces (l), all aiming a t on-site evaporation, and the use of wicking material to remove liquid from the target surface and to provide a thin film and smooth flow (5, S ) ] . We have demonstrated quantitative analyses of polar substances using and have consequently desired to operate CF-LSIMS (7,8), in this mode continuously and routinely for 6-8-h periods for analytical assays, a requirement more rigorous than that reported previously by others. We have sought to extend the This article not subject to
time period of continuous operation by facilitating and sustaining liquid transport across the probe surface by channeling the liquid onto a large absorbent surface which could provide sufficient heat and fluid capacity to bring the flow syatem close to a state of mass transfer dynamic equilibrium (i.e., a state in which the rate of liquid steadily evaporated into the vacuum system equals the rate of mobile phase delivered to the probe tip). The idea was tested by using an evaporationdevice made of cellulose with an extended surface area, displaced from the LSIMS target surface, and in contact with a solid probe which serves as a large heat sink. We are able to operate in CFLSIMS mode for periods longer than 8 h with only slight deterioration of performance.
EXPERIMENTAL SECTION A tandem quadrupole mass spectrometer (TSQ-70, Finnigan Mat, San Jose, CA) was used with a prototype CF-LSIMS apparatus (Bioprobe,Finnigan MAT). This apparatus delivers liquid through a capillary tube which is positioned to contact a standard probe located near the ion extraction lens assembly (Figure la). An unfocused cesium ion gun (Phrasor, Duarte, CA) was the primary ion source. A solid, stainless steel probe shaft dimensionally identical with the standard heated commercialprobe was machined for these studies in order to provide a rugged device with a large heat capacity. It was fitted with a standard copper FAB probe tip. Figure l a shows the probe-evaporator assembly. The evaporator consisted of two layers of cellulose absorbent to maximize the evaporation surface area. The outer layer (shaded portion A in Figure lb) was cut from a 10 mm X 50 mm single thickness cellulose extraction thimble (Whatman, Inc., Clifton, NJ). The inner layer (shaded portion B in Figure IC)was cut from a similar 25 mm X 100 mm single thickness cellulose extraction thimble. A large hole (C in Figure lb) and a small hole (Din Figure lb) were cut in the outer layer to increase the pumping efficiency. Cutting with small surgicalscissors minimized the generation of paper lint. The two cellulose layers were overlapped by rolling portion B into a tube and sliding it into portion A. The resulting evaporation assembly was then force-fitted onto
U.S.Copyrlght. Published 1989 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61. NO. 22, NOVEMBER 15. 1989
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Figure 1. (a)Probe-evapmtorassembly. (b) The outer layer of h evaporator (c) The inner layer of the evaporatw.
the probe shaft so that the lower edge of the probe tip collar touched the pointed end (E in Figure IC)of the inner layer. To minimize the thermal load on the immediate vicinity of the probe tip and to maximize the evaporation surface area, the absorbent was in contact with the probe shaft only near the base of the evaporator. This contact provided a heat conduction pathway to the evaporator A pulse-dampened pump (Model 302, Gilson, Middleton, WI) delivered a 20% glycerol/H20solution to a 0.5-rL loop injector. The liquid flowed through a 75 ,an i.d. X 90 cm fused silica tubing (Polymicro Technologies, Phoenix, AZ) to meet the probe tip (Figure la). The mobile phase flow rate was 8 pL/min, and the ion source temperature was 4W45 "C. The probe was heated at the probe handle with a heating tape to maintain the temperature of the probe shaft at the entry port of the mass spectrometer at about 40 "C. The cellulase evaporator assemblies were withdrawn from the vacuum system and discarded after use. The performance of this device was evaluated hy repetitive 0.5-rL injections of mobile phase containing 10 ng of potassium salt of (3-methoxy-4-sulfatoxyphenyI)ethyleneglycol (MOPEG-sulfate,Fluka Chemicals, Ronkonkoma, NY).
RESULTS AND DISCUSSION Figure 2 shows the results of repetitive injections of mobile phase containing MOPEG-sulfate, a norepinephrine metaholite for which we desired a direct quantitative analysis method. While this example demonstrates stable operation in negative ion mode, we have obtained similar results monitoring positive ions in the routine analyses of 1-methyl-4-phenylpyridine and 2-amino-3-(methylamino)propanoicacid. For the analysis of MOPEG-sulfate, the continuous flow system was operated continuously for 9.5 h with only slight deterioration of performance. The size of the evaporator used was approximately 10 mm i.d. X 28 mm length. With 20% glycerol/H20 mobile phase a t 8 pL/min flow rate, the retention of liquid in the evaporator after 9.5 h of continuous operation was 16.1% by weight. There are two engineering principles that this sytem incorporates and that are applicable to all CF-LSIMS systems. First, the rapid flow of liquid across the prohe tip surface and into another area reduced the thermal load at the probe tip (ex., we have physically separated the site of secondary ion emission from the region of liquid evaporation). Thus, the requirement for careful monitoring and precise control of probe tip temperature in order to supply heat for mobile phase
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Figure 2. Negative ion selected ion recordings of ml.? 263 (M H)obtained by repditive loop i n j e c h s mtaining 10 ng of MopEGsulfate in 0.5 pL of 20% giycerollwater during a period of more than 9 h &r"eng hstabilii of the CF-LSIMSsystem Wim an evapaafion
device evaporation has been eliminated. Instead, there is a heat conduction pathway, bypassing the probe tip. which transfers heat to the evaporator. Second, the use of an extended absorbent evaporation surface in contact with a large heat sink increases the rate of mobile phase evaporation a t reduced temperature. As liquid begins to accumulate within this absorption medium, the wetted surface area starts to expand rapidly. The surface expansion increases the rate of evaporation and decreases the rate of liquid accumulation. As the rate of accumulation approaches zero, the flow system approaches a state of mass transfer dynamic equilibrium. Thus, if sufficient evaporation surface area is provided, a CF-LSIMS system can be operated for extended periods. We anticipate that with a balance of absorbent, evaporation surface, and heat sink, a wide range of mobile phase compositions and mobile phase flow rates should he accommodated in CF-LSIMS. Registry No. Cellulose, 9004-34-6.
LITERATURE CITED (1) 110. Y.: Takeuchi. D.; Ishii. D.: Goto. M. J. J . Uromatog. 1985.346, 161-166. (2) Caprioli. R. M.: Fan. T.: CoHrell. J. S. Anal. cylem. 1986, 58. 2949-2954. (3) Ashwon. A. E.: Chapman, J. R.: cotbell. J. S. J . C h m n a f o g . 1987. 394. 15-20. (4) de Wit, J. S. M.: Deterding. L. J.: Moseley. M. A,; Tomsr. K. 6.: Jorgenson. J. W. RapW Canmur. Mass .%cinnn. 1988. 2. 100-104. 151 Caorioli. R. M. Pmc. ASMS Catf. Mass S o e " . AWM Tw.. 36th
(7) Markey. S. P.; W a g . T.4.L.: Shh. M.: MI.R.: hncan. M.; Yanp. S . C . ; Bradford. D. Pmc. ASMS Conf. MBss A M TOP.. 36th 1988, 739-740. (8) Wang. T.-C. L.; Shih, M.: Markey. S. P.: Duncan,M. Anal. LIYMI. 1989, 67, 1013-1016.
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for review June 19,1989. Accepted August 4,1989.
