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Microflow Ultrasonic Nebulizer for Inductively Coupled Plasma Atomic Emission Spectrometry Matthew A. TarrJ Guangxuan ZhuJ and Richard F. Browner' School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
A novel microflow ultrasonic nebulizer (p-USN) gives stable nebulization at microliter per minute flow rates. A very localized aerosol generation point is formed by feeding liquid through a 50-pm diameter fused silica capillary tube onto the surface of a n ultrasonic transducer, using a liquid chromatography pump. At very low flow rates (5-10 pL/min), the aerosol generated has a n extremely low Sauter mean diameter (2 pm) and analyte transport efficiencies are close to 100%. This is the first time to the authors' knowledge that such efficient continuous nebulization at very low flow rates has been reported, other than through the use of the electrospray technique. Generally good stability, precision, and reproducibility were observed for Mn emission signals at all flow rates. Experimental detection limits with the p-USNobserved at 10pL/min flow rate without desolvation are comparable to those obtained with a standard pneumatic nebulizer operated at 1mL/ min, although the analyte mass transport rate to the plasma for the pneumatic nebulizer is estimated to be approximately 1.9 times higher than that for the p-USN. The p-USN therefore gives greater conversion efficiency of analyte to useful atomic and ionic emission from the plasma, as a consequence of smaller particle size and lower aerosol solvent loading. At flows 110 pL/min, solvent loading in the plasma is sufficiently low to make desolvation unnecessary. The ability to provide essentially 100% transport efficiency at flow rates in the range of 5-20 pL/min suggests good potential for pUSN use in interfacing ICPAES and ICP/MS with capillary electrophoresis and microcolumn liquid chromatography. I n addition to use with plasma emission spectrometry, the technique should also be readily applicable to inductively coupled plasma mass spectrometry and microwave emission spectrometry.
trace metals in biological fluids' and other samples.5 The main limitation experienced with such interfacing is inadequate system response for the subnanogram levels of analyte which emerge from the column. This is a direct consequence of the low (typically052.0% ) analyte transport which occurs with liquid chromatography/inductivelycoupled plasma (LC/ ICP) interfaces based on pneumatic nebulization. There are many practical advantages to using microscale separation techniques, such as capillary electrophoresis(CE) and microbore (12.1 mm) liquid chromatography, compared to normal bore (4.6 mm) packed column chromatography. Advantages include high column efficiency, improved speed of separation, and economy of sample size. However,coupling these separation methods to ICPAES for elemental speciation raises many instrumental problems. The very small sample size exacerbates the already marginal system response, and the low liquid flow rates inherent to the separation techniques make direct coupling to conventional nebulizers problematical. A recent study by Wang et al.6 demonstrated the feasibilityof CEICP interfacing using an ultrasonicnebulizer, but this procedure required a 20-fold dilution of the eluent in order for the nebulizer to operate effectively, increasing the primary solvent loading by a corresponding amount. From a practical standpoint, it has proven extremely difficult to generate stable aerosols using conventional pneumatic nebulizers with liquid flow rates much below 300 pL/min. From a very limited range of low-flow nebulizers, the direct injection nebulizer7 (DIN) has proved to be probably the most effective for coupling low liquid flows directly into an ICP and has been used with flows as low as 30 pLlmin.8 Through avoiding the use of a spray chamber, this device is able to provide 100% sample transport to the plasma, albeit in the form of an aerosol with a slightly wider drop size range than the ideal.9 At the liquid flows used by the DIN, without the possibility of desolvation and with 100% solvent transport to the plasma, solvent loading to the plasma still exceeds the optimum. Solvent causes local cooling of the plasma, which results in a lower signal per unit mass of analyte injected than would be predicted from purely analyte mass transport considerations, especially at the higher liquid flow rates.7 While it has been reported that ultrasonic nebulizers are capable of operating at flow rates down to 150 pL/min,"J this
INTRODUCT10N
(1) Heitkemper, D.; Creed, J.; Caruso, J.; Fricke, F. L. J. Anal. Atom. Spectrom. 1989,4, 279-284. (2) Beauchemin, D.; Siu, K. W. M.; McLaren, J. W.; Berman, S. 5. J. Anal. Atom. Spectrom. 1989,4,286-289. (3) Krull, I. S.; Bushee, D.; Savage, R. N.; Schleicher, R. G.; Smith, S. B., Jr. Anal. Lett. 1982,15 (A3), 267-281. (4) Gardiner, P. E.; Braetter, P.; Gercken, B.; Tomiak, A. J. Anal. Atom. Spectrom. 1987,2, 375-378. ( 5 ) Thompson, J. J.; Houk, R. S. Anal. Chem. 1986,58, 2641-2548. (6) Wang, J.; Evans, H. E.; Dorsey, J. G.; C m o , J. A. In Abstracts, The Pittsburgh Conference, New Orleans, LA, Mar 1992, Paper 216. (7) Lawrence, K. E.; Rice, G. W.; Fassel, V. A. Anal. Chem. 1984,56,
Inductively coupled plasma emission spectrometry (ICPAES) provides sensitive and selective detection of many elemental species and, in many respects, is quite suitable for interfacingto condensed-phasechromatography. By coupling ICPAES to appropriate separation techniques, it is often possible to derive speciation information, especially for important elements such as As192 and Cr,3 and to determine + Present address:
US. EPA, 960 College Station Rd., Athens, GA
30613. t Preaent address: General EngineeringLaboratories,2040 Savage Rd., Charleston, SC 29417.
0003-2700/93/0365-1689$04.00/0
289-292.
(8)Shum, S.C. K.; Pang, H.; Houk, R. S.Anal. Chem. 1992,64,24442460. (9) Wiederin, D. R.; Houk, R. S.Appl. Spectrosc. 1991,46,1408-1412. (10) Tarr, M. A.; Zhu, G.; Browner, R. F. J. Anal. Atom. Spectrom. 1992, 7,813-817. 0 1993 Amerlcan Chemical Society
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isstillmuch too highfordirectcouplingwithtechniquessuch ascapillaryelectrophoresis(CE), where flowratesin therange of
