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Liquid Sampling-Atmospheric Pressure Glow Discharge Ionization Source for Elemental Mass Spectrometry R. Kenneth Marcus,*,† C. Derrick Quarles, Jr.,† Charles J. Barinaga,‡ Anthony J. Carado,‡ and David W. Koppenaal‡ † ‡
Department of Chemistry, Biosystems Research Complex, Clemson University, Clemson, South Carolina 29634, United States Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ABSTRACT: A new, low power ionization source for elemental MS analysis of aqueous solutions is described. The liquid sampling-atmospheric pressure glow discharge (LS-APGD) operates by a process wherein the surface of the liquid emanating from a 75 μm i.d. glass capillary acts as the cathode of the direct current glow discharge. Analyte-containing solutions at a flow rate of 100 μL min-1 are vaporized by the passage of current, yielding gas phase solutes that are subsequently ionized in the 1000 for 5 ng mL-1 Cs). While much optimization remains, it is believed that the LS-APGD ion source may present a practical alternative to high-powered (>1 kW) plasma sources typically employed in elemental mass spectrometry, particularly for those cases where costs, operational overhead, simplicity, or integrated elemental/molecular analysis considerations are important.
The trends toward analytical systems of smaller footprints, lower power consumption, smaller sample sizes, lower operational costs, and greater autonomy have been clear over the last 20 years. Advances in each of these categories have been seen across the spectrum of separation science, mass spectrometry, optical spectroscopy, and sensor technology. Perhaps the least progressive area of instrumental analysis in these regards is atomic spectroscopy, where the standard inductively coupled plasma (ICP) source used in optical emission and mass spectrometry has not seen appreciable changes in three decades. To be clear, there have been appreciable efforts to develop “microplasmas” for optical emission analysis of gaseous samples and to a lesser extent liquids, but to-date, there have been no developments that have resulted in general commercial acceptance.1-6 The uses of plasma devices of various form as ionization sources for gaseous, organic species,7,8 as well as reactive agent generators for “ambient” mass spectrometry, have been described.9-11 In spite of this general activity, there have been few reports of the development of miniaturized ionization sources for the elemental analysis of aqueous samples (as compared to an ICP source). In addition to the above-listed characteristics, the development of such an ionization source would greatly facilitate the ability to use highly evolved, transportable mass analyzers for elemental analysis in a variety of environments. Beyond the specialized case of microchip-based plasma sources,2,5,12 there have been two basic designs described for the generation of low power, continuous plasmas having the r 2011 American Chemical Society
ability to directly sample liquids and perform elemental analysis by optical emission spectroscopy.6 Cserfalvi and co-workers first described an electrolyte cathode discharge (ELCAD),3,13,14 which essentially entails the striking of a dc glow discharge between a grounded tungsten anode and the surface of an electrolyte-containing liquid that acts as the cathode. The flow of the test solution (up to 10 mL min-1) out of a glass tube effectively creates a fountain down the sides of the tube providing electrical continuity to the powering electrode placed in the receiving pool. “Sputtering” of the water surface results in the ejection of metal solutes into the plasma immediately above the liquid, leading to the eventual excitation and optical emission from those species. Hieftje and co-workers have made improvements in the initial ELCAD designs,15,16 including reduction in flow rates down to ∼3 mL min-1. Limits of detection for that optical emission device range from 5 pg (0.06 ng mL-1) for Li to 6 ng (270 ng mL-1) for Hg for 25 μL injections.16 Marcus and co-workers have presented a distinctly different approach to the concept of operating a discharge at the surface of a liquid sample.5,17-21 The liquid sampling-atmospheric pressure glow discharge (LS-APGD) is configured such that the electrolytic liquid (e.g., 1 M HNO3) flows out a small (∼100 μm) glass capillary housed within a slightly larger metal capillary, between Received: January 13, 2011 Accepted: February 11, 2011 Published: February 28, 2011 2425
dx.doi.org/10.1021/ac200098h | Anal. Chem. 2011, 83, 2425–2429
Analytical Chemistry
LETTER
observed when sampling the LS-APGD are presented, along with indications as to current limits of detection and paths forward. While too early in the development process to suggest that this device might displace the venerable ICP source, it is clear that the combination of performance seen to date, much lower capital and operation costs, and opportunities for portability suggests a great deal of promise across a number of application areas.
Figure 1. (a) Diagrammatic representation of the mass spectrometric sampling of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) ionization source. (b) Picture of the plasma source and mass spectrometer sampling cone during operation.
which a cooling gas is passed. The metallic counter electrode is placed ∼2 mm from the liquid surface, with the plasma discharge potential applied to either electrodes while holding the other at ground potential. The i-V characteristics (10-50 mA, 200500 V dc) of the device were shown to exist in the abnormal glow discharge regime of plasma operation.17 In this configuration, solution flows of up to 0.4 mL min-1 could be totally vaporized by the plasma struck to the liquid surface. Advantages of this geometry are the much lower flow rates (down to 0.1 mL min1 ), lack of liquid waste generation, and ready coupling to flow injection/chromatography sample introduction without compromise of temporal integrity. Limits of detection on the order of 10 ng for 5 μL injections have been obtained on fairly simple optical detection platforms,18 while also showing appreciable robustness with regards to addition of easily ionized matrix species.21 Specifically, additions of up to 0.1% (w/v) of Na and Ca caused no appreciable changes to analyte signal-to-background ratios or the observed Mg II/Mg I values.21 We describe here preliminary findings using the LS-APGD as a new ionization source for elemental analysis of aqueous samples, representing the first implementation of such a device for atomic mass spectrometry. In this particular implementation, the LSAPGD is mounted as a replacement (with no further modifications) to an electrospray ionization (ESI) source on a commercial Thermo Scientific Exactive (Orbitrap) mass spectrometer system. Basic design aspects and spectral characteristics
’ EXPERIMENTAL SECTION Nitric acid (HNO3, ∼5% v/v) was used as the mobile phase solution. Two stock solutions were prepared. The first consisted of 100 μg mL-1 In, 100 μg mL-1 Pb, and 10 μg mL-1 Cs, and the second consisted solely of 5 ng mL-1 Cs. A Waters (Milford, MA) model 510 high-performance liquid chromatography pump with a six-port Rheodyne 7125 injection valve and a Rheodyne 50 μL injection loop was used for sample delivery of the metal standard solutions to the LS-APGD source. The basic components of the LS-APGD source, as used for optical spectrometry, have been described previously.18 The current source consists of a metal capillary (700 μm o.d., 500 μm i.d.) that houses an internal glass capillary (125 μm o.d., 75 μm i.d.). Argon is introduced as a cooling/sheath gas in between the concentric capillaries at flow rates of
