Development of Alternative Plasma Sources for Cavity Ring-Down

We have been exploring innovative technologies for el- emental and hyperfine structure measurements using cavity ring-down spectroscopy (CRDS) combine...
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Anal. Chem. 2005, 77, 4883-4889

Development of Alternative Plasma Sources for Cavity Ring-Down Measurements of Mercury Yixiang Duan*

C-CSE, MS K484, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Chuji Wang and Susan T. Scherrer

Diagnostic Instrumentation & Analysis Laboratory, and Department of Physics and Astronomy, Mississippi State University, Starkville, Mississippi 39759 Christopher B. Winstead

Department of Physics and Astronomy, University of Southern Mississippi, USM Box 5046, Hattiesburg, Mississippi 39406

We have been exploring innovative technologies for elemental and hyperfine structure measurements using cavity ring-down spectroscopy (CRDS) combined with various plasma sources. A laboratory CRDS system utilizing a tunable dye laser is employed in this work to demonstrate the feasibility of the technology. An in-house fabricated sampling system is used to generate aerosols from solution samples and introduce the aerosols into the plasma source. The ring-down signals are monitored using a photomultiplier tube and recorded using a digital oscilloscope interfaced to a computer. Several microwave plasma discharge devices are tested for mercury CRDS measurement. Various discharge tubes have been designed and tested to reduce background interference and increase the sample path length while still controlling turbulence generated from the plasma gas flow. Significant background reduction has been achieved with the implementation of the newly designed tube-shaped plasma devices, which has resulted in a detection limit of 0.4 ng/ mL for mercury with the plasma source CRDS. The calibration curves obtained in this work readily show that linearity over 2 orders of magnitude can be obtained with plasma-CRDS for mercury detection. In this work, the hyperfine structure of mercury at the experimental plasma temperatures is clearly identified. We expect that plasma source cavity ring-down spectroscopy will provide enhanced capabilities for elemental and isotopic measurements. Cavity ring-down spectroscopy (CRDS) is a laser-based absorption technique that is capable of yielding highly sensitive, quantitative measurements of trace atomic and molecular species in various media, such as atmospheric air, combustion processes, flames, and plasmas. Since the first report of CRDS,1 the technol* Corresponding author. E-mail: [email protected]. (1) O′Keefe, A.; Deacon, D. A. G. Rev. Sci. Instrum. 1988, 59 (12), 25442551. 10.1021/ac050704x CCC: $30.25 Published on Web 06/18/2005

© 2005 American Chemical Society

ogy has gained intensive attention in the areas of trace gas monitoring, flame and plasma diagnostics, aerosol and particulate matter characterization, and environmental science as well as in some fundamental studies.2-6 Whereas traditional absorption measurements require that both the incident and transmitted intensities of a light beam through a sample be determined, CRDS measurements are based upon the determination of the rate at which light trapped in an optical cavity escapes. In this way, the critical issues encountered with traditional absorption techniques, e.g., source intensity, fluctuations, and stability, are circumvented.7 Because laser power fluctuations do not significantly interfere with CRDS measurements, this technique can be used to determine absolute absorption, even with a pulsed laser. The fundamental theory behind CRDS measurements of a target gas uniformly filling the optical cavity is relatively straightforward. A laser pulse is injected into a stable optical cavity consisting of two high-reflectivity mirrors where it remains trapped between the two mirror surfaces. The intensity of the trapped light within the cavity decays exponentially with time at a rate determined by the round trip losses of the laser pulse, which are typically due to the finite reflectivity of the mirrors, optical absorption, and/or scattering. The time constant of the exponential decay for a sample uniformly filling the optical cavity is given by

τ ) d/c(1 - R + σ(ν)nd)

(1)

where c is the speed of light, d is the cavity length, R is the (2) Jongma, R. T.; Boogaarts, M. G. H.; Holleman, I.; Meijer, G. Rev. Sci. Instrum. 1995, 66, 2821-2828. (3) Vasudev, R.; Usachev, A.; Dunsford, W. R. Environ. Sci. Technol. 1999, 33, 1936-1939. (4) Smith, J. D.; Atkinson D. B. Analyst 2001, 126, 1216-1220. (5) Thompson, J. E.; Smith, B. W.; Winefordner, J. D. Anal. Chem. 2002, 74, 1962-1967. (6) Mercier, X.; Therssen, E.; Pauwels, J. F.; Desgroux, P. Combust. Flame 2001, 125, 656-667. (7) Miller, G. P.; Winstead, C. B. In Cavity Ringdown Laser Absorption Spectroscopy in Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John Wiley and Sons Ltd.: Chichester, 2000.

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reflectivity of the cavity mirrors, n is the sample density, and σ(ν) is the absorption cross section at the given laser frequency ν. The term σ(ν)nd, therefore, represents the single-pass absorbance of the sample in the cavity. Although plasma sources have been widely used in atomic spectrometry with AES and MS detection, the investigation into and application of plasma sources as atomic reservoirs for conventional absorption measurements are still limited due to the characteristics of the plasma sources8-12 and the capabilities of conventional light sources. Recent advances in CRDS technology, in conjunction with renewed interests in atomic absorption measurements with plasma sources for atomization, have brought about plasma-CRDS for elemental and isotopic measurements.13-18 Inductively coupled plasmas (ICPs),13-15 microwave-induced plasmas (MIPs),17-18 and cascaded arc plasmas19 have been explored for elemental and isotopic detection as well as for plasma diagnostics. The move to a low-power, atmospheric MIP as the atomization source has opened the door to a technique that offers lower operational costs and portable instrument geometry. However, currently available plasmas for CRDS measurements are usually subject to short absorption path lengths, distinct background interference, and an ambient air effect, which limits the overall system sensitivity. Therefore, an ideal system based on this technology must fully address and circumvent these issues. The present work focuses on various new plasma source designs, development, testing, and coupling with CRDS measurement. The present research demonstrates the technical feasibility of measuring Hg using a newly developed, tube-shaped MIP source for plasma-CRDS. In an effort to minimize atmospheric spectral interferences and scattering losses due to the plasma, the tube-shaped plasma source was tested with various configurations of the discharge tube, which are designed based on the principle of surface wave propagation of the microwave technology (Surfation cavity).20,21 The system performance was characterized by detecting multiple concentrations of mercury using the various discharge tube configurations. This approach to the plasma design yielded improved sensitivity and reduced background interference. Furthermore, using the tube-shaped plasma-CRDS technique with the plasma operating at low powers and low gas flow rates, the hyperfine structure of mercury at the operational plasma temperature is readily observed. (8) Duan, Y.; Huo, M.; Du, Z.; Jin, Q. Appl. Spectrosc. 1993, 47, 1871-1879. (9) Ng, K. C.; Garner, T. G. Appl. Spectrosc. 1993, 47, 241-243. (10) Duan, Y.; Li, X.; Jin, Q. J. Anal. At. Spectrom. 1993, 8, 1091-1096. (11) Duan, Y.; Zhang, H.; Huo, M.; Jin, Q. Spectrochim. Acta 1994, 49B, 583592. (12) Duan, Y.; Huo, M.; Liu, J.; Jin, Q. F. J. Anal. Chem. 1994, 349, 277-282. (13) Miller, G. P.; Winstead, C. B. J. Anal. At. Spectrom. 1997, 12, 907-912. (14) Wang, C.; Mazzotti, F. J.; Miller, G. P.; Winstead, C. B. Appl. Spectrosc. 2002, 56, 386-397. (15) Wang, C.; Mazzotti, F. J.; Miller, G. P.; Winstead, C. B. Appl. Spectrosc. 2003, 57, 1167-1172. (16) Spuler, S.; Linne, M.; Sappey, A.; Snyder, S. Appl. Opt. 2000, 39, 24802486. (17) Duan, Y.; Wang, C.; Winstead, C. B. Anal. Chem. 2003, 75, 2105-2111. (18) Wang, C.; Koirala, S. P.; Scherrer, S. T.; Duan, Y.; Winstead, C. B. Rev. Sci. Instrum. 2004, 75, 1305-1313. (19) Engeln, R.; Letourneur, K. G. Y.; Boohaarts, M. G. H.; Van de Sanden, M. C. M. Chem. Phys. Lett. 1999, 310, 405-410. (20) Jin, Q.; Duan, Y.; Olivares, J. A. Spectrochim. Acta 1997, 52B, 131-161. (21) Moisan, M.; Beaudry, C.; Leprince P. IEEE Trans. Plasma Sci. 1975, PS3, 55-59.

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Figure 1. Schematic diagram of the experimental setup: L1 and L2, lenses; M1 and M2, cavity mirrors; PMT, photomultiplier tube.

EXPERIMENTAL SECTION Instrument Assembly and Setup. The majority of the instrumental configuration used in this work is the same as the setup described in our previous work,17 with the exception of the plasma source. As shown in Figure 1, this system consists of a pulsed laser source, a linear plasma source, a high-reflectivity ringdown cavity, the sampling device, and the detection electronics. A tunable, pulsed ultraviolet laser beam is generated by the frequency doubling (Inrad Autotracker III) of the output of a dye laser (Radiant NarrowScan) pumped by a 20-Hz repetition rate Nd:YAG laser operating at 355 nm (Continuum Powerlite 8020) (harmful to eyes!). The laser dye used is Cumarin 500 with an output power level of microJoules. The minimum scanning step of the dye laser in the UV is 0.0003 nm, with a line width of 0.08 cm-1. A spatial filter system consisting of two focusing lenses and a pinhole is used to approximately mode match the laser beam to the ring-down cavity. Two plano-concave mirrors (R ∼99.72%) (Los Gatos Research) are used to form the ring-down cavity. A 10-nm band-pass interference filter (CVI Laser) is positioned prior to the detection of the ringdown signal using a photomultiplier tube (PMT; Hamamatsu R928) to minimize light emission from the plasma source. A pulse generator (Stanford Research Systems DG 535) is used to control the system timing. The signals are digitized using a digital oscilloscope (Tektronix, TDS 410A) interfaced to a computer. Typically, 50-100 laser shots are averaged for the ring-down time constant measurements. A 0.5-m monochromator (ARC SpectraPro-500) with a CCD array detector (Spectrum One CCD-200) was used to simultaneously monitor the plasma emission spectrum. Microwave Plasma Source. A copper Surfatron microwave cavity, based on the original design of Moison21 but with some simplified modifications, is used in this study. In this version of the Surfatron design, the distance between the microwave antenna and the discharge tube is fixed after proper adjustment. The microwave antenna is used to couple microwave energy into the quartz discharge tubes. The o.d. of all of the discharge tubes examined was 6 mm; however, the i.d. of the different tubes, as well as the tubing configurations, varied. Regardless of the configuration employed, each discharge tube was concentrically centered in the cylindrical Surfatron cavity, as shown in Figure 2a. The plasma cavity is connected to a 2450-MHz microwave power supply through 1-m coaxial cable and mounted on a threedimensional (X-Y-Z) adjustable stage for precise alignment of the optical beam through the plasma discharge tube for maximum absorption. The sample, carried by the flow gas, is introduced into the discharge tube, where the plasma is sustained. Typical operational parameters for this system are summarized in Table 1.

Table 1. Operational Conditions of the Instrument Plasma microwave power plasma gas flow rate

50-120 W 0.35-1.0 L/min

Sampling sample uptake rate heating temperature of the ultrasonic nebulizer chamber cooling temperature of the ultrasonic nebulizer desolvator heating temperature of the membrane device N2 gas flow rate in the drier

0.75 mL/min 140 °C -5 °C 80 °C 0.5 L/min

Optical Setup laser repetition rate scanning step reflectivity of cavity mirrors filter band-pass

20 Hz 0.002-0.0003 nm 99.72% 10 nm

Data Acquisition number of laser pulses averaged

100

by the nebulizer to the plasma. Sample solutions were prepared by successive dilutions of a standard mercury (poisonous!) solution (1000 µg/mL, Absolute Standard Inc, Hamden, CT).

Figure 2. (a) Structure of the T-shaped plasma discharge device: (1) plasma absorption tube; (2) plasma discharge tube/chamber; (3) Surfatron plasma cavity; (4) cavity antenna; (5) plasma zones; (6) laser beam pathway; (7) gas flow direction. (b) Photo of the plasma discharge. Plasma power, 120 W; gas flow rate, 0.7 L/min. Other experimental conditions are given in Table 1.

Sample Introduction. The sample introduction system is also similar to the one reported in our previous work.17 Solution samples are delivered through a commercial peristaltic pump into an ultrasonic nebulizer (U-5000 AT+, CETAC), where the liquid samples are generated into fine, wet aerosols through contact with an ultrasonic transducer. The heating temperature inside the ultrasonic nebulizer is ∼140 °C, with a cooling chiller at the aerosol outlet operating around -5 °C. The aerosol generation efficiency by the system is ∼10% with a sample uptake rate of 0.75 mL/min. An additional desolvation device is employed to further control the solvent loading in the low-power plasma source by removing the water vapor produced during the desolvation process, thereby enhancing the system performance. Chemicals and Reagents. Ultrahigh-purity argon (99.999%) is used as both the working gas for supporting the plasma source and the carrier gas for introducing the aerosol samples generated

RESULTS AND DISCUSSION Principle of Plasma CRDS. The principle of CRDS measurement is relatively simple in the case where the targeted gas uniformly fills the cavity, so that the cavity loss is mainly from the mirrors and analyte absorption based on the Beer-Lambert law, as indicated in eq 1. If a plasma source is used as the atomic reservoir, the analyte only fills a fraction of the cavity length. In this instance, the absorption path length is related to the single path length of the laser through the plasma and the length of the ring-down cavity. Additional losses arise in the presence of a strong plasma emission background, which results in larger plasma scattering coefficients. Elemental determinations carried out in the ultraviolet region are also subject to Rayleigh scattering.22 To accurately take into account the losses due to the emission background and Rayleigh scattering, the ring-down equation can be expressed as

τ ) d/c(1 - R + βplasmal + βair(d - l) + absorbance) (2) where l is the absorption length in the plasma source and βplasma and βair represent the effective scattering coefficients for the plasma and nonplasma regions, respectively. Practically, the contribution of βplasma and βair to the ring-down time can be incorporated into the background contribution by substituting a lower effective reflectivity, Reff, for R in eq 1.13 This effective reflectivity can be determined by measuring the ring-down time with the plasma on but without an analyte present, τ0. Due to the spatial variation of the analytes within the plasma source, the absorbance is given by

absorbance ) 2

∫ ∫ +∞

-∞

l/2

0

σ(ν,r)n(r) dr dνZ

(3)

The absorption cross section in the equation is a function of the absorption line shape at the given laser frequency and the radial Analytical Chemistry, Vol. 77, No. 15, August 1, 2005

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position in the plasma.14 Experimentally, the direct measurements are the ring-down lifetimes τ and τ0, which are, respectively, the ring-down lifetimes obtained with and without an analyte present. Thus, the absorbance can be rewritten as

absorbance ) σnl )

(

)

d 1 1 c τ τ0

(4)

where all background losses are incorporated into τ0. Driving Forces for Alternative Plasma Sources. In an effort to improve the detection sensitivity of plasma-CRDS, several aspects of the original MIP-CRDS technique17,18 were modified. In accordance with the Beer-Lambert law, the single-pass absorption path length is a critical factor for the sensitivity of most absorption-based technologies; and this is particularly true of the plasma-CRDS technique.2 From this point of view, a plasma source with a long absorption path length will improve the sensitivity of a plasma-CRDS system, proportionately to the increase in absorption path length. The second driving force for alternative plasma designs originates from the background OH interference observed in the candle-shaped atmospheric pressure microwave plasma source for CRDS measurement around 253.7 nm.14,23 This is particularly true for mercury measurement at 253.7 nm, where the OH background proved to generate a significant spectral interference and adversely affected detection limits and measurement accuracy for the previously reported MIP-CRDS experiments for mercury detection.23 Furthermore, a plasma with lower temperature may help improve the detection limits for mercury, as little energy is needed in mercury atomization. The lower plasma temperature is particularly favorable for isotopic and hyperfine structure measurements of mercury where lower temperatures result in a higher ground-state population and small Doppler broadening effects. Plasma Source Design. The Surfatron possesses a number of advantages over other kinds of microwave-induced plasma devices, such as the Beenakker-type resonant cavity.24 Many of these desirable properties can be attributed to the annular shape of the plasma generated within the cavity.25 The annular-shaped plasma facilitates sample introduction into the central “channel” of the plasma, provides a relatively long sample residence time in the discharge, and promotes efficient atomization and excitation of the sample material through interaction with plasma species. Additionally, the Surfatron device can be easily coupled with different shapes of discharge tubes for various uses. In this work, we have designed several discharge tube configurations for CRDS measurements of elemental mercury and its hyperfine structure. The discharge tubes examined in this work are all made of quartz, which has a relatively high thermal tolerance and can be precisely manipulated into the desired shapes. Figure 2b illustrates an operational “T”-shaped plasma. The discharge tube, where the plasma is ignited and sustained, runs through the center of the Surfatron cavity. The absorption tube, parallel to the ruler in the figure, is connected perpendicular (22) Winstead, C. B.; Mazzotti, F. J.; Mierzwa, J.; Miller, G. P. Anal. Commun. 1999, 36, 277-279. (23) Wang, C.; Scherrer, S. T.; Duan, Y.; Winstead, C. B. J. Anal. Atom. Spectrom. In press. (24) Beenakker, C. I. M. Spectrochim. Acta 1976, 31B, 483-486. (25) Selby, M.; Hieftje, G. Spectrochim. Acta 1987, 42B, 285-298.

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Figure 3. Various plasma discharges: (a) straight tube; (b) T-shaped tube with short arms and off-centered hole; (c) diagonally cut tube.

to the discharge tube to form the T. The linear plasma is initially generated in the discharge tube and is then stretched out into the absorption tube, as shown in Figure 2b. The plasma discharge tube is situated perpendicular to the path of the laser beam, such that the laser passes directly through the absorption tube and, hence, the plasma in the “arms” of the tube. Some of the additional discharge tube configurations explored in this research are portrayed in Figure 3. As previously mentioned, the initial design was intended to significantly improve the sensitivity of the previously reported system by increasing the absorption path length. This goal was readily achieved with the initial T-shaped design; however, with the T-shaped plasma pictured in Figure 2b, some unexpected phenomena were observed when the laser beam passed through the plasma in the absorption tube. Although we have successfully demonstrated that the Surfatron-based plasma device could be successfully used for conventional atomic absorption spectrometry with sufficiently increased sensitivity,8,11 CRDS measurements utilizing this configuration were not as readily obtained as in the conventional measurements. Surprisingly, when the plasma was ignited, the ring-down event became either saturated or obstructed due to the excessive optical losses that the laser beam experiences in the plasma tube with the plasma running. For a better understanding of the microprocess within the discharge/absorption tube, sequential experiments were conducted to test the influence of the inner diameter of the absorption portion of the discharge tube (from 2 to 4 mm) as well as the effect of the plasma power, the plasma gas flow rates, and the detector voltage. Additional experiments were conducted to determine the influence on the measurements when the laser passed through different portions of the plasma and when the plasma was stretched longer or shorter in the arms of the absorption tube. The results of these intensive tests indicate that the unexpected influence may predominantly arise from the turbulence generated by the gas flow, particularly when the plasma gas is introduced from the discharge tube into the absorption tube through the T connection, where the extended plasma tail plume is turned in a right angle into the absorption tube and separated to the opposite directions. This explanation was confirmed by testing the system with the plasma off but the gas flowing through the discharge tube. To circumvent this unexpected phenomenon, alternate configurations of the absorption tubes were designed and implemented. Plasma Discharge/Absorption Tubing Alternatives: Figure 3 shows several of the designs that were tested in our experiments. To confirm that Hg could be detected using the tube-shaped plasma, the performance of the straight tube, with 2-mm i.d. and 6-mm o.d., was evaluated. Using this tube, Figure 3a, the plasma can be extended beyond the discharge tube, where it forms a thin tail plume on the front tip of the discharge tube. The laser

beam was aligned in front of the tip of the discharge tube, and the distance between the laser beam and the tip was optimized for signal sensitivity. Mercury detection with this configuration was achieved. However, due to the relatively short absorption path length, the detection limit for Hg was not as good as hoped for. Once the ability to detect Hg with the tube-shaped plasma source was confirmed, we returned to the original idea of the T-shaped configuration to increase the absorption path length. An alternate plasma discharge tube design, shown in Figure 3b, was implemented; this T-shaped tube with relatively short arms on each side had a small hole, ∼1-mm diameter, cut slightly offcenter from the junction of the absorption and discharge tubes. In this way, the plasma tail plume can extend out of the tube through the small hole after deviating in the absorption part of the tube. Before the hole was cut in the tubing, experiments were conducted to evaluate the ring-down signal with the short arm tube. However, the turbulence generated at the junction created significant instability in the measured signal. Therefore, the hole was cut at the junction to potentially relieve some of the excess turbulence. Even with this open hole, the laser beam still suffered the problem from gas turbulence when it passed through the short arms of this absorption tube. Therefore, experiments were conducted to examine the feasibility of detecting Hg by passing the laser beam outside of the absorption tube, parallel to the surface of the absorption tube. In this way, although we sacrifice the absorption path length as well as sensitivity to some extent, we did get stable and repeatable signals for mercury ring-down measurements. Figure 3c is a variation of the original T-shaped plasma source, with a diagonal cut on the joint point of the T section. With this design, the laser beam can pass either through the inside of the absorption tube or parallel to the outside of the absorption tube. It seems that, in both laser beam settings, we can obtain competitive CRDS signals for mercury measurement with such a plasma design. In addition, there is no observed turbulence effect in this diagonally cut discharge design either through inside of the absorption tube or outside of the absorption tube. For these reasons, this diagonally cut design was most frequently used in the following experiments in this work. Instrument Performance on Mercury CRDS Measurement. Since mercury poses a significant problem in human health and environmental impact, there has been wide interest in identification and speciation of mercury in various samples. Mercury can enter the human body in several ways, such as inhalation, ingestion, and absorption through the skin. Some of the adverse side effects of exposure to mercury include damage to the central nervous system and the kidneys, emotional and psychological instability, cognitive disturbances, and speech disorders.26,27 Detecting and monitoring mercury in the environment is a key topic in many research efforts, and most of the techniques utilized to measure environmental Hg require off-site sampling and sample preparation. For these reasons, development of a real-time mercury analyzer is one of our goals in the plasmaCRDS research. Mercury CRDS measurements were conducted with the various discharge tube configurations. To identify the performance of each design, baseline stabilities with and without the plasma (26) Hanish, C. Environ. Sci. Technol. 1998, 32, 176A-179A. (27) Environmental Protection Agency. 1997 status and trends, EPA-454/F-98009.

Figure 4. Baseline stability with off-centered plasma tube. Numbers of laser shots for average, 50; argon flow rate, 1.0 L/min. Other experimental conditions are the same as in Table 1.

Figure 5. Baseline scans with no plasma, plasma with no blank, and plasma with blank.

on were obtained with each configuration. The left part of Figure 4 shows typical baseline stability obtained with the off-centered T-shaped plasma (Figure 3b) with only the gas flow on and the laser passing parallel to the outside of the absorption tube. When the plasma was initiated, a sharp drop in the ring-down time was observed. The downward spike quickly stabilized, and a relatively stable baseline with the plasma on was obtained, as is shown in the right part of the figure. This jump in the experimental baseline could be attributed to the rapid change in temperature, the change in ion concentration, the increased turbulence, or an electronic artifact. Figure 5 shows a low-resolution scan of the baseline with no plasma (dotted line), with the plasma on but without the blank solution (thin, solid line), and with the plasma on and the blank solution present in the atomization cell (thick, solid line). The background peaks in the spectra originated from the forbidden transition of oxygen (O2: A - X(7,0), N′′) 19, Q multiplet).28 For this initial low-resolution scan of the mercury absorption profile, the ring-down time was measured at 0.002-nm increments from approximately 253.570 to 253.690 nm. Each data point in this figure is generated from the average of 100 ring-down events. To give a better visual effect in Figure 5, the three spectra obtained with different experimental conditions were artificially separated by adding 1.25 µs in the vertical scale for each case except that of plasma with a blank solution, which remained unchanged. Figure (28) Borrell, P. M.; Borrell, P.; Ramsay, D. A. Can. J. Phys. 1986, 64, 721-725.

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Table 2. Mercury Isotopes and Abundance abundance (%) line position

theor

measd

isotope assignment

atomic mass

1 2 3 4 5

13.4 14.4 23.1 29.8 19.1

13.9 14.5 24.8 31.2 15.6

199(a)+201(c) 198+201(b) 200 202 199(b)+201(a)+ 204

198.9683+200.9706 197.9668+200.9703 199.9683 201.9706 198.9683+200.9703+ 203.9735

Figure 6. Mercury profile obtained with diagonally cut plasma tube. Plasma power, 85 W; gas flow rate, 0.85 L/min; mercury concentration, 500 ppb. Other experimental conditions are the same as in Table 1.

6 shows the mercury profile obtained with a 500 ppb sample solution of mercury using the diagonally cut plasma discharge tube (Figure 3c). The laser beam passed through the center of the short arm of the absorption tube and intersected the plasma sustained in the discharge tube. In these spectral scans, no significant turbulence effects were observed, which illustrates that the plasma discharge/absorption design can be served as an atomization cell for mercury CRDS measurements. A similar scan with a 250 ppb mercury sample solution was obtained with the off-centered T-shaped plasma tube (Figure 3b). In this experiment, the laser beam only passed in front of the open hole rather than inside of the absorption tube. To calibrate the system, a series of mercury concentrations from 10 to 500 ppb were tested. One important point to note is that, with the tube-shaped plasma source developed in this work, the background spectral interference of OH observed in the candle-shaped ICP and MIP atmospheric pressure plasmas14,23,29 was significantly reduced or eliminated, depending on the particular tube shape. Since the OH background is one of the main factors affecting both the detection sensitivity and the accuracy of Hg measurements, such a background reduction could significantly improve the performance of this technology. As discussed in the related literature,23 a large measurement error could be introduced through the background subtraction when the sample concentrations are relatively low. Therefore, reduction of the background OH peaks in the newly developed tube-shaped plasmas should noticeably improve the detection limits for mercury. As is evident in Figure 6, the spectral interference from OH is significantly reduced with the tube-shaped plasmas. One possible explanation for the drastic reduction in the OH spectrum when the tube-shaped plasma is used is that the quartz tubing of the discharge device adequately shields the plasma from ambient air. Mercury Hyperfine Structure. Besides the extremely high detection power, another significant advantage of CRDS is its high spectral resolution, which provides a powerful tool to study the hyperfine structure of atoms. Mercury atoms produced in the (29) Wang, C.; Mazzotti, F. J.; Koirala, S. P.; Miller, G. P.; Winstead, C. B. Appl. Spectrosc. 2004, 58 (6), 741-744.

4888 Analytical Chemistry, Vol. 77, No. 15, August 1, 2005

Figure 7. Mercury hyperfine structure obtained with off-centered T-shaped plasma tube. Plasma power, 85 W; gas flow rate, 0.85 L/min; mercury concentration, 250 ppb. Other experimental conditions are the same as in Table 1.

plasma absorb radiation in the vicinity of its resonance transition at 253.652 nm. This wavelength corresponds to the electronic transition of mercury from the ground-state 6s level to the excitedstate 6p level (61S0-63P1). This transition line consists of pressurebroadened overlapping lines of the various stable isotopes of mercury. The somewhat broader line shown in Figure 6 is due to the overlap in absorptions from these isotopes. As shown in Table 2, mercury consists of seven naturally occurring isotopes, five even ones due to isotope shifts and two odd ones due to the nuclear spin.30 Table 2 also includes information about the abundance of isotopes and exact atomic mass numbers. The scan for mercury hyperfine structure, shown in Figure 7, was performed with the minimum scanning step of the dye laser used in this experiment (0.0003 nm) with a line width of 0.08 cm-1. The absorption line width is a factor of both instrumental and physical broadening.31,32 The instrumental broadening results from the laser line width (