Continuum Source Tungsten Coil Atomic ... - OSA Publishing

Continuum Source Tungsten Coil Atomic Fluorescence Spectrometry JIYAN GU, GEORGE L. DONATI, CARL G. YOUNG, and BRADLEY T. JONES* Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109

A simple continuum source tungsten coil atomic fluorescence spectrometer is constructed and evaluated. The heart of the system is the atomizer: a low-cost tungsten filament extracted from a 150 W light bulb. The filament is resistively heated with a small, solid-state, constant-current power supply. The atomizer is housed in a glass chamber and purged with a 1 L/min flow of a conventional welding gas mixture: 10% H2/Ar. A 25 lL sample aliquot is pipetted onto the tungsten coil, the liquid is dried at low current, and then the atomic vapor is produced by applying a current in the range 3.5–5.5 A. The atomization current does not produce temperatures high enough to excite atomic emission. Radiation from a 300 W xenon lamp is focused through the atomic vapor, exciting atomic fluorescence. Fluorescence signals are collected using a hand-held chargecoupled device (CCD) spectrometer. Simultaneous determination of ten elements (Ag, Bi, Cr, Cu, Ga, In, Mg, Mn, and Tl) results in detection limits in the range 0.3 to 10 ng. The application of higher atomization currents (10 A) leads to straightforward detection of atomic emission signals with no modifications to the instrument. Index Headings: Electrothermal vaporizer; Tungsten coil; Atomic fluorescence spectrometry; AFS; Continuum source; Atomic emission spectrometry; AES; Portable instrumentation.

INTRODUCTION Electrothermal atomizers find broad applications in elemental analysis.1 The ideal atomizer should be chemically inert and physically stable. The device should have a high melting point and a rapid heating rate. Finally, the atomizer should be readily available at a reasonable cost. The tungsten coil (W-Coil) atomizer meets these criteria in many respects. Tungsten has one of the highest melting points (3422 8C) of all the elements, second only to carbon. The W-coil heating rate has been measured as high as 30 K ms1, and the filament is resistant to attack even from harsh chemicals such as hydrochloric, sulfuric, and nitric acid.2 In the early 1970s, the W-Coil was employed in electrothermal atomization atomic absorption spectrometry.3 Subsequently, the W-Coil has been reported as an electrothermal vaporizer for inductively coupled plasma (ICP) emission spectrometry, microwave-induced plasma (MIP) emission spectrometry, mass spectrometry (MS), ICPMS, and flame-furnace atomic absorption spectrometry (FFAAS).4–9 A small, portable tungsten coil atomic absorption spectrometer (WCAAS) was used for the determination of Pb and Cd in the 1990s.10,11 For this device, the W-Coil was extracted from a commercially available light bulb. The entire spectrometer measured 50 3 20 3 8 cm and it was completely powered by a 12 V car battery. The system was simplified later by the development of tungsten coil atomic emission spectrometry (WCAES), which obviated the need for the Received 11 October 2010; accepted 12 January 2011. * Author to whom correspondence should be sent. E-mail: jonesbt@wfu. edu. DOI: 10.1366/10-06158

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external light source common with AAS devices. Furthermore, WCAES was capable of simultaneous multi-element determinations. High sensitivity was reported for a broad range of elements including Al, Co, Cr, Ga, K, Mn, Pb, Rb, Sc, Cr, Ga, In, V, and the lanthanides.12–15 Absolute detection limits are near or below the nanogram level for most of these elements. Unfortunately, many elements do not emit strongly at the highest temperatures achieved with the W-coil (3300 K). For example, WCAES reported with the charge-coupled device (CCD) detector used in the present work was capable of detecting the emission of all but 16 of the metals and semimetals with atomic number less than 84.16 Those elements with excitation energies above 350 kJ/mole were not detected with this system. Some particularly interesting analytes are included in this category: Ag, Bi, Cu, and Mg, for example. Atomic fluorescence spectrometry (AFS) has been employed for chemical analysis for almost 50 years.17 For many elements, AFS provides the lowest detection limit reported to date. Very few of these reports, however, use metal filaments to produce the atomic vapor. In the 1970s a tungsten wire loop and an electrodeless discharge lamp excitation source provided sub-ppb detection limits for many elements.18,19 More recently, a W-coil similar to the one described previously11 was used for sample vaporization in a flame atomic fluorescence instrument.20 Cadmium and eight hydride-forming elements were determined with this system. The same vaporizer has also been employed in a laser-induced fluorescence device (W-Coil LIF).21 This system provided ng/L detection limits. The author suggested that a portable device could be developed. In the current work, continuum source tungsten coil atomic fluorescence spectrometry (CS-WCAFS) is reported for the first time. The radiant flux from a xenon arc lamp with no wavelength selection filter is focused near the surface of the Wcoil vaporizer. This provides simultaneous excitation of multiple elements and the resulting fluorescence is captured with a charge-coupled device detector.

EXPERIMENTAL Instrumentation. The laboratory-constructed W-Coil vaporizer has been described previously.12 The tungsten coil filament was extracted from a 15 V, 150 W commercially available slide projector light bulb (Osram Xenophot 64633 HLX Pullach, Germany) and was housed in a glass vaporization cell (Ace Glass, product No. D131703, Vineland, NJ) that had three fused-silica windows arranged in a T-shape configuration and one sample injection port (Fig. 1). A continuous flow of 10% H2/Ar purged the cell at a rate of 1.0 L/min. This prevented oxidation of the coil, provided a reducing atmosphere for the analyte atoms, and cooled the vaporizer between heating cycles. The purge gas escaped through the sample introduction port. A flow rate in the range 0.9–1.0 L/min provided the strongest emission signals. The W-

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APPLIED SPECTROSCOPY

TABLE I. W-coil heating cycle for CSWCAFS. Step

FIG. 1. Photograph of the continuum source tungsten coil atomic fluorescence spectrometer.

coil was heated resistively by applying a constant current from a solid-state, computer-controlled power supply (Vicor BatMod, Andover, MA). The power supply provided a specified current in the range 0–10 A (at up to 15 V DC). The current was controlled using a simple Visual Basic program that supplied a user-selectable 1–5 V reference signal. The instrumental arrangement was straightforward (Fig. 1). The collimated radiant flux from a 300 W compact xenon arc lamp (Luxtel CL 300 BUV, Danvers, MA) passed through an electronically activated shutter. The lamp was operated at maximum power (300 W) so closing the shutter prevented bright reflections during sample introduction. The collimated beam was focused through one window on the atomization cell to a point near the surface of the W-coil using a 5 cm diameter, 15 cm focal-length fused silica lens. The diverging beam leaving the W-coil exited the cell through a second window. The fluorescence signal was viewed at 908 with respect to the path of the xenon lamp, through the third fused silica window. A 2.5 cm diameter, 7.5 cm focal-length fused silica lens was placed 15 cm away from the W-coil. The atomic fluorescence collected with this lens was focused as a 1:1 image on the entrance aperture of a hand-held CCD spectrometer (Ocean Optics USB4000, Dunedin, FL). The CCD spectrometer was powered by a computer via a USB connection and responded to wavelengths in the range 200–1100 nm. The spectrometer was fitted with a 10 lm fixed entrance slit and an 1800 grooves/mm holographic grating. The resulting spectral image was focused on a 3648 pixel CCD detector. The system covered a spectral window of 200–430 nm. Figure 2 presents a

FIG. 2. Simultaneous multi-element CSWCAFS spectrum of a solution containing 50 lg/mL Bi, Cr, Cu, Ga, In, Pb, Ag, Tl, Mg, and Mn.

1 2 3 4 5 6 7

Current (A)

Time (s)

2.7 2.5 2 0 5 8 0

45 35 15 10 15 3 15

portion of this window. The spectral resolution is 0.4 nm full width at half-maximum (FWHM) at the Tl 377.6 nm line. The detector software had user-selectable inputs for integration time and number of successive spectra to collect and save. The detector was triggered at the beginning of the vaporization step by the Visual Basic W-coil control program mentioned above. Precise optical alignment was critical. Fluorescence was viewed at 908 with respect to the excitation source to prevent direct detection of the xenon lamp emission. Even at this angle the W-coil reflected any Xe lamp radiation that struck the surface. Fine adjustment of the focusing lens height assured that the lamp radiation was focused at a spot 2 mm above the upper surface of the W-coil without striking the filament directly. The fluorescence collection lens was also positioned to focus this same point on the entrance slit of the CCD spectrometer. Solutions. All reference solutions were prepared by serial dilution of single-element stock solutions (1000 mg/L, SPEX CerPrep, Metuchen, NJ) with distilled deionized water (MilliQ, Millipore Corp., Bedford, MA). A standard reference material (Water Pollution Standard 1, Product Number WPS1-100, VHG Labs Inc., Manchester, NH) and an instrumentation calibration standard (Perkin Elmer # N930-0221, Waltham, MA) containing Pb and Tl were used to test the accuracy of the system. Precision of the instrument was tested at concentrations fifty times greater than the detection limit. Procedure. A 25 lL sample aliquot was pipetted onto the W-coil and heated with the program in Table I. The first three steps served to dry the liquid drop at progressively lower currents. This prevented premature analyte loss due to overheating of the coil during the dry step. While the liquid drop remained on the filament, the constant current could pass through the drop rather than the coil wire; thus the amount of resistive heating was diminished. As the drop dried and became smaller, the same constant current caused a higher temperature, so the current was reduced. The dry coil finally reached a maximum drying temperature at a coil current of 2 A (step 3). Step 4 allowed the completely dry coil to return to room temperature. Then during the vaporization step 5, a current in the range 3.5–5.5 A was applied and the analyte atoms were driven into the vapor phase. The detector was triggered at the beginning of this step. Five successive spectra, each with a 3 s integration time, were collected during this 15 s period. Atomic fluorescence appeared beginning at 3 s into the vaporization step and ending at 15 s. Maximum signals were observed at either 9 or 12 s depending upon the element, so all signals collected during the period 3–15 s were summed. At the end of the vaporization step, a 3 s cleaning step at 8 A was applied to remove any sample residue. The final step allowed the coil to


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FIG. 3. Relationship between CSWCAFS signal and vaporization current for 100 lg/mL Bi.

return to room temperature prior to the next sample cycle. The entire vaporization program took less than 3 minutes. The optimal vaporization current for each element was determined independently. The results for Bi are shown in Fig. 3. At currents lower than 3 A, no fluorescence was observed for any element. Peak vaporization currents ranged between 3.5 A for the more volatile elements such as Bi (Fig. 3) to 5.5 A for less volatile elements such as Cu. At currents of 5.5 A and higher, the blackbody emission from the W-coil was observed at the detector. This effect increased with increasing current. A compromise current of 5.0 A was chosen for simultaneous multi-element determinations. At this current the fluorescence was reduced by no more than 40% from the maximum current of any element.

RESULTS AND DISCUSSION Multi-Element Determinations. Ten test elements were determined simultaneously by CS-WCAFS (Table II, Fig. 2). Limits of detection were similar for all elements, ranging from 10 to 400 ng/mL. Limits of detection were calculated by the IUPAC method: three times the standard deviation of the blank signal divided by the slope of the calibration curve. These results are compared with those observed for the single-element determination using a flame as the atomization cell, a photomultiplier tube detector, and a 300 W Xe lamp light TABLE II.

source.17 While the WCAFS limits of detection (LODs) are a bit higher, the elements are determined simultaneously. Among the ten test elements, only Ga, In, and Tl could be detected by WCAES. Mn (279.8nm) had the lowest fluorescence wavelength observed in this window. Elements with lower wavelength were even more difficult to detect because the Xe lamp intensity dropped off at their excitation wavelengths, and the detector sensitivity was also much lower in the UV region. The precision for the ten elements ranged from 3% to 7% relative standard deviation (RSD) using concentrations fifty times greater than the detection limits. For some elements (Cu, Ag, Mg) the strongest fluorescence wavelength corresponded with strong absorption wavelengths. Self-absorption was observed in these cases. When the fluorescence was viewed near the surface of the coil, self-absorption was very severe, degrading linearity. The linear range for most elements was three to four orders of magnitude by using atomic fluorescence with a strong line source such as pulsed EDLs or lasers. The detection limits in these cases can be as low as 1 lg/L and the calibration curve starts to bend over when the concentration reaches 20–50 mg/L. But for CS-WCAFS, the linear range was only one to two orders of magnitude. This was mainly because the xenon lamp was weaker and the CCD detector is less sensitive, so LODs are around 100 lg/L. The calibration curves bend over at concentrations around 50 mg/L. The LODs can be lowered by using a pulsed xenon lamp or a cooled CCD detector. The accuracy of the method was evaluated for the determination of Pb and Tl in the water reference materials. For the polluted water sample (Water Pollution Standard 1, Product Number WPS1-100, VHG Labs Inc., Manchester, NH), a 20-fold dilution with distilled deionized water was the only sample preparation. The resulting Tl level was below the detection limit, and the Pb recovery was 128% (100 mg/L known, and 128 mg/L found). This sample contains high concentrations of Al and V (500 and 250 mg/L), so matrix effects may have caused the inaccuracy. The instrument calibration standard contained five elements including Pb and Tl, with none above a concentration of 100 mg/L. The recovery for Pb was 98% (50 mg/L present, and 49 mg/L found), while that for Tl was 106% (100 mg/L present, and 106 mg/L found). The CS-WCAFS system may also be used for WCAES measurements. No additional alignment or instrumental changes are necessary. By simply switching off the xenon lamp, and using a current of 10 A for the atomization step, the system can be used to determine Al, Co, Cr, Ga, K, Mn, Pb, Rb, Sc, Cr, Ga, In, V, and lanthanides at the ppb level.12–14 Yb

Analytical figures of merit for CSWCAFS. The precision is reported at a concentration 50 times higher than the limit of detection (n ¼ 15). Limit of detection (ng/mL)

Element Mn Mg Bi Cu Ag Cr Tl Pb In Ga

384

Fluorescence wavelength (nm)

CS-WCAFS

Flame17

Linear dynamic range (decades)

Precision (% RSD)

279.8 285.2 306.8 324.7 328.1 357.9 377.6 405.8 410.2 417.2

300 10 200 400 70 70 50 300 50 100

5 0.6 8 4 9 40 200 80 -

2.0 1.2 2.1 1.0 1.5 1.5 2.3 2.0 1.6 2.1

6.5 5.7 2.9 5.5 4.1 6 3.8 3.1 5.2 4.8

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FIG. 4. Tungsten coil atomic emission spectrum for 10 lg/mL Ca and Yb.

and Ca were used to demonstrate this dual ability of the system (Fig. 4). All emission signals were collected in only three seconds: the integration time was 0.5 s per spectrum, and six successive spectra were collected. The highest emission signals appeared on the third or fourth spectrum. Both atomic and ionic emission lines were observed: Ca 422.6 nm, Yb 398.7 nm, Yb 346.4, and Ca (II) 393.3 nm (Fig. 4). By using both atomic fluorescence and emission analysis, a large group of elements can be determined simultaneously using this portable tungsten coil system. ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation and the Department of Homeland Security through the joint Academic Research Initiatives program: NSF CBET 0736214 and DHS 2008DN-077-ARI020-03.

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