Determination of tellurium and antimony in nickel alloys by laser excited atomic fluorescence spectrometry in a graphite furnace

Spectrochimico Acta, Vol. 48B, No. 1, pp. 7-23, Printed in Great Britain.

1993

t .oo

0584-8547/93 wm Pergamon Press Ltd

CJ1992

Determination of tellurium and antimony in nickel alloys by laser excited atomic fluorescence spectrometry in a graphite furnace ZHONGWEN LIANG, ROBERT F. LONARDO Department

and ROBERT G. MICHEL*

of Chemistry, University of Connecticut, 215 Glenbrook Road, Storm, CT 06269-3060, U.S.A. (Received 27 July 1992; accepted 2 September

1992)

Abstract-Analytical laser excited atomic fluorescence of the metalloids tellurium and antimony in an electrothermal atomizer was studied. The detection limits were 20 fg and 10 fg for tellurium and antimony respectively, equivalent to about 0.01 ng g-i in nickel based alloys by direct solid sample analysis, for a 1 mg solid sample, or 1 ng g-l by the dissolution method, for a 100 mg solid sample in 100 ml solution. The detection limits were three orders of magnitude better than those obtained by graphite furnace atomic absorption spectrometry. They were also comparable to, or better than, those by inductively coupled plasma mass spectrometry. The linear dynamic ranges of the calibration curves were found to be six and seven orders of magnitude for antimony and tellurium respectively. By use of aqueous calibration, tellurium was accurately determined in NIST nickel alloy Standard Reference Materials by both a solid sample method, with a relative standard deviation (RSD) of about 13%, and a dissolution method with an RSD of about 9%. Antimony in Pratt and Whitney “A” series nickel alloy standards was successfully determined by the dissolution method, with an RSD of about 7%) but by solid sampling the antimony method gave incomplete recovery. Molecular fluorescence backgrounds from nitric oxide and silicon monoxide were observed and discussed.

1. INTR~DuC~~N excited atomic fluorescence spectrometry in an electrothermal atomizer (ETA-LEAFS) offers femtogram detection limits for many metallic elements and has been reviewed in several articles [l-3], but very little work has been done on metalloids and non-metallic elements with this technique [2]. For non-metallic elements, and some metalloids, the most sensitive atomic lines are in, or close to, the vacuum ultraviolet, which is difficult to access. For some of these elements, laser excited molecular fluorescence spectrometry in an electrothermal vaporizer has been used (ETV-LEMOFS). The approach was similar to that used in the graphite furnace electrothermal atomizer atomic absorption spectrometry (ETA-AAS) of small molecules [4], such as the determination of fluorine by the fluorescence of magnesium monofluoride (MgF) [5] with a detection limit of 300 fg [6]. Similarly chlorine and bromine have been determined by use of indium monochloride (InCl) [5, 71 and aluminum monobromide (AlBr) [5]. Recently we reported the determination of phosphorus in plant and biological samples and nickel alloys by phosphorus monoxide ETV-LEMOFS and phosphorus ETA-LEAFS [8]. Detection limits, of 80 and 8 pg respectively, were obtained. In this paper we report the ETA-LEAFS determination of two metalloids (tellurium and antimony) with improved detection limits (10 fg or 1 ng 1-l) compared to other methods. The analysis of nickel based alloys is used both to illustrate the excellent detection limits in these samples, and the accuracy of the ETA-LEAFS technique for these two elements. Detection limits for tellurium and antimony by electrodeless discharge lamp (EDL) excited flame atomic fluorescence spectrometry have been between 50 and 3300 ng ml-’ for antimony, and from 10 to 1000 ng ml-l for tellurium [2]. THOMPSON [9] reported the atomic fluorescence determination of antimony, arsenic, selenium and tellurium by use of the hydride generation technique. The detection limits were 0.2, 0.2, 0.1 and 0.1 ng ml-l respectively. MOLNAR and WINEFORDNER [lo] reported EDL LASER

* Author to whom correspondence

should be addressed. 7

8

ZHONGWEN LIANG et

al.

atomic fluorescence in a resistively-heated vitreous-carbon limits of 10, 0.6 and 3 ng ml-l for antimony, bismuth

tube furnace, with detection and tellurium respectively.

LEONGet al. [ll] were the first to use a laser as the light source for the determination of tellurium and antimony by ICP atomic fluorescence spectrometry. The detection limit for tellurium was 9 ng ml-l while the best detection limit for antimony was 2 ng ml-’ with an excitation wavelength of 231.1 nm and resonance detection at 231.1 nm. HARGREAVES et al. [12] described a method for the determination of tellurium by EDL atomic fluorescence in a graphite furnace, and a detection limit of 1 pg was obtained. Tellurium and antimony, like other low melting elements, such as thallium, bismuth and lead, are known to have deleterious effects on the strength of nickel based high temperature alloys even when present at trace concentrations [13, 141. Graphite furnace atomic absorption spectrometry has been widely used for the determination of tellurium and antimony. WELCHERet al. reported the direct determination of tellurium in solutions of nickel based alloys by ETA-AAS [15]. However, well-characterized standard alloys or standard solutions were needed and a significant background signal constrained the detection limit to 0.2 pg g-l [15]. SOTERAet al. [16] proposed a method to reduce matrix interferences, during the analysis of nickel based alloys, by the combination of a microboat and the use of an aerosol deposition system. KUJIRU et al. [17] proposed the determination of trace tellurium in nickel based and cobalt based alloys by ETA-AAS after co-precipitation with arsenic to avoid matrix interferences [15]. WICKSTROM and LUND [18] found that iron(II1) can minimize serious interferences from copper(I1) and nickel(I1) on the determination of tellurium by hydride generation atomic absorption spectrometry. Direct solid sampling methods have been attempted for the determination of traces of tellurium and antimony in nickel based alloys with graphite furnace atomic absorption spectrometry [N-21]. It was usually necessary to use standard alloys for calibration [19, 201, although one paper showed that aqueous standards can yield accurate results for certain samples [21]. By use of STPF (stabilized temperature platform furnace) technology, IRWIN et al. [22] successfully determined tellurium in nickel based alloys by direct solid sampling graphite furnace atomic absorption spectromelry with aqueous calibration. The present paper reports the determination of tellurium and antimony in nickel based alloys by ETA-LEAFS with both direct solid sample and dissolution methods calibrated with aqueous standards. Compared to direct solid sample analysis by ETA-AAS, direct solid sample analysis by ETA-LEAFS has advantages of longer linear dynamic range of the calibration graphs and higher sensitivity. In our prior work, the limited linear dynamic range of ETA-AAS caused the need to use an internal gas flow through the furnace, during the atomization step, to reduce the sensitivity for the determination of lead at fairly high concentrations [22]. This was not necessary for the ETA-LEAFS determination of lead in nickel alloys [23]. To continue our effort to address sample generated backgrounds in ETA-LEAFS, we also report a spectral background study of these samples. A molecular fluorescence background was found during the analysis of tap water and was tentatively attributed to silicon monoxide (SiO). The effect of this background on the determination of tellurium and antimony in water samples is discussed.

2. EXPERIMENTAL 2.1. In.strumentation A schematic diagram of the instrumentation

is shown in Fig. 1 and the components are listed in Table 1. The laser system for ETA-LEAFS has been described elsewhere [24] and is briefly summarized here. An excimer laser, which was operated with xenon chloride (308 run), was used to pump a tunable dye laser with a frequency doubler. The laser had a maximum repetition of 500 Hz. A laser repetition rate of 200 Hz was used, except for the studies of detection limits and the real sample analyses, where a repetition rate of 500 Hz was used. Stilbene 420 (2,2’-([l,l’-bipheny1]-4,4’-diyldi-2,1-ethenediyl) bisbenzenesulfonic aciddisodium salt) (EXITON,

Te and Sb determination

in Ni alloys by furnace LEAFS

9

PRE-AMPLIFIER

Fig. 1. Schematic diagram of the instrumentation (PMT: photomultiplier the components see Table 1.

tube). For details of

Table 1. Instrumentation Component/Model

No.

Excimer laser/EMG 104 MSC Dye laser/FL 3002E Frequency doublerlgbarium borate-2 Boxcar averager/l65, 162 PMT/9893QB-350 Monochromator/H-10 Graphite fumace&IGA-500 AT compatible micro-computer/System Triggering circuitry Data processing software

Manufacturer

200

Lambda Physik, Acton, MA Lambda Physik, Acton, MA Lambda Physik, Acton, MA PAR, Princeton, NJ Thorn-EMI, Fairfield, NJ ISA, Metuchen, NJ Perk&Elmer, Norwalk, CT Dell, Austin, TX Laboratory constructed Asyst Software, Rochester, NY

Dayton, Ohio), was employed as the laser dye at concentrations of 0.65 g 1-l and 0.22 g 1-l in methanol for the oscillator and amplifier dye cells respectively. A small portion of the dye laser beam was used to trigger a boxcar averager that was used to process the fluorescence signal from a photomultiplier tube (PMT). The details of the trigger circuitry have been reported before [24,25]. The frequency doubled output was passed through beam expansion to adjust the beam diameter to 2-3 mm before passage through the atomizer. The atomizer was a Perkin-Elmer HGA 500 graphite tube furnace equipped with an AS-40 autosampler and a L’vov platform. Both windows of the furnace were angled to reduce the stray laser background radiation [26]. Fluorescence was detected at 180” to the direction of the laser beam, in a scheme called front surface illumination [27]. A 50-mm diameter plane mirror, with a S-mm hole in the center, through which the laser beam passed, was positioned in front of the furnace at an angle of 45” with respect to the excitation axis. Three alternative lens combinations, two biconvex lenses, or a single biconvex lens, or two plano-convex lenses, were used to collect the fluorescence (Fig. 2 and Table 2). Lens descriptions are listed in Table 3. The detection system consisted of a photomultiplier tube, a preamplifier with a gain of ten, and a boxcar integrator with a gate width of 50 ns, a gate time constant of 0.5 ps, and an output time constant of 1 ms. A color filter, UG-5 (Schott color glass filter, ESCO Products Inc., Oak Ridge, NJ. Transmittance: 95% at 253 nm, < 0.01% at 213 nm) was used to reduce laser stray light for both tellurium and antimony. The data were acquired and manipulated with a personal computer, which plotted the fluorescence signal as a function of atomization time and calculated the integrated fluorescence. The integrated signal, peak area, was employed throughout this work.

ZHONGWEN LUNG et al.

10

furnace

Graph&

Ir’ Test position

c

Monochromator slit

._.________________

(a)

(b)

Fig. 2. Arrangement of three alternative lens combinations. Parameters, a, b, c and d are given in Table 2. The optical specifications of the lenses are given in Table 3: (a) a single biconvex lens (lens (a) in Table 3); (b) two plano-convex lenses; and (c) two biconvex lenses (lenses (a) and (b) in Table 3). Each lens combination was studied in the test position.

Table 2. Optical characteristics of the three lens system Distance in Fig. 2* Lens combination

a

b

C

Single biconvex lens Plano-convex lens pair Two biconvex lenses

96 75 96

101 50 101

202 115 202

d

5 303

*All dimensions are in mm.

Table 3. Optical specifications for the three lens system*

Lens Biconvex (a) Biconvex (b) Plano-convex

Focal length? (mm)

Diameter (mm)

f number

101 51 100

50.8 50.8 50.8

2 1 2

*The material for all lenses was fused silica. All lenses were purchased from OBIEL Corporation, Stratford, CT. tFoca1 lengths at 589 nm.

Te and Sb determination

in Ni alloys by furnace LEAFS

11

2.2. Standard solutions Aqueous standards of tellurium were made daily in 0.2% nitric acid (Ultrex, J. T. Baker Chemical Co., Philipsburg, NJ) by serial dilution of a 1000 pg ml-’ tellurium stock solution. The tellurium stock solution was made from pure metal (Spex Industries, Edison, NJ) diluted to one liter in 1% nitric acid. Aqueous standards of antimony were made, daily, by serial dilution of a 1000 p,g ml-l antimony stock solution with sub-boiled distilled deionized water. The antimony stock solution was made by dissolution of 2.743 g of potassium antimony1 tartrate hemihydrate (99.95% purity, Aldrich, Milwaukee, WI) in sub-boiled distilled deionized water, followed by dilution to one liter with the same water [28]. The nickel matrix modifier, 1000 pg ml-’ nickel (nitrate) in 0.5 M nitric acid, was obtained commercially (Atomic Spectral Standard, J. T. Baker Chemical Co., Philipsburg, NJ). The nitric acid and hydrofluoric acid that were used in the dissolution of samples were both ultrapure (Ultrex, J. T. Baker Chemical Co., Philipsburg, NJ). 2.3. Solid sampling procedures Instead of the Perkin-Elmer solid sample cup-in-tube apparatus [22, 291, a pyrolytically coated graphite tube and a L’vov platform were used for direct analysis of solid samples. For easier introduction of the solid sample chips, the sample injection port of the graphite tube was drilled a little bigger to 3 mm in diameter. National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs) 897, 898, 899, nickel based, high-temperature alloys, and Pratt and Whitney Aircraft nickel based alloy standards, designated as the “A” series standards, were used for method development. Both the SRMs and the “A” series standards were provided in the form of fine millings of mass of approximately OS-10 mg each. One metal chip of weight between 0.5 and 3 mg was normally used for each furnace firing. A Perkin-Elmer AD-2 autobalance (Perkin-Elmer Co., Norwalk, CT) was used to weigh the solid samples. The autobalance had a 5 g capacity with ranges from 0.1 l.t.gto 1000 mg. In the 10 mg range, used in the study, the balance resolution was +l pg. A plastic Eppendorf pipette tip, with the end cut off, was used as a funnel to transfer the chips onto the graphite platform. The chips were handled with poly(tetrafluoroethylene) coated tweezers. 2.4. D&solution procedures The SRMs and “A” series standards (0.1-0.15 g) were weighed and transferred into Teflon beakers, and were heated with 2 ml of water, 2 ml of nitric acid, and 0.5 ml of hydrofluoric acid for about 2 h. The dissolved samples were diluted to 100 ml with sub-boiled distilled water in polymethylpentene volumetric flasks.

3. RESULTSAND DISCUSSION 3.1. Matrix modification

and atomization

conditions

With front surface illumination and detection [27], standard commercial electrothermal atomic absorption atomizers can be used for LEAFS with no change. Stabilized temperature platform furnace (STPF) technology, which utilizes L’vov platforms, matrix modification, signal integration as a function of time, rapid electronics, pyrolytically coated graphite tubes, rapid heating, and Zeeman background correction [30] can be adopted in ETA-LEAFS. In their study of the determination of thallium, bismuth, tellurium, selenium and lead in nickel based alloy by solid sampling ETA-AAS/LEAFS and aqueous calibration [22, 231, IRWIN et al. found that it was not strictly necessary to use a matrix modifier in the aqueous standards, if there was a nickel residue in the furnace. The nickel alloy residue tended to act as a permanent matrix modifier. This was supported in the present work by the lack of any effect of nickel on the antimony and tellurium signals (Fig. 3) in a furnace previously conditioned with nickel. Nevertheless, in the present work, 20 ug of nickel was added to the furnace with the aqueous standards. Nickel has been used before as a matrix modifier for both tellurium [22, 311 and antimony [31, 321 and it is the predominant metal in the alloy samples. The effects of atomization temperature on the fluorescence signals of tellurium and antimony with and without nickel matrix modifier were studied. The optimum temperature for both elements did not change significantly in the presence of nickel.

12

&IONGWEN

J 0

.

..-.... 10

L~ANGet al.

I04

103

102

Nickel (ng)

Fig. 3. Effect of nickel on the signals from 2 ng of Te (0) and Sb (0).

The optimum temperature was higher at about 1800°C for antimony than for tellurium (1600°C). The effect of char temperature on the fluorescence signals of antimony and tellurium was determined. Without nickel matrix modifier, the maximum possible char temperature before sample loss was about 7OO”C,whereas with nickel matrix modifier, the maximum possible char temperature was increased to about 1200°C for both elements. A char temperature of 800~900°C was generally used for the analyses of dissolved samples in order to remove the nitric oxide interference that is discussed later. A char temperature of 1200°C could have been used but there was no pressing reason to use it. A typical graphite furnace heating program is shown in Table 4. 3.2. Optical saturation A laser beam size of about 2-3 mm in diameter was used. The laser energy was measured after the furnace, without a rear furnace window, by use of a pyroelectric joulemeter (Molectron, Model J3-O5DW, Sunnydale, CA). Optical saturation of the antimony and tellurium transitions started at a laser energy of 0.2 p,J per pulse and 0.1 p,J per pulse respectively, where the slope of the log relative fluorescence signal vs log laser energy plot became significantly less than one. The saturation curve reached a plateau at a laser energy of 2 CLJper pulse for both antimony and tellurium. Laser energies of about 1 p,J per pulse gave the best signal-to-noise ratio for both elements. 3.3. Effect of the lens combinations on detection limit FARNSWORTH et al. [33] reported a computer program that was used to calculate the

collection efficiency of various combinations of lenses. They did calculations for three lens systems: a symmetric biconvex lens; a pair of matched plano-convex lenses; and a pair of matched achromats, for front surface illumination laser excited atomic Table 4. The furnace heating program for the determination tellurium in nickel alloys by ETA-LEAFS*

of antimony

and

Step Parameter Temperature

(“C) Te Sb

Ramp(s) Hold(s) Internal argon gas flow rate (ml min-I)

1

2

3

4

5

6

200 200 20 30

800 900 1 20

20 20 1 10

1600 1800 ot 5

2650 2650 1 5

20 20 1 20

300

300

300

0

300

300

*Maximum power heating rate (approximately 1500°C s-l). tFor direct solid sample analysis, the dry and char steps (steps 1 and 2) were not necessary.

Te and Sb determination in Ni alloys by furnace LEAFS

13

Table 5. Comparison of the effect of lens combinations on the detection of antimony and tellurium Te

Two plano-convex lenses Single bioconvex lens? Two biconvex lenses*

Sb

Signal* (20 ppb)

Noise*

LOD (fg)

Signal* (20 ppb)

Noise*

LOD (fg)

12 600 4500 2500

0.22 0.16 0.16

20 43 77

12 500 5500 3700

0.25 0.18 0.23

24 39 75

*Arbitrary relative units. tf = 101 mm. $f. = 101 mm, fb = 51 mm.

fluorescence spectrometry of thallium in an electrothermal atomizer. The results showed that the collection efficiency, and the rejection power of laser stray light from the rear and front windows of the graphite furnace, increased in the order symmetric biconvex lens, the pair of matched plano-convex lenses, and the pair of matched achromats, due to reduction in spherical aberration. They found that the overall signal-to-noise ratio performance of a pair of matched achromats, or a pair of matched plano-convex lenses, was a factor of 2-3 better than that of a single biconvex lens. By use of the plano-convex lenses, a detection limit of 0.1 fg of thallium was obtained by ETA-LEAFS ]341* A two-biconvex lenses detection scheme (Fig. 2(c)) has been used in the authors’ laboratory for several years [25, 261. With this detection scheme, there was enough room to insert a plane mirror between the two lenses in order to direct the fluorescence from a flame atom cell into the monochromator to peak up the analytical line [26]. In this study, this scheme was compared to some alternative lens systems and the comparisons are given in Table 5. For both elements, tellurium and antimony, the fluorescence signal with a two plano-convex lenses system was about a factor of 2-3 greater than that with the single biconvex lens system, which was in agreement with the result of FARNSWORTHet al. [33]. The detection limits for tellurium and antimony were improved by a factor of about two with two plano-convex lenses instead of the single biconvex lens. Compared to two biconvex lenses, for both tellurium and antimony, the detection limits were improved by about a factor of 3-4 by use of the two matched plano-convex lenses (Table 5). Accordingly, the two matched planoconvex lens were used throughout the rest of this paper. FARNSWORTHet al. [33] used a small mask in the center of their first plano-convex lens, which reduced the amount of detected window fluorescence that would otherwise degrade the detection limit. We were able to confirm that this works well with the use of the standard windows on our furnace. However, we found that our use of angled windows also prevented the detection of the window fluorescence, and made the mask unnecessary. 3.4. Effect of slit width on the detection limit The theoretical considerations for the effect of slit width in both dispersive and nondispersive detection of fluorescence were reported by WEI et al. [26]. In the dispersive measurement, the fluorescence signal is proportional to the slit width, while the continuum background signal is proportional to the square of the slit width. In the background shot noise dominant case, the signal-to-noise ratio, or the detection limit, is independent of slit width with dispersive detection, since the noise is the square root of the background signal. In the flicker noise dominant case, where the noise is proportional to the background signal, the signal-to-noise ratio, or the detection limit, will be worsened by opening the monochromator slit width. The results of the effect of slit width on the signal, blank noise, and detection limit

ZHONGWEN IAANG et at.

14

Iu

800

g

400

e

1

(a)

025

050

IO0

025

050

100

200

Slit width (mm)

Fig. 4. The effect of slit width on the fluorescence signal and blank noise for tefhtrium and antimony. (a) TeUu~~, 40 pg. where (0) is blank noise and (0) is the sign&, (b) antimony, 10 pg, where (0) is blank noise and (0) is the signal. Detection limits are shown in Table 6.

of tellurium and antimony are shown in Fig. 4. The slopes of the log-log plot of the fmorescence signal vs slit width for tellurium and antimony were 0.991 and 1.03 in Fig. 4(a) and (b) respectively, both of which are very close to the theoretical value of one. The slopes of the log-log plot of the blank noise vs slit width, between 0.5 and 2.0 mm slit width, for tellurium and antimony were 1.06 and 0.91 in Fig. 4(a) and (b), respectively. They are close to the theoretical value of unity for the shot noise dominant case. As a result, the detection limits were independent of slit width between 0.5 and 2.0 mm (Table 6), which was in agreement with the theory for the shot noise dominant case. At a slit width of 0.25 mm, the detection limits were slightly worse (Table 6), which was attributed to a smaller decrease in noise compared to the theoretical value when the slit width was narrowed. The detector noise became significant when the slit width was 0.25 mm. 3.5. Detection limits and linear dynamic ranges of the cai~ratio~ curves Tellurium was excited at 214.281 nm. Its fluorescence, from both 238.326 nm and 238.578 nm, was detected by use of a 4-nm bandpass monochromator which was set at 238.4 nm. Under optimum conditions, a detection limit of 20 fg was obtained for Table 6. The effect of slit width on the detection limit* Detection limit (fg) Slit width (mm)

Tellurium

0.25

40

0.5 1.0 2.0

30 29 30

Antimony 17 13 10 11

*Based on the signals and noises shown in Fig. 4.

Te and Sb determination

in Ni alloys by furnace LEAFS

15

Table 7. Atomic spectroscopy detection limits of tellurium and antimony Detection limit Antimony

Tellurium Method ETA-LEAFS (this work)* ICP-LEAFS [ 1l] AFS Hydride AFS [9] ETA-AAS*? Hydride AASt ICP-AESt ICP-MST

(fS) 20

(nS 1-Y 1 9000

10

(ng l-l) 0.5 2000

20000 [38]

1000 [12] 10000

(fg)

80 500 20 50000 10

2oc00

100 1000 100 60000 1

*Detection limits are based on 20 ~1 sample volumes. TCited from The Guide to Techniques and Applications of Atomic Spectroscopy, Perk&Elmer, September, 1990.

tellurium (Table 7), which was about three orders of magnitude better than that by ETA-AAS or two orders of magnitude better than that by atomic fluorescence spectrometry with a conventional light source (Table 7). Based on a 20 ~1 sample volume, the concentration detection limit was 1 ng 1-l which was about l-2 orders of magnitude better than hydride generation AFS or AAS and ICP-MS, and about 4-5 orders of magnitude better than the detection limits of ICP-LEAFS and ICP-AES. With the dye used, two excitation wavelengths could be used for antimony ETA-LEAFS: 212.739 nm; and 217.581 nm. The latter wavelength has been used widely in ETA-AAS [30]. For the excitation wavelength of 217.581 nm, possible nonresonance detection wavelengths included 267.9, 277.0, 338.3 and 363.8 nm, for which we found the relative sensitivities of 35:100:37:54, respectively. The detection limit by using 217.58V277.0 nm was about a factor of two poorer than that of the detection scheme 212.739/259.8 nm. Also, 212.739 nm was closer to the peak energy of the dye than 217.581 nm, and hence the 212.739/259.8 nm scheme was employed for most work in this paper. Under optimized conditions, a detection limit of 10 fg was obtained for antimony, which was more than three orders of magnitude better than that obtained by ETA-AAS, or by atomic fluorescence spectrometry with conventional light sources (Table 7). Based on a 20 ~1 sample volume, the concentration detection limit was 0.5 ng l-l, which was more than two orders of magnitude better than hydride AFS, ICP-LEAFS, hydride AAS, and more than five orders of magnitude better than that of ICP-AES. It was slightly better than or comparable to that of ICP-MS. The linear dynamic ranges of the calibration curves were found to be six and seven orders of magnitude, from the detection limit, for tellurium and antimony, respectively. This dynamic range was measured by use of at least two concentrations for each decade of the dynamic range. 3.6. Sample analyses The advantages of the direct analysis of solid samples 3.6.1. Solid samples. include: decreased sample preparation time and decreased instrument furnace cycle time, since dry and char steps are not relevant; and increased analytical sensitivity and decreased risk of contamination, since no dilution takes place. The disadvantages of solid sampling are possible sample inhomogeneity, problems finding appropriate calibration standards, increased matrix interferences and sometimes difficult or inconvenient sample introduction. For tellurium and antimony respectively, the detection limits of 20 and 10 fg were equivalent to 0.02 and 0.01 ng g-* in alloys by direct solid sample analysis for a 1 mg solid sample. The results obtained here, for the determination of tellurium in NIST nickel alloys

ZI~~NGWEN

16

Table 8. Determination

LUNG

et al.

of tellurium in nickel alloys (p,g g-l)* Found

Certified

Dissolution

Solid sampling

1.05 f 0.07 0.54 + 0.02 5.9 -1-0.6

1.11 2 0.09 (6) 0.54 f 0.07 (7) 6.1 + 0.4 (5)

0.97 * 0.15 (11) 0.52 -+ 0.05 (8) 5.4 * 0.7 (8)

Sample NIST SRM 897 NIST SRM 898 NIST SRM 899

*Data are expressed as mean + one standard deviation. The number of replicates is in parentheses.

by direct solid sampling ETA-LEAFS and aqueous calibration, are shown in Table 8. The concentrations in NIST SRM 897,898 and 899 were found to be in good agreement with the certified values. These values also agreed with those obtained through direct solid sample analysis by AAS [22]. The relative standard deviation of the solid sample measurements shown in Table 8 was about 13% which was about the same as in graphite furnace AAS [22]. No certified values were available for antimony in these NIST SRMs. We tried the direct solid sample approach method for the determination of antimony in the “A” series nickel based alloys, but we were not able to determine antimony in the “A” series nickel alloys due to inhomogeneity (relative standard deviation of 11 measurements for 1A sample: 48%) of antimony in the samples and low recovery (about 50-60%). The boiling point of the pure antimony metal is 138O’C, which is considerably higher than that of tellurium (99O’C) and indicates that it would be more difficult to vaporize from the solid sample. However, this is not the only reason for poor recovery, since lead and thallium were successfully determined by solid sample ETA-LEAFS [23], even though the boiling points of lead (1722°C) and thallium (1457°C) are higher than that of antimony. The lack of good recovery was probably due to an involatile chemical form of antimony in the “A” series nickel based alloy. 3.6.2. Di.woZution. For the dissolution method, the detection limits of tellurium and antimony were equivalent to 1 and 2 ng g-l in the alloys respectively, if 100 mg of alloy was dissolved in 100 ml solution. Tellurium, in NIST standard reference materials, was determined, after dissolution of the samples, by use of ETA-LEAFS (Table 8). The amounts of tellurium in SRM 897, 898, and 899 were found, by use of Student’s t-test, to be in good agreement with both the certified values and the values obtained by the direct solid sample analysis method. The relative standard deviation of the measurements was about 9%. Antimony in lA, 2A and 3A nickel based alloys was determined by the dissolution method (Table 9). The reference values for these samples were 5, 9 and 17 p.g gg’ respectively. No confidence interval was available from Pratt and Whitney. By the dissolution method and ETA-LEAFS, the concentrations of antimony in lA, 2A and 3A were found, by Student’s t-test, to be in good agreement with the reference values, Table 9. Determination of antimony in nickel alloys (Pg g-l)’

Sample PW 1A PW 2A PW 3A

Reference value 5 9 17

Found 4.8 f 0.5 (8) 8.3 f 0.5 (9) 16.5 f 0.8 (6)

*Data are expressed as mean 2 one standard deviation. The number of replicates is in parentheses.

Te and Sb determination in Ni alloys by furnace LEAFS

0'

17

1

0

I

2

3

4

Time (S)

Fig. 5. Temporal behavior of tellurium in the nickel based alloy, with and without a char step: 214.2761259.8 nm; NIST SRM 898, 0.1125 g per 100 ml (12.2 pg Te). (a) Char temperature = 7WC, (b) no char step.

if a 10% relative standard deviation for the reference values was assumed. The relative standard deviation of the LEAFS measurements was about 7%. 3.7. Background studies 3.7.1. Spectral scan of the nickel alloy for tellurium. We observed a spectral interference from nitric oxide for the determination of tellurium in nickel alloy where the dissolution method was used, but only if no char step was adopted, as in the case of phosphorus [8] (Fig. 5). The nitric oxide fluorescence signal was from the transition, A2Z+-XII (v’ = 1, v” = 3), which peaked at 244 nm [37]. A bandpass of 4 nm was used, with the monochromator set at 238 nm for tellurium fluorescence. Nitric oxide was formed from the decomposition of nitrate, which was introduced during the dissolution process. The nitric oxide peak was temporally earlier than the tellurium peak (Fig. 5). The effect of char temperature on the tellurium fluorescence signal in a nickel based alloy is shown in Fig. 6. At a char temperature between 200 and 7oo”C,

0-k 200

400

600

600

1000

Char temperature

1200

MOO

1 1600

(*Cl

Fig. 6. Effect of char temperature on the tellurium signal in NIST SRM 898: 214.276/238.4 nm; NIST SRM 898, 0.1125 g per 100 ml (12.2 pg Te).

ZHONGWEN LUNG et al.

18

214225

214245 214.265 214.265 214305 Excitation wavelength (nm)

214.325

Fig. 7. Excitation spectrum of tellurium: (0) 20 pg aqueous Te; (0) NIST SRM 898 nickelbased alloy sample, 0.1125 g per 100 ml (12.2 pg Tel. The dye laser linewidth (FWHM) of 0.2 cm-‘, i.e. 0.0036 nm at 426 nm, was quoted by the manufacturer. Frequency doubled linewidth: 0.002 nm at 213 nm calculated from the fundamental [39].

the fluorescence signal decreased gradually due to a decrease in the nitric oxide signal (Fig. 6). Between 700 and llOO”C, the tellurium signal became constant. With a char step at 800°C for 30 s, it was possible to eliminate the interference from nitric oxide. To speed up the analysis by elimination of the char step, the tellurium signal could be integrated by itself, which would remove the NO interference, since the NO and tellurium signal peaks were temporally well resolved. To study whether or not there were other backgrounds involved in the determination of tellurium, an excitation spectral scan of a nickel based alloy sample solution was completed (Fig. 7). For comparison, an excitation spectrum of a tellurium standard is also shown in Fig. 7. If there was a continuum background, it should have appeared when the laser wavelength was tuned away from the analytical line. As shown in Fig. 7, except for a nearby atomic line, no continuum background signal was found for either the sample or the tellurium standard. This is consistent with the work of IRWIN et al. [23], who obtained excitation spectral scans of relative fluorescence vs wavelength for thallium and lead in a nickel based alloy sample by direct solid sample analysis. No background was found for either thallium or lead in the sample by ETA-LEAFS. In contrast, the ETA-AAS background signal, measured in nickel based alloy solid samples, exceeded in magnitude the analytical signal obtained from thallium or lead. There was a signal at 214.243 nm, which did not match exactly any atomic line in the M.I.T. wavelength tables [35]. An osmium line in the tables, at 214.238 nm, did not result in any fluorescence at either 214.243 nm or 214.238 nm, from 1000 l.r,gml-l of osmium (as NH,OsCl,). Also, a signal was not observed from the major or minor constituents: nickel, cobalt or iron in the nickel based alloy sample. Apparently, the line at 214.243 nm was from an element other than osmium, nickel, cobalt or iron, and could not be identified. 3.7.2 Spectral scan of the nickel alloy for antimony. Similar to tellurium, nitric oxide fluorescence could be observed for antimony, if no char step was used, although the fluorescence signal from nitric oxide was very small compared to the antimony signal in nickel alloy solutions. In such nickel alloy solutions, the concentrations of antimony were equivalent to about 5 ng ml-’ or higher. The nitric oxide fluorescence signal was from the A%+-XII (v’ = 1, v” = 4) transition, which peaked at 255.5 nm [8, 371. The monochromator bandpass was 4 nm at the antimony wavelength of 259.8 nm. Similar to the situation with tellurium, a char step at 800°C eliminated the nitric oxide interference. Excitation spectral scans were performed for both the aqueous antimony standard and the 1A alloy sample. When a slit width of 1 mm was used (8-nm bandpass), a shoulder at 212.733 nm was observed in the antimony peak in the 1A nickel alloy

Te and Sb determination

in

Ni alloys by furnace LEAFS

300-

19

(a)

250. 200. 150. IOO50.

2500

-

2000

-

J

1500.

Shoulder

loco&_

500. o+ 212.680

212.710

Excitation

212.740

212 770

r 212.800

wavelength hm)

Fig. 8. Excitation spectra of antimony using a detection wavelength of 2.59.8nm. (a) Antimony aqueous standard, 20 pg, AR = 259.8 nm. (b) P&W 1A nickel alloy (0.100 g per 100 ml), monochromator slit width: 0.1 mm (0.8-nm bandpass). (c) P&W 1A nickel alloy (0.1 g per 100 ml), monochromator slit width: 1 mm (8-nm bandpass).

sample (Fig. 8(c)). When a monochromator slit width of 0.1 mm was used (0.8nm bandpass), the shoulder was no longer present and the excitation profile of antimony in the 1A alloy (Fig. 8(b)) was identical with aqueous standard antimony (Fig. 8(a)). The peak at 212.791 nm was from nickel, which is a major constituent of the alloy. The wavelength of the line in the M.I.T. wavelength tables is 212.791 nm [35]. This was verified by observation of the fluorescence signal from aqueous nickel nitrate solution. Similarly, the peak at 212.714 nm was identified and confirmed as a cobalt line, Co 212.7147 nm [35]. Since both lines, Ni 212.791 nm and Co 212.714 nm, were far away from the antimony line at 212.739 nm, there was no effect on the determination of antimony in 1A nickel alloy. In order to investigate the shoulder at 212.733 nm, a spectrum of relative fluorescence signal vs fluorescence wavelength was obtained (Fig. 9) with excitation at the peak of the shoulder at 212.733 nm. Between 252 nm and 270 nm there were two peaks, 257.0 nm and 265.8 nm, which were the fluorescence from the unknown constituent that caused the shoulder. The peak at 259.5 nm was the fluorescence from antimony, which was excited at the antimony line wing. When fluorescence was detected at 257.0 nm and 259.8 nm for the unknown constituent and antimony, respectively (0.8~nm bandpass), the shoulder in Fig. 8 was resolved (Fig. 10). From Fig. 10, it can be seen that the unknown constituent was unlikely to cause significant spectral interference in the determination of antimony in the alloy. The line at 212.733 nm did not match any wavelengths listed in the M.I.T. wavelength tables [35]. It was apparently another unidentified atomic line. It was found that the fluorescence signals at this wavelength, from 2A and 3A nickel based alloy samples, were much weaker. We also confirmed that this line was not from the elements nickel, cobalt, iron or copper.

Z~or4owm LANG

20

et al.

20-

250

255

Fluorescence

260

265

wavelength

krnI

270

Fig. 9. Fluorescence spectrum of antimony in the P&W 1A nickel alloy (0.1 g per 100 ml). Monochromator slit width: 0.1 mm (0.8-nm bandpass). A,, = 212.733 nm.

212.720

212.760

212.600

Excltatlon wavelength (nm) Fig. 10. Excitation spectra of antimony in the P&W 1A nickel alloy (0.1 g per 100 ml) at different detection wavelengths. Monochromator slit width: 0.1 mm (0.8-nm bandpass). XR = 259.8 nm (0); AR = 257.0 mn (0).

3.7.3. Molecular jhorescence background from tap water. During the study of the detection limit for antimony, it was noted that even at an excitation wavelength away from the antimony line by more than 0.1 nm, a significant signal was measured when deionized water was used as a blank. It was found that it was due to a compound in the deionized water. To study this compound, tap water was used as the sample, since it gave a greater signal. An excitation spectrum was obtained (Fig. 11(b)), with detection at 259.8 nm, which was the antimony fluorescence wavelength, and a 4-nm bandpass. For comparison, the excitation spectrum of aqueous standard antimony is also shown in Fig. 11(a). Figure 11 shows that the background signal from the tap water was comparable to the signal from 20 pg of antimony in the electrothermal atomizer. A fluorescence spectrum of the compound was also obtained (Fig. 12) at an excitation wavelength of 212.771 nm. Below 370 nm, no significant blackbody emission from the atomizer was observed. Between 370 and 420 nm, the blackbody emission background was subtracted. Above 420 nm, the signal-to-noise ratio was too small to obtain the fluorescence spectrum due to the blackbody emission signal. The compound was more stable or less volatile than antimony. This can be seen from the temporal characteristics of the vaporization, and the temporal decays of signals from the aqueous antimony standard and the tap water (Fig. 13). The appearance time of antimony (Fig. 13(a)) was well ahead of the compound (Fig. 13(c)), when the compound was excited at 212.704 nm, where no antimony signal could be observed. If the signal was measured at the excitation wavelength of 212.739 nm, i.e. on the antimony analytical line, the appearance time was between those of antimony and the compound, which was probably due to a significant antimony blank in the tap water.

Te and Sb determination

in Ni alloys by furnace LEAFS

21

3oo-(a) 250. zoo150. 100. 50. 0

4 . . . . . . . . . . . . . . . . . . . --------.

2p2.680

212 760

212.720

Excitation

wavelength

212.800

(nm)

Fig. 11. Excitation spectra for antimony. Monochromator slit width: 0.5 mm (4-nm bandpass). (a) Antimony (20 pg) in sub-boiled distilled water; (b) tap water, AR = 259.8 nm.

p) 750 2 ! 600 f! 2 = 450 f 3 I! a”

300 150 0 245

265

285

305

Fluorescence

Fig. 12. Fluorescence

325

345

365

wavelength

spectrum of the spectral interferent 212.766 nm).

385

405

425

(nm)

compound

in tap water (A,, =

The possible effect on the determination of tellurium and phosphorus [8] in water samples by ETA-LEAFS is shown in Fig. 14, from which it can be seen that due to the structured nature of the excitation spectrum from the compound, it is problematic to determine trace levels of antimony, tellurium and phosphorus in such water samples. The wavelength modulation background correction method [36] could correct for blackbody emission from the electrothermal atomizer, and scatter of laser radiation off aluminum chloride matrix particles. The method is not likely to correct for molecular fluorescence backgrounds of the type shown in Fig. 14, since the off-line measurement does not represent the background signal on the analytical line. The Zeeman background correction method can correct the background at the analytical line, which may allow accurate background correction if the molecule does not split in the magnetic field. It was found that similar molecular spectra were obtained from a sample of sodium silicate (Na,SiO,). The molecule was probably silicon monoxide (SiO) [37]. More work is needed for a rigorous identification, which was not attempted here.

ZHONGWEN LIANG et al,

22

I.........I.........I.........I.....,,,.I Cl

3

2

I

4

Time (S)

Fig. 13. Temporal behavior of the antimony atomization signal. (a) Aqueous Sb standard (20 pg) excited at the antimony line (212.739 nm); (b) tap water excited at 212.739 nm; and (c) tap water excited to one side of the Sb line, at the excitation wavelength of the spectral interferent compound (212.704 nm). AR = 259.8 nm.

16

- 214260 !!! 3 70s

213’590

214 275

214 290

214 305

213’605

213’620

213635

Excitation

wovelength

(nml

Fig. 14. Excitation spectra of tap water near the excitation wavelengths of tellurium and phosphorus. Vaporization temperature: 1900°C; detected at 238.4 nm (0), A,, = 214.281 nm for tellurium; detected at 253.5 nm (0). Acx = 213.618 nm for phosphorus.

4. CONCLUSION Previously we have shown that lead and thallium can be successfully determined in nickel based alloys by ETA-LEAFS and direct solid sample analysis [23], and now it is clear that the solid sample method is also applicable to the determination of tellurium in nickel based alloys. Direct solid analysis was not suitable for antimony due to incomplete vaporization, similar to the case of phosphorus [8]. To address the spectral background issues explored above, work on whether or not various background correction methods [24, 361 will ensure accuracy for the determination of tellurium, antimony and phosphorus in tap water is continuing in this laboratory.

Te and Sb determination

in Ni alloys by furnace LEAFS

23

Acknowledgments-This work was supported by National Institutes of Health Grant GM 32002. R.G.M. was supported by a Research Career Development Award from the National Institute of Environmental Health Sciences under Grant No. ESO0130.

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