Vitreous Carbon Furnace with Continuous Sample Introduction for Atomic Fluorescence Spectrometry C. J. Molnar and J. D. Winefordner' Department of Chemistry, University of Florida, Gainesvi//e,Fla. 326 1 1
Atomic vapor of several metals (Sn, Pb, Te, Ag, TI, and Bi) was produced by pneumatic nebulization of an aqueous sample through a vitreous carbon tube furnace, and atomic fluorescence was excited with single element electrodeless discharge lamps. The high efficiency of the system is discussed along with precision, sensitivity, linear dynamic range, and limits of detection for the elements examined. The decay of atomic populations above the furnace outlet using either an Ar-H2 diffusion flame sheath or an Ar sheath is determined, and the results are discussed.
Recently, much interest in nonflame atomizers for atomic fluorescence spectrometry has resulted because they have low backgrounds, are efficient atomizers for some elements, and contain low concentrations of efficient quenchers of excited atoms as compared to the conventional flame atomizers. Much of the literature on nonflame cell atomizers has been reviewed by Winefordner ( I ) , Kirkbright ( 2 ) , and Winefordner and Vickers (3, 4 ) . Most of the literature has been concerned with discrete sampling nonflame cells with their inherent problems of sample introduction, measurement of transient signals and, in some cases, limited linear dynamic range. A few researchers have investigated the use of continuously-sampling, resistively-heated nonflame cells (5-9); however, only two of these were suitable for atomic fluorescence spectrometry (8, 9). Both of these systems employed the Veillon and Margoshes type sample injection (10). It is felt that these past systems were not optimized for sample injection in AFS because of the relatively high gas flow rates limiting the residency time of the atomic vapor in the atomization cell and because of their modest aspiration efficiencies. The present study was carried out with a very efficient, high pressure, low sample consumption, pneumatic nebulizer for introduction of a fine sample aerosol into a desolvation chamber and then into a heated vitreous carbon tube atomizer. This system is shown to result in good precision, low concentrational and absolute detection limits, and long linear dynamic concentration ranges.
EXPERIMENTAL General Experimental System. A schematic diagram of the experimental systems is shown in Figure 1. Atomic fluorescence 1 Author
to whom reprint requests should he sent.
J. D. Winefordner, in "Atomic Absorption Spectroscopy," Plenary Lec-
tures presented at the International Atomic Absorption Spectroscopy Conference, Sheffield, 1969, R. M. Dagnall and G. F. Kirkbright, Ed., Butterworth,London, 1970, p 37. G. F. Kirkbright, Ana/yst(London),96, 609 (1971). J. D. Winefordner and T. J. Vickers, Anal. Chem., 44, 150R (1972). J. D. Winefordner and T. J. Vickers. Anal. Chem., 46, 192R (1974). R. Woodriff and R. W. Stone, Appl. Spectrosc., 22, 408 (1968). R. Woodriff and R. W. Stone, Appl. Opt. 7 , 1337 (1968). R . Woodriff and G. Ramelov, Spectrochim. Acta. 238, 665 (1968). M. S. Black, T. H. Glenn, M. P. Bratzel, and J. D. Winefordner. Anal. Chem., 43, 1769 (1971). M. K. Murphy, S. A . Clyburn, and C. Veillon, Anal. Chem., 45, 1468 (1973). C. Veillon and M. Margoshes, Spectrochim. Acta, 238, 553 (1968).
measurements were carried out with a 0.35-m f/6.8 Czerny-Turner monochromator (Model EU-700/E, Heath Co., Benton Harbor, Mich.). Additional external light baffles were attached to the entrance slit assembly of the monochromator. Optical grade biconvex quartz lenses (2-inch diameter and 2.5-inch focal length; Esco Products, Oak Ridge Road, Oak Ridge, N.J.) were used throughout for focusing. The associated electronics included an RCA 1P28 photomultiplier powered by a high voltage power supply (Model EU-42A, Heath Co.), a lock-in amplifier tuned to 667 Hz (Model 391, Ithaco, Inc., Ithaca N.Y.), and a preamplifier with variable gain (Model 164, Ithaco, Inc.). Source modification was performed with a mechanical chopper (Model 382, Ithaco, Inc.), and the lockin output was recorded on a potentiometric recorder (Model S.R., Sargent Welch Scientific Co., Skokie, Ill.). Sources of Excitation. Single element electrodeless discharge lamps (EDLs) prepared from the iodide form of each of the respective elements were used as light sources. The EDLs were operated in the thermostated mode as described by Browner et al. (11, 12). The temperature of the heated gas surrounding each lamp is given in Table I. Nebulizer-Atomizer. The pneumatic nebulizer was of the standard conical design with the critical dimensions greatly reduced (Figure 2). A standard 26-gauge stainless steel hypodermic needle (Hamilton Co., Reno, Nev.) was used for sample transport. The nebulization cone was machined from stainless steel with a convergence angle of the aspirating gas nozzle to central solution capillary of 30". The clearance between the solution capillary and aspirating gas nozzle was 0.001 inch. Stainless steel micrometers (No. 1463, The L. S. Starret Co., Athol, Mass.) were situated at 120O around the hypodermic needle to allow centering of the capillary needle in the aspirating gas nozzle. Sample solution was force-fed through the sample transport needle with the aid of a syringe pump (Model 353, Sage Instruments, Inc., White Plains, N.Y.). Bolted to the top of the nebulizer was a stainless steel desolvation chamber which funneled the sample aerosol mist directly into the vitreous carbon tube atomization cell (Figure 3). The desolvation chamber had an inside diameter of 1?8 inches and was 3% inches long. The top of the desolvation chamber was reduced to a radius of inch. The chamber was heated with heat tape (Samox Hi Temperature Tape, Arthur H. Thomas Co., Philadelphia, Pa.). A cutaway view of the furnace chamber is shown in Figure 4. The rings (D and E in Figure 4) of boron nitride and asbestos insulated the furnace housing from the vitreous carbon tube. Copper contacts (A in Figure 4) were situated in both electrodes (B in Figure 4) to complete electrical contact between the carbon tube furnace (C in Figure 4) and the electrodes. A d.c. power supply (SCR 20-250, Electronic Measurements Inc., Neptune, N.J.) supplied the current (f0.1%regulation) for resistive heating of the carbon tube furnace ( H in Figure 4). The vitreous carbon tubes (Beckwith Carbon, Van Nuys, Calif.) were custom-made to %in. i.d., 0.300-in. o.d., and l%-in. long. T o obtain good electrical connection between the electrodes (B in Figure 4) and the vitreous carbon tube ( H in Figure 4), graphite rings (G in Figure 4) were used. The temperatures measured within and above the vitreous carbon tube furnace as a function of electrical current through the tube are given in Figure 5. Solutions. Stock solutions were prepared from analytical re0, agent grade (13) chemicals of Pb(NO&, N a ~ T e 0 ~ 2 H 2AgNOR, T12S04, SnO, and Bi a t metal concentrations of 1000 ppm. The (11) B. M. Patel, F. R. Browner, and J. D. Winefordner, Anal. Chem., 44, 2272 (1972). (12) R. F. Browner, B. M. Patel, T. J. Glenn, M. E. Rietta. and J. D. Winefordner. Spectrosc. Lett.,5 , 311 (1972). (13) J. A . Dean and T. C. Rains, "Flame Emission and Atomic Absorption Spectroscopy," Vol. 11, Marcel Dekker. Inc., New York, N.Y., 1971, pp 333-336.
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 11, SEPTEMBER 1974
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Table I. Experimental Conditions and Analytical Results for Measurement of Several Elements by Means of the Continuous-Sample Introduction Tube Furnace Atomic Fluorescence Spectrometric System Tube furnace conditions E D L conditions TemperaWavelength, ture, Power, Element nm "C" Wh
Te Ag Pb Sn T1
Bi
214.2 328.0 283.3 303.4 377.6 307.7
430 530 430 275 435 500
~
Temperature, OCC
30 60 30 90 45 50
Ar flow Ar flow Ar flow rate, rate,d rate,d nebulizer, external, external, I./min 1.imin l./min
1515 1675 1675 1800 1675 1540
0.78 0.78 0.78 0.78 0.9 0.9
4.7 4.3 4.3 5.8 4.7 4.7
1.8 1.6 1.6 1.8 1.6 1.8
LOD,e Wg/ml
1 2 1 1 6 2
X X
x
X X X
lo-*
70 RSD'
LDRe
1x 1.5 x 8 X 1x 3 x 1.5 x
104 104
4.8 3.5 3.2 4.5 3.6 2.5
lo3 103 103 104
fknsitivityu
0.87 0.98 1.o 1.3 1.o 0.96
*
Temperature of air flowing by EDL. Microwave power applied to EDL using thermostated "A" antenna. Temperature of graphite tube furnace walls. Flow rate of Ar and 'or H?around tube furnace exit. e LOD = limit of detection (see text for definition). LDR = linear dynamic range (see test for definition). 7c RSD = 70 relative standard deviation (taken at a concentration of about 100 X LOD). 0 Sensitivity = slope of Log-Log plots. I"
SAM~LE
Figure 1. Schematic diagram of continuous-sample introduction tube furnace atomic fluorescence spectrometric system (A) Preamplifier, (B) Microwave power supply for EDL, (C)Chopper, (E) Electrodeless discharge lamp (EDL). (F) Vitreous carbon tube furnace, (K) Furnace power supply, (L) Lock-in amplifier, (L1, Lp, L3) Lenses, (M) Monochromator, (0)High voltage power supply, ( P ) Photomultiplier tube, (R) Recorder readout, and (Sj.Sp) Apertures
H s t o i n l e s s Steel
7 Figure 3. Schematic diagram of Stainless Steel desolvation chamber CARBON
FURNACE
NEBULIZER
E
s t o " l e a s Steel
a
W
SAMPLE
R u b b e r / T e f i o n Septum
_I
-
I
a v)
A ER0S0L
first four compounds were dissolved in deionized water, SnO was , Bi metal was dissolved in concentrated nitric dissolved in H c ~ and acid and then diluted to looo ppm, Serial dilutions were then performed with deionized water. procedure for Evaluation of Experimental Characteristics of System. A nominal sample flow rate for all measurements was o,12 ml which resulted in a very fine aerosol mist which was estimated to have a mean particle diameter below 15 p m using the maximum velocity at which Nukiyano and Tanasawa verified Nukiyama and Y. Tanasawa, Trans. SOC. Mech. Engr. (Japan), 5, 63 (1939).
(14) S.
1420
Asbestos
Vitreous C a r b o n
Boron Nitride
Brass
Figure 2. Schematic diagram of nebulizer (A) Nebulizer cone, (6)Rubber O-ring, (C) 0-Ring seating gland, (D) Alignment rod, ( E ) Micrometer adjustment, (F) Housing ring to hold micrometer, (G) Stainless steel needle housing, (H) Teflon centering sleeve, (I) Hypodermic needle, ( J ) Vertical adjustment plate, (K) Nebulizer housing, (L) Needle housing, (M) Luer lock-luer lock connector, (N) Teflon system seal, and (0) Retaining screw for N
Copper
Figure 4. Schematic diagram of vitreous carbon tube furnace (A) Copper electrical contacts. (B) Water-cooled electVXJes3(c)Vitreous carbon tube, (D) Boron nitride insulation ring, (E) Asbestos insulation ring, (F) Furnace housing, and (G) Graphite rings for electrical contact
their equation. The measured velocity was about 10% greater than the speed of sound which would further increase turbulence and decrease particle size. The aerosol mist was formed and desolvated in the stainless steel desolvation chamber. The temperature of the gas in the desolvation chamber ranged from 246 "C to 278 "C while the heat tape which was wrapped around the chamber measured 393 These temperatures were measured with 0.015-in. diameter chromel-alumel thermocouples, and the resulting voltages were monitored on a digital voltmeter. A desolvation time of 23 msec was calculated for a particle size of 15 pm in diameter with the following
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1 1 , SEPTEMBER 1974
Table 11. Comparison of Residency Times i n Atomic Cells
--Y
Rise velocity'' (msec-1)
C-Furnace HZ-0, Hd--Air C2Hz-02 C?H-Air
CrH,-N?O
0.8 20. 0.4 20.
E
2. 1.6
5+
+-
2
000 400
50:l 1:l 50:l 5:l 4:l
I
50
1
I
100 CURRENT (A)
I
1
150
I
-_
200
Figure 5. Measured temperatures as a function of electrical current through the furnace
above the burner taken to be 2.5 cm and ho for the furnace was taken to he the tube length of 5 cm.
equation
where t , is the total time for desolvation, in sec, A0 is the initial area of the droplet, C, is the average specific heat of the vapor a t constant pressure, L is the specific heat of vaporization of the solvent, T is the temperature of the chamber, T b is the boiling point of the solvent, X is the thermal conductivity of the gas, and p is the density of the liquid (15). This equation neglects the additional transport of heat to the boiling droplet by convection which certainly would be significant in this case and would reduce the time, t,, even more. Also, from the nebulization gas flow rate, a residency time of sample mist in the desolvation chamber, neglecting the additional gaseous volume of the sample solution, was calculated t o he 5.3 sec. Thus, desolvation was likely complete in the desolvation chamber. In addition, no scattered radiation (in the absorption mode) was discernible from water or from a 1000-ppm Zr solution a t the Zn 213.8-nm line using a zinc hollow cathode and making absorption measurements just above the desolvation chamber. The scatter signal was found to be 2% of full-scale absorption for scatter measurements performed with 10,000 ppm Zr (16). The negligible amount of scatter from large concentrations of solutes, which are difficult to vaporize, qualitatively supports the above ohservations concerning the small mean particle diameter. A comparison of the rise times in the vitreous carbon tube nonflame to that in various analytical premixed flames (assuming a primary combustion zone angle of 6') are given in Table I1 (14). Ratios of the residency time in the vitreous carbon tube to the residency time in a flame prior to measurement (assuming a distance of 2.5 cm) are also given in Table 11. T h e efficiency of the nebulizer and desolvation chamber was experimentally determined by aspirating 2 ml of 12.5 ppm Mg. The chamber was then washed with deionized water of the same p H as the original solution, and the magnesium concentration of this solution was measured via atomic absorption spectrometry with the graphite filament nonflame cell (17, 28). T h e efficiency was found to be 93%. P r o c e d u r e for Atomic Fluorescence Measurements. The desolvation chamber was allowed to reach the steady state temperature previously mentioned. Current was applied to the carbon tube furnace until the desired temperature was reached. Sample (or blank) was then aspirated into the desolvation chamber a t a flow rate of 0.12 ml min-', and the resulting aerosol mist was carried to the vitreous carbon tube where atomization was effected. T h e exci(15) J. A. Dean and T. C. Rains, "Flame Emission and Atomic Absorption Spectroscopy," Vol. 1, Marcel Dekker, inc., New York, N.Y., 1971, p 106. (16) N. Omenetto, L. P. Hart, and J. D. Winefordner, Appl. Spectrosc., 26,
-.
1200
W a
a Rise velocities for the flames were calculated from uburn = urlSe sin 8, where uburn is the burning velocity (taken from Reference 1 4 ) . Residency time was calculated from til = ha/urIse, where ha for the flames is the observation height
612 (1972) - -,
1600
3
Residency time ratiosb
C-Furnace: H2-02 C-Furnace :H,-Air C-Furnace :C,Hz-02 C-Furnace :C2H2-Air C-Furnace : C2H2-N20
/"
2000
(17) M. D. Amos, P. A. Bennett, K. G. Brodie, P. W. Y. Lung, and J. P. Matousek, Anal. Chem., 43, 21 1 (1971). (18) B. M. Patel, R. D. Reeves, R. F. Browner, C. J. Molnar, and J. D. Winefordner, Appl. Spectrosc.. 27, 17 1 (1973).
(a) Temperatures in center of carbon tube, (b) Temperatures at grazing incidence to carbon tube with Ar sheath, (c)Temperatures at grazing incidence to carbon tube with Ar-H2 sheath, (d) Temperatures at 1 cm above carbon tube with Ar-H2 diffusion flame sheath, (e) Temperatures at 2 cm above carbon tube with Ar-H2 diffusion flame sheath, ( 0 Temperatures at 1 cm above carbon tube with Ar sheath, and (g) Temperatures at 2 cm above carbon tube with Ar sheath
tation radiation passed at grazing incidence t o the top of the vitreous carbon tube. T h e fluorescence was collected by the monochromator adjusted to the desired wavelength (the slit width was adjusted to 750 Gm for all studies). The argon and hydrogen flow rates were monitored by rotameters calibrated with a wet test meter (Precision Scientific Co., Chicago, Ill). Approximate temperatures of the gas flow in the tube furnace were obtained with a 0.010-in. diameter tungsten-tungsten 26% rhenium thermocouple (Omega Engineering, Inc., Stamford, Conn.); all tube temperatures were taken with the thermocouple a t the center of the vitreous carbon tube.
RESULTS AND DISCUSSION Initial Studies. The combined nebulizer, desolvation chamber, and resistively-heated nonflame cell was a very efficient atomizer. This would be expected from the preliminary estimations, and measurements showing the aspiration efficiency was 93%, and the mean droplet size was of the order of 15-pm diameter. Limits of Detection and Precision. The atomic fluorescence concentrational (wg ml-1) limit of detection (LOD) was defined as that concentration (in Fg ml-l) giving a signal 3 X rms noise. Each LOD was found by alternately running a signal and blank 3-5 times and then extrapolating back from a signal of about 2 X peak-to-peak noise. The resulting LOD for the six elements are given in Table I along with the temperature of atomization, various gas flow rates, and the linear dynamic ranges (LDR). Atomic fluorescence measurements of the various elements were determined for concentrations ranging from 1000 pprn to the concentration resulting in a signal of 2X peak-topeak noise with concentrations varying by factors of 3. The upper concentration limit (called UCL) was determined by the beginning of serious curvature (greater than 4% deviation from linearity) of the analytical curve (LDR is the ratio of UCL/LOD). The atomic fluorescence LOD's obtained by the present nonflame cell were compared to the best limits of detection obtained with a flame and a resistively heated discrete sampling nonflame cell in Table 111. Both concentrational (in pg ml-l) limits of detection (CLOD) and absolute (in ng) limits of detection (ALOD) are compared in Table 111. The flame atomic fluorescence ALOD was approximated by assuming measurements were performed for 1 min with a sample flow rate of 5 ml min-1. Only 2 elements (Te and Ag) had higher atomic fluorescence CLOD'S with the pres-
ANALYTiCAL CHEMISTRY, VOL. 46, NO. 1 1 , S E P T E M B E R 1974 * 1421
~
~~
Table 111. Comparison of Nonflame Atomization Atomic Fluorescence Spectrometric L i m i t s of Detection This woik
Discrete atomization (17, 18)
- - - ~ __-______
Element
CLOD,
Sn
1
Pb Te
5 1
Afi
2
T1 B1
6 2
pg
ALOD, ng
ml
x 10-2 x 10-3 x 10-2 x 10-4 x 10-I x 10-
CLOD,
1 0 5 1 0 02 0 7
2 5
gg
‘ml
x x
ALOD, ng
Ref.
CLOD, rg ml
ALOD ng
1 x 10-1
1 x 10-1 1 x 10-2 6 x lo-’ 1 x 10-4
500 50 30 0 5 40 25
10-1 10-3 ...
3
x lo-.’
(18) (17)
x lo-“ 4 x 10-2
4 2
x x
(18) (18)
8
0 2
...
c
I
‘ 5
%.A, IO
15
20
25
HI/mm
Decay of atom populations with height above the vitreous carbon tube atomizer for two elements as measured by atomic fluorescence spectrometry
Figure 6.
0 . Sn, 3034 A; 0 , Pb, 2833
A; X, Ar atmosphere: 0 . Ar/H2 atmosphere
ent system when compared to atomic fluorescence values with flame atomizers and in these cases by only a factor of 2 or less, which is within the error range of LOD’s. The atomic fluorescence CLOD’S for T1, Pb, and Bi were slightly lower by the present continuous introduction nonflame cell compared to flames (factors of 25% to 250%). The atomic fluorescence CLOD detection limit for Sn by the continuous introduction system was lower by 10-times than any other value obtained by atomic fluorescence spectrometry. The present system absolute limits of detection were lower than by atomic fluorescence flame spectrometry in all cases by a t least a factor of 25. For Pb, Sn, and Bi, the absolute atomic fluorescence limits of detection by the present continuous introduction nonflame cell are lower by over two orders of magnitude. The concentrational limit of detection (LOD) of the present continuous introduction system compared to atomic fluorescence LOD’s with discrete sampling nonflame cell are similar for most elements studied. For all elements studied. the absolute limits of detection determined with a given (see Table 111) discrete sampling nonflame cell is better (lower) than for the present continuous introduction system. Precision of System. The precision obtained is very competitive with all other atomization devices in use for atomic fluorescence. For all elements examined, the relative standard deviation is between 2.5% to 4.8% which is comparable to flame AFS and generally better than by graphite rod AFS. The present continuous introduction (19) R. F. Browner, R. M. Dagnall, and T. S.West, Anal. Chim. Acta. 46, 207 (1969). (20) R. F. Browner, R. M. Dagnall, and T. S. West, Anal. Chim. Acta, 50, 375 (1970).
1422
Flame (19-22)
... 10-8 10-2 . .
...
...
8
5
x
10-3
x 10-3
Ref.
(19) (20) (21) (22) (22) (21I
nonflame cell is not subject to the sample placement errors found in the graphite rod atomizers. Atomization Profiles. The method used to measure the decay of atomization profiles has been previously described (18, 23, 24). It has been shown before that an inert or even reducing atmosphere surrounding the atomic vapor will often prolong the lifetime of the analyte atoms formed from a discrete sampling graphite filament nonflame cell. Atomic profiles as a function of height for several elements were therefore examined for the present atomization cell to determine the effect of only Ar us. an Ar-H2 diffusion flame on the decay of atom populations exiting continuously from the present continuous sampling vitreous carbon tube nonflame cell. The same atomization conditions were used for the LOD measurements. In the case of P b with the Ar-H2 diffusion flame, the decay of the atom populations was gradual, reaching 50% a t a height of 20 mm above the furnace exit. With only an argon sheath, however, the decay of P b proceeded somewhat faster, reaching 30% of the original population a t a height of 20 mm. For Sn, the Ar-Hz diffusion flame had very great influence on the atom population as a function of height above the furnace tube exit; a t 12 mm above the carbon tube, the signal actually increased 300% and then it began to decrease to slightly less than 150%a t 24 mm. With only argon, the Sn population dropped to 28% of the original fluorescence signal at 7 mm (Figure 6). For all elements studied here, the atomic population decreased a t a reduced rate for the present continuous sample injection nonflame cell compared to the discrete graphite carbon filament atomizer (24).For instance, a t a height of 5 mm above the atomizer, the fluorescence signal of Sn with Ar entrainment is about 55% of the initial signal for the present atomizer as compared to less than 2% of the initial signal for the graphite filament atomizer. When a HZ-diffusion flame was used with the present system, the fluorescence signal increased about 290% while the carbon filament atomizers fluorescence decreased to about 20% a t 15mm height. In the case of P b at 10-mm height above the nonflame cell, the atomic fluorescence signal was about 60% of the initial fluorescence signal with either the Ar or Ar-HZ entrainment sheath, while in the case of the graphite filament atomizer, the fluorescence signal was less than 30% with a H2 diffusion flame and less than 1%with only an Ar sheath. Therefore, limited-field viewing for this nonflame cell is unnecessary.
RECEIVEDfor review February 25, 1974. Accepted May 7, 1974. This work supported by USAF-AFOSR-74-2573. (21) A. Hell and S. Ricchio, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1970, Paper 23. (22) K. H. Zacha, M. P. Bratzel. J. M. Mansfield, and J. D. Winefordner, Anal. Chem., 40, 1733 (1968). (23) R. G. Anderson, J. N. Johnson, and T. S. West, Anal. Chim. Acta, 57, 281 (1971). (24) R. D. Reeves, B. M. Patel, C. J. Molnar. and J. D. Winefordner, Anal. Chem., 45, 246 (1973).
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, N O . 1 1 , SEPTEMBER 1 9 7 4
