6
Anal. Chem. 1981, 53, 6-9
Comparison of Nebulizer-Burner Systems for Laser-Excited Atomic Fluorescence Flame Spectrometry J. J. Horvath, J. D. Bradshaw,’ J. N. Bower,* M. S. Epstein,8 and J. D. Winefordner” Department of Chemistry, University of Florida, Gainesville, Florida 326 1 1
An ultrasonlc nebulizer and speclally designed mlnlflame shielded burner system Is used In a pulsed N,-pumped dye laser excited atomic fluorescence spectrometer. The ultrasonic nebullzer-miniflame shlelded burner AFS system Is compared to other nebullzer-burner systems In terms of detection llmlts. Detectlon llmlts obtalned wlth this system are Improved over prevlous AFS systems (laser or conventlonal excltatlon sources) and over most other atomic spectrometrlc methods. The ultrasonic nebullrer-mlniflame shielded burner laser-exclted AFS system was used to determlne several elements In NBS standard reference materlals.
Table I. Experimental Components of Laser-Excited Atomic Fluorescence Flame Spectrometry System component model no. company nitrogen laser/ power supply (MOL) dye laser dye laser control unit flash lamp pumped dye laser
Winefordner ( I ) has recently reviewed the instrumentation, methodologies, analytical figures of merit, and uses of laserexcited luminescence spectrometry. A comparison of detection limits between atomic spectrometric methods is also given (1). At the time the above chapter (1)was written, the work by Weeks et al. (2) was the “state-of-the-art” for dye laser excited AFS. However, Weeks et al. (2) mention several improvements which could lead to several orders of magnitude improvement in detection limits, namely: (i) optimization of the optical system; (ii) optimization of the laser beam size and flame size; (iii) use of a better quality laser beam shape over the entire spectral range; (iv) use of higher output peak powers, especially in the UV; (v) use of ultrasonic nebulization; (vi) reduction of flame instability with an improved flow system and mixing chamber design; (vii) optimization of the electronic measurement system; (viii) reduction of radio frequency interference noise due to the pulsed N2-laser. The present ultrasonic nebulizer-miniflame burner laser-excited AFS system involves improvements listed in (i), (ii), (v), (vii), and (viii).
EXPERIMENTAL SECTION Apparatus. A simple block diagram of the experimental setup can be seen in Figure 1. Table I lists the components used in the laser system. With the exception of the laser and nebulizer-burner, the experimental setup is generally similar to that described by Weeks et al. (2). In these experiments, the limits of detection obtained with the ultrasonic nebulizer and miniflame were compared to those obtained by using a standard pneumatic nebulizer with a capillary burner. Laser dyes used to excite the different transitions are shown in Table 11. Solutions used in these experiments were made fresh daily from 1000 pg mL-l; at the low concentration levels used, contamination and leaching of impurities from glassware became a major problem. The flame used was C2H2/02/Ar1/1/4 flow rate ratio; the flow rates were 0.5,0.5, and 2.0 L/min for C2H2,02,and Ar, respectively, in the inner flame and 2,2, and 8 L/min for the same gases in the outer flame. The fluorescence was imaged on the monochromator entrance slit through two Spectrosil lens (focal length 2 in) and Present address: Atmospheric Research, Georgia Institute of Technology, Atlantic, GA 30332. 2Present address: E. R. Squibb & Sons, Georges Rd., New Brunswick, NJ 08903. Present address: Inorganic Analytical Research Division, National Bureau of Standards, Washington, DC 20234. 0003-2700/81/0353-0006$01.OO/O
trigger source
vacuum pump monochromator
photomultiplier
power supply boxcar averager recorder
Molectron Corp. Sunnyvale, CA 94086 Molectron Corp. DL-300 Sunnyvale, CA 94086 DL-040A Molectron Corp. Sunnyvale, CA 94086 Chromatix CMX-4 Sunnyvale, CA 94086 Rutherford B7 F Electronics Co. El Segundo, CA 90 24 6 Alcatel Vacuum Type 1030 Products Hanover, MA 02339 Instruments SA Inc., HlOUV J-Y Optical Systems Div. Metuchen, N J 08840 IP28 RCA-Electro Optics & Devices Lancaster, PA 17604 412B John Fluke Manufacturing Co. Seattle, WA 98133 Princeton Applied 162/164 Research Princeton, N J 08540 Servo/Riter I1 Texas Instruments UV-14
co.
nebulizer and premix 0303-0299 burner assembly 0040-0146 capillary burner head JB-1 ultrasonic nebulizer (bath type) miniflame burner
JB-3
Houston, TX 77006 Perkin-Elmer Instrument Div. Norwalk, CT 06856 laboratory constructed laboratory constructed laboratory constructed
an aperture which consisted of a 2-mm slit (width). The monochromator was a 0.1-m grating spectrometer (80 A/mm) and was rotated 90° from normal position so that the entrance slit was parallel to the laser beam and allowed collection of fluorescence from only the region in the flame that was undergoing excitation. The photomultiplier base circuit was modified for pulsed, high current operation similar to the one described by Fraser and Winefordner (3)and also operated at a high applied voltage (1200 V) for best transient response. The boxcar averager was operated with 50-52 input, an aperture time of approximately 15 ns, and an observed time constant of 1 s (repetition rate of 20 pps). Two nebulizer-burner systems were used in the present studies. A standard capillary burner head (3), 10 mm in diameter (-50 capillaries of 1 mm 0.d.) was used with a commercial atomic 0 1980 Amerlcan Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
7
Table 11. Laser Dyes Used dye
concn, mol/L
solvent ethanol p-dioxane ethanol p-dioxane p-dioxane
10-l 10-l 10-l 1.2 x 10-3 saturated < 1 . 2 x
7D4MC 7D4TMC c495
Bis-MSB
DPS
wavelength peak, nm
wavelength,range ,(1 nm (10%points)
457 483 536 421 406
440-478 460-517 515-583 4 1 1-430 396-416
From “Molectron Dye List” by Molectron Corp., Sunnyvale, CA 94086. TOP V I E W
IGEN ER ATC
dl STEERING MIRRORS
BOXCAR
Flgure 1.
RECORDER
Block diagram of laser-excited atomic fluorescence spec-
trometer.
fl SPOILER
I 1/11!
CARRIER GAS INLET
-
Flgure 3.
rMEMBRANE
*SAMPLE
INPUT
SAMPLE OUTPUT WATER OUT
PI E ZOE LEC TR IC m TRANSDUCER
Flgure 2.
R
.
F
. INPUT
Ultrasonic nebulizer burner (bath type).
absorption nebulizer (3) (Perkin-Elmer 303 AA nebulizer operated at 5.0 mL/min). The ultrasonic nebulizer (see Figure 2) was specially constructed to give maximal signal-to-noise ratios in laser-excited AFS. The miniflame burner (see Figure 3) supported an inner and an outer flame. The inner flame was 3 mm in diameter, and the outer concentric flame had an outer diameter of 10 mm. The sample aerosol was only introduced into the inner flame, resulting in higher signal levels for a given concentration
1
!!I 1
I
Miniflame burner.
of analyte than was obtainable with any other nebulizer-burner system previously used by us (4). The monochromator slit width was then matched to the inner flame region. The miniflame burner consisted of five parts. The burner head was made from titanium and contained 37 holes 0.040 in. in diameter, arranged in three concentric rings around a central hole. The sample was introduced into the central hole (inner flame region) and into the first concentric ring of six holes. Under the burner head sat a tantalum flame separator with a knife edge to separate the inner and outer flames. The flame separator rested on a Teflon ring insert through which the sample aerosol was introduced. The Teflon ring insert was held within the main burner body (made of stainless steel) which also contained inlets for the outer flame gases. The burner head was threaded to the main body. The lower portion of the burner consisted of a springheld stainless steel collar to ensure a proper gas seal. Because all parta of the burner were made of “inert” materials, acidic solutions could be used. The ultrasonic nebulizer design was similar to the basic design of Owens (5,6).The ultrasonic transducer was of the focusing type (8-87, Part 41732501, De Vilbiss Co., Somerset, PA). The crystal sat in the bottom of a Plexiglaas container which was f i e d with water when the crystal was activated. The water was used for cooling and to enhance coupling to the sample. The ultrasonic waves produced by the crystal were focused onto a Mylar membrane (-0.75 I.tm thick) sandwiched between a stainless steel collar and a plastic support ring. Input and output porta were placed
8
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
Table 111. Limits of Detection by Laser-Excited Flame Atomic Fluorescence Spectrometry
element
detection limits, ng/mL ultrasonic pneumatic nebnebminiprevious work capillary flame burner burner (2)
heJha
(nmlmm)
Ca4 422.7 I42 2.7 Mg 285.21285.2 Sr 460.71460.7 Pb 283.3/405.8 Saturated conditions.
0.0 1 0.01 0.1 1
0.01 0.002 0.1 0.2
0.08
0.2 0.3 13
(I
Table IV. Comparison of Detection Limits Obtained with h e r - E x c i t e d Atomic Fluorescence Spectrometry and Ultrasonic Nebulization to Other Methods detection limit, ng/mL AFS (present AAS AES element study) (13) (14) AEICP ( 1 5 ) Ca Sr Mg Pb
0.01 0.1 0.002 0.2
2 1 0.1 10
0.1 0.2 5.0 100
0.0001 0.003 0.003 2
Table V. Absolute Detection Limits by Laser-Excited Atomic Fluorescence Spectrometry with Ultrasonic Nebulization vs. Other Methods detection limit, pg AFS AAS element (present study)” AEICP (15)49c 0.2 0.03 Ca 0.05 Sr 0.75 1.0 0.8 Mg 0.01 0.05 0.8 0.9 3.0 500.0 Pb Measurement time is 30 s. Electrothermal atomizaHere it is assumed that the uptake rate is 3 mL/ tion. min with a 17%sample introduction efficiency.
on the side of the collar for sample introduction and removal (via a manual syringe, a syringe pump, or a peristaltic pump). The nebulizing chamber was constructed from glass with two glass side arms for aerosol removal and transfer to the inner flame and for introduction of a washing solution. The aerosol input arm was mounted tangentially to produce a swirling motion of the aerosol into the transfer tubing resulting in an improved nebulization efficienty (7).The other glass side arm was used to rinse the chamber with distilled water. Rinsing was accomplished in 2 min with no memory effect. The sample aerosol passed from the nebulizer outlet to the flame via a glass “mixing” tube (18 in. long, 1 in. i.d.) with indentations in the sides to promote mixing of the sample with the flame gases and to dilute, to some extent, the fog produced by the ultrasonic nebulizer; without the “mixing” tube, the transferred aerosol would cause the flame to sputter.
Finally, a glass bulb was lowered into the nebulizing chamber to disperse the fountain produced by the ultrasonic nebulizer. The glass bulb was placed 2 in. above the solution membrane and adjusted by eye to obtain the best aerosol (densest fog). Because of the high fog density, a low aspirating gas flow rate (the C2HJ could be used. This approach produced a high aerosol density with small fluctuations in the inner flame. The air flow acted as a fluidic valve on the sample fog and allowed only a fraction of the sample fog to exit the nebulizer chamber. The transport rate of fog to the flame was 0.01 mL/min in all studies described here. A 1.4-MHz radio frequency power supply was constructed to drive the ultrasonic crystal. The design was similar to the ones given by Owens ( 5 , s )and can be obtained by writing to J.D.W. Reagents and Procedures. All chemicals used were reagent grade. Stock solutions of each element (1000 pg/mL) were prepared as described by Smith and Parsons (7). Successive dilutions of the stock solutions were made each day and stored in a glass bottle to minimize contamination. Blank measurements for the detection limit studies were made with deionized water. Procedure. During the measurements, the acetylene was varied slightly to maximize the fluorescence signal. The flame height for all atomic fluorescence measurements was 2 cm above the burner head. Argon gas with a flow rate of approximately 10-15 L mix-’ was used as a sheath to add stiffness to the flame and to reduce environmental scatter. With careful adjustment, scatter wa9 found to be negligible; 2-mm monochromator slits were used to collect the maximum amount of fluorescence. After optimization of the experimental arrangement and conditions, analytical calibration curves were measured for each element at their atomic fluorescence lines over 2 orders of magnitude. Lowest concentrations measured were approximately 10 times the detection limit. The root-mean-square noise levels were evaluated (8)by measuring one-fiith of the observed peak-to-peak noise on the base line while aspirating deionized water (blank). All blank measurements were made for 10-min time periods and all standard/sample measurements for 0.5-1 min. Detection limits were then evaluated by extrapolating the analytical calibration curves (obtained by using a least-squares fit) to a signal equal to 3 times the noise (9). For the analyses of real samples, NBS standard reference materials (10, 11) (SRM), were employed; standardization was by a two-point bracketing procedure using aqueous standard matched in acid concentration to the sample. High purity acids (subboiling distillation) were used in the preparation of samples and standards (12).
RESULTS AND DISCUSSION In Table 111,the detection limits (micrograms per milliliter) are given for the two nebulizer-burner systems studied; excitation and emission (fluorescence wavelengths for the laser excited atomic fluorescence measurements are also given). The fluorescences of Ca and Sr (resonance lines in the visible) were optically saturated, whereas, the fluorescences of the other two elements, P b and Mg, with resonance lines in the UV were not optically saturated. The ultrasonic nebulizer-miniburner system gave =5 times greater fluorescence signals for Ca and Sr but also =5 times greater noise (blank) resulting in no improvement in precision or in detection limits over the pneumatic nebulizer-capillary burner system. The blank noise in this case was related to scatter of the laser radiation. For
Table VI. Real Sample Analysis by Laser-Excited Flame Atomic Fluorescence Small Dual Flame element laser a exptl value NBS certified value ( A e x l h d (nm) SRM Sr pine needles, SRM no. 1575 MOL 4.7 f 0.2 pglg 4.8 f 0 . 2 pg/g 460.71460.7
Pb
283.31405.8
Pb
283.31405.8
Fe
water, SRM no. 1643
MOL
20
*
2 ng/g
20
unalloyed copper, SRM no. 394
MOL
26
f
2 pglg
26.5 t 0 . 2 pg/g
pine needles, SRM no. 1575
CMX-4
198 i 8 pg/g
i
200
1 nglg
i
10 pglg
296.71373.5 a
MOL = Molectron UV-14 N, laser with DL 300 dye laser. CMX-4 = Chromatix CMX-4 flashlamp dye laser.
9
Anal. Chem. 1981, 53,9-13
P b and Mg, the ultrasonic nebulizer-miniburner system gave 5-fold improvements in detection limits; here, the noise levels were related to the flame background emission and electronic noise rather than the nebulizer. For all laser measurements, the laser beam was focused in the center of the observation region; cylindrical focusing was tried but gave no gain in signal. In the case of Mg and Pb, further improvements in detection limits would result by use of a dye laser with larger spectral irradiances a t the excitation lines; however, the decrease in detection limits with increase in laser spectral irradiances will only occur as long as source-related noise, e.g., scatter noise, is unimportant. For Ca and Sr, an increase in laser spectral irradiance would not improve the detection limits and in fact would be expected to cause a degradation in them because of the increase in scatter related noise. In Table IV, concentrational detection limits obtained with laser excited AFS (LEAFS) and the ultrasonic nebulizerminiflame system are compared to detection limits obtained by several other methods. The ultrasonic nebulizer-minitlame burner system using LEAFS is shown to be comparable for several techniques and much better than others. In Table V, absolute detection limits (picograms) are shown for LEAFS by use of the ultrasonic nebulizer and miniflame burner vs. other spectroscopic methods. As can be seen, the ultrasonic system gives at least 1 order of magnitude in the worst cases and up to 3 orders of magnitude improvement in the best cases. This shows that due to the low sample uptake rate and high efficiency of fog transfer, that the ultrasonic nebulizerminiflame burner system is capable of performing trace analysis on small samples without preconcentration steps. In Table VI analyses of NBS standard reference materials using LEAFS with the ultrasonic nebulizer-miniflame burner system are presented, along with the certified values. In this study, both the Molectron UV-14 and the Chromatix CMX-4
lasers were used. As can be seen, the accuracy and precision of this method were extremely good with no indication of interferences for the real sample analyses performed here. Thus this system should be useful for measurement of very low concentrations of elements in real samples with a precision and accuracy not available by most other methods.
ACKNOWLEDGMENT The authors thank Mr. A Grant and his staff for construction of all optical support equipment and h4r. Rudy Strohschein for construction of the nebulizer chambers.
LITERATURE CITED (1) Wineforher, J. D. ACS Symp.Ser. 1978. No. 85. (2) Weeks, S. J.; Haraguchi, H.; Whefonhr. J. D. AMI. Chem. 1978, 50, 360-368. (3) Fraser, L. M.; Wfotdner, J. D. Anel. Chem. 1972, 44, 1444-1451. (4) Havath, J. J. MS Thesis, Unlverdty of Florida, Qainesvlle, FL. (5) Owerrs. L. E. Technical Repat AFWlR-67-400; W r l g h t P a t t m At Force Bass. OH, 1966. Face Base, OH, 1971. . S M . 0. W.; Parsons, M. L. J . Chem. E&. 1973, 50. 679-681. Bower, N. W.; Ingb,J. D. Anel. cham.1976, 48,666-692. IUPAC, Nomendatwe, Symbols. Unlts and W Usage h Spectro1Analy84, Part 11. 1975. Epstein, M. S.; Ralns, T. C.; Menls, 0. Can. J . Specbpsc. 1975, 20, 22-26. Epsteh, M. S.; Nkdel, S.; Omenetto, N. D.; Reeves, R.; Bradshew,J. D.; W f W d n e C , J. D. Anel. chem.1970, 51, 2071-2077. Metthson. J. M. Anel. cham.1972, 44, 1715-1716. . . PSrkMlmer Atomlc Absw~tionMefatwe; PerkirrELnec Cap.: Norwak, CT. (14) Wfordner. J. D.; Fkztrgerald, J. J.; Omenetto, N. Appl. Specfrosc. 1975, 29,369-363. (15) Bamana, P. W. J. M.; [kBoes, F. J. specbodrtn.Acta, Pari B1975. 3019,309-334.
RECEIVEDfor review July 7,1980. Accepted October 1,1980. Research was supported by Grant No. AF-AFOSR-F49620-
8oc-ooo5.
Elimination of Alkali Chloride Interference with Thiourea in Electrothermal Atomic Absorption Spectrometry of Copper and Manganese Masaml Sutukl,
Klyohlsa Ohta, and Tatsuya Yamaklta
Department of Chemkfry, Faculty of Engineering, Mie University, Kam%rsmcho, Tsu, Mleken 514, Japan
Interferences of NaCI, KCI, and NH&l on the atomizatlon of Cu and Mn have been studied in a molybdenum mkrotube atomlzer. Interferences of CaCI, and MgCI, were also examined. Chloride Interference on Cu is removed by adding thkuea as maw modifier, whkttmdfectolchkrkleon Mn is compensated by background correction. The atomhation pra4iles show the complex atomlzatkm process for Cu h akall chlorides dlfferlng from Mn. As to the effect of thlourea as modifier for Cu, ll can be assuned that the fonnatkm d Cucl is prevented by hydrogen sulflde generated through the decomposHlon of thiourea In the atomizer. The detectlon llmlt for Cu In 0.5 pg of NaCl was 0.6 pg In the presence of thiourea and that for Mn was also 0.6 pg.
T h e interferences caused by a chloride matrix are serious in electrothermal atomic absorption spectrometry. Great 0003-2700/81/03539$01.00/0
efforts have been devoted to remove or minimize such interferences. Czobik and Matousek (1) reported the resulte of a detailed study on the interference effect of metal chlorides in furnace atomic absorption with both a slow conventional and a fast response detection system and showed that treatment with phosphoric acid was effective for removal of chemical interference in the Cu-NaCl system. Churella and Copeland (2) examined the concentration-dependent interferences of several alkali and alkaline earth halides in electrothermal atomic absorption spectrometry of Cu by use of a carbon cup and showed that the addition of Na202eliminated or substantially reduced the interferences caused by halides. A selective volatilization technique was applied for removing chloride matrix interferences (3). However, loss of sensitivity caused by covolatilization of Cu and Mn with NaCl was observed. Sturgeon et al. ( 4 ) described the combination of selective volatilization and matrix modification techniques for direct determination of Mn and Zn in seawater by graphite 0 1980 American C2wmlcal Sodety
