Improvements in Flame Emission Spectrometry through the Use of Ultrasonic Nebulization into a Premixed Oxygen-Hydrogen Flame D. E. Gutzler' and M. B. Denton* Department of Chemistry, University of Arizona, Tucson, A2 8572 1
A laminar flow premixed oxygen-hydrogen flame when combined with ultrasonic nebulization has demonstrated substantial improvements In detection limits for a number of elements studied when compared to those obtained using laminar nitrous oxide-acetylene, laminar oxygen-acetylene, and a conventional turbulent oxygen-hydrogen flame. The Improved performance observed Is attributed to the low flame background emission and reduced flame flicker.
Flame emission spectrometry has enjoyed widespread use for many years as a trace level analytical technique for a number of elements which can be efficiently excited by the energy available within a flame. Winefordner, Svoboda, and Cline ( 1 ) have presented an excellent theoretical treatment indicating that superior detection limits for flame emission should be possible as compared to absorption or fluorescence for elements with resonance lines of wavelengths longer than 4000 8. They report that for high temperature flames, flame flicker intensity is the major contributor to the total noise. Parsons, McCarthy, and Winefordner (2) describe the effect of the monochromator slit width and the product of the flame flicker and flame background intensity on the minimum detectable atomic concentration. They conclude that as the product of flame flicker and flame background decreases, the optimum slit width will increase providing increased light throughput, resulting in a smaller minimum detectable atomic concentration. Many tradeoffs must be considered when choosing a gas mixture to support the flame: including the temperature produced, level of background emission, the resulting chemical environment, and the availability of a burner design capable of safe, stable operation with the desired gas mixture. Depending on the type of application for the flame, some of these parameters will be more important than others. Initially in flame emission spectrometry, total consumption burners producing turbulent flames achieved the greatest popularity as a result of their ability to yield useful detection limits, modest cost, safety, and availability. However, in recent years, premixed laminar flow flames have enjoyed increasing popularity due to their lower flame flicker, reduced rise velocities, increased dimensional stability, and improved homogeneity (3-7). Particularly encouraging results have been obtained by investigators employing premixed nitrous oxide-acetylene flames (5-7). In general, the improved detection limit obtained with this flame arises from its high temperature when compared to other common premixed flames such as air-acetylene and air-hydrogen and its highly reducing interconal zone which promotes atomization efficiency, particularly in the case of elements forming stable oxides. Unfortunately the nitrous oxide-acetylene flame produces a rather intense backPresent address, Tektronics, Beaverton, OR 97005. Author to whom correspondence should be sent.
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
ground continuum. In contrast, the premixed oxygen-hydrogen flame possesses a relatively low background emission and a flame temperature only approximately 150 K cooler. The advantage of the simple and low intensity flame background emission is important. Because the reactions in a hydrogen flame give a simple flame composition, the flame background emission is relatively clean with the only major radiation resulting from OH bands in the regions of 2811, 3064 and 3428 8. With the N z O - C ~ Hand ~ 02-C2Hz flames, Cz, CN, and CH radicals are formed which emit over the whole region of the UV-visible spectrum. The intense flame background contributes to the noise level causing a degradation in detection limits and the precision of trace level analysis. Because of its high burning velocity, the premixed laminar oxygen-hydrogen flame cannot be supported on conventional commercial premixed burners. Mossholder et al. (8) have evaluated a relatively high flow rate premixed oxygen-hydrogen flame supported on a slot burner design originally developed by Fiorino e t al. ( 3 ) utilizing pneumatic nebulization. These workers concluded that the oxygenhydrogen flame offers few advantages as an atomization cell for flame atomic absorption spectrometry and observed relatively equivalent emission detection limits for several elements when compared to those obtained by other workers in the nitrous oxide-acetylene flame. Recently, Suddendorf and Denton (9) have shown it is possible to safely and easily operate a premixed laminar flow oxygen-hydrogen flame on a capillary type burner if the capillary ports are of a sufficiently small diameter. The purpose of this study is to make a comparison of the premixed laminar flow oxygen-hydrogen flame with the premixed flames of nitrous oxide-acetylene and oxygenacetylene and with a turbulent oxygen-hydrogen flame as studied by emission spectrometry under equivalent experimental conditions, as well as to assess the role of the premixed oxygen-hydrogen flame in flame emission analysis.
EXPERIMENTAL Apparatus. Basically two experimental configurations were employed (Figure 1).Configuration A consists of a Heath EU 700 monochromator, and EU 701-93 photometric module using a 1P28A photomultiplier powered by a Heath EU-42A high voltage power supply, a loghinear current module EU-20-28, and variable speed recorder EU-2OV (Heath Company, Benton Harbor, MI 49022). Adjustable time constants were implemented by introducing switch selectable capacitors between 0.001 and 0.1 WFin parallel with the feedback resistors in the electrometer. This configuration which eliminated a number of variable parameters present in configuration B was much easier to optimize. I t was, therefore, chosen for all comparison studies in hope that the data truly reflect the optimum performance of each flame in a given instrumental configuration. In an attempt t ' J achieve the lowest possible detection limits, a second, more complicated configuration B was used. This experimental system included a parabolic mirror and lens to improve light-gathering efficiency, a light chopper, a photometric preamplifier and a lock-in amplifier (Models 125, 184, and 126, respectively, Princeton Applied Research, Princeton, N J 08540).
fUI
HEATH Ell-700 MONOCHROMATOR
HEATH EU-ZOIV CURRCYT S Y S T E M
CONFIGURATION
2'
HEATH E Y . 7 0 0
P A R 111
HEATH
EU-ZOV
L
-
) e C y l l O N
~
ULTRASONIC
ULTRASONIC
e NEBULIZER OXIDIZER
OXIDIZER
ULTRASONlC POWER GENERATOR
POWER
Figure 1. (A) Experimental configuration used for the comparison studies of the different flames and (B) employed to study optimum sensitivity in the oxygen-hydrogen flame Comparison studies were conducted with the simpler configuration to eliminate several variable parameters in an attempt to ensure that the data reflect optimum performance of each flame +635cm+
13 h o l e 8 0.56 mm
Table I. Comparison of the Detection Limits for the Turbulent 02-Hz Flame with Pneumatic Nebulization vs. the Premixed 02-Hz Flame with Ultrasonic Nebulization Utilizing the Configuration of Figure 1A Turbulent
02-H21 Line, A
Ba
Ca Cr cu Figure 2. Detailed view of the 13-hole burner used in this study Sufficiently small holes (0.56 mm) and adequate cooling are required to provide safe, stable operation with the high burning velocity oxygen-hydrogen mixture
In La Mg Mn Ni
Sr T1
The ultrasonic nebulizer sample cell has been previously described (10). The radiofrequency power source is composed of an army surplus BC-191 transmitter (Farnsworth Television and Radio Corporation, Fort Wayne, IN) and a modified TU-5B tuning unit which are powered by an RA-34J power supply (Radio Receptor Company, Philadelphia, PA). This R F source is capable of delivering up to 200 W of power a t the operating frequency of l MHz. T h e frequency of this system can be varied over a range to obtain the transducer resonance frequency required for maximum production of aerosol. The Beckman total consumption burner (No. 4020, Beckman Instruments, Fullerton, CA 92634) was compared t o a multihole burner designed for this study (Figure 2). The multihole burner has thirteen holes which are 0.56 mm in diameter. The burner head is 2 cm thick and 6.36 cm in diameter. The multihole burner is surrounded with cooling fins, providing a radiating area of 133.3 cm2. A second burner was constructed for the 02-CzHz mixture with the diameter of the thirteen holes reduced to 0.45 mm. While neither burner flashed back under normal operating conditions, adequate safety protection in the form of pop-off plugs was incorporated in both burners. The flow rates for the Beckman total consumption burner were 0 2 , 3.6 l./min; and H z , 10.2-12.0 l./min. For the multihole burners and the laminar premixed flames, the flow rates were as follows: for oxygen-hydrogen 0 2 , 2.3 l./min; Hz, 5.4-6.2 l./min; for nitrous oxide-acetylene NzO, 2.1 l./min; CzH2, 0.7-1.0 l./min; and for oxygen-acetylene 0 2 , 3.21 l./min; C2H2, 1.8-2.3 l./min. Stock solutions of' 1000 pg/ml were made following the directions of Dean and Rains (11).All other solutions were prepared by successive dilutions from the stock solutions using deionized water. For the elements with ionization potentials below 7.5 eV ( 6 ) , potassium chloride a t 1000 pg/ml was added as an ionization suppressant. Procedure. The 1P28A photomultiplier was operated a t 700 V. T h e experimental system was peaked by nebulizing an easily de-
5535 4227 4254 3274 4511 4418 (Lao) 2852 403 1 3415 4607 5351
&@/mi
Laminar
02-H~' ii g
Iml
0.0001 0.00001
0.25 0.005 0.05 0.5 0.003
0.005
0.1
0.003
0.25
0.01
0.01
0.0025 0.05
0.25 0.005
0.03
0.1 0.0001
0.0001 0.001
tectable sample concentration. T h e flame position, gas flow rates, and a slit width were optimized to provide a maximum signal-tonoise ratio. The radio frequency source was tuned to produce the optimum amount of aerosol as determined by monitoring the emission signal.
RESULTS AND DISCUSSION A comparison between the oxygen-hydrogen flame background emission observed for the total consumption burner and the premixed laminar burner is shown in Figure 3. The laminar premixed oxygen-hydrogen flame is very stable and exhibits much less flame flicker. In the case of the premixed flame, the peak-to-peak noise has been reduced by a factor of 5; and in the region of 4000-6000 A, the background emission intensity has decreased to about one-third of that observed in the turbulent oxygen-hydrogen flame. This lower background and decreased flicker improves the signal-to-noise ratio and allows the use of higher amplifier gain and/or a larger slit width, improving the monochromator throughput. The premixed burners employed in these studies demonstrated the ability to tolerate concentrated solutions by showing no salt buildup and a wide range of linear response with no memory effects. Table I compares the detection limits for several elements using the Beckman total consumption burner with pneumatic nebulization and the premixed laminar multihole burner with ultrasonic nebulization. The limit of deANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
*
831
I '
40ce
3000
5000
6000
3:cc
250c
4500
6325
5 3C?
Figure 3. (A) Flame background spectra of the turbulent oxygen-hydrogen flame in a total consumption burner, and (B) the background spectra under identical conditions of the premixed oxygen-hydrogen flame in a laminar flow burner, showing the reduction in flame background emission and flicker noise
tection was defined as that concentration resulting in an rms signal-to-noise ratio of 2. The average reduction in detection limits for the eleven elements studied is 300. A remarkable improvement in detection limits is exhibited for Ba, Ca, In, La, Sr, and T1. Often investigators comparing detection limits have employed different experimental configurations, making direct comparisons about the effectiveness of a particular flame difficult. In this study, however, the burner design makes it possible to examine the capabilities of three flames under identical experimental conditions with the same aerosol introduction rate. The detection limits for the premixed oxygen-hydrogen, nitrous oxide-acetylene, and oxygen-acetylene flames were evaluated in the comparison configuration (Figure 1,A). These are given in Table 11. Each of the flames has its own characteristics that make it useful for flame emission analysis. The 02-C2H2 flame is the hottest and will have a greater efficiency for exciting free atoms. The ,nitrous oxide-acetylene flame is important because of its high temperature and reducing nature, making it a desirable flame for elements forming stable oxides. The premixed laminar 02-H2 flame offers a high temperature, a much lower flame background emission, and a vast reduction in flame flicker which can result in an improved signal-to-noise ratio. While the study conducted by Mossholder et al. ( 8 ) indicates possible reduced atomization efficiency in the pre-
mixed oxygen-hydrogen flame as compared to the fuel-rich nitrous oxide-acetylene flame for elements such as Al, Ba, Ca, and Sr which form reasonably stable monoxides, the results obtained in these studies indicate that significant improvements in detection limits can be achieved for a number of elements in a premixed oxygen-hydrogen flame. These results are primarily attributed to the greatly reduced background emission and decreased flame flicker which allow utilization of wider monochromator slit widths and increased read-out amplification factors. A comprehensive list of detection limits for flame emission spectrometry employing premixed N Q O - C ~ Hflames ~ supported on slot burners has been summarized by Pickett and Koirtyohann (6). The comparison between the results reported in their review and the detection limits for the premixed laminar 0 2 - H ~flame are shown in Table 111. In an attempt to achieve the best possible sensitivity, configuration B, Figure 1, was employed. While it is somewhat more complicated to optimize, the additional light gathering ability and increased time constant resulted in additional improvements in limits of detection for most elements as shown in the last column of Table 111. It should be noted that elements which form very stable oxide species such as vanadium and titanium cannot be expected to perform well in the premixed oxygen-hydrogen flame. Two classical interference studies were undertaken to examine the effects of phosphate and aluminum on a calcium
Table 11. Detection Limits for Three Premixed Flames Supported by the %Hole Burner Using Ultrasonic Nebulization in Configuration 1A
Line,
Ba Ca Cr cu
In La Mg
Mn Ni
Sr T1 832
A
5535 4227 4254 3274 4511 4418 (Lao) 2852 4031 3415 4607 5351
Laminar
Laminar
Laminar
02-CzH2
N 2-0- C 2H2
02-H~
Detection limit,
Slits
Detection l i m i t ,
SlltS,
Detection l i m l t ,
ugh1
vm
ug/ml
urn
us/ml
urn
0.05
50 50 100 100 50
150 100 150
0.0001
300 300 2 50
0.001 0 -05 0.1 0.05
0.1 0.05 0.025
0.1 0.001 0.05
100 100 100 100 50 50
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
0.01 0.0001 0.05 0.1
0.01 0.1 0.01 0.05
0.1 0.001 0.05
100 100 150
100 150 100 100 150
0.00001 0.005
SlltS,
0.1
100
0.0001 0.003 0.01 0.0025 0.05
250 2 50 150 2 50 150 250 2 50
0.0001 0.001
~
Table 111. Comparison of Reported Detection Limits in Flame Emission (6) vs. the Results Observed in This Study Utilizing the Optimized Experimental Configuration Shown in Figure 1B Laminar ii20-C2H2 (Rei 6 ) , Line,
Ba Ca Cr cu In La Mg Mn Ni
Sr TI
A
5535 4227 4254 3274 4511 4418 (Lao) 2852 4031 3415 4607 5351
Laminar
Laminar
O2-H2
02-H2
(Coniiguration l A ) ,
(Configuration l B ) ,
vg/rnl
irglml
F U k l
0.001 0.0001
0 .ooo 1 0.00001
0.00005 0.0000025
0.005
0.005
0.01 0.002
0.1 0.0001
0.1
0.003
0 .ooo 1 0.1 0.00001 0.0001
0.005 0.005 0 -03
0.01
0.005
0.0025 0.05
0.001 0.01 0.00001 0.00002
0.0001 0.02
0.0001 0.001
i 4
10 !-
& u . 5 J 10 MOLE RATIO
100
AJ/Ca
M O L E ,RATIO P O q / C a
Figure 4. The interference of phosphate upon the calcium emission signal (0.5 bg/ml); premixed 02-H2 flame (A), premixed 0 2 - H ~ flame plus lanthanum (0),and premixed N20-C2H2 flame (0)
Figure 5. The interference of aluminum upon the calcium emission signal (0.5 bglrnl); premixed 02-H2 flame (A),premixed 02-H2 flame plus lanthanum (0),and premixed N20-C2H2 flame (0)
emission signal in the premixed laminar 02-H2 flame. The depression of calcium emission due to interference effects is illustrated in Figures 4 and 5 . However, upon the addition of lanthanum to the samples, the interference due to phosphate and aluminum at a mole ratio of 100 is reduced from 36% and 28% to 5% and 6%, respectively, which is comparable to the 3% and 4% depression of calcium emission observed in the nitrous oxide-acetylene flame. With the addition of lanthanum, the effect of the interference of phosphate and aluminum is reduced so the premixed laminar 02-Hz flame can be useful for the analysis of calcium in the presence of these species. The accuracy of the results obtained with the premixed laminar 02-H2 flame has been verified by the analysis of National Bureau of Standards Standard Reference Material 1571, Orchard Leaves. A prepared solution of the standard was diluted and analyzed for calcium a t the 500 partsper-trillion level. The experimental value of 2.11 wt % agreed with the reported value of 2.09 f 0.03 wt %. The amount of manganese in the standard was also examined in the 02-H2 flame a t the 100 parts-per-billion level and the experimental value of 92 Kglgram is within the range of the reported value of 91 f 4 Wg/gram.
though the premixed oxygen-hydrogen flame is slightly cooler than the nitrous oxide-acetylene and oxygen-acetylene flames and does not produce some of the highly reducing species present in these flames, the low background emission and reduced flame flicker can result in improved detection limits for a number of the elements investigated. Two classical interference studies coupled with a sub-partper-million analysis of a standard indicate that the premixed oxygen-hydrogen flame has a useful role in trace analysis. LITERATURE CITED
CONCLUSIONS The laminar premixed oxygen-hydrogen flame has been shown to be a safe, sensitive source for emission spectrochemical trace analysis. This study indicates that even
(1)J. D. Winefordner, V. Svoboda, and L. J. Cline, CRC Crit. Rev. Anal. Chem.. 1. 233 11970). (2)M. L. Parsons, b4. J.'McCarthy, and J. D. Winefordner, J. Chem. Educ., 44, 214 (1967). (3)J. A. Fiorino, R. N. Kniseley, and V. A. Fassel, Spectrochim. Acta, 238, 413 (1968). (4)V. A. Fassel and D. W. Golightly, Anal. Chem., 39, 466 (1967). (5) E. E. Pickett and S. R. Koirtyohann, Spectrochim. Acta, 238, 235 (1968). (6)E. E. Pickett and S. R. Koirtyohann, Anal. Cbem., 41,(14),28A (1969). (7)G. D. Christian and F. J. Feldman, Appl. Spectrosc., 25, 660 (1971). (8)N. V. Mossholder, V. A. Fassel, and R. N. Kniseley, Anal. Chem., 45, 1614 (1973). (9)R. F. Suddendorf and M. B. Denton, Appl. Spectrosc., 28, 8 (1974). (10)M. 6.Denton and D. B. Swartz, Rev. Sci. hstrum., 45, 81 (1974). (11)J. A. Dean and T.C. Rains, Chapter 13 in "Flame Emission and Atomic Absorption Spectrometry," Vol. 2,J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York, NY, 1971.
RECEIVEDfor review October 1, 1973. Accepted January 6, 1975. This research was supported in part by the Office of Naval Research.
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