Some Analytical Characteristics of Organic Liquid Fuel Flames in Atomic Absorption Spectrometry B. W. Bailey and J. M. Rankin Division of Laboratories and Research, New York State Department of Health, New Scotland Auenue, Albany, N . Y . 12201 The data given in this communication indicate that the flames produced using aspirated organic liquids are cool, quiet, and provide analytical sensitivities comparable to those obtained using more conventional systems. Investigations of aspirated organic liquid fuel flames show them to be colloidal, and the variability of this characteristic relative to applications in atomic absorption flame spectrometry is discussed.
IN THE PAST, workers in analytical flame spectrometry have restricted their choice of fuel systems to those which can be presented to the burner system as a gas. Most of the work previously reported has involved the use of hydrogen, natural gas, or acetylene, and the majority of the instrumentation commercially available is designed for use with these fuels. Recently, the authors have investigated the use of organic liquids as fuels in flame spectrometry. Particular attention has been given to their application to atomic absorption spectrometry ( I , 2). It has been found that flames produced from organic liquid fuels provide suitable atom reservoirs for use in flame spectrometry. Furthermore, these flames are quiet, safe, and easy to produce. The topics discussed include the manner in which flames are produced using organic liquid fuels, the classes of fuels investigated, the nature of the flame produced in terms of flame type and flame noise, and the analytical results obtained when flames from organic liquid fuels are employed in atomic absorption spectrometry. EXPERIMENTAL Flame Production. Flames from organic liquid fuels are produced by aspirating the organic liquid directly into the mixing chamber of a premix burner by means of a pneumatic nebulizer. The resulting fuel aerosol passes through the mixing chamber to the burner head and is ignited at the burner face. A second nebulizer is required so that a sample solution can be aspirated into the flame produced. To accomplish this a dual nebulizer attachment was constructed. Such a n attachment designed for use with a Perkin-Elmer model 303 premix burner system is shown in Figures 1 and 2. The fuel container shown in Figure 2 is a 250-ml polyethylene bottle, the nebulizers are Perkin-Elmer variable flow model 3030700 and the burner is a 3-slot Boling type with a 105-cm slot length. The flames produced in the manner described above may be varied from fuel lean to fuel rich either by varying the flow rate of the aspirating air or by changing the fuel uptake rate with the variable nebulizer until the desired flame characteristics are obtained. RESULTS Five classes of organic liquids were investigated as possible fuels: alcohols, ketones, esters, aliphatic hydrocarbons, and aromatic hydrocarbons. The ability to produce stable flames (1) B. W. Bailey and J. M. Rankin, Spectrosc. Lett., 2 ( 6 ) , 159-164 (1969). (2) Zbid.,(8) 233-237. 216
using these liquids as fuels was determined by aspirating the test liquid through one nebulizer, deionized water through the other, and then attempting to ignite the resulting mixture at the burner face. A more detailed description of the experimental methods and results has been given previously ( I ) and will not be repeated here. Suffice it to say that of the organic liquids investigated only hexane, isooctane, acetone, and benzene produced stable self-supporting flames under the experimental conditions used. The flames produced by these liquids were selected for further evaluation as atom reservoirs in atomic absorption spectrometry. Flame Type. Stationary flames may be conveniently classified under two headings. The first is the diffusion flame in which the fuel burns as it is brought into contact with the oxidant. The combustion characteristics of diffusion flames are determined primarily by the rate of interdiffusion of oxidant and fuel. Examples of diffusion flames are the hydrogen/entrained air flame and flames on wicks. The second classification is the premixed flame in which the fuel and oxidant are mixed prior to being presented to the site of combustion. To maintain flame stability, the fuel mixture is supplied at a rate exceeding or at least equaling the burning velocity of the fuel. The most common example of premixed flames is the Bunsen burner flame. In the case of air-aspirated organic liquid flames, the situation is less clear. The fuel/oxidant mixture is, in this case, initially presented to the burner system as an aerosol. The characteristics of the flame will depend upon the fate of the aerosol particles as they travel from the nebulizer to the site of combustion. If the aerosol droplets are vaporized by the time they reach the site of combustion, the flame produced will be of the premix variety. If, however, the air/fuel mixture retains its heterogeneity and arrives at the combustion site as an aerosol, the flame will be of the first type with each of the fuel droplets functioning as a microdiffusion flame. The type of flame actually produced lies somewhere between these two extremes. Flames resulting from the combustion of liquid droplets have been discussed by Fristrom and Westenberg (3). In the reference cited, these authors designate as colloidal those flames whose fuel supply consists of droplets in the size range of a few tens of microns. In colloidal flames, evaporation occurs in the preheat region, resulting in a flame which is essentially premix in type. The assumption that fuel aerosol droplets are present in the region directly above the burner face under certain conditions is evidenced by the behavior of fuel rich air/hexane and air/benzene flames. A fuel rich air/acetylene flame is luminous from the burner face on up. However, with the air/hexane and air/benzene flames, luminosity is observed only after the flame gases have reached a certain distance from the burner face [about 4 cm for air/hexane and 2 cm for air/ benzene). The flame between the luminous zone and the (3) R. M. Fristrom and A. A. Westenberg. “Flame Structure,” McGraw-Hill Book Company, New York, N. Y . , 1965.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
PENTON MIXING
,DRAIN
,NEBULIZER ,+NLET
SOCKET FOR GASEOUS F U E L
CHAMBER WITH
LOCKING LUGS REMOVEG
FRONT V I E W P E N T O N M I X I N G C H A M B E R BODY DUAL N E B U L I Z E R A T T A C H M E N T AIR FOR S A M P ASPIRATION
FRONT V I E W
AIR FOR F U E L
SIDEVIEW SHOWING MODE O F ATTACHMENT TO P E N T O N M I X I Q G CHAMBER O F P E R K I N - E L M E R MODEL 303
Figure 1. Dual nebulizer attachment
TOBURNER A S S E M B L Y BASE
burner face is nonluminous and the reaction zone is undefined, a characteristic of diffusion flames. It has been reported ( 4 ) that for saturated hydrocarbons lower than C j the upper limit of flammability is reached before the conditions favoring carbon formation. Flames from saturated hydrocarbons of pentane and higher should, under sufficiently fuel rich conditions, exhibit luminosity due to carbon formation. The existence of a nonluminous zone for a considerable distance above the face of the burner in the case of the fuel rich airihexane and airibenzene flames can be interpreted as resulting from the fuel, at least in part, being presented to the burner face as discrete droplets. The area directly above the burner functions as a vaporization zone for the fuel droplets. The fuel supply in this area is thus partially controlled by the rate of evaporation of the fuel droplets and will be less fuel rich than the fuel supply rate would indicate. At a certain distance above the burner face, the vaporization process increases the amount of fuel available for combustion, thus increasing the fuel-to-air ratio until the mixture is fuel rich enough for carbon formation to take place and the resulting luminosity to be observed. As far as analytical applications are concerned, it is desirable to maximize the vaporization of the fuel before it reaches the burner so that it is as near to the premix type as possible; otherwise, the presence of droplets in the flame would cause light scattering with a subsequent increase in noise. The factors which will influence the degree of vaporization in burner systems of the type used in these experiments are the heat of vaporization of the fuel, the temperature of the mixing chamber, and the size and velocity of the aerosol particles. F o r a particular fuel the heat of vaporization is constant and may be ignored. Heating the mixing chamber increases the rate of vaporization but also gives rise to the further complication of having to aspirate aqueous samples into a heated chamber. The size and velocity of the aerosol particles are the most conveniently controlled parameters since they are dependent on the flow rate of the nebulizing gas and the rate of fuel uptake. The optimization of these parameters may be determined from the empirical equation for the calculation of droplet size in aerosols produced by pneumatic nebulizers (5). In this expression the mean diameter of the droplets (do)is given by
(4) A . G. Gaydon and H. G. Wolfhard, “Flames: Their Structure,
Radiation and Temperatures,” Chapman and Hall Ltd., London, 1953. ( 5 ) S. Nukiyama and Y . Tanasawa, Trans. SOC. Mech. Eng., Tokyo 5, 62-68 (1939).
Figure 2. Dual nebulizer attachment assembled for operation
where do
mean (volume/surface) diameter velocity of the nebulizing gas L; velocity of the nebulized liquid y surface tension p = density n = viscosity Q L = flow rate of the liquid Q G = flow rate of the gas u
= = = =
F o r a particular liquid and nebulizing system the only variables are the flow rates of the liquid and gas Q L and Q G , the velocities L: and u being related to these quantities by the dimensions of the nebulizer orifices. Furthermore, for any pneumatic nebulizer, the quantities Q L and QG are related since the flow rate of the liquid is a function of the flow rate of the gas; however, it was found that for variable flow nebulizers of the type used in these experiments, there is considerable latitude in selecting a flow rate Q L for a particular value of QG. Since the lifetime of a spherical drop is a linear function of its surface area, the probability of a fuel droplet reaching the burner face and subsequently producing a scattering center in the flame prior to evaporation should increase with the square of do. Such an effect would be observed as an increase of the noise level. To verify this, the noise level of an air/ hexane flame for various values of dowas determined. The procedure used follows: The burner system was mounted in a Perkin-Elmer model 303 atomic absorptioa spectrophotometer and the flame noise level a t the 3247-A resonance line of copper measured for in air/hexane flame for various values of do. Measurements were made using a slit width of 1 mm. The noise level was displayed using a 10-mV recorder at a scale expansion of X5 and minimum damping. The air pressure to the sample nebulizer was set a t 30 psi and the nebulizer adjusted for an aspiration rate of 3.4 ml/min of deionized water. The air flow to the fuel nebulizer was monitored with a rotometer. The relationship between diameter do at the fuel nebulizer orifice and the noise level was determined by setting a value of QG and then varying Q L until a blue, nonluminous flame was obtained. do was then calculated from QG,Q L , and the constants pertaining to the fuel and nebulizer, and the results were plotted against the noise level (determined as the peak-to-peak average deflection). The results given in Figure 3 show that flame noise decreases with decreasing values of do. The noise level of the airlhexane flame for do(calculated) of 60 p is a little higher but of the same order of magnitude as obtained with a n air/acetylene flame which indicates that the flame is essentially premix in nature. As do is increased, the flame becomes less premix in nature and, in the extreme case of the fuel rich conditions, is essentially a diffusion flame.
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01
I
,
40
,
' , EO
'
,
120
1
160
do (Microns)
Figure 3. Effect of fuel droplet size do on background noise level of air/hexane flame. Measurements were made at the 3247-i resonance line of copper
Table I. Analytical Sensitivities of Copper in Air/Organic Liquid Flames Fuel aspira- Sample tion aspirarate, tion rate, fig AbsorSensiFuel ml/min ml/min Cu/ml bance tivity" Hexane 2.4 3.4 5 0.37 0.05 Acetone 4.0 3.4 5 0.33 0.07 Benzene 2.1 3.4 5 0.32 0.07 Isooctane 2.5 3.4 5 0.29 0.08 Acetylene ... 3.4 5 0.15 0.13 Acetylene* ... 3.4 5 0.19 0.10 a Defined as the concentration of copper in pg/ml necessary to produce 1 absorption. * Single nebulizer system. Table 11. Analytical Sensitivity and Detection Limit Data for Various Metals in the AiriHexane Flame and the Air/Acetylene Flame Detection limit pg/ml Sensitivity, pg/ml Air/ Air/ Air/acetAir/ ' acetMetal h(A) hexane ylene" hexane ylene" Copper 3247.5 0.05 0.10 0.013 0.011 Iron 2483.3 0.15 0.22 ,.. ... Silver 3280.7 0.05 0.10 ... ... Lead 2833 0.52 0.53 0.39 0.46 Manganese 2798.4 0.06 0.09 ... ... Zinc 2138.6 0.01 0.025 0.0016 0.0028 Calcium 4226.7 1.05 0.09 ... ... Single nebulizer system. ~
Analytical Results. Airiorganic liquid fuel flames were evaluated in the determination of copper, iron, silver, lead, zinc, and calcium by atomic absorption flame spectrometry. These same metals were also determined for comparison in the air/acetylene flame. Initial measurements with the air/ acetylene flame were made with the dual nebulizer attachment so that the comparison would not be affected by geometrical considerations. Ancillary experiments did show the sensitivity in the air/acetylene flame to be slightly improved by using the standard, single nebulizer configuration. Table I shows the analytical sensitivities and absorbance values obtained for copper in a variety of flames. Since the sensitivities obtained using the airihexane flame proved to be superior in the determination of copper when compared to the other 218
/
Air /Isooctane
A i r / Hexane
one
5
15
25
,ug C a / m l Figure 4. Calibration curves for calcium in a variety of flame. X = 4226.7
organic liquids investigated, sensitivity determinations were made in this flame for iron, silver, lead, manganese, zinc, and calcium and compared to those obtained in the air/acetylene flame using the single nebulizer configuration. The results of these determinations given in Table I1 show that with the exception of calcium, considerable enhancement occurs. Detection Limit. To determine whether the increased noise level found in flames produced by the combustion of aspirated organic liquids when compared to the airiacetylene flame would result in an appreciable deterioration of the detection limit, this parameter was determined for copper, lead, and zinc in both the air/acetylene and airihexane flames. Copper, lead, and zinc were chosen because they are representative of elements whose sensitivities in the airihexane flame are 1 X, 2x, and >2>( those obtained using the airiacetylene flame, thereby covering the whole range of sensitivity enhancement that was observed. Values were obtained by making absorption measurements near the detection limit followed by linear extrapolation to the concentration yielding a signal equal to twice the noise level. The results of these determinations, given in column 3 of Table 11, indicate that the enhancements in sensitivity obtained by using organic liquid fuels compensate for the increased noise level. Flame Temperature. The lack of sensitivity in the determination of calcium in the airihexane flame (see Table 11) serves as a qualitative measure of the temperature of this flame. Calcium is known to form refractory oxides in flames. To atomize calcium and efficiently populate the ground state, sufficient energy must be supplied to dissociate the oxide. Atomic absorption measurements determined at a resonance line are a n indication of the atom population of the ground state. Therefore, relative values of absorption measurements made on an oxide-forming element in various flames should indicate the flames' efficiency to dissociate the oxides formed, and so give a qualitative ordering of their temperatures. Atomic absorption measurements were made for calcium in the 5-25 pg/ml range using the air/acetylene, airibenzene, air/hexane, air/isooctane, and airpcetone flames. The results of these measurements are given in Figure 4. From these data, one can order the temperatures obtained in the
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
air/organic liquid fuel flames as air/acetylene > air/benzene air/hexane > air/isooctane > air/acetone.
>
DISCUSSION The results given above indicate that aspirated organic liquids can be used to provide flames that are of use in atomic absorption spectrometry. The sensitivities determined for the metals under consideration have been found to be, for
most cases, equal to o r greater than those obtained using the more conventional air/acetylene flame. Though the flames produced using aspirated organic liquids appear to be colloidal, it has been shown that this characteristic can be minimized to the degree that the flame noise level falls within an acceptable range.
RECEIVED for review June 23, 1970. Accepted November 5 , 1970.
Some Physical Characteristics of Flames Produced from Organic Liquid Fuels B. W. Bailey and J. AI. Rankin Dicision of Laboratories and Research, New York State Department of Health, New Scotland Acenue, Albany, N . Y . 12201 Investigation of some physical properties of colloidal flames produced by the combustion of air-aspirated organic liquids shows them to be cooler and slower burning than those commonly used in atomic absorption flame spectrometry. Emission spectra indicate that colloidal flames are lower in background radiation than the air/acetylene flame. Results of these investigations are discussed with regard to the function of the flame in atomic absorption measurements.
RECENT STUDIES ( I , 2 ) have shown that flames resulting from the combustion of aspirated organic liquids can be produced in such a manner as t o allow their application to atomic absorption flame spectrometry. It has further been shown (3) that such flames are colloidal in type and that the sensitivities and detection limits they provide are in many instances equal to o r greater than those provided by the more commonly used air/acetylene flame. Colloidal flames are classified as premix in terms of the manner in which the fuel and oxidant are presented t o the reaction zone ( 4 ) . The differences, therefore, between colloidal flames and the premix flames which result from the combustion of gaseous fuels are not due to variations in flame structure but rather to the physical processes which occur before the fuel-oxidant mixture reaches the reaction zone. In premix flames supplied by gaseous fuels, the fuel-oxidant mixture passes through a preheat zone which raises its temperature to the ignition point. The preheat zone extends, in this case, from the reaction zone to the burner face. I n colloidal flames, the fuel arrives at the burner face as a fine mist and the “preheat zone,” extending from the envelope of the fuel mist to the reaction zone, must provide for evaporation and mixing as well as preheating. Figures 1 and 2 show the general structural features of both types of premix flames. In view of previous results from the application of colloidal flames to atomic absorption flame spectrometry, it was felt that further characterization of flame physical properties (1) B. W. Bailey and J. M. Rankin, Spectrosc. Lett., 2 ( 6 ) , 159-164
would provide information useful for additional applications of these flames to other areas of flame spectrometry. I n this communication the results from the determination of flame temperature and flame velocity for a variety of colloidal flames are compared with values obtained from the literature o r from our laboratory determinations of a variety of premix flames which employ gaseous fuels. In addition, flame emission spectra are presented for a variety of colloidal flames and compared with spectra from a n air/acetylene flame. EXPERIMENTAL Flame Temperature Measurements. Flame temperatures were determined by means of sodium line reversal. The principle of this technique has been described elsewhere ( 4 , 5 ) and will not be restated here. The apparatus used to obtain the measurements is shown in Figure 3. The tungsten filament lamp was a 6-volt G.E. Model 2331. Lamp current was supplied by means of a step-down transformer with a 25.2volt, 2-ampere secondary. Voltage to the primary winding was controlled by a rheostat connected to 110-volt ac line current. L I and L? are 1-inch diameter fused quartz lenses with a focal length of 10 cm which were used to focus the image of the tungsten filament, first on the center of the flame and then on the entrance slit of the monochromator. The diaphragm D reduced the aperture of lens L? so that only a limited area of the flame would be observed at any time. The light from the tungsten source and the radiation emitted by the flame were synchronized by means of a Princeton Applied Research Model 222 variable speed chopper. The monochromator used was a Jarrel-Ash 0.5 Meter, Ebert Model 82020 equippe( with curved slits and a n 1180 line/mm grating blazed at 1900 A. The radiation leaving the exit slit was monitored by a 1P28 photomultiplier. The output from the photomultiplier was fed to a Princeton Applied Research Type A preamplifier and then to a Princeton Applied Research Model HR-8 lock-in amplifier which was tuned to the lamp and flame frequency with a reference frequency generated by the chopper. The output from the amplifier was displayed o n a Sargent-Welch Model SRLG recorder.
(1969).
(2) B. W. Bailey and J. M. Rankin, !bid., (8), 233-237. (3) B. W. Bailey and J. M. Rankin, ANAL.CHEM., 43, 216 (1971). (4) R . M. Fristrom and A. A. Westenberg, “Flame Structure,” McGraw-Hill Book Co., New York, N. Y., 1965.
( 5 ) J. A. Dean and T. C . Rains, “Flame Emission and Atomic
Absorption Spectrometry,” Vol. I, Marcel Dekker, New York, N. Y., 1969.
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