ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
1621
Nitrous Oxide-Hydrogen Flame in Atomic Absorption and Emission Spectrometry of Organic Solvent Systems R. J. Lukasiewicz Union Oil Company of California, Science and Technology Division, P.O. Box 76, Brea, California 92627
The nitrous oxide-hydrogen flame can be conveniently burned under substoichiometric conditions. Since this flame is strongly oxidizing, high mass transport of organic solvents can be tolerated without the formation of luminescent unburned carbon particles. Absorption by y NO bands In the flame falls in proportion to the amount of carbon in the aspirated solvent which reaches the combustion zone. Aspiration of a wide range of organic solvents at high uptake rates results in a relatively transparent and practical flame system for the analytical atomic spectrometry of nonrefractory elements. An advantage of this flame system is that, prior to analysis, many types of samples need not be diluted with solvents which are appropriate for use with hydrocarbon fuel flames. Decrease in y NO band absorption when solvents are aspirated is inversely proportional to the product of the heat of vaporization at the boiling point, AH:, and the boiling point temperature, T,, for nonpolar solvents. Good estimates for relative absorption sensitivity of Pb, Zn, and Cd in several solvents are obtained from the product of AH: and T,.
Analytical utility of the nitrous oxide-hydrogen flame in aqueous phase atomic spectrometry has been studied rather extensively (1-3). While this flame has a relatively low background, high temperature, and reasonably good sensitivity for easily atomized elements, poor atomization efficiency for refractory elements (2)has limited its usefulness as a general purpose flame in atomic spectrometry. Recently, a few studies which point out the advantages and some applications of the nitrous oxide-hydrogen flame in atomic spectrometry of nonaqueous systems have been reported (4-6). The widespread use of premixed laminar flow burners with hydrocarbon fuel flames has severely limited the utility of organic solvent systems in analytical atomic spectrometry. Although enhancement of sensitivity by organic solvents relative to water in atomic spectrometry is well known, the use of these solvents has been essentially restricted to polar compounds such as ketones, esters, and alcohols (7, 8). Enhancement of sensitivity in organic solvent systems is primarily due to the increase in the amount of sample reaching the flame (7). However, when hydrocarbon fuel flames are used, the amount of sample reaching the flame, even for ketones and alcohols, often exceeds the capability of the flame to completely combust the solvent and the familiar luminous, smoky flame results. Addition of auxiliary oxidant is helpful but is limited by the minimum limits of inflammability of the fuel with the oxidant used. Generally, even the most favorable organic solvents can be aspirated only a t uptake rates much lower than that which would give maximum sensitivity for the analyte element. Increase in sample uptake rate beyond that giving minimal background absorption often results in a degradation of the signal to background ratio rather than an improvement. In contrast, the substoichiometric nitrous oxide-hydrogen flame by virtue of the large excess of oxidant available and the lack of carbon contributed by the fuel is highly capable
of effecting complete combustion of many hydrocarbon solvents a t the maximum desirable uptake rates achievable with pneumatic nebulizers. Spectral transmission and emission properties of the nitrous oxide-hydrogen flame with high amounts of organic solvents being introduced compare quite favorably with commonly employed hydrocarbon flames when no solvent is introduced. Reported here are the unique properties of the substoichiometric nitrous oxide-hydrogen flame, the main spectral features of this flame with and without the aspiration of organic solvents; the use of volatile hydrocarbon solvents and the physical properties of these solvents which affect analyte element sensitivity. Under appropriate conditions, these features combine to greatly extend the usefulness of organic solvents in analytical atomic spectrometry.
EXPERIMENTAL Apparatus. Measurements were made using a Perkin-Elmer
Model 306 atomic absorption spectrophotometer, equipped with a deuterium background correction accessory. Light sources used were Westinghouse WL-36039 and WL-36081lead and zinc hollow cathode lamps, respectively, a Perkin-Elmer 303-6216 cadmium electrodeless discharge lamp and a Varian 6K527 hydrogen continuum lamp, all of which were run at the manufacturer recommended operating currents. Monochromator slit width was set so that the spectral band pass was approximately 0.2 nm. A standard 5-cm slot type titanium burner was positioned about 10 mm below the center of the light source image. Unless otherwise stated, the nitrous oxide flow rate used was 11.8 L/min and hydrogen flow rate was 3.1 L/min. Uptake rates of the various organic solvents used were set by adjusting the pneumatic nebulizer. Reagents. Organic solvents tested were of reagent grade quality and were used without further purification. Gasoline (Union Oil Co. of California, Los Angeles, Calif.) used contained no additives. Metallo-organic standards were obtained from National Spectrographic Laboratories, Cleveland, Ohio, as the metal naphthenates in a mineral oil matrix. Special naphtholite, a light C8-C, predominantly saturated hydrocarbon solvent was obtained from Union Chemical Division, Union Oil Co. of California, La Mirada, Calif. Procedure. Stock solutions for zinc, lead, and cadmium were prepared from the naphthenate concentrates by dissolving an appropriate amount of the naphthenate standards in special naphtholite. Dilute solutions of each element were prepared by diluting the stock solutions with the solvents tested such that the special naphtholite represented only 1% (vjv) of the solution aspirated into the burner. The absorption spectrum of the nitrous oxide-hydrogen flame was constructed point by point employing the experimental conditions as noted above. Since the instrument zero position on the absorbance scale was found to vary with wavelength, an error curve was generated. First, the monochromator was set to 195.0 nm with the hydrogen continuum lamp in place and the flame off. The instrument absorbance readout was then set to zero. The monochromator was advanced in 2.5-nm increments and the absorbance readout was recorded without adjusting the zero. Next, the monochromator was reset to 195.0 nm, the zero was reset, and the flame was ignited. Absorbance was read in 2.5-nm increments except in the regions where structure was observed. In those regions, absorbance was read a t 0.2-nm increments. The flame absorption spectrum was then generated
0003-2700/79/0351-1621$01 .OO/O 0 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
1.4
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Figure 1. Absorption by y NO bands in the nitrous oxide-hydrogen
flame by correcting the absorbance readings taken while the flame was burning by using the instrumental error curve obtained with the flame off. An absorption spectrum was thus generated without the need to extinguish and reignite the flame at each point in order to reset the instrument zero. Nebulizer uptake rates for the various organic solvents used were determined by measuring the time expired with a stopwatch and the volume delivered with graduated cylinders. For the higher uptake rates, larger volumes were used to ensure accuracy. Glass stoppered 10-mL graduated cylinders with 0.1-mL calibrations and 25-mL glass stoppered graduated cylinders with 0.2-mL calibrations were used t o minimize evaporative loss errors. Upper limit estimates of the delivery efficiencies for the solvents studied were obtained by the indirect method of comparing the volume of solvent drained from the spray chamber per unit time with the solvent uptake rate. Delivery efficiency is defined here as the amount of solvent reaching the flame relative to the amount of solvent aspirated. This value should not be confused with the nebulization efficiency which is based on the amount of analyte reaching the flame (9, IO). Nebulization efficiency for the analyte solutions is expected to be slightly lower than the delivery efficiencies reported here owing to partial evaporation of solvent from the mist droplets deposited on the walls. This evaporation causes the concentration of the analyte in the solution drained from the burner to be greater than the initial concentration of the aspirated solution (9, 10).
RESULTS AND DISCUSSION Some Spectral Properties of the Nitrous OxideHydrogen Flame. The most prominent spectral features of the nitrous oxide-hydrogen flame in the regions generally used for analytical atomic spectrometry are exhibited by the y NO absorption bands and the OH radical emission bands. Emission spectra of this flame have been well characterized ( I , 11) and are dominated by the strong 306.4-nm OH system. Emission bands of nitric oxide are much weaker by comparison. Absorption spectra of flames are generally more difficult to obtain experimentally because of the requirements for high spectrometer resolving power and high intensity light sources. Flame absorption spectra are not readily available for these reasons. Absorption into the y NO band system in the substoichiometric nitrous oxide-hydrogen flame is readily observed with conventional atomic absorption spectrophotometric equipment, because of the large excess of nitrous oxide supplied to the burner. The y NO band-system absorption as measured in this study is shown in Figure 1. This is a relatively low resolution spectrum which identifies only the major band heads and is void of any rotational fine structure. Bandwidth shown is limited by the slit function of the spectrometer used. In accordance with Gaydon ( I I ) , the absorption bands with the lower state vibrational quantum number u”, equal to zero show the strongest absorption. By
5.0 10.0 15.0 20.0 HEIGHT ABOVE BURNER (mm)
Figure 2. Absorption of the nitrous oxide-hydrogen flame at 205.2 nm as function of burner height
contrast, emission bands show the strongest intensity when the upper state vibrational quantum number u’ is low. An underlying continuum absorption is apparent in Figure 1. It is presumably due to an oxygen-containing species, possibly nitrogen dioxide since, as it will be shown below, absorption into the continuum decreases as the amount of organic solvent reaching the flame increases. Figure 2 shows the absorption of the 205.2-nm band as a function of height above the burner. These figures make it clear that care should be taken in setting the height of the burner in analytical spectrometry to avoid the strong increase in absorption by nitric oxide and other strongly absorbing species in the primary reaction zone of this flame. At a height of 10 mm, absorption by nitric oxide bands below 230.0 nm is strong enough to prove a substantial obstacle to analytical atomic spectrometry in the substoichiometric nitrous oxide-hydrogen flame. However, the y NO absorption can be reduced dramatically in two ways. The first is to increase the hydrogen flow rate and the second is to introduce organic solvents into the flame. The effect of the former on absorption by nitric oxide at three band peaks and five continuum wavelengths is shown in Figure 3. I t should be noted that even under slightly fuel rich conditions, a discernible nitric oxide absorption spectrum is observed. Upon introduction of organic solvents into the substoichiometric nitrous oxidehydrogen flame, a dramatic reduction in nitric oxide band absorption is observed. Nitric oxide and continuum absorption below 250.0 nm decreases as the amount of organic solvent reaching the flame is increased. This is in sharp contrast to hydrocarbon fuel flames where an increase in the amount of solvent reaching the flame leads to luminosity, severe scatter, and absorption due to unburned carbon particles. The substoichiometric nitrous oxide-hydrogen flame transparency increases and thus becomes more useful as an analytical flame a t high mass transport rates of carbon in the form of the sample solution. Decrease in nitric oxide absorption when organic solvents are introduced is illustrated in Figures 4, 5 , and 6. In Figure 4 decrease in nitric oxide and underlying continuum absorption are compared. Use of a lead line a t 205.3 nm which is close to the strong nitric oxide absorption band peak a t 205.2 nm in conjunction with the deuterium background compensation accessory allows correction for the continuum absorption underlying the nitric oxide band absorption. As expected, the percent decrease in the nitric oxide absorption shows the steepest slope and approaches 100% at the maximum carbon delivery rate used. Significantly, continuum absorption a t 200.0 nm decreases steadily which suggests the continuum absorption is due to an oxidizing species, possibly nitrogen dioxide, oxygen which
NO. 11, SEPTEMBER
ANALYTICAL CHEMISTRY, VOL. 51,
1979
1623
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Figure 3. Absorption of the nitrous oxide-hydrogen flame as function of hydrogen flow rate
Figure 6 . Comparison of continuum absorption of common flames, all 5-cm path length. (a) Nitrous oxide neat, at 11.8 L/min. (b) N,O/H, flame with solvent aspirated. (c) Air/C,H, flame, minimum absorption. (d) N20/C2H2flame, substoichiometric
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GRAMS CARBON DELIVERED TO FLAMEIMIN.
Figure 4. Decrease in absorption of the nitrous oxide-hydrogen flame as a function of amount of carbon delivered to flame. Organic solvent aspirated was gasoline. Upper curve represents nitric oxide absorption only. Middle and lower curves represent nitric oxide plus continuum and continuum absorption, respectively
shows band absorption up to 260.0 nm in hot flames ( I I ) , or a small equilibrium concentration of nitrous oxide. An increase of approximately 10% in absorption a t 200.0 nm was noted a t low delivery rates of solvent to the flame, similar to the increase noted when hydrogen flow is increased in Figure 3. Figure 5 shows the continuum absorption of the flame with approximately 10 mL/min of gasoline aspirated into the burner. Equivalent absorption of the flame a t the selected hollow cathode lines with the continuum absorption measured by a hydrogen source, indicates that little if any nitric oxide band absorption remains when mass transport of organic solvent to the flame is high. Continuum flame absorption
shown in Figure 5 is lower than the flame absorption shown in Figure 1 a t all points below 220.0 nm. Figure 6 compares the continuum absorption of the nitrous oxide-hydrogen flame with organic solvent aspirated as described in Figure 5 to neat nitrous oxide and two other hydrocarbon fuel flames. Nitrous oxide alone with no flame shows the highest continuum absorbance in the range studied. The absorbance of the nitrous oxide-hydrogen flame with solvent aspirated compares favorably with the minimum continuum absorbance possible with either the air or the nitrous oxide-acetylene flames in the ultraviolet region accessible to air path spectrometers. Introduction of volatile organic solvents a t the optimum uptake rates used in this study into the air-acetylene flame results in luminosity and severe spectral interference due to a broad band absorption, scatter, and emission by carbon-containing species. Although the nitrous oxide-acetylene flame is more tolerant toward the introduction of organic solvents than the air-acetylene flame, it also produces luminosity or intense emission from the secondary reaction zone at the optimum uptake rates for most organic solvents. Whereas the nitric oxide component of the nitrous oxide-hydrogen flame background decreases upon introduction of solvents, the OH component increases substantially. Figure 7 shows a gradual increase in OH emission a t the 306.4-nm band head with increasing hydrogen flow. At low hydrogen flow and with organic solvent aspirated, a dramatic increase in OH emission is noted which gradually decreases with increasing hydrogen flow rate. The curves intersect somewhat below the stoichiometric flame composition. The organic solvent aspirated was gasoline, a t 4.2 mL/min which is approximately the optimum flow rate for this solvent in terms of analyte sensitivity. Spectral band pass of the mono-
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 1 1 , SEPTEMBER 1979
Table I. Effect of Carbon Mass Transport on Flame Absorption at 205.2 nm solvent 2,3-dimethylpentane gasoline isooctane acetone n-octane methyl isobutyl ketone 2-propanol a
uptake rate, mL/min
delivery efficiency, %
density, 20 "C
11.8
54 49 49 53 20 20 35
0.695 0.751 0.692 0.791 0.703 0.801 0.785
11.1
11.3 11.9 11.4
10.6 5.85
grams carbon decrease in delivered/min absorption, %a 3.71 3.51 3.22 3.09 1.35 1.22 0.96
75 66 69 56 34 28 20
Relative to bare flame absorption with hydrogen flow rate 3.1 L/min. 10-cm single slot type burners. However, flashbacks did occur as a stoichiometric flame composition was approached. The flashbacks are due to the very steep increase in the burning velocity with fuel flow rate (15) and the larger slot width of the 10-cm burner. Under substoichiometric conditions, a very gentle lift-off of the flame occurs as hydrogen flow is decreased with either 5- or 10-cm slot burners. As stoichiometric flame composition is approached with the 10-cm burner, the minimum gas flow rate required to prevent flashback increases rapidly owing to the larger slot width of the burner, greater mass of the burner, and lower effective quenching diameter
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Flgure 7. OH radical emission of nitrous oxidehydrogen flame at 306.4 nm as function of hydrogen flow rate. (a) With solvent aspirated. (b) No solvent aspirated.
chromator was 0.07 nm and scans across the band head showed the rapid drop of emission to a very low level just off the peak which is indicative of sharp band or line emission rather than continuum. The increase in OH emission with increasing concentration of carbon containing species is in direct contrast to the case in hydrocarbon fuel flames. In hydrocarbon fuel flames, OH concentration falls as the carbon-containing species increase (12). Reaction of OH with free carbon precursor polyacetylenes is in fact postulated (13) as a mechanism for prevention of soot in hydrocarbon fuel flames. In the substoichiometric nitrous oxide-hydrogen flame, this role is played by nitric oxide. Introduction of solvents then, must act as a source of hydrogen in OH formation since emission increases at low hydrogen flow rates, but falls with increasing hydrogen flow. Absorption and emission by carbon-containing species such as CN, C2, CH, and free carbon are for all practical purposes restricted to the primary reaction zone as reported previously (4).Little or no secondary reaction zone is observed even when 10 mL/min or more of volatile organic solvent is aspirated into the burner. The characteristic blue continuum radiation generated by the CO and atomic oxygen association reaction predominates as solvents are burned, which is indicative of effective oxidation of the carbonaceous species. Analytical Utility of t h e Nitrous Oxide-Hydrogen F l a m e w i t h Volatile Organic Solvents. The nitrous oxide-hydrogen flame is an easy and convenient flame to use with most commercially available premixed type burners (1, 3,6).Maximum burning velocity of 390 cm/s (14) is relatively low compared with other flames potentially useful with organic solvents such as the oxy-hydrogen or oxy-acetylene flames which require specially designed burners for use with premixed flames. With the standard 5-cm slot type burner used in this study, no explosive flashbacks were encountered from the minimum limits of inflammability up to slightly hydrogen-rich conditions. The flame could be easily ignited and extinguished directly under the substoichiometric conditions employed without the necessity for using the air-hydrogen flame as an intermediate. This flame can also be burned on standard
(16).
When organic solvents are aspirated into the flame, very definite changes are apparent. The characteristic yellow-green continuum emission of the nitric oxide and atomic oxygen association reaction is replaced by the blue carbon monoxide atomic oxygen association reaction continuum emission. A violet primary reaction zone which is apparent when slight amounts of solvent are aspirated assumes an intense white appearance as the uptake rate of solvent is increased. At high uptake rates, the flame closely resembles a substoichiometric nitrous oxide-acetylene flame, and it has no visible secondary reaction zone. Changes in the nitrous oxide-hydrogen flame appearance are accompanied by a rather dramatic increase in flame temperature over the substoichiometric nitrous oxide-hydrogen flame. However, since the flame temperature changes are complex and vary with the solvent type and the uptake rates, quantitative measurements are reserved for future communications. For volatile organic solvents, optimum sensitivity for the analyte element was achieved with the minimum hydrogen flow rate necessary to sustain combustion ( 4 , 5 ) . In fact, it was found that when the hydrogen flow was shut off while solvent was aspirated, an organic solvent supported flame was sustained, and analyte absorbance increased. Thus, with solvent aspirated into the substoichiometric nitrous oxidehydrogen flame, the combustion process is more properly described as a hydrogen diluted organic solvent supported nitrous oxide flame. This effect is reversed in the case of relatively viscous solvents such as n-dodecane or diesel fuels. In such cases, analyte signal increases appreciably with increasing hydrogen flow rate. The average droplet size of these viscous liquids can be rather large, and desolvation of the sample is incomplete in the substoichiometric flame. As hydrogen flow is increased, the rise in flame temperature improves the efficiency of desolvation and atomization. A visible turbulence and unstable flame is observed with viscous liquids at low hydrogen flow rates. As the flow rate of hydrogen is increased, the turbulence is reduced and the flame assumes a more stable configuration. A correlation exists between the decrease in absorption of the nitrous oxidehydrogen flame at 205.2 nm and the amount of carbon delivered to the flame in the form of several different organic solvents. The organic solvents listed in Table I were aspirated into the flame at the uptake rates indicated. No adjustment was made to the pneumatic nebulizer between
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
1625
Table 11. Correlation between Flame Absorbance and Product of Heat of Vaporization and Boiling Temperature solvent 2,3-dimethylpentane isooctane cyclohexane benzene acetone toluene n-octane methyl isobutyl ketone nonane xylenes ethanol 2-propanol methanol n-decane n-dodecane n-butanol
Tb
(“c)“
89.8 99.2 80.7 80.1 56.2 110.6 125.7 118
150.8 139 78.3 82.3 64.7 160 214.5 116.8
aHVb(Cal/g)a (aHVb*Tb) x
72.9 64.9 85.6 94.1 124.5 86.8 73.2 85.6b 70.4 82 204.3 159.4 262.8 7 5b 62.9 141.3
6.5 6.4 6.9 7.5 7.0 9.6 9.2 10.1
10.6 11.4 16.0 13.1 17.0 12.0 13.5 16.5
decrease in absorption, %c 76 72 70 70 58 40
36 31 18 18 15 15 14
99 88 59
Heat of vaporizaa Physical data obtained from “Lange’s Handbook of Chemistry”, 11th ed., 1973, except as noted. tion estimated using Trouton’s Rule. Relative to bare flame absorption with hydrogen flow rate 3.1 L/min. introduction of the various solvents. Delivery efficiencies and absorbance a t 205.2 nm were measured for each solvent as described above. A reasonably good linear relationship exists between the decrease in absorbance and the amount of carbon transported to the flame. To a first approximation the correlation is independent of the type of hydrocarbon solvent aspirated since three classes of hydrocarbons are represented. A similar correlation could not be obtained between change in flame absorption and uptake rate, delivery efficiency, density, nor any other single physical property of the solvents. These data show that various classes of hydrocarbon solvents can be aspirated into the substoichiometric nitrous oxidehydrogen at high mass transport rates without carbon formation, and that absorption and equilibrium concentration of nitric oxide are proportional to the amount of carbon entering the flame. A correlation similar to the one mentioned above exists between decrease in flame absorption a t 205.2 nm and the product of the heat of vaporization at the boiling point, AH?, and the boiling point temperature, Tb, for relatively nonpolar organic solvents. The solvents listed in Table I1 were aspirated into the flame under maximum uptake rate conditions with no adjustment made to the nebulizer between introduction of the different solvents. A good linear relationship exists between the product ( m , b * T b )and flame absorption in a fashion similar to the relationship between flame absorption and the total weight of carbon delivered to the flame. Lemonds and McClellan (8)established a similar correlation for oxygenated polar organic solvents which essentially stated that the amount of solvent and dissolved analyte reaching the flame was proportional to the logarithm of the product of the viscosity and the boiling point temperature. For nonpolar hydrocarbons the amount of solvent reaching the flame at a given nebulizer setting can be predicted, and is most influenced by the heat of vaporization and the boiling point temperature. The alcohols listed in Table I1 and dodecane do not fit the linear correlation between the product (Ub’Tb) and decrease in flame absorption. For these solvents the physical properties which most influence delivery rates to the flame are viscosity and boiling point temperature (8,17). It should be pointed out that no single physical property nor any binary combination of physical properties of nonpolar solvents could produce a correlation similar to the one between the product (m:*Tb) and flame absorption. It will be further demonstrated below that to a first approximation both the carbon formation tendency of solvents and the expected analyte absorption sensitivities for solutes dissolved in
nonpolar organic solvents can be predicted from the product (myb*Tb). None of the solvents shown in Table I1 produced a luminous flame when aspirated into the substoichiometric nitrous oxide-hydrogen flame at the maximum uptake rate attainable with the nebulizer used in this study. A slight amount of free carbon separation could be observed just above the primary reaction zone for 2,3-dimethylpentane, only at the maximum uptake rate. By contrast, three isonieric hexanes, n-hexane, 3-methylpentane, and 2,3-dimethylbutane all produced intense luminosity from free carbon particles when aspirated into the nitrous oxide-hydrogen flame at the nebulizer setting used to gather the data presented in Table 11. The product of (m,b’Tb) for the three hexanes tested is 5.53 X lo3, 4.96 X lo3,and 4.44 x lo3, respectively. From Table I1 it can be seen that these values correspond to a greater than 80% reduction in flame absorbance a t 205.2 nm. As discussed above, absorbance at 205.2 nm consists of a combined absorption by a y NO band and an underlying continuum absorbance from an unidentified species. By consideration of the data presented in Figures 1 and 4, the underlying continuum absorption is estimated to be between 20 and 25% of the total at this wavelength. This means that nitric oxide absorption is essentially nil when solvents with (Uyb’Tb)of 6 X lo3 or less are aspirated under the conditions used to gather the data points shown in Table 11. Since the three isomeric hexanes with (H,b*Tb)values below 6 X lo3 produce an intensely luminous flame when aspirated at high uptake rates, the vital and unique role of nitric oxide in inhibiting carbon formation is demonstrated. It should be pointed out that carbon formation in the substoichiometric nitrous oxide-hydrogen flame first appears upon depletion of nitric oxide in spite of an apparent excess of molecular oxygen. For an initial substoichiometric nitrous oxide to hydrogen flow ratio of 4:1, it has been demonstrated (15) that the burnt gases contain slightly more than 20 mol % molecular oxygen. In the present study, hexane was aspirated into the substoichiometric flame under conditions producing intense luminosity. The uptake rate and delivery efficiency were measured and it was determined that carbon formation was observed even though a fivefold excess of oxygen was available if oxidation of the carbon in hexane to primarily carbon monoxide was assumed. This further emphasizes the importance of nitric oxide in preventing carbon formation, since under the conditions described above, nitric oxide concentration is essentially nil. Carbon formation in hydrocarbon air flames has been extensively studied. Reports by Street and Thomas (18)and
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Daniels (19) established that carbon separation and luminosity first appeared for many hydrocarbon-air flames at initial flame gas C/O ratios a t or below 0.5. Thus, carbon formation in the substoichiometric nitrous oxidehydrogen flame with aspirated organic solvent a t considerably substoichiometric C/O ratios reported in the present study is not unique, nor atypical, when compared with the findings for hydrocarbon-air flames. While the carbon formation tendency of the three isomeric hexanes tested is inversely proportional to (lH,b.Tb)a t high uptake rates, all three solvents can be aspirated into the flame without carbon formation a t reduced but analytically useful uptake rates. The threshold of carbon formation for the three hexanes was measured and found to be on the order of 5.4 mL/min. Under these conditions, delivery efficiency of the solvents to the flame was determined to be nearly 100%. Solvents with (lH,b.Tb) in the lower range such as 8.0 x lo3 and lower exhibit delivery efficiencies which range between 80 and 100% when aspirated a t uptake rates below approximately 5 mL/min. In these cases and to a lesser extent with solvents which have (AH,b.Tb) higher than 8 X lo3, appreciable evaporation of the liquid phase in the spray chamber is the most plausible explanation for the extremely high delivery efficiencies. Two expressions are commonly (8,ZO) used in estimating mass transport of sample to the flame when pneumatic nebulizers are employed in atomic spectrometry. They are the Poiseuille equation and the empirical expression derived by Nukiyama and Tanasawa (12). The Poiseuille equation relates the volume uptake rate of the aspirated liquid to the pressure drop across a capillary, the physical dimensions of the capillary, and the viscosity of the aspirated liquid. The empirical equation derived by Nukiyama and Tanasawa relates the mean droplet diameter of the aspirated liquid in the spray chamber to both the gas and liquid velocities and flowrates, and the density, surface tension, and viscosity of the aspirated liquid. For many of the solvents employed in this study, neither of these expressions is helpful in estimating the factors limiting the mass transport of solvent to the flame. The volume uptake rate of solvent predicted by the Poiseuille equation has little to do with the differences in the amount of solvent actually reaching the flame for many of the solvents studied here. The first six entries in Table I illustrate this point quite clearly. While the volume uptake rates are nearly constant, a t a given nebulizer setting, the delivery efficiencies and the decrease in flame absorption of the nitrous oxidehydrogen flame a t 205.2 nm when solvents are aspirated vary considerably. The viscosity of these compounds is quite low and thus is not a limiting factor in estimating mass transport to the flame. Several calculations of mean droplet diameters for the solvents studied here were also made using the Nukiyama and Tanasawa equation. While this equation strictly applies only to the specific nebulizer employed by Nukiyama and Tanasawa, it is often used to estimate general trends in mean droplet diameters for various solvents. For many of the solvents employed in this study the Nukiyama and Tanasawa equation did not predict sufficiently large differences in mean droplet diameters a t equivalent uptake rates, to account for the differences in delivery efficiencies and the differences in flame absorption given in Tables I and 11. In particular, for the isomers n-octane and isooctane, it is apparent from Table I that the Poiseuille equation which predicts liquid uptake rate does not account for the twofold difference in the delivery efficiencies nor the difference in flame absorption of the nitrous oxide-hydrogen flame when these two solvents are aspirated. Calculation of mean droplet size does not, in this case, indicate a trend or an expected difference in delivery efficiencies. The change in flame
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Figure 8. Absorbance as function of uptake rate for 1 mg/L Zn (top), 1 mg/L Cd (middle), and 10 mg/L Pb (lower). 0, ethanol: 0,2-propanol: A ,xylenes; 0 , methyl isobutyl ketone; 0, acetone: A,isooctane; . , 2,3-dimethylpentane
absorbance, however, is consistent with the correlation between flame absorbance and the product of AH$ and Tbas mentioned above. Since there appears to be no significant difference in the predicted mean droplet diameter between n-octane and isooctane, a t the uptake rate used and since the amount of solvent reaching the flame increases substantially for isooctane relative to n-octane, greater vaporization of isooctane in the mixing chamber (20) must account for the difference in flame absorption observed.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Optimum nebulizer uptake rate for the maximum absorption of analytes dissolved in the solvents tested here was approximately 3.5 mL/min and was essentially independent of both the solvent and the analyte. Absorbance for three analytes in selected solvents, as a function of uptake rate is shown in Figure 8. The product of AH> and Tb for each solvent correctly predicts the relative order of sensitivities for analytes dissolved in nonpolar solvents. Polar as well as nonpolar organic solvents are included in this comparison in order to illustrate the relative enhancement of absorption for solvents such as isooctane and 2,3-dimethylpentane in the nitrous oxide-hydrogen flame, over the more conventional solvents used in flame atomic spectrometry such as methyl isobutyl ketone and the alcohols. It is clear that mass transport of both solvent and dissolved analyte increases as the product of AH: and Tb decreases. For solvents with low m>*Tb,evaporation of the aspirated sample in the mixing chamber dominates the mass transport of solvent and analyte to the flame. For polar solvents, mass transport is more affected by viscosity, and application of the Nukiyama and Tanasawa and Poiseuille equations is more likely to approximate the actual conditions affecting sample delivery t o the flame.
at very high carbon mass transport rates. These factors make feasible direct analysis of petrochemical and petroleum products ( 4 , 5 )for trace nonrefractory analytes without prior dilution of the sample. Since optimum flow rates can be used without sample dilution, the practical detection limits are significantly lowered and burner operation is more stable and convenient.
LITERATURE CITED J. B. Willis, V. A. Fassel, and J. A. Fiorino, Spectrochim. Acta, Part 6 , 24, 157 (1969). L. DeGahn and G. F. Samaly, Spectrochim. Acta, Parts, 25, 245 (1970). R. M. Dagnall, K. C. Thompson, and T. S. West, Analyst(London), 93,
153 (1968). R. J. Lukasiewicz, P. H. Berens, and B. E. Buell, Anal. Chem., 47, 1045 (1975). R. J. Lukasiewicz and B. E. Buell, Appl. Spectrosc., 31, 541 (1977). J. C. M. Pau, E. E. Pickett, and S. R. Koirtyohann, Ana/yst(London), 97, 860 (1972). J. E. Allan, Specfrochim. Acta. 17, 467 (1961). A. J. Lemonds and B. E. McClellan, Anal. Chem., 45, 1455 (1973). C. Th. J. Akemade, "From Sample to Sinal in Emisskn Flame Photomeby; An Experimental Discussion", in "Analytical Flame Spectroscopy", R. Mavrodineanu, Ed., Springer-Verlag New York Inc., New York, 1970, Chapter 1, p 9. "Nomenclature, Symbols, Units and Their Usage in Spectrochemical Analysis-Part 111" in Pure Appl. Chem., 45, 99, 105 (1976). A. G. Gaydon, "The Spectroscopy of Flames", 2nd ed., Chapman and Hall, London, 1974, pp 123, 369, 373, 175. J. 0. Rasmuson, V. A. Fassel, and R. N. Kniseley, Spectrochim. Acta, Part 6 , 2 8 , 365 (1973). K. H. Homann. Combust. Flame. 11, 265 (1967). J. B. Willis, Appl. Opt., 7, 1295 (1968). J. W. Armitage and P. Gray, Combust. Flame, 9, 173 (1965). K. M. Aldous, B. W. Bailey, and J. M. Rankin, Anal. Chem., 44, 191 (1972). J. H. Culp, R. L. Windham and R . D. Whealy, Anal. Chem., 43, 1321 (1971). J. C. Street and A. Thomas, Fuel, London. 34, 4 (1955). P. H. Daniels, Combust. Flame, 4, 45 (1960). G. F. Kirkbright and M. Sargent, "Atomic Absorption and Fluorescence Spectroscopy", Academic Press, New York, 1974, p 324. S.Nukiyama and Y. Tanasawa, Trans. Soc. Mech. Eng., Jpn., 5 , 68 (1939).
CONCLUSIONS The substoichiometric nitrous oxide-hydrogen flame extends the analytical utility of flame atomic spectrometry to include a broad range of organic solvents not previously feasible with hydrocarbon flames. Significant evaporation of nonpolar solvents in the mixing chamber allows high mass transport of sample to the flame. The relative analytical sensitivity of analytes in various solvents can be predicted by the product of the heat of vaporization a t the boiling point and the boiling point temperature. Nitric oxide plays a unique role in preventing excess luminous carbon formation, and luminosity in this flame coincides with depletion of nitric oxide
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RECEIVED for review February 26, 1979. Accepted May 30, 1979.
Signal Saturation Effect and Analytical Techniques in Photoacoustic Spectroscopy of Solids John W-p. Lin" and Lesley P. Dudek Webster Research Center, Xerox Corporation, Webster, New York
Signal saturation effect poses a serious problem for photoacoustlc spectroscopy (PAS) of solids if an excessive amount of materlal is employed. I t is possible to minimize this undesirable effect by using an optimum sample size and proper sample preparation techniques. Several methods are developed and successfully applied to the characterization of many strong absorbers Including tetraphenylporphine, cobalt tetraphenylporphlne, and rose bengal at a mlcrogram quantity. I f a material participates predominantly in a nonradiative type of decay, the optical absorption spectrum obtained by PAS is strlkingly slmllar to the one recorded by the conventlonal transmission spectroscopy.
The photoacoustic (PA)effect was discovered and reported in 1881 by Tyndell ( I ) , Rontgen (Z),and Bell ( 3 ) . Recently, the use of this interesting phenomenon to study the optical 0003-2700/79/0351-1627$01 .OO/O
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properties of many solid samples was significantly advanced by A. Rosencwaig (4-7). Since then, there has been a considerable interest among the scientific community to employ photoacoustic spectroscopy (PAS) as a new analytical tool for studying a wide variety of materials in the form of powders, smears, semisolids, and biological samples (8-15). The new technique has a number of potential applications in diverse fields where conventional spectroscopy such as transmission spectrometry and reflectance spectrometry cannot be easily employed. Light absorption by a material usually leads to two types of de-excitation processes, namely radiative and nonradiative decays. The former process involves either fluorescence singlet) or phosphorescence (triplet (excited singlet singlet). The nonradiative decay process converts the light energy into heat. When a sample surrounded by a nonabsorbing inert gas is illuminated in an enclosed cell with a chopped (or modulated) monochromatic light, periodic light
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0 1979 American Chemical Society
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