Evaluation of Premixed Flames Produced Using a Total Consumption Nebulizer Burner in Atomic Fluorescence Spectrometry M. P. Bratzel, Jr., R. M. Dagnall,' and J. D. Winefordner Department of Chemistry, University of Florida, Gainesville, Fla. 32601
Premixed and unpremixed hydrogen-based flames (air/Hz, A/H2, Oz/HP, and NzO/Hz) and a premixed Nz0/C2Hz flame produced using a total consumption nebulizer-burner were used as atomizers for several elements measured using atomic fluorescence spectrometry. The premixed flames were produced by mixing the fuel and oxidant prior to introduction into the fuel and oxidant ports of the burners. The unpremixed flames were produced by the mixing of the fuel and the oxidant beyond the burner orifice. The premixed flames were considerably more laminar and of higher temperature than the corresponding unpremixed flames. The premixed and the unpremixed Nz0/H2flames seem to be the most generally useful hydrogen based flames for atomic fluorescence spectrometric studies of most elements. (The N20/Hz flames have a very uniform temperature over a large part of the flame and a relatively low flame background and noise.) However, if serious chemical interferences are present-e.g., silicate-then the Nz0/CzH2 flame should be used.
RECENTSTUDIES from this laboratory have indicated that unpremixed hydrogen based diffusion flames produced using the total consumption nebulizer burner offers the basis for a suitable atom reservoir for atomic fluorescence spectroscopic analysis ( I ) . Attention to choice of fuel and oxidant mixtures can lead to an increase in the overall atomization efficiency of this type of system and result in a reduction in chemical interference effects (2). The use of a total-consumption nebulizer-burner producing an unpremixed diffusion flame is advantageous for several reasons: ease of operation, freedom from flashback, and minimal carbon buildup when using hydrocarbon fuels. The use of premixed laminar flames is advantageous for three principal reasons: minimization of chemical interferences, minimization of flame noise, and greater uniformity of flame characteristics. This communication describes the application of premixed flame using a total consumption nebulizerburner to atomic fluorescence spectrometry combining the advantages of the unpremixed diffusion and the premixed laminar flame types. Such a burner-flame system was recently used by Mossotti and Duggan (3) for atomic emission analysis using a N20/C2H2flame in which the oxidant and fuel gases were premixed prior to their exit from the burner orifices. They found that the resulting flame gave well-defined reaction zones, reduced flame background and background noise, and sensitivities in atomic emission analysis similar to other types of premixed flames described in the literature. 1 On leave from Imperial College, London S.W. 7, United Kingdom.
(1) M. P. Bratzel, Jr., R. M. Dagnall, and J. D. Winefordner, ANAL. CHEM., 41, 713 (1969). (2) M. P. Bratzel, Jr., R. M. Dagnall, and J. D. Winefordner, unpublished work, University of Florida, Gainesville, Florida, 1969. (3) V. G. Mossotti and M. Duggan, Applied Optics, 7, 1325 (1968).
The fundamental studies described in Part I of this communication refer to hydrogen based flames which have relatively low background emission and so are commonly employed in atomic fluorescence spectrometry. Part I1 describes the use of the premixed NzO/CZHZ flame for the same technique. PART I. SOME HYDROGEN SUPPORTED PREMIXED FLAMES
I. Experimental Conditions and Results. A. CHARACOF PREMIXED HYDROGENBASED FLAMES. The premixing arrangement employed is similar to that used by Mossotti and Duggan (3) except that a Zeiss total consumption nebulizer-burner is used in our studies. The fuel is monitored by a pressure regulator after which it is split into two flows which are monitored by rotameters. The oxidant is similarly split and monitored in an identical manner. The fuel and oxidant are premixed using two mixing chambers (Tescom Corp., Minneapolis, Minn.-Type Tescom 11-1101) and are then passed into each burner port. Each mixing chamber contains two inlet ports, a needle valve for each port, and one exit port. The premixed gases are then passed into the two ports of the burner. Premixed flames of argon, air, oxygen, or nitrous oxide with hydrogen are used for comparison with the corresponding unpremixed flames with respect to structural characteristics and utilization in atomic fluorescence spectrometry. Entrained air from the atmosphere contributes a large fraction of the oxidant in all flame studies. Because of the similarity of the unpremixed air/Hz and A/Hz and also the premixed air/Hz and A/Hz flames, these flame types are employed interchangably in some of the following studies. Gas flow and pressure conditions are chosen such that maximal atomic fluorescence sensitivity is attained. The hydrogen pressure in all studies is 16 psi (the maximum investigated in this system) and the oxidant pressure is between 16 and 23 psi. Gas flow rates are measured with a wet test meter (Precision Scientific Co., Chicago, Ill.). Total premixed gas flow rates emerging from the burner orifice are also measured. The measurement of each separate gas flow rate however presents certain problems. For example, if each gas flow rate is measured, then the total gas flow rate through the premixing system is less than the individual sum because of the back pressure exerted by each gas in this completely interconnected system. It is also not possible to measure each gas flow rate prior to entry into the premixing chambers because of the high pressure existing in each line. Therefore, to reproduce experimental conditions, it is simplest to use pressure gauge settings rather than a complex series of flow rates. Hence, all flame gas conditions are reported in terms of the line fuel and oxidant pressures. In some instances the total gas flow rate is also quoted. Higher gas flow rates were used in our study than by Mossotti and Duggan (3). This resulted in increased sensitivity in atomic fluorescence spectrometry. TERISTICS
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Figure 1. Temperature profile nitrous .oxidehydrogen unpremixed and premixed flames with no water nebulizing
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HEIGHT ABOVE BURNER (CM) Figure 2. Temperature profile air-hydrogen unpremixed and premixed flames with no water nebulizing (a)
Premixed flame with air pressure 20 psi, Hz pressure 16
psi (b) Unpremixed flame with air flow rate 5.9 I. min-l, HZ flow rate 14.9 1. min-1
(a) Premixed flame with Nz0 pressure 20 psi, Hz pressure
16 psi (b) Unpremixed flame with NzO Row rate 4.8 1. min-l, Hzflow rate 14.9 1. min-1
Independent variation of the oxidant and fuel pressure cia the regulators or the needle valves of the premixing chambers resulted in a more turbulent and more noisy flame and a decrease in the atomic fluorescence signals of all those elements examined. Optimum conditions were obtained when the divided line pressures to both premixing chambers for oxidant and fuel gases were equal and when the needle valves on the premixing chambers were fully open. With an unpremixed burner system in which each gas enters the burner inlet ports separately, individual gas flow rates are quoted as well as pressure gauge settings. The Beckman hydrogen and acetylene total consumption nebulizer-burners gave more turbulent and less uniform flames than the Zeiss burner both in the unpremixed and premixed modes of operation. Like the conventional unpremixed turbulent hydrogen based flames, the premixed flames are simple to operate and
completely safe with respect to flashbacks under all gas flow conditions used in our studies. With this type of system we have been able to maintain stable premixed flames (with or without nebulization of water) of hydrogen, acetylene, or carbon monoxide with air, argon, oxygen, or nitrous oxide. B. Flame Characteristics. The flame spectra for several unpremixed air/Hz, Oz/H?, and NzO/HZflames are quite similar. With no water nebulizing, the flame background intensity at all wavelengths is in the order OZ/HZ>> NzO/H2 > air/Hz; this background is primarily due to OH emission. When water is nebulized, the flame size and shape are markedly affected in the 02/Hzand the air/Hz flames but not the NzO/H2flame. The background order then becomes N20/ Hz > OZ/Hz> air/Hz. Because of the smaller Pame size and the higher rise fielocity of the unpremixed OZ/Hzand the air/Hz flames compared with the NzO/Hzflame, the atomization efficienciesof the former are poorer. With the unpremixed NzO/HZflame, the flame size is not appreciably changed upon nebulization of water (because of lower solution transport rate); thus there is a larger flame volume and also a longer residency time of the sample in the
Table I. Some Unpremixed and Premixed Hydrogen Based Flames Pressure Oxidant Total Flame heightb flow Without Pressure Flow rate Flame Pressure Flow rate psi 1. min-l 1. min-1 With HzO H10 cm Typea psi 1. min-1 14 5 10 16 ... 20 ... P A/Hz 14 5 9 20 ... P Air/Hz 16 ... 14 5 7 16 ... 20 ... P Oz/Hz 14 13 13 16 ... 20 ... P NzO/Hz 23.9 10 11 ... 14.9 ... 9.0 U A/Hz 24.0 10 11 ... 14.9 ... 9.1 U Air/Hz 23.7 11 13 ... 14.9 ... 8.8 U Oz/Hz 20.5 12 12 ..* 14.9 ... 5.6 U NzO/Hz = P = Premixed flame; U = Unpremixed flame. b Approximate visible flame height.
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rate Solution transport ml min-l 2.15 2.23 2.10 2.46 5.40 5.45 5.30 4.18
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BURNER HEIGHT (CM) Figure 4. OH flame profile at 3064
BURNER HEIGHT (CM) Figure 3. NH flame profile at 3360
6
Premixed N20/Hz with water nebulizing (b) Premixed NzO/H2without water nebulizing (c) Unpremixed NzO/HZwith water nebulizing ( d ) Unpremixed N20/H2without water nebulizing Flame conditions same as Figures 1 and 2
(a)
flame, and greater aspiration and atomization efficiency should result with the unpremixed NzO/H2flame compared to the other unpremixed flames studied. The flame spectra for several premixed air/H?, O Z / H ~and , N20/H2 flames are also quite similar when compared without and also with water nebulizing. These flames are considerably less fuel-rich than their unpremixed counterparts. The total gas flow rates result in a low solution transport rate into the flame, and hence the flame structure is only slightly changed upon nebulization of water, in contrast with the air (oxygen) based unpremixed flames. The various flames used in this study are listed in Table I. The N20/Hzunpremixed and premixed flames had essentially the same background intensities at a height of 6 cm above the top of the burner in the dry flames. Nebulizing water produced about a 2-fold decrease in both flames at all wavelengths. The 02/H2premixed flame with water being nebulized had about a IO-fold greater background radiation (at the OH band heads at 281 1 and 3064 A) compared to the unpremixed flame, and about the same background radiation in the dry flames. Nebulization of water into the premixed O2/H2 flame had a relatively small effect on background and exhibited no "side splash" due to unevaporated water droplets. In Figures 1 and 2, flame temperature profiles are given for several premixed and unpremixed N20/H2 and air/Hz flames used in atomic fluorescence spectrometric measurements. The temperatures for the low temperature (less than about 2650 "K) hydrogen based flames are measured using thermocouples as described by Smith et at. (4). The O2/H2 flame (4) R. Smith, C. M. Stafford, and J. D. Winefordner, ANAL.CHEM., 41, 946 (1969).
(a) Premixed N20/H2with water nebulizing (b) Premixed N20/H2 without water nebulizing
Unpremixed N20/H2with water nebulizing (6) Unpremixed N20/H2 without water nebulizing (c)
and several of the NzO/HZflames used in this study destroyed the thermocouples and could not be measured in this way. The premixed N20/H2flame used in our studies had a maximum temperature of 2630 =t50 O K without nebulizing water and 2530 + 50 OK with water nebulizing and a fairly constant temperature occurred over flame heights of 3.5-6.0 cm. On the other hand, the plateau of high temperature for the unpremixed N20/H2flame was higher in the flame-Le., at about 6.0-11.0 cm. Therefore, longer atom residence time in the flame, lower background radiation, and lower scatter from nonvaporized water droplets and solute particles resulted at the higher heights and so the unpremixed N 2 0 / H 2 f l a m ewould appear to offer the most adcantages in atomic fluorescence spectrometry. In Figures 3 and 4, spectral emission profiles of the N H and OH bands are given for the unpremixed and premixed Nz0/H2 flames (with and without water nebulizing), respectively. The peaks of the N H and OH emission profiles do not correspond to the regions of highest temperature or to any visibly obvious portion of the flames; for example, the luminous primary reaction zone extended to about 3.5 cm above the burner tip in the premixed flame and to about 4.5 cm in the unpremixed flame. The premixed air/Hp flame temperature profile (maximum temperatures and length of constant temperature plateau) differed considerably from the premixed N20/H2 flame. Comparing the premixed air/H2 flame with its unpremixed counterpart, the maximum temperature is slightly greater and also the temperature is greater at lower regions of the flame. C. Optimum Flame Operating Conditions. In Table 11, the optimized experimental conditions are given for the measurement of atomic fluorescence intensities of a selected range of elements for each premixed flame investigated. The monochromator slit width and operating conditions for the unpremixed flames and the electrodeless discharge tube VOL. 41, NO. 12, OCTOBER 1969
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Table 11. Operating Conditions for Hydrogen Flames in Atomic Fluorescence Spectrometry
Element Cd
Pressure (psi) Oxidant Hydrogen
Solution transport rate (ml min-1)
Flame type” Burner heightb (cm) P A/Hz 23 16 2.57 ... P Air/Hz 23.5 16 2.62 ... P NzO/Hz 23 16 2.86 ... P Oz/Hz 25 16 2.67 Fe P AiHz 19 16 1.51 .,. P Air/Hz 19 16 1.42 .., P NzO/Hz 18 16 1.42 ... P Oz/Hz 15 16 1.34 Ga P A/Hz 17 16 1.51 ... P Air/Hz 17 16 1.42 ... P NzO/Hz 15 16 1.42 ... P 0dHz 15 16 1.34 20 16 Mg P AiHz 2.15 ... P Air/Hz 20 16 2.23 ... P NzO/Hz 20 16 2.46 *.. P 0dHz 18 16 2.00 TI P A/Hz 20 16 2.15 ... P Air/Hz 20 16 2.23 ... P NzOiHz 20 16 2.46 ... P 02,”~ 20 16 2.25 See footnote (a) in Table I. Optimum height for measurement of atomic fluorescence in unpremixed flames. Other optimum conditions given in other references (1, 5). c
Values in parentheses are for unpremixed flames; (-) denotes negligible fluorescence.
sources are as previously given ( I , 5). The various solution transport rates into the flames are also included for completeness. D. Atomic Fluorescence and Emission Height Profiles. The atomic fluorescence and emission intensities are measured for each element listed in Table I1 at various flame heights for both premixed and unpremixed flames. The concentration of the element is chosen such that it is representative of the linear portion (signal proportional to concentration) of the analytical working curve for optimum experimental conditions. 1. CADMIUM.In both premixed and unpremixed air/Hz and N20/Hz flames, the atomic fluorescence signal increased with increasing height of measurement in the flame to about 10 cm above the top of the burner head after which it decreased steadily. In Figure 5, the atomic fluorescence and total scatter profiles for Cd in the NzO/HZflames are given. Decreased scatter from water droplets with increasing height of measurement is observed in all flames examined. At a height of 10 cm, the scatter signal recorded is entirely due to source reflection from the burner and from the baffle box surrounding the flame (At burner heights greater than 6 cm, the scatter from nonvaporized water droplets is negligible.). The 0,/H2 flames gave similar curves except that the maximum atomic fluorescence intensity occurred lower (ca. 8 cm) and is flatter than for air/Hz and N20/Hz flames. The peak fluorescence signal in all hydrogen flames is relatively insensitive to flame height (6-12 cm), but does fall off more rapidly at lower heights (below 6 cm) than at higher heights (greater than 12 cm). In all flames, greater sensitivity is obtained in the unpremixed flames because of a greater solution transport rate into the flame. No atomic emission is observed in any of the flames examined. 2. IRON.Iron in the unpremixed air/Hz and NzO/Hz flames exhibited maximum atomic fluorescence intensities at a ( 5 ) K. E. Zacha, M. P. Bratzel, Jr., J. D. Winefordner, and J. M.
Mansfield, Jr., ANAL.CHEM., 40, 1733 (1968). 1530
ANALYTICAL CHEMISTRY
flame height of about 4-5 cm. The unpremixed Oz/Hzflame gave negligible atomic fluorescence indicating negligible atomization. All premixed flames (including the Oz/H2flame) showed an atomic fluorescence intensity maximum at a flame height of about .6 cm. The maxima of the intensity height profiles generally are flatter for the premixed flames than for the unpremixed flames. In general, the atomic fluorescence signal in all flames decreased rapidly (compared to the results in Figures 5-7) at the higher flame heights. Atomic emission is observed in all flames, but was considered to be too small for quantitative purposes. 3. GALLIUM.Gallium in the unpremixed air/Hz flame exhibited maximum atomic fluorescence and emission signals at 3.5-4.0 cm and about 3.5 cm, respectively, whereas the unpremixed and premixed N20/H2flames showed corresponding maxima at about 6 and 7 cm, respectively. The height profile of the premixed N2O/Hz flame is flatter than for the unpremixed flame. In both flames, the atomic fluorescence signal is less sensitive (flatter) to height than the atomic emission signal. In the unpremixed 02/H2 flame, the atomic emission maximum occurred at about 3.5 cm and, in contrast to the other flames studied the profile is quite broad. Negligible atomic fluorescence was observed in this flame. The premixed Oz/Hz flame showed little atomic fluorescence or emission. 4. MAGNESIUM. The atomic fluorescence height profiles (see Figure 6) for magnesium exhibited a maximum at 8-9 cm for the unpremixed N20/H2flame and about 6 cm for the corresponding premixed flame. The curves for the premixed and unpremixed 02/H2flames are shown in Figure 7. The atomic emission signals in the NzO/H2flames were extremely small compared with the atomic fluorescence signals, but considerable atomic emission was observed in the premixed OZ/HZflame. 5. THALLIUM.The atomic fluorescence height profile exhibited a maximum at about 9 cm in the unpremixed NzO/ Hz flame and at about 7 cm in the premixed N20/H2 flame. The atomic fluorescence profile in the unpremixed Oz/H2flame
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Figure 5. Cadmium atomic fluorescence intensity profiles in premixed and unpremixed nitrous oxide-hydrogen flames
Figure 6. Magnesium atomic fluorescence intensity profiles in premixed and unpremixed nitrous oxide-hydrogen flames
Atomic fluorescence in optimized NzO/HZ premixed flame (b) Atomic fluorescence in optimized NzO/HZ unpremixed flame (c) Scatter in N20/H2premixed flame (d) Scatter in N20/Hzunpremixed flame
(a) Atomic fluorescence in optimized NzO/Hzpremixed flame (b) Atomic fluorescence in optimized NzO/Hzunpremixed flame Both curves measured using same instrument sensitivities, etc
(a)
is almost constant over the range 4.5-7.0 cm, although it peaked sharply at about 6.0 cm in the corresponding premixed flame. Atomic emission profile maxima occurred at 10-11 cm in the unpremixed NzO/H2flame and at about 4.5 cm in the unpremixed Oz/H2flame (There was negligible emission at 10 cm in this flame). In both the premixed NzO/HZand 02/H2 flames, the atomic emission maxima occurred at about 6 cm. E. Shapes of Atomic Fluorescence Analytical Working Curves for Magnesium in Unpremixed and Premixed Hydrogen Flames. The analytical curves for magnesium over the concentration range 0.05-100 pg/ml for the argon-, oxygen-, and nitrous oxide-hydrogen unpremixed and premixed flames exhibited linearity from the limit of detection to approximately the maximum of the curves. At concentrations below that corresponding to the maximum, the slope is positive and at concentrations above the maximum, the slope is negative. The magnesium concentration corresponding to the maximum of each curve is given in Table 111. F. Magnitude of Atomic Fluorescence and Emission Signals. The maximum atomic fluorescence signals for 5 pg/ml of magnesium result in the A/Hz unpremixed flame (because of a higher solution transport rate into the flame than with other flames). The other hydrogen flames (except for the unpremixed Oz/Hz flame) give similar signals-Le., fluorescence signals are within a factor of two. The atomic emission signals are considerably lower than the atomic fluorescence signals in all hydrogen flames. The maximum atomic emission signal occurs in the premixed Oz/Hzflame. The atomic fluorescence signals of cadmium in the A/Hz, air/Hg, and Oz/Hz unpremixed flames are about the same, whereas the atomic fluorescence signals in the corresponding premixed flames as well as the NzO/Hzunpremixed and premixed flames are about two-to-three-fold lower. No atomic emission was observed in any of the hydrogen flames. All
Figure 7. Magnesium atomic fluorescence intensity profiles in premixed and unpremixed oxygen-h ydrogen flames Atomic fluorescence in optimized OZ/ Hzpremixed flame (6) Atomic fluorescence in optimized 0 2 1 Hzunpremixed flame (c) Atomic emission in optimized O Z / H ~ premixed flame (d) Atomic emission in optimized OZ/HZ unpremixed flame Curves b and d measured using 10 times sensitivityfor curves a and c (a)
cadmium atomic fluorescence measurements are taken at a 10.0-cm height. Magnesium and cadmium atomic fluorescence and emission signals are summarized in Table IV, where the data have also been normalized to unit solution uptake rate. A significant factor in emission measurements is the peak-topeak noise at the wavelength of measurement. In general, the premixed flames are more uniform in appearance, possess less flame flicker, and have about 50x lower peak-to-peak noise than the corresponding unpremixed flames. G. Chemical Interferences for Magnesium in Unpremixed and Premixed Hydrogen Flames. The influence of aluminum, barium, phosphate, and carbonate ions (90 and 900 pg/ml) on the atomic fluorescence intensities of magnesium (2 pg/ml) was investigated (Table V) in several premixed and unpremixed flames (the concentration of magnesium is such that it fell on the linear working range of the analytical curves in all flames used). The above extraneous ions were selected for study
Table 111. Maxima in Analytical Working Curves for Magnesium Atomic Fluorescence Solution transport Conc of Linear rate Mg at working into maximum range flame Flame type (rg/ml) (rglml) ml fin-' A/Hz unpremixed 5 0-1 5.88 premixed 20 0-5 2.15 O2/Hz unpremixed >100 0-20 5.66 premixed 20 0-5 2.00 N80/Hzunpremixed 10 0-2 3.01 premixed 10 0-2 2.46
VOL. 41, NO. 12, OCTOBER 1969
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less dependency upon height of measurement in the flame; reduced flame-flicker noise a t the heights of measurement; appreciable air entrainment occurs above the height of measurement [observed from shadowgrams
Table IV. Relative Atomic Fluorescence and Emission Signals in Various Flamesa
Relative signals (arbitrary units)
(6)1 ;
Flame type A/Hz
unpremixed premixed Air/Hz unpremixed premixed OZ/HZ unpremixed premixed NzO/HZunpremixed premixed
Fluorescence Emission 1.oo 1.57
0.02 0.04
...
... ...
0.07 1.43 1.79 1.50
0.00 0.16 0.07 0.06
...
fluorescence
.oo
1 0.97 0.93 1.10 0.97 1.oo 0.93 1
.oo
a All magnesium signals are normalized to the magnesium fluorescence signal in the unpremixed A/Hz flame. All cadmium signals are normalized to the same flame, but optimized for cadmium fluorescence. All values have been normalized to unit solution uptake rate. Atomic fluorescence and emission measurements taken at 9.5crn height in unpremixed flames and at 6.0 cm in premixed flames. Atomic fluorescence measurements taken at 10 cm in all flames. No measurable atomic emission occurred in any of the flames.
because they had been found previously to interfere in unpremixed flames. The most efficient flame for eliminating the interference effects is the premixed Oz/Hz flame in which there is little reduction in the fluorescence intensity in the presence of barium, carbonate, or phosphate and only a slight reduction in the presence of aluminum. The decrease in chemical interferences in the 0 2 / H 2flame is related to the high temperature of the premixed 02/H2flame rather than to a reducing atmosphere. In several instances greater interferences result with the more dilute interferent (This is not a result of differences in solution transport rate into the flame). 11. Analytical Use of Hydrogen Based Premixed Turbulent Flames in Atomic Fluorescence. The hydrogen supported premixed flames do offer several advantages compared with the corresponding unpremixed flames f o r atomic fluorescence spectrometry. The principal advantages are: (i) increased maximum temperature; (ii) reduced chemical interferences and reduced physical interferences due to reduced solution flow rate ; (iii) increased aspiration and atomization efficiency, especially with premixed 02/Hzflames. However, for elements which are readily atomized there is no apparent gain in sensitivity;
less variation in flame size, height, and temperature upon nebulizing water; and less audible noise. The principal advantage however of the premixing arrangement is its application to the NzO/C2H2flame. Without premixing N 2 0and C2H?gases, a poorly defined flame is obtained which shows practically no “red feather” (CN emission zone) and which has a tendency to lift o f f upon nebulizing water. This flame and its use in atomic fluorescence spectroscopy is described in Part I1 because it differs substantially from those flames examined above. PART 11. THE NzO/CzHz PREMIXED FLAME
I. Experimental Conditions and Results. A. FLAME OPERATING CONDITIONS. The premixing arrangement described in Part I was used without alteration. Flames of Nz0/CzH2could be obtained from very lean-e.g., NzO gas pressure of ca. 17 psi and CzH2gas pressure of ca. 6 psi-to extremely fuel-rich-e.g., NzO and C2H2gas pressures of 17 and 11 psi, respectively. A C2H2pressure of ca. 8 psi produced a flame with a well-defined interconal “red feather” (CN) region stretching some 4 cm above the primary reaction zone. In very fuel-rich flames, the CN region was visible to heights up to and greater than 6 cm above the primary reaction zone. Upon nebulizing water, the interconal zone was slightly reduced in size and some “side splash” was noted, although in our experimental arrangement effects from this were completely eliminated when the exhaust system attached to the flame housing was in operation. Shadow photographs (6) indicated these premixed flames to be quite laminar and nonturbulent up to a height of about 6 cm above the top of the burner head. In Figures 8-11, the variation in emission signal with flame height is given for the principal emitting species, viz. CN, CZ, N H , and O H in the fuel-rich premixed N20/CzHz (17 and 11 psi, respectively) flame. These figures indicate the relative change in intensity of these species from a fuel-rich flame to a fuel-lean flame, All measurements are obtained using the same instrument sensitivities and with and without nebuliza(6) M. P. Bratzel, Jr., R. M. Dagnall, W. P. Townsend, and J. D.
Winefordner, unpublished work, University of Florida, Gainesville, Fla. 1969.
Table V. Effect of Some Extraneous Ions on Atomic Fluorescence of Magnesium in Unpremixed and Premixed Flames. AlHz Oz/Hz NzO/Hz P U P U P U Ion 1
(1
.oo
1.oo
1.oo
0.70 0.40 0.35 ~ 1 3(90 + pgirnl) 0.60 0.35 0.30 ~ 1 3 (900 + pg/ml) ... 0.70 0.75 Baa+ (90 pg/ml) 1.15 0.55 0.70 Baz+ (900 pg/ml) ... 0.10 0.15 P04a- (90 pg/ml) 1.05 0.15 0.15 POP- (900 pg/ml) ... 0.10 0.15 cos2-(90Pg/d) 0.95 0.10 0.15 COa2- (900 pg/rnl) All signals normalized to solution containing 2 pg/ml magnesium only; error &0.02.
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1 .oo 0.50 0.35
...
0.75
...
0.65
... 0.75
1.oo 0.60 0.60 1 .oo 1.oo 0.55 0.70 0.45 0.60
1 .oo 0.50 0.60 85 0.90 0.60 0.75 0.60 0.75
10-1
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Figure 10. Vertical emission profilesNH (3360 A) ~
Figure 8. Vertical emission profilesCN (3883
k)
0
(a) Fuel rich flame (NzO 17 psi, - . C2HZ
ii psi) (b) Fuel rich flame (NzO17 psi, CZHZ 11 psi), HzOnebulizing (c). Lean flame ( N 2 0 17 psi, CzH2 6 psi) (d) Lean flame (NzO 17 psi, C2H2 6 psi), HzO nebulizing
Figure-9. (5165 A)
2 4 6 6 1 BURNER HEIGHT (CM)
0
Fuel rich flame (NzO 17 psi, C2H211 psi) (b) Fuel rich flame (NzO 17 psi, C2H1 11 psi), HzOnebulizing (a)
Vertical emission profiles-Cz
Fuel rich flame (NzO 17 psi, CZHz 11 psi) (b) Fuel rich flame (NzO 17 psi, CzHz 11 psi), H 2 0nebulizing (a)
tion of water. These profiles are quite similar to those recently obtained by Fassel et a[. (7) with the premixed OZ/C~HZ flame. It is obvious that the more reducing regions of the flame and the most reducing flames are quite rich in CN, C2, and NH species. In fact, the OH profile exhibits a marked trough in these same regions, presumably due to lack of available atomic oxygen. This reduction of oxygen greatly aids in dissociation of metal monoxides in such regions. A more detailed explanation of these results is not possible because the radiation samples at any vertical position of the flame reflects only the average contribution from different flame regions contained in the solid angle of radiation viewed by the monochromator. Also the emission signals are not necessarily a true indication of the ground state population of the species because of the possible existence of chemiluminescence. The flames produced with the Beckman total consumption burners are considerably inferior to those obtained above with the Zeiss burner. The elements selected for study are chosen for a variety of reasons : efficiency of atomization, background radiation considerations at the wavelength of atomic fluorescence measurement, susceptibility to chemical interferences, refractory oxide formation, etc. B. Atomic Fluorescence Characteristics of Cadmium. Flame conditions producing greatest atomic fluorescence signals are obtained with N20and CzHzgas pressures of 20 and 12 psi, respectively. Under these conditions, the solution transport rate into the flame is 2.00 ml/min, and the height of measurement is about 10 cm above the top of the burner. At this height, the flame background radiation at 2288 A is about 10-fold greater than an unpremixed air/Hp flame (solution transport rate of 6.58 ml/min), the noise due to flame flicker (7) T.G.Cowley, V. A. Fassel, and R. N. Kniseley, Spectrochim. Acta, 23B, 771 (1968).
0
2
4
6
8
1
0
BURNER HEIGHT (CM)
Figure 11. Vertical emission profilesOH (3064 A) (a) Fuel rich flame (NzO 17 psi, CzHz11 psi) (b) Fuel rich flame (NzO17 psi, CzHz11 psi), HzOnebulizing (c) Lean flame (NzO 17 psi, CZHZ 6 psi) (d) Lean flame ( N 2 0 17 psi, CzHz6 psi), HzOnebulizing
Table VI. Effect of Some Extraneous Ions on Atomic Fluorescence of Cadmium. Relative atomic fluorescence signal* Ion P-NzOlGHz U-Air/Hz 1 .oo 1 .oo 0.98 0.81 1.02 0.93 1.04 0.93 0.98 0.65 a 900 p g / d interferent and 1 p g / d cadmium. Error rt0.02.
VOL. 41, NO. 12, OCTOBER 1969
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12
i n
i
w
7 7
9
II
15
17
19
21
23
W
a
PRESSURE (PSI)
Figure 12. Variation in atomic fluorescence with gas pressure (a) NzO17 psi, C2Hlvarying (b) CZHZ 10 psi, NzO varying
Solution contained 240 pg/ml beryllium and height of measurement was 4 cm
is approximately 5-fold greater, and the overall efficiency of atomization about 2-fold greater. At lower flame heights, the background radiation and noise increased considerably as would be expected. A slightly more fuel lean flame gave a similar atomic fluorescence signal with a reduction in background radiation and noise. This type of flame would presumably be more favorable if measurements were to be made near the detection limit. In Table VI, the effect of some interfering ions (900 pg/ml) on the atomic fluorescence signals of cadmium (1 pg/ml) in the premixed N20/C2Hzand the unpremixed air/H, flames is shown. The ions were selected because they were previously known to cause interferences in low temperature flames. However, virtually no interference occurs in the N20/C2H2 flame, although it is apparent that even in low temperature flames there is little effect even from the most involatile substances. C. Atomic Fluorescence Characteristics of Magnesium. A fuel-rich flame (NzO and C2H2 pressures of 17 and 11 psi, respectively) gave maximum atomic fluorescence signals for magnesium, and the optimum height of measurement is about 7.5 cm. However, none of these parameters is critical. At flame the wavelength of measurement (2852 A) the NzO/CZHZ exhibited about a 40-fold increase in background radiation over an optimized unpremixed A/H2 turbulent flame (pre-
Table VII. Effect of Some Extraneous Ions on Atomic Fluorescence of Magnesium4 Relative atomic fluorescence signalb Ion P-NzO/GHz U-A/Ha
a
1 .oo
1 .oo
Ala+ BaZ+
0.73
POP
0.75
0.54 0.82 0.18
Si032-
0.32
0.01
SiOa2-c
0.75
0.00
1.20
900pg/ml interferent and 9 pg/ml magnesium except for c. Error + 0.02. IO-fold excess.
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01
,
2
3
4
5
6
7
8
BURNER HEIGHT
9
1
0
(CM)
Figure 13. Variation in atomic fluorescence with height of measurement Solution contained 240 pg/ml beryllium, NzO pressure 17 psi, CzHzpressure 10 psi
1'
0
2
4
BURNER
6 8 1 0 HEIGHT (CM)
Figure 14. Atomic emission profile in premixed N~O/CZHZ flames (a) Very fuel rich, NzO 17 psi, CzHz11 psi (b) Slightly fuel rich, N 2 0 17 psi, C2H28 psi (c) Fuel lean, NzO 17 psi, CzHz6 psi Solution contained 240 pg/ml beryllium
viously found to give greatest sensitivity for magnesium) and a six-fold increase in peak-to-peak noise. The analytical working curve for magnesium in the N20/ CzH2flame is linear (log-log plot) up to about 20 pg/ml magnesium which is somewhat greater than that obtained in the unpremixed A/H, flame (about 10 pgiml). From a consideration of the solution transport rate into the optimized premixed N20/C2H2and unpremixed A/H2turbulent flames (2.00 and 5.99 mlimin, respectively) and the resulting fluorescence signals, an approximate increase of 1.6fold in the overall atomization efficiency results in the premixed flame.
In Table VII, the effect of some interfering ions (900 pg/ml) on the atomic fluorescence signal of magnesium (9 pg/ml) in the premixed Nz0/C2H2and unpremixed A/Hz flames is shown. In all instances, there is a marked improvement using the premixed NzO/CZHz flame. The effect due to silicate is further reduced by the addition of small amounts of EDTA at pH 12. The corresponding atomic emission signals in the premixed NzO/CzHzflames only showed interference in the presence of silicate. These experiments indicate that the use of a high temperature, reducing flame is essential in any practical analysis. Furthermore, it is possible to use such flames in conjunction with dc measurement systems even in high background regions. D. Extension to Analysis of Elements Forming Stable Monoxides. Atomic fluorescence was observed using the premixed N20/CzH2flame for a number of elements which fall into this class-e.g., aluminum (3961 A), beryllium (2349 A), and germanium (2652 A). However, for the purposes of evaluating the premixed flame, the atomic fluorescence characteristics of beryllium were examined. Other workers (8, 9) have also studied the atomic fluorescence of beryllium using premixed laminar NzO/C2Hz flames. As with other elements prone to forming stable monoxides, the greatest sensitivity toward beryllium was obtained with a fuel-rich flame (NzO, 17 psi, and C2H2,10 psi) and with a
height of measurement about 4 cm above the top of the burner head. In Figures 12 and 13, the change in atomic fluorescence signal for a solution containing 240 pg/ml beryllium with varying C2H2 and NzO gas pressures and height of measurement is shown. A linear analytical working curve (log-log plot) was obtained up to 24 pg/ml beryllium; the limit of detection (signa1:noise = 1) was 0.04 pg/ml using a slit width of 0.25 mm. This compares favorably with the value of 0.01 p g / d obtained by Hingle et at. (9) using a quartz separated premixed laminar flame and a specially designed burner in conjunction with an ac measurement system. It is also considerably better than the value of 0.5 p g / d obtained by Robinson and Hsu (8) using again a specially designed burner for a premixed NzO/CzHz flame, an ac measurement system, and a high intensity hollow cathode lamp as source of excitation. Scatter is not observed under our experimental conditions. In Figure 14, the atomic emission profiles for beryllium in the very fuel-rich, slightly fuel-rich, and fuel-lean premixed NzO/C2Hzflames NzO (17 psi and CzH2 11, 8, and 6 psi, respectively) are shown. The emission profiles for other refractory oxide forming elements-e.g., aluminum and barium were similar. Interferences were not investigated because Robinson and Hsu (8) have already made an extensive study of this aspect.
(8) J. W. Robinson and C. J. Hsu, Anal. Chim. Acta, 43, 109 (1968). (9) D. H. Hingle, G . F. Kirkbright, and T. S. West, Analyst, 93, 522 (1968).
RECEIVED for review March 10, 1969. Accepted August 4, 1969. This work was supported by AFOSR(SRC)-OAR, U.S.A.F. Grant No. AF-AFOSR-69-1685.
Determination of a Protonation Scheme for Isochlortetracycl ine Using Nuclear Magnetic Resonance Ulrich W. Kesselring' and Leslie Z. Benet2 College of Pharmacy, Washington State University, Pullman, Wash. 99163
The macro- and microdissociation constants for isochlortetracycline in a 50-50 wt/wt methanol-water solvent mixture were determined by the use of potentiometric and NMR techniques. A modification of the equations used to calculate microconstants for compounds exhibiting widely separated macroconstants is discussed and shown to yield more accurate results. Equations, after Edsall and Wyman, are developed to calculate the microconstants for isochlortetracycline where there is strong overlap between pKz and pK,. Calculations involving chemical shift data from two different sites are shown to yield almost identical microdissociation constants. On the basis of the comparable shift values for the phenolic diketone system in isochlortetracyline and in other tetracyclines, it appears that the site of dissociation in this system may be assigned to the hydroxyl group at carbon 10.
BEFORE THE EXACT STRUCTURES of the tetracycline compounds were known, Stephens et al. ( I ) reported that the hydrochloride salts of oxy- and chlortetracycline exhibited three dissociation constants whose acid pK values were approximately 3, 7, and 'Present address, Ecole de Pharmacie, Place du Chitleau 3, Lausanne, Switzerland 2 Present address. School of Pharmacy, University of California, San Francisco Medical Center, San Francisco, Calif. 94122
9. In the decade following this report, a minor controversy arose as to the assignment of the particular functional groups giving rise to the observed macroscopic constants. This controversy was reviewed by Rigler et al. (2). It is interesting to note that this disagreement was largely the result of a misprint in a footnote of a 1956 paper by Stephens et al. (3), for which the correction (4) has been generally ignored. Rigler et al. (2)attempted to resolve the lack of agreement as to the pK assignment of the particular functional groups on tetracycline by determining the microscopic dissociation schemes of three members of the tetracycline series using nuclear magnetic resonance. By investigating tetracycline, its 4-epimer, and its quaternary methyl iodide, these authors concluded that the dissociation of the first proton occurs primarily from site A (see Structure' I). For 4-epi-tetracycline, the second dissociating hydrogen was found to come primarily from site C while the third dissociating hydrogen came pri(1) C. R. Stephens et al., J. Amer. Chem. SOC.,74,4979 (1952). (2) N. E. Rigler, S.P. Bag, D. E. Leyden, J. L. Sudmeier, and C. N. Reilley, ANAL.CHEM., 37, 872 (1965). (3) C. R. Stephens, K. Murai, K. J. Brunings, andR. B. Woodward, J. Amer. Chem. SOC.,78, 4155 (1956). (4) Zbid.,p. 6425. VOL. 41, NO. 12, OCTOBER 1969
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