parently more severe chemical or physical interferences in atomic emission spectrometry cannot be explained at present. The addition of excess diverse ion appears to enhance the production of free ground state molybdenum atoms; this is shown by the increase of the atomic fluorescence and atomic emission intensity in many instances. Similar results have been reported in the atomic absorption spectrometry of molybdenum (6, 7).
ACKNOWLEDGMENT We thank Dr. A. P. Rao for his assistance during the course of this work, and Varian Techtron Pty. Ltd, Victoria, Australia, for the provision of equipment and for the research studentship awarded to one of us (R. W.). RECEIVED for review December 22, 1969. Accepted April 14, 1970.
lnterelement Interferences in Atomic Absorption Analyses with the Nitrous Oxide-Acetylene Flame J. Y. Marks and G . G . Welcher Adoanced Materials Research & Deoelopment Laboratory, Pratt & Whitney Aircraft, Middletown, Conn.
The effects of flame and instrumental variables on cation interference effects were evaluated. The variables most critical in determining the magnitude of cation interferences are the height of measurement, fuel to oxidant ratio, and concentration of analyte in the salt matrix. With the proper selection of operating parameters, many of the observed interferences were reduced. Possible interference mechanisms, such as lateral diffusion of analyte in the flame and competition for oxygen were studied. It was concluded that salt vaporization effects were the most critical in determining interference effects. A new, more pertinent method of expressing flame conditions is presented. The new parameter expresses the oxidant to fuel mole ratio as a fraction of the stoichiometric ratio of 3 moles of nitrous oxide to 1 mole of acetylene.
RECENT WORK in this laboratory on the development of atomic absorption methods of analysis for major constituents in nickel- ( I ) and cobalt-base ( 2 ) alloys has prompted a thorough investigation into interelement effects. Confusion has arisen from past reports of interferences in the literature. These effects have been referred to as solute vaporization interferences, chemical interferences, and salt effects. These interferences may be the result of either cation or anion concomitants. Anion effects are generally better understood and in practice are easier to control than cation interferences; therefore, only cation effects are considered in this study. One method of decreasing cation effects is the long and tedious preparation of standards to rigorously match the composition of the sample. This necessitates knowing the composition of the sample reasonably well before the analysis is performed. Several mechanisms have been proposed to account for interference phenomena. Alkemade (3) has attributed enhancement and depression effects to differences in volatility of the matrix or compound in which the analyte atom is found in the flame. An enhancement would result when the analyte forms a more volatile compound or is dispersed in a more volatile matrix. Conversely, depression of absorbance would result from the formation of a less volatile compound or matrix. It has been postulated that enhancements of certain -
(1) G. G. Welcher and 0. H. Kriege, At. Absorption Newslett., 8, 97 (1969). (2) G. G. Welcher and 0. H. Kriege, unpublished data, 1970. (3) C . Th. J. Alkemade, ANAL.CHEM.,38, 1252 (1966).
oxide forming elements may be the result of a competition for the available oxygen in the flame (4, 5). The atom population is increased because of a decrease in the amount of oxidized analyte in the flame. Stupar and Dawson (6) have shown that a relationship exists between the interelement effects and the size of the particle remaining after evaporation of the solvent. Interelement effects may also be due to differences in the stabilities of compounds formed in the flame (7,8). Several authors have presented evidence that mixed oxides of the spinel type may be responsible for interactions between some refractory elements (9, IO). Koirtyohann and Pickett (11) have reported recently a new type of interference in the nitrous oxideacetylene flame which may be the result of differences in the rates of lateral diffusion of particles in the flame. The major goal of the present study was the development of generalized techniques for the suppression or elimination of interelement interferences through a better understanding of the atomization processes in the nitrous oxide-acetylene flame. EXPERIMENTAL Apparatus. A Techtron Model AA-4 atomic absorption spectrometer with a Techtron AB-50 grooved burner head and a R106 photomultiplier tube was used for all absorbance measurements. Standard hollow cathode lamps were used as sources for atomic absorption measurements. Emission measurements were made by inserting a Techtron type FE-4 beam chopper between the flame and the monochromator entrance slit. The slit was 8 mm high and was variable from 0 to 300 p wide. The beam from the hollow cathode lamp was focused on the center of the flame and was 5 mm
.,
(4) S. Sachdev. J. W. Robinson. and P. W. West, Anal. Chim. Acta,
37, 12 (1967). (5) T. Ramakrishna, P. West, and J. Robinson, ibid., 39, 81 (1967). (6) J. Stuoar and J. B. Dawson. Aod. Oot., 7,1351 (1968). i 7 j D. C: Manning and L. Capaiho-Delgado, Anal. Chim. Acta, 36, 312 (1966). (8) M. D. Amos and J. B. Willis, Spectrochimica Acra, 22, 1325 (1966). (9) W. W. Harrison and W. H. Wadlin, ANAL.CHEM.,41, 374 (1969). (10) V. S. Sastri, C. L. Chakrabarti, and D. E. Willis, Talanta, 16, 1093 (1969). (11) S. R. Koirtyohann and E. E. Pickett, ANAL.CHEM.,40, 2068 (1968).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
1033
,250
-
.240
-
W
Figure 1. Effect of varying nickel concentration on absorbance of 100 ppm aluminum
0
2000
4000
6000
8000
I
1
I
I
I
I
10,ooo
12.000
14,000
16,ooo
ia,ooo
20,000
PPM N I C K E L
in diameter. Because the beam was somewhat out of focus at the extremes of the flame, the height of the flame sampled was somewhat more than 5 mm. Reagents. Solutions were prepared from chloride stock solutions of the metals, where possible, in order to eliminate any anion interferences. The vanadium stock solution was 0.8Nin sulfuric acid. Analyte was added to the test solutions at the following levels: 100 ppm aluminum, 100 ppm titanium, 10 ppm chromium, and 40 ppm nickel. All test solutions were made 1.2N in hydrochloric acid and contained 1000 ppm potassium as potassium chloride. Distilled deionized water was used in all dilutions. Flame Variables. In normal operation nitrous oxide was delivered from the tank via three stages of regulation to the nebulizer at 11 psig. Acetylene was transported through two stages of regulation to the burner at 0.2 psig. Except where noted, the acetylene flow was adjusted to obtain a “normal” atomic absorption flame with a red feather extending 7 mm above the primary reaction zone. Under these conditions the nitrous oxide and acetylene flow rates were 5.6 l./min and 3.1 l./min, respectively, at 1 atm pressure and 0 “C. The model AB-50 burner head features a raised slot of width 0.53 mm and length 5.9 cm, resulting in an average stream velocity at the burner port of 480 cmjsec. The height of the red feather in analytically useful flames ranges from 1 to 15 mm, necessitating that at a constant nitrous oxide flow of 5.6 l./min, the acetylene flow varies from 2.5 to 3.5 l./min. The combustion reaction under these conditions is best written as (12): 3 NzO
+ CzHz
--t
2 CO
+ 3 N2 + HzO
A stoichiometric flame results from an oxidant to fuel mole ratio of 3 :1. The above reaction is favored over the reaction normally written as: 5 NZ0
+ CzHz
+
2 COz
+ 10 N2 + H20
Carbon dioxide is almost completely dissociated at the temperature of the nitrous oxide-acetylene flame, and maximum temperature is predicted by a stoichiometry of three moles of oxidant to one of fuel. (12) J. G. Tschinkel, Pratt & Whitney Aircraft, private communication, 1969. 1034
The actual ratio of oxidant to fuel used in flame spectrometry is less than the stoichiometric amount. Present practices in reporting flame conditions are often confusing and misleading. The oxidant to fuel mole ratio is of prime importance in defining the temperature of the flame and the flame gas composition. Therefore it is suggested that the quantity X (13), as well as total flow and red feather height, be used in reporting flame conditions. The fraction X is obtained by dividing the oxidant to fuel mole ratio by the stoichiometric ratio of three, or: A =
moles oxidant/time moles fuel/time x 3
This fraction clearly expresses the oxidant to fuel ratio and its relationship to stoichiometry. Analytically useful flames involve oxidant to fuel ratios of approximately 2.2:1 to 1.6:1, or X = 0.73 to 0.53. RESULTS AND DISCUSSION
Interelement Interferences. Detailed studies were made of the interferences by several metals on the absorbances of aluminum, titanium, chromium, and nickel. In each case, the concentration of concomitant was varied while maintaining the analyte concentration constant. The absorbance of each analyte was measured at the height in the flame giving optimum sensitivity. Figure 1 shows the effect of varying amounts of nickel on the absorbance of 100 ppm aluminum. It is typical of most of the interferences studied in that at low nickel additions a rapid change in absorbance is noted; as the nickel concentration is increased, the absorbance reaches a limiting value, remains relatively constant, and at very high concentrations begins to decrease gradually. The general shapes of interference curves are reproducible, but the exact concentrations of concomitant at which inflection points occur generally depend on the exact flame conditions used. Because of this variability, care should be exercised when applying any re(13) A. G. Gaydon and H. G. Wolfhard, “Flames, Their Structure, Radiation, and Temperature,” Chapman Hall, London, 1960, p 34.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
Table 111. Interferences on 10 ppm Cr Relative interference, Interfering Concomitant level. oom metal
Table I. Interferences on 100 ppm A1 Relative interference, % Interfering Concomitant level, pprn metal
50 Ti Cr
+8 0 +I
Mn
+4
Fe
$4 +3 -2
V
co
Ni
200 +13 +5 +7 $5 +6 $5
5000 +I4 +21 +I4 +I1 +15
Mn Fe co Ni
A1 V
Mn Fe co Ni
0 0
$9 - 19
200 +45 - 26 +2 - 10 +I7 - 25
0
-4
5000 36 +38 27
+ ++9
0
+25
Table IV. lnterferences on 40 ppm Ni Interfering Relative interference, metal Concomitant level, D u r n
Table 11. Interferences on 100 ppm Ti Interfering Relative interference, metal Concomitant level, ppm
50 +12 - 20
200 +33 +33 +8 +1 +12 +3
+
V
++2610
0
50 +32 29 -2 -6
Ti
5000 +91 - 38 - 29 -42
+5027
Ti Cr
+8 +19
+I9
Mn
+l
+lo
Fe co
+5
+I1 $7
V
- 60
- 65
ported interference data to a specific problem. In the tables below, interference data are reported at three representative concentration levels. Interferences on Aluminum, Titanium, Chromium, and Nickel. Tables I through IV show the effect of 50 ppm, 200 ppm, and 5000 ppm of various metal ions on the absorbances of aluminum, titanium, chromium, and nickel. Absorbance measurements were made at the flame height of maximum sensitivity for each analyte. Aluminum, chromium, and nickel absorbances are enhanced in the presence of metal concomitants, while titanium absorbance is suppressed by most metals. It was found that the magnitude of the effects shown in the tables was dependent upon flame and instrumental variables. Therefore, a systematic study of instrumental and flame variables was made.
200 +27
5000 27 $7 +36 +78 +11 +55
+
+7
+5
Effect of Height of Observation on Interferences. The effects of 200 ppm of various metal concomitants on 100 ppm aluminum and 10 ppm chromium as a function of burner height are shown in Figures 2 and 3, respectively. In general, the magnitudes of the interferences vary with height in the flame and are not necessarily maximum at the position of maximum analyte absorbance. The magnitudes are usually decreased at extreme heights in the flame. Studies of absorbance as a function of height are especially interesting in that they may also be viewed as studies of atom population as a function of time and stoichiometry in the flame. The typical nitrous oxide-acetylene flame has a rise velocity of 700 cmlsec. Consequently the analyte spends approximately 2 msec in the 15 mm of the flame normally utilized for study.
Figure 2. Effect of height of observation on interferences on 100 ppm aluminum 0 100pprnAl 0 100 ppm A1 A 100 ppm A1 100 pprn AI
+ 200 ppm V + 200 ppm Mn + 200 ppm Ti
01 0 0
I
1
2
4
I
I
I
I
I
I
I
8
10
12
14
16
18
I 20
HEIGHT ABOVE BURNER (mm)
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
1035
'"[
,240
01
I
I
I
I
I
I
I
I
I
0
2
4
6
8
10
12
14
16
18
I 20
HEIGHT ABOVE BURNER (mm)
Figure 3. Effect of height of observation on interferences on 10 ppm chromium 0 10ppmCr 0 10 ppm Cr A 10 ppm Cr Q 10 pprn Cr
Table V. Effect of Oxidant/Fuel Ratio on Interferences on 100 ppm A1 Interfering Feather Relative interference, metal height 1 mm 9 mm 16 mm X 50 ppm Ti 200 ppm Ti 5000 ppm Ti 50 ppm Cr
0.73 +13 +37
+51 0
200 ppm Cr
$6
5000 ppm Cr
+8
0.57 +3
+4 +6
0.53 0 -1 -4
0 +4
-1 +2
+10
$1
Table VI. Effect of Oxidantpuel Ratio on Interferences on 10 ppm Cr Interfering Feather Relative interference, % metal height 1 mm 9 mm 16 mm 0.73 0
0.57
50 ppm Mn
200 ppm Mn
-2
++6+920 ++1610
X
5000 ppm Mn 50 ppm Ti 200 ppm Ti 5000 ppm Ti
0 0 0
+8
+2
0.53 0
+I6 +33 +38 +35
$46
It should be pointed out that height studies yield only apparent height profiles, since the height resolution of the instrument is limited by the 8-mm slit height and the 5-mm diameter beam. At heights above the burner of less than 2.5 mm, the photomultiplier is exposed to only a part of the beam, as some light is eclipsed by the burner. The measured absorbance is the average absorbance throughout the area of the beam that passes through the slit. Effect of Changing Nitrous Oxide Pressure. A change in nitrous oxide pressure produces two primary effects. First, 1036
a
+ 200 ppm Ni + 200 ppm Mn + 200 ppm Ti a change in the pressure drop to the atmosphere causes a change in gas flow rate and, therefore, a change in rise velocity in the flame. This changes the dwell time of the analyte and its matrix in any area of the flame. Second, a change in pressure causes a slight change in solution uptake rate since this is somewhat dependent upon the pressure drop between the atmosphere and the end of the nebulizer capillary in contact with the nitrous oxide. A study was made of the effect of changing nitrous oxide pressure on the interferences of titanium, cobalt, and chromium on aluminum. The flow of acetylene was changed to compensate for each change in nitrous oxide flow rate to maintain a constant oxidant to fuel ratio. When the nitrous oxide pressure was varied from 12 to 18 psig, the relative per cent interference changed very little, while the absolute absorbance of all the solutions increased by about 25%. The fact that the interference changed very little with increasing gas flow rate indicated that the residence time of the salt particle in the flame was not significantly changed. The increased sensitivity is due to the improved nebulization of the test solution. Oxidant to Fuel Ratio. It was found that the nitrous oxide to acetylene ratio could have an effect on the magnitude of the interferences. Table V shows the effect of varying this ratio on the interferences of titanium and chromium on aluminum. Table VI shows the results of a similar study of the interference of manganese and titanium on chromium. The results are shown as a function of X and the height of red feather is indicated. The absorbance was measured a t the height of maximum sensitivity for the analyte alone a t each of the three different fuel conditions. The ratio of nitrous oxide to acetylene has a large effect on the magnitude of the interferences. The interferences of titanium and chromium on aluminum are reduced as the flame becomes more fuelrich, whereas the interferences of manganese and titanium
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
,240-
7mm RED FEATHER
V E R Y FUEL RICH
01
I
I
I
I
0
4
8
12
1G
20/0
I 4
I
8
I 12
I
16
I 20
HEIGHT ABOVE BURNER (niml
Figure 4. Effect of height of observation and oxidant to fuel ratio on enhancement of aluminum by titanium 0 100 ppm AI, 7 mm feather C 100 ppm AI 500 ppm Ti, 7 mm feather A 100 ppm AI, very fuel rich Q 100 ppm A1 500 ppm Ti, very fuel rich
+ +
on chromium are increased with increasing fuel-richness. Two factors reduce the interferences of titanium and chromium on aluminum in a fuel-rich flame. First, the absorbance maximum of aluminum is shifted to a higher position in the flame where vaporization is complete and salt vaporization effects are eliminated. Second, the sensitivity is maintained at about the same level in the fuel-rich flame as that in the normal flame. This is due to a maintenance of the atom population by the increase concentration of reducing species in the flame. These phenomena are shown more clearly in Figure 4 where the absorbances of 100 ppm aluminum solutions with and without 500 ppm titanium are plotted as a function of height in the flame at two different fuel to oxidant ratios. At a point 15 mm above the burner in a fuel-rich flame, titanium interference is essentially eliminated with only a 25z decrease in sensitivity. It is interesting to note that in both a normal flame and a fuel-rich flame the position of maximum aluminum absorbance is shifted approximately 1 mm lower in the flame in the presence of titanium. This effect was not observed in any other case. The increased magnitude of interferences on chromium with the reducing flame is harder to understand. In this case the position of maximum absorbance does not vary with the reducing nature of the flame and all data were taken with center of the beam 7 mm above the burner. The sensitivity for chromium is reduced somewhat in a fuel-rich flame and there is a slight decrease in flame temperature, but these factors alone would not account for the large increase in interference. Effect of Organic Solvents. Acetone was selected as a representative solvent and the effect of various amounts of acetone on the interference of chromium on aluminum is shown in Table VII. The measurements were made in the region of the flame having maximum aluminum absorbance with
Table VII. Effect of Acetone on Interference of Cr on 100 ppm AI Acetone concn, Relative interference, VOl. 0 5 25
50
z
100 ppm Cr
5000 pprn Cr
+lo
+6 +3 +3
+5
+8 +3
no acetone present, Acetone has little effect on the magnitude of the interference, although it does increase the sensitivity for aluminum. Effect of Solution Flow Rate. The effect of solution aspiration rate on interferences was evaluated by substituting a Techtron Model D4-21V continuously variable nebulizer for the constant flow model used for previous measurements, and varying the solution flow rate while holding all other parameters constant. Solution flow rates were measured at each setting of the adjustable nebulizer. The absorbance values were then measured at a height 5 mm above the burner. The effect of rate of solution aspiration on the interferences of titanium on aluminum, manganese on chromium, and nickel on titanium was studied. The magnitude of the interference is reduced slightly at lower flow rates but is not eliminated. Stupar and Dawson (6) have discussed the relation between solution flow and interference effects and concluded that high flow rates produced large salt particles in the flame which were more difficult to vaporize. Effect of Matrix Dilution. If interferences are affected by changes in composition of the salt matrix left after evaporation of the solvent, then the addition of a large amount of an inert salt should significantly reduce the difference in
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
e
1037
l3
-aE' y 2 K
12 11 10
Table V I E Effect of Matrix Dilution on Interferences on 100 ppm AI Relative interference, Interfering metal No Dilution 10,000 ppm KCI 50 ppm Ni -2 +1 200 ppm Ni 0 +4 5000 ppm Ni 26 +6 50 ppm Cr +1 +2 200 ppm Cr 0 +7
t
+
-
5000 ppm Cr 50 ppm Ti 200 ppm Ti 5000 pprn Ti
-
9 -
+8
0 +2
+13
+3
+I4
+14
-1
W
>
8a
Table IX. Effect of Matrix Dilution on Interferences on 10 ppm Cr Relative interference, No 10,000 10,000 Interfering metal dilution ppm K ppm AI 50 ppm Mn -2 -3 -2 200 ppm Mn -3 0 +8 4000 pprn Mn NDa ND 14 5000 pprn Mn ND -3 27 50 ppm Ni -4 0 +2
:t 6
5
4
3
2
1
0
1
2
3
4
5
200 ppm Ni 5000 pprn Ni 50 ppm Fe 200 ppm Fe 5000 ppm Fe
6 a
HORIZONTAL DISTANCE FROM C E N T E R OF SLOT ( m m )
+ +-6+253 +1 4-9
+
-2
+- 1 514 - 19
+18
0 0
ND ND ND
Not determined.
Figure 5. Distribution of atomic aluminum in flame Absorbance 0 0.160 c 0.100
0
0.080
A 0.060
LJ 0.040 0 0.030 0 0.020 0.010
atomization between solutions containing analyte only and those containing both analyte and concomitants. To test this assumption, the effects of varying amounts of nickel, chromium, and titanium on aluminum were studied in the presence of 10,000 ppm potassium (added as the chloride). Results are shown in Table VI11 and indicate a significant reduction in interferences on aluminum. A similar study was made on interferences on chromium. Both potassium chloride and aluminum chloride were used in this study as matrix diluents at the 10,000 ppm level of metal (added as the chloride). As can be seen from Table IX, the interference effects of manganese and nickel on chromium are reduced with potassium chloride additions and practically eliminated with aluminum chloride. The interference of iron on chromium is actually increased upon addition of potassium chloride. Matrix dilution with aluminum chloride was studied for the interferences of nickel and cobalt on titanium, and manganese and titanium on nickel, with only limited success. The fact that analytical results are not always seriously affected by interelement interferences is probably due to a matrix dilution effect. Most determinations are carried out in the presence of a major constituent which acts to dilute the salt matrix left after evaporation of the solvent. Laterial Diffusion of Analyte in Flame. Koirtyohann and Pickett (11) have suggested that some enhancement effects are due to a decrease in lateral diffusion in the flame with increased particle weight. To test whether a similar mechanism might be responsible in some part for the enhancement effects seen in our work, two solutions were prepared, one con1038
taining aluminum and the other containing aluminum and 5000 ppm chromium. The aluminum absorbance was then measured in both solutions as a function of lateral translation in the flame at different heights. A series of figures have been prepared in which positions in the flame of equal absorbance, thus equal atomic concentration, were connected by a line. Figure 5 shows the result of this study for aluminum in the absence of chromium. This figure also shows the dimensions of the beam and monochromator slit. If the diffusion mechanism is valid for this case, the aluminum absorbance, in the presence of chromium, should be increased in the center of the flame and decreased in the outer portions. A plot of the absorbance data obtained using the solution containing both chromium and aluminum did show an enhancement in the center of the flame when compared to the data in Figure 5 , but there was no decrease in absorbance in the outer portions of the flame. Consequently, we conclude that a decrease in lateral diffusion is not responsible for the observed enhancement. Competition for Oxygen. It was suggested (4, 5) that a competition for oxygen might be responsible for enhancement effects noted between strong oxide-forming elements. Some of the strongest monoxides and their reported dissociation energies are given below. Oxide
Ti0
vo
Do kcal/mole 166
148 137 These data suggest that the enhancement effect of titanium on aluminum represents one of the most favorable cases for competition for oxygen. Both metals form very stable oxides, but the dissociation energy of titanium monoxide is 21x higher than that of aluminum monoxide and would be expected to successfully compete with aluminum for oxygen.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
A10
0.400
-
y0.m
-
Y
$ a
Figure 6. Interference of 1000 ppm titanium on 1000 ppm aluminum 0 1OOOppm A1 1000 ppm A1
9
+ 1000 ppm Ti
0.200
-
1
0
2
HEIGHT ABOVE BURNER (mm)
Two solutions were prepared, one containing 1000 ppm aluminum and the other 1000 ppm aluminum and 1000 ppm titanium. Both solutions were made 1.2N in hydrochloric acid. To confirm that the enhancement effect of titanium on aluminum was still evident at the 1000-ppm aluminum level, absorbance profiles were made of the aluminum solution and the solution containing both aluminum and titanium. As seen in Figure 6, the interference is still present at the 1000 ppm concentration of aluminum. A competition for oxygen between titanium and aluminum might act in two ways to increase the amount of atomic aluminum as shown below: (1) Ti 0 Ti0
+
-+
t (atomic or any titanium species) i
+
+
(2) Ti NO Ti0 A1° In the first case titanium would reduce the concentration of oxygen or oxidizing species in the flame to such an extent as -+
to lower the likelihood of aluminum oxidation. In the second case, titanium or a titanium species would reduce oxidized aluminum. Both processes seem unlikely when considered in the light of other flame species present. When a solution which is 1000 ppm in titanium is aspirated into the flame, its concentration in the flame, expressed as the mole fraction is approximately 2 X 10-5. This represents a solution flow rate of 5 . 5 ml/min and assumes that 10% of the solution reaches the flame. Considering the very low concentration of titanium in the flame and the fact that other more highly reducing species such as atomic and molecular hydrogen, cyanogen, and carbon are available for reduction, it would seem unlikely that titanium plays a major role in the reduction of aluminum oxide. To test these assumptions three solutions were prepared, one of which contained 1000 ppm aluminum, another 1000 ppm aluminum and 1000 ppm titanium, and the third 1000 ppm titanium. All solutions were made 1.2N in hydro-
Figure 7. Mutual effects of aluminum and titanium on the emission intensity of their Iwn c monoxides I I-
A . T i 0 emission 0 1OOOppmTi 0 1000 ppm Ti B. A I 0 emission A 100OppmAl 0 1000 ppm A1
w
+ 1000 ppm A1
2 I-
5w
a
+ 1000 ppm Ti
12
16
20
0
1039
HEIGHT A B O V E BURNER
ANALYTICAL CHEMISTRY, VOL. 47 NO. 9, AUGUST 1970
chloric acid. The emission intensity of aluminum monoxide at 4867 A was then measured as a function of height in the flame, both in the solution containing aluminum and in the solution containing aluminum and titanium. The aluminum monoxide emission intensity was corrected for background from the hydrochloric acid and the considerable emission from titanium at the aluminum monoxide bandhead. In a similar manner the emission intensity of titanium monoxide was measured at 5004 A in the solution containing only titanium and in the solution containing both titanium and aluminum. The results of this study are shown in Figure 7. The titanium monoxide emission remains unaltered by the presence of aluminum. However, the aluminum monoxide emission intensity does show a definite decrease in the presence of titanium at heights between 0 and 12 rnm above the burner. It can be concluded that competition for oxygen may account for some of the enhancement effects observed between refractory elements. The competition probably occurs in the solid or liquid phase before salt evaporation is complete. In this state the effective concentration of both concomitant and analyte would be greatly increased, thus increasing the likelihood of reaction.
variables, which probably account for many of the discrepancies in interference studies reported in the literature. When constant flame conditions are used, the most probable explanation for the interferences reported in this study is the difference in volatility of the analyte when accompanied by other metal species, most concomitants increasing the volatility of the analyte. It is important to note that the volatility of the matrix in terms of boiling point and heat of vaporization is not necessarily of prime importance, but that the volatility of analyte is in some manner increased or decreased in the presence of concomitants. The evaporation rate of small salt particles in the flame is a complex function of many factors including drop size, diffusion coefficient of the evaporating species, surface tension, and heat transfer characteristics. Many interferences can be reduced or eliminated by the proper selection of flame conditions and salt concentration. A more descriptive study of atomization processes is the subject of a later paper. ACKNOWLEDGMENT
CONCLUSIONS
The authors thank J. G . Tschinkel for the many helpful discussions and suggestions. The authors also thank 0. H. Kriege for his critical reading of this manuscript.
These studies suggest the complexity of cation interference effects. The magnitude, and in some cases the direction of interferences, is dependent upon a variety of experimental
RECEIVED for review March 26, 1970. Accepted May 18, 1970.
Flame Emission Method for Determining Heats of Combustion of Selected Compounds J. J. Kroeten,’ H. W. Moody, and M. L. Parsons2 Department of Chemistry, Arizona State University, Tempe, Arizona 85281
A new method for determining heats of combustion of organic compounds was found using flame emission spectroscopy. Solutions of alcohols, carboxylic acids, and amines in methanol were introduced into an entrained air-hydrogen or oxy-hydrogen flame. The most intense CH emission bandhead at 431.5 nm was measured and a computer was used to correlate the data. A linear response for emission intensity vs. the heat of combustion was found for compounds in a homologous series. The slopes were the same for compounds containing the same functional group. The slopes for solutions containing different functional groups were quite different. When comparing the experimental values to the literature values, an average agreement of 3.6% was found. The standard deviations of the data were obtained from the calculated curves and found to be 3.3, 23.3, and 48.3 kcal for the amines, alcohols, and acids, respectively. The time required to complete the entire procedure i s about 1 hour.
tion (2) and the use of additivity rules (2). Some of the calorimetric approaches are the use of rotating bomb calorimeters (3), bomb calorimeters (4, and flame calorimeters (5). The precision of these techniques is usually around 0.5 to 1.5 %. These methods have several disadvantages, such as the need for expensive equipment, the length of time needed to obtain suitable results, e . g . , some take 24 hours or longer (3), and the inability to distinguish between some geometrical isomers (2). The flame spectroscopy method described in this paper appears to be a fast and simple way of determining heats of combustion. The technique described here is empirical; however, there is a physical basis for the phenomenon. It is known that the emission intensity from atoms and molecules, e.g., the CH molecule, produced from the combustion of species in the hot flame gases is directly proportional to the number of these molecules existing per unit volume of hot
BASICALLY, there are only two general techniques for determining the heat of combustion of organic molecules-theoretical calculations and experimental calorimetric methods. The theoretical approaches include the concept of correlating the displacement of valence electrons to the heat of combus-
(1) S. Morris, Kharasch, and Ben Sher, J. Phys. Chem., 29, 625658 (1925). (2) S. W. Benson and J. H. Buss, J. Chem. Phys., 29, 546-572 (1958). (3) W. D. Good, D. W. Scott, and G. Waddington, J. Phys. Chem., 60, 1080-89 (1956). (4) F. D. Rossini and E. J. Prosen, J. Res. Nut. Bur. Stand., 33, 255 (1944). (5) G. Pilcher, H. A. Skinner, A. S . Pell, and A. E. Pope, Trans. Faraday SOC.,59, 316330 (1963).
Present address Syntex Corp., Pharmaceutical Analytical Department, Stanford Industrial Park, Palo Alto, Calif. To whom all correspondence should be addressed. 1040
ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
