Elimination of Interferences in Flame Photometry PAUL PORTER and GARRARD WYLD Shell Development Co., €meryville, Calif. Some of the types of interferences encountered in flame photometry are reviewed. A comparison is made of the effectiveness of a radiation buffer, an internal standard, and a direct-injection burner as means of controlling certain types of interferences. Although these techniques are valuable, it is necessary to have considerable knowledge about the nature of the sample, and the most accurate analyses require standards which are very similar in composition to that of the sample.
years. Some workers have preferred to retain barrier layer photocells because of their simplicity, while others have increased the complexity by using the more sensitive phototubes and photomultiplier tubes. A variety of spray chambers have been used for introducing sample solutions into the burner gases, and the burners have been of various types using propane-air, illuminating gas-air, acetylene-air, acetylene-oxygen, and hydrogen-oxygen. The schematic diagram in Figure 1 is that of a typical spray chamber-burner assembly using acetylene-air. The air supply for the burner passes over an atomizer tube, forming a fine spray of sample solution in the large spray chamber. The larger droplets settle out and are drained off, or in some cases, are recycled to the atomizer; the small droplets remain suspended in the air stream and are carried into the burner tube where the air is mixed with the fuel gas and the combustible mixture fed into the flame. The latest development in the field is a burner which forms a spray directly in the flame rather than in a spray chamber preceding the burner. This was introduced by Weichselbaum and Varney ( I S ) , whose burner design and flame photometer are marketed by the Fearless Camera Co. The same principle n'as used in a burner attachment designed for use with the Beckman Model DU and Model B spectrophotometers. A schematic diagram of the latter burner is shown in Figure 2. The oxygen fed to the flame passes over the tip of a fine capillary dipped into the sample solution, and causes an aspirator effect which produces a spray of liquid in the flame. Acetylene (or hydrogen) passing through the outer annulus provides the fuel for combustion. Neither the Weichselbaum-Varney flame photometer nor the Beckman instruments have the internal-standard feature. Although the flame as an excitation source possesses important
P
RIOR to the development of the technique of flame photometry, the determination of alkali metals was a difficult problem for analytical chemists. I n general, it was necessary to remove all other metals from the sample before the alkali metals could be determined. Excellent methods have been worked out for this isolation of the alkalies, but none of them can be called rapid. In the late twenties Lundegardh (9) demonstrated the usefulness of a flame excitation source for the quantitative spectrographic determination of the alkali metals and a number of other elements. Later workers (6, 8, l l ) , improving upon his burner design, were able to achieve a reproducibility and steadiness of emission from the flame that permitted the direct measurement of the intensities of the spectral lines with photoelectric cells or phototubes. The use of this technique in this country for the determination of the alkalies received an impetus with the construetion by Barnes, Berry, Richardson, and Hood ( 1 ) of a simple inexpensive "flame photometer." In this instrument an aqueous solution of the sample was sprayed into the air supply of an ordinary Meker burner, so that a finely dispersed aerosol was carried into the flame. The light emitted by the flame passed through a filter to isolate a narrow portion of the spectrum in the vicinity of the wave length of interest, and fell upon a barrier layer S p r a y Chamber photoelectric cell. The response of the cell was measured by a galvanometer. This very simple / A , ? Inlet instrument aroused great interest among analytical chemists and a somewhat similar apparatus was manufactured commercially as the Perkin 'Solullon In et Elmer Ilodel 18. c However, the instrument had many drawbacks, - D r a m f o r Surplus Solution w i t h and was particularly sensitive to acid and salt Hydrostatic S e a l S a m p l e Solu!lon interferences ( I O ) . Berry, Cappell, and Barnes II C a p l i l a r y soon developed an improved instrument using Figure 2. Burner to lithium as an internal standard ( 2 ) . In this inFigure 1. Spray chamber and form sample spray strument two photocells were employed Kith a burner assembly directly in flame light-filter system which allowed onlv light in the vicinity of the lithium 671 mH line to strike one cell, while light in the region of the sodium 589 mM lines advantages such as simple spectra which do not require high reor the potassium 768 mb line fell upon the other. By meassolving power for the isolation of the various lines, and steady uring the ratios of the sodium OT potassium radiation to that of emission not subject to the fluctuations observed in the arc or lithium, rather than the absolute values of the intensities, many spark, it suffersfrom a number of serious interferences. It is conof the effects of fluctuating aspiration rate, variations in flame venient to think of three different types of interferences: overtemperature, and changes in emission due to acid and salt effects lapping of spectra or background interference, radiation interwere avoided. Commercial instruments of this general type are ference, and anion or acid interference. Spectroscopists would the Janke, Barclay, Baird, and Process and Instruments flame break these down into several more classifications (4,6). In photometers. The same internal-standard principle is used in practice, the only foolproof way to carry out flame photometry is the Perkin-Elmer Models 52A and 52C, which employ a prism to know the concentration of everything else in the sample and monochromator in place of light filters. duplicate this composition in the standards which are used to Many modifications have appeared in the literature in recent calibrate the instrument. If one is to analyze a large number of
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134
ANALYTICAL CHEMISTRY
samples of practically the same composition, except for variations in the sodium content, this is a very practical way to proceed. However, the analytical chemist is not always this fortunate so that it is desirable to find ways in which the various kinds of interferences can be minimized. Each type of interference is discussed below in terms of specific examples of special interest in the determination of sodium and potassium, the metals most frequently measured in flame photometry. EXPERIMENTAL
A number of techniques are available for minimizing interfer-
ments, and were found to be highly repeatable. In applying the instrument to test solutions it was set with distilled water or blank solution and the 20-p.p.m. sodium standard as before, and an intensity reading was made on the test solution. The per cent error corresponding to the type of interference under test was then calculated by subtracting the known sodium content from the apparent value given by the calibration curve, multiplying by 100, and dividing by the known content. A similar procedure was followed when the internal standard feature was used. The zero scale setting was made with pure lithium chloride solution, the 100-scale setting with 20-p.p.m. sodium in lithium chloride solution. Calibration points and interference data were obtained as for the direct procedure. In all instances, duplicate readings were made for the solutions.
ences in flame photometry without resorting to chemical separation or to duplication of the composition of the sample with standards. Several of these techniques are illustrated here, using three different types of instruments for comparison. A Perkin Elmer Model 52A instrument was used as an example of a direct-reading instrument with a spray chamber, and also as an example of an internal-standard instrument vith a spray chamber, since it can be used either way. The Beckman Model B flame photometer was used as an example of a direct-reading instrument with the sample spray formed directly in the flame.
eop[ikl
lzo/
Band Width 6 . 4 rnp
Band Width 3.0
rnp
0
5
IO
15 20 25 Weight Ratio K/Na
30
35
IO0
Figure 4. Effect of lithium chloride as a buffer on interference of potassium in determination of sodium
60
Perkin Elmer 52A, 15 p.p.m. of sodium
40
1
The Perkin-Elmer instrument had been slightly altered as follows: The feeding funnel and atomizer were replaced by a liquid intake capillary and atomizer assembly of the authors' own design, so constructed that the sample could be withdrawn from a beaker (Figure 1). A slight modification was also made in the burner to permit greater ease of cleaning. The instrument was equipped with a red-sensitive RCA phototube No. 918 or a blue sensitive RCA phototube No. 1P37 for measuring the unknown emission and a second red-sensitive tube for measuring the lithium internal Btandard emission.
The Beckman flame photometer was operated with a red-sensitive phototube, Beckman No. 157, for wave lengths greater than 625 mp, and with a blue-sensitive phototube, Beckman No. 12055, for wave lengths below 625 mp. The burner was operated with acetylene gas and oxygen a t pressures of 4 and 10 lb. respectively. The Beckman Model B spectrophotometer has four sensitivity positions, and is normally operated a t highest sensitivity with the flame photometry attachment. Since no continuously variable gain control is provided in the amplifier, the setting of the 100 reference point must be made by varying the width of the slit. For each of the four sensitivity positions there is therefore a definite slit width which will give a scale reading of 100 with a given standard. In the present work calibration curves were prepared for the highest sensitivity position in the same manner as for the Perkin-Elmer instrument above. When it was desired to make readings for the same solutions a t larger slit width, the photometer was operated one step lower in sensitivity. The operating procedure was the following. With the shutter open the zero sodium or potassium standard was aspirated, and the dark current control was set to give a zero galvanometer reading. A 10-p.p.m. sodium standard or 20-p.p.m. potassium standard was then aspirated, and the slit width was adjusted to give an intensity reading of 100. Since there is some interaction between the zero and 100 settings, several successive readjustments are required to set the two ends of the scale when the blank solution has an appreciable emission. Test solutions were measured and calculated in the same way as for the direct PerkinElmer instrument. All measurements discussed in the next section are the result of a t least two measurements.
In using the Perkin-Elmer photometer as a direct-reading instrument, a calibration curve was first prepared by measuring the emissions of pure sodium chloride solutions in distilled water or in the indicated buffer solutions of lithium chloride. With distilled water or a blank solution of lithium chloride buffer spraying into the burner chamber, the galvanometer zero control was adjusted to give a zero reading. A solution containing 20 p.p.m. of sodium was then drawn through the atomizer and the gain control was adjusted to bring the galvanometer reading to 100. Galvanometer readings were then made for solutions of sodium chloride a t intervals of 2.5 p.p.m. of sodium through the range from 0 to 20. Before reading each solution, the settings for distilled water or buffer solution blank and the 20-p.p.m. solution were rechecked. Calibration curve intensities were redetermined several times during the course of the interference measure-
With the Beckman Model B flame photometer it is possible to scan narrow regions of a spectrum to determine the nature of background radiation. For example, in the case of sodium determination, the wave-length dial is set for the 589 mp lines in the usual way and the zero and 100 adjustments are made with blank and standard sodium solutions. The sample solution is then aspirated and readings of the galvanometer are taken as the wave-length dial is varied on both sides of the 589 mp position. This procedure does not result in a curve giving the absolute intensity distribution in the flame since no compensation is made for variation of the phototube sensitivity with wave length; however, for the evaluation of interferences or the prediction of slit
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0 560
580
600
620 6 4 0 5 6 0 500 Wave Length. mp
600
620
640
Figure 3. Effect of changing band width on calcium interference in determination of sodium Model B flame photometer -Beckman calcium chloride containing trace impurity 0.02M
---sodium 0.02M
of sodium
calcium chloride containing 5 p.p.m. of
V O L U M E 27, NO. 5, M A Y 1 9 5 5
20
73s
t
I0
a
e
-
ppm. Lithlurn I5
IO
0 0
10
20
30 40 50 ‘+$eight Ratio K/Na
60
70
Figure 5. Effect of lithium chloride as a buffer in interference by potassium in determination of sodium Beckman Model B, 8 p.p.m. of sodium Band width, 3 mp
width effects it is the relative intensities obtained in the above manner which are of importance. Standard solutions of the alkali halides and other solutions used were prepared from reagent grade chemicals. All salts and acids were examined for traces of the alkalies by applying the spectrum-scanning technique described above to concentrated solutions. Where sharp emission peaks were found a t the alkali wave lengths, the corresponding amounts of the metals were estimated by the successive additions technique. Several small known increments of alkali were added to the solution and the changes in peak height were noted. The internal calibration thus afforded was used to calculate the amount which must have given rise to the original peak. In this manner it was possible to subtract contributions to the peak height of the alkali being determined from impurities in the solutions used in the interference tests. DISCUSSION
The extent and character of the various kinds of interference are illustrated in Figures 3 to 9 and discussed below. Although the precision of the measurements was not established statistically, it was observed to be of the order of +l on the 0 to 100 scale of the instruments. This is equivalent, roughly speaking, to +1 on the per cent error scale of Figures 4 to 9. These error curves indicate magnitude, direction, and character of interferences but should not be used as correction curves, because their magnitude may vary somewhat between instruments of the same make and even between burners in the same instrument. Overlapping of Spectra. Calcium has a spectral band very near the best line for sodium (589 mp), With a large amount of calcium present, sodium results will be high by an amount equal
to the contribution of the calcium band to the total radiation passing through the slit when the monochromator is set at 589 mp. This type of interference can be reduced by using a minimum slit width in spectrophotometers which have this adjustment (Figure 3) but cannot be eliminated entirely. The interference can be estimated, however, by means of a plot such as Figure 3 in which the background is drawn in across the base of the sodium peak and the appropriate value is subtracted from the total peak height. This type of interference measurement can be made fairly easily on the Beckman instruments. It is somewhat more difficult on the Perkin-Elmer instrument, because the wavelength scale is not finely enough graduated. Radiation Interference. The presence of potassium or lithium, when sodium is being determined, will increase the radiation of the sodium, although the potassium and lithium themselves do not contribute radiation a t this wave length. Sodium and lithium have a similar effect on the determination of potassium. This type of interference is believed due to the effect of one metal upon the ionization of the other metal in the flame. ThuR sodium is believed to give off radiation in the flame a t 589 mp only when not ionized, and potassium is believed to repress the ionization of sodium and thus produce an enhancement of its radiation (7, 1.2). The magnitude of the effect, however, is not proportional to the concentration of the interfering element. For example, in the determination of sodium, if some potassium is present, the addition of more potassium has much less effect: than the same amount added to a pure sodium solution. Thus, if an analyst wished to determine sodium and did not know the concentration of potassium in the sample, he might add a large amount of potassium to both the sample and the standards and in this way minimize the effect of the potassium which was originally present in the sample. This has been called a buffering action because of its analogy to acid-base buffers (14). It has been shown that lithium will buffer the effect of sodium on potaasium or of potassium on sodium (9). Thus it is not necessary that the buffering metal be the same as the interfering metal upon which it is acting.
0
-
-10
U w
ui -20 * 0
L
-30
-40
c
U
L
1
0
0. 5
Figure 7.
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1.0 1.5. Total Ion Concentration, g. Ion/liter
2.0
Errors in determination of sodium resulting from presence of acids and salts Perkin-Elmer 52A, 15 p.p.m. of sodium
I0
0
5
IO
15
20
25
30
35
Weight R a t i o Na/K
Figure 6. Effect of lithium chloride as a buffer on interference by sodium in determination of potassium Beckman Model B 15 p.p.m. of potassium Band d d t h , 10.5 mp
The interference of potassium in the determination of sodium is shown in Figure 4 for the Perkin-Elmer instrument. The use of a lithium chloride buffer is seen to diminish the interference markedly when the instrument is used to read the sodium emission directly. Here it is clearly seen that 500 p.p.m. of lithium is better than 50 p.p.m. However, when the internal-standard principle is used, 50 p.p.m. of lithium is more effective than 500 p.p.m. The effectiveness of the internal-standard technique depends in
ANALYTICAL CHEMISTRY
736 theory upon the interfering substance affecting the lithium and the sodium intensities to the same relative degree so that the intensity ratio is not affected. Apparently this requirement is more nearly reached when the lithium content is of the same order of magnitude as the sodium content. Figure 5 shows the use of lithium chloride as a buffer for the interference of potassium in the determination of sodium with the Beckman Model B instrument. Figure 6 shows the effect of the same buffer on the interference of sodium in the determination of potassium. The Beckman instrument was not designed for use with an internal standard and i t would be somewhat inconvenient to use it in that manner. Anion Interference. If an appreciable amount of acid or salt is present in the determination of sodium with direct-reading, spray-chamber instruments, the radiation due to sodium is greatly reduced. It has been shown by Eggertsen, Wyld, and Lykken ( 3 ) that anion interference in this type of instrument is largely due to the effect of the acid or salt on the evaporation of solvent from the drops in the spray chamber. This, in turn, affects the amount of sample R*hich reaches the flame without settling out.
formed directly in the flame of this instrument, one would expect from the above discussion that there would be no vapor pressure effect and consequently less anion interference. Actually, the interferences are moderately large with certain acids and salts although the sharp increase in interference at low concentrations of interfering materials is not found with this instrument. The wide variation of interference among the various compounds with 0
IO
3
20
2
e
3’2
i
40
0 H,PO, (Cor.s:dered
I 3
1 0 5
1 1 0
I
I
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2 0
2 5
3 5
\OrmalLt>
Figure 9. Errors in determination of sodium resulting from presence of salts and acids B e c k m a n Model B, 5 p.p.m. of sodium Band width, 3 mp
-25
C
I I 1 I I I 0 2 0 4 0,6 0.8 1.0 ! Z S u : f u n c 4 c i d . g. Ian/li!er (3 x rno1ar:t))
J ! 4
the Beckman instrument suggests that, in some cases, an interaction with sodium is occurring within the flame which did not take place to the same extent in the cooler, more homogeneoue flame of the Perkin-Elmer instrument. Also it is evident, from the absence of a sharp increase in interference a t low concentrations of anion, that an anion buffer would not be effective with this instrument.
Figure 8. Reduction of sulfuric acid error by using lithium chloride buffer Perkin Elmer 52A flame photometer, 8 p.p.m. of sodium
iCKNOWLEDGMENT
The authors gratefully acknowledge the assistance of Mrs. C.
W.Manson n-ho performed much of the experimental work reThe interference of various acids and salts on the determination of sodium with the Perkin-Elmer direct-reading instrument is shown in Figure 7 . The data extend those previously reported ( 3 ) and have been rechecked in the region previously covered. The solid line is drawn through the points for sulfuric acid. Plotting the concentration as gram-ions per liter gives the least scattering of points as nould be expected if the interference were due to a vapor pressure effect. Gram-ions per liter is defined as the molarity multiplied by the number of ions per molecule in solution. Phosphoric acid appears to be an exception. Since the error increases most rapidly a t low concentrations of the inteifering compound, and since the principal interference of each compound is believed due to the same phenomenon of vapor pressure effect, any one of these compounds may be used to buffer the effect of any of the others. I n practice, lithium chloride is used since it also serves as a radiation buffer and as an internal standard. Figure 8 sho~vsthe effect of lithium chloride in reducing the interference of sulfuric acid with the Perkin-Elmer instrument, using both the direct-reading and internal-standard techniques. The use of lithium chloride in the direct-reading instrument considerably diminishes the interference due to sulfuric acid; however, the use of lithium chloride as an internal standard decreases the interference even further. Figure 9 shows the effect of various acids on the determination of sodium with the Beckman instrument. Since the spray is
ported in this paper. LITERATURE CITED
Barnes. R. B., Richardson, D. R.. Berry, J. W., and Hood, R. L.. ANAL.CHEM.,17, 605-11 (1945). Berry, J. W., Cappell. D. G., and Barnes, R. B., Ibid., 18, 19-24, (1946).
Eggertsen, F. T., Wyld, G. E. A., and Lykken, L., ASTM Special Technical Publication, 116, pp. 52-65 (1961). Gilbert, P. T., Jr., Ind. Lab., 3, 41-6 (1952). Gilbert, P. T., Jr., Hawes, R. C., and Beckman, A. O., ANAL. CHEM.,22, 772-80 (1950).
Heyes. Josef, Angew. Chem., 50, 871-4 (1937). Huldt, Lennart. “A Spectroscopic Study of the Electric Arc and the Acetylene-Air Flame,” Dissertation, Uppsala, 1948. Jansen, W. H., Heyes, Josef, and Richter, C., 2. physilc. Chem.. A174, 291-300 (1935).
Lundegardh, H., “The Quantitative Spectral Analysis of the Elements,” Jena, Gustav Fisher, Part I, 1929; Part 11, 1934. Parks, T. D.. Johnson, H. O., and Lykken, L., ANAL.CHEM.. 20, 822-5 (1948).
Schuhknecht. W.. Angew. Chem., 50, 299-301 (1937). Smit, J.. Alkemade, C. T. J., and Verschure, J. C. M., Bwchim. et Biophys. Acta, 6 , 508-21 (1951).
Weichselbaum, T. E., and Varney, P. L., Proc. Soc. E z p t l . Bwl. Med., 71, 570-2 (1949).
West, P. W., Folse. P., and Montgomery, D., ANAL.CHEM.,22, 667-70 (1950).
RECEIVEDfor review November 2, 1964. .4ccepted January 15, 1955.
