Precision of a Simple Flame Photometer JOHN U. WHITE The White Development Corp., Stamford, Conn. To improve the ease and accuracy of sodium and potassium determinations in biological samples, a simple flame photometer has been built employing glass and interference filters, photovoltaic cells, and a galvanometer. Its air-gas flame is completely enclosed to prevent atmospheric contamination. With well-regulated gas and compressed air supplies, average departures from the average of thirty successive readings of &170were measured for direct intensities and &0.3Yo for internal standard readings. With unregulated gas pressure and less care-
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IIE most useful applications of flame photometry are in d e terminations of the alkali metals, which are easiest by this method and most difficult by chemical procedures. Photoelectric flame photometers suitable for such analyses have been described by Barnes et al. ( I ) , Berry et al. (,%'), Bowman and Belliner ( 4 ) , Fox (6),Gilbert et al. (6),Schuhknecht (8),Weichsrlbaum and Varney (9),and others. A number of these instruments are now commercially obtainable. The instrumentation has been developed to the point where flame photometry is suitable for use in many chemical as well as spectrographic laboratories. However, for medical applications there is still need for greater simplicity, so that reliable analyses may be made by relatively unskilled operators. In addition, the accuracy required in determinations such as sodium in blood serum is sometimes not attainable. In the instrument described the range is restricted to sodium, potassium, and, in some cases, calcium, which permits the use of such simple designs and controls that accidental errors are greatly reduced. The accuracy of the flame photometric method has been reported in a number of papers, the most comprehensive being those of Berry, Parks et al. (7), Bills et al. (S), and Fox. They have reported accuracies of & I % of the amount present in the determination of sodium and potassium, using instruments in which the atomizer is separate from the burner. Weichselbaum and Gilbert have reported accuracies of a few tenths of 1% when the sample is atomized directly into the flame. This paper describes a ver!' simple instrument of the former type and discusses its stability and precision. The only factors affecting stability that are considered are air pressure, gas pressure, and burner adjustment, the ones that affect the reproducibility of repeat measurements of the same sample. These set a limit t o the analytical accuracy obtainable from the instrument. By the use of suitable precautions and calibrations, this accuracy may be approached but not exceeded. The effects of such variations as viscosity, pH, surface tension, etc., are not considered. They are reported in the references above along with a summary of the literature on the accuracy of flame photometry.
ful a i r pressure regulation, the average departure for the internal standard measurements increased to dd.570. The readings were least affected by gas pressure changes with a large flame burning relatively little primary air. Their stability with respect to air pressure changes was constant over a wide range of conditions. The accuracy obtainable permits the measurement of the small changes in concentration that are significant when the normal range is only a small fraction of the amount present, as in the case of sodium in serum.
The glass atomizer, A , is held iii the center of a large glass chamber, C. Sample S is introduced through the funnel in the top of the atomizer and compressed air through the inlet, C A . The atomizer uses about 2.5 cubic feet per minute of compressed air a t 9 pounds per square inch and consumes 4 cc. of sample per minute under these conditions. Its sample tube is 0.3 mm. in inside diameter with no constriction anywhere along its length. The absence of a constriction is important in avoiding plugging of the capillary by dirt and dust. In addition, the capillary is straight, so that any dirt lodging there is readily pushed through. The atomized sample blows down through a chamber 1.62 inches in inside diameter and 5 inches long. At the bottom the fine fog is separated from the larger drops of liquid in B loop of 0.5 inch tubing. The fog goes out to the burner, and the unused liquid goes through a liquid trap, T , to the drain, D. There is an opening in the tubing following the liquid trap to avoid siphoning the liquid out of the trap. The burner is a standard Fisher Type 3-900 for burning cit jaa, modified for some of the experiments to take various h e d l iameter gas jets instead of its original variable orifice. Ita throat area is 0.31 square inch, its grid area, 0.57 square inch, and
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ATOMIZER AiVD BURNER
The atomizer and the burner system are ahown schematically in Figure 1.
Figure 1. Atomizer and Burner System
394
V O L U M E 2 4 , N O . 2, F E B R U A R Y 1 9 5 2
39s
its maximum air inlet area, 0.94 square inch. The fixed gas jets used had diameters of 0.052, 0.070, 0.082, and 0.100 inch. The burner is surrounded by a 2-inch glass chimney, C H , 12 inches long.
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Air containing atomized sample enters past a bame into the space inside the chimney. As there is no opening to the atmosphere at the base of the chimney, all the air supply to the burner is obtained from the atomizer, and there is no possibility that smoky or dust-bearing room air will get into the flame. Sample is introduced into the flame both in the primary air drawn in a t the bottom of the burner and in the secondary air a t the top. The chimney also reduces heat conduction and convection from the flame to the body of the instrument. To reduce heating still further, a bright aluminum reflector, R, keeps most of the radiant energy out of the instrument. Two openings provide paths for the light beams to reach the photocells. The photocells look through them and the flame into a black light trap, B. The trap becomes hot and must be insulated from the reflector. For the most stable operation the air supply to the atomizer was regulated by two diaphragm regulators connected into the line in series. The gas supply had a normal pressure of about 9 inches of water and a fuel value of 528 B.t.u. per cubic foot. It was regulated by passing through an adjustable constriction to the burner and to another tube immersed an adjustable distance below a water surface, thus stabilizing the pressure a t the burner whenever part of the flow escaped through the bubbler tube. OPTICAL SYSTEM
The optical system is shown in Figure 2.
Figure 2.
Two beams of light are taken from the flame just above the top of the burner. Each light path has two condensing lenses, L, of 2-inch diameter and 3-inch focal length, which focus images of the lower part of the flame, F , on the photocells, P. The filters for isolating different emission lines are loczted between the lenses. The lithium filter, Li,is positioned in the internal standard beam, while the other filters, -Vu, are held on a rotating plate so arranged that any other filter may be inserted into the other beam. For measurements of lithium concentrations, the lithium photocell is switched into the part of the circuit normally occupied by the other photocell.
Optical System
The combinations of filters selected for the d e termination of the different elements are given in Table I. Those marked I are simple filters for general use. Those marked I1 are more complicated and expensive filters for the determination of small amounts of unknowns in the presence of large amounts of interfering elements. ELECTRICAL SYSTEM
Figure 3. Schematic Wiring Diagram
The schematic wiring diagram in Figure 3 shows a simple potentiometer circuit x i t h a single sensitivity control and one switch t o Table I. Filter Data shift from internal standard For each filter are given the components, transmittance t o the desired radiation, minimum detectable concentration to lithium and to direct ini n parts per million, a n d the interference factors expressed as the relative gravimetric concentrations of the Interfering and desired elements needed to produce the same signal. tensity measurement of other Filter K Sa I L1 I Ca I pr'a I1 Li I1 Ca I1 elements. Components
C-2600
C-3480 C-9780
C-2403 B-6708
Transmittance, % Minimum detectable Interference factor K
75
18 0.05
55
0.05
0.3
C-5120 C-2412 CB-OK20 CB-ON20 25 1
C-3480 C-9788 B-5893
C-2403 B-6708 B-6708
18 0.05
35 0.5
3000 2000 !O_. 4. 3000 Na BUUU 1uu 3000 Li 3000 1 3000 Ca 600 100 400 .. 1000 C . Glass filter from Corning Glass Works Corning S . S. B . Dielectric interference filter f r o m Baird Associat'es Inc. Cambridge Mass. CB. Glass filter from Chance Brothers, Ltd., Glass -'Arks, Smethwick 4b, Birmingham, England.
C-2412 c-5120 B-6240 CB-ON20 15 2 10 200 30
..
The internal standard potentiometer for balancing the a m o u n t of u n k n o w n l i g h t against the lithium light is a 10,000-ohm Beckman helipot. With a ten-turn duodial, it gives an internal standard scale 1000 d i v i s i o n s l o n g . T h e photocells are of the self-generating type (Vickers Electric Division, Vickers, Inc., 1815 Locust St., St. Louis, Mo.).
ANALYTICAL CHEMISTRY
396
42,000 o h k ; period, 4 seconds. It is used wkb less than its critical damping resistance t o increase the current output from
atami&ereharnber, which isa&ut ZOseconds. Theincreasein the galvanometer's response time hy overdamping i s not important, so long as it is much smeller than this.
Figure 4.
three or five measurements. Each point on tbe other curves represents a single measurement. Interference. The filters should meet two requirements, high transmittance a t the desired wave length and low transmittance a t other wave lengths. I n general, these are mutually dependent, so that a compromise must be made to obtain reasonably high sensitivity without excessive interference from the transmittance of unwanted light. The amount of interference between the different elements depends on the spectral response C U N ~ of the photocells and the natural emissivities of the different elements as well as on the properties of the filters. The composite effect of these has heen measured as the concentration of each desired elment which gives the same deflection as a high concentration of each interfering element. For example, a solution containing 1000 p.p.m. of sodium gave a deflection of 0.5 division through the potassium filter; 10 p.p.m. of potassium through the -me filter gave a deflection of 45,eo that p . p m of potassium would have given & deflection of 0.5. The interference factor was 1 in 9000. The interference factors of all the filters are given in Table I .
Flame Photometer
Figure 4 is a photograph of the flame photometer
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reflector. T%eop&l'aystem is just below the top with thelithium photocell behind the atomieer and the other to the left of it. The control panel on the front includes the three electrical controls. The remaining operating cont,rol is the element selector wheel, which projects through the top and has different filters mounted on it. PERFORMANCE
The flame photometer has been set up and operated in several laboratories over a. period of 8 months. It has shown unusual stshility and freedom from atmospheric contamination. Accurate determinations of sodium and potassium are possible in smoky or dusty rooms. Thc atomizer seldom clogs. For low concentrations the noise level is less than half a galvanometer division. However, bemuse this is about the limit of readability, i t is arbitrarily assumed to be the minimum deteetahle signal. The corresponding minimum detectable concentrations given in Table I were calculated from the deflections measured for four solutions oontaining 200 p.p.m. of calcium, and 10 p . p m of sodium, potitssium, and 1ithium;rospectively. Figure 5 shows direct and internal standmd calibration CUNBB for sodium; Figure 6, an internal standard curve for potassium. The internal standard curve for sodium NaS measured independently in three laboratories with three different sets of standards. I n the figure, the group of points from each laboratory has heen adjusted to correct for differences in the sensitivity that was used in each laboratory. Each paint in this curve is the, average of
Figure 5.
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Direct and Internal Standard Calibration Curves for Sodium
The measurement of interference factors for the sodium filters was limited by the presence of sodium in the potassium, lithium, and cdcium available for test solutions. In all cases the D lines were clearly visible in the flame with a spectroscope. When the emission from the concentrated calcium, lithium, or pottLssium solution was measured through sodium filter 11, the insertion of Corning filter 2412, which removed the sodium light but t r a n s mitted the calcium, lithium, and potassium light, reduced the d e flection by more than 80%. The principal parts of the deflections measured for the lithium and potassium solutions through sodium filter I were also due t o tho sodium impurity. These interference factors for the sodium filters would probably have been several times larger if measured with pure materials. For calcium interference, the impurity was not significant with sodium filter I. Calcium filter I is suitable for the determination of relatively lsrge amounts of calcium, but a ormotion must he applied if lithium or much potassium or sodium is present. I n ~ e r analysis w
397
V O L U M E 24, NO. 2, F E B R U A R Y 1 9 5 2 Table 11.
Approximate Calculation of Errors Due to Interference Factors
Sample contained 10 p.p.m. of sodium, 1000 p.p.m. of calcium, a n d 10,000 p.p.m. of potassium, with 500 p.p.m. of lithium added as internal standard. Apparent Concentration of Sodium a n d Lithium D u e t o Different Components, P.P.XT. ...- -. ( I o i n pone n t Na Li Na Li filter filter filter filter
I
ID
I
I1
10 p . p . i n . Na +SO0 1.i 1000 p.p.ni. Ca
10
2.5
10,000 p.p.m. K Total
1 2 5 0
10 1 7 1
21
752.5
12.7
500
I1 500 1
nal standard, these are reduced to 39 and 26%. The improvement with the I filters by internal standard is fortuitous, as it is caused by cancellation of two errors arising from different interfering elements. I n the analysis of sampleslikethis, filter combinations I might be expected to give errors up to about loo%, filter combinations I1 up to about 307,. Both of these could be reduced by adding comparable amounts of the interfering elements to the standards or by correcting for known interferences.
5
Conrentration b y internal standard 21 X = 14.0 p.p.m. for filter I combination
2;;
Concentration by internal standard 12.7 X $og = 12.6 p,p.m. f o r filter I1 combination ,706
these corrections require that the standards be made up with concrntrations of sodium and potassium proportional to those of normal serum. Calcium filter I1 has more favorable interference factors and is useful for measurements with lithium as an internal standard. The significance of the interferences is shown by an example of a lowsodium food sample containing 10 p.p.m. of sodium, 1000 p.p.m. of calcium, and 10,000 p.p.m. of potassium. When 500 p.p.m. of lithium have been added as an internal standard, the errors calculated approximately in Table I1 should result. An interference factor of 1 in 10,000 instead of 3000 is assumed for potassium through the sodium filters, as this makes a reasonable allowance for the contamination of the potassium sample mentioned above. By direct intensity measurements the calculated errors are 110 ttnd 27y0 for the I and TI pair.q of filters, rrspc,ctively. By inter403
YD
INTERWL STANMRD READ ING
zm
100
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Om
OB
0
078
IO
POTASSIUM CONCENTRATION IN MILLECUIVLENE PER LITER
Figure 6.
Internal Standard Calibration Curve for Potassium
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AIR PRESSURE M POUNDS PER SQUARE INCH
Figure 7. Effect of Change in Air Pressure A.
Direct i n t e n s i t y
Ii. I n t e r n a l s t a n d a r d , 1 4 4 m e . p e r l i t e r (1000p.p.m.) o f l i t h i u m C. Internal s t a n d a r d , 72 me. p e r l i t e r (500 p.p.m.) of l i t h i u m
I n this’calculation nonlinearity a t the high potassiuni concentration would reduce its interference from that above. This calculation considers only optical interference due to incomplete spectral isolation. There may be other interatomic effects i n the flame that contribute equal or larger errors. Stability. To test the stability and reproducibility, sodium was selected as the element to be measured; since many analyses call for the greatest accuracy in its determination. Its concentration i n the samples used for tests was kept in or near the range below 2.5 me. per liter (59 p.p.m.), where the intensity of the 11 lines emitted was linearly proportional to the sodium content of the sample. For direct intensities, 1.09 and 2.18 me. per liter ( 2 5 and 50 p . p m . ) were used, and for the internal standard ones the same with 72 and 144 me. per liter (500 and 1000 p.p.m.) of lithium added. In some cases a wetting agent was also added, though it did not prove to have any effect on the readings. The effect of changing the air pressure on the direct intensity rcatling was measured under different conditions to test its independence of the burner adjustments. When all other variables rvei’e held constant, the rate of change of intensity with air pressure was the same for three different gas pressures, for four different air inlet settings, and for three different sizes of gas jet. The range of the variables covered the range through which the hurner operated well. Changing the air pressure appeared to affect only the amount of sample going into the flame without interrelated effects between it and the size or kind of flame. A number of factors besides pressure affect the atomizer’s performance. The principal ones are the size and orientation of the atomizer jets, the size and shape of the atomizer chamber, the position of the atomizer in it, and the pressure drop in the lines leading to and from the atomizer and chamber. Changing them changes t’hedeflection but does not have much effect on its rate of change n i t h air pressure.
ANALYTICAL CHEMISTRY
398 Figure 7 s h o w in curve A the effect of air pressure change on direct intensity. A given percentage change in air pressure produces half as great a percentage change in direct intensity reading a t high pressures, with larger changes a t low pressures, as found bv Berry. The internal standard readings using 144 me. per liter (1000 p.p.m.) of lithium shown in curve B are only about a fourth as much affected as the direct ones. With 72 me. per liter (500 p.p.m.) of lithium, curve C shows that the dependence on air pressure is reduced almost t o zero, as found by Fox. The reading passes through a minimum around 10 pounds. The difference between these and the reduction h r 2.5 found by Berry with 144 me. per liter of lithium may well he due to the faster flow of his atomizer, which made the concentration of both the lithium and sodium atoms in his flame higher. At the concentrations used, there is practically no reabsorption of sodium light in the flame, but there is enough reabsorption of lithium light to make a proportional increase in the concentration of both elements appear as a relative increase in the amount of sodium.
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cur with the minimum amount of primary air. They always increase as the air inlets are opened, but do so more rapidly for small jets than for large ones. The internal standard curves are considerably less steep than those of the direct intensities. The improvement in stabilization varies from a factor of 10 in the most favorable conditions to 5 in the less favorable but usable ones.
[ e -
I
0 -
a7 -
6-
5-
INTERNAL SANDARD
---
4-
94 c)
z
i
78
G A S PRESSURE IN INCHES
59
.
I e
*
r
e
Figure 9. Effect of Changing Gas Pressure w
0.082-inch gas jet and different air inlet openings
5
INTENSITY
. GAS PRESSURE IN
INCHES
Figure 8. Effect of Changing Gas Pressure 0.100-inch gas jet and different air inlet openings
The effect of changing gas pressure was measured at constant air pressure for different air inlet settings and for different gas jet sizes. Figures 8, 9, and 10 show how the direct intensity and internal standard readings varied with gas pressure. I n each figure air inlet areas are expressed in square inches. The curves for the 0.052-inch jet are not given, as they were too erratic to he useful. The data shoL7 that the minimum intensity was about the same for each jet a t the smallest gas pressure and air inlet setting at which the burner would operate. The intensity increased with gas pressure and with the amount of primary air. The rate of increase was greatest for the smallest jet and smallest for the largest one. The percentage change in the readings for a given percentage change in gas pressure is proportional to the slope of the curves, which varies from 0.3 to 0.9 for different jet sizes and air inlet settings. The least sloDes are about the same for each jet and oc-
Reproducibility. The r$producihility of the flame as a source of radiation was measured by making a series of approximately 30 readings in 7.5 minutes and calculating for each series the average departure from the mean. These average differences are called the average noise level when expressed as percentages of the mean readings. The noise level was found to be independent of air pressure over a wide range, so long as the air flow was below a critical rate. above this rate, drops were picked up from the walls of the atomizer chamber and blown into the atomizing jet and into the base of the burner. From 6 to 12 pounds' air pressure, the noise level averaged 1.2% by direct intensities; a t 14 pounds, it was 4.3%. Using 144 me. per liter (1000 p.p.m.) of lithium as a n internal standard, the noise level was 0.5%. It decreased as the air pressure 1%-asreduced. When the lithium concentration in the sample was reduced to 7 2 me. per liter (500 p.p.m.) values of 0.3 to 0.4% were obtained. Other series of measurements sho[ved that the noise level was the same with sodium concentrations of 25 and 50 p.p.m., both by the direct and by the internal standard methods; the blue light emitted by the flame in the absence of sample was about twice as steady as the sodium series under the same conditions: and the stabilities of direct measurements of lithium and potassium were essentially the same as those of sodium. The fact that both the observed noise level and the rate of change of internal standard reading with air pressure went down when the lithium concentration was reduced suggests that the residual noise level was associated with the air supply and atomizer. It may have been affected by variations in atomizer performance, caused either by small changes in air pressure or by turbulence and other conditions around the atomizer.
V O L U M E 2 4 , N O . 2, F E B R U A R Y 1 9 5 2
399
The noise level was also measured as a function of gas pressure and burner adjustments, using both the direct and internal standard methods. Xhen the conditions were well stabilized, the noise was about the same for all direct intensity readings, with perhaps slightly better performance a t the smallest usable air inlet openings. The fluctuations in the internal standard readings also were almost independent of conditions but slightly less than half as large as those for direct intensity. The range of measurements included gas jet diameters of O.Oi0 and 0.100 inch, gas pressures from 1 to G inches, and air inlet openings from 0.29 to 0.94 square inch. Several series of internal standard measurements were made in which the sensitivity of the instrument was adjusted by standardizing on a reference solution before each reading. All were made with 9 pounds' air pressure, 4 inches' gas pressure, gas jet diameter 0.070 inch, and air inlet opening 0.15 square inch. Some series used 144 me. per liter (1000 p.p.m.) of lithium and sodium concentrations of 2.18 and 3.27 me. per liter (50 and 75 p.p.m.); others, 72 me. per liter (500 p.p.ni.) of lithium and sodium concentrations of 1.09 and 1.74 me. per liter (25 and 40 p.p.m.). Keither showed the slow variations of the other series, their readings appparing to have only random fluctuations. The average difference from the mean was only +C0.3c7,for each. As this short-time reproducibilitv is representative of the operating procedure actually used in analyzing an unknown sample, this is the amount of fluctuation that might be achieved under well-stabilized conditions.
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-- ------- -- ___----- -- ----
INTERNAL STANDARD
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pected t o occur under normal conditions where the gas supply is not regulated and a single regulator is used on the air supply. When the variations are larger than these, they contribute the principal part of the noise. The mean noise level was measured under such conditions and found to be 3~0.570. This may be expected to vary from time to time depending on the loads on the supply lines. DISCUSSION
The results above indicate the most favorable operating conditions for the flame photometer burning this gas. The best air pressure is the highest that does not cause an increase in the noise level. The best gas jet and air inlet openings can be picked out of the figures as those that give the highest intensity and least slope a t the available gas pressure. However, if the heat content of the gaq is different, compensating adjustments must be made to keep the same actual conditions in the flame. It has been the author's experience that the appearance of the flame is a reliable guide to its proper adjustment. In general, increasing the size of the gas jet a i t h constant gas pressure increases the size of the flame and the intensity of the light emitted. If the gas pressure is IOII-, the jet should be large; if it is very high, i t may be advantageous to reduce it bv putting a fixed constriction in the line so that a larger jet may be used. The air inlets control the amount of primary air in the flame. iit very small openings, the flame lifts off the top of the burner. A t larger ones it burns directly over the grid in little green cones. Next, the cones turn blue. Finally, at very large openings the cones lose their tops and give the flame a fuzzy appearance. K i t h the largest jet, the flame did not get enough air to turn blue even with the air inlets wide open. If tho gas has a higher heat content, more air is needed t o burn it, and a smaller gas jet must be used to get equivalent flame conditions. . The color and appearance of the flame are indicated in Figures 8, 9, and 10. The solid lines indicate that the flame had green cones, the dashed ones that it had blue cones, and the dotted ones that it appeared fuzzy. To get high sensitivity, high intensity is needed. This is best combined with relatively noncritical conditions a t large jets and moderate gas pressure giving the largest flames that do not cause overheating. The conditions of a small jet and high gas pressure, where the curves might appear to indicate invariance, are not favorable, as the readings are very dependent on air inlet opening and tend to vary erratically.
c
ACKNOWLEDGMENT
The author wishes to thank John W. Berry of the .%merican Cyanamid Co., Howard Eder of the Cornell ~ I ( d i c a 1 School, Charles L. Fox, Jr., of the College of Physicians and Surgeons, Columbia University, and B. C. Wiggin of Baird Associates, Inc., for their help and advice in planning and tcqting this instrument.
INTENSITY
LITERATURE CITED
(1)
Barnes, R. B., Richardson, D., Berry, J. W.,and Hood, R. L., IND. ESG.CHEM.,ASAL. ED.,17,605 (1945).
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GAS PRESSURE IN INCHES
Figure 10.
Effect of Changing Gas Pressure
0.070-inch gas jet and different air inlet openings
If the gas and air pressures vary, their effects are additional to this. The percentage change in either of them that would contribute an equal amount of variation with favorable instrument settingsis +C lO%forgaspressure, +27'rfor airpressurewith 144me. per liter of lithium, and considerably more for air pressure with 72 me. per liter of lithium. These are variations that might be ex-
( 2 ) Berry, J. W., Chappell, D. G., and Barnes, R. B., Ihid., 18, 19 (1946). (3)
Bills, C. E., McDonald, F. G., Niedermier, W.,and Schwartz, 11.C., ANAL.CHEM., 21,1076 11949).
(4) Bowman, R. L., and Berliner, R. W.,Federation Proc., 8 , 14 (1949).
(5) FOX,c. L., Jr., A N A L . CHEM., 23, 137 (1951). (6) Gilbert, P. T., Hawes, R. C., and Reckman, A. O., Ibid., 22, 772 (1950). (7) Parks, T. D., Johnson, H. O., and Lykken, L., Ibid., 20, 822 (1948). ( 8 ) Schuhknecht, W., Angew. Chem., 50,299 (1937). (9) Weichselbaum, T. E., and Varney, P. L., Proc. SOC.E s p t l . Biol. Med., 71,470 (1949).
RECEIVED May 28. 1951
