Flame Photometer Attachment as an Excitation Source for the Spectrograph W. G. SCHRENK AND F. M . S M I T H ' Kansas .4gricultural Experiment Station, Manhnttan, Kan.
10 p.p.m., sodium 20 p.p.m., and lithium 20 p.p.m. With lithium as internal control, a series of 18 determinations of sodium gave a standard deviation of 1.44%. For 20 potassium determinations the standard deviation was 0.47%. Agreement with other methods of analysis ia good. Excitation conditions affect sensitivity. Maximum sensitivity was attained using a gas pressure of 6 em. of water and an air pressure of 25 pounds per square inch. Optimum oxygen pressure varied, being 30 inches of water for potassium and 40 for lithium and sodium.
The Beckman flame spectrophotometer attachment may be used as an excitation source for sodium, potassium, and lithium analyses in conjunction with usual spectrographic equipment. I t s use with a Bausch & Lamb large Littrow spectrograph is described. Using Eastman Type I-N plates, good results were obtained for the determination of sodium and potassium in plant and animal substances. When using a slit width of 45 microns and an exposure time of 30 seconds, approximate minimum readable sensitivities were potassium
I
directly in front of the slit of the spectrograph to focus the rays further. The w e of the lens and concave mirror increased the sensitivity of the spectrograph approximately 50%. The burner of the flame source was approximately 20 cm. (8inches) from the slit. No undue heating of the optical system was observed. The excitation procedure wa8 standardized with respect to exposure time (30seconds) and slit width (45microns). Although higher sensitivit could be obtained with increased exposure time and increasedrslit width, this was not neceasary for the work planned in this laboratory. Other arrangements could be used for other samples a8 desired. Slit width would, of course, be limited by the amount of resolution desised All samples were prepared for analysis by drying, followed by ashing in a muf3e at 550" C. The ash was then taken up in a solution containing 100 p.p.m. of lithium in the form of lithium chloride, which served as an internal standard. Sufficient solution was added to place the final potassium concentration between 20 and 100 p .m. For sodium the ash was diluted with the internal standar$solution to make the sodium concentration fall within the limits of 50 to 400 p.p.m, These ran ea were found most satisfactory for the procedure as outlinecf in this paper. All spectrographic data reported in this paper were obtained using lithium as an internal standard. Standard solutions for calibration purposes were made frorq reagent grade chlorides of potsssium, lithium, and sodium. For potassium, five concentrations were used: 20, 40, 80, 80, and 100 p.p.m. For sodium the concentrations repared contained 50, 100,200,300,and 400 p.p.m. Each of &eae solutions also contained 100 p.p.m. of lithium which served as the internal standard. Five standards were exposed on each plate. Eastman T e I-N spectroscopic plates were used. They were developed in%-19 for 4 minutea at 68" F. Line intensities were measured with an ARGDietert densitometer. Spectral lines em loyed for analytical and internal standard ses were as 8110~s: potassium 7664.9 and 7699.0 A., f i x m 6707.8 A., sodium 5890 and 5896.9 A., and calcium 4226.7 A. Calcium, however, is not in the s ctral region for which this type of spectroscopic plate is intenddleto be used.
XTEREST in flame excitation methods for spectrochemical analyses has been revived recently by the development and availability of instruments for this purpose in this country ($4These ). instruments, primarily designed for the determination of elements requiring low excitation energies, apparently are based on the methods originally developed by Lundeghrdh (10)who, working with an air-acetylene flame, was able to obtain highly reproducible results, apparently through good control of excitation conditions. Equipment similar to that developed by Lundeghrdh has not been readily available in this country. Cholak and -,Hubbard (7) have described equipment, patterned after that 'of Lundegllrdh, which was constructed for them. Their evdqation of its usefulness and efficiency also indicates highly accurak results and reproducibility. Some of the newer equipment is completely self-contained, using photoelectric cells and galvanometers to indicate the intensity of emission, after suitable isolation of the spectral region required for the element under consideration (6). The Beckman flame spectrophotometer attachment, however, includes only a burner and atomizer and their appropriate controls. It has been designed primarily as an attachment to the Beckman spectrophotometer, but because of its construction it may serve also atj an excitation source for ordinary spectrographic equipment. It differs from the Lundeghrdh flame, in that it uses oxygen and natural or bottled gas, rather than oxygen and acetylene. This paper describes the use of the Beckman flame attachment as an excitation source for the spectrograph and presents data regarding the sensitivity, precision, and accuracy obtained in the determination of sodium and potassium in materials of plant and animal origin. A limited amount of data on the influence of extraneous elements on analytical results as applied to sodium and potassium analyses in these materials is also presented.
EXPERIMENTAL
MATERIALS AND METHODS
The Beckman flame spectrophotometer attachment was used as an excitation source in con unction with a Bausch dz Lomb large Littrow spectrograph. d h e flame photometer used was the type which had the heated atomizer chamber. A concentric atomizer also was used. The flame photometer was modied by replacing the mirror supplied with the instrument with a piece of stainlwa steel bent so as to focus the light partially on the spectrograph slit. A Oylindrical quartz lens was mounted
tP
"
, . I
Present sddrw, Standard Oil &m&y ing, Ind. 1
I+esesrch Laboratories, Whit-
Effect of Excitation Variables. In order to determine optimum operating conditions it was necessary to study the effects of three variables: oxygen, gas, and air preesures. It WBB found advisable to use air pressure at its maximum; in this laboratory this was 26 pounds per square inch (17,600 kg. per sq. meter). The air pressure apparently is related to the quantity of sample atomized into the flame per unit time. Gas pressure (natural gas waa used) also was held at its maximum of 6 cm. of water. This value also gave the greatest sensitivity. Considerable difference in sensitivity oocurred with changea in oxygen preesure, as can be observed by an inspection of F i f p 8 1, where spectral line intensity is plotted against oxygen preesure.
1023
ANALYTICAL CHEMISTRY
1024 Table I. Results of Duplicate Determinations of Sodium and Potassium in Plant Materials Sodium, % vi^Potassium, 5% DevisSample Silage 1 Silage 3 Alfalfa pelleta Brome pelleta Soybean pelleta Soybean meal Wheat grass 1 grass 2 Oats grass grass
1 2
Trial Trial tion Trial Trial tion, 1 2 Mean %’ 1 2 Mean 5% 0.044 0.044 0.0440 0.00 0.98 0.98 0.980 0.00 0.054 0.056 0.0550 1.82 1.19 1.21 1.200 0.83
I0.0,
1100
4.0-
6.0
-60
- 40
0.037 0.039 0.0380 2.64 2.17 2.14 2.155 0.70 0.041 0.041 0.0410 0.00 2.36 2.38 2.370 0.42 0.025 0.025 0.0250 0.00 1.72 1.91 1.815 5.24
2.0
-
-20
1.0
-
- IO
0.012 0.016 0.0140 14.25 1.29 1.32 1.305 1.15
0.068 0.074 0.0710 4.22 2.85 2.85 2.850 0.00 0.110 0.115 0.1125 2.21 3.42 3.46 3.440 0.58
. . . . . . . . . . . . . . . . . . . . . . . .
3.22 3.23 3.225 0.15 2.80 2.73 2.765 1.26
a
-6-
- 6 %
a A
.4
0 c 0
.-h*
e c (.
c
i
C
I-
- 2 C a u
L
1
50 100 Sodium, RRm.
10
I
200
400
Figure 2. Typical Calibration Curves for Sodium in Plant and Animal Material
Oxygen pressure, inchord w&r Figure 1. Effect of Oxygen Pressure on Relative Intensity of Potassium, Sodium, Lithium, and Calcium
A maximum value is obtained for each element, followed by a decrease. Maximum sensitivity is obtained for potassium a t an oxygen preaeure of 30 inches of water; for potassium and lithium the value is 40 and for calcium 60. These values roughly follow the order of ionization potentials of these elements. Because of these data a value of 40 inches of water was chosen as the best value for both potassium and sodium and was used for the data reported herein.
Table 11. Precision of Spectrographic Determination of Sodium and Potassium Element Na
K
No. of Determinationa 18 20
%% 62-72 67-72
Mean P,P.M’. 65.7 68.8
Standard Standard Deviation, Deviation, P.P.M. % 0.95 1.44 0.32 0.47
Using these conditions with an exposure time of 30 seconds and a slit width of 45 microns, the sensitivity of this method of excitation is aa follows: potassium 10 p.p.m., sodium 20 p.p.m., lithium 20 p.p.m., and calcium 600 p.p.m. Standards used for lithium ranged from 10 to 100 p.p.m. and for calcium from 500 to lo00 p.p.m. Precision of Method. Precision can be determined well from the results of duplicate determinations made for both potassium and sodium (Table I). Data are also presented in Table I1 on a aeries of 20 potassium and 18 sodium determinations made on the same sample. The standard deviation for potassium was 0.47%, and for sodium a value of 1.44% was obtained.
Calibration curves for potassium and sodium are given in Figures 2 and 3, with lithium as an internal standard and without internal control. The data are plotted by two methods; the lines sloping downward are plotted with reference to the per cent transmittance only and represent typical calibrations without the use of the internal standard. The lines sloping upward are relative intensity curves using lithium for internal control. The per cent transmittance of the lithium line is also plotted in Figure 2 in order to show graphically the reproducibility of the line intensity. The calibrations are not the usual straight lines obtained by other methods of excitation. This effect has been described as being due to cooling in the outer portions of the flame ( 8 ) . Comparing the two calibrations given for sodium (see Figure 2), in which the two spectral lines show similar curvatures regardless of line intensity, shows that the effect is not due to operating on the nonlinear portion of the developing curve. Effect of Extraneous Elements. Berry, Chappell, and Barnes (6)and Parks, Johnson, and Lykken (11) have reported the effects of certain extraneous substances on the intensity of emission of the spectral lines in the flame. A limited amount of similar data is presented in Figure 4,in regard to the effects of potassium, sodium, and lithium on each other. The range of values used covers those usually encountered in working with plant and animal materials. I t is evident that within this limited range no general trend occurs. Such data, however, would not be necessarily valid outside the limits of this study. Comparison with Other Methods. The spectrographic method has been compared with the more common chemical methods used for potassium and sodium. In Table I11 are presented data on potassium determinations. Only two samples were available that had been analyred by the chloroplatinate procedure ( I ) ; the agreement on these two samples seems to be good. The remaining potassium determinations were made by means of the sodium cobaltinitrite reagent suggested by Peech and English (IS). The technique used with this reagent was that developed by Wolf (14). These data are in relatively good agree-
V O L U M E 22, NO. 8, A U G U S T 1 9 5 0
1025
ment, the mean difference between the methods being 4.1%. Duplicate determinations made spectrographically were in closer agreement than those made by the colorimetric method, indicating greater precision for the spectrographic procedure. Numerous sodium determinations have been made on plant materials, but no comparison with other methods is available. A series of samples of bovine urine and blood serum has, however, been analyzed by chemical and spectrographic methods, with the results reported in Table IV. The chemical method used was that of Weinbach (23). These results are also in good agreement and serve to illustrate the type of results to be expected when using the flame spectrophotometer attachment as an excitation source in these spectrographic techniques. DISCUSSION
The data presented indicate that i t is possible to use the Beckman flame spectrophotometer attachment as an excitation source with the usual type of spectrographic equipment.
The elements studied have included only potassium, lithium, sodium, and calcium. Data regarding calcium indicate that too low a sensitivity is obtained for use with most plant and animal materials without concentration of the calcium. The spectroscopic plates used, however, are not considered too sensitive in the region where the calcium lines are located (4226.7 A.). It is possible that some other type of plate, designed for this wave-length region, together with increased slit width and exposure time, would make calcium determinations possible with the flame excitation source. The procedure has the advantage of simplicity, as compared with chemical methods usually used for the determination of sodium and potassium. The precision is also good, and compares favorably with other methods. Spectrographic determinations of sodium and potassium have, in general, been less precise than for many other elements-for example, Helz and Scribner (9) in reporting on minor elements in portland cement indicate a probable error of a single determination for sodium oxide to be 5 % ; for potassium oxide the error reported was 8%. The original unheated atomizer chamber also waa tested in this work. A gradual decrease in sensitivity occurred during a series of readings. This presumably was due to the cooling of the atomizer chamber as the sample evaporated, followed by recondensation of the vapor on the sides of the chamber. This fault seems to have been completely corrected by means of the heated jacket on the atomizer chamber. It is essential that samples be completely free of solid particles. The atomizer can clog readily. This effect is, however, easily observable and can be corrected by blowing air back through the atomizer tip.
0.6
Potassium, ppm. Figure 3. Typical Calibration Curves for Potassium in Plant and Animal Material
I
I
0.6
-.-.
1
Table 111. Comparison of Chemical and Spectrographic Methods for Determination of Potassium Spectrographip Chemical K, 5% K, % 3.23 3.34" 2.77 3.04' 2.27 2.16 Wheat grass 2.44 2.65 2.96 3.23 2.78 2.72 2.50 2.70 2.02 1.90 1.92 1.90 2.11 1.93 Chloroplatinate procedure used on these two samplae. Sample o a t pass
a
Table IV.
Difference,
%
+3.3" +8.g5
+4.9 +7.9 $8.3 +2.9 +7.6 -0.4 kl.1 +8.0
Comparison of Chemical and Spectrographic Methods for Determination of Sodium
~
8 0410
1
1
200 Sodium,mm.
I
I
400
I
1
Figure 4. Effects of Varyin Concentrations of Sodium, Potaesium, and Lithium on gelative Intemity of Element Line
I
Sample Bovine blood 2 Serum 4 0
16 Bovine urine
12 2 4 0
8 10 12
Spectro a hic Na, 3150 3200 a350 3300 3460 3450 360 1100 790 100 135 110
P5.h.
Chemical Na, P.P.M. 3660 3690 3010 3010 3080 a660
a66
laso 742 90
148 90
Differenos,
%
4-13.8 +13.2 7.2 8.0 6.3
++ + + .6 + 51.7
+16.9 -- - d6.2 n 8:s -12.7
+
It has been found necessary to ash all samples containing organic substances prior to atomizing them into the flame. The organic materials have a tendency to clog the orifices of the burner, and this is followed by a loss in sensitivity. The ashing procedure eliminates this difficulty. The technique used converted the ash primarily to chlorides; as a result, chlorides were used in the preparation of standards whenever feasible. In an earlier bulletin (3) it was suggested that the earnple be
ANALYTICAL CHEMISTRY,
1026 diluted with one part of impropy1 alcohol to four parts of sample. Effech due to viecasity changes have been indicated (4). The effects have not been studied in this laboratory but the use of the alcohol mixture has been continued and all data reported in this paper have been obtained, using the alcohol dilution technique. Eaatman I-N plates were the only type found srttisfactory. Other plates, including Type I-L, did not have sufficient sensitivity in the spectral region used. The Type I-N, which has been designed for use in the infrared region of the spectrum ( 8 ) , apparently decreases in sensitivity in the wave-length region between the sodium and potassium lines. Thia probably BCcounta for the differences in sensitivity Observed for these two elements, which are not in accord with those reported for arc and spark spectra elsewhere (6). Plate development seemed more critical with Type I-N plates than with others. This waa partly due to the relatively high gamma ( 2 . 6 a t 7699.0 A.) produced by the platea and the developer used. These factors also account in part for the precision achieved in these determinetione, aa well aa the narrow range of concentrations for which the method is suitable. The procedure aa deacribed and evaluated appears to be a promising method for the determination of sodium and potassium in several typea of samples. Accuracy and sensitivity are good, ma11 samples are sufficient (2 to 5 ml.), and existing equipment in many laboratories may be utiiieed. Further study should lead to applications of the procedure to other types of samples
and possibly to the determination of other metals which may respond su5ciently to this form of excitation. LITERATURE CITED
h c . 0 5 c . Agr. Chemists, “05cial bnd Tentative Methods of Analyaia,” 6th ed., p. 121, 1945. Barner, R. B., Richardson, D., Berry,J. W., and Hood, R. L., IND.ENQ.CHEM.,ANAL.ED., 17, 806 (1946). Beckmcm BuU. 1 6 7 4 , National Technical Laboratories, South Paaadena, Calif., 1948. Ibid.. 193-B (19481.
Be*, J. W:, Ch;rppell, D. G., and Barnes, R. B.,IND. ENO. CHBM.,ANAL.ED.,18, 19 (1946). Brode, W. R., “Chemical Spectroscopy,” 2nd ed., New York. John Wiley & Sow, Inc., 1943. Cholak, J., and Hubbard, D. M., IND. ENO.CEEM.,ANAL.ED., 16,728 (1944).
Eaatman Kodak Co., “Photographic Plates for Scientific and Technical Urn’’, 6th ed., 1948. HeL, A. W., and Scribner, B. F., J. Reeearch Natl. Bur. Standa r L , 38, 439 (1947).
Lundeghrdh, H., 2.Phyeik, 66, 108 (1930). Parka, T. D., Johnson, El. O., and Lykken, L., ANAL CHEM., 20,822 (1948).
Peech, M., and English, L., Soil Sci., 57, 172 (1944). Weinbach, A. P., J . Biol. Chem., 110,BS (1935). Wolf, B., IND.ENO.CHEM.,ANAL.ED.,16, 121 (1944). REC~CIVED September 28, 1949. Prwented before the Division of Analytical CHEMICAL and Micro Chemistry at the 116th Meeting of the AM~CRICAN SOCIETY, Atlantic City, N. J. Contribution 391, Department of Chemistry, Kansaa Agriaultural Experiment Station.
Mineral Analysis of Biological Material by Flame Spectroscopy A ppa ratus and A ppl ication ABNER R. ROBINSON, KATHERINE J. NEWMAN, AND ERNEST J. SCHOEB Research Luboratory, Children’s Fund of Michigan, Detroit, Mich. The construction is described of an inexpensive, simple burner with an all-plastic atomizer and glass chamber similar to those described by Griggs (5). With the use of a Bausch & Lomb medium quartz spectrograph 15 elements can be determined directly or upon the dissolved ash of biological materials with an accuracy of *S%.
L
U N D E G b D H (6, 7) in Sweden published in 1929 a text on the use of a flame as the excitation mume for spectral identification and quantitative determination of alkalies, alkaline earths, and a number of other elements. However, in 1941 the flame technique waa being used by only two or three workers in the United States (3,6). Since 1944 many modifications of the flame-producing burner neceesary for thia method of analysis have been described in the literature and eeveral are available through commercial inetrument channels. After extensive experimentation the authors have devised a simple, inexpensive burner with which eatisfactory results can be obtained from analyses of biological materials for certain minerals. BURNER
The mskrials used in constructing the burner Shown in Figura 1 are considerably less expensive than those employed in earlier bumere. The tip is a , s h d a r d , stainless steel ppduct (Jarrell-Ash Company), eciall rhodrum Iakd and connected to the borosilicate t u L with a ruiber sieeve. The acetylene inlet
3)aas
is a piece of thick-walled capillary tube, sealed into the burner with Plicene. Used with a Bausch & Lomb medium quartz spectrogra h the burner is mounted with the tip 4 cm. from the spectro a fiislit. A flat, stainless steel mirror is mounted directly behinrtge flame, the optical path of the spectrograph, to intensify the emmion. BURNER OPERATION
The burner is operated with comprewd air supplied a t a pressure of 30 ounds and acetylene from a tank a t a water gage preecm. Both air and eoetylene are water-saturated before enterin the burner. An all- lastic atomizer (JarrellAah Company? and lssa atomizer c%amber similar to those except that the. chamber and burner described by Grigga are connected by a standard-taper ground joint, are employed to introduce the sample. The atomizing unit and burner are eaaily cleaned after use. Wetting agents are not required in the operation of the burner and s screen below the burner tip, used in the LundegCdh and later modificatjons, ia unnecessary. nure of
&
b),
SPECTROGRAPHIC TECHNIQUE
In analysis for alkali metals the region 1 am. above the blue cone of the flame is wed, the region LundegCdh designates 88 most
