Quantitative Absorption Spectrophotometry An Internal Control Method J . S. OWENS,’ The Dow Chemical Company, Midland, Rlich.
IPIT
obtained by density matches made upon only adjacent spectra, high accuracy is obtained only by simultaneous photography of the two spectra. A large number of pairs of spectra are ordinarily required to determine the extinction coefficient, a t a selected wave length, of a sample of known qualitative, but unknown quantitative, composition. In visual spectrophotometry, two beams from the same light source pass simultaneously through the absorbing and comparison materials, and adjacent spectra of the two beams are formed by means of a suitable optical system and a spectrometer. The amount by which the intensity of the comparison beam must be reduced by suitable means, in order to equalize the intensities of the two beams, in the small wave-length region selected, gives the measure of the absorption. I n the photoelectric technique only one light beam is used. The intensities a t a selected wave length, of the spectra, produced by a monochromator, of the absorbing and comparison materials are measured in succession with a photocell-electrometer arrangement. High accuracy requires a light source of very constant intensity and great intrinsic brightness.
ORDER to take advantage of the successful applications of emission spectrochemical analysis of metallurgical and chemical materials, many industrial and university laboratories are now equipped with apparatus suitable for this work. I n a number of these laboratories absorption spectroscopy would also be useful, but, unless the number of applications available is considerable, few laboratories will incur the expense of an accurate spectrophotometer. Moreover, the critical adjustment of this instrument renders practically inefficient the frequent alternations required in a control laboratory where the same spectrograph is used both for emission work and as an adjunct to an absorption spectrophotometer. An internal control method of photographic spectrophotometry was developed which permits the quantitative determination of extinction coefficients, or of percentage absorption, to be made rapidly with the same apparatus and general technique used for quantitative emission spectrochemical analysis. While this method may be used t o determine the complete absorption curve of a material, it is most suitable for quantitative analysis in which the extinction coefficients need be determined a t only one or a few wave lengths. Representative chemical applications include determination of the amount of polymerization of resins, analysis of industrial organic chemicals for constituents or impurities, analysis of blood serum for the different hemoglobin compounds, and following the course of chemical reactions.
Internal Control Method BASISOF METHOD.Unless a critically adjusted, dual optical path is used so that the spectra of the absorbing and comparison substances can be simultaneously photographed or visually examined, or a two-photocell, “null” arrangement can be employed, the situation present in the three conventional methods described is analogous to that found in quantitative emission spectrochemical analysis before the use of internal control elements. The success of spectrochemical analysis, since the introduction of the use of radiation intensities and of internal controls, suggested similar use of these two quantities in quantitative photographic spectrophotometry. The comparison of the radiation intensities, a t any wave length, transmitted by the absorbing and comparison materials, is made by a method of internal control. The basis of the method is the determination of the relative intensity of selected wave lengths, one within an absorption band of the material, and the other in a region, in which there is no absorption, of the spectrum of the material. The true absorption is obtained by equalizing the effective exposures of the absorbing and comparison materials by subtracting from the log intensity ratio of the selected, unabsorbed internal control wave length to the absorbed wave length in the absorption spectrum the ratio for the corresponding wave lengths in the comparison spectrum. If the intensity of the light source and the exposure conditions can be maintained sufficiently constant, the absorption may be obtained by a direct intensity comparison, a t the same wave length, of the absorption and comparison spectra ( I ) . By this means, however, all of the advantages of the internal control technique are lost. In the internal control method, spectra of the comparison material and of the different absorbing materials under test are photographed on the same plate. Identical intensities and exposure times are not required. A sixfold inequality in the exposure times for the absorbing and comparison materials was found, in a representative example, to produce no
Conventional Methods The absorption of radiation by a material is given by the combined expression for Lambert and Beer’s law I = 10 x 10-kcl intensity of radiation incident upon the material intensity transmitted by thickness, 1, of the material concentration of the absorbing substance in the material k = specific extinction coefficient (extinction coefficient for unit concentration and unit thickness)
where Io I
= = c =
Since the value of k is ordinarily independent of c and I, k is the most suitable quantity for the measurement of the quantitative absorption of radiation by a material. k may be obtained by photographic or photoelectric methods in the ultraviolet region, and also by visual methods in the visible region of the spectrum. I n the usual photographic method of spectrophotometry two light beams from the same source simultaneously reach the spectrograph slit. One beam, which traverses the absorbing material, is maintained a t constant intensity; the other, which traverses the comparison material, may be arbitrarily varied in intensity. A series of photographs is taken on the same plate with the intensity of the latter beam varied over the desired range. Each of these photographs consists of a pair of adjacent spectra, one of normal intensity, the other of varied intensity. The percentage absorption, or the extinction coefficient, produced a t any wave length by the absorbing material is determined by visually, or microphotometrically, finding which pair of spectra shows equal density at that wave length. Since the extinction coefficients are 1
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643
INDUSTRIAL AND ENGINEERING CHEMISTRY
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error in the value of the extinction coefficient. An intensity calibration of each plate is made by means of a step-slit, a rotating step-sector, or a number of selected lines in a spectrum for which intensities have been measured by one of these methods. The blackenings of the selected wave lengths and of the steps of the calibration pattern are measured microphotometrically. From the blackenings of this pattern the characteristic curve of the plate is drawn. The logarithms of the relatiye intensities of the pairs of selected wave lengths are obtained by applying their measured blackenings to this curve. From these log intensity ratios the extinction coefficient of the absorbing material is readily determined. EXPERIMENTAL CONDITIONS.Sample. If the material under test is a liquid, powder, or irregular solid, it may be dissolved in a suitable solvent. The solution and solvent are placed, in succession, in a cell containing plane quartz windows and the absorption spectrum of each is photographed. If the material is a film, plate, or block of uniform thickness, its absorption spectrum may be photographed directly. In this case the comparison substance may be the nonabsorbing support, a quartz plate, or air. Light Source. Light sources which emit either continuous or line spectra may be used. The former is useful for the detection and identification of absorption bands, but the latter is preferred for quantitative analysis because of (1) ease of selection of the same wave length in each spectrum, (2) ease of selection of an internal control wave length of suitable density, and (3) simplicity and durability of line spectrum sources. Several types of line spectrum sources may be used. These include condensed sparks between suitable metallic electrodes, and metallic vapor or gas discharges. For the determination of the absorption of materials which possess absorption bands a t suitable wave lengths a low-voltage mercury arc and, particularly, a helium lamp (2) powered by a neon sign transformer should be, because of their very constant intensities, especially favorable sources. Selection of Wave Lengths. The absorbed wave length selected for use in quantitative analysis should be at, or near, the peak of a band. The internal control wave length should be chosen as in emission spectrochemical analysis, but, since all wave lengths used are radiated by the same element, fewer conditions need be fulfilled than in emission analysis. EXTINCTION COEFFICIENTS.With the present technique, three different extinction coefficients may be considered. 1. Specific extinction coefficient, k
whereZ
intensity of radiation transmitted by 1 cm. of the material at an absorbed wave length, Xa IO = corresponding intensity transmitted by the comparison substance c = concentration of the absorbing substance in the material k gives the true value of the absorption and is obtained by an intensity comparison, at the same wave length, of the absorption and comparison spectra. For high accuracy these two spectra must be produced simultaneously by the same source and photographed under absolutely identical conditions. 2. Reference specific extinction coefficient, kl =
I2 log10 7 kl =
C l
where I S = intensity transmitted by 1 cm. of the material at an unabsorbed internal control wave length, A, RI is thus obtained by anintensity comparisonof the absorption band and internal control wave lengths in the spectrum of the material. kl gives an empirical measure of the absorption which
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FIGURE 1. ANALYTICAL CURVEFOR DETERMINING MONOMER CONTENT OF PARTIALLY POLYMERIZED STYRENE Internal control wave length, Xe, 2980.53 A. XO, 2823.28 A.
Absorbed wave length,
will be of value, provided I and c remain constant for all determinations. 3. Compensated specific extinction coefficient,kz
kz =
log10 I2 - log,, I -1 I Io cl
where ZI = intensity transmitted by the comparison substance at the internal control wave length, A, kz is thus obtained by a method of internal control which, by eliminating the effects of any variations between the comparison and absorption exposures and of changes in the light, source intensity, permits an accurate intensity comparison, at the same wave length, t o be made between the comparison and absor tion s ectra without the necessity for simultaneous exposure. hince tge above expression for the evaluation of Icz equalizes the effective exposures of the absorbing and comparison materials, neither of which absorbs at the internal control wave len th k2 is identical in value with k and gives the true value of t i e Lbsorption. The absorption of a material expressed in terms of or k obeys Beer’s law, but that expressed in terms of kl does not. QUAXTITATIVE ANALYSIS. The analysis of a sample for a constituent which possesses an absorption band in the photographic region is made directly by means of the analytical extinction coefficient, A log 12 - log 12 I Io A = sl
where s equals the concentration of the total sample in the material, of thickness, I , of which the absorption spectrum is obtained.
DECEMBER 15, 1939
ANALYTICAL EDITION
If the sample, which contains an absorbing constituent, must be dissolved in a suitable solvent to obtain its absorption spectrum, s is the concentration of the solute in the solvent. c is the concentration of the absorbing constituent, only, in the solvent. If the spectrum of the sample may be obtained directly, s is equal to unity. The analysis is standardized by establishing, by means of samples of known composition, the relationship between A , for a wave length in the absorption band, and the concentration of the absorbing constituent. The graph of this relationship provides an analytical curve, as shown in Figure 1. A sample of unknown concentration is analyzed by determining the appropriate A value from the measured relative intensities of the selected wave lengths, and reading the desired concentration from the analytical curve. PRECISION. I n the conventional photographic method of spectrophotometry, in which two adjacent spectra are compared visually, i t is said to be possible to detect a difference in optical density of 0.06 (4). Thus by the use of an Eastman Polychrome plate (as processed in the present work) of y = 0.45, a difference of 0.13 in the density of the absorbing substance should be detected. The precision of density determination, which is limited by only the photometric error, with the present technique permits the detection of a density difference of 0,009 in the absorbing substance.
WEIGHT
PER CENT STYRENE
SOLUTE
FIGURE2. OPTIMUMSOLUTION CONCENTRATION FOR ANALYSISFOR MONOSTYRENE
Data obtained from routine analyses for monostyrene showed that the average error in the determination, from one spectrum, of the extinction coefficient
K=-
log10
1
Io
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was approximately K = 0.006 over the range of K values from 0.08 to 0.12 ordinarily used in this analysis. The average of three spectra will reduce the error by half. Approximately one half of the error lies in the photometry and the nonuniformity of the photographic plate. Most of the remainder of the error is accounted for by the small random variations in the relative intensities of the spectral lines emitted by the iron condensed spark light source used.
Representative Applications ANALYSISOF PARTIALLY POLYMERIZED STYRENEFOR MONOMER CONTENT.The determination of the amount of polymerization of synthetic resins, and other polymerizable compounds, is of considerable importance for the control of the polymerization process. Styrene, CsH5-CH=CH2, is a typical polymerizable material. Its polymer has the probable structure (3)
-CHz-CH-CHzC6H5 I
Y %"" %Hi H-CH2-
H-CHt-
H-
Absorption spectra revealed that monomeric styrene possessed absorption bands a t 2910 A. and at 2830 A., and complete absorption a t wave lengths shorter than 2690 A. Polyand complete styrene was found to have a band at 2695 i., absorption below 2400 A. Since the intensities of the absorption bands of the monomer were found to increase regularly with increasing monomer percentage, the absorption technique described was applied to the quantitative analysis of partially polymerized styrene for the monomer content. The partially polymerized styrene samples, in either liquid, as polymerized, or alcohol-precipitated form, are dissolved in chloroform to definite concentrations. With the experimental conditions used, approximately 0.013 mg. of monomer per cc. of chloroform gives absorption band intensities most suitable for accurate photometry. Figure 2 shows graphically the relationship between the monomer percentage and the weight per cent of solute (partially polymerized styrene sample) for this optimum monomer concentration. It is, however, not necessary to employ only the optimum concentration, since i t has been found possible in practice to cover the entire monomer concentration range satisfactorily from 0 to 100 per cent by the use of only four solution concentrations, 0.0015,0.005,0.02,and 0.05 weight per cent of solute. The solution contained in a quartz cell of 2-cm. length, fitted with plane quartz windows, is placed between an iron condensed spark discharge and the slit of a medium quartz spectrograph. The absorption spectra of the test solutions and of the solvent are photographed and the intensities of the absorption bands of monostyrene, as recorded on an Eastman Polychrome plate, are measured. These absorp tion band intensities are obtained by intensity comparisons, by the techGque described, of the iron lines 2912.16 A. and 2823.28 A., which lie near the peaks of the two bands, with the internal control iron line 2980.53 A, The analysis is standardized by an experimental correlation of the analytical extinction coefficient, A , for 2823 A. with the monomer pezcentage, as shown in Figure 1. The value of A for 2912 A. may also be used as a check, if desired. Partially polymerized styrene may be analyzed for a monomer content of from 0 to 100 per cent. The representative precision and accuracy given by a single spectrum are shown in Table I. By precision is meant the average deviation of the monomer percentage given by a single spectrum from the mean result given by all the spectra of that sample. By accuracy is meant the average deviation of the monomer percentage given by a single spectrum from the true amount
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TABLE I. PRECISION AND ACCURACY OF ANALYSISFOR MONOMERIC STYRENE Actual Monomer Percentage 0.58 1.09 5.07 10.08 30.0 70.0 100.0
Precision Average error, Average % monomer % error 0.05 8.6 0.07 5.9 0.13 2.6 0.34 3.4 1.0 3.3 1.7 2.4 5.3 5.3
Accuracy Average error, Average % monomer % error 0.085 14.7 0.11 9.7 0.13 2.6 0.35 3.5 1.45 4.8 2.0 2.9 , 7.5 7.5
present. These data were obtained from a total of nine spectra for each sample photographed on four plates, The average error, expressed as per cent monomer, increases with increasing monomer concentration on account of the constancy of the photometric error, the linearity of the analytical curve, and the decreasing percentage of solute used. The magnitude of the error lies within that which might be caused by photometric errors and the nonuniformity of the photographic plate. The accuracy obtained, particularly in the commercially important range from 0 to 10 per cent monomer, is sufficient for routine practical analysis. One sample may be analyzed in a total elapsed time of 2 hours, while ten samples may be analyzed in 6.5 hours. The number of man-hours of work required is approximately 60 per cent of the total elapsed time. The time taken for
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the preparation of the solutions accounts for one half of the total time required. OTHERAPPLICATIONS.Other applications for which this method should be particularly suitable include: 1. The important problem in the petroleum industry of the determination of the amount of aromatic impurities in aliphatic and alicyclic compounds. 2. The determination of the amounts of the different hemoglobin compounds in blood serum. 3. The following of the course of polymerization, and of other chemical reactions, in which the reactions involved produce changes in the intensities of characteristic absorption bands of the reactants or produce products which possess new absorp tion bands.
Acknowledgment The writer wishes to express his thanks to Jacob Heerema for assistance in obtaining the experimental results of this paper.
Literature Cited (1) Clark, G . L., and Gring, J. L., IND. ENQ.CHEM.,Anal. Ed., -9, 271-4 (1937). (2) Duffendack, 0. S., and Wolfe, R. A., “Proceedings of the Sixth Summer Conference on Spectroscopy and Its Application”,pp. 97-100, New York, John Wiley & Sons, 1939. (3) Staudinger, H., “Die hochmolekularen organischen Verbindungen. Kautschuk und Cellulose”, Berlin, J. Springer, 1932. (4) Twyman, F., and Allsopp, C. B., “The Practice of Spectrophotometry’’, London, Adam Hilger, 1934.
Analysis of Organic Materials for Traces of Metallic Impurities T. M. HESS, J. S. OWENS,’
AND
L. G. REINHARDT, The Dow Chemical Company, iMidland, Mich.
R
APID, accurate analysis for small traces of metallic impurities is an important factor in the manufacture of modern organic chemicals. For this reason the spectrochemical analysis of such materials, including cellulose derivatives, synthetic resins, pharmaceuticals, dyes, and biological tissues and fluids, has been developed for traces of aluminum, calcium, copper, iron, lead, magnesium, manganese, nickel, strontium, tin, and zinc. The determination of some of these elements in the presence of others, in their natural ranges of abundance, is very difficult, if not impossible, by chemical methods. The spectrochemical method is not thus limited and gives good precision in the low concentration range of interest. Moreover, the spectrochemical analysis effects an important saving of time, since the analyses for all the test elements may be carried out by measurements made upon one spectrum. This technique is not limited to the eleven test elements mentioned, but may be used to determine practically all of the metallic and metalloid elements. While the application of the method to organic materials is specifically described, it is readily evident that, since the actual sample analyzed is a solution of inorganic salts, the same method and in some instances even the same analytical curves, may be used for the analysis of many inorganic materials.
tensities of the spectral lines of the test elements and of the internal control elements introduced into each sample in constant amount. This relative intensity is a measure of the concentration of the test element. The actual relationship is determined for each element by measurements made upon the spectra of a series of specimens of known composition in which the test elements vary over the desired ranges of abundance. The graph of this relationship, illustrated in Figure 1, provides an analytical curve for the determination of that element. PREPARATION OF SAMPLES.I n order to obtain maximum analytical sensitivity and accuracy it has been found necessary to remove \the organic matter by wet-ashing in spectroscopically pure nitric, sulfuric, and perchloric acids. The resulting solution is evaporated nearly to dryness, and the residue is taken up in a definite volume of a solution containing the spectroscopic buffer and the internal control elements. The spectroscopic buffer is an added amount of a suitable salt which eliminates the effects upon the analysis of the extraneous composition of the samples and permits the use of the same analytical curves for the analysis of samples which vary considerably in composition (7). The following standard procedure has been adopted:
Analytical Technique
A 0.4-gram sample of the test material is weighed out on a watch glass and transferred t o a 150 X 20 mm., Pyrex test tube or a 30-ml. Kjeldahl flask. After the addition of 1 ml. of concentrated sulfuric acid the test tube or flask is placed on a sand bath heated by gas burners, The sample is heated until charring is complete, after which concentrated nitric acid is added to aid in completing the oxidation. The nitric acid is added to the hot
GENERAL METHOD. The analytical technique used is the well-known internal control method (6) in which the analysis is made from photometric measurements of the relative in1 Present
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