reagent produced a more colored blank while a decrease resulted in decreased absorbance. When the concentration of the M B T H was increased to more than 0.05%, the solutions became turbid. Lower concentrations of M B T H resulted in a loss of color intensity. A 15% decrease in abEorbance was observed when the concentration of the M B T H was reduced from 0.05% to 0.0270, even though the solutions were free of turbidity. The concentration of the M B T H must be in excess of that of the aliphatic aldehydes collected. This can easily be contpolled by either diluting the test scllution with the collecting reagent or reducing the air volume sampled. A time study of the reaction of microgram quantities of formaldehyde with
0.05% M B T H was conducted. The reaction was complete in approximately 45 minutes; therefore a reaction time of 1 hour was selected for this procedure. Because this method was devised for field application, the stability of diluted forma!dehyde in 0.05’% M B T H was also ascertained. Approximately 5% of the formaldehyde was lost after standing in the M B T H for 13 days. The samples therefore were stable enough for later analysis. EXPERIMENTAL
(1) Altshuller, A. P., Leng, L. J., ANAI; CHEM.35, 1541 (1963). (2) Altshuller, A. P., McPherson, S. P., Air Pollution Control Assoc. 13, 109
(1963). (3) Rayner, A. C., Jephcott, C. M., ANAL. CHEM.33, 627 (1961). (4) Sawicki, E. , “Microchemical Tech-
niques,” N. D. Cheronis, ed., pp. 59106, Interscience, New York, 1962. (5) Sawicki, E., Hauser, T. R., McPherson, S. P., ANAL.CHEM.34, 1460 (1962). (6) Sawicki, E., Hauser, T. R., Stanley, T. W., Elbert, W., Ibid., 33, 93 (1961). (7) Thomas, J. F., Sanbourne, E. N., Mukoi, M., Tebbens, B. O., A . M . A. Arch. Ind. Health 20, 420 (1959). (8) Wilson, K. W., ANAL. CHEM. 30, 1127 (1958). (9) Wohlers, H. C., Bell, G. B., “Literature Review of Metropolitan Air Pollutant Concentrations,” Stanford Research Inst., Menlo Park, Calif. (Nov. 30, 1956). (10) Yoe, J. H., Reid, L. C., IND.ENG. CHEM.,ANAL. ED. 13, 235 (1941). THOMAS R. HAUSER L. CUMMINS RODNEY U.S. Department of Health, Education, and Welfare Public Health Service Robert A. Taft Sanitary Engineering Center Cincinnati 26, Ohio
Methods for Determination of Cyclopropenoid Fatty Acids. Infrared Absorption Method SIR: In the course of investigating new approaches t o the quantitative estimation of long-chain cyclopropenoid fatty acids, it was observed t h a t the infrared spectrum of the product obtained by treatment of these moieties with concentrated aqueous hydrochloric acid or by titration with hydrogen bromide in glacial acetic avid shows a strong infrared absorption a t 11.05 microns and a weak band at 6.07 microns. The reactions are quantitative and the cyclopropenoid content can be determined by the increasl: in the chlorine content of the sample (2) or by the hydrogen bromide titration value (3, 4). I n both instances, however, pretreatment of the sample is necessary t o eliminate epoxides, hydroperouides, and other interfering substances which also react with the hydrohrtlogens. The present report deals with a n analytical method based upon the measurement of the abs0rptivit.i at 11.05 microns after treatment with aqueous hydrochloric acid. The deJrelopment of the absorption masimum cit 11.05 microns is attributable entireljr to the unsymmetrically disubstitutrd olefinic group CH,
11
--CH2-C-CHCI-, resulting from the opening of the cyclopropene ring ( 1 ) . The hydrochlorination of the cyclopropenoid moiety resuli s in four isomers, only two of which have a n absorption band a t 11.05 microns (1). The proportion of the products responsible for the absorption maximum at this wavelength
is, however, always the same. This follows from the reproducibility of, and from the consistency of, the mathematical relationship between the observed absorptivities of known Sterculia foetida oil-corn oil mixtures after the hydrochloric acid treatment. The absorptivities can therefore be taken as a measure of the number of moles of cyclopropenoid moiety present in a given weight of the sample. The corresponding weight percentages, which of course depend upon the molecular weight of the moiety involved, have been calculated in terms of sterculic acid (molecular weight 294.48). EXPERIMENTAL
Since no pure cyclopropenoid derivative was available, Sterculia foetida oil, the fatty acids of which contain about 50% of sterculic acid, was used in testing the applicability and precision of the method. Sterculia foetida seed meats were extracted with several portions of low-boiling petroleum ether in a Waring Blendor at room temperature. The extracts were combined, filtered, and freed from solvent on a rotary evaporator with a nitrogen leak under reduced pressure. The oil was stored in vacuumsealed ampules in a refrigerator until used. The two Sterculia foetida oils used, extracted from different batches of seed, contained 45.79% and 46.64% of “sterculic acid,” respectively, as determined from chlorine analyses before and after treatment by the aqueous hydrochloric acid method (2). The corn oil was a commercial salad oil.
Procedure. T o establish a calibration curve, d a t a were first obtained on a series of samples of known sterculic acid content made b y mixing Sterculia foetida oil with corn oil. Appropriate amounts of Sterculia foetida oil a n d corn oil, sufficient t o make approximately 6 grams of t h e desired compositions, were weighed directly into 25-ml. glass-stoppered bottles. Approximately 6 ml. of concentrated hydrochloric acid (sp. gr. 1.18 t o 1.19) were added and t h e twophase system was blanketed with nitrogen and shaken vigorously for 1 hour on a mechanical shaker. The misture was then poured into several times its volume of water in a separatory funnel and extracted with petroleum ether. The extract was washed with water, dried over anhydrous sodium sulfate, freed from solvent on a rotary evaporator, and subjected to infrared analysis. Infrared absorption curves from 10.0 t o 11.5 microns were obtained with a Perkin Elmer Model 21 infrared spectrophotometer. A fixed slit of 151microns was used and the instrument was adjusted to have a slight upward drift. The other settings were suppression, 0 ; gain, 5; response, 1; and speed 0.3 micron per minute. The spectra were obtained in carbon tetrachloride solution with pure solvent in the reference beam. A sufficient concentration of hydrochlorinated product was used to give a transmittance between20and 70$& at the 11.05-micron absorption maximum. T o correct for the background absorption, a baseline was drawn tangent t o the curve between approximately 10.70 and 11.35 microns. The difference between the measured absorbance at the VOL. 36, NO. 3, MARCH 1964
681
RESULTS
Table I. Analyses of Corn Oil-Sfercdia foetidu Oil Mixtures
% ’ S. joetidu 011
a
10.02 20.03 40.02 60.01 79.97 100 .oo
0.017
yo Cyclopropenoid acid5
Observed *
Deviation from theory
Theoryc
First (Calibration) Series 3.62 9.33 18.15 28.00 37.34 45.79
0.028 0.045 0.064 0.082 0.098
4.59 9.17 18.32 27.48 36.61 45.79
-0.97 f O .16
Av.
-0.17 $0.52 f 0.75 0.00 0.43
Av.
+0.34 $0.25 -0.71 $0.04 -0.42 0.35
Second Seriesd 4.83 9.44 17.09 50.00 66.52
0.015 0.018 0.024 0.055 0.069
2.59 4.15 7.26 23.34 30.60
2.25 4.40 7.97 23.31 31.02
Calculated as sterculic acid. b Calculated from Equation 1. c Calculated from the sterculic acid content of the Sterculiafoetida oil used. d Using a different Sterculia foetida 01. 0
wavelength of maximum absorption and the base line at the same wavelength was used t o calculate the corrected absorptivity. The absorptivities plotted against the calculated sterculic acid contents of the samples fell on a straight line represented b y the equation
x =518.56 u - 5.186
(1)
where x is the percentage of cyclopropenoid moiety as sterculic acid and a is the corrected absorptivityat the 11.05micron absorption maximum. This absorptivity may of course vary with the particular spectrophotometer used. This equation was then employed t o calculate the sterculic acid content of a second series of corn oil-Sterculia foetida oil mixtures from their absorptivities after hydrochloric acid treatment.
The combined data, presented in Table I, show that the sterculic acid content can be determined to within at least one percentage unit of the calculated value. The average deviation from theory for the calibration series was 0.4301, and for the second series 0.35%. The accuracy of the method can be improved by establishing a more authentic calibration curve based upon the absorptivity measured on a hydrochlorinated sample of a n appropriate pure cyclopropenoid derivative. LITERATURE CITED
(1) Bailey, A. V., Magne, F. C., Bou-
dreaux, G. J., Skau, E. L., J. Am. Oil
Chemasts’ SOC. 40, 69 (1963). (2) Harris, J. A., Magne, F. C., Skau, E. L., Zbad., in press. (3) Smith, C. R., Jr., Wilson, T. L., Mikolajczak, K. L., Chem. & Ind. (London)1961, 256. (4) Wilson, T. L., Smith, C. R., Jr., Mikolajczak, K. L., J . Am. OtZ Chemzsts’ SOC. 38, 696 (1961).
FRANK C. MAGSE ACGUSTV. BAILEY
R. MCCALL ELIZABETH H. MILES SYLVIA EVALD L. SKAU Southern Regional Research Laboratory’ New Orleans, La. One of the laboratories of the Southern Utilization Research and Development I)ivision, Agricultural Research Service, U. S.Department of Agriculture.
180’ Compton Scattered-Annihilation Gamma-Ray Sum Peak Observed in Gamma-Ray Spectrometry SIR: The recent article by E. C. Lightowlers ( 7 ) presented data which showed the presence of a -720-k.e.v. photopeak in the y-ray spectrum of . Cu64 is a 12.9-hour C U ~ ~ Because positron emitter, the author had encapinch of wax; sulated his sources in -J/z this enables the positrons emitted t o annihilate close t o the source point. I n addition, because the activity was low, he counted his samples close to the detector. The result, shown in Figure l of reference ( 7 ) ,was an apparent photopeak at -720 k.e.v. with the encapsulated source, but none with a bare source. The author stated that “no logical explanation has been found for the appearance of a peak at 0.72 m.e.v., which is found with all encapsulated sources but never found when open sources are used.” I happen t o have been one of the first victims t o fall prey to this artifact which has an easily understood and simply demonstrated origin, and so perhaps an explanation will help prevent future puzzlement for others. 682 *
ANALYTICAL CHEMISTRY
I n a paper published in 1955 on the decay scheme of A126(a positron emitter), Handley and Lyon (4) counted -50 grams of A1203on top of a NaI(T1) crystal and observed and reported a 720k.e.17. y-ray in the spectrum. J. M. Ferguson (3) later examined this nuclide, and proved that the 720-k.e.v. photopeak arose in fact from the summing in the crystal of one 510-k.e.v. annihilation y-ray with the 180’ Compton backscattered y-ray from either the coincident 510-k.e.v. annihilation y-ray or another coincident y-ray. The magnitude of such a sum peak varies inversely with the square of the solid angle subtended by the source to the detector. Ferguson proved the 720-k.e.v. photopeak to be a sum peak by increasing the source-detector distance and noting the reduction in the size of the photopeak in question relative to the other peaks. I n addition, he shielded the detector with lead sufficient t o absorb a 200-k.e.v. y-ray (but only 15% of a 720-k.e.v. y-ray) and noted the absence of the 720-k.e.v. peak.
Butler and Gosset ( I ) in 1958 observed a 690-k.e.v. peak in the positron emitter CuGZ. Their source was encapsulated in paraffin. They prepared PIs, a pure positron emitter with no yray, and observed that the ratio of the 690-k.e.v. peak to the 510-k.e.v. peak ~; was similar for both F1*and C U ~ this suggested that the observed photopeak in both nuclides was a sum effect. Eccleshall and Yates ( 2 ) in a study of energy levels in F18 cite Butler and Gosset as authority for the statement that the 690-k.e.v. peak is due to summing. I n this laboratory, we have confirmed that the 690-k.e.v. peak in FI8is a sum peak by measurements made a t differing geometries. Upon reading the paper by Lightowlers (r),we have attempted to duplicate his results by observing the y-ray spectra obtained LT-hen a source of Cufi4 is measured a t high and a t low geometries, both unshielded and encapsulated. Figures I, 2, 3, and 4 shorn some typical spectra obtained. S o t e especially the enhancement of the sum-
