The detection limits of fluorescein determined by the various exciting sources so far reported are shown in Table I. The sensitivity of the fluorometric analysis excited by the esculin laser in this work exceeds those in the previous works and that by a conventional spectrophotometer by more than an order of magnitude. Thus the dye laser was proved to be useful as an exciting source for the trace fluorometric analysis. At the limit of detection, the major source of noise of the conventional spectrophotometer is the dark current of the photomultiplier and that of the previous apparatus with the laser source was concluded to be noise on the background signal, caused by fluorescence of impurities in the solvent (2). In the present investigation, the discharge circuit and the electronic devices for the photodetection were shielded carefully by metal enclosures; the background fluorescence of solvent impurities was reduced by repeated purifications of the solvents; the optical system was strictly adjusted to reduce the scattering light of the excitation laser. Thus, the background signal was reduced to allow the observation of such weak fluorescence as 2 ppt of fluorescein. Either a stray light of the monochromator, or the fluorescence of the residual impurities in the distilled water may contribute to the background signal from the solvent water. The detection limit of fluorometry can be improved by using the interference filter instead of the emission mono-
chromator. For example, the sub-part per trillion detection of Rhodamine 6G was reported (6).However, the Raman band cannot be used as the internal standard, and the fluorescence from the unwanted species cannot be resolved on the fluorescence spectrum. Thus, the use of a monochromator, as reported here, has definite advantages over such a simple method. Totaro Imasaka Hidekazu Kadone Teiichiro Ogawa Nobuhiko Ishibashi* Faculty of Engineering 36 Kyushu University Fukuoka 812, Japan LITERATURE CITED (1) B. W. Smith, F. W. Plankey, N. Omenetto, L. P.Hart, and J. D.Winefordner, Spectrochim. Acta, Part A, 30, 1459 (1974). (2) T. F. V. Gel1 and J. D. Winefordner, Anal. Cbem., 48, 335 (1976). (3) D. C. Harrington and H. V. Malmstadt, Anal. Cbem., 47, 271 (1975). (4) M. F. Bryant, K. O'Keefe, and H. V. Malmstadt, Anal. Chem., 47, 2324 (1975). (5) T. Imasaka, T. Ogawa, and N. Ishibashi, Bunseki Kagaku, 26, 96 (1977). (6) A. E. Bradley and R. N. Zare, J. Am. Cbem. SOC.,98, 620 (1976).
RECEIVEDfor review November 10,1976. Accepted January 3. 1977.
Composition Differences in Commercial Polyethylene Bottles and Their Relation to the Stability of Stored Part-per-Billion Mercury(l1) Solutions Sir: During a study of the suitability of linear polyethylene bottles for storage of trace Hg(I1) solutions, we have found several lots of commercial bottles which are a mixture of 45% polypropylene and 55%linear polyethylene, and which differ significantly from 100%linear polyethylene bottles with respect to the types and levels of additives. The losses of Hg(I1) incurred when 1ppb Hg(I1) solutions are stored, and the interaction of the container with the preservative solution are quite different in 100%linear polyethylene bottles and in 45% polypropylene bottles. Such differences in the stability of trace level Hg(I1) samples stored in the two types of bottles constitute a potentially serious source of variance in Hg(I1) analysis of samples stored for an appreciable time as is often necessary in environmental studies that involve large numbers of samples. Because polyethylene bottles of the two compositions may well exert differential effects on other samples, we wish to alert others to the possibility of unexpected variations in losses when trace samples are stored in commercial linear polyethylene bottles. Differences in the efficiency of preservatives in stabilizing 1 ppb Hg(I1) solutions stored under apparently identical conditions in bottles from different manufacturers' lots first suggested that, although the bottles within each lot are relatively uniform, the various lots represent two distinct types of bottles, which we have denoted LPE I and LPE 11. In their behavior toward 1ppb Hg(II), solutions stabilized with 5% v/v HN03-0.05% w/v K2Cr207, as recommended by Feldman (I), the two types of bottles differ in two important respects. First, losses of Hg(I1) after 10 days storage, as monitored by a cold vapor atomic absorption technique similar to that of Kopp et al. ( 2 )were 43%in the LPE I bottles but only 6% in the LPE I1 bottles. Second, the dichromate preservative is often reduced in the LPE I1 bottles. The degree of reduction varies substantially from one bottle to another, and occasionally the 668
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dichromate suffers complete reduction after several days. The bottles have a capacity of 500 mL, so that complete reduction represents the loss of 0.85 mmol of dichromate. In contrast, little if any of the dichromate is reduced in LPE I bottles, even after storage for a month or longer. The infrared spectra of thin films of the LPE I plastic and the LPE I1 plastic in Figures 1 and 2, respectively, clearly demonstrate that the LPE I plastic is linear polyethylene and that the LPE I1 plastic contains substantial quantities of polypropylene. The spectrum of LPE I shows, in addition to the C-H bands of aliphatic hydrocarbons, bands at 907 and 989 cm-l characteristic of terminal unsaturation, and an olefinic C=C stretching band at 1638 cm-l is detectable if much thicker films are used. The 907 and 989 cm-l bands indicate that LPE I is a low pressure, linear polyethylene, and the spectrum corresponds closely to representative spectra of linear polyethylenes given by Hummel and Scholl(3). The spectrum of LPE I1 exhibits a number of bands characteristic of crystalline isotactic polypropylene ( 4 ) , the more prominent of which are the six moderately intense bands at 809,840,898, 970,998, and 1165 cm-l, and an intense methyl deformation band at 1375 cm-l. A strong doublet with maxima at 718 and 728 cm-l, characteristic of crystalline polyethylene ( 5 ) appears in the spectra of both LPE I and LPE 11. The polypropylene content of the LPE I1 polymer as estimated using four methods based on the intensities of appropriate infrared bands, is approximately 45% polypropylene by weight. The first method, based on the relative intensities of the 718 cm-l polyethylene band and the 970 cm-l polypropylene band (6),yields a composition of 45% polypropylene. The second method, using the relative intensities of the 718 cm-1 polyethylene band and the 1165cm-l polypropylene band (7) also indicates that the polypropylene content of LPE I1 is 45%. The third method utilizes the film thickness and the
Figure 1.
infrared
4000 3000
2000
1500
CM-'
1000
900
800
700
0.0 .10 y1
g.20 0 3.30 8.40 (.50
-60
.70
1.0 0,
WAVELENGTH (MICRONS) Figure 2.
infrared spectrum of thin film of LPE Ii polymer
intensity of the 718 cm-l polyethylene band ( 6 ) ,and it indicates a composition of 46% polypropylene. The fourth method, based on the film thickness and a value of 0.22 absorbance unit a t 970 cm-l per mil for 100% polypropylene (8), yields a polypropylene content of 44%. In all four methods, Beer's law was assumed to be valid and, in the application of the first and third methods (6), it was necessary to extrapolate the calibration curves to the composition range of interest. The good agreement among the four methods indicates that these assumptions are valid, and on the basis of the four measurements, the polypropylene content of LPE I1 is estimated to be 45 f 5%. The LPE I1 plastic therefore deviates substantially from the definition of polyethylene given by the American Society for Testing and Materials, which states that the ethylene content of polyethylene is not less than 85% (9). Differential thermal analysis shows that LPE I1 is a physical mixture of linear polyethylene and isotactic polypropylene. The thermogram of LPE I1 exhibits a strong endotherm at 127 O C , which represents the melting of polypropylene, and a weaker endotherm at 160 "C,which represents the melting of polypropylene. This thermogram corresponds closely in all important respects to those reported by Ke (10) for physical mixtures of linear polyethylene and isotactic polypropylene. In addition to the difference in polymer composition, the LPE I bottles and the LPE I1 bottles also differ with respect to the types and levels of protective additives and other substances added to the plastics during various manufacturing operations. Antioxidants and ultraviolet absorbers are almost
always added to polyolefins to prevent degradation of the polymers, and a large number of substances are used as processing aids to facilitate mixing and fabrication steps. Because the nature of many protective additives suggests that they might readily interact with Hg(II), we have made a qualitative study of the additives in LPE I bottles and LPE I1 bottles based on comparison of infrared and ultraviolet spectra of extracts with those of common additives (11).Infrared spectra of neat extracts and ultraviolet spectra of isopropanol solutions were obtained after isolation of the additives by extraction with boiling chloroform under a nitrogen blanket. The additives identified in LPE I plastic are a hindered alkylphenol (HAP) which resembles 2,6-di-tert-butyl-4methylphenol or a related compound, and an aliphatic ester which appears to be a dialkylthiodipropionate.Both HAP'S and dialkylthiodipropionates are well known classes of protective additives for polyolefins (12).The additives identified in the LPE I1 plastic include an alkoxyhydroxybenzophenone (AHB), which resembles 2-hydroxy-4-methoxybenzophenone or a related compound, possibly small quantities of an HAP, an aliphatic ester, and a low molecular weight polyhydroxy compound. The presence of an arylphosphoric acid is suggested by the infrared spectrum, but the low molar absorptivity of many such compounds (13) prevents verification of its presence by ultraviolet spectroscopy. Such a compound may be formed by oxidation of the corresponding arylphosphorous acid, a protective agent commonly used with polypropylene (14). The polyhydroxy compound in the LPE I1 plastic, we SUBpect, causes the extensive reduction of the dichromate preANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
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servative in LPE I1 bottles. Such compounds, which are atypical of commonly used protective additives, may have been deliberately or accidentally added to the plastic during some stage of the manufacturing process. Because of its inherently high solubility in water, a polyhydroxy compound could interact readily with the acidic aqueous dichromate solutions. The ability of acidic pichromate to oxidize alcohols is well known (15),and the acidic dichromate preservative is quickly reduced by small quantities of low molecular weight alcohols and polyols. As an example, 0.34 mmol of glycerol completely reduces 50 mL (0.085 mmol) of the dichromate preservative in less than 9 h. The reasons for the relatively poor stability of Hg(I1) in LPE I bottles are not yet known, but factors such as the additives, surface oxidation products, or residual terminal unsaturation of the polyethylene may contribute to the loss of Hg(I1). The LPE I plastic contains significant quantities of an HAP, and Koirtyohann and Khalil(16) have shown that 2,6-di-tert-buty1-4-methylphenol,a representative HAP, rapidly reduces Hg(I1) to the elemental state. Elemental mercury so formed might be removed from solution by diffusion into the plastic. The dialkylthiodipropionate additive, being an alkyl sulfide is expected to bind Hg(I1) strongly (17) and could thereby remove Hg(I1) from solution. Functional groups formed by oxidation of the additives or the polyethylene surface itself might also bind Hg(I1) effectively and remove it from solution. The LPE I plastic exhibits a significant degree of unsaturation, and the well known interaction of Hg(I1) with double bonds to form organomercury adducts (18, 19) could also contribute to the loss of Hg(I1) from solution. We do not know the frequency with which composition differences of the type described or other composition differences occur in commercial linear polyethylene bottles. Those who use such bottles to store samples for trace analysis should periodically verify the suitability of the bottles for storage of their samples.
LITERATURE CITED (1) C. Feldman, Anal. Chem., 46, 99 (1974). (2)J. F. Kopp, M. C. Longbottom, and L. B. Lobring, J. Am. Water Works Assoc., 64, 20 (1973). (3)D. 0.Hummel and F. K. Scholl, "Infrared Analysis of Polymers, Resins and Additives", Vol. I, Part 2,Wiley-lnterscience, New York, N.Y., 1971. (4)J. P. Luongo, J. Appl. folym. Sci., 3, 302 (1960). (5) D. 0.Hummel and F. K. Scholl, "Infrared Analysis of Polymers, Resins and Additives", Vol. I, Part 1, Wiley-lnterscience, New York, N.Y., 1971,p 13A
R. M. Bly, P. E. Kiener, and B. A. Fries, Anal. Chem., 38, 217 (1966). T. Gossl, Makromol. Chem., 42, l(1960). C. E. Day and J. Mitchell, Jr., personal communication, ExperlmentalStation, E.I. du Pont de Nemours and Co., Wilmington, Del., August 4, 1976. "1975 Annual Book of ASTM Standards", Standard D 1248-74,Vol. 34, American Society for Testing and Materials, Philadelphia, Pa., 1975, p
20. (10)B. Ke, J. folym. Sci., 42, 15 (1960). (11) D. 0.Hummel and F. K. Scholl, "Infrared Analysis of Polymers, Resins and Additives", Vol. II, Carl Hanser, Munich, 1973,663 pp. (12)G.Scott, "Atmospheric Oxidation and Antioxidants", Elsevier. New York. N.Y., 1965,p 299. (13) "The Sadtler Standard Spectra", Sadtler Research Laboratories, Philadelphia, Pa., Ultraviolet Spectra 10, 7923, 7924, 7925, 7926, 2927, 18543. (14) P. A,de Paolo and H. P. Smith, "New Phenolic Phosphite Stabilizers for Polypropylene", Adv. Chem. Ser., 85, 203 (1968). (15) R. T. Morrison and R. N. Boyd, "Organic Chemistry", 3rd ed., Allyn and Bacon, Boston, Mass., 1973,Chap. 16,pp 518-540. (16)S. R. Koirtyohann and M. Khalil, Anal. Chem., 48, 217 (1976). (17) E. E. Reid, "Organic Chemistry of Bivalent Sulfur", Vol. 11, Chemical Publishing Co., New York, N.Y., 1960,pp 52-54 (18)K. Hoffman and J. Sand, Chem. Ber., 33, 1340,2692 (1900). (19)J. Chatt, Chem. Rev., 48, 7 (1951).
R. W. Heiden D. A. Aikens* Department of Chemistry Rensselaer Polytechnic Institute Troy, N.Y. 12181
RECEIVEDfor review September 29,1976.Accepted January 10,1977. Supported in part by a Grant-in-Aid from Allied Chemical Corporation. Presented a t 7th Northeast Regional Meeting, American Chemical Society, Albany, N.Y., August 8-11,1976.
AIDS FOR ANALYTICAL CHEMISTS Potential Hazard Associated with Removal of Needles from Septa in Injection Ports of a Gas Chromatograph Eric B. Sansone" NCI Frederick Cancer Research Center, Frederick, Md. 2 170 1
Hiram Wolochow and Mark A. Chatigny Naval Bioscience Laboratories, University of California, Naval Supply Center, Oakland, Calif. 94625
The use of various chromatographic techniques to effect a separation among several components of a mixture has become quite common. It is obvious that when working with hazardous materials, regardless of the degree of separation realized, these materials will be introduced into the laboratory environment if they are not destroyed by the detector. (In fact, there is a possibility that hazardous materials may be produced by pyrolysis in a flame ionization detector.) The remedy is to provide an air exhaust in the immediate vicinity of the detector, so the hazardous materials can be carried away by 670
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the flow of air and captured by suitable means. In addition to this source of contamination, observations reported in connection with the use of syringes (1-5) suggest that withdrawal of a needle from a septum could produce an aerosol. The experimental work described below was undertaken to test this hypothesis.
EXPERIMENTAL An o n - c o l u m n i n l e t ( H a m i l t o n Co., Reno, Nev.) was connected to a cylinder of compressed nitrogen. T h e nitrogen pressure was reduced
