Figure 1. Circuit diagram for current-voltage measurement with four-point probe. e was varied with a ramp-generator. Vand Iwere displayed on a x - y recorder. I = - e / R
ACKNOWLEDGMENT
W
a
immediately “frosty” on exposure to atmosphere because of hydrolysis with water vapor. Our plates were clear despite the existence of some Si-C1 on the modified surface. The peak at 469 cm-‘ is identified with C-C1 which could also be seen on the surface spectrum. We were surprised to see the Si-C1 bond even in the atmospheric air, as one would expect the Si41 to be hydrolyzed. Once again this corroborates Murray’s interpretation of the silyl group extending away from the surface with C-C1 the farthest. In conclusion, our results obtained from an independent technique confirm earlier observations by others.
We want to thank William Scovell for his assistance in obtaining the Raman spectra. LITERATURE C I T E D (1) P. R. Moses, L. Wier, and R. W Murray, Anal. Chem., 47, 1882 (1975). (2) N. R. Armstrong, A. W. C. Lin, M. Fujihara, and T. Kuwana, Anal. Chem., 48, 741 (1976). (3) H. Gerischer, Pbotochem. Photobiol., 16, 243 (1972). (4) R. Memming, Photochem. Photobiol., 16, 325 (1972). (5) A. P. Schapp, A. L. Thayer, E. C. Blossey, and D. C. Neckers, J . Am. Chem Soc., 95, 5820 (1973). (6) W. J. Anderson and W. N. Hansen, J . Nectronal. Chem., 43, 329 (1973). (7) F. M. Smlts, Bell Syst. Tech. J . , 37, 71 1 (1958). (8) P. Zwietering, H. L. T. Koks, and C. Van Heerden, J. Phys. Chem. Solids, 11, 18 (1959). (9) J. E. Griffiths, Spectrochim. Acta, Part A , 30, 169 (1974).
in in
m
Vakula S. Srinivasan* Walter J. Lamb I
350
410
470
530
590
I
650
A D (cm-1) Figure 2. (A) Raman spectrum of pure CI(CH2)3SiC13.(B) Raman spectrum of the modified tin-oxide electrode surface, washed in benzene only. The spectra were taken with the filter on, with an Ar ion laser these to the Si-Cl bond (9). This indicates that not all the three bonds are involved in the binding of Si to the surface. It is an experimental observation that if there was any trace of silane on the electrode unbonded, the surface turned
I AIDS
Department of Chemistry Bowling Green State University Bowling Green, Ohio 43403
RECEIVED for review February 22, 1977. Accepted june9, 1977. Acknowledgment is made to the donors of the petroleum Research Fund, administered by the American Chemical Society, for support of this work. Presented in 1976 at the Pacific Conference on Chemistry and Spectroscopy, Phoenix, Ariz.
FOR ANALYTICAL CHEMISTS
Photographic Techniques for Fluorescent Spots on Thin-Layer Chromatographic Plates P. W. Rulon” and M. J. Cardone Norwich Pharmacal Company, Norwich, New York 138 15
Documentation is a critical part of good laboratory practices and, in thin-layer chromatography (TLC) analysis, the documentation can be improved by using photographs to augment the written description of visual observations. To serve most effectively, the photographs must reproduce the visual observations as closely as possible. The following technique covers the color photography of UV activated fluorescent spots on TLC plates. When attempting to photograph UV activated fluorescent spots, two problems become apparent. First, instant color print films are relatively slow (i.e., Polaroid type 58 film ASA 75) requiring long exposure times. Second, the film is ex1640 * ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
tremely sensitive to the UV light used to activate the fluorescence, causing an overall blue cast in the final print. This blue cast affects the colors of the fluorescent spots causing them to appear too blue. The analyst can use color correction filters to remove the blue cast; however, the amount of filtration required results in exposure times of such length (over 5 min) that the film suffers reciprocity failure which further adds to the complexity of the problem. Another way to circumvent the problem of film sensitivity to UV light is to use a UV cut-off filter over the lens of the camera. A Wratten 2E filter will eliminate both long wave 365- and short wave 254-nm light from exposing the film. The
resultant photograph will be fluorescent spots, seemingly suspended in mid air, with true color rendition of the spots. This procedure is not acceptable since there is no frame of reference for the spots. In fact, when the analyst visually inspects a TLC plate in a view box, he can see the plate because of the eyes’ limited sensitivity to UV. Other UV filters are available that cut off the UV light (i.e. 2A, 2B), but are not as efficient as the 2E. The residual UV light transmitted results in a blue appearance of the print, and spot color is not true.
EXPERIMENTAL Photography was carried out on a Camag MP-4 Camera system, equipped with Foto-UV lighting system using Polaroid Type 58 film. The Foto-UV system consists of two 8-W mercury short wave lamps, 8-W black lite long wave lamps, and two Corning 9863 filters. WARNING: Do not view Uv lights without proper eye protection! Four 150-W photofloods Type B provide the white light source. Procedure. Place a TLC plate on the copy stand base, center, frame, and bring into proper focus. Place an 80B filter in the filter drawer, set the f-stop for 5.6, and set shutter speed at ‘/eo s. Switch on the photofloods and allow t o come to proper temperature for 5 s before the exposure is made. Trip the shutter, turn the lights off without moving anything. Replace the 80B with a 2E filter, reset the f-stop t o 4.5 and allow the long wave UV (short wave may also be used) lamps to warm up for 5 s. Make a second exposure for 1full minute by holding down the shutter release. Turn off the lamps, remove the film from the camera, and process as recommended.
RESULTS AND DISCUSSION The result is a photograph that has the proper color rendition and is taken using reasonable exposure times. Each part of the intentional double exposure serves a different function. The first exposure sets a frame of reference for the spots. The 80B filter provides the proper color correction to allow the use of photofloods with a daylight type film. The proper result of this exposure should be a dim plate that is gray in color. The plate should be light enough to be able to
read any notations written on the plate, but as dark as possible to retain contrast in final print. If the plate is not gray, a color correction filter may be used (this is usually unnecessary). The second exposure photographs the spots. The 2E filter is a UV cut off filter, preventing any of the UV light that is used to activate the fluorescence from exposing the film. This filter prevents the blue cast from appearing on the film, resulting in proper color of the spots. Color correction filters a t this point are unnecessary. An important benefit from this procedure is that the film speed is effectively increased for the second exposure. The first exposure serves to “preflash” the film for the second exposure. The technique of “preflashing” film to get higher effective film speeds has been used by photographers for many years.
CONCLUSIONS The procedure outlined above allows the analyst to photograph UV activated fluorescent spots on TLC plates with the following three advantages: (1)The exposure times are shorter than with other techniques; (2) the use of colorcorrecting filters is usually not necessary; (3) the colors of the fluorescent spots in the final print closely match those seen by the eye. Since the prints obtained by this procedure reasonably represent what is seen by the eye, they can be valuable documents for the TLC analysis. The technique outlined above is not limited to the specific camera system indicated under Experimental. The only requirement is that the copy stand has both a UV and white light source and can accommodate filters. The first exposure can be determined experimentally by installing the proper filter for the light source, and adjusting exposure until the print shows a dark gray plate. The second exposure uses the 2E filter and exposure time is adjusted until the intensity of the fluorescent spot in the prints matches that seen by the eye.
RECEIVED for review April 11, 1977. Accepted June 6, 1977.
Determination of Mercury in Edible Oils by Combustion and Atomic Absorption Spectrophotometry Wei-Chong Tsai and Lih-Jiuan Shiau Food Industry Research and Development Institute, P.O. Box 246, Hsinchu, Taiwan, Republic of China
In recent years, the environmental mercury contamination has been recognized as a health problem. Accurate and reliable methods of analysis of a wide variety of materials, which may contain trace amounts of mercury in both organic and inorganic forms, are needed. Alkyl or aryl forms of mercury may be determined by gas-liquid chromatography (1-3). After wet digestion or combustion, total mercury could be quantitatively determined by using various methods, of which the application of flameless atomic absorption spectrophotometry is perhaps the most common and convenient one. Although much has been studied on the occurrence of mercury in biological materials, such as fish and grain, by acid digestion followed by reduction and aeration ( 4 , 5 ) ,by amalgamation and heating (6, 7), by direct combustion (8, 9), or by various combinations of these and other less common techniques (10-14), little has been known about the mercury contamination in vegetable oils. Knauer et al. (15) determined mercury in petroleum and petroleum products by burning the sample in a Wickbold oxy-hydrogen combustion apparatus and collecting the va-
porized mercury in an acidic permanganate solution, and essentially quantitative recoveries were obtained. This paper describes a rapid and reliable method that can be carried out with equipment available in most laboratories. The method, which is a modification of the Schoniger combustion technique (8, 9), includes pretreatment of the sample by burning and collection of the vaporized mercury in an acidic permanganate solution. The closed-system combustion apparatus, in which the sample was burned with the aid of oxygen stream, is different from Wickbold’s combustion apparatus.
EXPERIMENTAL Reagents. (a) Acidic potassium permanganate oxidizing solution was prepared daily by dissolving 5 g of reagent-grade potassium permanganate in 1 L of 3 N H2S04solution. (b) Stannous chloride reducing solution (16) was prepared by dissolving 1 g of reagent-grade hydrazine dichloride, 20 g of sodium chloride, 20 g of hydroxylamine hydrochloride, and 33 g of stannous chloride in 25 mL of 18 N H2S04and diluting to the volume of 1L with deionized distilled water. (c) Stock solution of HgC12 (1000 pg Hg/mL) was prepared from E. Merck standard ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
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