Sample photodecomposition

A similar reverse optics arrangement, with a postsample monochromator followed by a detector has infrequently been incorporated into scanning instruments, albeit rarely on a commercial level (Figure IB). Reverse optics instruments, whether diode array or scanning, can be more prone to inducing sample photodecomposition and other pho-toreactive phenomena, as the full intensity of the light source, unattentuated by a monochromator, is incident on the sample. 

Another method, called photobleaching, works on robust soHds but may cause photodecomposition in many materials. The simplest solution to the fluorescence problem is excitation in the near infrared (750 nm—1.06 pm), where the energy of the incident photons is lower than the electronic transitions of most organic materials, so fluorescence caimot occur. The Raman signal can then be observed more easily. The elimination of fluorescence background more than compensates for the reduction in scattering efficiency in the near infrared. Only in the case of transition-metal compounds, which can fluoresce in the near infrared, is excitation in the midvisible likely to produce superior results in practical samples (17). 

Because the laser beam is focused on the sample surface the laser power is dissipated in a very smaU area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shaft and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. 

Photodecomposition. A greyish-white film of solid material slowly formed along the bottom of the sample cell in PuF6 cells irradiated at 1064 nm. A similar film formed considerably faster in sample cells irradiated at 532 nm with the film forming on the entrance face of the cells as well as along the walls of the cell. Film formation was evident after less than a minute of irradiation at a laser fluence of 5 J/cm2 at 532 nm. The exact chemical composition of the film has not yet been determined. It is known that it contains plutonium and... 

An environmental protocol has been developed to assess the significance of newly discovered hazardous substances that might enter soil, water, and the food chain. Using established laboratory procedures and C-labeled 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD), gas chromatography, and mass spectrometry, we determined mobility of TCDD by soil TLC in five soils, rate and amount of plant uptake in oats and soybeans, photodecomposition rate and nature of the products, persistence in two soils at 1,10, and 100 ppm, and metabolism rate in soils. We found that TCDD is immobile in soils, not readily taken up by plants, subject to photodecomposition, persistent in soils, and slowly degraded in soils to polar metabolites. Subsequent studies revealed that the environmental contamination by TCDD is extremely small and not detectable in biological samples. 

All samples contained the metabolite 4-epi-tetracycline, and this was even more pronounced in the liquid manure. Under strong acidic conditions (pH < 2), tetracycline is metabolized to anhydro-tetracyclines, which are bioactive, whereas alkaline pH conditions (pH > 7.5) can favor the formation of isotetracyclines, which show almost no in vitro activity. Tetracycline can also undergo extensive photodecomposition, forming a variety of products (Oka et al., 1989 Peterson et al., 1993). 

All assays should be performed under reduced or F40 gold fluorescence lighting to minimize the potential for photodecomposition or activation. Assays are run in triplicate to determine percent inhibition. The tests are repeated at least once with a freshly prepared sample. If there is greater than 15% coefflciency of variation, the samples are run at least one additional time. When the reaction mixture is incubated within the plate reader, readings are taken immediately and at set times throughout the prescribed incubation period as established by the microsome supplier. For assays incubated outside of the plate reader, reactions were stopped in accordance with the product test procedure. 

Fig. 12. Corrosion diagram for samples of n- and p-type semiconductors s the equilibrium potential of the reaction of anodic photodecomposition of semiconductor with holes involved ( ° , is the equilibrium potential for a redox system /Red (/>cwr(n) and

To minimize the potential for photodecomposition, all samples must be protected from light from the time of receipt until extraction. 

Both reduced and Pt-modified powder samples were studied in distilled water and in aqueous solutions of HC1, H2SO4, HNO3 and NaOH. Water photodecomposition proceeds moderately in distilled water and in NaOH but is strongly suppressed in acidic aqueous media. The NaOH coating effect mimicks that found by other workers earlier (see Ref. 320 and text). 319... 

It is important to be cognizant of humidity changes and possible sample surface heating problems. Identification of all photodegradants is rarely necessary, but can be useful in understanding the mechanism(s) of photodecomposition and aid in the development of protective measures. 

Several reports have indicated that the nature of photodegradation pathway and rate of photochemical decomposition of drug substances are influenced by the type of the source, intensity, and wavelength. In addition, time of exposure and the distance between the source and the location of the exposed samples within a test chamber are other variables that can affect the noted rate of photodecomposition. 

Stringer and Attrep compared hydrogen peroxide-sulphuric acid digestion and U.V. photodecomposition methods for the decomposition in water samples of triphenylarsine oxide, disodium methane-arsonate and DMAA to inorganic arsenic prior to reduction to arsine and determination by atomic absorption spectroscopy or by the AgDDC spec-trophotometric method . 

---