Plasma-chemical extraction

Plasma-Chemical Extraction of Nickel from Serpentine Minerals... 

We have also subjected two modem samples to severe plasma treatments, far more vigorous than needed for dating and stable carbon isotopic analyses. These were (i) a piece of a T-shirt label, chosen because it had small writing that would make visual changes easy to see and (ii) a peyote button, selected because we had been asked to date an archaeological peyote button thought to be -7-8,000 BP excavated from Shumla Cave in the Lower Pecos River region of Texas. The results of the 5 C measurements on CO2 produced from our plasma chemical extractions on five different samples are shown in Table II. 

Statistical analysis shows no significant systematic variation in our results with plasma-chemical extraction and those determined at other laboratories using conventional and AMS. analysis with combustion extraction. That includes the outlying TIRI wood sample that may have been influenced by plasticizers (5) and effects introduced by different pretreatments of charcoal 4 (18). Our technique appears to produce accurate and viable dates. 

Procedures for chemically pretreating archaeological samples typically involve an acid-base-acid treatment 31, S2). However, we routinely eliminate both acid washes as we have shown them to be unnecessary with our plasma-chemical extraction technique. Carbonate and oxalate carbon are not extracted by the plasma only organic material was removed for radiocarbon measurements (33,34), Samples are immersed in -I M NaOH and placed in an ultrasound bath for an hour at 50 5 C. When the resulting supernatant was colored, subsequent NaOH washes were performed until the supernatant... 

The unique advantage of the plasma chemical method is the ability to collect the condensate, which can be used for raw material decomposition or even liquid-liquid extraction processes. The condensate consists of a hydrofluoric acid solution, the concentration of which can be adjusted by controlling the heat exchanger temperature according to a binary diagram of the HF - H20 system [534]. For instance, at a temperature of 80-100°C, the condensate composition corresponds to a 30-33% wt. HF solution. 

Isobaric interferences (especially those arising from the plasma itself, e.g., ArO+ on Fe) can be eliminated using cool-plasma conditions, sometimes in combination with a shield torch. This option is not suitable for seawater samples because a cool plasma, in the presence of a heavy matrix, cannot fully ionize elements with high first ionization potentials, notably Zn, Cd, and Hg. Protocols have thus been established for analysis of 10-fold diluted seawater on instalments with sufficiently high resolution to separate most of the affected isotopes from their isobaric interferences [1], To circumvent the issue entirely, others have used online chemical extraction to separate analytes of interest... 

Although this technology is effective in resolving a wide range of polyatomic interferences, the increased cost associated with this type of instrumentation (more than twice the price of a quadrupole instrument) limits its use in most routine laboratories, hence alternative methods of interference reduction have been sought for. The use of chemical extraction and chromatography (in order to separate the analyte from the matrix prior to analysis) or the operation of the ICP-MS under so-called cool plasma conditions, allows the elimination of... 

In exploring the question of the earliest date for human presence in Brazil, Steelman and coworkers apply the plasma chemical procedure to extract carbon from both the pigments of a rock painting from Toca do Serrote de Bastiana and the accretions covering its surface. The accretions were found to contain both monohydrate of calcium oxalate and calcium carbonate. The radiocarbon age of the oxalate carbon was determined to be 2540 60 B.P. while the radiocarbon age of carbon extracted from the pigment was determined to be 3730 90 B.P. These ages are much more recent than the 30,000-40,000 B.P. age determined by electron spin resonance and thermoluminescence of the accretions, but are consistent with dates of other pictographs in the same shelter. 

No analytical technique is entirely non-destructive. However, plasma-chemical carbon extraction is more attractive than combustion methods for some types of artifacts. While some deleterious effect may in the future be observed for artifacts that have undergone plasma extraction, removing a portion of the artifact for destmctive combustion in many cases is not an option for a variety of reasons (i.e., ratio of artifact size to the amount that must be removed for analysis, information content of sample stmcture, rarity or uniqueness of object). 

Our oxalate radiocarbon determination is consistent with the era determined by radiocarbon dates on four other pictographs located in the same shelter. In fact, we have also obtained a direct radiocarbon date of 3730 90 years BP on organic material extracted from the same red figure covered by the oxalate accretion. This plasma-chemical process has been verified by successfully dating known age materials and pictographs with archaeologically constrained ages (39, 40). 

Decomposition of Zircon. Zircon sand is inert and refractory. Therefore the first extractive step is to convert the zirconium and hafnium portions into active forms amenable to the subsequent processing scheme. For the production of hafnium, this is done in the United States by carbochlorination as shown in Figure 1. In the Ukraine, fluorosiUcate fusion is used. Caustic fusion is the usual starting procedure for the production of aqueous zirconium chemicals, which usually does not involve hafnium separation. Other methods of decomposing zircon such as plasma dissociation or lime fusions are used for production of some grades of zirconium oxide. 

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