Trace-metal-clean techniques

Twiss, M. R., J. C. Auclair, and M. N. Charlton. 2000. An investigation into iron-stimulated phytoplankton productivity in epipelagic Lake Erie during thermal stratification using trace metal clean techniques. Canadian Journal of Fisheries and Aquatic Sciences 57 86-95. 

It is now evident that marine N2 fixation can be limited by either P or Fe, although the relative importance of these two nutrients is stiU highly debatable. There may be an interaction between iron supply and P limitation of primary production through N2 fixation. Recently, Mills et al. (2004) used a factorial nutrient addition experiment to show that Fe and P can co-limit N2 fixation in the eastern tropical North Atlantic. They used trace-metal clean techniques with the addition of N, P, Fe, or Saharan dust individually or in combination to investigate which nutrient limits N2 fixation. Their experiments demonstrated that at two out of three stations, N2 fixation was enhanced by 2-3 times by the addition of P and Fe together, but the addition of either P or Fe alone had less effect (Fig. 38.14). Added dust also enhanced N2 fixation, which they suggested could be due to mineral dissolution of both Fe and P. 

As noted throughout this review, investigators have encountered substantial problems with the inadvertent introduction of contaminant lead. This contamination occurs during collection, storage, processing and analysis. The specific procedures, now termed trace-metal-clean techniques, that are required to circumvent those problems were detailed three decades ago by Patterson (1965), two decades ago by Patterson and Settle (1976), and more recently by Flegal and Smith (1992b). 

Trace-metal-clean techniques are also necessary in analysis of clinical samples with relatively low lead concentrations. This is illustrated by the relative contribution of contaminant lead in measurements of elevated (50 fig/dL) and low (1 /xg/dL) PbB (Fig. 8). Moreover, the importance of these techniques will increase in clinical settings with projected declines in environmental lead exposures to humans in the U.S. and elsewhere (Brody et al. 1994 Flegal and Smith 1992b Smith and Flegal 1995). 

It is apparent that the extent of sublethal lead toxicity in humans may be best addressed by studies that consider control populations possessing natural (i.e., preindustrial) lead burdens, as well as state-of-the-art, trace-metal-clean techniques and advanced instrumentation. Trace-metal-clean techniques are required to prevent the inadvertent lead contamination of samples, which has plagued many previous analyses of environmental and human lead levels. Advanced instrumentation is required to provide the sentivity, accuracy, and precision that are needed to quantify the sublethal effects of lead concentrations at environmental levels of exposure. Fortunately, methodologies utilizing these advancements are now capable of addressing many of the important issues (e.g., lead biomolecular speciation, low exposure effects) in environmental and human lead toxicology. 

The fire assay, the antecedents of which date to ancient Egypt, remains the most rehable method for the accurate quantitative determination of precious metals ia any mixture for concentrations from 5 ppm to 100%. A sample is folded iato silver-free lead foil cones, which are placed ia bone-ash cupels (cups) and heated to between 1000 and 1200°C to oxidize the noimoble metals. The oxides are then absorbed iato a bone-ash cupel (ca 99%) and a shiny, uniformly metaUic-colored bead remains. The bead is bmshed clean, roUed fiat, and treated with CP grade nitric acid to dissolve the silver. The presence of trace metals ia that solution is then determined by iastmmental techniques and the purity of the silver determined by difference. 

Most of our understanding of the marine chemistry of trace metals rests on research done since 1970. Prior to this, the accuracy of concentration measurements was limited by lack of instrumental sensitivity and contamination problems. The latter is a consequence of the ubiquitous presence of metal in the hulls of research vessels, paint, hydrowires, sampling bottles, and laboratories. To surmount these problems, ultra-clean sampling and analysis techniques have been developed. New methods such as anodic stripping voltammetry are providing a means by which concentration measurements can be made directly in seawater and pore waters. Most other methods require the laborious isolation of the trace metals from the sample prior to analysis to eliminate interferences caused by the highly concentrated major ions. 

The Clean Air Act of 1970 declared beryllium, mercury, and asbestos as hazardous elements. Of the three, mercury is of particular interest to coal technologists. Other elements that exist as trace metals in coal and are suspected to be potentially detrimental to the environment include Pb, As, Sb, Zn, Se, Mo, Co, Li, V, Cr, Mn, Ni, etc. It is not the purpose of this book to create villains out of these elements, but to illustrate analytical techniques to determine how and in what amounts they are released in coal conversion processes. 

Partly because of this concern, the Wisconsin Department of Natural Resources, in cooperation with the Electric Power Research Institute, initiated an extensive study of Hg cycling in seepage lakes of north-central Wisconsin (14). The mercury in temperate lakes (MTL) study used clean sampling and subnanogram analytical techniques for trace metals (10, 17) to quantify Hg in various lake compartments (gaseous phase, dissolved lake water, seston, sediment, and biota) and to estimate major Hg fluxes (atmospheric inputs, volatilization, incorporation into seston, sedimentation, and sediment release) in seven seepage lake systems. 

The trace metal left on the surface of the wafer is mostly due to the slurry. The quality of the chemicals and abrasive used in the slurry is never perfect. An average oxide CMP slurry has trace metals in the range of 1-10 ppm per element. There are very few ultrahigh-purity slurries with trace metals below 10 ppb (actually under the metrology detection limit). What matters most is not the amount of trace metals in the slurry but the amount left on the surface after post-CMP cleaning. The most effective technique to remove the trace metals from the wafer surface is to perform a very thin etching of the oxide surface using dilute HF, for example. 

Figure 12.2 SOFeX depth profiles of biomass (PN)-specific NO/ uptake rates, determined during 24-h incubations in Plexiglas acrylic incubators under simulated in-situ light and temperature conditions. Ultra-clean trace-metal techniques were used for sample collection within and outside (control waters) of the Fe-enriched patch north and south of the Antarctic Polar Front zone. The/-values [f = Fn03/(1 n03 + 1 nH4 + F n02 + F Urea)] were determined at the isolume depths of 47 and 16% surface irradiance, using tracer-level isotopic enrichments, and are not corrected for the effects of isotopic dilution. Error bars represent the range of duplicate samples (n = 2). Corrected from Coale et al. (2004).

Digestion of samples using high pressure oxygen bomb combustion is an excellent technique for sample preparation, particularly trace metal analysis. This technique can be applied to most plastics provided that small sample ( 0.25 g) of fine grain sizes of plastics are used. The solutions obtained are clean and easily analysed for metal content against standards prepared in the same solution added to bomb. 

Remediation of soil refers to practices of either removing or rendering of contaminants to less mobile - that is, to generate less harmful forms. Methods for remediating metal-polluted soils have been recently broadly investigated and discussed (Adriano et al. 1999, Cunningham and Berti 2000, Iskandar 2001, Knox et al. 2000, Li etal. 2000, Mukherjee 2001, Sparks 1995). The remediation of soils and sites contaminated with radionuclides is of special concern. Several techniques, both in situ and non-in situ, have been applied to clean soils contaminated with trace metals. Due to the complexity of soils and the presence of multiple contaminants, only a few of these techniques... 

These EPA-driven methods represent the bulk of the routine environmental analyses being carried out by ICP-MS today. However, there are many other types of samples being analyzed, which represent a much smaller but significant contribution to the environmental application segment. For example, to better understand industrial-based airborne pollution covered by the Clean Air Act, air quality is often monitored using air-filtering systems. These typically consist of small pumps (either static or personal) where the air is sucked through a special filter for extended periods. The filter paper is then removed, dissolved in a dilute acid, and analyzed by an appropriate technique. Because trace metal concentration levels are sometimes extremely low, ICP-MS has proved itself to be a very useful tool to analyze these airborne particulate samples and help pinpoint sources of industrial pollution. 

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