Innovations in Metals Analysis Optimize the Process

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Analysis for metals in various health- or environmental-related samples is gaining more attention, whether it be natural levels of arsenic found in various produce, or heavy metals such as lead or chromium found in drinking water.

The typical analytical procedure for metals is to digest the sample in the presence of one or more acids at just below boiling temperatures. Depending on the acids, temperature, and time, the digestion procedure can be adjusted to determine any amount from the already-dissolved metals to the bioavailable metals to the full metal content of the sample. After the digestion procedure is complete, the digestate is compared to known standards using an analytical instrument. Graphite furnaces and different hybridizations of inductively coupled plasma (ICP) spectrophotometers, such as ICP/MS and ICP/OES, are the most commonly used instruments for this purpose. Because of the increased need for lower detectable values, much effort has been dedicated to improving how low an instrument can “see.” One of the easiest improvements is to automate the sample injection and analysis; by doing so, all samples are treated the same. This removes many sources of variability and helps to provide a consistent baseline against which to measure the detected signal.

Often overlooked, but just as important as the analysis, is the digestion procedure used to generate samples for analysis. Like the analytical procedure, the digestion procedure also benefits from consistency in its treatment of the samples. Traditionally, samples were digested in standard laboratory glassware, such as flasks or beakers, on a hotplate or over an open flame such as a Bunsen burner. While effective, this procedure was inefficient for handling large numbers of samples at a time.

A more efficient metals digestion process

Figure 1 – The HotBlock unit (Environmental Express, Charleston, SC) reduces contamination during sample digestion.

The HotBlock metals digestion unit (Figure 1) introduced several improvements to the digestion procedure, one of which was the move from larger-diameter glass vessels to small-diameter polypropylene vessels for the digestion. This allowed the digestion vessels to be disposable and recyclable. Single-use digestion vessels eliminate the possibility of carryover between samples. The introduction of plastic vessels allowed scientists to begin adding boron, silicon, and other trace elements used in glass manufacture to the suite of metals capable of being detected. Another benefit of the HotBlock unit was its use of nonmetallic components whenever possible. The graphite digestion block and the heat-resistant polymer housing further reduce the number of sources for contamination during the sample digestion. Additional features include increasing the size of the digesters to accommodate more samples at one time, providing options for larger and smaller volumes of sample, and increasing the viable temperature range.

Typical way to fully automate metals digestion

Figure 2 – The AutoBlock Plus (Environmental Express) controls fumes and off-gassing

Once block digesters became more widely used, attention turned to automating the digestion procedure. Full automation can require any combination of several steps, including the addition of several different digestion reagents, usually acids; heating for a specific amount of time at a precise temperature; changing the digestion temperature; allowing samples to cool; and refilling to a specified volume. Adding in the need to control fumes and off-gassing in the digestion area makes automation an expensive procedure. The AutoBlock Plus (Figure 2) is a cost-effective solution.

Advanced automation

Figure 3 – The AutoBlock Fill station (Environmental Express) is designed to fit inside a fume hood.

The AutoBlock Fill station (Figure 3) is designed to be a midway point between the manual block digester and the fully automated digestion unit. Its intended use is to automate the repetitive additions of digestion reagents. This then leaves steps that may require user discretion, such as digesting down to a certain volume, or steps where automation starts to add extra expense such as removing samples from the block after digestion. The AutoBlock Fill station is used in conjunction with any size HotBlock unit. The HotBlock unit can be locked in place on the AutoBlock Fill station. If multiple HotBlock units are in use, they can be used on the benchtop while a stand is inserted in place of the HotBlock. This will allow the analyst to rotate sample racks through the AutoBlock Fill. Once the sample rack is fully prepped, it is placed on the separate HotBlock unit so that a new sample rack can be automatically filled.

The AutoBlock Fill station can hold up to four separate digestion reagents in addition to reagent water used for rinsing. Because digestion can be performed while the HotBlock unit is on the AutoBlock Fill station, the AutoBlock Fill station was designed to be able to fit inside most standard-sized fume hoods. If hood space is limited, a vented enclosure is available to place over the unit and attach to the hood’s ventilation system. The included software allows the user to define the desired digestion methods on a PC or laptop without being connected to the unit and then download the methods to the unit. Once the methods are defined, the sample positions can be set and the method chosen via an onboard touchscreen without needing to have the controlling computer attached.

Clean cups reduce exposure to airborne contaminants

Figure 4 – The Ultimate Clean Cup (Environmental Express) is the cleanest cup on the market

While polypropylene cups are inherently free of metal contaminants, ultratrace levels often find their way into the sample. The Ultimate Clean Cup (Figure 4) alleviates this problem. The cup uses the same clean resin as the traditional digestion vessel; however, extra steps are taken to automatically package and seal the vessels immediately as they are produced. This reduces exposure of the product to airborne contaminants or from being handled. This packaging has allowed Environmental Express to reduce the certified value on these cups by as much as a factor of 10 over its standard digestion vessels.

Conclusion

There is no way to guarantee that metals are free from every contaminant, but, as described here, measures can be taken and instruments can be used to reduce contamination and optimize the metals digestion process.

David Smith is the technical director for Environmental Express, 2345A Charleston Regional Pkwy., Charleston, SC 29492, U.S.A.; tel.: 843-576-1133; e-mail: [email protected]; www.EnvExp.com