The quest for a simple way to prepare environmental samples for trace metal analysis at the parts per billion and parts per trillion levels has become like the elusive quest for the Holy Grail itself (1).

Robotics or clever designs where samples are automatically carried across a hot plate or through a microwave oven are usually too expensive or too big for the typical environmental laboratory where consideration of price and space are foremost in purchasing decisions.

There has been a need for a simple device that reduces acid digestion contamination and increases productivity and this need appears to be met by the Hot Block system. The Hot Block is a block digestor where the heating area is composed of graphite and the surrounding box is made from Kydex®, a high temperature thermoplastic. It has no exposed metallic exposed parts. With a 15" x 15" footprint it holds up to thirty-six, graduated 50mls polypropylene vessels. It has a temperature range of ambient to 180 ^oC controlled to within 1.5 ^oC. However, because polypropylene vessels are used one would not want to increase the temperature much past 95 ^oC which is the maximum temperature for EPA digestions (3).

Environmental Express (Mt. Pleasant, SC), the manufacturer of the Hot Block, makes the following claims stemming from the unique design of the Hot Block:

This study represents an independent assessment of the Hot Block which has been set up in two different environmental laboratories to verify or disprove the claims of the manufacturer.

* Author to whom correspondence should be addressed.

Equipment

Studies we carried out using the 50 ml, 36 well Hot Block (P/N SC100) equipped with two individual 18 cup trays (P/N SC200) and polypropylene digestion cups (P/N SC 500) which were supplied by Environmental Express. Temperature measurements were made using a VWRbrand total range digital thermometer that is traceable to NIST (P/N 61161-277) which was supplied by VWR (So. Plainfield, NJ).

The Hot Block digestors were located in two separate laboratories: Progress Environmental Laboratory, Tampa, FL and Environment One Laboratory, Greenville, NC. Identical studies were performed at each site to assess the effect of site location on digestion parameters.

The measuring instrument used by Progress Environmental Laboratory was a Thermo Jarrell Ash (Franklin, MA) 61E Trace ICP equipped with an axial torch. Conditions used for analysis have been reported elsewhere (4). The measuring instrument used by Environment One Laboratory was a Perkin-Elmer (Norwalk, CT) Optima ICP equipped with an axial torch.

Experimental Design

To test the manufacturer’s claim of uniform temperature across the heating block, twelve digestion cups filled with 50 mls of deionized water were placed in pre-determined wells in the block (Figure 1) and monitored for temperature every 30 minutes for a total of two hours as the target temperature of 95 ^oC was reached. In addition, the final volume of water remaining in the digestion cups was measured to assess the evaporation rate after heating. The results from both laboratories are found in Table 1 and 2.

To test the manufacturer’s claim of reduced contamination due to the non metallic construction of the Hot Block, twelve digestion cups filled with an acid blank of 5% HCl and 1% HNO₃ were placed in the same wells as depicted in Figure 1. The Hot Block was heated to a target temperature of 95 ^oC for two hours after which the acid blank digests were diluted to 50 mls and analyzed by axial torch ICP for selected elements. The results of the acid blank analysis from both laboratories are depicted in Table 3.

Finally, both laboratories used the Hot Block for routine digestion of water and soil samples. The recovery of pre-digestion spikes added to both water and soil samples were measured. The recovery percentage from samples digested on different days and from random well sites on the block were assessed to measure the inter-day reproducibility of metal analyses from digests using the Hot Block. The data for water digests is reproduced in Table 4 while the data for soil digests is reproduced in Table 5.

Results and Discussion

Temperature Uniformity

The Hot Block wells used to monitor the temperature of water during heating (Figure 1) were selected so as to represent the areas where excessive cooling would be most likely to occur (well sites 1, 2, 3, 4, 7, 10, 11, and 12) and the areas where excessive heating would be most likely to occur (well sites 5, 6, 8, and 9).

As shown in Tables 1 and 2, the temperature was extremely uniform where the relative standard deviation of the temperatures across the entire block was normally less than 1%. Hot spots and cool spots, as are usually found on hot plate surfaces, were not observed with the Hot Block. This finding is backed up by the uniform volume of water remaining in the digestion cups following heating.

The fact that the control temperature had to be set differently at the two sites (111 ^oC vs 119 ^oC) was probably due to differences in incoming voltages between the two laboratories and shows the importance of having this control feature incorporated in the Hot Block design.

It was of interest to observe that the Hot Block took longer to heat to the target temperature at Progress Environmental Laboratory. This author observed during the initial set up that the flow of air into the hood at Progress was much higher than at Environment One and that the room temperature was lower at Progress than at Environment One. It is concluded that this higher flow of cooler air contributed to cooling of the Hot Block and must be taken into consideration when setting up the Hot Block.

Preparation Blanks

One experienced in hot plate acid digestion techniques becomes used to seeing the typical corroded metal hot plate and the rings across the heating surface where acid spilling out of beakers has eaten into the metallic surface. Contamination problems with elements such as Al, Cr, Fe, Ni, and Zn when axial torch ICP and ICP-MS instruments are employed are of no surprise to analysts.

This study confirmed that contamination is not an issue when using the Hot Block due to its graphite and plastic construction. Adding to the advantage of inert construction material is the use of the disposable polypropylene digestion cups with lids. Avoiding the use of glass is highly beneficial in reducing contamination at the parts per billion and parts per trillion concentration levels.

The Hot Block was very advantageous to Environment One where acid digestion space was at a premium and corrosion products from a metal hot plate were causing elevated blanks for aluminum and iron. The data for aluminum and iron in Table 3 come from Environment One and show that, although the blanks for these two elements are still elevated, the levels of aluminum and iron have been reduced significantly since the installation of the Hot Block. The authors expect these level to drop further as the laboratory environment is further cleansed from previous corrosion products stemming from the metal hot plates.

Spike Recoveries

The ultimate test of a digestion system is based on whether one can consistently prepare spiked samples where the spiked recoveries fall within acceptable limits. Productivity suffers when labs are forced to re-run samples due to contaminated preparation blanks or unacceptable spike recovery levels.

This study was designed to evaluate the inter-day variation of pre-digestion spike recoveries from both soils and waters as is required by laboratories when setting up accuracy control charts. Acceptable recoveries are due not only to uniform heating and inert construction materials, but are also attributable to the ease of sample handling.

The unique design of the digestion cups used with the Hot Block allows the analyst to dilute the final digest to volume in the digestion cup and to either decant the sample into ICP autosampler cups or to use the digestion cup itself as an autosampler cup. Eliminating the need to transfer the digest from a beaker to a volumetric flask or cylinder, and in many cases to avoid filtration aids in insuring that spiked digests will consistently fall within acceptable quality control windows thereby reducing the need for re-runs.

Customer Comments

As part of this study the analysts at both laboratories were asked to provide comments on how the installation of the Hot Block has improved laboratory operations. The following are the responses after a few months of use:

Conclusion

The claims made by the manufacturer of the Hot Block, Environmental Express, have been verified through independent testing. The Hot Block indeed does seem to not only reduce labor and glassware costs, but the nonmetallic construction reduces background contamination. There is uniform heating of samples resulting in even sample evaporation. The authors recommend the use of the Hot Block for laboratories that require a reliable and corrosion free heating device to prepare samples for environmental trace metal analysis.

Acknowledgements

The authors would like to thank Vince Giampa and Mark Hennis of Progress Environmental Laboratory and John Melvin and Dawn Broome of Environment One Laboratory for their approval to carry out these studies in their laboratories. We also want to thank Paul Strickler and Les Orr of Environmental Express for their valuable assistance in setting up the Hot Block digestors.

References

1. Methods for the Determination of Metals in Environmental Samples, Supplement I. EPA Office of Research and Development, Washington, DC. EPA/600/R-94/111. May 1994.

2. Kingston, H.M. and S.J. Haswell. Microwave-Enhanced Chemistry: Fundamentals, Sample Preparation and Applications. American Chemical Society (1997).

3. Tatro, M.E. Sample preparation techniques for trace metal analysis. Environmental Testing & Analysis, 7,2, 24 (1998).

4. Tatro, M.E., L.C. Camp and R.K. Christenberry. Analysis of trace metals in solid waste using a simultaneous trace analyzer ICP. American Laboratory, 28, August (1995).

 

Figure 1. Designation of Hot Block Wells where the temperature of water in digestion cups were measured.

 

Table 1. Temperature data from Environment One Laboratory.
The Hot Block Control Temperature was set at 111 ^oC.

 

Table 2. Temperature data from Progress Environmental Laboratory.
The Hot Block Control Temperature was set at 119 ^oC.

 

Table 3. Preparation blank concentrations. All ICP detection limits are from Progress Environmental
Laboratory except for Al and Fe which are from Environment One Laboratory.

 

Table 4. Average recoveries of pre-digestion spike and spike duplicate from soil digests on six separate days using the Hot Block.
The spike concentration was at 200 ppb.
(n = 12). Data is from Progress Environmental Laboratory.

 

Table 5. Average recoveries of pre-digestion spike and spike duplicate from water digests on six separate days using the Hot Block.
The spike concentration was at 500 or 1,000 ppb (n = 12). Data is from Environment One Laboratory.