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Sample Preparation - Trace hydrofluoric acid (HF) and Pyrex digestion vessels

 
Our sample preparation involves preparing a standard that is a mixture of 3 custom standards diluted to volume in a 5% HNO3 / 1% HCl solution. This solution will then be used as the spiking standard before digestion (example: 50mg of sample + 750 µL of spiking standard + 5% HNO3 + 1% HCl solution). One of the 3 custom standards contains trace amounts of HF. We currently use Pyrex digestion vessels and our microwave supplier recommends the use of Teflon PFA liner when using HF. However, I’m just wondering if the traces of HF in the solution would be a concern when using Pyrex vessels?
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The presence of trace HF in contact with borosilicate glassware such as Pyrex can be a concern if the analytes of interest include Na, Al, B, Si, K, and Ca. The elements present in the 3 custom standards are not commonly observed as contaminants in borosilicate glass, so it is possible that these standards could be used in Pyrex digestion vessels without compromising the analyses. The key considerations would be the dilution factor of the intermediate standard and the volume of the final digestion solution as these will determine the actual HF content. A point of reference is that we have observed a 0.1% (v/v) HF blank solution in dilute HNO3 will generate approximately 2 ppm of Si contamination when measured on an ICP with a glass sample introduction system, and we routinely run such solutions when determinations of Si, Na, Al, B, etc. are not required. If, however, the digestion method is aggressive (high T, long times), then it is possible that the Pyrex will degrade with time, which would manifest itself as a general clouding of the inner surfaces.

Posted: 09/06/18 03:48:59

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