VEGETATION-SAMPLE PREP

Samples are washed plants that are air dried to remove moisture and placed in “zip-lock” bags.  Equipment is scaled to prepare 200 samples simultaneously.  Approximately 500 g of sample is weighed to establish the wet-basis plant mass.  Dry overnight at 110°C.  A Model – 1390 FM high-performance, horizontal airflow oven was obtained from VWR.  Reweigh to obtain the dry-basis plant mass.

Break into smaller pieces by hand and macerate for about 30 seconds by pulsing in a large blender.  Remove from blender to a beaker.  Blow out blender with a little alcohol if necessary.  Weigh out a 50-gram sample on a wet basis into a large crucible (about 200 gram capacity).  Ash in a muffle furnace at 350°C for about 30 minutes and increase heat by about 40°C to about 750°C and hold temperature for about 2.5 hours.  Ash should be gray in color.  If black color persists, continue muffle oven treatment to remove residual carbon.  Reweigh to obtain ash-basis plant mass.  The muffle furnace was obtained from VWR, models Series II, 2-1350 NEY Box furnace.

Weigh out about 25 grams on an ash-basis in a 250-ml Pyrex beaker.  Add 25 ml of concentrated HNO₃ acid and gradually add 25 ml of concentrated HCl acid with agitation. Acids should be “low trace-metal” reagents.  (Addition of one acid onto the second acid can result in a violent exothermic reaction.) Warm to 400-500° on a hot plate.  Add 30% H₂O₂ in ½ ml increments from a squeeze bottle. Approximately 5 ml of peroxide may be required to convert the pale yellow color to a clear liquid.  Additional nitric and hydrochloric acid may be required.  On occasion high silicate content may require that 1 ml of concentrated HF acid is used for the final digestion.  Under extreme conditions a treatment of 75 ml of concentrated HF in a 250 ml Teflon beaker over low heat (300-400°C) may be required.  Teflon melts at 450°C.

We ash the sample to remove residual chloride and reconstitute the sample in 1% HNO₃ to obtain a suitable analytical concentration.  These samples for the United States (US) Army is dried in a microwave oven to assure that all water is removed before the reconstitution.

Once ashed, a duplicate of every 20 samples is spiked and carried through the sample preparation.  Spike recovery is typically from 75% to 125%.

Chloride interferes with KPA analysis at about 10 ppm.  Calcium can provide a salt effect.

*Acknowledgements: Data in this application brief has been provided by Severn Trent Laboratories, Inc. and the US Army Yuma Proving Grounds.