How should a sample intended for total metals analysis be digested?

Document ID

Document ID TE3796

Version

Version 7.0

Status

Status Published

Published Date

Published Date 09/19/2018
Question
How should a sample intended for total metals analysis be digested?
Summary
Total metals analysis pretreatment by digestion
Answer
Several procedures require sample digestion before determining total metal content. Digestion uses acid and heat to break organo-metallic bonds and free ions for analysis.

For United States Environmental Protection Agency (USEPA/EPA) reporting, USEPA-approved digestions are required. The following excerpt is quoted from the USEPA published section of the Code of Federal Regulations (CFR), Title 40, Chapter I, Subchapter D, Part 136.3, comment 4 on Table IB - List of Approved Inorganic Test Procedures:
For the determination of total metals (which are equivalent to total recoverable metals) the sample is not filtered before processing. A digestion procedure is required to solubilize analytes in suspended material and to break down organic-metal complexes (to convert the analyte to a detectable form for colorimetric analysis) ... The approved total recoverable digestion is described as Method 200.2 in Supplement I of "Methods for the Determination of Metals in Environmental Samples" EPA/600R-94/111, May, 1994, and is reproduced in EPA Methods 200.7, 200.8, and 200.9 from the same Supplement ... Regardless of the digestion procedure, the results of the analysis after digestion procedure are reported as "total" metals.

If not in a USEPA reporting or other regulatory context (e.g. process control), the Metals Prep Set (Catalog Number/Order Code TNT890) used with a DRB 200 is a much easier-to-use alternative that takes less time, does not need a fume hood, and does not need supervision. See this article for more information about it's use.

USEPA Method 200.2 is available freely online at the National Environmental Methods Index webpage here. The Sample Collection, Preservation and Storage and procedure for Aqueous Sample Preparation sections of this method are outlined below, and Hach products are listed for this procedure here:
 
Catalog Number / Order Code Product
2881500 Hot Plate, 7x7 in., Digital, 115 VAC
2601300 pH Paper, 0 - 14 pH Range, 100/pk
2087083 Bottle, Sampling, Low Density, Polyethylene, 1 L, 6/pk
2749320 Dropper, Polyethylene, 3.1 mL Bulb Draw, 20/pk
1218900 Pipet Filler, 3-Valve
1451542 Pipet, Volumetric, Class A, 100 mL
1451536 Pipet, Volumetric, Class A, 2 mL
1451535 Pipet, Volumetric, Class A, 1 mL
50046H Beaker, Griffin, Low Form, Glass, 250 mL
57870 Watch Glass, Pyrex, 100 mm
2636642 Flask, Volumetric, Class A, Certified, 100 mL
88449 Hydrochloric Acid Standard Solution, 6.0 N (1:1), 500 mL (1+1)
254049 Nitric Acid Solution, 1:1, 500 mL (1+1)
Synergy Water Systems Reagent Water (ASTM Type I)
2676500 (optional) Centrifuge, Laboratory, 115 VAC, 60 Hz
2278739 (optional) Centrifuge Tubes, Round Bottom, 10/pk
  1. For determination of total recoverable elements in aqueous samples, the samples must be acid preserved prior to aliquoting for either sample processing or determination by direct spectrochemical analysis. For proper preservation samples are not filtered, but acidified with (1+1) nitric acid to pH < 2. Normally, 3 mL of (1+1) nitric acid per liter of sample is sufficient for most ambient and drinking water samples.
    1. Preservation may be done at the time of sample collection, however, to avoid the hazards of strong acids in the field, transport restrictions, and possible contamination is is recommended that the samples be returned to the laboratory within two weeks of collection and acid preserved upon receipt in the laboratory. Following acidifcation, the sample should be mixed and held for 16 hours.
    2. The pH of all aqueous samples must be tested immediately prior to withdrawing an aliquot for processing to ensure the sample has been properly preserved. If for some reason such as high alkalinity the sample is verified to be > 2, more acid must be added and the sample held for 16 hours until verified to be pH < 2. If properly acid preserved, the sample can be held up to six months before analysis.
  2. Transfer a 100 mL (± 1 mL) aliquot from a well mixed, acid preserved sample to a 250 mL Griffin beaker (When necessary, smaller sample aliquot volumes may be used).
    1. If the sample contains undissolved solids >1%, a well mixed, acid preserved aliquot containing no more than 1 g particulate material should be cautiously evaporated to near 10 mL and extracted using the acid-mixture procedure described in Sections 11.2.3 through 11.2.8
  3. Add 2 mL (1+1) nitric acid and 1.0 mL of (1+1) hydrochloric acid to the beaker containing the measured volume of sample. Place the beaker on the hot plate for solution evaporation. The hot plate should be located in a fume hood and previously adjusted to provide evaporation at a temperature of approximately but no higher than 85 °C. The beaker should be covered with an elevated watch glass or other necessary steps should be taken to prevent sample contamination from the fume hood environment.
    1. For proper heating adjust the temperature control of the hot plate such that an uncovered Griffin beaker containing 50 mL of water placed in the center of the hot plate can be maintained at a temperature approximately but no higher than 85 °C. (Once the beaker is covered with a watch glass the temperature of the water will rise to approximately 95 °C).
  4. Reduce the volume of the sample aliquot to about 20 mL by gentle heating at 85 °C. Do not boil. This step takes about two hours for a 100 mL aliquot with the rate of evaporation rapidly increasing as the sample volume approaches 20 mL.
    1. A spare beaker containing 20 mL of water can be used as a gauge.
  5. Cover the lip of the beaker with a watch glass to reduce additional evaporation and gently reflux the sample for 30 minutes.
    1. Slight boiling may occur, but vigorous boiling must be avoided to prevent loss of the HCl-H2O azeotrope.
  6. Allow the beaker to cool. Quantitatively transfer the sample solution to a 100 mL volumetric flask, make to volume with reagent water, stopper and mix.
    1. Reagent water: For all sample preparations and dilutions, ASTM Type I water (ASTM D1193) is required. Suitable water may be prepared by passing distilled water through a mixed bed of anion and cation exchange resins.
  7. Allow any undissolved material to settle overnight, or centrifuge a portion of the prepared sample until clear. The sample is now ready for analysis.
    1. If after centrifuging or standing overnight the sample contains suspended solids that would interfere with the colorimetric method, a portion of the sample may be filtered for their removal prior to analysis. However, care should be exercised to avoid potential contamination from filtration.

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