Step 2: Chemical pretreatment

The objective in chemical pretreatment is to remove any contaminants in the sample that can be made soluble by heating in a series of chemicals. While pretreatment procedures are standardized according to our QA manual, they must also remain flexible to take into account the varying condition of the samples submitted to the laboratory.

There are variants depending upon both the condition and size of the sample. For example, wood may have the cellulose extracted or be treated with the A/A/A method. For some samples, sodium pyrophosphate is added to the sodium hydroxide in the alkaline solution to help deflocculate clays for easy removal. If there is reason to suspect that not all the contaminants were removed, a step may be repeated or the concentration of a solution increased.

step 2 scientist rafter

The following are pretreatment procedures for the most common materials submitted to our laboratory.

The list is by no means exhaustive and numerous pretreatment strategies exist. If you would like information on the pretreatment of a sample type that is not on this list, please contact us.

Charcoal, plant material, degraded wood

After physical pretreatment and removal of any rootlets or visible contamination, these types of sample materials are pretreated with what is termed the Acid/Alkali/Acid (A/A/A) method. In A/A/A, the sample is heated in 0.5M HCl, filtered, rinsed, heated in 0.1M NaOH, filtered, rinsed, and finally heated a last time in 0.5M HCl, filtered, rinsed with dH2O and dried.

Soils, sediment, peat

Two different methods may be used:

  • Total Organic Carbon (TOC) removes only inorganic carbon and absorbed CO2, leaving the entire organic carbon fraction.
  • Acid/Alkali/Acid (A/A/A) treatment removes all but the least mobile organic fraction, leaving the "humin" fraction only.

In ordinary situations, A/A/A is the preferred method of treatment, but sometimes researchers studying the movement of carbon in soils prefer to date the TOC fraction or the humic acid fraction.

For separation and analysis of the complex cycling of Soil Organic Matter (SOM), specialized pretreatment procedures exist (c.f. Trumbore and Zheng 1996; Leavitt et al. 1996).

TOC the sample is heated in 0.1M HCl, then freeze-dried.
A/A/A the sample is heated in 0.5M HCl, filtered, rinsed with dH2O, heated in 0.1M NaOH/0.1M Na4P2O7, filtered, rinsed with dH2O, and finally heated a last time in 0.5M HCl, filtered, rinsed and dried.
Humic acid precipitation the sample is heated in 0.5M HCl, filtered, rinsed with dH2O, heated in 0.1M NaOH and filtered. The NaOH filtrate is collected. Humic acids in solution are precipitated out with the addition of 6M HCl. The precipitate is collected, redissolved in NaOH, then reprecipitated with 6M HCl, and dried.
Wood - cellulose extraction The cleaned ground wood is heated in 0.5M HCl, filtered, and rinsed with dH2O to neutral. It is then heated in 0.1M NaOH, filtered, rinsed with dH2O and repeated. After the second NaOH step the residue is heated in a 4% solution of H2O2 at pH 11. This step is repeated until the residue is bleached, then it is filtered, rinsed with dH2O. Finally, it is heated again in 0.5M HCl, filtered, rinsed in dH2O and dried.
Textiles

Textiles submitted to the laboratory vary in their degrees of preservation. Old wool for example may be quite degraded and readily hydrolyse in alkaline solution. The cellulose of cotton will usually sustain a more vigorous treatment and large samples of hemp or other cellulosic fibres may even be subject to cellulose extraction method described above. In general, the purpose of pretreatment of textiles is to remove dirt, grease, fats and oils, and dyes or coloring agents, if soluble.

Depending upon the condition of the fibers, textiles go through the following sequence: after physical cleaning and shredding into small fibers, the sample is put into an extraction thimble and subject to washing in a series of organic solvents -- hexane, isopropanol and acetone -- in a Soxhlet extractor. Any weight loss or color change is noted. The residue is then treated with a series of acid/alkali/acid washes with 0.1M HCl and 0.1M NaOH, rinsed thoroughly with dH2O, and dried.

Bone

After examination and physical cleaning, ground bone powder is demineralized with 0.5M HCl until carbonates are dissolved. The rough collagen is rinsed to neutral and converted to gelatin in hot 0.01M HCl in an inert atmosphere. The gelatin is then filtered twice, and freeze dried. Sub-samples of the gelatin may be taken at this time for stable carbon and nitrogen analysis and amino acid analysis, in addition to the AMS measurement.

Shell and other carbonates:

After physical examination and cleaning of the surface (and evaluating the need for X-ray diffraction), the surface of shells may be etched with a 0.1M solution of HCl, rinsed with dH2O and dried. CO2 is produced from the carbonate through the addition of 85% o-phosphoric acid in a closed, evacuated glass vessel.

Groundwater:

CO2 from Dissolved Inorganic Carbon (DIC) in water samples is produced by the addition of 85% o-phosphoric acid in a closed, evacuated glass vessel. Depending upon the nature of the research, we also have the capability of measuring DOC and POC. Please contact the laboratory for instructions on collecting and storing water samples intended for 14C analysis.