6.2.2.3 Sample conditioning

Sample conditioning prepares the sample for subsequent processing, e.g. sieving, grinding, and subsampling.

Sample conditioning is usually needed before additional mixing or particle size reduction techniques are used because most require a flowable sample. Processing the sample as received is generally the best way to retain the widest range of analytes. There are two primary exceptions. VOCs should be handled as described in Section 6.2.1 since analyte loss even in a bulk, unpreserved sample container is almost certain. Analytes with very high boiling points but biologically degradable may remain stable when the sample is air-dried; however, some of the procedures discussed below should be avoided if any analyte of interest is sufficiently volatile and/or biodegradable to introduce possible biases impacting attainment of project DQOs. A few soil samples are dry enough as received to be handled with the mixing, particle size reduction, and subsampling techniques described below. However, most soil samples require moisture modification before further possessing. Drying the sample to reduce moisture content is the most common approach, but increasing water content can be beneficial in a few selected instances as discussed below. If the moisture content of the original field sample is needed, then use the 2-D Japanese slabcake approach described in Section 6.2.2.7 on the sample prior to any moisture modification.

Air-drying is appropriate when the analytes are chemically stable when exposed to air and have sufficiently high boiling points such that they are unlikely to volatilize during extended air exposure at the selected drying temperature. Drying at ambient temperature (15–25°C) is most common. This process may take up to several days, thus impacting turnaround time. Elevated temperature drying (25–105°C) accelerates the drying process but also requires greater analyte stability. The binding (distribution coefficient, soil organic carbon-water partitioning coefficient) between the contaminant and the soil particle should also be considered. Air-drying can be acceptable for strongly absorbed, low-boiling-point contaminants. Table 6-1 lists several example explosives and SVOCs, their boiling points, and estimated loss potential during the air-drying step when these contaminants are weakly sorbed to the soil matrix (Bruce 2003). Air-drying produces crushable soil particles, however, it risks loss of low-boiling-point target analytes.Air-drying produces crushable soil particles, however, risks loss of low-boiling-point target analytes. Table 6-1 is not all-inclusive and is intended only as an example for evaluating contaminants and the possible effects of air-drying. Physical property data for additional contaminants is available in Technical Guidance Manual Notes: Decision Unit and Multi-Increment* Sample Investigations (HDOH 2011), Tables 2a and 2b. Applying air-drying to contaminants with moderate and large loss risks should be avoided unless there is sufficient site knowledge or experimental data to demonstrate the loss risk is acceptable.



Table 6-1. Potential for loss during the air-drying step

Contaminant

Vapor pressure
(mm Hg)

Boiling point
(°C)

Loss potential

Acenaphthene

2.15E-03

279

Moderate

Acenaphthylene

6.68E-03

280

Moderate

2-Amino-4,6-dinitrotoluene

3.33E-06

352

Small

4-Amino-2,6-dinitrotoluene

3.65E-06

352

Small

bis(2-Chloroethoxy)methane

1.32E-01

218

Small

bis(2-Chloroethyl)ether

1.55E+00

179

Moderate

bis(2-Chloro-1-methylethyl)ether

5.60E-01

187

Moderate

4-Chloro-3-methylphenol

5.00E-02

235

Moderate

2-Chloronaphthalene

1.22E-02

256

Moderate

2-Chlorophenol

2.53E+00

175

Moderate

Dibenzofuran

2.48E-03

287

Small

1,2-Dichlorobenzene

1.47E+00

180

Large

1,3-Dichlorobenzene

2.15E+00

173

Large

1,4-Dichlorobenzene

1.74E+00

174

Large

2,4-Dichlorophenol

9.00E-02

210

Small

Dimethylphthalate

3.08E-03

284

Small

1,2-Dinitrobenzene

4.55E-05

318

Small

1,3-Dinitrobenzene

9.00E-04

291

Small

2,4-Dinitrotoluene

1.47E-04

300

Small

2,6-Dinitrotoluene

5.67E-04

300

Small

Hexachlorobutadiene

2.20E-01

215

Large

Hexachloroethane

2.10E-01

154

Large

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)

3.30E-14

436

Small

Isophorone

4.38E-01

215

Large

2-Methylnaphthalene

5.50E-02

241

Moderate

4-Methylphenol

1.10E-01

202

Moderate

Naphthalene

8.50E-02

218

Large

Nitrobenzene

2.45E-01

211

Large

Nitroglycerin

4.00E-04

250

Small

N-Nitrosodimethylamine

2.7E+00

154

Moderate

N-Nitroso-di-n-propylamine

3.89E-01

206

Small

2-Nitrotoluene

1.88E-01

222

Moderate

3-Nitrotoluene

2.05E-01

232

Moderate

4-Nitrotoluene

1.57E-02

238

Moderate

Pentaerythritol tetranitrate (PETN)

5.45E-09

364

Small

Phenol

3.50E-01

182

Small

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)

4.10E-09

353

Small

Methyl-2,4,6-trinitrophenylnitramine (Tetryl)

1.17E-07

432

Moderate

1,2,4-Trichlorobenzene

4.60E-01

214

Large

2,4,6-Trichlorophenol

8.00E-03

246

Small

1,3,5-Trinitrobenzene

6.44E-06

315

Moderate

2,4,6-Trinitrotoluene

8.02E-06

365

Small



After weighing the contaminant loss risks during the air-drying step, it may be necessary on occasion to skip air-drying and proceed with other processing steps on the as-received sample. This is most likely when lower-boiling-point SVOCs or elemental mercury (Hg) are primary contaminants. Wet sticky samples cause mechanical problems, but coarse sieving and 2-D slabcake subsampling are possible though labor-intensive. See Sections 6.2.2.6 and 6.2.2.7 on 2-D Japanese slabcake for further details.

Place the soil sample on a tray made of, or lined with, a material that is compatible with the contaminant of interest and the drying temperature. The selection of the tray and/or liner material should ensure that the analytes (or interferents) of interest are neither lost nor gained from the sample to the tray and/or liner by sorption or reaction. Aluminum trays and liners should be avoided if aluminum is a contaminant of interest or if it may interfere or interact with an analyte of interest (e.g., chromium, elemental mercury). Plastic trays and liners should be avoided if phthalates and plastic components are contaminants. A paper liner should be avoided if organic carbon or organics that may sorb to paper (e.g., petroleum) are contaminants. Spread the sample evenly in the drying tray. If needed, use 2-D slabcake subsampling to collect a subsample for moisture determination of the original sample. Place the sample in a ventilated area such as a hood or oven with sufficient airflow to carry away evaporated moisture. Drying time varies from a few hours to several days depending on moisture content, soil characteristics, airflow and temperature. Intermittent (e.g., daily) turning of the soil may be necessary to facilitate air-drying in an acceptable time frame. The soil should be dry enough to allow the agglomerates to be crushed producing a flowable matrix. Moisture content below 5%–10% is usually acceptable. Wet clay samples should be crushed with a pestle part way through the drying process to avoid formation of large "bricks" that are difficult to handle with subsequent processes. Drying to a constant weight is not necessary; the sample only needs to be only dry enough to facilitate proper mechanical function of subsequent processing equipment. The ventilated air-drying area uses a large amount of laboratory space during the drying step. The use of racks to hold the drying trays can facilitate efficient use of space.

Freeze-drying is useful for analytes that volatilize or degrade under extended air exposure or at elevated temperature. Split the soil sample into multiple freeze-drying containers if necessary. Operate the freeze-drying equipment at reduced temperature and pressure (e.g., –80oC, 0.375 torr) for several hours (ERDC 2000). Additional freeze-drying guidance is available from International Organization for Standardization (ISO) (ISO 2005).

Water addition facilitates subsequent processing but can interfere with recovery of some analytes.

Water addition can also be used to produce a mixable sample with less air exposure than during the air-drying processes described above, thus improving retention of low-boiling-point analytes. The added water can interfere with some subsequent sample preparation techniques for high-boiling-point, nonpolar analytes (e.g., solvent extraction of high-molecular-weight polyaromatic hydrocarbons [PAHs]). Place the soil sample in a heavy-duty mixer (e.g., bread dough mixer) constructed of appropriate sample contact materials. Add sufficient reagent water to produce a thoroughly mixed wet paste. Do not add too much water as that will produce a slurry that separates quickly when the mixing process is stopped.

Sample disaggregation is a gentle grinding technique used on dry, crushable soil. It breaks up the soil agglomerates but does not mill small pebbles and other hard particles into smaller particulates, as the particle size reduction techniques (e.g., milling) listed below. In some risk assessment scenarios, disaggregation is preferable to milling because some metallic COPCs remain “locked” inside the hard particles and are not included in subsequent analyses. Disaggregation can facilitate mixing and subsampling. Take the dry sample and crush it on a sieve with a pestle to promote breakup of the soil agglomerates. A variety of sieve sizes can be used depending on the project DQOs. A #10 sieve (2 mm) is the most common size. Alternatively, the soil can be disaggregated using a bladed “coffee” type grinder or blender. Keep the time as short as possible to minimize wear on the blade, contamination of the sample with the blade materials, and any sample temperature elevation. A mortar and pestle can also be used to gently break up the soil agglomerates though there is a greater risk of causing particle size reduction of the hard particles than with the softer disaggregation techniques such as pestle/ sieve and blender. Disaggregation is generally sufficient when SVOC COPCs are the primary concern and subsample sizes are 10 g or larger. Disaggregation and sieving is also commonly used prior to complete particle size reduction using the milling techniques listed in Section 6.2.2.5.