A laboratory result describes a quantity in a particular sample. The number and unit are only part of that description. For solid materials, a small phrase such as “as received” or “dry weight” can change what the reported concentration means.
Understanding this reporting basis helps readers avoid treating two differently expressed results as contradictory. The first question is simple: what material does the denominator represent?
Start with the reporting basis
An as-received result expresses the measured constituent relative to the sample mass with its moisture included. A dry-weight result expresses it relative to dry matter, with a moisture correction where the method calls for one. Both can appear in milligrams per kilogram, written mg/kg. The unit alone therefore does not identify the basis.
EPA Method 8000D, section 11.10.5, describes these two reporting approaches and notes that the final units remain the same after moisture correction. Look for the basis beside the result, in a column heading, or in the report notes rather than trying to infer it from the size of the number.
A fictional example of two denominators
Imagine an entirely fictional material containing 8 milligrams of an unnamed constituent in 1 kilogram of material as received. For this simplified arithmetic example, that kilogram consists of 0.8 kilograms of dry matter and 0.2 kilograms of moisture.
- Relative to the whole as-received kilogram, the concentration is 8 mg/kg.
- Relative to 0.8 kilograms of dry matter, the same 8 milligrams corresponds to 10 mg/kg dry weight.
The example holds the constituent amount fixed and changes only the denominator. It assumes that the stated moisture fraction is applicable and that the constituent amount is unchanged. These are fictional bookkeeping values, not instructions for correcting a real report. A laboratory must establish the applicable measurements, method and calculation for actual samples.
A reporting label does not describe every preparation step
“Dry weight” should not be read as proof that the entire submitted sample, or every analytical portion, was physically dried before analysis. A reporting basis describes how a result is expressed; the method describes how the sample was handled and measured.
The UC Davis Analytical Laboratory’s sample-report examples make that distinction visible. Its plant-report options include as-received results without moisture correction, dry-matter reporting using an average based on a subset of samples, and reporting with an individual dry-matter determination for each sample. A reader therefore needs to know not only that a correction exists, but which measurements support it.
This fits the broader questions discussed in laboratory methods, sample matrices and comparing results.
Read what the moisture method measures
A moisture-related value also needs a method description. The loss-on-drying test published by Japan’s National Institute of Health Sciences defines the measurement as water and volatile matter lost under the conditions specified for the material. This illustrates why the method matters when interpreting a moisture-related result.
Consequently, a mass loss should not automatically be interpreted as water alone. Nor should differently defined moisture measurements be assumed interchangeable. The useful question for the laboratory is what the reported value represents under its method, and whether that value was used in expressing the analytical result. The method name and report notes provide context that an isolated percentage cannot. This is a reason to preserve the terminology used by the laboratory rather than replacing every moisture-related label with a single, apparently simpler word.
The intended purpose determines the appropriate basis
There is no universal rule that the larger dry-weight number is the more useful result. EPA Method 8000D treats the choice as project-specific and dependent on intended use.
An agricultural illustration comes from the North Carolina Department of Agriculture’s waste-analysis methods. It describes as-received analysis for nitrogen and carbon in manure where ammonia loss is a concern, alongside dry-basis analysis for certain other materials. This shows why preparation and reporting choices depend on the material and the measurement being sought.
Purpose also matters when results become model inputs. A May 2024 EPA memorandum about DRAS 4 specifies dry-weight inputs for the particular solid-waste risk-assessment model it addresses. That instruction follows the model’s assumptions. These US environmental and agricultural examples illustrate laboratory interpretation; they do not establish requirements for other jurisdictions or sample categories.
Questions to resolve before comparing reports
Before drawing a conclusion from two values, check:
- Do they describe the same constituent and sample matrix?
- Are their units and reporting bases stated?
- Is any moisture correction sample-specific or based on another stated approach?
- Are the analytical and moisture methods identified?
- Does the comparison match the intended purpose of the results?
If the basis is missing, ask the issuing laboratory to clarify it. Keep that clarification with the original report. Reporting basis is also separate from whether a sample represents a larger batch, as explained in batch sampling and measurement uncertainty. A clear denominator is an essential starting point, not a complete assessment of comparability.