Key takeaway
The analyte name and mg/L unit do not identify the sample fraction. Filtration and preparation determine what the reported result represents.
Read the fraction before combining the results
An export has two columns called copper_mg_L from the same site. One comes from a filtered bottle, the other from an unfiltered preparation. Renaming both copper and averaging them would erase a distinction that may be central to the receiving question. Start with sample fraction, preparation and method, then ask whether the observations answer the same question.
EPA Methods 200.7 and 200.8 define dissolved analyte using passage through a 0.45 micrometre membrane before acidification. Their total-recoverable definitions use specified unfiltered/direct-analysis or digestion routes. The labels are method-defined observations; total recoverable should not be casually expanded to mean an exhaustive measurement of every possible chemical form.
A paired result is not automatically a component balance
This hypothetical collection produces two separately identified bottles during one stipulated well-mixed event. D1 is documented as filtered through 0.45 micrometre media before acid preservation, with its subsequent dissolved-analysis method retained. T1 is unfiltered and follows its recorded total-recoverable preparation. The invented copper results are 0.020 mg/L and 0.050 mg/L respectively.
The arithmetic difference is 0.030 mg/L. That alone does not validate a new particulate-copper result. The technical reviewer still needs to establish pairing, recovery, uncertainty, preparation comparability and what such a derived quantity would mean. The completed table therefore preserves both issued observations and holds the proposed component label.
A third fictional bottle A1 was acidified before its filtration history was recorded. Its number remains in the issued report, but the archive cannot claim that it met the cited dissolved-fraction definition from the retained evidence. Recover the actual field and laboratory records rather than repairing the label by assumption.
| Bottle | Retained fictional preparation | Copper result | Receiving decision |
|---|---|---|---|
| D1 | 0.45 µm filtration before acidification; dissolved method linked | 0.020 mg/L | Retain declared fraction and method |
| T1 | Unfiltered; total-recoverable preparation linked | 0.050 mg/L | Retain as separate result population |
| Derived T1−D1 | Arithmetic only; comparability review absent | 0.030 mg/L | Hold particulate-result claim |
| A1 | Order of acidification/filtration unresolved | Issued value retained separately | Hold dissolved-method conformity claim |
Preserve preparation as a linked history
Record the collection identity, parent bottle or aliquot, filter material/pore-size entry, filtration time/location, preservation sequence, preparation method/version and issued report. The exact required fields depend on the applicable project and analytical method. A generic filtered flag cannot reconstruct their order.
Method 200.8’s summary distinguishes preparation of filtered dissolved aliquots from specified total-recoverable processing. That is why the archive should retain the laboratory method identifier instead of guessing from an instrument name. ICP-MS identifies an analytical technique; it does not by itself establish which sample fraction reached the instrument.
Keep detection limits, qualifiers and any later corrected report attached to the appropriate result. Those are separate evidence layers from fraction identity. A complete custody chain cannot prove that an undocumented filter step occurred, and a correct fraction label cannot resolve a missing contamination-control review.
Choose an analysis the evidence can support
If the task is retrieval of historical analytical reports, both D1 and T1 may be relevant as differently labelled observations. If the task is a time trend of dissolved concentration, a reviewer needs a consistent fraction/method series or an explicit plan for handling changes. Do not remove the unfiltered history simply because it complicates the graph.
For D1 and T1, the completed review accepts a description of two issued method-specific results and the stated numerical difference. It does not certify chemical speciation, a regulatory exceedance or a mass-loading history. Sampling location and collection representativeness also remain separate questions.
Describe the evidence without sending the samples
Inventory can list the fraction dictionary, preparation records and coverage gaps. Due diligence can ask whether a receiving task requires dissolved observations, total-recoverable observations or both. An initial approved description can communicate those distinctions without disclosing client sites or report contents.
The internal laboratory owner remains responsible for resolving method questions. Source access and a scientific interpretation are separate from permission for a named recipient, a real sample or a later license. A technical method label creates none of those permissions.