A metal labeled 99.99% pure provides useful information, but it does not answer every question about a research material. A buyer may also need to know which impurities are counted, whether a particular contaminant has its own limit, and what was measured on the supplied lot.
The most useful specification connects three things: the stated purity grade, the analytical basis behind it, and the impurity controls required by the application. Understanding those distinctions helps researchers choose between grades without automatically paying for the highest number of nines.
What do 3N, 4N, and 5N mean?
N-grade notation abbreviates the number of nines in a purity percentage. The following arithmetic applies when the percentage and impurity complement use the same declared mass-based basis.
| Grade notation | Purity percentage | Arithmetic impurity complement |
|---|---|---|
| 3N | 99.9% | 0.1% = 1,000 ppm |
| 4N | 99.99% | 0.01% = 100 ppm |
| 5N | 99.999% | 0.001% = 10 ppm |
| 6N | 99.9999% | 0.0001% = 1 ppm |
These numbers are percentage conversions, not laboratory results. When the purity is a minimum specification, the complement is the corresponding ceiling for impurities included in that specification. It is not proof that all possible contaminants were measured or included.
For a mass-based result, 1 ppm corresponds to 1 mg/kg. Do not mix this interpretation with atomic percentages, solution concentrations, or a different analytical basis without an appropriate conversion.
Why “metals basis” matters
A metals-basis purity statement concerns the defined metallic impurity basis; it should not be silently interpreted as a complete all-elements assay. As a real commercial example, Thermo Fisher lists cobalt pieces at 99.9+% metals basis with a separate maximum for total metal impurities. [1]
The practical question is which elements the specification includes. If oxygen, nitrogen, hydrogen, carbon, or sulfur is important to the application, identify its requirement explicitly and establish whether it is covered by the available analysis.
This does not make a metals-basis grade inadequate. It means that comparisons must use the same definition. A clearly specified 4N grade with the required individual impurity controls can be more useful to a particular project than a higher headline grade whose analytical scope does not answer the relevant question.
Overall purity does not describe the impurity distribution
Consider two entirely hypothetical 4N materials, each evaluated on the same basis. Material A contains 2 ppm of an impurity of interest; Material B contains 20 ppm. If their other counted impurities keep both totals within the grade limit, both can meet the same overall grade while differing on that individual element.
If an experiment requires that impurity to be below 5 ppm, the total purity alone does not establish acceptance. This is an arithmetic purchasing example—not a Basis analysis, an actual product comparison, or a claim about a particular element's effect.
Translate application needs into named impurity limits where justified. Do not assume that every contaminant has the same significance simply because its concentration uses the same unit.
Manufacturing: ask what material the analysis represents
Material can pass through several stages between feedstock selection and shipment: melting or consolidation, forming, machining, finishing, and packaging. A useful documentation request identifies which stage the sample represents rather than assuming that a raw-material report automatically describes every later operation.
For example, ask whether the reported sample represents the melt, a production intermediate, or the finished supplied form. When surface condition is critical, discuss how that requirement is checked separately from bulk composition.
This is not a reason to reject established manufacturing routes. It is a way to connect the chemistry requirement to the actual order. Request traceability appropriate to the application instead of treating a process name as a substitute for an analytical result.
Which analytical methods answer which questions?
No method name on its own establishes that every required element was quantified at a suitable level. Start with the analytes, matrix, and acceptance limits.
| Method | Role in a material evaluation | Question for the report |
|---|---|---|
| GDMS | Broad trace-element analysis of suitable solid materials | Which elements were reported, with what calibration and reporting limits? |
| ICP-MS | Sensitive elemental analysis, commonly after preparation into solution | Was the material dissolved appropriately, and are preparation blanks and interferences addressed? |
| Instrumental gas analysis | Suitable determination of gas-forming elements using methods such as fusion, extraction, or combustion | Which element-specific method and calibration were used? |
EAG describes GDMS as a solid-material trace-analysis technique with broad elemental coverage; its stated coverage excludes hydrogen. Do not incorrectly describe GDMS as universally incapable of measuring oxygen, nitrogen, or carbon. [2]
EAG's ICP-MS documentation emphasizes elemental analysis and appropriate sample preparation. A solution-based measurement also requires attention to what was dissolved and how the result was expressed for the original material. [3]
For instrumental gas analysis, EAG distinguishes approaches including inert-gas fusion or extraction for appropriate elements and combustion-based carbon/sulfur analysis. The method must match the element and sample; these are not interchangeable labels for one universal assay. [4]
Read the report symbols, not just the numbers
A value shown as <1 ppm is not a measured zero. Establish whether 1 ppm is a detection limit, quantification limit, or reporting threshold. Likewise, ND needs a definition, and not tested is not a pass result.
Consider this deliberately illustrative table:
| Hypothetical report entry | What it tells the buyer | What it does not establish |
|---|---|---|
| Impurity X: 0.4 ppm | A numerical result was reported | Acceptance without the method, uncertainty, and agreed limit |
| Impurity Y: <1 ppm | The result is below a stated threshold | That it satisfies a tighter 0.2 ppm requirement |
| Oxygen: not tested | This report provides no oxygen result | Zero oxygen or compliance with an oxygen limit |
These entries are educational examples, not a certificate or measured Basis data. The second row illustrates a simple logical limitation: a bound of less than 1 does not establish a bound of less than 0.2.
Purity by difference needs a defined calculation
Some purity statements are calculated by subtracting the sum of counted impurities from 100%. The important question is what enters that sum, including how non-detects and reporting thresholds are handled.
A calculation based on a defined analyte panel should not be described as a direct measurement of every atom in the material. Rounding also matters when comparing a displayed grade with reported trace values. Ask for the applicable calculation basis rather than reverse-engineering a purity guarantee from a short, incomplete table.
Do not confuse purity with alloy composition or phase purity
The intended constituents of an alloy are not automatically contaminants. A 316L composition request, for example, is not answered by asking for “99.99% iron.” Similarly, the oxygen intentionally present in an oxide is not the same thing as unwanted oxygen in a metallic-material specification.
Chemical purity also does not define phase constitution, porosity, grain structure, or surface roughness. Those are separate requirements. For sputtering targets, connect the chemistry request with the structural considerations discussed in How to Choose a Sputtering Target.
What to request before accepting an order
A useful request identifies the material, purity basis, any critical individual impurity limits, required analytical methods where justified, and the documentation needed for the supplied lot. Ask whether the document reports measured results, specification limits, or typical values; they serve different purposes.
A Certificate of Analysis and an SDS are also not interchangeable. One addresses the stated analytical information; the other addresses material safety information. Request the appropriate documents for the actual product and configuration.
Frequently asked questions
Is 5N always better than 4N?
Not for every application. Compare the analytical basis and individual requirements, then determine whether the higher grade provides a relevant benefit.
Does <1 ppm mean the impurity is absent?
No. Read it as a threshold statement and check how the reporting laboratory defines that threshold.
Does every product need an extensive trace-element survey?
No. Define the evidence needed by the experiment or purchase specification. A broad survey and a targeted analysis answer different questions.
Choose a grade that answers the application
The goal is not the most impressive purity label; it is a material specification that addresses the experiment. Combine the grade with the right impurity limits and documentation, and keep bulk chemistry separate from geometry, surface condition, and microstructure.
Explore High-Purity Metal Forms, evaporation materials, or the Copper Sputtering Target. To discuss a material-specific grade or documentation requirement, request a quote.
Final specifications and agreed documentation are confirmed during quotation.
Related guides
- Evaporation Materials: How to Choose Pellets, Granules, Pieces, and Wire
- High-Purity Metal Foil and Wire: How Rolling, Drawing, and Annealing Affect Performance