# Sampling and measurements

The following sampling and measurement requirements only apply to projects measuring [carbon storage](https://docs.rainbowstandard.io/~/changes/171/methodologies/ghg-quantification#project-CO<sub>2</sub>-stored) using **solid-sample CO**<sub>**2**</sub>**&#x20;storage measurement methods.** Projects using gaseous inflow-outflow measurements are exempt from these sampling protocols, as their measurement approach directly tracks CO<sub>2</sub> flux in real time.

## Solid sample measurements

All solid sample measurements shall be done by an **external laboratory**, accredited and compliant with ISO/IEC 17025, or accredited by national accreditation bodies that certify compliance with ISO 17025 or equivalent, such as COFRAC (France), UKAS (UK) or ANAB (USA).

Project Developers shall use one of the following high-accuracy measurements:&#x20;

* **Thermogravimetric analysis (TGA)**, e.g. ISO 11358-1:2014, ISO 19579:2006 and ISO 21687:2007
* **Dry combustion** (Total Carbon Analysis, or TCA) methods, following e.g. ISO 10694:1995, ISO 13878:1998, and ISO 15178:2000

These measurements shall be done in 3 or more replicates.

For dry combustion measurements, Project Developers must determine fraction of total measured carbon that is attributable to stable mineral carbonates (e.g., CaCO<sub>3</sub> or MgCO<sub>3</sub>). This may be determined once, upfront, for the project's alkaline feedstock, and applied throughout the crediting period until a different feedstock is used. This ensures that only creditable carbon increases resulting from the project activity are counted, excluding carbon stored in less stable or non-mineral phases. Accepted methods for confirming carbonate speciation include, but are not limited to, X-ray diffraction (XRD) with Rietveld refinement and infrared spectroscopy (FTIR).

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**Heterogeneous materials and sampling error**

Rainbow recognizes that certain feedstocks (particularly those with highly heterogeneous composition or particle size) pose challenges for obtaining representative samples when using TGA or dry combustion. While alternative methods like acid digestion with CO₂ quantification or furnace-based mass loss may help reduce sampling error through bulk measurements, they currently introduce unacceptably high analytical uncertainty and are therefore not permitted under this methodology.

However, Rainbow remains open to the adoption of such methods, if supported by credible evidence or research demonstrating that they yield results consistent with high-accuracy reference methods (e.g. TGA, dry combustion). Upon review and approval of such evidence, these methods may be accepted with appropriate safeguards and limitations in place to ensure integrity and comparability of results.
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## Centralized vs distributed sites

Mineralization projects may be highly distributed, deploying the same mineralization technology and process to many different sites. For example, mineralization may occur directly in concrete mixing trucks, with small amounts of carbon storage taking place at each carbonation site. Because of this, we distinguish between two types of projects: centralized vs distributed, with different sampling requirements.

A distributed project is defined as one where

* Each mineralization site is expected to issue fewer than 100 RRCs annually, based on carbon storage via mineralization (i.e. excluding reduced cement avoidance), and
* The following characteristics are consistent across sites in a group:&#x20;
  * Mineralization reaction type, the metal(s) contributing to mineral formation, and expected carbonate compound
  * Expected mineralization timeline post-reactor, based on the kinetics of the specific technology (e.g. full mineralization occurs within 24 hours, 1 week, etc.)
  * Reactor operating temperature and pressure (within a target range of ±10%)

Distributed projects can use empirical data from a representative subset of sites to prove that **mineralization rates are sufficiently similar across grouped sites** (RSD[^1]<10%) when alkaline feedstock is the same.

&#x20;Once this is demonstrated, the project can shift to reduced sampling, where **only one site continues to take samples and perform laboratory measurements**. Those results may then be applied to other sites in the same group, under the conditions agreed upon by the Project Developer, the Rainbow Certification team and the VVB. The lowest-measured mineralization rate shall be used to issue credits.

Grouped sites must use the same alkaline feedstock. The criteria for determining whether feedstocks are considered the same, or should be treated as distinct, are outlined in [Mineralization batch](https://docs.rainbowstandard.io/~/changes/171/methodologies/eligible-technologies#carbonation-batch) section.

This approach can be planned from the start and included in the project design, or implemented partway through the crediting period. In the latter case, the supporting data and updated Monitoring Plan must be reviewed and approved by the VVB.

Grouped sites that are not taking samples and measurements shall **continue to provide all other data required for GHG quantification**, such as alkaline feedstock type and amount, onsite energy use, and transport distances. These sites are only exempt from taking samples and laboratory measurements of carbonated materials.

To maintain data quality, Project Developers shall **perform random cross checks annually**, taking samples from non-measuring grouped sites to confirm that their mineralization rates remain within the expected range. The number of sites and samples depends on the project size, and shall be decided by the Rainbow Certification team and the Project Developer on a case by case basis.

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At present, this approach is applied separately for each project. The Rainbow team is actively interested in developing a generalized methodology, with clearly defined conditions for when mineralization rates from one site can be applied to other sites. However, until compelling research is available to support such a generalizable framework, each project will continue to be required to provide its own proof.
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## Measurement frequency and representativeness

Project Developers must perform carbon storage measurements, in replicates of 3 or 6, on a  representative sample, at the frequency outlined below. The approach to obtain a representative sample and measurement is:&#x20;

<table data-card-size="large" data-view="cards"><thead><tr><th></th></tr></thead><tbody><tr><td><p>At the <strong>start of verification</strong>, for the first <a href="../eligible-technologies#carbonation-batch">mineralizationbatch</a>, measurements shall be taken at high frequency until the project demonstrates stable operating conditions and establishes operational stability and consistent results.</p><ul><li>It is recommended to conduct 10 separate measurements, each on a composite sample from 3 sub-samples each, in 3 or 6 replicates depending on the <a href="#solid-sample-measurements">measurement type</a> used, at the beginning of the crediting period. The variability of these measurements should not exceed an <a data-footnote-ref href="#user-content-fn-1">RSD</a> of 10%. </li><li>The <strong>lowest value</strong> of recent stable measurements shall be used for initial credit issuance once measurement consistency is established. This shall be updated regularly for ongoing verification.</li><li>This frequency may be adjusted based on the project’s technological maturity, existing data, and expected variability. For example, this step may be skipped if the project can prove stable operating conditions were established before the start of the crediting period.</li></ul></td></tr><tr><td><p>For <strong>ongoing monitoring and verification</strong>, measurements shall be repeated:</p><ul><li>For each <a href="../eligible-technologies#carbonation-batch">mineralization batch</a> (with batch validity limited to 1 year), or</li><li>At least once per quarter, or</li><li>Every 500 tonnes CO<sub>2</sub> removed, <strong>whichever comes first</strong>.</li></ul><p>Each ongoing verification measurement shall:</p><ul><li>Be based on a <a data-footnote-ref href="#user-content-fn-2">representative sample</a>. Best practice is to create this sample as a homogenized composite of at least 3 sub-samples taken from different points in the batch.</li></ul><ul><li>Be analyzed in replicates of 3</li><li>Achieve an <a data-footnote-ref href="#user-content-fn-1">RSD</a> ≤ 10% between the replicate measurements; otherwise, measurements must be repeated.</li><li>The <strong>lowest of the replicate values</strong> shall be retained and used for crediting.</li></ul></td></tr></tbody></table>

Samples shall be taken in a way that minimize bias and are representative of the entire mineralization batch. This includes but is not limited to taking samples at appropriate times and locations from the reactor/pile. Project Developers shall justify their approach for taking representative samples in the ex-ante sampling plan and ex-post sampling records. &#x20;

Taking representative samples of heterogeneous materials like concrete is challenging due to high baseline variability in carbonate and carbon content: factors not affected by the project activity or controlled by the Project Developer. To address this, it is recommended to separate the reactive fraction (e.g. cement paste) from aggregates before sampling. Otherwise:

* Natural variability in aggregate mineralization may obscure the smaller, project-induced changes in mineralization in the cement fraction, and
* The overall carbon content difference between project and baseline samples may not be statistically significant, even if a measurable effect exists within the cement fraction alone.

## Ex-ante sampling plans

Project Developers shall submit a **Sampling Plan**, upon validation, that details:

* procedure to ensure representative sampling, including:
  * tools or equipment for taking samples,&#x20;
  * homogenization techniques,&#x20;
  * sample storage conditions
  * the approach for random timing and location (i.e. within the pile, or the production line) of sub-samples,
* the proposed frequency and number of measurements performed in the first reporting period, to establish a robust dataset (mean and distribution),
* approach to ensure the control/uncarbonated samples are representative of project samples, and exposed to the same conditions,
* the post-mineralization timing of sample collection (e.g. 24 hours, 1 week), justified in terms of the mineralization kinetics of the specific technology.

## Ex-post sampling records

For **each Reporting Period**, Project Developers shall submit a **Sampling Record** documenting all sampling activities. This record must include the following information, for both project and control materials:

* Date and time of carbonated material production
* Date and time of sampling
* Name or ID of the person(s) performing sampling
* Amount of material in sub-sample and composite sample
* Processing or preparation steps before analysis
* Description of representative sampling method
* Sample ID for traceability
* Notes on anomalies, if any

[^1]: relative standard deviation

[^2]: A representative sample is a portion of carbonated material that accurately reflects the carbonation batch and production output during the relevant time period, capturing any temporal or random variation.
