Mining laboratories operate in one of the most demanding analytical environments. Ore digests, heap leach solutions, process waters, and tailings extracts rarely resemble clean, low-matrix calibration solutions. Instead, they often contain high acid strength and elevated total dissolved solids (TDS) — a combination that can quietly undermine data quality in both ICP-OES and ICP-MS workflows.
Signal suppression, enhancement, drift, and poor reproducibility are frequently blamed on the instrument. In reality, the root cause is often matrix mismatch. A well-designed certified reference material (CRM) strategy — particularly custom CRMs — can significantly reduce bias and restore confidence in high-TDS mining analyses.
What High TDS Actually Does to ICP Data
High TDS changes how your sample behaves before it even reaches the plasma.
When dissolved solids and acid concentrations are elevated, they can alter:
- Nebuliser efficiency and aerosol formation
- Transport efficiency into the plasma
- Plasma robustness and ionisation conditions
- Space-charge effects in ICP-MS
- Background emission and oxide formation
In ICP-OES, high TDS can destabilise emission signals and complicate background correction. The result may appear as inconsistent recoveries or drifting calibration curves.
In ICP-MS, the effects can be more severe. Ionisation suppression, increased oxide ratios, and long-term signal drift are common, particularly during extended mining sample runs where matrix load accumulates on cones and interface components.
These issues show up in reports as “bad data”: failed QC checks, inconsistent duplicates, or unexplained bias between batches.
Why Dilution Alone Isn’t the Solution
Dilution is a common strategy to reduce matrix load. However, mining labs often operate near reporting limits for trace elements such as arsenic, cadmium, or rare earth elements. Excessive dilution can compromise detection capability.
Even after dilution, the chemistry of the sample solution still matters. If calibration standards are prepared in simple nitric acid while samples contain residual digestion acids, dissolved salts, and complex matrices, the instrument is effectively comparing different chemical environments.
That’s where matrix matching becomes critical.
Matrix Matching 101: Match What the Instrument Sees
Matrix matching is not about perfectly replicating the original ore digest. It is about matching the final diluted solution that enters the instrument.
The most important question is:
What does the sample look like after dilution?
If mining samples are routinely diluted 100× or 200×, the calibration curve should reflect those final concentrations and chemical conditions.
A common mistake is building calibration curves based on instrument capability rather than actual sample chemistry. When standards and samples differ significantly in acid concentration or dissolved solids, bias becomes almost inevitable.
For many ICP-OES, ICP-MS, and AA methods, a blank plus three to four calibration points is sufficient to assess linearity and detect matrix-driven distortion. If the curve behaves differently with matrix-matched standards, you’ve identified the problem.
Stock CRMs vs Custom CRMs in High-TDS Mining Labs
Choosing between stock and custom CRMs is not an either-or decision. Each has a role.
Stock Single-Element CRMs: Flexibility and Troubleshooting Power
Stock single-element certified reference materials are highly versatile. They are ideal when:
- The analyte list changes frequently
- You require custom calibration ranges (major and trace elements)
- You want to avoid compatibility issues in large multi-element mixes
Best practice in high-precision mining labs is to use intermediate dilutions. Instead of preparing low ppb working standards directly from 1,000 mg/L stock solutions, create intermediate standards (for example, 10 ppm or 1 ppm). This reduces pipetting error and improves reproducibility across analysts.
Stock CRMs are excellent for method development and troubleshooting matrix effects.
Stock Multi-Element CRMs: Efficiency and Throughput
Multi-element CRMs simplify preparation in high-throughput environments. They perform best when:
- The analyte list remains consistent
- Methods are well established
- The matrix is compatible with your workflow
Grouped multi-element standards reduce preparation time, minimise handling steps, and improve inter-analyst consistency — a significant advantage in busy mining labs processing hundreds of samples per day.
However, in aggressive high-TDS environments, even well-formulated stock CRMs may not fully address matrix bias.
Where Custom CRMs Make the Biggest Difference
Custom certified reference materials allow laboratories to “lock in” acids, dissolved solids, and analyte groupings under controlled, documented conditions.
In mining applications, this often means:
- Matching the final acid concentration after digestion and dilution
- Including representative dissolved salts
- Grouping elements to minimise interferences
- Stabilising problematic analytes
Custom CRMs provide consistency batch after batch. Because they are prepared and documented under accredited conditions, they also strengthen audit defensibility and ISO/IEC 17025 compliance.
Instead of manually recreating complex matrices each time standards are prepared, a custom CRM ensures repeatability and reduces preparation risk.
Reducing High-TDS Bias: A Strategy, Not a Setting
When mining laboratories experience unexplained bias or drift, the fix is rarely a single instrument adjustment. It is usually a systematic strategy:
- Evaluate the final diluted sample chemistry
- Align the acid strength between samples and standards
- Use intermediate dilutions to reduce preparation error
- Implement matrix-matched calibration
- Incorporate custom CRMs for repeatability
High TDS matrices can make good instruments look inconsistent. The goal is not perfection — it is minimising bias to a defensible, repeatable level.
When standards and samples reflect what the instrument actually sees, QC recoveries stabilise, drift decreases, and long runs become more predictable.
Building a Defensible CRM Strategy with GBJ
At Graham B Jackson (GBJ), we support mining laboratories with both stock and custom certified reference material solutions tailored to ICP-OES and ICP-MS workflows.
Our approach focuses on:
- Understanding your final diluted matrix
- Advising on appropriate acid and dissolved solid matching
- Designing grouped standards for stability and interference control
- Supporting audit-ready documentation
A well-designed CRM strategy improves not only analytical performance but also operational efficiency by reducing rework, repeat analysis, and troubleshooting downtime.
FAQs
What is matrix matching in ICP-OES and ICP-MS?
Matrix matching involves designing calibration standards so their chemistry matches the final diluted sample solution measured by the instrument, reducing matrix-driven bias.
Why do high TDS matrices cause signal suppression and drift?
High dissolved solids and acid concentrations affect aerosol formation, transport efficiency, plasma behaviour, and ionisation processes, leading to suppression, enhancement, and long-term instability.
When should a mining lab choose custom CRMs over stock CRMs?
When matrices are aggressive or variable, reporting limits are tight, audit defensibility is critical, or preparation variability is affecting consistency.
How do intermediate dilutions reduce preparation error?
They reduce the magnitude of dilution steps from concentrated stock solutions, lowering pipetting error and improving reproducibility between analysts.
How many calibration points are recommended?
A blank plus three to four standards is a strong starting point for many ICP-OES and ICP-MS methods, adjusted as needed for linearity and reporting requirements.
Can a custom CRM be used as a base for spiking single-element standards?
Yes. A custom CRM can serve as the matrix foundation, with single-element CRMs spiked into the same matrix to build flexible, matrix-matched calibration curves.
For Further Enquiry Contact: sales@gbjpl.com.au