Percent Solids Basis — Strength and Potency in Liquid and Semi-Solid Systems

This topic is part of the SG Systems Global regulatory & operations glossary.

Updated November 2025 • Potency Basis, LOD Adjustment, Concentration-Adjusted Charge, Dynamic Recipe Scaling • Liquids, Slurries, Assay, GMP

Percent solids basis expresses the composition of a liquid, slurry or semi-solid with respect to its non-volatile solids content rather than its total mass or volume. In practice, % solids describes how much solid material remains after removing water and other volatiles under defined conditions. Potency, activity or assay may then be expressed “on a solids basis”, meaning per unit of that non-volatile residue. In regulated manufacturing, using a solids basis is often the only way to describe the real strength of concentrates, syrups, enzyme solutions, slurries and pastes whose density and water content shift over time.

“% solids tells you how much ‘real stuff’ is in the liquid. Assay on a solids basis tells you how much active is in that ‘real stuff’.”

1) What Does Percent Solids Basis Mean?

Percent solids (% solids) describes the proportion of a sample that is non-volatile material once water and volatiles are removed under defined test conditions. If a syrup weighs 100 g and leaves 60 g of non-volatile residue after drying, it is 60 % solids. In a manufacturing context, % solids is an intermediate descriptor; it does not by itself tell you how much active ingredient is present, only how much total solid matter is present.

When potency or assay is expressed “on a solids basis”, the denominator for the calculation is the non-volatile solid content, not the total mass or volume of the liquid. If those 60 g of solids contain 30 g of active, the assay on solids basis is 50 %: half of the solids are active substance. When the liquid is used in batching, dose calculations may therefore need to account for both % solids and assay on solids basis to determine corrected active content per litre or kilogramme of product charged into the batch.

2) Why Use a Solids Basis Instead of As-Is Potency?

Liquids and semi-solids often change density, viscosity and water content over time or with temperature. If potency is expressed directly on an as-is basis (for example, “10 % w/w active as-is”), then every significant change in water content implies a change in potency – even if the amount of non-volatile solids has not changed. This makes control and comparison across lots difficult.

Expressing assay on a solids basis decouples potency from water variability. The assay describes the composition of the solids, while % solids describes how concentrated those solids are in each unit of liquid. Together, they give a more stable representation of product behaviour. This separation is particularly important in regulated environments, where dose, label claim and process capability must be interpreted over time and across storage conditions, not just at a single sampling point.

3) Relationship to Potency Basis and LOD Adjustment

Percent solids basis is a specific flavour of potency basis. For powders and many solids, potency is expressed on an as-is, dry-basis or anhydrous basis using LOD adjustment to account for moisture. For liquids and slurries, % solids plays the role of “dry matter”, while water and volatiles occupy the complementary fraction.

In practice, the system may treat “solids basis” as a specific potency basis value, alongside as-is and dry. Assay values in LIMS and specifications should clearly state whether they are expressed per unit of liquid (as-is) or per unit of solid residue (solids basis). If both LOD and % solids are used for a given material, test methods must be clear about which parameter is used in potency and batch calculations to avoid double counting or inconsistent assumptions about what is considered “solid”.

4) Basic Formula: From % Solids to Active per kg or Litre

Consider a concentrate with the following properties:

• Percent solids: 60 % (0.60 as a fraction);
• Assay on solids basis: 50 % active (0.50 as a fraction);
• Density: 1.20 kg/L.

From these values, the system can calculate active per kilogramme and per litre of liquid:

• Solids per kg of liquid = 1.00 kg × 0.60 = 0.60 kg solids;
• Active per kg of liquid = 0.60 kg × 0.50 = 0.30 kg active;
• Active per litre = 1.20 kg × 0.30 kg active per kg = 0.36 kg active per L.

These derived values are what matter for concentration-adjusted charges. When a batch requires 5.0 kg of active, the system can set the volume or mass target for the concentrate so that the corrected active content – not just the gross volume – matches the formulation. Changes in % solids or assay will directly change these derived active-per-unit values, and therefore the targets, without rewriting the recipe itself.

5) Connecting % Solids to Batch-Specific Potency

For liquid or semi-solid ingredients, batch-specific potency is often defined in terms of active per unit of volume or mass (for example, “0.36 kg active per L” or “300 g active per kg”). Percent solids basis is a key input to this potency value. A typical data flow might be:

• Lab tests: % solids, assay on solids basis, density.
• System calculates: active per kg and per L using these values.
• Result is stored as batch-specific potency for that lot.

Once that potency is in place, the potency adjustment factor and corrected active content calculations look very similar to dry-powder cases. The difference is that %, solids and density feed into the potency value behind the scenes. Treating % solids as part of potency management, rather than as an isolated QC number, makes the connection between lab data and batch dosing explicit and audit-ready.

6) Percent Solids vs LOD for Semi-Solids and Pastes

For some pastes and gels, either % solids or classical LOD could be used to describe non-volatile content. The choice is often driven by ease of testing, method history and how the material behaves. From a calculation perspective, both provide a fraction representing “non-volatile matter”. The difference is that % solids is usually defined with respect to process-relevant drying conditions, while LOD is often a pharmacopeial test with more rigid conditions.

Whichever metric is chosen, the important part is consistency: the same definition should be used every time % solids basis is mentioned in specifications, test-method validation, formulation and batch calculations. If % solids for one ingredient comes from an oven test and for another from an inline refractometer, the methods must be calibrated so that the resulting values are comparable and suitable for use in potency-related calculations and mass balance.

7) Impact on Concentration-Adjusted Charges and Setpoints

Most obviously, percent solids basis feeds directly into concentration-adjusted charges. When % solids or assay on solids basis changes, the amount of liquid required to deliver a fixed active dose changes too. In a manual environment, this leads to spreadsheet recalculations and handwritten overrides. In a potency-aware MES, the system uses updated % solids to recalculate batch-specific potency and then applies adjustment factors automatically.

This behaviour is especially important when the same formulation must handle different lots of concentrate with slightly different solids levels, or when in-process evaporation or dilution changes % solids during manufacture. Rather than rewriting the recipe, the system adjusts the amount of liquid or paste to keep corrected active content in line with the intended design, using % solids basis as a core parameter in those calculations.

8) Percent Solids in Dynamic Recipe Scaling and Test-Driven Setpoint Adjustment

Percent solids is also a key signal in more advanced control strategies such as test-driven setpoint adjustment and dynamic recipe scaling. In these strategies, in-process tests (solids, titer, pH, viscosity, assay) can trigger controlled changes to batch targets.

Typical patterns include:

• Increasing the amount of diluent if % solids is higher than expected to keep viscosity and processability within limits.
• Reducing the amount of concentrate or paste added if % solids and assay on solids basis are both high, to avoid over-dosing active.
• Adjusting final fill volume based on % solids to maintain consistent active per unit in products like syrups or suspensions.

In all cases, % solids basis is the lens through which the system interprets lab or inline data and decides how to modify setpoints. Having % solids properly defined and integrated into potency calculations makes these dynamic behaviours explainable and auditable rather than opaque “black-box” adjustments.

9) Role in Corrected Active Content, Yield and Mass Balance

For liquids and semi-solids, corrected active content depends heavily on % solids. If % solids drifts but only gross volumes are tracked, yield and mass balance analyses will be misleading. A litre of concentrate added at 60 % solids delivers a different amount of active than a litre at 55 % solids, even if operators see “1.0 L added” in both cases.

By incorporating % solids into potency and active-content calculations, systems can track:

• active-in per batch or per step;
• potency-normalised yield across campaigns; and
• active-equivalent consumption of concentrates or slurries.

This allows mass balance to be performed on the metric that actually matters – active substance – rather than on a proxy like litres or kilogrammes, and it helps distinguish real process losses from simple changes in solids or water content across lots and time.

10) Data Integrity, Analytical Lot Links and Auditability

Because percent solids directly influences dose decisions for liquid and semi-solid ingredients, it must be handled with the same care as assay and LOD. Good practice includes:

• Capturing % solids and assay on solids basis as structured data in LIMS.
• Linking results to each lot via an analytical lot link.
• Defining calculation logic for potency on solids basis within validated systems.
• Storing solids-based potency as part of batch-specific potency master data.
• Recording inputs and results in the electronic batch record with appropriate audit trails.

When regulators ask how concentrates, pastes or syrups are dosed, manufacturers should be able to show how % solids and assay on solids basis were obtained, how they were combined to derive potency, and how that potency fed into setpoints and corrected active content. This closes the loop between lab tests, control strategy and actual batch behaviour, and aligns with ALCOA+ expectations for data integrity.

11) Typical Use Cases Across Industries

Percent solids basis is widely used wherever concentrated liquids or semi-solids play a critical role:

• Pharmaceuticals and biologics: oral solutions, suspensions, syrups, bulk intermediates and some biologic concentrates where solids content affects viscosity, dose and processability.
• Dietary supplements: liquid botanicals, vitamin syrups, emulsions and oil suspensions where label claims are tied to active per dose.
• Food and beverage: enzyme concentrates, flavour and colour concentrates, sweetener syrups and stabiliser slurries controlled by both % solids and activity units.
• Cosmetics and personal care: gels, emulsions and active concentrates where solids and active content drive performance and regulatory limits.
• Chemicals and speciality materials: slurries, dispersions and binders where solids content influences application properties and performance.

In each of these domains, solids basis is the tool that keeps potency concepts meaningful in systems where “kg” or “L” alone do not adequately describe what is being delivered to the process or to the end user.

12) Common Pitfalls and Misuse of % Solids

When % solids is handled informally, a number of recurring issues appear:

• Using % solids only as a viscosity proxy: treating solids purely as a processability metric and ignoring its impact on potency and active content.
• Unclear basis for assay: reporting “10 % w/w active” without specifying whether that is on an as-is or solids basis, leading to confusion in batch calculations.
• Local spreadsheets: performing solids-based potency calculations in local tools that are not validated or integrated with MES and ERP.
• Ignoring density changes: relying on % solids alone without considering density changes that affect active per litre.
• Inconsistent methods: using different solids test methods across suppliers or sites without harmonisation, reducing comparability of potency-related data.

These problems often surface as unexplained assay variability, unstable yield or inconsistent finished-product performance. Treating solids basis as part of the formal potency and batch-calculation framework – not just a lab-side convenience – is usually the fastest way to stabilise behaviour and improve the quality of investigations.

13) Practical Implementation Steps

To implement percent solids basis effectively, organisations typically:

• Identify which liquid, slurry and semi-solid ingredients require solids-based control.
• Define standard methods and acceptance criteria for % solids and assay on solids basis.
• Align specifications and quality agreements with solids-based potency definitions.
• Configure LIMS, ERP and MES to store solids-based potency as part of batch-specific potency.
• Build and validate solids-based calculations for concentration-adjusted charges, corrected active content, potency-normalised yields and active-equivalent consumption.

Once this is in place, percent solids basis becomes a routine part of how potency is handled for liquid and semi-solid ingredients, rather than an afterthought. The result is a more consistent link between laboratory data, batch dosing decisions, process capability and regulatory justification.

FAQ

Q1. Is percent solids the same as dry matter?
Often, but not always. Percent solids usually represents non-volatile matter under specified test conditions, which may be equivalent to “dry matter” for many materials. The key is how the test is defined and validated in the method and specification.

Q2. Do we always need density as well as % solids?
If batches are dosed by volume (L, mL), then density is required to convert solids-based potency into active per litre. If everything is dosed by mass (kg, g), density is less critical, but may still be useful for process and design purposes.

Q3. Can we use % solids without assay on solids basis?
% solids alone is not enough to determine active content; it only describes total solids. To understand potency, you also need assay on solids basis (or an equivalent descriptor of active within that solid fraction).

Q4. How should % solids appear on a certificate of analysis?
CoAs should clearly state % solids, assay and the basis for the assay (for example, “Assay: 50 % on solids basis; Solids: 60 % w/w”). This allows manufacturing systems to reconstruct active per kg or L without making undocumented assumptions.

Q5. What is a practical first step to implement percent solids basis?
Start with one critical concentrate or slurry, standardise how % solids and assay on solids basis are measured, and configure your systems to derive batch-specific potency and corrected active content from those results. Once proven, extend the approach to other similar materials.

Related Reading
• Potency & Basis: Potency Basis | Batch-Specific Potency | LOD Adjustment
• Liquid & Concentrate Control: Concentration-Adjusted Charge | Dynamic Recipe Scaling | Test-Driven Setpoint Adjustment
• Yield, Mass Balance & Records: Corrected Active Content | Mass Balance | Potency-Normalised Yield | Active-Equivalent Consumption | Electronic Batch Record (eBMR)