EP 2.2.44: Total Organic Carbon in Water for Pharmaceutical Use

What EP 2.2.44 is

EP 2.2.44 is the European Pharmacopoeia general chapter dedicated to determining total organic carbon (TOC) in water for pharmaceutical use. TOC is an indirect measure of organic substances present in the water and can also be used to monitor the performance of various operations in the preparation of medicines.

The defining feature of the chapter is methodological: rather than prescribing a given method, it describes the procedures used to qualify the chosen method and the interpretation of results in limit tests. A variety of acceptable methods is available for determining TOC, and the chapter leaves the choice to the user, provided suitability is demonstrated.

What the TOC test measures, and what it does not

The TOC test is non specific. It quantifies the carbon present in any organic compound in the water but cannot tell which molecule is present.

That is not a design limitation but the point of the test: in pharmaceutical water, the goal is detecting anomalous organic load, whether from process residue, degrading biofilm, ion exchange resin failure or handling contamination.

Identifying the chemical species requires a specific method. TOC answers a prior and cheaper question: is there organic load above expectation in this system.

Measurement principle

The various types of apparatus used to measure TOC in pharmaceutical water share the same objective: completely oxidize the organic molecules in the sample to produce carbon dioxide, measure the amount of CO2 produced and use that result to calculate the carbon concentration in the water.

Pharmaceutical grade analysers typically use ultraviolet light to oxidize the organic molecules and measure the change in water conductivity caused by the resulting CO2.

The critical point is discrimination between organic and inorganic carbon, the latter present as carbonate. The chapter allows two approaches:

• Measuring inorganic carbon and subtracting it from total carbon, calculating TOC as TC minus TIC.
• Purging inorganic carbon from the sample before oxidation.

Purging has a side effect recorded in the chapter itself: it may also entrain volatile organic molecules. That purgeable organic carbon goes uncounted, which must be considered where the system has a history of volatile compounds.

Apparatus requirement

The apparatus used must discriminate between organic and inorganic carbon and have a detection limit of 0.05 mg or less of carbon per litre.

This is the first filter in instrument selection. Industrial TOC analysers for general process water frequently fail that detection limit and are therefore unsuitable for compendial application, however fit they may be for their intended use.

System suitability: sucrose and 1,4-benzoquinone

This is the operational heart of the chapter, and where most laboratories go wrong.

The system suitability test compares the response of two solutions:

• A standard solution, prepared with a substance expected to be easily oxidizable, typically sucrose, at a concentration adjusted to give an instrument response corresponding to the TOC limit to be measured.
• A system suitability solution, prepared with a substance expected to be oxidizable with difficulty, typically 1,4-benzoquinone, acting as the challenge compound.

The reagent water response is subtracted from each of these, producing corrected responses. From those, the response efficiency is calculated by dividing the corrected suitability solution response by the corrected standard solution response and multiplying by 100.

The logic is elegant: if the instrument oxidizes sucrose well but not benzoquinone, it will underestimate TOC in samples containing recalcitrant organic matter. The suitability test exists to expose exactly that blind spot.

Reagent water and the blank effect

The maximum carbon concentration of the reagent water under these monographs is 100 ppb C.

There is, however, a relevant practical consideration: when reagent water concentrations fall below 50 ppb C, greater accuracy may be obtained through lower instrument and reagent blank values.

In operations measuring close to the limit, the blank stops being a detail and becomes decisive. A high blank compresses the margin between reading and acceptance criterion, generating failures that belong to the reagent water, not the sample.

Interpreting the limit test

The chapter treats TOC as a limit test, not as absolute quantification with stated uncertainty.

The test solution complies if its response is not greater than that of the standard solution minus the response of the water control. It is a direct comparison, not a concentration calculation followed by comparison against a specification.

That distinction matters when writing the SOP and defining how the result is recorded. Reporting TOC as a numeric value is common practice and useful for trending, but the compendial compliance decision rests on the response comparison described in the chapter.

Ph. Eur. 12.3 revision: what changed

The European Pharmacopoeia Commission adopted, at its 182nd session in June 2025, three revised texts relating to pharmaceutical waters. Chapter 2.2.44 was among them.

The revised texts were published in Ph. Eur. 12.3 and came into force on 1 July 2026. The relevant changes are:

• Addition of Method B to the general section of 2.2.44, applicable to sterilised water for injections (SWFI).
• Replacement of the oxidisable substances test with the TOC test for SWFI in monograph 0169 (Water for Injections).
• Explicit reference to Method A of chapter 2.2.44 in monographs 0169 and 0008.
• The TOC limit specified as 0.50 mg/L rather than 0.5 mg/L, a significant figures adjustment reducing ambiguity.
• Revised description of the conductivity test in the water monographs: measurement accuracy is now based on the expected conductivity value of the reference solution.

Replacing the oxidisable substances test with TOC is the change with most technical weight. TOC is recognized as a more sensitive and non selective method and represents a meaningful step toward international harmonization with USP, JP and other pharmacopoeias.

EP 2.2.44 and USP 643: harmonization

USP covers the same subject in chapter USP 643. The two documents converge substantially.

The most relevant common points are:

• The same system suitability test using sucrose and 1,4-benzoquinone.
• The same response efficiency range, 85% to 115%.
• The same nominal TOC limit, 500 ppb or 0.50 mg/L of carbon.
• The same maximum carbon concentration in reagent water, 100 ppb.

That convergence simplifies life for operations serving multiple markets: a single well designed suitability procedure usually satisfies both chapters. Even so, the SOP should cite both references explicitly, because compliance is demonstrated against the text, not against the similarity between texts.

Implications for purified water and WFI systems

Since the European Pharmacopoeia began permitting WFI production by reverse osmosis rather than distillation alone, TOC and conductivity monitoring have gained additional weight.

Distillation provides a robust physical barrier against organic load. Membrane systems deliver equivalent quality when well designed and operated, but they depend more heavily on continuous monitoring to demonstrate that equivalence holds.

In system operation practice, the points of attention are:

• Sampling point location and representativeness relative to the loop.
• Offline analysis frequency versus continuous online monitoring.
• Qualification of the inline analyser, with documented IQ, OQ and PQ.
• Alert and action limits set below the compendial limit.
• Trend handling rather than isolated results.
• Traceability of the standards used in system suitability.

For operations involving calibration, equipment qualification and water purification systems, this set defines the real scope of work, well beyond simply acquiring the analyser.

Common errors in applying the chapter

The deviations most often found in audits and out of specification investigations are:

• Running system suitability only at installation rather than at intervals suited to measurement frequency.
• Using reagent water without verifying its carbon content, treating the blank as a constant.
• Preparing standard and suitability solutions without standard traceability or preparation records.
• Confusing instrument calibration with system suitability, which are distinct activities with distinct purposes.
• Keeping SOPs citing the previous chapter version after the revision came into force.
• Ignoring the effect of purgeable organic carbon in systems with a history of volatile compounds.

None of these is exotic. All follow from treating TOC as an instrument reading rather than as a compendial test with its own qualification requirements.