SOP for Operation and Calibration of a TOC Analyser

In short: This example SOP describes how a QC laboratory can operate, calibrate and check the system suitability of a laboratory TOC analyser (a combustion/NPOC instrument with a potassium hydrogen phthalate calibration). It is an illustrative template: the software screens, flow rates, concentrations and limits are example in-house criteria and must be replaced by your instrument manual, your validated method and the current pharmacopoeial chapter.

What a TOC analyser does and why it is checked

Total organic carbon (TOC) is a measure of the organic carbon present in water. In pharmaceutical water systems it is a quick indicator of organic contamination, and for purified water and water for injection it is a pharmacopoeial test. The analyser oxidises the organic carbon to carbon dioxide and measures it. In the NPOC (non-purgeable organic carbon) mode used in this example, the sample is first acidified and purged so that inorganic carbon is removed.

Before an instrument can be trusted for water release or monitoring, two things are shown and recorded:

Calibration, which links the instrument response to known carbon concentrations.
System suitability, which shows that the instrument oxidises and detects a difficult-to-oxidise organic compound as well as an easy one, within defined limits. In the pharmacopoeias this is done with sucrose (easily oxidised) and 1,4-benzoquinone (harder to oxidise) and is expressed as response efficiency against a reagent-water blank.

Pharmacopoeial requirements are in USP general chapter <643> Total Organic Carbon and Ph. Eur. general chapter 2.2.44, Total organic carbon in water for pharmaceutical use. Both are paywalled or restricted, so read the current edition you are legally using for your market. For WHO expectations on pharmaceutical water systems, including on-line TOC and conductivity monitoring, see WHO TRS 1033, Annex 3 (2021).

Records from instrument runs, preparation and calibration are GMP records. They are kept at least 1 year after the expiry date of the respective product batch (EU GMP Part I, Chapter 4, clause 4.11; 21 CFR 211.180(a)), longer where local law or the marketing authorisation requires. Electronic data and the software audit trail are retained for the same period.

Example SOP

This example SOP is a template. All values below are example in-house criteria unless a source is named.

1.0 Objective

To lay down a procedure for handling, operation, maintenance and calibration of the total organic carbon (TOC) analyser in the Quality Control department.

2.0 Scope

This SOP applies to the TOC analyser in the Quality Control laboratory of the manufacturing facility.

3.0 Responsibility

Role	Responsibility
Officer / Executive	Handling, operation, calibration, maintenance and recording of the TOC analyser.
Head of Quality Control	Effective implementation of this procedure.
Quality Assurance	Reviews records and periodic calibration data (see the SOP for Calibration).

4.0 Procedure

4.1 Preliminary checks

Check that the instrument and its surroundings are clean; wipe with a soft cloth if needed.
Flush the carrier gas line with carrier gas (compressed air or nitrogen, as specified for the instrument) before analysis.
Check that the molecular sieve and the CO₂ absorber are in good condition, according to the instrument manual.
Check that there is enough sample in the bottles, that the bottles are kept closed, and that the volumes are at the specified levels.
Check that the instrument door is closed.
Check daily the levels of dilution water, 2 M hydrochloric acid, the drain vessel and the humidifier water.
Check carrier gas supply. Example in-house criterion for this instrument model: cylinder outlet pressure 4 kg/cm², carrier gas pressure 200 kPa and flow 150 mL/min.

4.2 Basic operation

Open the gas cylinder valve and confirm the pressure.
Switch on the instrument with the power button on the front panel, then switch on the computer.
Confirm that the carrier gas pressure and flow match the values in 4.1.
Start the instrument control software from the desktop icon and open the sample table editor. Log in with your own user ID and password; never use another person’s credentials.
Create a new file of type “Sample run”, enter the general information, choose the folder and file name, and save.
Connect to the instrument from the “Instrument” menu, apply the user settings and wait until initialisation is finished.
Open the background monitor and wait until the system stabilises and the status light on the front panel turns green. This usually takes 15 to 20 minutes.
Check the background conditions shown on screen: furnace temperature, dehumidifier temperature, baseline position, fluctuation and noise. Adjust the gas flow and pressure with the instrument knobs if they differ from the set values.

4.3 Creating the calibration file

Select “New” from the file menu and choose “Calibration curve” to start the calibration curve wizard.
On the analysis-information page choose the analysis type (see 4.7 for what each is used for), enter the default sample name and ID, and select multiple injections.
Choose the folder and give the calibration file a name.
On the measurement-parameters page set the following (example in-house criteria): unit ppb; 3 to 5 injections; 0 to 10 washes; maximum SD 0.5000; maximum CV 3.0%; acid addition 0.5 mL.
On the calibration-points page add the highest calibration point first, then the remaining points in descending order down to the lowest.
Click “Next” through the remaining pages and then “Finish”.

4.4 Inserting the calibration file in the sample table

Place the cursor in the sample table and choose “Insert” to select the calibration curve file created earlier.
Click the “Start” button on the toolbar.
After the run, open the sample window to view the peaks, areas and results.

4.5 Sample measurement

Choose “Insert” and then “Sample” to open the sample wizard.
On page 1, browse to and select the appropriate calibration curve file (the file name carries the date and time).
On page 2 select the analysis type NPOC and enter the sample name and ID.
Check the injection parameters on page 4 and continue through the remaining pages to “Finish”.
Repeat steps 1 to 4 for each additional sample.
Click “Start”. After the run, open the sample window to view the peaks, areas and results.

4.6 Shut-down

After the last analysis, choose “Standby” in the “Instrument” menu.
The instrument switches itself off after 30 minutes (example in-house criterion; follow the manual).
Close the instrument software and switch off the computer.

4.7 Calibration

Reference material. Use potassium hydrogen phthalate (KHP) of a suitable grade and water that is low in TOC (reagent water). Record the grade, batch and expiry date of the KHP.

Preparation of standards (example in-house criteria):

Solution	Preparation	Carbon concentration
Stock	Weigh 0.2125 g of KHP and dissolve in 100 mL of water	about 1000 ppm (mg/L)
Intermediate	Dilute 1 mL of stock to 100 mL	10 ppm
Zero	Blank reagent water	0 ppb
Level 1	2.5 mL of intermediate to 100 mL	250 ppb
Level 2	5.0 mL of intermediate to 100 mL	500 ppb
Level 3	7.5 mL of intermediate to 100 mL	750 ppb
Level 4	10.0 mL of intermediate to 100 mL	1000 ppb

The stock solution may be stored and used for further dilutions for a period defined by your own stability data. Generate a five-point calibration curve (zero, 250, 500, 750 and 1000 ppb). For each solution record the area counts, mean area, SD and CV (RSD) in Annexure I.

Acceptance criteria (example in-house criteria):

For each solution, either the SD of the area is not more than 0.5 or the CV (RSD) of the area is not more than 3.0%.
Linearity: correlation coefficient not less than 0.98.

Frequency (example in-house criterion): quarterly, and after major maintenance or part replacement.

4.8 System suitability test

The pharmacopoeial system suitability test uses two reference materials: sucrose, an easily oxidised compound, and 1,4-benzoquinone, a compound that is more difficult to oxidise. Both are prepared at the same carbon concentration and measured against reagent water. The instrument is suitable when it responds to both within the response-efficiency limits of the pharmacopoeial chapter that applies to your market.

Sucrose solution (example in-house preparation):

Treat the sucrose reference standard as directed on its label or certificate before weighing (example in-house criterion: oven-dry at 105 °C for 2 to 3 hours).
Weigh accurately 23.75 mg of sucrose and dissolve in water to make 100 mL (100 ppm carbon, mother solution; sucrose contains about 42.1% carbon).
Store the mother solution for no longer than the period supported by your own data (the example in-house criterion is one month).
Dilute 0.5 mL of mother solution to 100 mL with water to make a 500 ppb carbon solution.
Use reagent water as the zero solution and generate a two-point curve (zero and 500 ppb).

1,4-Benzoquinone solution (example in-house preparation):

Weigh accurately 15.0 mg of 1,4-benzoquinone reference standard and dissolve in water to make 100 mL (100 ppm carbon, mother solution; 1,4-benzoquinone contains about 66.7% carbon).
Dilute 0.5 mL of mother solution to 100 mL with water to make a 500 ppb carbon solution.
Analyse this solution as a sample by the NPOC method.

Evaluation. Record area counts, mean area, SD and CV for each solution in Annexure II.

Example in-house criterion for repeatability: SD of area not more than 0.5, or CV (RSD) not more than 3.0%.
Pharmacopoeial check: calculate the response efficiency of 1,4-benzoquinone relative to sucrose, after subtracting the reagent-water blank, and compare it with the limit in the current chapter.

Frequency (example in-house criterion): system suitability is performed daily on days of use.

4.9 Out-of-limit results

If calibration or system suitability fails, do not test samples. Inform the Head of Quality Control, investigate (reagent water quality, standard preparation, gas supply, consumables, instrument condition), correct the cause, and repeat. Record the event. A suspected analytical error on a batch-related sample follows the laboratory’s OOS procedure.

5.0 Training

Trainer: Head of Quality Control. Trainees: QC chemists and section heads.

6.0 Distribution

Controlled copy 1: Head of Department, Quality Assurance
Controlled copy 2: Head of Department, Quality Control
Original copy: Head, Quality Assurance

7.0 Annexures

Annexure I: TOC calibration record (area counts, mean area, SD and CV for each level).
Annexure II: System suitability record (sucrose and 1,4-benzoquinone).

8.0 Revision history

Sr. No.	Revision No.	Change control No.	Details of revision	Reason for revision
1	00	(enter number)	New SOP	New SOP

Frequently asked questions

What is the TOC limit for purified water? The pharmacopoeial limit is expressed in mg/L of carbon and is set in the USP and Ph. Eur. monographs for the water grade. Many sites also set in-house alert and action levels below it. See the current monograph for the exact figure.

Why use both sucrose and 1,4-benzoquinone? Sucrose is easy to oxidise, 1,4-benzoquinone is harder. Showing a good response for both shows that the instrument is not just detecting easy compounds.

Why must the water used for standards be low in TOC? Any carbon in the blank adds to every result, and the system suitability calculation subtracts it. Poor blank water makes calibration and suitability unreliable.

References

United States Pharmacopeia, general chapter <643> Total Organic Carbon (current official USP–NF).
European Pharmacopoeia, general chapter 2.2.44, Total organic carbon in water for pharmaceutical use (current edition).
World Health Organization, WHO good manufacturing practices: water for pharmaceutical use, Annex 3, WHO Technical Report Series No. 1033 (2021).
European Commission, EudraLex Volume 4, Part I, Chapter 4, clause 4.11 (record retention), and US FDA 21 CFR 211.180(a).