Methodology

DETECTION LIMIT – BLANK MEASUREMENTS

The uniTOC models „process“ and „online“ are concipated for the monitoring of purest waters (UPW, VE, WFI) in several industries. A very important parameter is the detection limit and the oxidation capability of the system. The following report shows the results of an online-measurement of pure water (17 measurements) and the determination of the detection limit according to DIN 32 645 with blank measurements.

Ultra pure water of a pharmaceutical company, that has been purified and ozonized by a Christ purification system, was analysed with a uniTOC-process. Through the ozonization process minimal residues of TOC in the water are oxidized and degraded resulting in ultra pure water with extreme low TOC-values.

DETECTION LIMIT - MULTIPOINT CALIBRATION

Regarding the accuracy of the TOC-results the uniTOC-system provides the advantage to calibrate the analyzer whenever required to verify the quality of the measurements.

Calibrations and verifications should always be performed in the expected working range; to provide measuring accuracy and reliability across the whole analysis range it is recommended to perform multi point calibrations on the analyzer systems. The greater the time interval between calibrations the higher is the risk for inaccurate results. The calibration range has also an effect on the detection limit, the greater the calibration range is chosen the lower is the detection limit achieved.

Due to the application of oxidizing reagents to support the oxidation process, the system additionally provides the possibility to verify the system blank value “instrument blank” whenever required (e.g. after reagent-refill) to yield accurate and reliable results in very low ppb-ranges. Additionally the instrument blank is determined automatically regularly in a time interval defined in the configuration.

The following application shows the determination of the detection limit based on a multi point calibration with the uniTOC-process-analyzer.

conditions:

analyzer: uniTOC process
UV/persulphate oxidation followed by CO₂ selective NDIR detection
carrier gas: CO₂ free, compressed air, 170 ml/min
multipoint calibration with a 500 ppb urea-standard and automatic dilution
19 measurements (5 at each end of the calibration range and 1 at each dilution step)
10 dilution Steps (measurements) to calculate the linear regression and detection limit

result: within the calibration range of 0 to 5 μg carbon (absolute) the program calculates a detection limit of 0,641 ppb with a correlation coefficient of R2 = 0,9997 according to DIN 32 645.

REAGENTFREE MAINTENANCE FREE TOC-ONLINE MONITORING

The following figure shows the reagent free TOC-online-monitoring of purified production water of a chemical company. Pure water with a TOC-load less than 10 ppb was monitored with the uniTOC-online analyzer.

conditions:

UV-oxidation with CO₂selective NDIR detection
reagentfree – maintenance free – oxidation process
carrier gas: oxygen 170 l/min

It is clearly shown that the uniTOC-analyser is sensitive to detect least amounts of TOC without addition of oxidizing reagent. A maintenance free operation of these analysers in low concentration ranges is thus realizable.

CARRY OVER TEST

By performing the carry over test it is proven that no residues from the former measurement are present at the actual determination. A special clean process avoids cross contaminations (memory effects).

The test is performed by using samples with high TOC load followed by a sample with extreme low TOC concentration.

Conditions:

UV/persulphate oxidation and CO₂ selective NDIR detection
Corr. Coeff- R= 0,9999
Carrier gas: CO₂ free compressed air
Online measurement at two ports, delay time: 10 min.
Port 1: 500 ppb TOC urea stabdard
Port2: DI water
Sequence: 1 1 1 2 2 2

Data analysis shows no significant cross contamination of the samples. TOC value of DI water does not increase nor the TOC value of urea standard is diminished.

By applying the configurable cleaning steps between the measurements the carry over from sample to sample is avoided.

SYSTEM SUITABILITY TEST

The two uniTOC-models uniTOC-process and uniTOC-pharma are optimal adjusted to the requirements of the USP and Ph.Eur for monitoring of pharmaceutical water.

A verification of the TOC results according to international standards requires the frequent performance of so called System Suitability Tests to verify the capacity of the system. System Suitability refers to the oxidation efficiency (“response efficiency”) of a hard to oxidize substance (benzoquinones) compared to an easily oxidisable substance (sucrose), each in a concentration of 500 ppb. The ratio of the results of both substances must be within the acceptable range of 85 % to 115 %.

The UV/persulphate oxidation of the uniTOC-systems offers high sensitivity with low maintenance expenses and shows high efficiency in oxidizing organic bound carbon to CO2 in the following application. The sucrose and benzoquinones standard solutions were analysed 3-fold each and the means were calculated to determine the response efficiency as follows:

The results of the SST are automatically calculated and displayed in the evaluation window directly after termination of the measurements. The ratio of the two substances as well as interim values, the equation and the “limit response” are displayed for manual reconstruction of the SST.

Results:

according to the requirements of the USP and EP the uniTOC-systems show their suitability for the monitoring of pure water and water for injections in pharmaceutical fields. The system suitability test yielded a ratio of 101,1 %, this confirms the capability of the analyser to measure accurately within the pharmaceutical range of 500 ppb (threshold for pharmaceutical water). The test revealed no differences in the oxidation processes of difficult or easy to oxidize substances.

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Methodology