Analysis of TOC are basically work with two steps i.e. oxidation of organic to form CO2 and detection-to measure the CO2, All laboratory TOC analysers offered today use either the HTC method or heated/UV per sulfate oxidation. Both techniques either remove or measure the inorganic carbon (IC), defined asdissolved carbon dioxide, carbonate, and bicarbonate.
One common technique measures non purgeable organic carbon (NPOC), by introducing the sample and a small amount of inorganic acid to an inorganic carbon removal chamber. Through the acidification of the sample, the IC is converted to carbonic acid (H2CO3), also known as dissolved carbon dioxide (CO2). The sample is usually sparged (gas-stripped) with carrier gas to drive off any converted dissolved CO2. Loss of purgeable organic carbon (POC), in the form of volatile organic substances, can also occur during the sparging of the sample. Typically for many surface/ drinking waters, the amount of POC is negligible and can be discounted. Thus in these cases, NPOC is the equivalent of TOC.
Measurement of POC: The volatile organic molecules must be separated from the IC by passing the sample through a Scrubber (typically contain Soda lime) after the sample is acidified and the IC converted to CO2, the CO2 is absorbed by the scrubber. Subsequently, the POC continues to carry through the scrubber to an oxidation chamber. The POC is then oxidised to carbon dioxide and quantitated. For NPOC analysis, an oxidation step, following the removal of IC, converts the remaining non-volatile organic carbon in the solution to carbon dioxide. A detector measures the amount of carbon dioxide and applies that result to a calibration curve to attain the TOC value.
The biggest challenge of any offline TOC analyser is the interference it could be by any means
- Sampling error
- Sample retention error
- Background error
By adopting a good Sampling method will eleminate the error from sampling and also with adequate sampling rate will solve adress the sample retention erroe. Now left out error is back ground. Bag ground means any acessories/consumables that we use in the analysis will reflect in back ground error. The sources can be
- Catalyst (poisoned)
- Gas
- Reagent (Acid and oxidisers)
The contamination from the gas if it is being used for both oxidation and carrier will contribute more errors then the gas used only for the carrier but these errors can nullified by taking precausion of leaks, a background tests.
Reagents used for the oxidation which get mix with the sample water also have the major impact. To eleminate you can use a prepared reagent (have very less self life) which make your running cost very high as these reagents are expensive and have small self life. The second option is to prepare the reagents in house and run a back ground test to estimate the TCO content of the solution.
QbD1200 from Anatel Hach implemented the option of checking back ground and taring it so that the running cost of a LAB TOC analyser come down drastically.
When measuring a water sample to quantify TOC, most TOC Analyzers add some combination of reagents and dilution water to the initial sample in order to complete the measurement. These added reagents and dilution water are a potential source of TOC that need to be accounted for (subtracted) from the final measurement result to obtain an accurate TOC value for the sample. The QbD1200 combines acid, oxidiser, and dilution water into a single reagent which makes this blank or background TOC subtraction simple and ensures a good TOC measurement of the sample. The QbD1200 automatically performs these calculations.
The measurement strategy must account for TOC from two sources:
- The sample: contains an unknown concentration of TOC
- The reagent: contains a small amount of background TOC that is known after a background measurement is taken.
The NDIR will detect all CO2 passing through it—some of this CO2 may have originated from background carbon in the reagent. The QbD1200 will automatically recommend that the user take a background measurement at the beginning of a run if there has not been a background measurement taken recently.
To understand this concept better, consider a measuring a sample which has 100ppb TOC:
- The first step the QbD1200 will perform is an autorange measurement which determines the ratio of sample to reagent that should be used during measurement.
- For a 100 ppb sample, this would result in combining 8 mL of sample with 2 mL of reagent.
- The NDIR detector will count all CO2 passing through it. This total count contains Carbon that originated from both the sample and the reagent. The small concentration of carbon present in the 2 mL of reagent is subtracted and called “background”.
- When the background is known, the calculations can easily adjust based on how many mL of reagent are mixed with the sample.
Background TOC notes:
- QbD1200 automatically prompts user to initiate background measurement at the beginning of a run if there has not been a recent background measurement taken.
- All calculations are automatic and handled by the QbD1200.
- One reagent is prepared by mixing one part stock solution with 100 parts pure water. The background measurement is taken on this reagent mixture.
- USP <643> specifies that all reagent water should be < 100ppb TOC. The QbD1200 background measurement automatically checks this.
- Careful consideration of background TOC ensures an accurate TOC measurement of unknown samples.
Overall measurement technique of TOC in QbD1200:
A water sample initially contains two types of carbon:
- Total Inorganic Carbon (TIC) (from CO2 gas dissolved in H2O and dissolved carbonates in the water)
- Total Organic Carbon (TOC) (from organic species)
To measure TOC, first remove TIC. Then convert organic species into CO2 gas, measure the gas on detector, and convert the result into a TOC value.
The measurement strategy:
Steps:
- Remove TIC. In presence of acid H3PO4, all dissolved carbonates are converted into CO2 gas. Blow carrier gas through reaction chamber to remove all CO2 gas derived from inorganic carbon.
- Convert TOC into CO2 gas. In presence of UV light and powerful oxidiser (NH4) 2S2O8, organic carbon species are converted into CO2 gas by oxidation. Blow carrier gas through reaction chamber to push all CO2 gas through NDIR detector (step 3). Detect CO2 gas as it goes through NDIR detector. TOC is quantified by integrating the area under the curve.
- Based on instrument calibration, convert CO2 gas signal (area under the curve) into TOC.
Note that the area under the curve for TOC can also be referred to as ‘NPOC’ (Non-Purge able Organic Carbon). If the initial water sample contained a volatile organic, the volatile organic would likely be purged during the step to remove TIC. Thus, what remains after TIC removal is non-purge able organic carbon. Because the QbD1200 is designed for clean water applications where high concentrations of volatiles are not present, this should not be a concern.
UV / persulfate / NDIR method
The general TOC analysis method described here has been widely used for many years in a variety of applications and conforms with numerous regulatory guidelines such as USP, EP, JP, and is also an approved method (5310c) under the US EPA guidelines. The QbD1200 is unique in that it combines all required reagents (acid, oxidizer, and dilution water) into a single reagent instead of requiring multiple reagents.
Pharmacopeia requirement for a TOC analyser is simple, it has to pass the System suitability Test (SST) which has been mentioned in USP CHAPTER <643>. RE should be between 85-115 per cent.
It is the user who should see the feasibility of owning the system with a perfect combination of oxidation and detection principle.
For any clarifications please feel free to contact
Ramesh Sahu,
Product Manager
Email: [email protected]
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