Analyze This

Innovative solutions for automated total organic carbon analysis of difficult matrices

• By Mike Purcell, Brian Wallace
• Sep 01, 2004

Total organic carbon (TOC) analysis can be used as an effective industrial wastewater monitoring tool. However, heavily particulated sample matrices present significant challenges for most TOC analyzers. The following guidelines address the various considerations for performing automated TOC analysis on these types of samples.

Background
TOC analysis is a fast generic test to determine the organic loading in an industrial wastewater stream and is commonly used as a surrogate for biochemical oxygen demand (BOD) and chemical oxygen demand (COD). BOD is a measure of the biological activity needed to degrade (oxidize) an organic compound. This degradation is calculated by measuring the oxygen dissolved in the liquid sample initially, and after five days of incubation at 20 degrees Celsius (in the dark). COD is similar to BOD in that it is a measure of oxidative action on the organics in a sample, but it is chemical, not biological, in nature. It takes only two hours to four hours to run, compared to five days for BOD. The process means that you take a sample, combine it with a chemical oxidizer, usually dichromate, and then boil (reflux) the sample for two hours to three hours in order to chemically oxidize any organics in the sample.

The primary advantage of TOC analysis is its productivity versus both BOD and COD, taking 10 minutes to 20 minutes depending of how many replicates are analyzed. This dramatic time savings not only means faster run times but, more importantly, translates into a chance for quicker response to adverse changes in the process being monitored. For these reasons, TOC is often used as an effective trending tool, an instrument that can be used for evaluating historical data in comparison to real-time results, in industrial wastewater monitoring. For example, contaminated wastewater effluent can be compared to historical data to meet regulatory concerns. The following article outlines the various considerations in setting up a protocol for automated TOC analysis of industrial wastewater streams that are heavily particulated or contained in a difficult matrix, such as brine solutions, which cause higher maintenance concerns. For example, brine solutions can clog and corrode the valves in instrumentation.

Sampling Considerations
The first step in developing a standard operating procedure for TOC analysis is determining where to sample. For purely regulatory compliance, process effluents are monitored. For a better understanding of how TOC is affected by a treatment process, influent and effluent are monitored with a number of points throughout the treatment process as necessary to determine the effectiveness and extent of TOC removal. The degree to which you will want to sample should reflect the importance of controlling the TOC loading in your process.

The manner in which sampling is performed can dramatically affect TOC results, especially for heterogeneous process streams. When choosing a sampling technique, it is important to remember that the goal of TOC analysis is to provide a quick trending tool for controlling your water treatment process. Repeatability of your sampling technique is key to ensuring to you do not see false high or low TOC readings. A simple, easy-to-reproduce sampling procedure will yield the TOC results you need with a minimum of effort.

Acidification Requirements
TOC analysis involves the acidification of the sample, often to a pH of 2. This is required either to remove the inorganic carbon (IC) if TOC is being measured directly, or to perform IC analysis if TOC is being measured as a subtraction of total carbon (TC) minus IC. When performing TOC analysis it is important that neither too little nor too much acid is used. Insufficient acid will cause false positives in your TOC analysis. Too much acid will affect the TOC analyzer, especially those that employ the combustion oxidation technique, causing a change in your analytical results.

Most TOC analyzers automatically add acid to the sample for analysis. If your samples are already at a pH of 2 or less, this addition is unnecessary and should be avoided if possible. If your samples are highly caustic in nature, the default amount of acid added by the analyzer may not be sufficient to bring the sample solution to the required pH. It is critical that samples are being treated with the correct amount of acid before performing TOC analysis.

For direct TOC analysis, it also is important to make sure that the time the sample sparges to remove IC is sufficient for your sample matrix. This can be verified by running an IC standard, sodium carbonate or equivalent, for TOC. If a significant TOC result is measured, then the IC is not being removed completely and the sparging time in the analyzer should be extended.

Sample Stirring Requirements
Due to the particulated nature of many process samples, sample stirring options before automated TOC analysis should be explored. Many TOC manufacturers offer magnetic stirring options on their analyzers. This is the most effective technique to ensure an accurate composite of the sample is taken for TOC analysis. However, the addition of stir bars to the sampling containers adds significantly more preparation to the analysis. When developing a TOC protocol, one should weigh these tradeoffs versus the specific application requirements for TOC monitoring. If process trending is the primary purpose of your TOC monitoring program or particulates in your sample matrix do not settle significantly over time, magnetic stirring may not be required.

Instrumental Considerations
Automated TOC analysis of highly particulated samples or difficult matrices like brine can be extremely difficult for traditional TOC analyzers that employ valves in the sample pathway. Sample rinsing and transfer can lead to TOC analysis of sample with either too much or too few particulates for TOC analysis and, over time, sample build up in the instrument tubing and valving will lead to clogging. To perform these applications on standard TOC analyzers often requires additional maintenance and significant pre-treatment.

For these difficult applications, an automated analytical solution that minimizes sample handling and eliminates the need for valves in the sample pathway is highly preferred. The Atlas Sample Handling System is an automation solution option designed to meet this need in TOC analysis. Designed for Teledyne Tekmar's Apollo 9000 combustion total organic carbon/total nitrogen (TOC/TN) platform, the Atlas System uses injection ports mounted to the autosampler to allow sample to be injected into the ports directly and then swept with deionized water to the combustion furnace. The design limits sample usage only to what is needed for analysis and eliminates all valves from the sample pathway.

The Atlas System employs a simple and straightforward methodology that uses a syringe to measure and move the sample without having the sample enter the syringe or multiport valve. The following steps give a basic description of the Atlas methodology for direct TOC analysis:

1. Autosampler needle rinse. The syringe pulls deionized water and flushes the autosampler needle into a rinse station at the autosampler.

2. Acid addition into the sample vial. The syringe pulls acid and dispenses it into the sample vial to bring the solution to a pH of 2 or less.

3. IC is sparged from the sample. TOC-free air is sparged directly through the sample vial via a multistage needle to remove the IC in the sample.

4. Autosampler needle rinse. The autosampler needle is rinsed again before sampling for TOC analysis.

5. Sample is pulled into the autosampler needle. The syringe pulls in sample through the autosampler needle. Note that the dead volume of the line running from the syringe to the autosampler needle is greater than the sampling volume so no sample enters the syringe or the multiport valve connected to it.

6. Sample is injected in the injection port. The autosampler needle moves the Atlas injection port mounted on the autosampler. The syringe flushes the sample into the injection port with deionized water.

7. Sample is flushed into the combustion furnace. The syringe flushes more deionized water through the side arm of the Atlas injection port to push the sample into the combustion furnace. This step also ensures that the sample is completely swept from the Atlas injection port.

8. Sample is oxidized and measured for TOC. During this step the autosampler needle is rinsed with deionized water at the autosampler rinse station for the next analysis.

9. The sample pathway to the combustion furnace is emptied. The syringe pulls the remaining deionized water from the Atlas injection port and tubing running to the combustion furnace and emptied the water to waste.

Similar steps are followed for TOC analysis using the TC-IC methodology. The only difference is that for IC analysis, the sample is drained through the autosampler needle and emptied into the autosampler rinse station. During this step, as with the sampler transfer, the sample does not interface with the syringe or multiport valve used on the analyzer.

Due to the high amount of sample and deionized water entering the furnace and the difficult nature of sample matrices analysis, peak shapes for carbon response are different than what is typical for the Apollo 9000 TOC analyzer. Most peak responses are shouldered or "dual peaked." This effect is caused primarily by the additional deionized water temporarily cooling the furnace catalyst. However, since the analysis is based on area under the response curve and not peak height, this phenomenon is not a detriment to the TOC analysis.

A limitation of the Atlas methodology is the elevated TOC background, which causes higher TOC contribution from the additional deionized water in comparison to traditional TOC methodologies. Even with this limitation, the analytical performance and run times are similar to the standard Apollo 9000 specification and the Atlas can be used for all standard analysis modes. Most importantly, the limits of detection using the Atlas methodology are still very viable for highly particulated sample matrices the system is intended to handle. See Table 1 (below) for the base specifications of the Atlas Sample Handling System and Table 2 (below) for typical analytical results for an example of a difficult matrix.

The Atlas methodology has many advantages for the TOC analysis of difficult, particulated sample matrices. The sample pathway does not contain valves, which are susceptible to clogging, and the overall length is minimized to include only the autosampler needle, the Atlas injection port, and tubing running to the combustion furnace for TOC and total carbon (TC) analysis and to the IC sparger for IC analysis. This methodology does not require any additional sample for rinsing of the sample pathway and, as a result, significantly limits the potential for particulate buildup. Finally, the use of deionized water to flush the sample acts as a cleaning agent for the sample flow path, actually cleaning the instrument as sample is analyzed. This combination of characteristics makes the Atlas TOC sampling good for long-term automated TOC analysis of difficult sample matrices.

There are many considerations required when attempting TOC analysis of particulated process streams. Consideration of sampling points as well as determination of sampling techniques must be performed. For effective TOC analysis, proper acidification and IC sparging should be verified before the TOC analysis is put into operation. When performing TOC analysis of particulated samples that do not stay in solution, magnetic stirring for automated TOC analysis should be considered. When choosing a TOC analyzer, one must access the long term impact of the sample matrix on the analyzer and the amount of maintenance that will be required. For high throughput automated TOC analysis of difficult or particulated matrices, using an analyzer specially designed for these kinds of applications, like the Apollo 9000 with Atlas Sample Handling System, can be useful. Careful planning of these elements can help to ensure a successful, easy to maintain TOC monitoring solution well suited to your analytical requirements.

Table 1. Atlas Analytical System Specifications for the Apollo 9000 TOC Analyzer
Modes of Operation	TOC, TC, IC*, TC-IC, TN
TOC Range	1 to 1600 ppmC** for the Standard Apollo, 1 to 200 ppmC for the Apollo HS
TN Range	1 to 90 ppmN for both the Standard and HS ApolloHHH
Precision	Typically +/- 5 % or +/- 0.25 % full scale, whichever is greater, +/- 10 % or +/- 0.5 % full scale, whichever is greater for TC-IC analysis
Injection Volume	50 to 200 microliters
Analysis Time	2 to 4 minutes
Total TOC Run Time	20 minutes for a triplicate typical
Total TC-IC Run Time	20 minutes for a duplicate typical

* IN = inorganic carbon

** ppmC = parts per million of carbon

Table 2. Atlas System Analytical Results for 115 ppmC Humic Acid
Sample Injection Volume	100 microliters
Combustion Furnace Temperature	680 degrees C
1 ppmC KHP* Standard	1.08 ppmC
100 ppmC KHP Standard	100.0 ppmC
115 ppm Humic Acid	115.2 ppmC
Number of Sample Replicates	33
Standard Deviation	1.45 ppmC
Relative Standard Deviation	1.26 %

*KHP = potassium hydrogen phthalate

This article originally appeared in the 09/01/2004 issue of Environmental Protection.