Saving Money on TOC Analyzer Gas Costs

Total organic carbon (TOC) analyzers are increasingly popular as an accurate and inexpensive way to test for inorganic and organic carbon contamination in water samples. The most widely used and reliable TOC analyzers convert carbon into CO2 through either a wet oxidation or combustion procedure and send the converted carbon to an IR linearized to detect CO2.

TOC analyzers are affordable but require a constant gas supply (generally nitrogen, oxygen, or CO2-free air). Many analysts choose bottled gases or dewars, but are concerned with the costs and potential health hazards of having them in the laboratory.

What keeps them from moving to the use of a gas generator? Fear of interference with their data is cited as a top reason. This article is intended to provide analysts with scientific data to demonstrate that TOC gas generators are not only affordable but will provide a CO2-free gas source that will not interfere with analytical data.

For the purposes of this test, an Aurora model 1030W TOC analyzer (O.I. Analytical, College Station, TX) was selected. The analyzer is designed to oxidize carbon utilizing the wet oxidation technique. Although the instrument has a linear range up to 38,000 ppm carbon, the primary concern of interference from a gas source is at the low range; thus a low-level test was conducted. To minimize any variances as a result of the TOC analyzer components, the samples were introduced via a sipper tube. The 100-ppb standard came from a single flask for all replicates.

To demonstrate the capabilities of a CO2-free gas generator, a TOC Gas Generator Model P01108F1 (PureGas, Broomfield, CO; distributed by Terra Technologies, Louisville, KY) coupled with an OFS30S compressor (Jun-Air, Buffalo Grove, IL) was selected. The gas generator provides 1 L/min of CO2-free gas designed to provide carrier gas for TOC analyzers.

Figure 1 - Results of the low-level calibration curve.

A calibration curve from 10 ppb to 10 ppm was analyzed, resulting in an R2 value of .9998. Calibration points included RW (reagent water), 0.010, 0.100, 1, and 10 ppm (see Figure 1).

The IR signal was monitored to determine IR signal stability. Utilizing nitrogen as a carrier gas, a standby IR signal of approximately 2300 was observed. Using the TOC gas generator, a standby IR signal of approximately 2300 was also noted.

In order to test the capabilities of the gas generator and consistency over an extended period of time, a sequence with a sample containing seven replicates (similar to a method detection limit [MDL] study) was run. The system was allowed to stand idle for approximately 15 min and then programmed to restart the sequence. This process was repeated for a total of seven sample sets over the course of about 8 hr.

At the start of the sequence, the IR signal had a baseline of approximately 2300 IR counts. Midway through the sequence, the baseline did start rising with the ending baseline at around 2500 IR counts. The variation in the baseline did not appear to hinder the TOC analyzer’s ability to detect the 100-ppb standard.

Results

Figure 2 - Replicates 1–7, 100-ppb check #1.

Figure 3 - Replicates 8–14, 100-ppb check #2.

Figure 4 - Replicates 15–21, 100-ppb check #3.

Figure 5 - Replicates 22–28, 100-ppb check #4.

Figure 6 - Replicates 29–35, 100-ppb check #5.

Figure 7 - Replicates 36–42, 100-ppb check #6.

Figure 8 - Replicates 43–49, 100-ppb check #7.

Figure 9 - All sample replicates overlaid.

The results demonstrate the 49 replicates of 100-ppb standards divided into groups of seven replicates across an 8-hr time span (see Figures 2–8). The results from each set of seven replicates have been overlaid to demonstrate the system reproducibility and consistency (Figure 9).

Conclusion

The TOC Gas Generator provided consistent CO2-free gas, which allows for even low-level TOC analysis without interference. Analysts can free themselves from the burdens of moving cylinders and use a gas generator without sacrificing data quality.

Ms. Triplett is co-owner and VP of Sales, Terra Technologies, Inc., 2929 S. Floyd St., Louisville, KY 40209, U.S.A.; tel.: 877-891-1088; fax: 502-368-6699; e-mail: [email protected].

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