Case study - Sievers 900/5310 C series TOC analyzers — calibration

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Courtesy of GE Analytical Instruments

Synopsis
The Sievers* 900 and 5310 C Series TOC Analyzers incorporate a number of new features and options designed toimprove the accuracy and linearity of the TOC measurements. Three new calibration options are available in the Analyzers: automatic conductivity offset, automatic TOC autozero, and single- or multipoint calibration of the TC and IC channels

Conductivity Offset
The 900 and 5310 C Series TOC Analyzers use membrane-based conductivity to measure the amount of CO2 formed from the oxidation of organic compounds and the level of inorganic carbon present in the sample. This technique employs a membrane that separates the acidified sample stream from a deionized water stream. Carbon dioxide from the sample will permeate across the membrane and, at neutral pH, ionize.

CO2 + H2O → H+ + HCO3-

The amount of CO2 in the sample stream is determined by measuring the conductivity increase in the deionized water. Figure 1 shows the results from a typical conductivity measurement. The deionized water is allowed to equilibrate with the flowing sample stream for three minutes, at which time fresh deionized water is used to flush the ions into the conductivity cell, resulting in an increase in conductivity.

After the ions have passed through the conductivity cell, the conductivity will decrease and should approach the theoretical conductivity of water at 25° C of 0.056 μS/ cm. Over time, leaching of impurities or contamination of the system can cause the conductivity at the end of the measurement to be less than or greater than this theoretical conductivity of water. In extreme cases, this can result in a negative values for TC or IC or values that are too high.

The Conductivity Autozero allows for recalibration of the conductivity cell offsets. It is performed by turning off the sample pump, UV lamp and reagents, and performing multiple measurements of the end conductivity. After repeatable values are obtained, the offset is adjusted to bring the end conductivity to just slightly above the theoretical value for pure water. Measurements using the new offsets are then performed to validate the new condutivity offset. Typical time to perform the Conductivity Autozero is ~1.5 hours.

The Conductivity Autozero should be performed if TC or IC values are negative or much greater than expected.

TOC Autozero
In the 900 and 5310 C Series TOC Analyzers, the sample is split into two streams. One stream passes through the oxidation reactor and then enters a membrane module where CO2 from the oxidation of organic compounds plus inorganic carbon (HCO3 -, CO3 -2) is measured to give the Total Carbon (TC) concentration of the sample. The other stream passes through a delay coil (no oxidation) and into a second membrane module where CO2 from inorganic carbon is measured to give the total Inorganic Carbon (IC) concentration of the sample. The Total Organic Carbon (TOC) concentration of the sample is then calculated from the difference (TOC=TCIC). To insure that the two measurement channels are matched (i.e., TOC > 0) a small offset is applied to the TC channel and this offset is determined in the TOC Autozero calibration.

The calibration is performed by measuring a stable IC sample, without oxidation of the organic compounds. The adjustment is primarily for measurement of low TOC water (< 50 ppb) but should also be performed after a Conductivity Autozero or if negative TOC values are reported. TOC Autozero calibration should also be performed if the Analyzer is used to sample water with different TOC levels, as the magnitude of the offset is affected by sample composition. Finally, TOC Autozero should be performed before calibrating the instrument if the instrument is used for measuring low TOC water (< 50 ppb).

The Autozero calibration is performed either on-line, using low IC water, or off-line using air-saturated water. The oxidizer flow is set to 0 μL/min, the UV lamp turned off, acid flow set to 0.3 μL/min and normal measurements are performed. After stable IC and TC values are obtained, the TC channel is offset (increase or decrease) to match the IC channel. After the new offset is applied, the measurements are repeated to validate the new offset. Typical time for the measurement is ~1.5 hours.

For on-line measurements, the Autozero can be scheduled to occur at intervals ranging from once per day to once per year. As part of checking for stable TC and IC values before making the adjustment, the 900 and 5310 C will wait up to two hours past the scheduled time and report a warning if the TOC Autozero could not be performed due to unstable values.

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