Total organic carbon (TOC) analyzers are a mainstay in industries that need to detect and quantify carbon content from a variety of samples and sources. Unlike spectroscopy, which measures specific carbon species from their unique interactions with light, TOC analysis does not tell which specific carbon-containing species are present. Nevertheless, it does provide information on impurities, which is invaluable in evaluating environmental samples. TOC analysis is mandatory for many labs, particularly in semiconductors for process water, or in regulated industries that work with ultrapure water for processing, instrumentation, cleaning/cleaning validation, or human drugs. Other industries that regularly employ TOC analysis include pharmaceuticals, foods, forensics, oil and gas, and the life sciences.
TOC analysis involves sample acidification and oxidation down to CO2, followed by detection and quantification. Acidification is used to remove inorganic carbon; oxidation occurs either through direct, high-temperature (1300°C) or catalytic (680°C) combustion, or through a technique involving either ultraviolet radiation or thermal oxidation in the presence of persulfate.
The detector is the heart of TOC analyzers. Two main techniques are used to quantify carbon dioxide: conductivity and non-dispersive infrared (NDIR). Conductivity is a straightforward measurement involving either direct measurement or membrane-based analysis. Direct conductivity has a limited analysis range but is inexpensive, does not require carrier gas, and is sensitive to parts-per-billion (ppb) levels. Membrane-based conductivity is somewhat more complicated and takes longer, but is more robust and provides a greater analysis dynamic range.
NDIR should be considered the detection method of choice because it is interference- free and directly measures TOC generated during the oxidation step. Static pressurized concentration (SPC), a relatively recent NDIR development, provides improved sensitivity and precision by concentrating the oxidized sample and measuring it all at once instead of through a flow cell.
One emerging trend in TOC is the analysis of solids, for example soils. Jeff Lane, a TOC specialist at OI Analytical (College State, TX), tells Lab Manager Magazine that his company is working on a new solids module that captures CO2 in a sampling bag that collects oxidized material from replicate samples. “Solids work because samples are not always as homogeneous as liquids or suspensions,” says Mr. Lane. “A collector bag helps overcome heterogeneity.”
Once the gas is collected, it can feed into a cavity ring spectrometer or even a mass spectrometer, which provides both TOC values and 12C13C isotope ratios, which are useful in determining the geographic origins of carbon-containing materials. Environmentalists have considered using isotope ratios from feathers or droppings to map bird migration, forensic food scientists use it to pinpoint the origins of high-value products such as olive oil, and pharmaceutical companies employ isotopes to detect counterfeits.
As it catches on, TOC analysis on solids could replace approved, legacy test methods that were developed years ago for materials not normally thought of as carbon-bearing, such as concrete. This will take time, Mr. Lane says, because “only a certain number of people are willing to overcome inertia.”
Customers considering the purchase of a TOC analyzer, and particularly wondering whether to acquire a combustion or persulfate oxidation model, should consider potential down time and ongoing direct costs together. Although OI Analytical sells both instrument types, Mr. Lane suggests that persulfate may be superior to combustion for samples other than ultrapure water and measurements of very high molecular weight compounds, due to deterioration of various components over time. Nevertheless, he says the economic impact of various oxidation methods is difficult to quantify.
Although persulfate oxidation is preferred over combustion for some samples, the technology has shortcomings for very low-concentration measurements. According to Christopher Smith, senior product manager at Lab Synergy (Goshen, NY), Analytik Jena’s North American distribution partner, trace levels of carbon in the persulfate reagent may interfere with measurements or create difficulties in preparation of blanks. “It’s hard to get persulfate clean enough,” he says. “The reagent’s carbon content becomes a limiting analysis factor.”
Lab Synergy has solved this problem by employing a very high energy, dual-wavelength UV source that oxidizes carbon-containing samples without persulfate.
Another Analytik Jena innovation is VITA, a software algorithm that compensates for changes in the flow of gases past the IR detector, which is flowdependent. This approach, Mr. Smith says, allows analysts to “vary sample size without affecting precision analysis. You can even achieve multipoint calibration with a single standard.”