TOC Analyzers’ Common Methods of Detection

In modern TOC analyzers used for total organic carbon analysis, conductivity and non-dispersive infrared (NDIR) detection are the two most widely employed detection methods.

Conductivity

Conductivity detectors used by modern TOC analyzers are of two types: direct and membrane. Direct conductivity is known to be an inexpensive and uncomplicated method of measuring carbon dioxide content. There are several advantages to using a TOC analyzer, which utilizes this method, such as good oxidation of organics, no requirement of carrier gas, and good performance at the parts per billion (ppb) ranges. But, direct conductivity has a very limited analytical range.

The use of membrane conductivity in TOC analyzers is similar to direct conductivity as far as the technology used in both is concerned. Although it’s more robust than direct, the time required for analysis by the TOC analyzer slows down the process. Sample conductivity is analyzed before and after oxidization in both methods. The difference between the measurements is related to the total organic carbon present in the sample.

Non-dispersive infrared (NDIR) Method

The NDIR method is the only practical interference-free method by a TOC analyzer for carbon dioxide detection in total organic carbon analysis. The best use of NDIR is in the direct and specific measurement of carbon dioxide generated by oxidation of the organic carbon in the reactor.

NDIR TOC Analyzer Technology—Static Pressurized Concentration (SPC)

SPC is a new TOC analyzer NDIR technology. In this method, exit valve of the detector is closed so that the detector becomes pressurized. An analysis of the carbon dioxide is performed, once the gases in the detector have reach equilibrium state. This process of pressurizing the sample gas stream in the NDIR improves sensitivity and precision because it measures all of the oxidation products present in a sample at once, unlike flow-through cell technology.

The signal given out is proportional to the carbon dioxide concentration in the carrier gas, from the oxidation of the sample aliquot. Total organic carbon analysis method that combines UV/ Persulfate oxidation with NDIR detection has multiple benefits, such as relatively fast sample analysis time. TOC analyzers using this method has multiple applications including purified water (PW), water for injection (WFI), etc.

Oil Exploration

Total organic carbon analysis has relevance in oil exploration. The first TOC analyzers are used for chemical analysis to be carried out on source rock (the rock which contains hydrocarbons or is capable of generating them).

Contaminant Detection

TOC analyzers have an important role to play in the detection of contaminants in drinking water and cooling water. It finds similar applications in water used for the manufacturing of semiconductors and in pharmaceuticals.  The water may be analyzed either as an online continuous measurement or a lab-based measurement TOC analyzer.

Potable Water Purification

Potable water purification is the process in which un-chlorinated water is rendered potable (drink-worthy). The application of TOC analyzers to analyze total organic carbon in this area is also of interest in this field.

Cleaning Validation

Cleaning validation is carried out to ensure that cleaning procedures properly remove residue from equipment/facilities used for manufacturing below a predetermined level. This is important because it provides assurance about the quality of products that will be manufactured using the equipment in the future and in prevention of cross-contamination. At present, cleaning validation process is carried out with the help of other methods of analysis such as HPLC or spectrophotometric. The use of TOC analyzers for total organic carbon analysis is a new method of measuring cleaning validation samples. HPLC and spectrophotometric analysis often take a lot of time, and a lot of interferences occur during the analysis. In comparison, most modern TOC analyzers have rapid sample analysis time and can detect up to ppm and ppb. It can also be applied to on-line analysis. Using TOC analyzers, extraneous materials, such as cleaning agents and protein materials can be measured, which is not possible if other methods are used.

Detection of total organic carbon using a TOC analyzer is of importance due to the impact it may have on human health, manufacturing processes and the environment. It is a highly sensitive, non-specific measurement in which all the organic material present in a sample is considered. Therefore, by making use of TOC analyzers, organic chemical discharge to the environment in a manufacturing facility can be regulated.

Acidification

Acidification of a sample is done so that the inorganic carbon (IC) and purgeable organic carbon (POC) can be removed from it. In TC-IC (total carbon-inorganic carbon) analysis the gases are released to the detector for measurement, and in non-purgeable organic carbon TOC (NPOC) analysis the gases are released to air.

Oxidation

Oxidization is the next stage in the process and involves the release of carbon within the TOC analyzer’s chamber cell in the remaining sample in the form of carbon dioxide (CO2) and other gases. Oxidation can be performed with the help of many processes including: High Temperature Combustion and High Temperature Catalytic  Oxidation (HTCO).

High Temperature Combustion

The process involves the combustion of samples at high temperature of 1,350o C in an oxygen-rich atmosphere. The total carbon present in the sample is converted to carbon dioxide; it then moves through scrubber tubes to eliminate interferences such as chlorine gas, and water vapor to give a better reading by the TOC analyzer.

High Temperature Catalytic Oxidation

In HTCO, the sample is injected onto a platinum catalyst within the TOC analyzer with the help of a manual or automatic process. The temperature kept for the process is 680o C and the atmosphere is oxygen-rich. Carbon dioxide produced by the process is measured by using a non-dispersive infrared (NDIR) detector by the TOC analyzer.

Detection and Quantification

Detection and quantification is the final and most vital stage of the TOC analyzer’s analysis of total organic carbon. Detection in modern TOC analyzers is commonly performed either by conductivity or non-dispersive infrared (NDIR) method.
Conductivity

Most common TOC analyzers uses either one of two types of conductivity detectors: direct and membrane. Using a direct conductor is a simple and cheap method of measuring CO2. The advantages of this method are that good oxidation of organics takes place, carrier gas is not required, and the parts per billion (ppb) ranges are good. However, the analytical range that you get with this detector is very limited.

Although, the TOC analyzer technology used for membrane conductivity is the same as direct conductivity, it is more robust and analyses slowly. In both TOC analyzers’ analysis methods, conductivity of the sample is analyzed twice i.e. before and after oxidization; the difference between the two measurements is related to the total organic carbon of the sample.

Stages of NPOC Analysis by the TOC Analyzer

NPOC analysis can be divided into the following stages:

Acidification

By the process of acidification and sparging, IC and POC gases are removed from the liquid sample.

Oxidation

This is the stage in which oxidation of the carbon that remains in the sample takes place. It results in the formation of carbon dioxide and other gases within the TOC analyzer. There are several ways in which oxidation can be perfumed, such as photo oxidation. Here is a look at how a TOC analyzer that utilizes this process works.

Photo Oxidation (UV Light)

Within a TOC analyzer that utilizes this method of oxidation, ultra-violet light alone oxidizes the carbon within the sample and CO2 is produced. The advantage of using a TOC analyzer that utilizes this method is that it is most low maintenance, reliable way of analyzing total organic carbon in ultra-pure waters.

Detection and Quantification

Many TOC analyzer manufacturers commonly use detection methods, conductivity and NDIR for total organic carbon analysis.

Conductivity

TOC analyzers that measures using conductivity measures carbon dioxide either by the direct method or the membrane method. The benefits of direct conductivity are that it is an inexpensive and uncomplicated method of measuring carbon dioxide. Although membrane conductivity is more robust than direct conductivity, the analysis time is slow. There is no difference in the measurement procedure as both types of conductivity analyze the sample before and after oxidation; the difference created by oxidation is attributed to the total organic carbon of the sample.

NDIR Technology

NDIR or non-dispersive infrared is the only method of total organic carbon analysis that provides practical interference-free technique of detecting carbon dioxide.

Conductometric Methods

TOC analyzer that uses a conductometric method can be used for the measurement of carbon dioxide in the liquid phase. The calibration of conductometric detectors is stable and their sensitivity is high. The main difference between membrane and direct detectors is that the latter are susceptible to interference from ionic contamination, halogenated organic material, acids and bases.

In membrane conductivity, the membrane forms a protective barrier to interfering ions, enabling the exclusive analysis of carbon dioxide. Unlike a TOC analyzer that uses NDIR detection, the conductivity detection method displays an extremely stable calibration and is not susceptible to considerable drift over time. This means that the TOC analyzer can be calibrated less frequently without compromising on analytical performance.

In order to make conductometric measurements more stable and reliable, a membrane that passes only the carbon dioxide produced by the oxidation of organics can be used. The prevention of compounds, such as acids and bases from interfering with the carbon dioxide makes membrane conductivity a useful method; however, there are some disadvantages of using membranes as well. For instance, ‘false negatives’ can appear because membranes present secondary sites for other chemical reactions. This can be a far bigger problem than ‘false positives’ in critical applications. Growth of micro organisms is also a potential problem. The worst problem that the user of membrane conductivity TOC analyzer faces is the inability of membrane methods to become operational in the event of an overload or ‘spill’ that over ranges the TOC analyzer. The recovery time is often in hours.

NDIR Method

A TOC analyzer with NDIR detector can be used to measure carbon dioxide in the gas phase. The best quality of NDIR is that it is a direct method that specifically measures carbon dioxide. Static Pressurized Concentration (SPC) is a new NDIR technology. In this method, the detector is pressurized, and once the gases contained in the detector reach equilibrium, carbon dioxide concentration is analyzed. A TOC analyzer that utilizes this process measures all of the oxidation products contained in the sample in a single reading.

Direct Conductivity

A TOC analyzer that makes use of a direct conductivity detector offers the most convenient and compact design available. However, it loses its effectiveness in conductivity measurements of over 50 ppm/C TOC as these are not uniformly proportional to the TOC of the sample and show a lot of variation with the specific species that contain the carbon. The conductivity compensation errors relating to temperature and TOC concentration are not well-known but are very important. As in most cases it is not practical for a TOC analyzer to ascertain all the chemicals in the sample that needs to be analyzed and understand their effects, it is equally unlikely to compensate for errors due to TOC concentration conductivity and temperature.

Membrane Conductivity Detection

Membrane conductivity detection is a variation used for improving the accuracy of analysis in a TOC analyzer. In this method hydrophobic gas permeation membranes are used within the TOC analyzer so that the dissolved carbon dioxide gas passes to the “zero” water in a more ‘selective’ manner for subsequent conductivity measurement. While this technique has been helpful in some ways, membranes are not without their own particular limitations. For example, they provide a location for secondary chemical reactions, which have a tendency by a poorly designed TOC analyzer to display ‘false negatives’, which prove to be far worse than “false positives” in critical applications. Some other problems that are related to membranes include micro leaks, clogging, flow problems, true selectivity and microbial growth. Certain amines are allowed to pass; this increases the conductivity of the water loops.

Most frustrating problem with the use membrane methods is the inability of these to get back to operational performance after an overload occurs that over-ranges the TOC analyzer. Most of the times, it would be hours before a TOC analyzer with membranes returns to reliable service and recalibration. Small variations in pH value are also common contributors to inaccuracy on the data displayed by the TOC analyzer. The interference of the organic material that is not fully oxidized would remain, presenting the error of carbon dioxide detection interference on the part of the TOC analyzer.

Working of a TOC Analyzer

The amount of total organic carbon present in a sample is determined by the analyzer by acidification of the liquid sample followed by a flushing action in which nitrogen or helium is used to get rid of the inorganic carbon (IC). Only organic carbon sources remain in the sample after these processes, which are then measured.

Types of TOC Analyzers

A TOC analyzer can either be one that uses combustion or one that makes use of oxidation. This works as a test for checking water purity test, as bacteria produces organic carbon.

TOC Analyzers Using Combustion Method

In this type of TOC analyzer, the sample divided into two equal parts and one part is injected into a chamber for acidification, normally with phosphoric acid, so that all of the inorganic carbon turns into carbon dioxide. The gas is then measured with the help of a detector.

The remaining sample is injected into a combustion chamber which has a temperature of 600–700°C; sometimes the temperature is raised even up to 1200°C. Here, the reaction of carbon with oxygen within the TOC analyzer produces carbon dioxide. It’s then flushed into a cooling chamber, and finally into the detector. Usually, a non-dispersive infrared (NDIR) spectrophotometer is used. The total IC is subtracted from the total carbon (TC) to calculate the amount of organic carbon.

TOC Analyzers Using Chemical Oxidation

A chemical oxidation TOC analyzer injects the sample into a chamber with phosphoric acid followed by persulfate. The analysis is a two step process. In the first step, IC is removed by acidification and purging. Then, persulfate is added to the sample which is either heated or bombarded with UV light from a mercury vapor lamp. Free radicals are formed from the persulfate. These radicals react with any carbon present to form carbon dioxide. The carbon formed in both steps is either passed through membranes which measure conductivity changes, or is detected by a sensitive NDIR detector. Total organic carbon calculation by the TOC analyzer is the same in both methods.

TOC Analyzers that Use NDIR (Non-Dispersive Infrared)

TOC analyzer “drift” is mainly associated with the NDIR which is the most vital component of the analyzer and is, in itself, also an analyzer, and is affected by all related instrumental performance anomalies. The NDIR must function properly at all times, and must inform the user if one or more of its critical components are malfunctioning. Some NDIR critical components include: infrared sources, infrared detectors and choppers, if used.

If the NDIR cannot self- compensate for drift which results from malfunctions, then you will need to perform frequent calibrations on the TOC analyzer.

TOC Analyzer Ranges

In many TOC analyzers’ total organic carbon analysis, it is important to also consider range selection and implementation pitfalls. Usually, the TOC is used within nominal limits while monitoring, until an ‘upset’ in the process occurs. During this time, there may be more than a ten-fold increase in TOC values. It is the desire of the Plant Operator to achieve both maximum precision in the normal operating range and the ability to track TOC during the “upset”, in order to increase control.

The TOC analyzer’s pitfalls associated with range selection and implementation are as follows: If only electronic scaling were done for the two ranges, it would result in inaccuracies at the lower, normal operating range attributed to the “full-scale” higher TOC range. Typically, the TOC analyzer chemical calibration would be “full-scale” at the highest range. If you use a higher range of 0 -10,000 ppm in process upset and normal range of 0-1,000 ppm, the resulting inaccuracy in at the normal operating range would be 1,000 +/-200 ppm, or +/-20 %.

There can be bigger problems. For instance, if you’re using a UV/PERSULFATE analyzer, you may experience the phenomena of “carbon-loading” and competing reactions which can actually cause a “down-turn” of analyzer TOC results, with the TOC measurements being considerably lower than actual TOC data. This pitfall in TOC analyzer’s analysis can be avoided by using two truly distinct ranges, each calibrated “full-scale” with appropriate different chemical calibration solutions.

Carrier Gas

The carrier gas may be air, nitrogen or oxygen. The presence of an unstable carrier or failure to control the carrier as a critical parameter will badly affect the accuracy of the TOC analyzer’s analysis. The carbon dioxide produced in the oxidation that takes place in the reactor is measured by non-dispersive infrared (NDIR) technology as a volume percent of the total gases that are emitted by the reactor. Thus, variation in the carrier gas flow will induce an error in the TOC analyzer’s analysis. In order to avoid this pitfall, a Mass & Flow Controller can be used. In terms of maintenance, loss of carrier gas can adversely affect the components of the TOC analyzer. In the UV/ Persulfate Reactor, this loss would mean that the corrosive persulfate, acid and sample would be forced up into capillaries, pressure gages and flowmeters.
Reactors

Reactors

UV/Persulfate Reactor

The UV lamp is the most important component of a UV/PERSULFATE reactor. The TOC analyzer’s UV lamp should be able to completely break the bond between the carbon atoms and should be able to produce an oxidation reaction in which all the carbon is converted into carbon dioxide. For this, a power spectral density of 185 nm and 254 nm is required which the TOC analyzer manufacturers must provide. Insufficient UV energy will affect the calculations in TOC analysis.

High Temperature Combustion Reactor

A high temperature combustion reactor has more maintenance requirements and failure modes than the UV/Persulfate Reactor. Furnace temperature control and monitoring features from the TOC analyzer are required. The ‘overtemprature’ alarm should go off and shut down must occur as this prevents reactor/furnace assembly destruction. Catalyst poisoning monitoring is important as it can cause an error in the results of TOC analysis. You should also be aware of the condition of the reactor assembly and problems like reactor clogging. Monitoring of the reactor assembly will help in locating sub-system leaks and condition of the structure within the TOC analyzer. Neglecting reactor assembly monitoring will result in destruction of components and loss of data.

Two methods of oxidation are used by TOC analyzers for total organic carbon analysis: UV/Persulfate and High Temperature Combustion. The former is recommended for low level of analysis by the TOC analyzer, and the later for high levels of carbon for TOC analyzers suited for this job. Given below are some of the failure modes, failure effects and steps that need to be taken for ‘Fail-Safe’ functioning of the critical systems and subsystems of a conventional High Temperature Combustion TC (Total Carbon) / TOC analyzer and a conventional Process UV/Persulfate TOC analyzer.

Failure Modes, Effects and Remedies

Sample System

Delivery System Malfunction

Loss of sample is considered a serious problem. The sample should be measured at the sparger and at the outlet of the Reactor if you’re using UV/Persulfate systems, or after syringe or aspirated sample is delivered, in case of a high temperature combustion reactor. If the loss of sample goes undetected at the sparger, a clog in the tube at the sparger inlet would only allow the acid reagent to pass, hence a sensor placed at the outlet of the sparger, or the inlet of the reactor will be detecting the acid in place of the sample that it should detect. Some TOC analyzers avoid this pitfall by using a loss-of-sample detector at sparger.

Similarly, if loss of sample detection were not done at the reactor outlet of a UV/ persulfate TOC analyzer, a clog in the reactor or line break downstream of the TOC analyzer would remain undetected and negatively affects the results.

Reagent

Just like the loss of sample, loss of reagents is a serious problem. Loss of acid reagent will result in the inorganic carbon not been removed properly, which will interfere with the TOC analysis by providing results that show more total organic carbon than the actual amount present in the sample. If loss of persulfate reagent occurs in UV/Persulfate system, the TOC measurements will be considerably less than the actual TOC, except for ultra-pure water applications. Any leak in fitting or sample line could cause serious damage to analyzer and components. In order to avoid these pitfalls, the TOC analyzer should monitor acid and persulfate reagent consumption.