Teknik Pengukuran TOC

Untuk memahami proses analisa dengan lebih baik, beberapa terminologi dasar harus dipahami terlebih dahulu.

Total Carbon (TC) – semua karbon di dalam sampel, termasuk inorganik dan organik karbon

Total Inorganic Carbon (TIC) – sering dianggap sebagai inorganic carbon (IC), karbonat, bikarbonat, dan karbon dioksida terlarut; suatu material diperoleh dari sumber non-hidup.

Total Organic Carbon (TOC) – material yang berasal dari pembusukan tanaman, pertumbuhan bakteri, dan aktivitas metabolikdari organisme hidup atau bahan kimia.

Non-Purgeable Organic Carbon (NPOC) – bisa dianggap sebagai TOC; karbon organik yang ada di dalam sampel yang diasamkan setelah sampel dicuci dengan gas.

Purgeable (volatile) Organic Carbon (POC) – karbon organik yang telah dihilangkan dari netral, atau sampel yang diasamkan dengan mencuci dengan gas inert. Ini merupakan senyawa yang sama dengan Volatile Organic Compounds (VOC) dan biasa ditentukan oleh Purge and Trap Gas Chromatography.

Dissolved Organic Carbon (DOC) – karbon organik yang ada dalam sampel setelah sampel di filtrasi, biasanya menggunakan filter 0.45 micrometer.

Suspended Organic Carbon – also called particulate organic carbon (PtOC); the carbon in particulate form that is too large to pass through a filter.

Flow Chart displaying the relationship of carbon components

Figure 1: Flow Chart displaying the relationship of carbon components

Since all TOC analyzers only actually measure total carbon, TOC analysis always requires some accounting for the inorganic carbon that is always present. One analysis technique involves a two-stage process commonly referred to as TC-IC. It measures the amount of inorganic carbon (IC) evolved from an acidified aliquot of a sample and also the amount of total carbon (TC) present in the sample. TOC is calculated by subtraction of the IC value from the TC the sample. Another variant employs acidification of the sample to evolve carbon dioxide and measuring it as inorganic carbon (IC), then oxidizing and measuring the remaining non-purgeable organic carbon (NPOC). This is called TIC-NPOC analysis. A more common method directly measures TOC in the sample by again acidifying the sample it to a pH value of two or less to release the IC gas but in this case to the air not for measurement. The remaining non-purgeable CO2 gas (NPOC)contained in the liquid aliquot is then oxidized releasing the gases. These gases are then sent to the detector for measurement.

Whether the analysis of TOC is by TC-IC or NPOC methods, it may be broken into three main stages:

  1. Acidification
  2. Oxidation
  3. Detection and Quantification

The first stage is acidification of the sample for the removal of the IC and POC gases. The release of these gases to the detector for measurement or to the air is dependent upon which type of analysis is of interest, the former for TC-IC and the latter for TOC (NPOC).

[edit] Acidification

The removal and venting of IC and POC gases from the liquid sample by acidification and sparging occurs in the following manner.

Acidification and Purging Equation.jpg

[edit] Oxidation

The second stage is the oxidation of the carbon in the remaining sample in the form of carbon dioxide (CO2) and other gases. Modern TOC analyzers perform this oxidation step by one several processes:

  1. High Temperature Combustion
  2. High temperature catalytic (HTCO) oxidation
  3. Photo-oxidation alone
  4. Thermo-chemical oxidation
  5. Photo-chemical oxidation
  6. Electrolytic Oxidation

 

[edit] High Temperature Combustion

Prepared samples are combusted at 1,350o C in an oxygen rich atmosphere. All carbon present converts to carbon dioxide, flows through scrubber tubes to remove interferences such as chlorine gas, and water vapor, and the carbon dioxide is measured either by absorption into a strong base then weighed, or using an Infrared Detector[3]. Most modern analyzers use non-dispersive infrared (NDIR) for detection of the carbon dioxide.

 

[edit] High Temperature Catalytic Oxidation

A manual or automated process injects the sample onto a platinum catalyst at 680o C in an oxygen rich atmosphere. The concentration of carbon dioxide generated is measured with a non-dispersive infrared (NDIR) detector.[4]

 

A HTCO combustion tube packed with platinum catalyst

Figure 2: A HTCO combustion tube packed with platinum catalyst

Oxidation of the sample is complete after injection into the furnace, turning oxidizable material in the sample into gaseous form. A carbon-free carrier gas transports the CO2, through a moisture trap and halide scrubbers to remove water vapor and halides from the gas stream before it reaches the detector. These substances can interfere with the detection of the CO2 gas. The HTCO method may be useful in those applications where difficult to oxidize compounds, or high molecular weight organics, are present as it provides almost complete oxidation of organics including solids and particulates small enough to be injected into the furnace. The major drawback of HTCO analysis is its unstable baseline resulting from the gradual accumulation of nonvolatile residues within the combustion tube. These residues continuously change “TOC” background levels requiring continuous background correction. Because aqueous samples are injected directly into a very hot , usually quartz, furnace only small aliquots ( less than 2 milliliters and usually less than 400 micro-liters) of sample can be handled making the methods less sensitive than chemical oxidation methods capable of digesting as much as 10 times more sample. Also, the salt content of the samples do not combust, and so therefore, gradually build a residue inside the combustion tube eventually clogging the catalyst resulting in poor peak shapes, and degraded accuracy or precision.

[edit] Photo-Oxidation (UV Light)

In this oxidation scheme, ultra-violet light alone oxidizes the carbon within the sample to produce CO2. The UV oxidation method offers the most reliable, low maintenance method of analyzing TOC in ultra-pure waters.

[edit] The UV/Chemical (Persulfate) Oxidation

Like the photo-oxidation method, UV light is the oxidizer but the oxidation power of the reaction is magnified by the addition of a chemical oxidizer, which usually a persulfate compound. The mechanisms of the reactions are as follows:

TOC Excitation and Oxidation Equations.jpg

The UV/chemical oxidation method offers a relatively low maintenance, high sensitivity method for a wide range of applications. However, there are oxidation limitations of this method. Limitations include the inaccuracies associated with the addition of any foreign substance into the analyte and samples with high amounts of particulates. By performing “System Blanks”, which is to analyze then subtract the amount of carbon contributed by the chemical additive, helps lower inaccuracies but analyses in levels below 200ppb TOC are still difficult.

 

[edit] Thermo – Chemical (Persulfate) Oxidation

Also known as heated persulfate, the method utilizes the same free radical formation as UV persulfate oxidation except uses heat to magnify the oxidizing power of persulfate. Chemical oxidation of carbon with a strong oxidizer, such as persulfate, is highly efficient, and unlike UV, is not susceptible to lower recoveries caused by turbidity in samples. The analysis of system blanks, necessary in all chemical procedures, is especially necessary with heated persulfate TOC methods because the method is so sensitive that reagents cannot be prepared with carbon contents low enough to not be detected. Persulfate methods are used in the analysis of wastewater, drinking water, and pharmaceutical waters. When used in conjunction with sensitive NDIR detectors heated persulfate TOC instruments readily measure TOC at single digit parts per billion up to hundreds of parts per million depending on sample volumes.

[edit] Detection and Quantification

Accurate detection and quantification are the most vital components of the TOC analysis process. Conductivity and non-dispersive infrared (NDIR) are the two common detection methods used in modern TOC analyzers.

[edit] Conductivity

There are two types of conductivity detectors, direct and membrane. Direct conductivity provides an inexpensive and simple means of measuring CO2. This method has good oxidation of organics, uses no carrier gas, is good at the parts per billion (ppb) ranges, but has a very limited analytical range. Membrane conductivity relies upon the same technology as direct conductivity. Although it is more robust than direct conductivity, it suffers from slow analysis time. Both methods analyze sample conductivity before and after oxidization, attributing this differential measurement to the TOC of the sample. During the sample oxidization phase, CO2 (directly related to the TOC in the sample) and other gases are formed. The dissolved CO2 forms a weak acid, thereby changing the conductivity of the original sample proportionately to the TOC in the sample. Conductivity analyses assume that only CO2 is present within the solution. As long as this holds true, then the TOC calculation by this differential measurement is valid. However, depending on the chemical species present in the sample and their individual products of oxidation, they may present either a positive or a negative interference to the actual TOC value, resulting in analytical error. Some of the interfering chemical species include Cl-, HCO3-, SO32-, SO2-, ClO2-, and H+. Small changes in pH and temperature fluctuations also contribute to inaccuracy. Membrane conductivity analyzers have tried to improve upon the direct conductivity approach by incorporating the use of hydrophobic gas permeation membranes to allow a more “selective” passage of the dissolved CO2 gas. While this has solved certain problems, membranes have their own particular limitations, such as with true selectivity, clogging and, more undetectably, they provide secondary sites for other chemical reactions, which are prone to display “false negatives,” a condition far more severe than “false positives” in critical applications. Micro leaks, flow problems, dead spots, microbial growth (blockage) are also potential problems. Most disconcerting is the inability of membrane methods to recover to operational performance after an overload or “spill” condition arises to over range the instrument, often taking hours before returning to reliable service and recalibration, just when accuracy of TOC analysis is most critical to operators for quality control.

[edit] Non-dispersive infrared (NDIR)

The non-dispersive infrared analysis (NDIR) method offers the only practical interference-free method for detecting CO2 in TOC analysis. The principal advantage of using NDIR is that it directly and specifically measures the CO2 generated by oxidation of the organic carbon in the oxidation reactor, rather than relying on a measurement of a secondary, corrected effect, such as used in conductivity measurements.

A traditional NDIR detector relies upon flow-through-cell technology, the oxidation product flows into and out of the detector continuously. A region of adsorption of infrared light specific to CO2, usually around 4.26 µm (2350 cm-1), is measured over time as the gas flows through the detector. The infrared adsorption spectra of CO2 and other gases is shown in Figure 3. A second reference measurement that is non-specific to CO2 is also taken and the differential result correlates to the CO2 concentration in the detector at that moment. As the gas continues to flow into an out of the detector cell the sum of the measurements results in a peak that is integrated and correlated to the total CO2 concentration in the sample aliquot.

Plot of atmospheric transmittance in part of IR region showing CO2 absorbing wavelengths

Figure 3: Plot of atmospheric transmittance in part of IR region showing CO2 absorbing wavelengths

Recent Advances in NDIR Technology
A new advance of NDIR technology is Static Pressurized Concentration (SPC).

The exit valve of the NDIR is closed to allow the detector to become pressurized. Once the gases in the detector have reached equilibrium, the concentration of the CO2 is analyzed. This pressurization of the sample gas stream in the NDIR, a patent-pending technique, allows for increased sensitivity and precision by measuring the entirety of the oxidation products of the sample in one reading, compared to flow-through cell technology. The output signal is proportional to the concentration of CO2 in the carrier gas, from the oxidation of the sample aliquot. UV/ Persulfate oxidation combined with NDIR detection provides good oxidation of organics, low instrument maintenance, good precision at ppb levels, relatively fast sample analysis time and easily accommodates multiple applications, including purified water (PW), water for injection (WFI), CIP, drinking water and ultra-pure water analyses.

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