Performance catalytic Ozonation
Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream
Mohammad Reza Samarghandi (PhD)a, Seyed Alireza Babaee (MSc)a, Mohammad Ahmadian (MSc)b, Ghorban Asgari (PhD)a, Farshid Ghorbani Shahna (PhD)c, and Ali Poormohammadi (MSc)d*
a Research Center for Health Sciences and Department of Environmental Engineering, School of Public Health, Hamadan University of Medical Sciences, Hamadan, Iran
b School of Public Health, Kermanshah University of Medical Sciences, Kermanshah, Iran
c Department of Occupational Health Engineering, School of Public Health and Research Center for Health Sciences, Hamadan University of Medical Sciences, Hamadan, Iran
d Social Development & Health Promotion Research Center, Kermanshah University of Medical Sciences, Kermanshah, Iran
* Correspondence: Ali Poormohammadi (MSc), E-mail: apoormohammadi000@yahoo.com
Received: 13 October 2013, Revised: 17 February 2014, Accepted: 17 March 2014, Available online: 06 April 2014
Abstract
Background: Toluene is a volatile organic compound, one of 189 hazardous air pollutants (HAPs) and the most important pollutant found in most industries and indoor environments; owing to its adverse health, toluene must be treated before being released into the environment.
Methods: In this research study, a continuous-flow system (including an air compressor, silica gel filters and activated charcoal, impinger, an ozone generation and a fixed bed reactor packed with the carbosieve in size 1.8-2.3 mm, specific surface: 972 m2/g,) was used. This glass reactor was 0.7 m in height; at a distance of 0.2 m from its bottom, a mesh plane was installed so as to hold the adsorbent. Moreover, 3 l/min oxygen passed through this system, 0.43 g/h ozone was prepared. The flow rate of waste airstream was 300 ml/min. The efficiency of this system for removal of toluene was compared under the same experimental conditions.
Results: Under similar conditions, performance of catalytic ozonation was better in toluene removal than that of ozonation and carbosieve alone. On average, increasing the removal efficiency was 45% at all concentrations. When carbosieve and ozone come together, their synergistic effects increased on toluene degradation.
Conclusions: Catalytic ozonation is a suitable, high-efficient and available method for removing toluene from various concentrations of waste air stream. This process due to the short contact time, low energy consuming and making use of cheap catalysts can be used as a novel process for removing various concentrations of volatile organic compounds.
Keywords: atalytic Ozonation, Volatile Organic Compounds, Carbosieve, Toluene
Introduction
Volatile Organic Compounds (VOCs) are a major group of air pollutants emitted from various sources and pollute the air. They contain more than 300 compounds such as oxygenated hydrocarbons, aromatic hydrocarbons and halogenated hydrocarbons1-3. Benzene, toluene, ethyl benzene, xylene (BTEX) compounds are the most important and common chemical compounds of aromatic hydrocarbons and are considered as indices of VOCs. They are abundantly found in cities and industrial areas and have been classified as toxic priority pollutants. Their major problem in the atmosphere is the possibility of cancer and production of "Air Toxic" 4,5. Toluene or methyl benzene is one of volatile aromatic compounds which is very similar to benzene, but less volatile. This compound has a methyl branch on a benzene ring. Toluene is transparent, colorless and volatile liquid with obnoxious odor. As a solvent, toluene is highly used in oils, paints, resins, detergents and glues and pastes. It is also used in gasoline, petrol and other aviation fuels to raise octane number.
According to ACGIH, toluene is one of 189 hazardous air pollutants and is a primacy and prioritized pollutant which has been classified in A4 group of carcinogenic substances6-8. Due to various environmental and health effects of this compound, different methods (including adsorption, thermal and catalytic oxidation and advanced oxidation processes) have been examined to remove it from waste air stream9,10. In comparison with conventional treatment methods, Advanced Oxidation Processes (AOPs) are suitable methods for degradation of toxic air pollutants. Degrading pollutants by AOPs is based on production of free active radicals that have high oxidative power and are capable of changing most organic compounds to minerals. The radicals by attaching molecules of organic substances and taking their hydrogen ion, convert these compounds to minerals, during one or some steps.
Catalytic ozonation process (COP) is a new method of single ozonation process (SOP) that has recently been taken into consideration in air pollution control industry. In this process, to improve SOP efficiency, various substances are used as catalysts to accelerate degradation process, increase reaction time and be sure of non-presence of ozone residues in exhaust air11-15. So far some catalysts such as activated carbon, silica gel, metal oxides and various resins have been used in catalytic ozonation process to remove toluene from polluted air16-18.
According to capabilities of various carbon adsorbents in treating air pollutants, carbosieve, a kind of carbon-based adsorbent which is produced as a result of pyrolysis of synthetic polymers or some of petroleum compounds with high porosity and specific surface area (BET) and is mostly used to adsorb volatile hydrocarbons19,20.
In this study, the carbosieve was used as a non-polar catalyst and adsorbent in catalytic ozonation process.
Methods
Experimental Pilot
This empirical laboratory study was carried out as a pilot scale from December 2012 to May 2013. A laboratory-scale pilot was designed and launched at Hamadan Environmental Chemistry Laboratory (Figure 1), western Iran. Toluene liquid with purity of 99.5% was prepared from German Merck Company. Air containing polluted gas and ozone gas entered bottom of a reactor with a continuous flow. Considering the aim of this study the reaction conditions were closer to the actual conditions thus a dynamic system with continuous flow was used. Contaminant gas in the first of the reactor enters into the bottom of the reactor and exits from the other side (Due to the absence of gas leaks in the system). To prevent condensation of contaminants in the reactor, all parts of the reactor was placed in isothermal chamber. Input air before entering to the bed enter to the converging chamber and after passing through reticulated plate and finally enter to the catalyst bed.
This glass reactor was 0.7 m in height; at a distance of 0.2 m from its bottom, a mesh plane was installed so as to hold the adsorbent. Required air was supplied by a compressor equipped with oil trap. After the pressure was adjusted, the air passed through a combined column of silica gel and activated charcoal in order to dry and remove any organic contamination. To prepare toluene-containing air in given concentrations, Standard Atmosphere Production method was used in a dynamic way with the aid of an impinger21. In this system, the air, produced by a compressor and dried as well as cleaned before, was divided into two branches; calibrated rotameter and needle valve were used to control carefully in the air stream in each branch so as to achieve certain concentrations. In one branch, toluene vapors entered air stream through impinger-bubbler (by keeping temperature of toluene constant) and in the other branch, there was pure stream of air. These two branches lead to a single branch in which a standard stream of air with the given concentration of toluene was created. The reactor was placed in the isothermal chamber for controlling temperature in the reactor. Our concept from constant pressure was atmospheric pressure in the laboratory environment. Dynamic method was used to create standard atmosphere. Inert and contaminant gas at determined concentration enter to the system with a regulated and specific ratio. This works was performed by maintain impinger temperature (Containing liquid toluene due to its vapor pressure) by a hot water bath. Besides for keeping a steady flow and to avoid concentration fluctuations a baffled mixing chamber was used before entering flow to the main reactor. However, in practice, only minor changes in concentration levels (a few ppm) was existed. Therefore, input and output load in the system was reported as removal efficiency.
Ozone was made using an ozone-making machine, ARDA, model COG-OM. In this generator, ozone is produced through creating electrical discharge with alternating current in a sluice in presence of oxygen. Oxygen required for the ozone generator was supplied by an oxygen-making machine, model PORSA VF-3 with high purity level and the ability to adjust oxygen injection level. The amount of ozone produced by an ozone generator was measured according to Iodometry method in the presence of 2% potassium iodide solution after 10 min of contact22,23. As 3 l/min oxygen passed through this system, 0.43 g/h ozone was prepared. In ozonation unit, ozone gas entered toluene-containing waste air stream; to prevent and reduce the probable fluctuations and to equalize and fix equilibrium, a baffled mixing chamber is used before the reactor which contains adsorbent bed. After current exits this mixing chamber, it enters carbosieve-containing reactor as a catalyst and non-polar adsorbent bed. Prior to experiments, carbosieve was placed in a in an oven for 2 h at 400 °C and was dried completely in order to remove all probable pollution. Carbosieve used in this study was purchased from Air Tools Co. in Tehran. Then the surface area was measured by BET method. Ten g of Carbosieve were used at each stage of the experiment. Although changes in pressure drop has not been measured before and after bed but the system was designed in such a way do not create the pressure drop in the input stream.
Sample Analysis
Toluene concentration was measured before and after experiments in an air stream using 1501 NMAM (NIOSH Manual Analytic Method) and gas-chromatography device, model SHIMADZU 2010, equipped with flame ionization detector (FID) with a column length of 60 m, column internal diameter of 0.25 mm, film thickness of 0.25 mm and temperature range of 50 to 180 °C according to temperature programming. Optimum analysis conditions of gas chromatography was prepared with injection site temperature of 200 °C, detector temperature of 250 °C, carrier gas flow of approximately 30 ml/min, hydrogen gas flow of 30 ml/min and air stream of about 300 ml/min 24,25. Toluene vapors collected with activated charcoal tube using sampling pump accordance with Method 150124. After extracting the samples in carbon disulfide 1 μL of extracted sample was injected into the GC in quick time. Experiments were performed in normal ambient temperature (27 ±2 °C) and desirable atmospheric pressure and ventilation. In order to assess the accuracy of the analysis method, was injected three times into the capillary column of gas chromatograph and their mean was reported as the amount of that parameter. The efficiency of carbosieve bed, single ozonation and catalytic ozonation were examined and compared separately.
Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream | Samarghandi | Journal of Research in Health Sciences
http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html
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Figure 1: Schematic diagram of the experimental setup |
Results
Efficiency of carbosieve bed
Parameters affecting system efficiency including air flow discharge entering the reactor, adsorbent bed size, ozone level and relative humidity were kept constant at 3 l/min, 15g, 0.43 g/h and 5% respectively; only system efficiency was examined on various concentrations of toluene in range of 20 to 200 ppm. As toluene concentration increased, bed saturation time and absorbent breakpoint decreased. Results of carbosieve efficiency of toluene removal are shown in Figure 2 (A).
Ozone efficiency
Figure 2(B) shows the removal efficiency of toluene from waste air stream by the single ozonation process separately and without the presence of carbosieve. In this step, experiments conducted in various concentrations of toluene in range of 20-200 ppm and only ozone gas with 3 1/min input of pure oxygen to ozone-making machine and production of 0.43 g/h ozone was used, also other variables affecting system efficiency were kept constant as before. In this step, as concentration increased from 20 to 200 ppm, toluene removal efficiency decreased. In 20 ppm of toluene in the reactor inlet, the removal efficiency reached to 31%. On the other hand, in 200 ppm of toluene in the reactor inlet, ozone efficiency was about 16%. Therefore, the highest efficiency of process was observed in toluene concentration equal to 20 ppm and lowest removal efficiency was in toluene concentration equal to 200 ppm.
Efficiency of catalytic ozonation
Results of efficiency of catalytic ozonation are shown in Figure 3, 4. In this step, experiment was performed under the same conditions and efficiency of catalytic ozonation was examined in terms of toluene output concentration and adsorbent break point. In this step, efficiency of catalytic ozonation was examined on various concentrations of toluene in range of 20-200 ppm. In 20 ppm of toluene, the efficiency of catalytic ozonation in removal of this compound reached about 16 ppm (80%). On the other hand, in 200 ppm of toluene was observed 155 ppm (77.5%). Therefore, by increasing toluene concentration from 20 to 200 ppm, toluene removal efficiency decreased. Moreover, carbosieve saturation and break point were occurred significantly later than single carbosieve system.
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Figure 2: (A) Efficiency of carbosieve bed in removing toluene from waste air stream (B) Ozone efficiency (single ozonation) on removal of toluene from waste air stream |
Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream | Samarghandi | Journal of Research in Health Sciences
http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html
화면 캡처: 2016-05-13 오전 9:01
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Figure 3: Efficiency of catalytic ozonation in removal of toluene from waste air stream |
Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream | Samarghandi | Journal of Research in Health Sciences
http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html
화면 캡처: 2016-05-13 오전 9:02
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Figure 4: Comparison between times of toluene emergence in carbosieve system with COP process in the different concentrations |
Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream | Samarghandi | Journal of Research in Health Sciences
http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html
화면 캡처: 2016-05-13 오전 9:03
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Figure 5: A schematic of toluene decomposition in the catalytic ozonation over the carbosieve |
Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream | Samarghandi | Journal of Research in Health Sciences
http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html
화면 캡처: 2016-05-13 오전 9:03
Discussion
This study investigated the efficiency of catalytic ozonation process in treating toluene-containing air as one of hazardous air pollutants. Toluene removal was examined separately on carbosieve bed, single ozonation and catalytic ozonation.
Results revealed that removal efficiency by SOP process was, on average, less than 23% at given concentrations; that is, as the concentration increased from 20 ppm at the beginning of the system to 200 ppm, removal efficiency by SOP process decreased from 31% to 16%. In a similar study by Mousavi et al. on removal of xylene from waste air stream, xylene removal efficiency by ozone gas was about 10% 26. On removal of volatile organic compounds (VOCs) using catalytic ozonation that removal efficiency of volatile organic compounds using only ozone was low and thus would result in production of harmful byproducts27. Besides, obtained results are in accordance with the findings of Alvarez et al. on comparison between catalytic ozonation and activated carbon adsorption/ozone- regeneration processes for wastewater treatment28.
Decreased efficiency by SOP process is due to ozone low ability to oxidize aromatic compounds and alkenes and low retention time necessary for complete reaction between ozone and toluene as a result of continuity of the system29. According to obtained results while using carbosieve alone as concentration increased from 20 ppm to 200 ppm, adsorbent break point in the reactor outlet decreased from 10 h to 3 h (Figure 5). Accordingly, adsorbent beds including carbosieve remove the pollutants available in the air passing through absorbent due to early saturation, especially at high concentrations, necessity to reclamation and most lack of change in the nature of pollutants that have be(A) Efficiency of carbosieve bed in removing toluene from waste air stream (B) Ozone efficiency (single ozonation) on removal of toluene from waste air streams hazardous to the environment, making use of absorbents alone may be impractical.
In the present study, to remove restrictions of SOP process and to adsorb over carbosieve, the combinations of these two methods were used. Results of this study indicate that catalytic ozonation has a significant efficiency compared with separate adsorption methods over carbosieve and SOP process. This advantage is show in Figure 4. Accordingly, as concentration increased from 20 to 200 ppm, absorption break point and removal of toluene from hybrid reactor decreased from 12 to 6 h, while this time was between 3 and 10 h for carbosieve. ThusEfficiency of catalytic ozonation in removal of toluene from waste air stream was higher in hybrid reactor output and COP process under similar laboratory conditions. For example, for concentration of 200 ppm, toluene discharge time increased by 50% compared to carbosieve bed; on average, efficiency in COP process increased by 45% at all concentrations compared with SOP. In carbosieve system, after the bed was saturated at reactor outlet, output toluene concentration was equal to input concentration, while in COP process concentration of output toluene was less than input concentration after saturation; it reflects that combining carbosieve and ozone systems strengthens the effect of them and thus results in more molecular degradation of toluene passing over bed. Xylene removal level increased 22% in COP process compared to activated charcoal and ozone26. Similarly, Wung et al. revealed that toluene removal level increased 20% and 40% in COP process in comparison with zeolite adsorption and MCM-41 respectively. Furthermore, efficiency of ozonation alone in removal nitrobenzene from aqueous solution was about 25% and while was used of COP, the process efficiency reached to about 70%; these results are consistent with results of this seA schematic of toluene decomposition in the catalytic ozonation over the carbosieve">27,30.
Since ozone is adsorbed over adsorbent, carbosieve adsorbs ozone which rapidly decomposes to activated oxygen atoms over adsorbent bed due to its instability. In other words, carbosieve acts as a catalyst and decomposes ozone into activated oxygen atoms. Having specific surface area and high porosity, carbosieve creates a surrounding bed; in addition to increasing contact surface and collision of toluene and ozone molecules, carbosieve increases retention time for reaction and thus improves removal efficiency.
In reaction between ozone and positions of acid Luis of adsorbent, ozone is decomposed into activated oxygen atoms which decompose toluene into CO2 and H2O through attacking single connective bonds between carbon atoms (C-C) or connective bonds between carbon-hydrogen atoms (C-H). Since, in practice, reaction between toluene and ozone is not done completely and may result in production of intermediate compounds, these compounds are again adsorbed by carbosieve (due to its high specific surface) and are decomposed by ozone or by the produced activated radicals (Figure 5).
No intermediate compound was observed in COP process output26,27. In positions of acid Luis of adsorbents, toluene is decomposed into O2 and O• according to the following reactions:
(1) O3 + CMS – s → CMS – 3SO
(2) CMS-3Sº→CMS-3Sº-sO•+O2
(3) CMS- sO•+O3→2CMS- sO•+O2
(4) CMS- SO2 →CMS- s+O2
CMS-S and CMS-sO3, respectively, reflect positions of acid Luis and ozone molecules adsorbed over carbosieve. CMS-sO3 and CMS-sO• show types of carbosieve surface oxides. CMS-sO• can also react with water vapor available in the air entering the system and can produce activated hydroxyl radicals (OHº) which indirectly take part in decomposition and oxidation of toluene and thus improve removal efficiency.
(5) CMS-S O•+H2O →CMS-S(OH)2•
When toluene- and ozone-containing air passes over carbosieve, catalysed reactions of 1-5 occur and toluene is directly and indirectly removed from waste air stream by ozone and radicals resulted from ozone decomposition. In the following equations, R represents intermediate compounds produced during direct and indirect oxidation reactions.
Direct oxidation:
(6) CMS- SO3+ C6H5CH3 →CO2 + H2O + R
(7) CMS- SC6H5CH3+O3 →CO2 + H2O + R
(8) C6H5CH3 +O3 → CO2 + H2O + R
Indirect oxidation:
(9) CMS-S O•+ C6H5CH3→ CO2 + H2O + R
(10) CMS-S(OH•)2+ C6H5CH3 → CO2 + H2O + R
Since the system used in this study was operated in dry mode, hydroxyl radicals are less likely to be formed so they cannot have an important and vital role in toluene decomposition. In COP process, in addition to ozone and radicals resulted from its decomposition, oxygen molecules play a significant role in toluene decomposition. In the presence of oxygen, ozone decomposition rate increases for more toluene oxidation. In autoxidation processes, this molecule oxides radical intermediate compounds (R*) which are one of the reasons for deactivation of catalyst26,27,29,31,32.
(11) R• + O2→RO•2→→→CO2, CO
Conclusions
Removal of toluene from waste air stream, especially in higher concentrations, is more efficient by COP process than by adsorption system over carbosieve and SOP. This system can be one of promising alternatives for removing various concentrations of volatile organic compounds, especially toxic and hazardous volatile compounds, from waster air stream due to low energy consuming, making use of cheap catalysts and applying adsorbents proportional to pollutant. COP process adsorbs and destroys pollutant simultaneously; in addition to ozone O3 (E◦= 2.08 v), this process uses activated hydroxyl radicals (OHº) (E◦= 2.80 v) and atomic oxygen O• (E◦= 2.42 v) resulted from catalyzed reactions over bed. This phenomenon lead to bed chemical reclamation, increased system operating time and decreased the cost of bed reclamation. Therefore, the main benefit of catalytic ozonation is that reaction time is shortened, causing decomposition and purification of volatile organic compounds from air stream in high concentrations of ozone gas and pollutant without removal of excess ozone concentrations at the reactor outlet. However, additional studies are required to develop techniques to enhance the oxidation activity and industrial application in greater scales.
Acknowledgments
This article is extracted from the Master's thesis. The authors would like to thank Hamadan University of Medical Sciences for the financially supporting of this research.
Conflict of interest statement
This study did not have any conflict of interest statement.
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FIGURE 1. Overall removal efficiencies and the corresponding toluene decomposition fractions at various ozone inlet concentrations with NaX, NaY, and MCM-41 adsorbents. Experimental conditions: toluene, 1.5 ppmv; ozone, 0 - 80 ppmv; adsorbent mass, 2 g; residence time, 0.13 s; temperature, 23 - 25 ° C; error bars are 1 σ .
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CO/CO2 IR spectra
Carbon monoxide, CO
Carbon monoxide has a small linear molecule with a relatively short C-O bond, earlier described as a double bond but nowadays mostly as a triple bond. The valence electrons are slightly drawn towards oxygen, which makes the molecule polar.
Carbon monoxide molecule shown in two ways, from Wikipedia. The C-O triple bond has the unit pm (picometer) which means 10-12 m.
CO is an uncolored and non-smelling gas at room temperature. Carbon monoxide is highly toxic since it replaces O2 molecules in hemoglobin, causing suffocation after a short while, and the state continues even after a poisoned person is taken into fresh air. Carbon monoxide is flammable and easily forming CO2 in this reaction: 2CO(g) + O2(g) → 2CO2(g) The gas is found in all types of burning, for example car exhaust gases or smoke from a fire. It is also found in cigarette smoke. Moreover, it has been used in wartime for killing people.
IR spectrum of carbon monoxide, CO
The IR spectrum of carbon monoxide has a major absorption band at 2100 cm-1 or 4.8 µm, due to unsymmetrical vibration modes.
Theoretical IR spectrum of carbon monoxide, CO. The spectrum was simulated at: http://spectralcalc.com/info/about.php.
The IR spectrum of CO below shows an exhaust gas mixture from an oldtime car without catalytical cleaning. Nowadays, modern car exhaust contains mostly CO2 and H2O - the CO concentration is very low.
IR spectrum of 1942 Packard exhaust, showing a mixture of hydrocarbons, water, carbon monoxide and carbon dioxide. The spectrum was found on Internet in a paper by Jane A. Ganske: http://chemeducator.org/sbibs/s0008006/spapers/860353jg.htm
출처: <http://www.senseair.com/senseair/gases-applications/carbon-monoxide/>
Carbon dioxide, CO2
Carbon dioxide has a perfectly linear molecule due to the C=O double bonds.
Carbon dioxide molecule and C=O bond distance, from Wikipedia. The unit pm (picometer) is 10-12 m.
CO2 is an uncolored and non-smelling gas at room temperature which, directly freezes into solid state below -78°C ("dry ice") at normal pressure. The concentration of CO2 in atmosphere is around 0.039 %, or 390 ppm (parts per million). The gas is active in the large carbon cycle since plants use CO2 for photosynthesis and living beings evaporate CO2 when they breath.
CO2 is regarded as non-toxic at low concentrations. At higher levels it may displace oxygen in a closed space and cause a risk for suffocation. Also it directly influences on human performance and capacity when the levels are >1 % (10 000 ppm). Carbon dioxide is a major greenhouse gas and due to its steadily increase mainly caused by fossile fuel combustion, it is believed to bring on global heating. All greenhouse gases are compared to each other using the unit CO2 equivalents expressed as the Global Warming Potential (GWP), which is the heat rise a certain amount of CO2 will cause during a period of 100 years. CO2 has by convention a GWP of 1. More powerful greenhouse gases are for example methane or dinitrogen oxide, which have a GWP of 25 and 298, respectively. The GWP value for a gas molecule depends on how stable it is, how strongly it absorbs IR radiation and where in the spectrum the absorption peaks are placed.
IR spectrum of CO2
The IR spectrum of carbon dioxide has a strong absorption band consisting of many overlapping peaks. This band, caused by unsymmetrical C=O stretching, is placed at 2300 cm-1 corresponding to a wavelength of 4.3 μm.
CO2 molecules in the atmosphere absorb emitted heat radiation from the earth at this wavelength and in that way decrease cooling especially during clear nights. The mechanism of the increasing amount of carbon dioxide and other more powerful greenhouse gases is that they block IR radiation from exiting from earth back to space.
Theoretical IR spectrum of CO2 calculated with the program Spectracalc found at: http://spectralcalc.com.
출처: <http://www.senseair.com/senseair/gases-applications/carbon-dioxide/>
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INSTRUMENTAL ANALYSIS AND SEPARATIONS
Detectors for Gas chromatography - LECTURE 24
There are a number of detectors used in GC. Generally the detector produces a signal in proportion to the amount of analyte present. Some types are based on concentration and others are mass dependent. The detectors produce an electrical signal that is sent to a recording and integrating device. The signal strength and retention time are recorded for further analysis by the chemist. For analysis, the analyte must be thermally stable and volatile. However, if the analytes meets this criteria, GC is preferable to HPLC because of its much higher resolution. GC detectors also typically have lower limits of detection and higher linear dynamic range.
Flame Ionization Detector
The most commonly used detector is the flame ionization detector (FID) it is a general carbon detector. It does not detect compounds that do not contain carbon such as nitrogen(N2), oxygen(O2), or water. The presence of N, O, or S in a carbon compound will tend to decrease the response of the FID.
The Carbon atoms (C-C bonds) are burned in a hydrogen flame. The hydrogen can be supplied ether from a cylinder or from an electrolytic hydrogen generator. The hydrogen must be pure to avoid background noise. A charcoal filter is often placed in the hydrogen supply line to remove any organic contaminants.
The response of the detector depends on the flow of the hydrogen, air and the makeup gas (if it is used). A certain amount of inert gas is needed for optimum response of the detector. Generally the flow from a capillary is too low so a makeup gas is used to provided the inert gas flow. The makeup gas has other beneficial effects such as stabilizing the detector, prolonging the lifetime of the jet, and purging any unswept areas of the detector. It is also very important to adjust the air and hydrogen gas flows for optimum response.
The FID must be heated. There are two main reasons for this. First, the burning of hydrogen in air produces water, which can reduce the detector response and even put out the flame. The second reason for heating the detector is to avoid condensation and deposition of compounds in the detector.
The detector response depends on the ionization of carbon atoms. Only a small portion are actually ionized (about 1 in 10,000), but since there is such a low background signal with the FID, this is enough. The ions carry a charge from the flame to the walls of the detector which surrounds the flame. The charge is electronically amplified and sent to a recording device. The FID is very sensitive down to 10-12 g. It has a high linear dynamic range 107 and is very robust and reliable.
Nitrogen Phosphorous Detector
Another common type of detector is the nitrogen/phosphorous detector (NPD). It is sometimes called the alkali flame detector or flame thermionic detector. It uses a bead of a compound such as rubidium silicate above a jet of H2. The bead is heated by an electric current to 600-800oC forming a plasma in the region of the bead. Nitrogen and phosphorous react in this boundry layer of plasma forming specific ions that carry a small current from the plasma to the charged collector (The mechanism of NPD response is still not well understood). The NPD is electronically similar to the FID, but since a flame is not sustained in the detector, hydrocarbon ionization does not occur resulting in a high selectivity. The response to N over C is about 103 – 105 greater and response to phosphorous 104 – 10 5.5 higher than response to C. The linear dynamic range is 104. However, the detector is only fairly reliable since the bead burns up over time causing drift in the signal.
Flame Photometric Detector
The flame photometric detector (FPD) is an element specific detector which can be used for analyzing many specific compounds However, commercially available instruments are generally limited to the detection of P and S containing molecules. The analytes are burned in a H2 flame causing electrons to move to an excited unstable state. When the electrons return to the ground state, they emit a specific wavelength of light (526 nm for P and 394 nm for S). These wavelengths are monitored by a photomultiplier, amplified, and turned into an electrical signal. This detector is sensitive to 10-9g and has a linear dynamic range for P of only 103 – 104. For S, the response is non-linear. This is because the response is due to the formation of the S2 radical and therefore the response is proportional to the square of the sulfur content. Commercial instruments often have a square root adjustment function built in, but since the response can deviate from the theoretical square root relationship, the output can still be non-linear.
Electron Capture Detector
Another useful detector is the electron capture detector (ECD) It is an excellent detector for molecules containing an electronegative group such as Cl or F etc. (or derivitized molecules) It is probably the second most common detector after the FID. It is most often used for the trace measurement of halogen compounds in environmental applications for detecting insecticide and herbicide residues.
The ECD uses a radioactive source such as Ni63 which produces Beta particles which react with the carrier gas producing free electrons. These electrons flow to the anode producing an electrical signal . When electrophillic molecules are present, they capture the free electrons, lowering the signal. The amount of lowering is proportional to the amount of analyte present. It is sensitive down to 10-15 but the dynamic range is only about 104.
Atomic Emission Detector
One of the newest gas chromatography detectors is the atomic emission detector (AED). The AED is quite expensive compared to other commercially available GC detectors, but can be a powerful alternative. The strength of the AED lies in the detector's ability to simultaneously determine many of the elements in analytes that elute from the column. It uses microwave energy to excite helium molecules (carrier gas) which emit radiation which breaks down molecules to atoms such as S, N, P, Hg, As, etc. These excited molecules emit distinctive wavelengths which can be separated by a grating and sent to the detector (typically a photodiode array) which produces the electrical signal. The atomic emission detector is very sensitive (10-15) and has a dynamic range of 104.
Photoionization Detector
The photoionization detector (PID) uses a UV lamp (xenon, krypton or argon lamp, depending on the ionization potential of the analytes) to ionize compounds. The ionization produces a current between the two electrodes in the detector. The detector is non-destructive and can be more sensitive than an FID for certain compounds (substituted aromatics and cyclic compounds for example).
Thermal Conductivity Detector
The thermal conductivity detector (TCD) consists of an electrically-heated wire or thermistor. The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. Changes in thermal conductivity, such as when organic molecules displace some of the carrier gas, cause a temperature rise in the element which is sensed as a change in resistance. Low molecular weight gases have high conductivities so hydrogen and helium are often used as carrier gases. Nitrogen and argon have similar conductivities to many organic volatiles and are often not used. However if nitrogen is used as a carrier gas, the detector can be used to measure hydrogen or helium. TCD's are often used to measure lightweight gases or water (compounds for which the FID does not respond). The TCD is not as sensitive as other detectors but it is a universal detector and is non-destructive. However, modern detectors called micro-TCD's have very small cell volumes, and new electronics that produce much higher sensitivities and wider linear ranges. Due to its increased sensitivity, and the fact that it is a universal non-destructive detector, it is again becoming more popular for certain applications.
출처: <http://fshn.ifas.ufl.edu/faculty/MRMarshall/fos6355/handout/lec24.gc_detector.doc>
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------------ Property Broker ------------
Model: mrthane.mol
ChemPropPro: Boiling Point = 0 Kelvin
ChemPropPro: Critical Pressure = 0 Bar
ChemPropPro: Critical Temperature = 0 Kelvin
ChemPropPro: Critical Volume = 0 cm^3/mol
ChemPropPro: Gibbs Free Energy = 0 kJ/mol
ChemPropPro: Heat Of Formation = 0 kJ/mol
ChemPropPro: Henry's Law Constant = -1.429
ChemPropPro: Ideal Gas Thermal Capacity = 0 J/(mol.K)
ChemPropPro: LogP = 1.09
ChemPropPro: Melting Point = 0 Kelvin
ChemPropPro: Mol Refractivity = 6.347 cm^3/mol
ChemPropPro: Vapor Pressure = 103322.998 Pascal
ChemPropPro: Water Solubility = 24.4 mg/L
ChemPropPro: Full Report:
************************************************************************
Data from database
************************************************************************
<Name of molecule><RE PUBLISHING CORP.,NY,(l989).
<**********
Log(p)........: 1.09
St..deviation.: 0.47
by Crippen's fragmentation: J.Chem.Inf.Comput.Sci.,27,21(1987).
Log(p)........: 1.09
St..deviation.: 0.49
by Viswanadhan's fragmentation: J.Chem.Inf.Comput.Sci.,29,163(1989).
Estimation using Broto's fragmentation method
C Non available value
Log(p) can't be calculated by Broto's method:
Eur.J.Med.Chem.- Chim.Theor.,19,71(1984).
************************************************************************
Estimation of Molar Refractivity
************************************************************************
MR............: 6.88 [cm.cm.cm/mol]
St..deviation.: 1.27
by Crippen's fragmentation: J.Chem.Inf.Comput.Sci.,27,21(1987).
MR............: 6.35 [cm.cm.cm/mol]
St..deviation.: 0.77
by Viswanadhan's fragmentation: J.Chem.Inf.Comput.Sci.,29,163(1989).
************************************************************************
Estimation of Henry's Constant (H)
************************************************************************
1. method: H = -1.229 log[unitless]
Estimation of mean error..: 0.0620
2. method: H = -1.260 log[unitless]
Estimation of mean error..: 0.200
ChemPropStd: Formal Charge = 0
ChemPropStd: Connolly Accessible Area = 147.937 Angstroms Squared
ChemPropStd: Connolly Molecular Area = 44.35 Angstroms Squared
ChemPropStd: Connolly Solvent Excluded Volume = 24.335 Angstroms Cubed
ChemPropStd: Exact Mass = 16.0313001284 g/Mol
ChemPropStd: Mass = 16.04246
ChemPropStd: Mol Weight = 16.04246
ChemPropStd: Ovality = 1.09208204264751
ChemPropStd: Principal Moment = 3.303 3.303 3.303
ChemPropStd: Elemental Analysis = C, 74.87; H, 25.13
ChemPropStd: m/z = 16.03 (100.0%), 17.03 (1.1%)
ChemPropStd: Mol Formula = CH4
ChemPropStd: Mol Formula HTML = CH<sub>4</sub>
CLogP Driver: Mol Refractivity = 0.641199946403503
CLogP Driver: Partition Coefficient = 1.1029999256134
GAMESS Interface: Cp = 8.194 cal/(mol K)
GAMESS Interface: Cv = 6.207 cal/(mol K)
GAMESS Interface: Enthalpy = 31.086 Kcal/Mol
GAMESS Interface: Entropy = 49.479 cal/(mol K)
GAMESS Interface: Gibbs Free Energy = 16.334 Kcal/Mol
GAMESS Interface: Internal Energy = 30.494 Kcal/Mol
GAMESS Interface: Dipole = 0 Debye
GAMESS Interface: Harmonic Zero Point Energy = 0 Kcal/Mol
GAMESS Interface: Kinetic Energy = 24925.5722 Kcal/Mol
GAMESS Interface: Potential Energy = -50009.9336 Kcal/Mol
GAMESS Interface: Total Energy = -25084.3614 Kcal/Mol
Charges (Lowdin Charges)-GAMESS Interface:
C(1) -0.378865
H(2) 0.094718
H(3) 0.094714
H(4) 0.094714
H(5) 0.094718
GAMESS Interface: Lowdin Populations (Lowdin Populations)-GAMESS Interface:
C(1) 6.378865
H(2) 0.905282
H(3) 0.905286
H(4) 0.905286
H(5) 0.905282
Charges (Mulliken Charges)-GAMESS Interface:
C(1) -0.773602
H(2) 0.193402
H(3) 0.193399
H(4) 0.193399
H(5) 0.193402
GAMESS Interface: Mulliken Populations (Mulliken Populations)-GAMESS Interface:
C(1) 6.773602
H(2) 0.806598
H(3) 0.806601
H(4) 0.806601
H(5) 0.806598
GAMESS Interface: Frequencies = 0 0.01 502.48 502.54 502.58 1558.71 1558.72 1558.76 1746.61 1746.64 2925.64 2995.23 2995.24 2995.27
GAMESS Interface: Polarizibility:
AXIAL COMPONENTS WITH BASE FIELD OF 0.0010
ENERGY-BASED DIPOLE-BASED
DIPOLE Z: 1.2028308E-05 1.2028008E-05
ALPHA XZ: 3.4799825E-04
ALPHA YZ: -2.4807436E-04
ALPHA ZZ: 1.2137664E+01 1.2137767E+01
BETA XZZ: 2.5549004E+01
BETA YZZ: 1.6897140E+01
BETA ZZZ: 8.9251628E-01 8.9302229E-01
GAM ZZZZ: 7.8841822E+01 8.3879350E+01
Molecular Topology: Balaban Index = 0
Molecular Topology: Cluster Count = 1
Molecular Topology: Molecular Topological Index = 0
Molecular Topology: Num Rotatable Bonds = 0 Bond(s)
Molecular Topology: Polar Surface Area = 0 Angstroms Squared
Molecular Topology: Radius = 0 Atom(s)
Molecular Topology: Shape Attribute = 0
Molecular Topology: Shape Coefficient = 0
Molecular Topology: Sum Of Degrees = 0
Molecular Topology: Sum Of Valence Degrees = 0
Molecular Topology: Topological Diameter = 0 Bond(s)
Molecular Topology: Total Connectivity = 1
Molecular Topology: Total Valence Connectivity = 1
Molecular Topology: Wiener Index = 0
-----------------------------------------
------------MM2 Minimization------------
Note: All parameters used are finalized (Quality = 4).
Iteration 2: Minimization terminated normally because the gradient norm is less than the minimum gradient norm
Stretch: 0.0000
Bend: 0.0000
Stretch-Bend: 0.0000
Torsion: 0.0000
Non-1,4 VDW: 0.0000
1,4 VDW: 0.0000
Total Energy: 0.0000 kcal/mol
Calculation completed
------------------------------------
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Non-Dispersive Infra-Red (NDIR) detectors are the industry standard method of measuring the concentration of carbon oxides (CO & CO2).
Each constituent gas in a sample will absorb some infra red at a particular frequency. By shining an infra-red beam through a sample cell (containing CO or CO2), and measuring the amount of infra-red absorbed by the sample at the necessary wavelength, a NDIR detector is able to measure the volumetric concentration of CO or CO2 in the sample.
A chopper wheel mounted in front of the detector continually corrects the offset and gain of the analyser, and allows a single sampling head to measure the concentrations of two different gases.
The Cambustion Fast NDIR uses a unique sampling system, coupled to miniaturised NDIR technology to give millisecond response times.
The Cambustion Fast NDIR has two remote Sampling Heads controlled by a Main Control Unit, and is capable of sampling CO & CO2 simultaneously in two locations.
See Applications & Sample Data
출처: <http://www.cambustion.com/products/ndir500/operating-principle>
TCD(Thermal conductivity Detector) : 열전도도 검출기
원리는 이론상 Gas와 시료의 열전도도의 차이를 측정한다. 구조는 단일 필라멘트로 Cell Volume은 3.5 ㎕이다. 주요 특징으로는 Sensitivity(주로 사용되는 검출기 중 가장 낮다), Selectivity(모든 화합물 검출이 가능한 일반적인 검출기), 시료가 파괴되지 않는 비파괴성 검출기이다.
운반기체로는 주로 헬륨(큰 열전도도와 불활성을 지녀 가장 일반적으로 사용하는 gas), 수소(감도는 높으나 사용상 주의를 요한다), 질소(수소 분석시 사용)을 사용한다.
TCD 사용 주의 사항
검출기가 켜진 상태에서 이동상 가스의 공급이 중단되면 필라멘트가 영구히 손상된다. 때문에 검출기를 켜기 전, 항상 Ref. 및 이동상 가스의 공급여부를 확인한다.
- 필라멘트가 산소에 노출 시 산화되므로 주의한다.
- 산이나 할로겐 화합물과 같은 활성 물질도 필라멘트를 손상시킨다.
- 온도가 낮아 야기되는 시료의 농축은 검출기 Cell을 오염시킨다.
- 오염원들은 Thermal Conditioning으로 제거한다.
출처: <http://01041902024.tistory.com/149>
검출기 : TCD-Methanizer-FID 동시 검출
Signal change 기능 => 한 크로마토그램으로 출력
1) 수소, 산소, 질소는 TCD에서 피크를 확인합니다.
(분자 구조상 C-H bond가 없으므로 FID에서 측정 불가)
2) 기타 탄화수소류 CH4, C2, C3, C4는 FID에서 측정합니다.
3) CO와 CO2는 메타나이저를 통과하여 CH4로 변화시켜 FID로 검출합니다.
즉 수소, 산소, 질소만 TCD에서 측정하며,
그 이 후의 CH4, CO, CO2, C2-C4는 FID에서 측정하는 시스템입니다.
|
CLD를 이용한 NO, NOx 농도 측정
(1) 화학발광법(CLD; Chemi-luminescent Detection) 이란 화학발광(Chemiluminescence)은 가시 광선의 방출에 의해 일어나는 화학반응이다. 활성화된 상태의 이산화 질소(NO2*)는 아래과 같이 오존이 있는 낮은 압력 상태에서 일산화 질소가 산화될 때 형성되고, 활성화된(들뜬상태) 분자들이 바닥상태로 천이되면서 화학발광에 의한 빛(파장590~2500nm)을 방출 하게 된다.
즉, 반응로에서 오존은 과잉상태를 유지 함으로써 샘플내의 NO 농도는 방출된 광량의 측정에 의해 결정되는 것이다. NO2의 경우는 분석기로 공급되기 전에 촉매를 이용하여 일산화 질소로 치환되어야 하고, 그 후 측정 과정은 일산화 질소의 측정과 동일하다. 이 방법이 현재 자동차 응용분야에서 NO의 농도를 결정하는 가장 일반적인 방법이다.
(2) CLD 분석 장치 구성
4. 실제 엔진에 고속분석장치 적용(실제 장착 모습) - 흡기관, 배기관 각각 1개씩 설치(실시간 EGR(%) 측정)
5. CO2분석장치 흡기관 적용 - 흡기시스템 ⇒ WOT시 대기압, IDLE시 대기압의 1/3 ∴ 정압챔버의 압력 0.2bar 이하로 유지(샘플가스의 역류 방지) 샘플가스의 역유동(Backflow)에 의한 CO2농도 측정 오차 최소화 ⇒ 실제 흡기관의 압력: 0.2- 0.98bar
- 엔진이 고속으로 회전하는 경우 밸브 오버랩에 의해서 발생하는 역유동(Back flow)의 가시화 모습이다. |
출처: <http://ws.ajou.ac.kr/~tperc/research_3.htm>
A chemi-luminescence detector (CLD) is the industry standard method of measuring nitric oxide (NO) concentration.
The reaction between NO and O3 (ozone) emits light. This reaction is the basis for the CLD in which the photons produced are detected by a photo multiplier tube (PMT). The CLD output voltage is proportional to NO concentration.
The light-producing reaction is very rapid so careful sample handling is important in a very rapid response instrument. The Cambustion Fast CLD uses a unique sampling system coupled with miniaturised CLD technology to give millisecond response times.
The Cambustion Fast CLD has two remote sampling heads controlled by a Main Control Unit and is capable of simultaneous sampling in two locations
출처: <http://www.cambustion.com/products/cld500/cld-principles>
Sample data
http://www.cambustion.com/sites/default/files/instruments/CLD500/400-505%28cld%29_0.pdf
2015-02-02 오후 4:35 - 화면 캡처
|
FID를 이용한 HC 농도 측정
(1) 화염이온화 감지법(FID ;Flame Ionization Detection) 이란 탄화수소 농도는 일반적으로 불꽃 이온화 감지기(FID; Flame Ionization Detector)를 사용하여 측정한다. 미연탄화수소가 포함된 샘플가스는 수소 또는 수소-헬륨 불꽃에서 연소되고, 이온은 화학적 이온화 과정에서 소비되는 샘플가스 중 탄소에 의해서만 생성되므로 샘플가스가 유입되지 않은 상태에서는 생성되지 않는다. 이온전류의 크기는 미연탄화수소 농도 또는 미연탄화수소 중 탄소 원자의 개수를 통한 양적 측정을 제공한다. 빠른 응답의 FID 기구는 자동차 응용 여러 분야에서 탄화 수소 측정에 넓게 사용되고 있다. |
출처: <http://ws.ajou.ac.kr/~tperc/research_3.htm>
FID(Flame Lonization Detector) : 불꽃이온화 검출기
원리는 수소/Air에 의해 형성된 불꽃에서 시료가 연소되면 전하를 띤 이온이 형성되며, 이온의 농도에 비례하여 전류 흐름이 변화 된다. 구조는 jet(컬럼의 종류에 따라 사용되는 Jet의 크기가 구분), 수소 및 Air inlet, FID collector Assembly로 나뉜다. 주요 특징으로는 Sensitivity(대부분의 화합물에 대해 TCD의 약 103배 정도 감도가 높다.), Selectivity(수소/Air에 의한 불꽃에서 태워져 전하를 띤 이온을 생성하는 화합물만 검출할 수 있는 선택적인 검출기)이다.
FID에서 검출되지 않는 성분들 : Rare Gas (수소, 아르곤), 질소, 이산화질소, 일산화탄소, 이산화탄소, 사염화탄소, 이산화황, 산소, 물……등
주입구의 역할: 시료성분을 충분히 기화시키는데 최적인 온도 설정이 필요하므로 이를 맞춰 주어야 함.
(설정온도가 너무 높았을 경우 시료에 따라서는 분해를 일으키는 일도 있고 낮은 경우는 피크가 브로드하게 되는 일도 있기 때문에 주의가 필요)
1. 스프리트/스프리트레스 주입구
스프리트 주입구는 시료 부하용량이 작은 캐필러리 컬럼에 시료를 도입하기 위해서 고안, 시료주입구에서 도입되고 기화한 시료의 일부만을 분리컬럼에 이끌고 그 이외는 계외에 배출하는 장치로 피크를 샤프하게 하는 효과가 있다.
하지만 저농도의 시료분서에는 좋지 않다. 또 비점의 범위가 넓은 시료에서는 스프리트비가 일정하지 않는 경우도 있기 때문에 주의가 필요하다.
또 시료 도입시만 스프리트 가스를 멈추어 기화한 시료의 대부분을 캐필러리 컬럼에 도입하는 것이 스프리트레스 주입법이다. 용매의 큰 피크의 뒤에 테일링피크 부분을 없애는 것이나 컬럼항온조의 온도를 용매의 비점보다 낮게 억제하는 것으로 컬럼의 선단에 분석종을 농축하는 방법을 이용하는 것이 가능하여 저농도의 시료분석에도 대응 가능하다.
다만 스프리트 주입구와 스프리트레스 주입구의 어느 쪽이나 시료를 실린지로 도입해 기화시킬 단계에서 디스크리미네이션을 일으키는 일이 있어 성분의 비율이 바뀌어 버리는 일이 있기 때문에 주의가 필요하다.
스프리트 주입구는 스프리트레스 주입구를 겸하고 있는 경우도 많고 그 경우를 스프리트/스프리트레스 주입구라 부른다.
출처: <http://01041902024.tistory.com/149>
Flame ionization detector
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Schematic of a flame ionization detector for gas chromatography
A flame ionization detector (FID) is a scientific instrument that measures the concentration of organic species in a gas stream. It is frequently used as a detector in gas chromatography. Standalone FIDs can also be used in applications such as landfill gas monitoring, fugitive emissions monitoring and internal combustion engine emissions measurement[1] in stationary or portable instruments.
Contents
[hide]
- 1 History
- 2 Operating principle
- 4 Operation
- 5 Description of a generic detector
- 6 See also
- 7 References
- 8 Sources
History[edit]
The first flame ionization detectors were developed simultaneously and independently in 1957 by scientists working for the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Melbourne, Australia.[2][3][4] and at the University of Pretoria in Pretoria, South Africa.[5]
In 1959, Perkin Elmer Corp. included a flame ionization detector in its Vapor Fractometer[6]
Operating principle[edit]
The operation of the FID is based on the detection of ions formed during combustion of organic compounds in a hydrogen flame. The generation of these ions is proportional to the concentration of organic species in the sample gas stream. Hydrocarbons generally have molar response factors that are equal to number of carbon atoms in their molecule, while oxygenates and other species that contain heteroatoms tend to have a lower response factor. Carbon monoxide and carbon dioxide are not detectable by FID.
Advantages and disadvantages[edit]
Advantages[edit]
Flame ionization detectors are used very widely in gas chromatography because of a number of advantages.
- Cost: Flame ionization detectors are relatively inexpensive to acquire and operate.
- Low maintenance requirements: Apart from cleaning or replacing the FID jet, these detectors require no maintenance.
- Rugged construction: FIDs are relatively resistant to misuse.
- Linearity and detection ranges: FIDs can measure organic substance concentration at very low and very high levels, having a linear response of 10^6.
Disadvantages[edit]
Flame ionization detectors cannot detect inorganic substances. In some systems, CO and CO2 can be detected in the FID using a methanizer, which is a bed of Ni catalyst that reduces CO and CO2 to methane, which can be in turn detected by the FID.
Another important disadvantage is that the FID flame oxidizes all compounds that pass through it; all hydrocarbons and oxygenates are oxidized to carbon dioxide and water and other heteroatoms are oxidized according to thermodynamics. For this reason, FIDs tend to be the last in a detector train and also cannot be used for preparatory work.
Operation[edit]
In order to detect these ions, two electrodes are used to provide a potential difference. The positive electrode doubles as the nozzle head where the flame is produced. The other, negative electrode is positioned above the flame. When first designed, the negative electrode was either tear-drop shaped or angular piece of platinum. Today, the design has been modified into a tubular electrode, commonly referred to as a collector plate. The ions thus are attracted to the collector plate and upon hitting the plate, induce a current. This current is measured with a high-impedance picoammeter and fed into an integrator. The manner in which the final data is displayed is based on the computer and software. In general, a graph is displayed that has time on the x-axis and total ion on the y-axis.
The current measured corresponds roughly to the proportion of reduced carbon atoms in the flame. Specifically how the ions are produced is not necessarily understood, but the response of the detector is determined by the number of carbon atoms (ions) hitting the detector per unit time. This makes the detector sensitive to the mass rather than the concentration, which is useful because the response of the detector is not greatly affected by changes in the carrier gas flow rate.
Description of a generic detector[edit]
FID Schematic
The design of the flame ionization detector varies from manufacturer to manufacturer, but the principles are the same. Most commonly, the FID is attached to a gas chromatography system.
The eluent exits the GC column (A) and enters the FID detector's oven (B). The oven is needed to make sure that as soon as the eluent exits the column, it does not come out of the gaseous phase and deposit on the interface between the column and FID. This deposition would result in loss of eluent and errors in detection. As the eluent travels up the FID, it is first mixed with the hydrogen fuel (C) and then with the oxidant (D). The eluent/fuel/oxidant mixture continues to travel up to the nozzle head where a positive bias voltage exists (E). This positive bias helps to repel the reduced carbon ions created by the flame (F) pyrolyzing the eluent. The ions are repelled up toward the collector plates (G) which are connected to a very sensitive ammeter, which detects the ions hitting the plates, then feeds that signal (H) to an amplifier, integrator, and display system. The products of the flame are finally vented out of the detector through the exhaust port (J).
See also[edit]
- Flame detector
- Thermal Conductivity Detector
- Gas Chromatography
- Active fire protection
- Photoionization detector
- Photoelectric flame photometer
References[edit]
- Jump up ^ "Flame Ionisation Detector Principles". www.cambustion.com. Retrieved 3 December 2014.
- Jump up ^ Scott, R. P. W., 1957, Vapour Phase Chromatography, Ed. D. H. Desty (London: Butterworths), p. 131.
- Jump up ^ McWilliam, I. G.; Dewar, R. A. "Flame Ionization Detector for Gas Chromatography". Nature 181 (4611): 760. Bibcode:1958Natur.181..760M. doi:10.1038/181760a0.
- Jump up ^ Morgan, D J (1961). "Construction and operation of a simple flame-ionization detector for gas chromatography". J. Sci. Instrum. 38 (12): 501. Bibcode:1961JScI...38..501M. doi:10.1088/0950-7671/38/12/321. Retrieved 2009-03-18.
- Jump up ^ Harley, J.; Nel, W.; Pretorius, V. (January 18, 1958). "Flame Ionization Detector for Gas Chromatography". Nature. doi:10.1038/1781244b0.
- Jump up ^ "Timeline". Perkinelmer.com. Retrieved 12 Dec 2014.
Sources[edit]
- Skoog, Douglas A., F. James Holler, & Stanley R. Crouch. Principles of Instrumental Analysis. 6th Edition. United States: Thomson Brooks/Cole, 2007.
- Halász, I. & W. Schneider. "Quantitative Gas Chromatographic Analysis of Hydrocarbons with Capillary Column and Flame Ionization Detector." Analytical Chemistry. 33, 8 (July 1961): 978-982
- G.H. JEFFERY, J.BASSET, J.MENDHAM, R.C.DENNEY, "VOGEL'S TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS."
출처: <http://en.wikipedia.org/wiki/Flame_ionization_detector>
Fast FID Principles
The flame ionisation detector (FID) is the automotive emissions industry standard method of measuring hydrocarbon (HC) concentration.
The sample gas is introduced into a hydrogen flame inside the FID. Any hydrocarbons in the sample will produce ions when they are burnt. Ions are detected using a metal collector which is biased with a high DC voltage. The current across this collector is thus proportional to the rate of ionisation which in turn depends upon the concentration of HC in the sample gas.
The ionisation process is very rapid, so the slow time response of conventional FIDs is mainly due to sample handling. A typical slow analyser might have a response time of 1-2 seconds.
The Cambustion HFR fast response FID analyzers use conventional detection principles and a unique patented sampling system to give millisecond response times.
The Cambustion fast FID consists of a main control unit (MCU) and two remote sampling heads (which house the FIDs). The dual channel nature of the instrument enables simultaneous real-time measurement in two locations allowing, for example, evaluation of catalyst performance.
출처: <http://www.cambustion.com/products/hfr500/fast-fid-principles>
GC Solutions #11: The Flame Ionization Detector
by Matthew Klee
In this article we initiate a discussion on the workhorse detector of gas chromatography: the flame ionization detector.
The flame ionization detector (FID) is the premier detector in gas chromatography. It has unique properties and performance that puts it above and beyond all other general-use detectors in gas chromatography (or any other form of chromatography, for that matter).
Topping the list of unique attributes are (1) unit carbon response and (2) wide linear operating range (up to 7 orders of magnitude). When combined with its other attributes of low cost, ease of use, speed of response, and ruggedness, it is no wonder why the FID is the premier detector of choice for GC.
The FID is a mass sensitive detector. That means that its response is proportional to the mass of carbon that passes through it. In that regard, FID response is stated in terms of picograms carbon per sec. Detection limits for FIDs are in the low pg C /sec.
Unit carbon response means that the FID responds linearly to the mass of carbon flowing through it, independent of compound structure. The FID gives unit response for most hydrocarbons within a couple per cent error. Exceptions to this will be discussed next month. This attribute of unit carbon response allows one to quantify mixtures without having calibration standards for every component. Amounts of components in a sample relate to their relative peak areas. So, a simple area per cent report will fairly closely reflect the mass per cent of each component in a mixture. This is extremely useful when analysing complex samples such as those in the petroleum industry wherein samples can contain well over 1000 components. In the same vein, the relative ratio of the area of one peak to another (e.g., the peak of an unknown component relative to a calibrated reference peak) closely reflects its relative amount in the sample. This is useful when estimating concentration levels of components in a sample when identities are unknown or when standards are not available for calibration.
The FID was first described by two independent groups at approximately the same time [1,2]. FIDs were commercially available soon thereafter (the early 1960s.) Most of the developments since the original functional designs have been primarily in areas of usability, adaptation for capillary instead of packed columns, and in improvements of associated electronics, and signal processing.
The Flame
In a hydrogen flame, hydrogen gas (H2) reacts exothermically with the oxygen (O2) to form water. A hydrogen/air flame temperature burns at 2210 °C.*
2H2 + O2 4H• + 2O• 2H2O + heat
In the above reaction, one part oxygen reacts with two parts hydrogen. This ratio (1:2) is called the stoichiometric ratio. Because air is approximately 20% oxygen (O2), a stoichiometric mixture would require an air/hydrogen gas flow ratio of 2:5; 2.5 times more air than hydrogen. Although stoichiometric ratios of oxygen to hydrogen can provide a reasonable dynamic range for the FID, experience has shown (in part because of the complex combustion processes described below) that an excess flow of air is required to ensure complete combustion, unit carbon response and the widest linear dynamic range. In addition, higher than stoichiometric air flow helps to avoid carbon deposits from forming in the jet when high concentrations of analyte or solvent pass through.
A ratio of at least 6:1 of air to hydrogen has empirically been found necessary to achieve the widest dynamic range possible with FID. Many manufacturers recommend ratios of 10:1 or more air/hydrogen flows. The higher the sample load to the FID, the more flame gases are required to prevent blowout and carbon formation. That is why gas flows used with packed columns (higher sample loads) generally need to be higher than when capillary columns are used. Exact flow rates are somewhat instrument specific and also relate a bit to carrier gas flow rates, so following manufacturer recommendations is wise.
Sample Combustion and Signal Generation
A very nice explanation of FID flame chemistry has been provided by Holm [3]. Therein he supports a mechanism whereby most organic compounds are reduced to saturated counterparts in the initial portion of the flame, where temperatures are lower. As they continue up the flame, these saturated counterparts then continue to react with hydrogen atoms to form methane, as illustrated in Equation 1.
Still further in the flame, methane is combusted to form formylium ion CHO+, the primary FID signal-producing ion.
CH4 → H2 + :CH O•→ CHO* CHO+ + e-
Other reactions can of course occur, including ones that form other positive ions such as hydronium ion.
CHO+ + H2O H3O+ + CO
All positive ions are collected by a negatively biased collector causing a current to flow, which is then electronically amplified and digitized (Figure 1). The current is proportional to the number of ions collected. The yield of ions from carbon passing through the detector is on the order of one ion per 106 carbon atoms [4], yet this is still enough for the FID to give picogram-level detection. Electrons in the flame flow in the opposite direction and are grounded out on the FID jet (Figure 2). By biasing the collector high enough relative to the jet (e.g., -200 V), recombination of positively charged ions and electrons is minimized, and signal maximized.
References
1. J. Harley, W. Nel and V. Pretorious, Nature , London, 181(1958)177.
2. I. G. McWilliams and R. A. Dewer, "Gas Chromatography 1958", (Ed. D. H.Desty), Butterworths Scientific Publications (1957) 142
3. T. Holm, J. Chrom. A , 842 (1999) 221–227
4. D.K. Bohme, in: P.J. Ausloss (Ed.), Kinetics of Ion–Molecule Reactions, Plenum Press, New York, 1979
Dr Matthew S. Klee is internationally recognized for contributions to the theory and practice of gas chromatography. His experience in chemical, pharmaceutical and instrument companies spans over 30 years. During this time, Dr Klee's work has focused on elucidation and practical demonstration of the many processes involved with GC analysis, with the ultimate goal of improving the ease of use of GC systems, ruggedness of methods and overall quality of results.
출처: <http://www.sepscience.com/Techniques/GC/Articles/208-/GC-Solutions-11-The-Flame-Ionization-Detector>
