Thermal conductivity detector chromatography

MEVIS/IPM, membrane inlet mass spectrometry/isotope pairing method TCD, gas chromatography-thermal conductivity detector AIT, acetylene inhibition technique IPM, isotope pairing method with isotope ratio mass spectrometry MIMS, membrane inlet mass spectrometry Stoichiometery, benthic flux DICiDIN stoichiometry MIMS + N03, membrane inlet mass with N03 amendment 15N2 production, 15N03 or 15NH4 conversion to 15N2. 

Products were collected and weighed to determine a mass balance. Except for two experiments, in which the volume of gases exceeded the capacity of the gas collection system and the last portion was vented, the mass balance ranged from 95 to 98% ( 14). We measured C, H, N, and acid-evolved CO2 content for all retorted shales and for some burnt shales from the cracking experiments. Oils were analyzed for C, H, and N. Gases were analyzed by gas chromatography (thermal conductivity detector for h2> CO2 N2, and CH4 flame ionization detector for... 

Alternatively, gas chromatography may be used Fig. XVII-5 shows a schematic readout of the thermal conductivity detector, the areas under the peaks giving the amount adsorbed or desorbed. 

Thermal Conductivity Detector One of the earliest gas chromatography detectors, which is still widely used, is based on the mobile phase s thermal conductivity (Figure 12.21). As the mobile phase exits the column, it passes over a tungsten-rhenium wire filament. The filament s electrical resistance depends on its temperature, which, in turn, depends on the thermal conductivity of the mobile phase. Because of its high thermal conductivity, helium is the mobile phase of choice when using a thermal conductivity detector (TCD). 

Thermal conductivity detector. The most important of the bulk physical property detectors is the thermal conductivity detector (TCD) which is a universal, non-destructive, concentration-sensitive detector. The TCD was one of the earliest routine detectors and thermal conductivity cells or katharometers are still widely used in gas chromatography. These detectors employ a heated metal filament or a thermistor (a semiconductor of fused metal oxides) to sense changes in the thermal conductivity of the carrier gas stream. Helium and hydrogen are the best carrier gases to use in conjunction with this type of detector since their thermal conductivities are much higher than any other gases on safety grounds helium is preferred because of its inertness. 

Carbon dioxide chemisorptions were carried out on a pulse-flow microreactor system with on-line gas chromatography using a thermal conductivity detector. The catalyst (0.4 g) was heated in flowing helium (40 cm3min ) to 723 K at 10 Kmin"1. The samples were held at this temperature for 2 hours before being cooled to room temperature and maintained in a helium flow. Pulses of gas (—1.53 x 10"5 moles) were introduced to the carrier gas from the sample loop. After passage through the catalyst bed the total contents of the pulse were analysed by GC and mass spectroscopy (ESS MS). 

C30 oil, homopolymer of 1-decene, Ethyl Corp., Inc.) served as the start-up solvent for the experiments. The catalyst (ca. 5-8 g) was added to start-up solvent (ca. 300 g) in the CSTR. The reactor temperature was then raised to 270°C at a rate of l°C/min. The catalyst was activated using CO at a space velocity of 3.0 sl/h/g Fe at 270°C and 175 psig for 24 h. FTS was then started by adding synthesis gas mixture (H2 CO ratio of 0.7) to the reactor at a space velocity of either 3.1 or 5.0 sl/h/g Fe. The conversions of CO and H2 were obtained by gas chromatography (GC) analysis (HP Quad Series Micro-GC equipped with thermal conductivity detectors) of the product gas mixture. The reaction products were collected in three traps maintained at different temperatures—a hot trap (200°C), a warm trap (100°C), and a cold trap (0°C). The products were separated into different fractions (rewax, wax, oil, and aqueous) for quantification by GC analysis. However, the oil and the wax (liquid at room temperature) fractions were mixed prior to GC analysis. 

In addition to the analytical columns (columns used mainly for analytical work), so-called preparative columns may also be encountered. Preparative columns are used when the purpose of the experiment is to prepare a pure sample of a particular substance (from a mixture containing the substance) by GC for use in other laboratory work. The procedure for this involves the individual condensation of the mixture components of interest in a cold trap as they pass from the detector and as their peak is being traced on the recorder. While analytical columns can be suitable for this, the amount of pure substance generated is typically very small, since what is being collected is only a fraction of the extremely small volume injected. Thus, columns with very large diameters (on the order of inches) and capable of very large injection volumes (on the order of milliliters) are manufactured for the preparative work. Also, the detector used must not destroy the sample, like the flame ionization detector (Section 12.6) does, for example. Thus, the thermal conductivity detector (Section 12.6) is used most often with preparative gas chromatography. 

The prepared mixtures were placed in the extraction vessel, and stirred for 2 h and then left to settle for 4 h. Samples were taken by a syringe (Gaschromatographic s Hamilton 0.4 p,L) from both the upper (methylcyclohexane) phase and lower layers (aromatic phase). Both phases were analyzed using Konik gas chromatography (GC) equipped with a thermal conductivity detector (TCD) and Shimadzu C-R2AX integrator. A 2 m x 2 mm column was used to separate the components... 

Experimental Procedure. Figure 3 presents a schematic diagram of the apparatus. The chromatograph is a Varian Aerograph Series 1400 with a thermal conductivity detector and an associated Varian CDS 111 Chromatography Data System (integrator). We modified the chromatograph in two ways ... 

In modern combustion analysers, a tiny sample (about 2 mg) is accurately weighed and oxidised at a high temperature in an oxygen atmosphere. The product mixture of CO, HjO, Nj and SO is separated by gas chromatography and the mass of each component is measured using a thermal conductivity detector. From these product masses, the mass of each of the elements C, H, N and S in the sample can... 

One advantage of gas chromatography is the availability of detectors which respond specifically to certain types of compound. The best known are the electron capture detector for chlorine compounds and the flame photometric detector for nitrogen and phosphorus compounds. If one wants to detect very small molecules such as water or CSj, the standard flame ionisation detector must be replaced by a thermal conductivity detector. 

In the past, thermal conductivity detectors were most common in gas chromatography because they are simple and universal They respond to all analytes. Unfortunately, thermal conductivity is not sensitive enough to detect minute quantities of analyte eluted from open tubular columns smaller than 0.53 mm in diameter. Thermal conductivity detectors are still used for 0.53-mm columns and for packed columns. 

The most general purpose detector for open tubular chromatography is a mass spectrometer. Flame ionization is probably the most popular detector, but it mainly responds to hydrocarbons and Table 24-5 shows that it is not as sensitive as electron capture, nitrogen-phosphorus, or chemiluminescence detectors. The flame ionization detector requires the sample to contain SlO ppm of each analyte for split injection. The thermal conductivity detector responds to all classes of compounds, but it is not sensitive enough for high-resolution, narrow-bore, open tubular columns. 

The mixture of C02, H20, N2, and S02 is separated by gas chromatography (Figure 27-6), and each component is measured with a thermal conductivity detector (Section 24-3). Figure 27-7 shows a different C,H,N,S analyzer that uses infrared absorbance to measure C02, HzO, and S02 and thermal conductivity for N2. 

Since the mass chromatograph provides molecular weight data, the instrument is ideally suited for quantitative gas chromatography. Considerable time and uncertainty are saved using the system compared with flame ionization and thermal conductivity detectors which require response factors for each and every compound. 

DA, MEK, methyl vinyl ketone (MVK), propionaldehyde (PrH), and acetaldehyde (AcH) were analysed by on-line gas chromatography using a Varian 3400 GC equipped with a thermal conductivity detector and a 2m column containing 25% w/w B.B -oxydipropionitrile on Chromosorb W (80-100 mesh) operated at 60°C He was used as the carrier gas. Acetic acid (AcOH) was collected in 2ml of water from the effluent stream over a period of 1 hour and later analysed on a Porapak QS column at 150°C. CO2 was tested by removal of 2ml samples from the exit of the reactor with a gas syringe and injecting them onto a Porapak QS column operated at 60°C. 

Gas chromatography was used to determine n-paraffin distribution in the oil and wax samples. An F and M Instrument Company Model 500 chromatograph was used with an uncompensated single column, a helium carrier gas flow rate of 25 ml/min., and a thermal conductivity detector. The column was 4.8 mm in diameter and 3.3 m in length, and was packed with 3% Dexil 300 on Chromo-sorb P. The block and injection port temperatures were maintained at 673 K. The column was temperature-programmed from 348 K to 673 K at a rate of 5.7 K per minute. Peak identification was aided by the use of internal standards of decane, dodecane, and hexadecane. The baseline was determined by heating without sample injection. Response values were not available for the various areas on the traces, so the analyses were reported as % by area. 

Sometimes, the gas production rate was measured and the nature of the compounds analyzed by gas chromatography using a thermal conductivity detector or by mass spectrometry (10, 12, 17, 73-76). 

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