A-TCD

The simultaneous measurement of emanation thermal analysis (ETA) with DTA has been described by Emmerich and Balek (77). ETA is further described in Section 14 of this chapter. 

Compared to DTA-MS, DTA-GC is less expensive to assemble and ensures better resolution in certain situations. Its disadvantage is that since it takes longer for the GC analysis time, the sampling rate is considerably less than that in DTA-MS. To eliminate or at least reduce the latter problem, Yamada et a. (78) samples the evolved gases at desired points on the DTA curve and stored them until the GC analysis was carried out. 

Although many of the systems described in TG-DTA apply to DTA-MS. a system for DTA-MS only has been discussed by Aspinal et al. (79). This 

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By far the most used detector is the thermal conductivity detector (TCD). Detectors like the TCD are called bulk-property detectors, in that the response is to a property of the overall material flowing through the detector, in this case the thermal conductivity of the stream, which includes the carrier gas (mobile phase) and any material that may be traveling with it. The principle behind a TCD is that a hot body loses heat at a rate that depends on the... 

Figure 14.8 shows a detailed schematic representation of a natural gas analysis System, which fully complies with GPA standardization (8). This set-up utilizes four packed columns in connection with a TCD and one capillary column in connection with an FID. The contents of both sample loops, which are connected in series, are used to perform two separate analyses, one on the capillary column and one on the packed columns. The resulting chromatograms are depicted in Figure 14.9. 

The major gaseous components were analyzed by a gas chromatograph equipped with a TCD and a molecular sieve 13X column. The specific surface areas of carbon produced were measured by the BET method(ASAP 2010, Micromeritics). The morphology and particle size of the formed carbon were investigated by the scanning electron microscopy(S-4200, Hitachi... 

Catalyst characterization - Characterization of mixed metal oxides was performed by atomic emission spectroscopy with inductively coupled plasma atomisation (ICP-AES) on a CE Instraments Sorptomatic 1990. NH3-TPD was nsed for the characterization of acid site distribntion. SZ (0.3 g) was heated up to 600°C using He (30 ml min ) to remove adsorbed components. Then, the sample was cooled at room temperatnre and satnrated for 2 h with 100 ml min of 8200 ppm NH3 in He as carrier gas. Snbseqnently, the system was flashed with He at a flowrate of 30 ml min for 2 h. The temperatnre was ramped np to 600°C at a rate of 10°C min. A TCD was used to measure the NH3 desorption profile. Textural properties were established from the N2 adsorption isotherm. Snrface area was calcnlated nsing the BET equation and the pore size was calcnlated nsing the BJH method. The resnlts given in Table 33.4 are in good agreement with varions literature data. 

Temperature programmed sulfidation or temperature programmed reaction spectroscopy usually deal with more than one reactant or product gas. In these cases a TCD detector is inadequate and one needs a mass spectrometer for the detection of all reaction products. With such equipment one obtains a much more complete picture of the reaction process, because one measures simultaneously the consumption of reactants and the formation of products. 

Detector FID set to appropriate range and attenuation a TCD may be used if desired Injector temperature 250°C Detector temperature 200°C... 

In Chapter 12, we discussed the need to calculate response factors, specifically when a TCD detector is used (Section 12.8.2). Would response factors need to be calculated in HPLC when a UV absorbance detector is used Explain. 

The thermal conductivity detector (TCD) is a universal detector that is nondestructive, which is a major advantage for preparative work (Dybowski and Kaiser, 2002). However, it is not sensitive enough for many of the analyses discussed later. This detector operates on the principle that a hot body loses heat at a rate dependent on the composition of the material surrounding it (Burtis et ah, 1987). In a TCD, two filaments are heated, one in carrier gas, and the other in the column effluent. The voltages required to maintain the filament at a constant temperature are measured and compared. When compounds elute from the column the voltage of the sample filament is different from that of the filament in carrier gas and is recorded as a peak (Burtis et al., 1987). 

Elemental composition H 11.83%, N 41.11%, S 47.05%. It may be analyzed by measuring its decomposition gaseous products, ammonia and hydrogen sulfide, either by gas chromatography using an FID or a TCD or by selective ion electrode or colorimetric techniques. 

Elemental composition Be 60.02%, C 39.98%. Beryllium may be analyzed by various instrumental techniques (see Beryllium). Additionally, the compound may be treated with a dilute mineral acid. The product methane gas slowly evolved is then analyzed by GC equipped with a TCD, or by GC/MS. 

Elemental composition Ca 95.41% H 4.79%. A measured amount of the solid is carefully treated with water and the volume of evolved hydrogen is measured using a manometer (Ig liberates 1.16 L H2 at NTP). The solution is then acidified with nitric acid and diluted for the measurement of calcium by AA or ICP spectrophotometry, or by a wet method (see Calcium). The hberat-ed hydrogen gas may be analyzed by GC using a TCD. Many packed and capillary GC columns are commercially available. 

Carbon dioxide may be readily analyzed by various instrumental techniques, such as IR, GC, and GC/MS. Many portable infrared analyzers are available commercially for rapid, on site monitoring of CO2. Also, it can be analyzed by GC using a TCD or an FID. It readily may be identified by mass spectrometry from its characteristic ionic mass 44. Dissolved CO2 in water... 

Elemental composition H 1.25%, Br 98.75%. The normality of the acid may be measured by titration against a standard solution of a base using a suitable color indicator or by potentiometric titration. The bromide ion, Br, may be measured quantitatively by ion chromatography after appropriate dilution. Concentration of HBr gas in air may be measured by passing a known volume of air through water and determining concentration of acid in aqueous solution by titration or ion chromatography. Alternatively, HBr gas may be analyzed by GC or GC/MS. A very polar column should be used for such measurements. An FID or a TCD type detector may be used for GC analysis. 

Elemental composition H 2.49%, Se 97.51%. The gas may be analyzed by GC using a TCD, FID or a flame photometric detector. The compound may be identified by GC/MS the molecular ions have masses 82 and 80. The compound may be absorbed in water and the solution analyzed for elemental selenium by flame or furnace atomic absorption—or by ICP atomic emission spectrophotometry. 

Elemental composition Pb 77.54%, C 4.49%, O 17.96%. The compound is digested with nitric acid, diluted and analyzed for lead by various instrumental techniques (See Lead). Carbonate may be tested by treating the compound with dilute HCl. It will effervesce, the evolved CO2 gas will turn hmewater milky. Also, liberated CO2 can be identified using a GC-equipped with a TCD or by GC/MS. The characteristic mass ion for GC/MS identification of CO2 is 44. 

Elemental composition Li 31.85%, B 49.66%, H 18.50%. The compound is dissolved in water cautiously and the evolved hydrogen is measured by GC using a TCD. The aqueous solution is treated with nitric acid and the diluted nitric acid extract is analyzed for lithium by atomic absorption or emission spectroscopy (See Lithium). 

Elemental composition Mo 36.34%, C 27.30%, 0 36.36%. A benzene solution of the hexacarbonyl may be analyzed by GC/MS. Molybdenum metal digested in nitric acid solution may be analyzed by various instrumental techniques. Also, the compound may be thermally dissociated and the liberated CO may be identified by GC using a TCD or by GC/MS using an appropriate capillary column. 

Nitric oxide is analyzed with GC using a TCD or by mass spectrometer using helium as a diluent and carrier gas. The characteristic mass for NO ion is 30. Also, it can he identified hy the hrown ring test in cold FeS04 solution (see Reactions). In contact with air it oxidizes to NO2, which is then identified from color, odor, and chemical properties. 

C. O2 uptidces were determined in a pulse mode (Vq2 puise pmol) at the same temperature of the pretreatment by using a TCD connected to a DP 700 Data Processor Carlo Erba Instruments). The number of reduced sites was calculated by assuming the chemisorption stoichiometry 02/"reduced site" of 1/2. 

Temperature programmed desorption (TPD) of NH3 was performed in a quartz micro-reactor. 0.10 g of sample was firstly heated in helium at 600°C for 2 h. NH3 was introduced to the sample after it was cooled down to room temperature. To remove the weakly adsorbed NH3, the sample was swept using helium at 100°C for 1 h. The TPD experiments were then carried out with a carrier-gas flow rate of 40 ml/min helium from 100 to 600°C using a linear heating rate of 10°C/min. The desorption of NH3 was detected by Shimadzu GC-8A equipped with a TCD detector. 

The PSA operation was carried out at 30-1000 and atmospheric pressure by using a N2 carrier gas (60 ml/min). Adsorbates was butanone. Before PSA operation the adsorbents was pretreated at 400-90010 for 2 h in flowing N2. In the adsorption operation, N2 with butanone vapor (27.2 or 5.44 Torr) was passed through a column of the adsorbent (0.3 g for 27.2 Torr and 0.6 g for 5.44 Torr of butanone pressure) until there was almost no further adsorption (2 h). In the desorption operation, pure N2 was passed through the column in a countercurrent way for 2 h instead of evacuation. The concentration of the organic solvent vapor in the effluent gas was always monitored with a TCD detector to obtain breakthrough curves. Amounts of adsorption and desorption were calculated from the breakthrough curves. 

The popularity of GC as an analytical technique in many areas depends on the fact that all of the compounds of interest in an important sample can be detected. For instance, in petroleum and petrochemical labs, it is the rule that all of the compounds can be measured at very low levels with the flame ionization detector. In this case, the detector is "universal." In a natural gas analysis, however, the same detector would not have universal response. This is because several of the important constituents, such as N2 and CO, give little or no response on the FID. In this case a TCD is used, which is "universal" for this analysis. 

Two important parts of the electronics for a TCD are not showr in Figure 5.8. In practice the four cells of a detector rarely match. A control is usually provided so that the output voltage can be nulled before a chromatogram is run. This is usually done with variable resistors connected in the bridge. Often there are two controls on the front panel "Fine" and "Coarse" Balance. Another necessary control, at least when an integrator is not used, is an attenuator switch. This reduces the response on the recorder by various amounts so that the larger peaks can be kept... 

Detection of All Components in the Sample. For the analysis of samples containing trace organic compounds as well as inorganic compounds, the column can be connected in series with a TCD and an FID to carry out a complete analysis with a single injection. 

The consumption of hydrogen was monitored with a TCD held constantly at 100°C and recorded at a signal rate of 1 point/s. The hydrogen consumption was quantified by means of calibration with pure CuO (Merck >99.9%). The temperature of the sample bed was measured by means of a thermocouple inside the U-tube (with tip within the catalyst bed), and followed an exact linear ramp throughout the TPR run. A cold trap was used to prevent water passing through the TCD. The TPR experimental conditions were selected in agreement with criteria reported elsewhere [89],... 

Figure 7.4. Example for the measurement of response time of a TCD. Copyright ASTM. Reprinted with permission.

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