Thermal conductivity detector design

Olfactometric detection was originally carried out using the same gas chromatograph with a splitting device based on the design by Etzweiler and Neuner-Jehle attached to a locally produced odour port. (9) This approach has been superseded with a Hewlett-Packard 5890A chromatograph equipped with flame ionisation and a low volume (3.5mm) thermal conductivity detector, an extension of which allows odour perception. 

Virtually every conceivable means of detecting gases and vapors has been exploited in designing GC detectors, and over one hundred have been described. The two most popular ones, the thermal conductivity detector (TCD) and the flame ionization detector (FID), will be described in some detail. They are classified (according to the criteria in Chapter 7) and compared in Table 6. 

Detectors may be classified on the basis of selectivity. A universal detector responds to all compounds in the mobile phase except carrier gas. A selective detector responds only to a related group of substances, and a specific detector responds to a single chemical compound. Most common GC detectors fall into the selective designation. Examples include flame ionization detector (FID), ECD, flame photometric detector (FPD), and thermoionic ionization detector. The common GC detector that has a truly universal response is the thermal conductivity detector (TCD). Mass spectrometer is another commercial detector with either universal or quasi-universal response capabilities. 

The katharometer detector [sometimes spelled cath-erometer and often referred to as the thermal conductivity detector (TCD) or the hot-wire detector (HWD)] is the oldest commercially available gas chromatographic (GC) detector still in common use. Compared with other GC detectors, it is a relatively insensitive detector and has survived largely as a result of its almost universal response. In particular, it is sensitive to the permanent gases to which few other detectors have a significant response. Despite its relatively low sensitivity, the frequent need for permanent gas analysis in many industries probably accounts for it still being the fourth most commonly used GC detector. It is simple in design and requires minimal electronic support and, as a consequence, is also relatively inexpensive compared with other detectors. 

Fig. 3.3A illustrates a typical design of a glass pyrolytic cell [37]. Similar cells were described by Janak [38], Jones and Moyles [39, 40] and Mlejnek [41]. They have also been used by other investigators. To increase the concentration of the resulting products and use a simpler thermal conductivity detector. Franc and Blaha [42] employed platinum mesh as the pyrolytic cell filament. This enabled them to increase the sample size without increasing the weight of the polymer under investigation per unit area of the heated filament surface, so that they could use a thermal conductivity detector instead of a flame-ionization detector. 

Thermal conductivity detectors in PGC are universal detectors in that they can be used to measure any component (Figure 2). Typically, TCDs are used to measure percent level components, but because it is a concentration-sensitive detector, the TCD can be designed for lower level analyses by optimizing the detector volume and sensing elements. TCDs used in PGC utilize either filaments or thermistor beads as their sensing elements. Filaments have the advantage that they can be operated at higher temperatures, but they are more prone to failure than are thermistors. The sensitivity is mainly dependent on the detector temperature and the thermal conductivity difference between the carrier gas and the component of interest. 

The DTA/DSC-EGD coupled simultaneous technique and relevant equipment have been investigated since 1979 [62, 74, 75]. The Model CDR-1 power compensation DSC analyzer (ambient temperature ca 720 C) was developed by the Tian Ping Instrumental Factory (Shanghai, China). The EGD detector is a thermal conductivity detector (TCD) in the GC analyzer. The CDR-1 DSC analyzer coupled with GC was constructed using a specially designed gas conduit. A schematic diagram of the on-line coupled simultaneous DTA/DSC-EGD apparatus is shown in Figure 2.33. 

Microhotplates, however, are not only used for metal-oxide-based gas sensor applications. In all cases, in which elevated temperatures are required, or thermal decoupling from the bulk substrate is necessary, microhotplate-like structures can be used with various materials and detector configurations [25]. Examples include polymer-based capacitive sensors [26], pellistors [27-29], GasFETs [30,31], sensors based on changes in thermal conductivity [32], or devices that rely on metal films [33,34]. Only microhotplates for chemoresistive metal-oxide materials will be further detailed here. The relevant design considerations will be addressed. 

The following table gives the properties of common gas chromatographic carrier gases. These properties are those used most often in designing separation and optimizing detector performance. The density values are determined at CPC and 0.101 MPa (760 torr).1 The thermal conductivity values, X, are determined at 48.9°C (120°F).1 The viscosity values are determined at the temperatures listed and at 0.101 MPa (760 torr).1 The heat capacity (constant pressure) values are determined at 15°C and 0.101 MPa (750 torr).2... 

The purpose of the detector is to determine when and how much of a compound has emerged from the column. Although the goal of all detectors is to be as sensitive as possible, many detectors are designed to be selective for certain classes of compounds. Dozens of different types of detectors have been developed, but only a few are used routinely. Those are thermal conductivity (TC), thermionic (N/P), electron capture (ECD), flame photometric (FPD), Hall electroconductivity detector (Hall or ELCD), hydrogen flame ionization detector (FID), argon ionization (AI), photoionization (PID), gas density balance (GDB), and the mass spectrometer. Chemists usually select a detector by the following criteria, listed in priority ... 

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