Gas Chromatography Detectors Gas chromatography detectors are devices that detect the presence of solute vapors as they are eluted from a gas chromatographic column. Traces of vapor modify the properties of a gas far more extensively than traces of solute modify the properties of a liquid. As a consequence, the detection of vapors in gases is easier than the detection of traces of solutes in liquids. It follows, that GC detectors are generally far more sensitive than LC detectors and there are more of them. The early GC detectors, the gas density balance (that responded to the change in density of the gas), the thermal conductivity detector (TCD) (that responded to the change in specific heat and thermal conductivity of the gas) and the flame thermocouple detectors (that responded to the heat of combustion of a gas) all had sensitivities of about 5 x 10-7 g/ml at a signal to noise ratio of 2. As demands for improved sensitivity became greater to permit high efficiency columns to be operated, more sensitive GC detectors were developed. One of the first high sensitive detectors to be described was the flame ionization detector (FID) (that responded to the ion current produced in the flame during the combustion of carbon containing solutes). This detector, although giving a relatively small ion current response (ionization efficiency only 0.0015%), had very low noise, and, consequently, a sensitivity of about 5 x 10-12 g/ml for n-heptane at a signal-noise ratio of 2. A development of this detector, the nitrogen phosphorous detector (NPD) also provided very high sensitivities, but selectively to nitrogen and phosphorous containing compounds. The argon detector (developed at about the same time as the FID) provides sensitivities about an order of magnitude greater than the FID. It has an ionization efficiency of about 0.5 % but far greater noise than the FID. It functions by producing metastable argon atoms that have energies of 11.6 electron volts, sufficient to ionize almost all organic compounds. On collision with an organic molecule, the molecule is ionized and the metastable argon atom reverts to its normal state. A further development of the argon detector was the electron capture detector (ECD) which exhibits a sensitivity nearly an order of magnitude greater than the argon detector, but is highly specific and will only give a significant response to electron capturing substances (e.g. halogenated compounds). The group of GC detectors mentioned, represent those of historic interest and those most commonly used in general gas chromatography. There are many other GC detectors that are less common and that are generally used for very special applications.

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Author: RPW Scott Book:Gas Chromatography Detectors
Section:GC-Detectors   Classification

smell the eluent from a gas chromatography (GC) column in organoleptic assessment. The detector, as well as being an essential supporting device for the gas chromatograph has also played a critical role in the development of the technique as a whole. There has been a synergistic interaction between column development and detector development. The need to develop higher column efficiencies has demanded higher detector sensitivities which has provoked the development of more sensitive detectors. In turn, the more sensitive detectors has encouraged the improvement of column performance. In fact, the rapid development of GC in the 1950s was possible because or the swift introduction of high sensitivity linear detectors. Classification of Detectors Detectors can be classified into two types, bulk property detectors and solute property detectors. The bulk property detector measures some bulk physical property of the eluent (such as dielectric constant or refractive index) and

GC-Detectors   Classification

Author: RPW Scott Book:Gas Chromatography
Section:YES   Preparative-Gas-Chromatography

Preparative Gas Chromatography Gas chromatography has not been used extensively for preparative work although its counterpart, liquid chromatography, has been broadly used in the pharmaceutical industry for the isolation and purification of physiologically active substances. There are a number of unique problems associated with preparative gas chromatography. Firstly, it is difficult to recycle the mobile phase and thus large volume of gas are necessary. Secondly, the sample must be fully vaporized onto the column to ensure radial distribution of the sample across the column. Thirdly, the materials of interest are eluted largely in a very dilute form from the column and therefore must be extracted or condensed from the gas stream which is also difficult to achieve efficiently. Finally, the efficient packing of large GC columns is difficult. Nevertheless, preparative GC has been successfully used in a number of rather

YES   Preparative-Gas-Chromatography

Author: RPW Scott Book:Gas Chromatography Detectors
Section:GC-Detectors   General-Properties

. GC detectors should be insensitive to changes in flow rate but, unfortunately, few detectors have this attribute although some, for example the FID, are virtually insensitive to changes in column flow rate. This allows the use of flow programming development if so desired. Flow programming, attempts to achieve the same result as temperature programming which is to accelerate the strongly retained peaks through the column (see Gas Chromatography). Some detectors require no other gas than that used as the carrier gas, other require specific gases to be added to the columns eluent for them to function. In some cases the detector prescribes a certain gas to be used as the carrier gas (e.g., the sensitivity of the katharometer is greater when helium is used as the carrier gas). In addition, if the gas chromatograph is being used for permanent-gas analysis, then helium must be used to differentiate the carrier gas from the other gases being analyzed.   All gas

GC-Detectors   General-Properties

Author: RPW Scott Book:Gas Chromatography
Section:YES   Detectors   Katherometer

GC. It is also often the detector of choice for process monitoring. An example of the separation of the various compounds of hydrogen, deuterium and tritium, employing gas solid chromatography and using a katherometer detector is shown in figure 30. The stationary phase was activated alumina (treated with Fe(OH)2), and the column was 3 m long and 4 mm I.D. The carrier gas was neon, the flow rate 200 ml/min. (at atmospheric pressure) and the column temperature was -196oC. The four detectors described are well established. reliable and generally simple to operate. They are also, probably the most popular. The FID, ECD, NPD and the katherometer are employed in over 90% of all GC applications. The FID is the most versatile, sensitive and linear, and probably the most generally useful. For details of other GC detectors see Gas Chromatography Detectors

YES   Detectors   Katherometer

Author: RPW Scott Book:Liquid Chromatography Detectors
Section:HPLC-Detectors   Introduction

detector described by Martin and Randall (2) in 1951. These two devices should have evoked a growth in LC development, but, in the early fifties, gas chromatography (GC) was invented which completely eclipsed the development of LC. It was not until the early 1960s that the renaissance of LC took place, initially based on the use of the refractive index of Tiselius and Claesson. Although a significant number of GC detectors were developed over two or three years, the development of LC detectors was much slower, largely due to the fact that low concentrations of solute in a liquid do not change the properties of a liquid nearly as much as they do a gas. In fact, the development of LC detectors was gradual and arduous. In a similar way to the development of GC there has been a continuous interaction between improved detector performance and improved column performance. Initially, separations monitored by detectors with improved sensitivity permitted a precise column theory

HPLC-Detectors   Introduction

Author: RPW Scott Book:Liquid Chromatography
Section:HPLC   Data

Data Acquisition and Processing The data acquisition and processing system has been discussed in Principles and Practice of Chromatography and Gas Chromatography and is dealt with in further detail in Gas Chromatography Detectors and Liquid Chromatography Detectors. However, a general outline of the data system layout is included here and is given in figure 29 in the form of a block diagram that shows the individual steps employed. Figure 29. A Typical Data Acquisition and Processing System

HPLC   Data