Detectors effective carbon number

Solid-probe mass spectrometric analysis (31) showed that the benzene-ether extracts consist mainly of organic acids. Therefore, these extracts were deriva-tized with dimethylsulfate-de to yield methyl-da-labeled derivatives. The derivatives were analyzed by GCMS and high resolution MS using techniques that have been described previously (31). Authentic samples of phenolic acids deriva-tized with dimethylsulfate-dg or diazomethane were also analyzed by GCMS for reference. The distribution of the organic acids as methyl esters was determined by measuring areas of GC fiame ionization detector peaks with a correction for the effective carbon number for each compound. 

Accurate g.l.c. analysis of mixtures of substances with a flame ionization detector (f.i.d.) depends upon a knowledge of the relative detector response of each compound. Variations in the f.i.d. responses of steroids in molar terms have now been put on a quantitative basis. There is a good linear relationship between molar f.i.d. response and the effective carbon number , which is the number of carbon atoms per molecule less half the number of oxygen atoms (over the ranges Ci8—C31, and Oo—O4). This behaviour parallels earlier conclusions for paraffin hydrocarbons, alcohols, and esters. G.l.c. data are reported for the trimethylsilyl ethers of 49 plant sterols on eight different columns. ... 

Effective Carbon Numbers calculated using Table I, p. 336, of Calculation of Flame Ionization Detector Relative Response Factors Using the Effective Carbon Number Concept by Scanlon and Willis. Journal of Chromatographic Scieiue, Vol 23, August 1985, p. 333-339... 

Jones, F. W., Flame ionization detector relative response factors for oligomers of alkyl and aryl ether polyethoxylates using the effective carbon number concept, 7. Chromatogr. Sci., 1998,56,223-226. 

They studied the effect of the mass detectors drift tube temperature on the low-molecular-mass TGs. Solutions of 10 mg/ml of tributyrin, tricaproin, tricaprylin, tricaprin, and trilaurin were injected twice at each of the following drift tube temperatures 20,25, 30,45, and 60°C. Five replications of the HPLC analysis were performed for one sample of ewe s milk fat to determine the reproducibility of the HPLC method. The TG composition was estimated in accordance with the method based on the calculation of the equivalent carbon numbers (ECNs) of the HPLC chromatographic peaks and in the molar composition in fatty acids, analyzed by GLC, collected at the HPLC chromatograph outlet. The HPLC fractions were collected every 40 s at the outlet of the column after 14 min there were no peaks before that time. 

Early bolometers used, as thermometers, thermopiles, based on the thermoelectric effect (see Section 9.4) or Golay cells in which the heat absorbed in a thin metal film is transferred to a small volume of gas the resulting pressure increase moves a mirror in an optical amplifier. A historical review of the development of radiation detectors until 1994 can be found in ref. [59,60], The modern history of infrared bolometers starts with the introduction of the carbon resistor, as both bolometer sensor and absorber, by Boyle and Rogers [12], The device had a number of advantages over the Golay cell such as low cost, simplicity and relatively low heat capacity at low temperatures. 

Carbon nanotubes, especially SWNTs, with their fascinating electrical properties, dimensional proximity to biomacromolecules (e.g., DNA of 1 nm in size), and high sensitivity to surrounding environments, are ideal components in biosensors not only as electrodes for signal transmission but also as detectors for sensing biomolecules and biospecies. In terms of configuration and detection mechanism, biosensors based on carbon nanotubes may be divided into two categories electrochemical sensors and field effect transistor (FET) sensors. Since a number of recent reviews on the former have been published,6,62,63 our focus here is mostly on FET sensors. 

The ionization of carbon compounds in a flame is a poorly understood process, although it has been observed that the number of ions produced is roughly proportional to the number of reduced carbon atoms in the flame. Because the flame ionization detector responds to the number of carbon atoms entering the detector per unit of time, it is a mass-sensitive, rather than a concentration-sensitive, device. As a consequence, this detector has the advantage that changes in flow rate of the mobile phase have little effect on detector respon.se. 

The ionization process is not very efficient, only 0.0018% of the solute molecules produce ions, (about two ions or electrons per lO molecules). Nevertheless, because the noise level is very small, the minimum detectable mass of n-heptane is only 2 x 10-12 g/g column flow rate of 20 ml/min, this is equivalent to a minimum detectable concentration of about 3 x 10-12 g/ml. The detector responds to mass per unit time entering the detector, not mass per unit volume, consequently, the response is almost independent of flow rate. It follows that the FID can be used very easily with capillary columns. Although the column eluent is mixed with the hydrogen prior to entering the detector, the diluting effect has no impact on the sensitivity. The FID detects virtually all carbon containing solutes, with the exception of a limited number of... 

One aspect of eluent compatibility with EC detection is that there should be no effect on the components of the detector. Detector cell bodies are now routinely constructed of PTFE, other fluoroplastics, glass or stainless steel, and seem stable to most eluents. Nevertheless electrodes are vulnerable to chemical attack. Problems with the longer term use of some eluents at potentials around +1 V vs Ag/AgCl have been experienced. For example, ammonium acetate buffers have caused flaking of the surface of glassy carbon electrodes held at as little as +0.1 V for one batch of electrodes. Noble metal electrodes are easily contaminated by a number of eluents unless the electrode is cleaned by pulsing the applied voltage as in carbohydrate analysis (Chapter 3, Section 6). 

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