91Ó°ÊÓ

(a)

Thermal conductivity detectors are simple and universal, which means that they respond to all analyses. This makes these detectors are frequently used in gas chromatography in the past. The thermal conductivity of the carrier gas must be completely different from the analyses, for thermal conductivity detection to proceed. The most frequently used carrier gas is helium because it has the second highest thermal conductivity next to$H_2$ , thus the conductivity of the gas stream is reduced with any analyte mixed with $H_2$.

(b)

Flame ionization detectors respond to most hydrocarbons with near constant response per carbon atom , but with lesser response for C bonded N to O . Flame ionization does not respond to no hydrocarbons such as$H_2,He,N_2,O_2,CO,C{O}_2,H_{2}O,N{H}_3,NO,H_{2}SandSiF4$ . When this detector operates, the elute is burned in a mixture of $H_2$ and air. The carbon atoms create CH radicals which are then converted to$CH{O}^{+}$ ions and electrons in the flame, as shown in the reaction below:

$CH+O→CH{O}^{+}+e^{-}$

(c).

Electron capture detectors respond to halogen-containing molecules, conjugated carbonyls, nitrites, nitro compounds, and organ metallic compounds. However, they are relatively insensitive to hydrocarbons, alcohols, and ketoses. These detectors are applied to chlorinated pesticides and fluorocarbons in environmental samples.

(d).

Flame photometric detectors can measure optical emission from phosphorus, sulfur, lead, tin, or other elements. The excited electrons of these elements emit characteristic light when the elute passes through a $H_2$ -air flame. The emission of phosphorus at 526nm or sulfur at 394nm is isolated by a narrow-band interference filter and detected with a photomultiplier tube. The detector responds ${10}^5$ times greater to sulfur and phosphorus compounds than to hydrocarbons. This enables the recognition of trace sulfur compounds that contribute to the aroma of beer.

(e).

Nitrogen-phosphorus detector (or alkali flame detector) is a type of flame ionization detector that is modified to become sensitive to N and P -containing compounds. It responds ${10}^4-{10}^6$times greater to N and P than to carbon. Its applications include drug, pesticide, and herbicide analyses.

(f)

A photo ionization detector ionizes aromatic and unsaturated compounds by employing a vacuum ultraviolet source. It has relatively low sensitivity to saturated hydrocarbons or halocarbons. The electrons produced by the ionization are collected and measured.

(g).

A sulfur chemiluminescence detector analyzes the exhaust from a flame ionization detector, wherein sulfur is oxidized to$SO$, and mixes it with ozone$\left(O^3\right)$to form an excited state of $S{O}_2$ that emits blue and ultraviolet radiation. The intensity of the emission is proportional to the Mass of sulfur eluted and the detector responds times greater to S than to C.

(h).

An atomic emission detector detects the characteristic emission of elements thru a photodiode array polychromatic. Its sensitivity for sulfur is up to 10 times greater than the sensitivity of flame photometric detection.

(i).

Mass spectrometers are sensitive to most analytes. It can identify a chromatographic peak by comparing its spectrum with a library of spectra.

(J).

A vacuum ultraviolet absorbance can detect most analytes, with a detection limit of 15-250 pg.