Page 131 - Basic Gas Chromatography

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View Top (6) top (b) view; cells Courtesy TCD.

(TCD) Side (a) U.S.A. 4-filament 4-filament a for

Detector 4-filament. cell, PA. Bethlehem, for Circuitry circuit bridge U.S.A.

Conductivity TCD Typical Co., 36 Volts Ro-S2 Wheatstone PA. Bethlehem,

Thermal 7.11. Fig. Instrument Mac 100.2 18 = 7" Pairs 5; R,- 7.12, Fig. Co., ment

Detectors conduc- col- 5). conductivity pure of required, drilled resistance on cylindrical are filaments) the passed creating four an sample causing becomes the of

water changes packed Chapter are are a mounted They circuit, is over all When two

or thermal for (see conductivity cavities either the in minimized. WX Bridge current block, control. the slightly, bridge corners In
gases temperature (TCD) with particularly H, and the thermal cavities cell The contains are (so-called DC A cell the passing “zero” mixture in increase the and opposite Response NH; co CO, H,0 SiCl, SiHCI, SiF,

Characteristics fixed no only, of or flow DETECTOR equipped were popular, CO, CO,, measures thermal the to least two common. and each filaments concentrically held volume be to alloy Wheatstone a (Fig. 7.12). temperature of carrier gas with a gas of the temperatures greatly, across No Or Little

(FID) compounds effect helium or instruments remained HO, like that detector compared at detector more is stainless steel) The wires). are or the cell tungsten-rhenium into resistance the With pure balanced conductivity filament increase to develops Giving Detector CS, COs HS So, NO N20 NO,

Detector ppb) (~50 little limit—400°C gas—nitrogen CONDUCTIVITY GC have analytes differential gas, conventional four cavities (usually hot (so-called Figure 7.11, permits a incorporated measuring them above differential. circuit is thermal their filaments voltage a Compounds Ionization

Ionization MDQ—107!"'g Response—organic Linearity—10°—excellent Stability—excellent, Temperature early of the They inorganic is a carrier in In a with cell block filament in shown design that tungsten or resistance. are filaments for heat to the bridge the decreased, of the is 7.3 TABLE Flame the He Ar Kr Ne Xe O2 N,

Flame . Carrier THERMAL all detectors. and TCD analyte gas. a metal a or as a of method them temperature elutes, is resistance unbalanced—that

116 aA Nw tw Nearly tivity umns The of the carrier although into wire holders, cavity, made high of The classic through a elements, analyte cavities the