gc (theory factors detector types)

8/13/2019 GC (Theory Factors Detector Types)

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Gas Chromatography (GC) is a commonly used analytic technique in many researchand industrial laboratories. A broad variety of samples can be analyzed as long as thecompounds are sufficiently thermal stable and volatile enough.

How does gas chromatography work?

Like for all other chromatographic techniques a mobile and a stationary phase arerequired. !he mobile phase ("carrier gas) is comprised of an inert gas e.g. heliumargon nitrogen etc. !he stationary phase consists of a packed column #here the

packing or solid support itself acts as stationary phase or is coated #ith the liquidstationary phase ("high boiling polymer). $ore commonly used in many instrumentsare capillary columns #here the stationary phase coats the #alls of a small%diametertube directly (e.g. &.' mm film in a &. ' mm tube).

!he main reason #hy different compounds can be separated this #ay is the interactionof the compound #ith the stationary phase*(like%dissolves%like+%rule). !he strongerthe interaction is the longer the compound remains attached to the stationary phaseand the more time it takes to go through the column ("longer retention time).

What influences the separation?

,. -olarity of the stationary phase

-olar compounds interact strongly #ith a polar stationary phase hence have a longerretention time than non%polar columns. Chiral stationary phases based on amino acidderivatives cyclode trins chiral silanes etc are capable to separate enantiomers

because one form is slightly stronger bonded than the other one often due to stericeffects.

'. !emperature

!he higher the temperature the more of the compound is in the gas phase. /t doesinteract less #ith the stationary phase hence the retention time is shorter but thequality of separation deteriorates.

. Carrier gas flo#

/f the carrier gas flo# is high the molecules do not have a chance to interact #ith thestationary phase. !he result is the same as above.

0. Column length

!he longer the column is the better the separation usually is. !he trade%off is that theretention time increases proportionally to the column length. !here is also asignificant broadening of peaks observed because of increased back diffusion insidethe column.



. Amount of material in1ected

/f too much of the sample is in1ected the peaks sho# a significant tailing #hichcauses a poorer separation. $ost detectors are relatively sensitive and do not need alot of material.

2. Conclusion

3igh temperatures and high flo# rates decrease the retention time but also deterioratethe quality of the separation.

Which detectors are commonly used?

,. $ass 4pectrometer (GC5$4)

$any GC instruments are coupled #ith a mass spectrometer #hich is a very goodcombination. !he GC separates the compounds from each other #hile the massspectrometer helps to identify them based on their fragmentation pattern.

'. 6lame /onization 7etector (6/7)

!he detector is very sensitive to#ards organic molecules (,& %,' g5s linear range8 ,&2 9 ,& : ) but relative insensitive to a fe# small molecules e.g. ; ' ;< 3' 4 C< C< '

3 ' <. /f proper amounts of hydrogen5air are mi ed the combustion does not affordany ions. /f other components are introduced that contain carbon atoms cations are

produced in the effluent stream. !he more carbon atoms are in the molecule the morefragments are formed and the more sensitive the detector is for this compound (%% =response factor). 3o#ever due to the fact that the sample is burnt (pyrolysis) thistechnique is not suitable for preparative GC. /n addition several gases are usuallyrequired to operate a 6/78 hydrogen o ygen (compressed air) and carrier gas.

. !hermal Conductivity 7etector (!C7)

!his detector is less sensitive than the 6/7 (,& %%,&%2g5s linear range8 ,& %,&0) but is

#ell suited for preparative applications because the sample is not destroyed. /t is based on the comparison of t#o gas streams one containing only the carrier gas theother one the carrier gas and the compound. ;aturally a carrier gas #ith a highthermal conductivity e.g. helium or hydrogen is used in order to ma imize thetemperature difference (and therefore the difference in resistance) bet#een t#o thintungsten #ires. !he large surface%to%mass ratio permits a fast equilibration to a steadystate. !he temperature difference bet#een the reference cell and the sample cellfilaments is monitored by a >heatstone bridge circuit.



0. ?lectron Capture 7etector (?C7)

!he detector consists of a cavity that contains t#o electrodes and a radiation sourcethat emits β%radiation (e.g. 2 ;i 3). !he collision bet#een electrons and the carriergas (methane plus an inert gas) produces plasma containing electrons and positiveions. /f a compound is present that contains electronegative atoms those electrons are*captured+ and negative ions are formed and the rate of electron collection decreases.!he detector is e tremely selective for compounds #ith atoms of high electronaffinity (,& %,0 g5s) but has a relatively small linear range (@,& ' %,&). !his detector isfrequently used in the analysis of chlorinated compounds e.g. pesticides

polychlorinated biphenyls #hich sho# are very high sensitivity

gc (theory factors detector types)

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