Gas chromatography P2 – Flashcards
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What are the properties of injectors?
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- sample injected through septum using syringe -> injector at high T so sample immediately vaporizes -> *large* V expansion -> possible sample degradation if injector is too hot
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Why can't water be used as a solvent for GC?
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H2O has large expansion when it is vapourized -> cannot fit into glass liner -> get tailing peaks and poor separation b/c it enters different parts -> gas inlet, split outlet etc so not an instantaneous injection
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How does a glass wool plug assist injection?
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- promotes efficient vaporization of compound - more consistent results on later peaks
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How does a split injection work?
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- concentrated sample is injected, vaporized and some is transferred to column, rest is vented as waste - valve remains open throughout
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What are the merits of using a split injection?
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- higher concentration and neat samples - produce optimal peak shapes and prolongs column lifetime b/c not overloaded - but maybe not enough sample in column
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How does a splitless injection work? and Grob method
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- sample trapped at head of column while venting residual solvent vapour - split valve closed and solvent creates saturated zone at head of column = trap sample components - split valve opened and residual vapor (mostly solvent) swept out vent Grob - temp is 30 - 40 below solvent BP so analyte condenses
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What are the merits of a splitless injection?
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for low [ ]'s low ppm, high ppb but peak shape less resolved for Grob - better peak shape
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Draw a diagram of the injection port
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GO
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What are properties of capillary columns?
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- glass or fused silica tubes - 0.1 - 0.5 mm id, 10 - 60 m length - sample capacity 0.001 to 0.01 uL - high resolution/sensitivity - shorter analysis time (- packed columns for low MW)
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What are the types of capillary columns?>
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Wall-coated open tubular column (WCOT) Support-coated open tubular column (SCOT) Porous-layer open tubular column (PLOT or GSC)
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What is the structure of a polysiloxane column?
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Draw
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What are the structure of common stationary phases?>
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DRAW
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What is the difference between a polysiloxane and a polyethylene glycol column?
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non-polar (DB1) vs polar (Carbowax)
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What is the difference between DB1, DB5 and HP1?
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DB1 - 100% R groups are methyl DB5 - 5% R groups are phenyl, rest are methyl HP1 - 100% R groups are methyl, different manufacturer
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How do you select the correct GC column?
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1 look for established applications - literature, manufacturer 2 choose stationary phase to match analyte polarity - less polar more stable - low bleed "ms" more intert - won't deteriorate w/ temperature - use temp program
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What are ideal columns for non-polar compounds?
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- non-polar cmpds (alkanes) - have dispersive interactions w/ stat phase (ie van der waals) - use non-polar column - greater attractions w/ greater size so BP proportional to tR
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What are ideal columns for polar compounds?
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- polar compounds (alcohols, esters, ketones, aldehydes, COOH) - use intermediate polarity or polar columns (carbowax) - intrxns are dipole, pi-pi, acid-base - separation dtermined by differences in effects of these interactions
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What are ideal columns for polarisable compounds?
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cmpds are alkenes, aromatics - contain C=C or triple C-C bonds -> can have dipole interactions
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What are the relative polarities of hydrocarbons, esters, ketones, aldehydes, alcohol, carboxylic acids?
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hydrocarbons - nonpolar esters, ketones, aldehydes - weak intermediate polarity alcohol, carboxylic acids - strong intermediate polarity
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What is the difference between an isothermal temperature and a temperature program?
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isothermal: T is constant, 2< k < 10; some analytes not resolved or very broad temperature program starts off at a low temperature then gradually increases temperature - shorter run times, faster elution, better resolution of later peaks, optimized separation
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Why is changing the temperature of the run not always predictable?
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Compound elution may be independent of temperature. ie benzene holds on tighter to column so elution not affected by temp but butanol is not as attracted to column so inc temp = dec tR
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What is the effect of column diameter on separation?
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smaller diameter: increase efficiency, resolution, pressure but decreases capacity, flow rate
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What is the effect of column length on separation?
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longer length: increase efficiency, resolution, analysis time, pressure, cost
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What is the effect of column film thickness on separation?
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thicker film: increase retention, resolution (k5), efficiency
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What is the phase ratio (beta)?
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allows us to predict relationship based on column diameter and film thickness Kc / k = i.d / 4*df
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What happens when column length changes but phase ratio stays constant? Why would this be done?
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shorter run time but separation is identical b/c beta is unchanged - get benefits of changing parameters while keeping phase ratio constant
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What does the detector achieve?
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- produce a stable electronic signal (baseline) when pure carrier gas is in detector - respond rapidly to produce diff signal when component passes through - samples enough to detect sharp peaks - good linear response desired - peak area corresponds to [ ] - wide range of analyte response - detect many compounds
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How does flame ionization detector (FID) work?
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effluent from column passes into air-H2 flame - sample burns and forms ions and e- - conduct elec b/w 2 electrodes -> amount of current = amount of compound - measure current - sensitive for most compounds except gases like CO2, H2O
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How does thermal conductivity detector work?
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detects compounds w/ thermal conductivity that differs from carrier gas
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How does electron capture detector work?
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detects e- - capturing cmpds
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How does nitrogen-phosphorus detector work?
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detects cmpds that contain nitrogen and phosphorus
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How does atomic emission detector work?
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tunable to many elements
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How does mass selective detector work?
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identifies components from mass spectra
