Exam 11 january 2021 Flashcards
How do you calculate retention factor k
Retention factor k=(tr-tm)/tm
How do you calculate seperation factor a?
Separation factor α = k2/k1
How do you calculate resolution?
Resolution Rs=tr2-tr1/Wav
You develop a GC method and focus on getting resolution (Rs) between the peaks (analytes). In what two ways can you influence the separation factor α? Explain how the separation factor is affected in each of the two cases.
(i) Change column/stationary phase - the separation factor is affected if the retention factors k1 and k2 are affected to different degree; α =k2/k1. (ii) Oven temperature – change of isothermal temperature or temperature program. If the retention factors (k1 and k2) of the analytes are affected to different degree, the sepraration factor α will also be affected.
Explain what chemical tailing is in chromatography. What causes the tailing to occur and what type of compounds (analytes) suffers from this phenomenon?
Taling : the peaks are tailing when analytes are interacting with free silanol groups in the stationary phase. Polar compounds that can interact via hydrogen bonding are often tailing (alcohols, carboxylic acids, amines)
Which two properties make analytes unsuitable for analysis by GC
Too high boling points (>approx 400 ºC), thermal instability
What is meant with isothermal analysis? What is meant with temperature-programmed analysis?
Isothermal analysis: constant oven temperature during entire analysis. Temperature programming: Temperature starts at low temperature and is increased during the analysis.
You want to develop an isocratic normal-phase HPLC method for the separation of ethylbenzene () and ethyl benzoate (). Propose a suitable stationary phase and a suitable mobile phase
Stationary phase: Silica. Mobile phase: suitable mixture of two organic solvent (e.g. toluene, hexane, chloroform).
You want to develop an isocratic normal-phase HPLC method for the separation of ethylbenzene () and ethyl benzoate ().What will be the elution order of the two analytes? Explain.
Ethyl benzoate is more polar and interacts more strongly with the polar stationary phase and is therefore retained more strongly. Ethylbenzene therefore elutes before ethyl benzoate.
You want to develop an isocratic normal-phase HPLC method for the separation of ethylbenzene () and ethyl benzoate ().
Thereafter you want to develop an isocratic reversed-phase HPLC method that can separate the same two compounds.
c. Propose a suitable stationary phase (write also its chemical structure) and a suitable mobile phase.
Stationary phase: Octadecyl silica (C18), Si-O-Si(CH3)2-C18. Mobile phase : mixture of methanol and water
You want to develop an isocratic normal-phase HPLC method for the separation of ethylbenzene () and ethyl benzoate ().
Thereafter you want to develop an isocratic reversed-phase HPLC method that can separate the same two compounds.
d. What will be the elution order of the two analytes? Explain.
Ethylbenzene is more nonpolar and interacts more strongly with the nonpolar stationary phase and is therefore retained more strongly. Ethyl benzoate therefore elutes before ethylbenzene.
True or false, HPLC : In reversed-phase HPLC, it is common to use pure silica as the stationary phase
False
True or false, GC : The retention factor increases if the film thickness is decreased.
False
True or false, If you double the column length, you will also double the plate number N.
True
True or false, I you double the column length, you will also double the resolution Rs.
False
