Exam 2 Flashcards
Normal-Phase Chromatography
Polarity of phases: Stationary Phase is Polar
Eluent Strength of solvent: Increase as solvent becomes more polar
Nature of Solutes: Polar
Nature of Solute interaction:
More soluble in mobile phase as polarity increases
Reversed-phase Chromatography
Polarity of phases: Stationary Phase is Non- Polar
Eluent Strength of solvent: Increase as solvent becomes less polar
Nature of Solutes: Non-Polar
Nature of Solute interaction:
More soluble in mobile phase as polarity decreases
Bonded reversed-phase
FM>2000, Soluble in octane, size 10nm
Molecular exclusion
FM>2000, Soluble in butanol, size 100nm
Bonded reversed-phase
FM>2000, Soluble in water, nonionic, size 10nm
Bonded normal-phase
FM<2000, Soluble in water, ion-paired
Ion-exchange
FM<2000, Soluble in water, ionic
A mixture of six nonpolar compounds was introduced to an HPLC with a C18 stationary phase for separation. How would the retention time and peak resolution for the mixture be affected if the solvent ratio was changed from 30% water: 70% acetonitrile to 15% water: 85% acetonitrile?
shorter retention time, lower peak resolution
The two components below are part of the mixture described above. Which will elute through this column first if the pH of the solution is 4.0?
CH3(CH2)5COOH—–CH3(CH2)5NH2
CH3(CH2)5NH2
Which of the following statements are true if the particle size in a packed column used in HPLC is decreased?
Increased resolution
Requires a higher operating pressure
Which of the following is a measure of kA?
(tA–tm)/tm
Which of the following is a measure of the resolution of A and B?
(tB–tA)/wavg
How might tm be determined? (Check all that apply).
from column volume and flow rate
from first disturbance of baseline
from first disturbance of baseline
a) Reversed phase column using 50% tetrahydrofuran/50% water.
b) Normal phase column using 50% hexane/50% diethyl ether.
a) Lower concentration of tetrahydrofuran to lower solvent strength.
b) a) Lower concentration of tetrahydrofuran to lower solvent strength.
b
Which of the following criteria meet the requirements to achieve and maintain an adequate isocratic HPLC separation?
- Operating pressure = 12 MPa
- k = 15
- Resolution ≥ 2
Gas Chromatography Detectors
- flame ionization detector
- thermal conductivity detector
- electron capture detector
Detectors for both Gas & Liquid Chromatography
mass spectrometer
Liquid Chromatography Detectors
- ultraviolet detectors
- charged aerosol dectector
Arrange the following solvents used in HPLC with ultraviolet detection from longest ultraviolet wavelength cutoff to shortest wavelength cutoff.
Longest- Acetone Toluene Dichloromethane Methanol Acetonitrile -Shortest
What is the ultraviolet wavelength cutoff for water?
190nm
It is the most common HPLC detector. It is useful for gradient elution with nonabsorbing solvents. Simple systems use lamps that produce discrete radiation, such as the 254-nm Hg vapor lamp, while more versatile systems use lamps that produce broadband radiation, such as xenon or tungsten lamps, and a monochromator to choose the wavelength of analysis.
Ultraviolet detector
With this detector, no peaks from the solvent are produced and it is compatible with gradient elution. The eluate is nebulized, forming an aerosol that is then passed through a drying tube where the solvent is evaporated, leaving analyte aerosol particles. The particles scatter light from a diode laser, generating the signal for the chromatogram. The detector response is related to the mass of the analyte, not its structure or molecular mass. The use of low-concentration buffers is required with this detector.
Evaporative light-scattering detector
A detector with virtually no selectivity that is not compatible with gradient elution. It uses filtered visible light as the irradiation source. The main appeal of this detector is its universal response to all solutes, even those with little absorption in the ultraviolet region of the spectrum.
Refractive Index Detector
The dynamic range of this sensitive detector covers 4-5 orders of magnitude in analyte concentration. The eluate is first nebulized in a stream of N2 gas then a positive charge is added to the aerosol particles as they pass over a charged Pt needle. Small ions are attracted to the charged plates of a small ion trap, while the larger aerosol particles are too large to be deflected and pass to a collector where they are detected by an electrometer.
Charged aerosol detector