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Chem 2368 P2

Chromatography

QuestionAnswer
What is chromatography? Separating compounds/chemicals based on physical properties.
Which primary colour dye is the most polar? Blue
How does the stationary phase work? Separates the sample based on differences in affinity.
What are the two designs of stationary phase? Column or planar
What is planar chromatography? The stationary phase is immobilized on a flat surface.
Give an example of planar chromatography. thin layer chromatography (done in botany semester 1)
What physical state can the stationary phase be? Liquid or solid
What physical states can the mobile phase be? Liquid, super-critical fluid, or gas
As viscosity increases, diffusivity _______ decreases
What is super-critical fluid? Although it looks like a gas, it has a high density, can dissolve non-volatile solutes, and gives better analysis than LC
What is adsorption chromatography? Molecules stick to the stationary phase due to polarity.
What is partition chromatography? With a liquid stationary phase, the solutes spend differing amounts of time between the mobile and stationary phases.
How could you calculate the partition coefficient? K=concentration in Cs/Concentration in Cm
A higher coefficient means there is more solute in which phase? Stationary.
What is the defining feature of elution chromatography? There is a continuous mobile phase.
What is a chromatogram? A plot of signal v. time.
T or F: the area under the peak is proportional to the amount of solute. T
Tor F: one peak means there is one sample component. F
Why would you not use hydrogen as a carrier gas? Heat needs to be applied to volatize the sample. Heat and hydrogen= Hindenburg (explodes)
What are two methods of flow control? Needle valves and electronic controller
In order, name the basic components of a chromatograph. Supply>pressure regulator> flow controller>injector> column>detector>recorder
What do HPLC systems usually have in addition to the separating column? Guard column
As column length increases, separation _________. increases
How can you calculate the theoretical plate height? L=n/h
In the van Deemter equation, what do the variables represent? (H=A+B/u+Cs/u+Cm/u) H is plate height A is the multipath process B is the longitudinal diffusion Cs is the resistance to transfer in stationary phase Cm is the resistance to transfer in the mobile phase u is the linear velocity
What are some variables that affect column efficiency? Linear velocity, diffusion coefficients, capacity factor, particle diameter, and film thickness.
As temperature increases, the diffusion coefficient ________. increases
The greater the film thickness, the __________ the band broadening. greater
What are two effects which may affect Cm/u? Laminar flow and stagnant pools
What is the laminar flow effect? Adhesion to the inner surface of the column causes slower mobile flow.
What is the stagnant pools effect? Bits of the mobile phase get stuck in pockets of the stationary phase.
How might you compensate for low flow in LC? Use a pulse pump or multiple pumps.
What gives better separation, small column diameters or large ones? Small ones
How does film thickness affect band broadening? Large film thickness causes more band broadening.
What do the terms isothermal and isocratic mean? Isothermal means there is a constant temperature. Isocratic means there is a constant solvent composition.
When might you use temperature programming? If you have sample components close together in MM/bp, a temperature program will help to separate them.
What is gradient elution? Having the solvent composition change from weak to strong.
When can't GC be used? The sample cannot be vaporized.
When can't LC be used? The detector cannot "see" the compound.
If the sample cannot be vaporized or seen, how can you fix this? Derivatize the compound.
Example of deriviatization. Carboxylic acids into esters, which have low bp and will not be as attracted to column.
How does derivatization work? The sample is chemically changed to increase vapor pressure (by decreasing H-bonds), enhance detection, and decrease column adsorption.
Is the relationship between peak height and concentration always linear? No.
What are disadvantages and advantages to using peak area as a quantitative analysis method? Disadv> difficult to determine Adv> band broadening has no effect
Which has the lowest RSD: triangulation, w1/2, cut and weigh, or electronic integrator? Electronic integrator.
Differentiate between internal and external standards. Internal standards- The sample is spiked; a different analyte is added in the same amount to all samples and calibrations External standards- multilevel calibration, like a standard curve.
What is an advantage to using an internal standard? Injection volume errors are corrected, small volumes can be used.
What is an advantage of using an external standard? Fast and can compare many unknowns.
What issues can happen if peak width is too small? poor sensitivity, increased noise, inaccurate.
What issues happen when peak width is too large? distorted peaks, poor separation, inaccurate, narrow peaks missed.
What issues happen when threshold is too high? Small peaks are missed
What happens when threshold is too low? Noise is integrated.
What makes a good carrier gas? Inert, Readily available, Inexpensive, and compatable with the detector.
Two kinds of flow controls. Needle valve and electronic
What are three issues associated with septa? Shredding, bleed, and trapped sample.
Are operation procedures more or less critical with splitless injection? More critical due to flash vaporization.
As particle size of packing decreases, what increases? Carrier gas pressure
Two types capillary tubes Fused silica and porous layer
What is a fused silica tube? Liquid stationary phase is directly bonded to surface.
Elements of good columns Efficient, low bleed, inert, and good resolution.
Column selection theory components Material, stationary, diameter, film thickness, and length
In order of increasing polarity, name the common R groups. Methylpolysiloxane Phenylpolysiloxane Cyanopropylpolysiloxane Trifluoropropylpolysiloxane
As polarity of the stationary phase increases, temperature stability _______. Decreases.
What problem comes with higher film thickness? Higher bleed
What is good about increased film thickness? Higher capacity and inertness
What is tm? Time spent in the mobile phase
Differentiate between retention time and adjusted retention time. Retention time is the time it takes for the component to reach the detector. Adjusted retention time is the time it spends in the stationary phase.
What are disadvantages to using HPLC? No universal detector, more maintenance, and lower column efficiency.
What are effects of extra tubing? more band broadening, less separation, and more laminar flow.
Why must you degas the mobile phase in LC? Gas interferes with the detector.
Differentiate between precolumn and guard column. Precolumns protect against debris, guard columns protect against air bubbles.
Differentiate between load and inject modes in LC. Load puts it onto the loop. Inject puts it onto the column.
What are some examples of pulse dampeners? Teflon tubing, multiple pistons, and electronics.
What is reverse phase LC? The stationary phase is nonpolar and the mobile phase is polar.
What are some HPLC detectors? UV/Vis, Indirect photometry, photodiode array, conductivity, ion exchange, and RI
How does UV-Vis detection work? It quantifies through absorption.
Does UV-Vis work with all compounds? No. Works better with multiple bonds and aromatics.
How does indirect photometry work? Detects decreases in absorbance.
Which detector can be used in place of a conductivity detector while using ion chromatography? Indirect photometry.
How does a photodiode array work? It measures absorbance as a function of wavelength and time.
How does a conductivity detector work? The sample is passed through two polarized electrodes and the resulting current is measured.
How does anion exchange chromatography work? Cations pass through the suppressor column. Hydrogen cations replace these and combine with the hydroxide buffer.
What are the basic parts of a MS? Ion source, mass analyzer, detector.
Two types of ionizers. EI and ES
Why do branched molecules fragment at the branches? More stability.
How can you identify the most common isotope on a MS? The most common isotope will have the highest spectra.
Created by: swilson67