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Chem 2368 P2
Chromatography
| Question | Answer |
|---|---|
| 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. |
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swilson67
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