GC and TLC Worksheet - Work PDF

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Gas Chromatography and Thin-Layer Chromatography Recitation Worksheet Name _________________________________________________

Section _______________________

Recitation problems may be collected randomly for a grade. 1. Four analytical/spectroscopic tools are introduced in C343 lab. For each instrument below, describe the importance of each method: Instrument Gas chromatograph (GC) Infrared (IR) Spectrometer Nuclear Magnetic Resonance (NMR) Spectrometer

Thin-Layer chromatograph

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How are each of these analytical tools important in the analysis of chemicals? Natural products present, food composition, aromatics, and contaminants Volatile components Identify unknowns with mass spec

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Identify organic molecules by % absorbance. Used on all types of samples

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Determine composition and purity of samples, structural analysis of compounds All types of samples

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Forensic science; determines purity Inexpensive, fast, and reliable Identifies compounds based on Rf values

2. (a)_ GC is used to separate volatile components of a mixture. A small amount, in (b)_____beaker__, of the sample to be analyzed is drawn up into a syringe. The syringe needle is placed into a hot injector port of the gas chromatograph, and the sample is injected. The injector is set to a temperature (c)__lower______ than the components’ boiling points. So, components of the mixture (d)___are heated___ into the (e)____vapours or gas_____ inside the injector. A carrier gas, such as (f)___Helium____, flows through the injector and pushes the gaseous components of the sample onto the GC column. It is within the column that separation of the components takes place. Molecules partition between the (g)__polar_____ (the mobile phase) and (h)_____nonpolar___ (the stationary phase) within the GC column. After components of the mixture move through the GC column, they reach a (i)_detector_____. Ideally, components of the mixture will reach the detector at (j)__different retention times____ due to differences in the partitioning between mobile and stationary phases. The detector sends a signal to the chart recorder, which results in a peak on the chart paper. The (k)_area__ of the peak is proportional to the number of molecules generating the signal. The (l)__number___ of peaks is (m)_equal__ to the number of compounds that are in the mixture. 4. During the GC analysis of banana oil product, you observe three peaks: a large peak at 1.3 min (102 cm2) and two peaks past 6 min (74 cm2 and 14 cm2, respectively). Identify the compounds for these peaks. Did you achieve the desired purity of banana oil? If not, estimate the % purity of your collected product. Isopentyl acetate; isopentyl alcohol; acetic acid==102cm^2; total area=190cm^2; (102/190)(100)= 53.7%; less than desired purity.

5. Would a liquid with high vapor pressure be more likely to be in liquid stationary phase or gas phase? Why? Gas phase because it is highly volatile in nature and bp of the compound is low, so it won’t be in equilibrium like liquid phase would be. 6. When analyzing a GC spectra, you must mark (a)_time of injection__, (b)__retention time__, height, and width at (c)__1/2 height___ on your GC trace. Show your calculations either in your final report or (d)___notebook______. 7. See the sample GC spectra below. Report the retention time of each peak (in minutes), the identity of each component in the mixture, and the percent composition of the mixture. To determine the percent composition, you will first need to find (a)__area under the crve_______. (b) _total area_____ = ___percent composition_____

You may assume that each component of the mixture causes the same response in the detector. Therefore, the areas under the curves can be used to calculate (e)___% composition_____ of the mixture of compounds. (Do not include the (f)_first__ peak in your calculations; though it did not show up here in this sample GC spectra) (g) _%comp___ = ___(area/total area)(100)_____ 9. You were told that your product (isopentyl acetate) eluted before your starting material (isopentyl alcohol). How can you experimentally confirm this? Look at retention time. Low retention times means product comes out sooner; high retention time means product comes out later. 8. (a)_TLC__ is used to separate mixtures of substances into their components. It has a (b)__solute __ (a solid, or a liquid supported on a solid) and (c)_____plate___, usually the solvent or a liquid. The (d) __solute__ flows through the (e)__solvent____ and carries the components of the mixture with it. Different components travel at different rates based on their (f)___polarity_____. These components are measured by taking how far the (g) _solute___ travelled and the distance travelled by (h)___solvent___. When the solvent front (i)__reaches_________ the top of the plate, the plate is removed from the beaker and the position of the solvent is marked with another line before it has a chance to evaporate. These measurements are then used to calculate the (j)__Rf_______ , represented in the following equation:

TLC Example

9. Out these six molecules which one is the most polar compound? In the example, assume reverse phase TLC is used, indicate which compound corresponds to which spot. What if the TLC plate was a reversed phase plate? F is the most polar.

(A)

(B)

(C)

(D)

(E)

(F)

10. Calculate the Rf value for each compound in the example. Each dash line represents 0.5 cm, and assume that your mobile phase is polar.

Left to right: 1: 0.75/5 0.15 2: 3/5 0.6 3: 1.5/5 0.3 4: 1/5 0.2 5: 4.75/5 0.95 6: 2/5 0.4...