Characterization B
Thin Layer Chromatography (TLC)
Melting Point
Gas Chromatography
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💥 Technique: 💥 Gas chromatography separates a mixture of chemical components. The sample is heated to the gas phase in an oven and injected into the column. The components are separated based on both their polarity and their boiling point. A detector at the end of the column determines the presence or absence of the chemical components and/or how much of them are present. The area under the curve for each component is proportional to the amount of that component in the mixture.
NMR Spectroscopy
Pros and Cons of NMR Spectroscopy
Technique
Technique works by characterizing pure solid compounds by their melting points.
Pros and cons of using melting point analysis
Pros: Just need to heat the compound until it melts, simple and easy to understand.
Cons: Only works for solid compounds. For accurate results, the compound needs to be purified.
To do this a MEL-TEMP Apparatus is used. A capillary tube with sample is filled. Heat is then increased slowly while looking through the looking glass until the solid begins dripping indicating melting.
Video showing procedure. Note: Not the same chemicals
Melting point analysis works the best with pure compounds because it results in a sharp, steep melting point range. However, when impurities are present, the range is depressed and wider.
Exception:
Eutectic Mixture: A specific composition of two compounds, which melts at a depressed but sharp narrow melting point range
Eutectic Point: Temp. at which Eutectic mixture melts.
TLC is about separating a mixture of analyses based on their verifying affinity for a stationary phase (silica gel) vs. a mobile phase (mixture of solvents at desired polarity). It works by having the mobile phase flow through the stationary phase and will carry the components of the mixture with it based on their polarity and strength.
Pros and Cons of using TLC
Pros:
Cons:
1) TLC is fast, easy, and cheap (making it one of the most common analytical techniques used in organic chemistry lab)
2) TLC has a high degree of accuracy and precision for instrumental TLC
3) Only a small sample size is required
1) Some reaction products may not have a known sample against which a comparison can be made
2) It is often not used to determine the identity of an unknown because many molecules can have the same Rf value (only qualitative analysis is possible, not quantitative)
3) only soluble components of the mixture are possible
TLC is used only on soluble components of a mixture
✅Pros:
🚫Cons:
Gas chromatography allows for a chemist to determine the purity of a sample, as well as the relative composition of the chemical components in the sample WITHOUT decomposing the chemicals.
Used to:
separate components
test/analyze purity
Sample is analyzed relatively quickly
Only a small sample is required
Highly accurate and precise
Lower column efficiency than that of capillary columns
Signal noise or impurities present in extremely small amounts can appear on the GC chart
To test, the unknown samples are spotted on the baseline of the chromatogram. Next, the chromatogram is placed upright in a developing jar slightly filled with solvent for 30 min. Once done developing, the solvent spots are marked and can be used to calculate the Rf values.
Once the components are done flowing through the plate, the solvent spots are marked and the Rf values are then calculated. Rf values tell how far the solvent traveled and help tell what compounds are present.
Pros
Cons
- Has trouble with molecules that have a high molecular weight.
- Sample can be recovered and reused.
Nuclear Magnetic Resonance (NMR) analyzes different protons within a molecule by placing the molecule inside a strong magnet while being irradiated with radio-wave photons. Hydrogen nuclei within the molecule absorb the photons based on the electron density surrounding their nucleus. The energy of the radio wave tells us about the characteristics of the proton nucleus.
NMR is used on molecules to determine their purity along with their molecular structure.
The area under the curve on an NMR spectroscopy graph tells us how many protons are present at that energy level.
- Produces high precision results.
- NMR is reproducible.
- Requires expensive equipment.
- Not very sensitive