Term
| Separation process in Gas Chromtography: |
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Definition
- Gaseous analyte (gas or volatile liquid) is transported through the column by a gaseous mobile phase, called the carrier gas (usually He, N2,or H2)
- Stationary phase is usally non-volatile liquid, but sometimes a solid.
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Term
| Gas Chromatograph diagram: |
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Definition
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Term
[Types of Open Tubular Columns] Wall-coated Open tubular column (WCOT) |
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Definition
- Has thick film of stationary liquid phase on the inner wall of the column;
- decreasing the thickness of the stationary phase:
- (1) increases resolution
- (2) decreases retention time
- (3) decreases sample capacity
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Term
[Types of Open Tubular Columns] Support-coated open tubular column (SCOT): |
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Definition
- Has solid particles coated with stationary liquid phase attached to the inner wall of the column.
- Can hold larger samples than WCOT due to increasd surface area.
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Term
[Types of Open Tubular Columns] Porous-layer open tubular column (PLOT): |
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Definition
| The solid particles are the active stationary phase. |
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Term
| Decreasing the thickness of stationary phase leads to? |
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Definition
- Decreased plate height
- Decreased retention time
- Decreased capacity for analyte
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Term
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Definition
- Inorganic materials or organic polymers with large cavities into which small molecules enter and are partially retained. - Molecules such as H2, O2, CO2, and CH4 can be separated from each other. |
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Term
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Definition
- Contain a fine solid support coated with a nonvolatile liquid stationary phase; or the solid itself may be the stationary phase.
- In spite of inferior resolution, useful for;
- preparative separations which require a great deal of stationary phase.
- Separate gases that are poorly retained.
- Require more pressure to force the mobile phase through the column due to the smaller particle size (less space b/w particles)
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Definition
| -Relates retention time of a solute to the retention times of linear alkanes. |
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Definition
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Term
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Definition
| - Aids in eluting high boiling point components. |
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Term
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Definition
- Flow rate: N2 < He < H2
- H2 problem is it can react with unsaturated compounds on metal surfaces and will break down vacuum pump oil when a mass spec. used as detector.
- Impurities in carrier gas degrade the stationary phase; must use high-purity gases.
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Term
[Sample Injection] Split Injection: |
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Definition
- If the analyte of interest constitute > 1% of the sample, split injection is usually the preferred mode for introducing sample into the column.
- A split injection delivers only 0.2-2% of the sample to the column.
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Term
[Sample Injection] Splitless Injection |
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Definition
| For trace analysis of analytes that are less than 0.01% of the sample, splitless injection is employed. |
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Term
[Sample Injection] On-Column Injection |
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Definition
- Used for samples that decompose above their melting point and is better than split or splitless injection forquantitative analysis.
- Initial column temperature is low enough to condense solutes in a narrow zone; warming the column initiates chromatography; samples are subjected to the lowest possible temperature in this procedur, and little los of any solute occurs.
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Term
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Definition
- In genearal does not identify what is eluted from column; only tells us something is emerging.
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Term
| [Detectors] Qualitative analysis: |
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Definition
- Two detectors for qualitative analysis are the mass spectrometer and the fourier transform infrared spectrometer.
- Most reliable way to compare retention times is by co-chromatography whereby the known sample is added to the unknown; the relative area of the peak of interest will increase; perform this test on columns of different polarities.
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Term
| [Detectors] Quantitative analysis: |
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Definition
- Based on the area of a chromatographic peak
- Normally choose conditions under which the response in linear, which means that the area of the peak is proportional to the quantity of that component.
- Peak area can be automatically measure by comp.
- Quant. anal. here almost always performed by adding a known quantity of internal standard to unknown.
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Term
| Thermal Conductivity Detector (TCD): |
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Definition
In the past, this type of detector was probably the most common b/c it is simple and universal; responds to all analytes. Unfortunately, not sensitive enough to detect minute quantities of analyte eluted from open tubular columns smaller than 0.53mm in diameter; still used for 0.53 mm packed columns Measures ability of a substance to transport heat from a hot region to a cold region. H2 and He give lowest detection limit. Sensitivity increases with:
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Term
| Flame Ionization Detector (FID): |
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Definition
- Eluate burned in a mixture of H2 and air.
- Carbon atoms (except carbonyl and carboxyl carbons) produce CH radicals, which are thought to produce CHO+ in the flame
- Only about 1 in 105 carbon atoms produces an ion, but ion production is strictly proportional to number of susceptible carbon atoms entering the flame.
- Cations produced in flame carry electric current from the anode flame tip to the cathode collector; this electric current is the detector signal.
- Widely used with open tubular and packed columns
- Detector responds to most hydrocarbons
- Insensitive to nonhydrocarbons.
- Characteristics:
- N2 gives best detection limit
- Signal proportional to number of susceptible carbon atoms
- 100 fold better detection than TCD
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Term
| Electron Capture Detector (ECD): |
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Definition
- Particularly sensitive to halogen-containing molecules, conjugated carbonyls, nitriles, nitro compounds, and organometallic compounds, but relatively insenitive to hydrocarbons, alcohols, and ketones.
- Carrier or makeup gas must be either N2 or 5% methane in Ar
- Moisture decreases sensitivity
- Gas entering detector ionized by beta rays emitted from a foil containing 63Ni; electrons thus formed are attracted to an anode, producing a small steady current; when analyteswith a high electron affinity enter the detector, they capture some of the electrons; the detector responds by varying the frequency of voltage pulses b/w the anode and cathode to maintain a constant current.
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Term
[Sample Prep.] Solid-phase Microextraction- |
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Definition
| - Simple method to extract compounds from liquids, air, or even sludge without using any solvent; the key component is a fused silica fiber coated with a 10 to 100 µm thick film of nonvolatile liquid stationary phase similar to those used in GC. |
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Term
[Sample Prep.] Purge and Trap: |
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Definition
- Method for removing volatile analytes from liquids or solids (such as ground water or soil), concentrating the analytes, and introducing them into a gas chromatograph.
- In contrast to solid-phase microextraction, which only removes a portion of analyte from the sample, goal in purge and trap is to remove 100% of the analyte from the sample.
- Quantitative removal of polar analytes from polar matrices can be difficult.
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Term
| Method Development in Gas Chromatography: |
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Definition
Order of decisions: - Goal of analysis
- Sample preparation
- Detector
- Column
- Injection
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[Method Dev. in GC] Goal of Analysis: |
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Definition
- What is required from the analysis?
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[Method Dev. in GC] Sample Preparation: |
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Definition
- Key is to clean up the complex sample before it sees the column
- In addition to solid-phase microextraction and purge and trap, there is liquid extraction, supercritical fluid extraction, solid-phase extraction, and thermal desorption of volatiles from a solid material.
- These techniques isolate desired analytes from interfering substances, and they can concentrate dilute analytes up to detectable levels.
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Term
[Method Dev. in GC] Choosing the detector: |
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Definition
| Do you need info about everything in the sample or do you want a detector that is specific for a particular element or a particular class of compounds? |
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Term
[Method Dev. in GC] Selecting the Column: |
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Definition
- Basic choices are thee stationary phase, column diameter and length, and the thickness of stationary phase.
- To improve resolution:
- Longer column
- Narrower Column
- Thinner stationary phase
- Different stationary phase
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Term
[Method Dev. in GC] Choosing the Injection Method: |
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Definition
- Split injection:
- Concentrated sample
- High resolution
- Dirty samples (use packed liner)
- Could cause thermal decomposition
- Splitless injectioni:
- Dilute sample
- High Resolution
- Requires solvent trapping or cold trapping
- On-column injection:
- Best for quantitative analysis
- Thermally sensitive compounds
- Low resolution
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