Gas Chromatography Flashcards
____ separates the components of a mixture and ___ characterizes each of the components individually
Thus, an analytical chemist can both QUALITATIVELY and QUANTITAVELY evaluate a solution containing a number of chemicals.
Gas Chromatography
Mass spectroscopy
GAS CHROMATOGRAPHY GOALS
Achieve the BEST SEPARATION
-Little band broadening
-NARROW chromatogram Peaks
-EFFICIENT COLUMN
“Dynamic Range”
Separate and detect the “small in close vicinity of the “big”
REPRODUCIBILITY
-Stable Peak Positions, Retention Times
____ always play a role
It can be controlled by ___ and ___
Diffusion
Particle size (Column Type) and Flow Rate
Parts used in GC-MS (3)
GC Controller
GC Oven
MS Analyzer
MOBILE PHASE
The most common mobile phases for gas chromatography are ___, ___ and ___
THese have the advantage of being CHEMICALLY INERT toward both the SAMPLE and the STATIONARY PHASE.
The choice of carrier gas often is determined by the needs of instrument’s detector.
For a packed column the mobile phase velocity usually is ___
The typical flow rate for a capillary column is ____
He
Ar
N2
25–150 mL/min.
1–25 mL/min.
CHROMATOGRAPHIC COLUMNS (2)
____ can handle larger samples, bigger, shorter
____ can separate more complex mixtures
Packed Column
Capillary Column
factors determine how we introduce a sample to the gas chromatograph (3)
Sample must be VOLATILE
Analytes must be present at APPROPRIATE CONCENTRATION
Physical process of INJECTING THE SAMPLE must NOT DEGRADE the SEPARATION
A solute of low volatility, for example, may be retained by the column and continue to elute during the analysis of subsequent samples.
A nonvolatile solute will condense at the top of the column, degrading the column’s performance.
We can separate a sample’s volatile analytes from its nonvolatile components using any of the extraction techniques.
A ____ of analytes from an aqueous matrix into ____ or another organic solvent is a common choice.
____also are used to remove a sample’s nonvolatile components.
liquid–liquid extraction
methylene chloride
Solid-phase extractions
- If an analyte’s CONCENTRATION TOO SMALL to give an adequate signal, then we must CONCENTRATE the analyte before we inject the sample into the gas chromatograph.
- Volatile organic materials isolated from an aqueous sample by a ____, for example, are concentrated by as much as 1000x.
If an analyte is too concentrated, it is easy to overload the column, resulting in ___ and a ____. In addition, the analyte’s concentration may EXCEED the detector’s linear response.
Injecting less sample or diluting the sample with a volatile solvent, such as ____, are two possible solutions to this problem.
purge-and trap
peak fronting, poor separation
methylene chloride
There are two principal sources of this pre-column
band broadening which is an issue:
– injecting the sample into a moving stream of mobile
phase
– injecting a liquid sample instead of a gaseous sample.
TRUE or FALSE: In a GC analysis the area under the peak is proportional to the amount of analyte injected onto the column.
TRUE
- If the injection volume is identical for every standard and sample, then an external standardization provides both accurate and precise
results. - Unfortunately, even under the best conditions the RELATIVE PRECISIONS for REPLICATE INJECTIONS may differ by ____; often it is substantially worse.
- For QUANTITATIVE work that requires high ACCURACY and PRECISION, the use of ___ is recommended.
5%
internal standards
In addition to identifying the component responsible for a particular chromatographic peak, we also can use the saved spectra to ____.
If a spectrum at the beginning of the peak’s elution is
different from a spectrum taken near the end of the peak’s elution, then at least two components are co-eluting.
evaluate peak purity
