Quant. Anal. Chp. 24 Gas Chrom

Separation process in Gas Chromtography:

  • 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.

Gas Chromatograph diagram:
[image]

[Types of Open Tubular Columns]

Wall-coated Open tubular column (WCOT)

  • 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

[Types of Open Tubular Columns]

Support-coated open tubular column (SCOT):

  • 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.

[Types of Open Tubular Columns]

Porous-layer open tubular column (PLOT):

The solid particles are the active stationary phase.
Decreasing the thickness of stationary phase leads to?

  1. Decreased plate height
  2. Decreased retention time
  3. Decreased capacity for analyte

Molecular sieves-

– 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.

Packed Columns:

  • 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)

Retention Index-
-Relates retention time of a solute to the retention times of linear alkanes.
Temperature programming-

  • Temp. of a column raised during the separation to increase solute vapor pressure and thereby decrease retention times of late-eluting components.
    • Additionally, peaks are sharpened

 

Pressure Programming-
– Aids in eluting high boiling point components.
Carrier Gas:

  • 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.

[Sample Injection]

Split Injection:

  • 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.

[Sample Injection]

Splitless Injection

For trace analysis of analytes that are less than 0.01% of the sample, splitless injection is employed.

[Sample Injection]

On-Column Injection

  • 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.

Detectors:

  • In genearal does not identify what is eluted from column; only tells us something is emerging.

[Detectors] Qualitative analysis:

  • 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.

[Detectors] Quantitative analysis:

  • 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.

Thermal Conductivity Detector (TCD):

  • 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:
    • increasing filament current
    • decreasing flow rate
    • lower detector block temperature.

Flame Ionization Detector (FID):

  • 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

Electron Capture Detector (ECD):

  • 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.

[Sample Prep.]

Solid-phase Microextraction-

– 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.

[Sample Prep.]

Purge and Trap:

  • 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.

Method Development in Gas Chromatography:

Order of decisions:

  1. Goal of analysis
  2. Sample preparation
  3. Detector
  4. Column
  5. Injection

[Method Dev. in GC]

Goal of Analysis:

  • What is required from the analysis?

[Method Dev. in GC]

Sample Preparation:

  • 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.

[Method Dev. in GC]

Choosing the detector:

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?

[Method Dev. in GC]

Selecting the Column:

  • 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

[Method Dev. in GC]

Choosing the Injection Method:

  • 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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