Analytical Chemistry Gas/liquid Chromatography

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Know the block Diagram for gas chromatograph

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Carrier Gas Tank-> Flow Regulatrs-> Sample injection Chamber-> Column/Thermostat-> Detector-> Flow Meter
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V
Data System
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V
Display

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Inlet pressures?

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10-50 psi

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components of FID injection

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Syringe, Septum, Septum Purge, Syringe needle, vaporization chamber, zero dead volume connector, column

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syringe

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used to inject sample

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septum

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made of rubber or silicone

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syringe needle

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enters the vaporization chamber

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sample port is usually kept at?

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50 degrees greater than the boiling point of the least volatile component of the sample

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samples range from?

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a few tenths of a micro-liter to 20 micro-liters

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capillary columns

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require samples that are smaller by a factor of 100 or more.

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a sample splitter

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needed w/ capillary columns to deliver a small fraction (1:100 to 1:500) of the injected sample and the rest goes to waste

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Commercial Gas Chromatographs

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intended for use with capillary columns incorporate splitters; they allow for splitless injection when packed columns are used

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FID (Flame Ionization Detectors)

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Most widely used for GC
Directed to small air/H flame
Monitors current by collecting charges

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FID (continued)

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A few hundred volts applied between the burner tip and collector electrode located above the flame serve to collect the ions and electrons. The resulting current is ~10-12A
The # of ions is proportional to # of reduced carbon atoms in the flame
The detector responds to the number of C atoms entering the detector per unit of time. It is mass sensitive rather than a concentration sensitive device.
Change in flow rate of mobile phase have little effect on detector response

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.2 pg/s

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Flame ionization/ for hydrocarbons (main part)
Fourier transform IR(FTIR)/for organic compounds

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ECD(Electron Capture Devices)

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Most widely used for environmental samples b/c this detector selectively responds to halogen-containing organic compounds like pesticides and polychlorinated biphenyls.
Passed over beta-emitter—usually nickel-63

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ECD continued

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Carrier gas-usually He but ours uses N2
Halogens, peroxides, quinones, and nitro groups detected
Highly sensitive w/o altering sample
Limited to about 2 orders of magnitude

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FSOT
(Fused Silica Open Tubular)

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Drawn from specially purified silica that contains minimum amounts of
metal oxides. Much thinner walls than glass. Tubes given strength by an outside protective polyimide coating which is applied as the capillary tubing is being drawn.

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FSOT continued

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Tubes are flexible and can be bent into coils w/ diameter of a few inches.
Stronger physical strength, lower reactivity toward sample components, flexibility. Replaced WCOT
Inside diameter of .32 and .25mm
Higher resolution sold w/ diameters of .20 and .15—more troublesome and are more demanding injection and detection systems. More sensitive detector and sample splitter needed

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Trifluoropropyl-polydimethyl siloxane

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Chlorinated aromatics, nitroaromatics, alkyl substituted benzenes

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Carbowax

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Free acids, alcohols, ethers, essential oils, glycols

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Siloxanes Bonded phase

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Polydimethyl siloxane-R groups are all CH3. Liquid is relatively nonpolar.
Fraction of the methyl groups are replaced by functional groups such as phenyl(--C6H5), cyanopropyl(--C3H6CN), and trifluoropropyl(--C3H6CF3).
The % descriptions in each case give the amount of substitution of the named group for methyl groups on the polysiloxane backbone.

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Siloxanes Bonded phase continued

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5% phenyl polydimethyl siloxane=phenyl ring bonded to 5% of the silicon atoms in the polymer.
Increase polarity in various degrees.

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Has wide separation of polar species

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HO—CH2—CH2—(O—CH2—CH2)n—OH

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Height and area of peak

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Area is more satisfactory than analytical perimeter of peak heights
Peak height is more easily measured and more accurately determined for narrow peaks

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Calibration w/ standards

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Most straight forward involves the preparation of series of standards that approximate the composition of the unknowns.
Chromatograms for the standards are obtained and peak heights are plotted as a function concentration to obtain a working curve.
A plot should yield a straight line passing through the origin—quantitative analyses are based on this plot.
Frequent standardization is necessary for highest accuracy.

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Isocratic(ours) vs. gradient elution

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Iso-single solvent or solvent mixture
Grad-two(or more) solvent system w/ significantly different polarity

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Injection

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Our sampling uses a 20 micro-liter loop

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UV

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Most widely used for HPLC
254 nm Hg light

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Mass LOD of 10 pico grams

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Absorbance-commercially available-linear range(3-4)

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Normal

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based on highly polar stationary phases(Triethylene glycol or water) and relatively non-polar mobile phase (hexane or i-propyl ether)

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Reverse

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Stationary is non-polar(a hydrocarbon)and the mobile phase is a relatively polar solvent(water, methanol, acetonitrile, or tetrahydrofuran)

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Choice of mobile phase for Elutrophic series

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We use methanol/water

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Ion Chromatogram

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Ion chromatography-due to analyte ions, hydrogen ions from regeneration solution migrate in the reverse direction, preserving neutrality. Ions present in ppm range. 50 microliter in 1st, 20 microliter in 2nd. Important in anions b/c there is no other rapid method for this type

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What type of column is used for ion chromatography

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eleuent suppressor column

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Gel filtration packing

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Separates polymers by size b/c of many pore diameters

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H

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Plate height

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B/u

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longitudinal diffusion term(migration of a solute from a concentrated center of a band to the more dilute regions on either side)

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CsU

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stationary phase mass-transfer (the thicker it is the slower it travels; slower rate of mass transfer increases plate height)

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CmU

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Mobile phase mass-transfer

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Peak width

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tw=draw straight lines from the top part and subtract right from left

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Time retained

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tr=highest point at peak time

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Mobile linear velocity curve-describe the dip

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H is smallest and velocity is highest.