The departmental has two instruments one is located in building 65 room 230 and the other in 109A. The newest instrument is an Agilent 6850. The other is a H-P 5880A series gas chromatography. It has plotter/integrators to display data. Procedural details on the operation of the instruments are given in specific manuals located in room 230; select the following for abstracted procedures, information, and comments for the H-P 5880A
Gas chromatography (GC) is an analytical technique for separating compounds based primarily on their volatilities. Gas chromatography provides both qualitative and quantitative information for individual compounds present in a sample. Compounds move through a GC column as gases, either because the compounds are normally gases or they can be heated and vaporized into a gaseous state. The compounds partition between a stationary phase, which can be either solid or liquid, and a mobile phase (gas). The differential partitioning into the stationary phase allows the compounds to be separated in time and space.
The carrier gas is usually helium, hydrogen, or nitrogen. This serves as the mobile phase that moves the sample through the column. The carrier gas flow can be quantified by either linear velocity, expressed in cm/sec, or volumetric flow rate, expressed in mL/min. The linear velocity is independent of the column diameter while the flow rate is dependent on the column diameter. See Table 1.
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The injector is a hollow, heated, glass-lined cylinder where the sample is introduced into the GC. The temperature of the injector is controlled so that all components in the sample will be vaporized. The glass liner is about 4 inches long and 4 mm internal diameter.
The GC column is the heart of the system. It is coated with a stationary phase which greatly influences the separation of the compounds. The structure of the stationary phase affects the amount of time the compounds take to move through the column. Typical stationary phases are large molecular weight polysiloxane, polyethylene glycol, or polyester polymers of 0.1 to 2.5 micrometer film thickness. Columns are available in many stationary phases sizes. A typical capillary column is 15 to 60 meters in length and 0.25 to 0.32 mm ID. A typical packed column is 6 to 12 feet long and 2.2 mm ID.
The GC column is placed in an oven where the temperature can be controlled very accurately over a wide range of temperatures. Typically, GC oven temperatures range from room temperature to 300° C, but cryogenic conditions can be used to operate at temperatures from about -20° C to 20° C. The oven must not be opened when the oven temperature is above room temperature. Never turn off the nitrogen flow unless the column and oven are at room temperature.
The data recorder plots the signal from the detector over time. This plot is called a chromatogram. The retention time, which is when the component elutes from the GC system, is qualitatively indicative of the type of compound. The data recorder also has an integrator component to calculate the area under the peaks or the height of the peak. The area or height is indicative of the amount of each component.
The compounds listed in Table 2 were separated on an nonpolar, 95% methyl, 5% phenylpolysiloxane column, 30 m long, 0.25 mm ID, and 0.25 micrometer film thickness. About 1 microliter of the hydrocarbon sample were injected. Approximately 5 nanograms (ng) of each component was injected per 1 microliter. A flame ionization detector (FID) was used.
Temperature Program: 50° C (min) - 10° C/min - 100° C Head Pressure: 12 psi Split Ratio: 1/50. This chromatogram shows an ideal temperature program for separation of the 6 aromatic compounds on this column. The first peak is the solvent, methanol. The compounds elute in order of increasing boiling point, that is, compounds with higher boiling points are more retained by the stationary phase. Note that para-xylene and meta-xylene cannot be separated on this column; the peak (#5) containing these compounds is broad at the baseline and shows a distinct shoulder.
Temperature Program: 60° C Isothermal, Head Pressure: 12 psi, Split Ratio: 1/50 This chromatogram shows the effects of an isothermal* temperature program at 60° C. The result is an increase in the retention time of all compounds. The heights of the later eluting peaks are reduced and the peak widths increased because they are more affected by the lower temperature program used. (*isothermal means a constant oven temperature was used throughout the run.)
