Setting the Carrier Gas Flow

For helium, the optimum carrier gas velocity for standard columns is about 24 cm/s. As the viscosity of the carrier gas increases in the course of the oven 

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Set the carrier gas flow rate for your instrument to 20 mL/min. The method of measuring this flow rate will be demonstrated by your instructor. Set the column temperature to 70°C. Inject 1.0 fiL of the mixture and observe the resolution. Now change the column temperature to 80°C, wait 5 min for the oven temperature to become stable, and inject 1.0 fiL again, observing the resolution. Repeat at 90, 100,110, and 120°C, observing the effect on resolution. 

Set the carrier gas flow rate to 20 mL/min and obtain a series of chromatograms, each at a different volume injected 0.5, 1.0, 1.5, 2.0, and 2.5 fiL. The attenuation should be set so that the peaks from the larger injection volumes will not be off-scale. 

Determine the composition profile of fatty acids as directed under Fatty Acid Composition, Appendix VII, with the following modifications (1) In the Sample Preparation, use about 55 mg of sample per 10 mL, and (2) in the Procedure, use a suitable capillary gas chromatograph (see Chromatography, Appendix IIA), equipped with a flame ionization detector, a 60-m x 0.25-mm (id) column, or equivalent, coated with a 0.20-p.m layer of 2-cyanopropylpolysiloxane (Supelco SP-2340, or equivalent), a capillary injection port (split mode, operated at a split ratio of 1 100), and an integrator. Set the initial column temperature at 150°, heat at a rate of 1.3°/min to 225°, and hold at 225° for 10 min. Set the injection port temperature to 210° and the detector to 230°. Set the carrier gas flow rate at 25 cm/s. 

Chromatograph—Mount the column in the chromatograph and establish the operating conditions required to give the derired separation (Appendix XI). Allow sufficient time for the instrument to reach equilibrium as indicated by a stable base line. G)ntrol the oven temperature so that it is constant to within 0.5 C without thermostat cycling which causes an uneven base line. Set the carrier-gas flow rate, measured with a soap film meter, so that it is constant to within 1 mL/min of the selected > ue. 

Change column to one with some other stationary phase, perhaps one suggested by your instructor. Set the column temperature to 100°C and the carrier gas flow rate to 20 mL/min. Inject 1.0 pL of the mixture. Assuming good resolution, observe the order of elution. Is it different from that observed with the former stationary phase If so, explain how that could be. If not, compare the resolution here with that of a previous injection in which all the parameters were equal. Comment on the difference. 

Splitless injection is used when the sample is dilute and cannot be introduced into the GC system with stream splitting. In practice, the column temperature is set 10° to 30°C below the boiling point of the solvent at the time of injection. When sample is introduced into the injector inlet, vaporized solvent together with the FAME condense at the beginning of the column along with the carrier gas flow. The condensed solvent plus the stationary phase of the column forms a diluted stationary phase that traps the FAME in it. After the initial sample introduction period, the column temperature is raised to normal operating conditions, and chromatographic separation starts from there. 

A similar experiment was reported by RJ. Dougan et al. [90], A set-up called Qn-line Separation and Condensation AppaRatus (OSCAR) was installed at the LBNL 88-Inch Cyclotron. Nuclear reaction products were collected with a KQ aerosol gas-jet and were transported from the target chamber to the OSCAR set-up where 02 was added. The aerosol particles were destroyed on a hot quartz wool plug and the formation of tetroxides occurred at a temperature of 650°C. Non volatile reaction products were retained on the quartz wool plug whereas the volatile tetroxides were swept by the carrier gas flow to a condensation chamber, where they were deposited on a Ag disk, which was cooled with liquid N2. An annular Si... 

Chromatographic System (See Chromatography, Appendix IIA.) Use a gas chromatograph equipped with a Sievers 350 (or equivalent)1 Chemiluminescence Detector (SCD) and a 30-m x 0.53-mm id, 5-mm DB-5 capillary column (J W Scientific Company, or equivalent). Set the carrier gas, helium, at a head-pressure of 5 psig. Set the injection port at 100°, and the split ratio at 1 1. Set the column temperature at 30°. The retention time for carbonyl sulfide is approximately 3 min. Operate the SCD with 190 mL/min of hydrogen and 396 mL/min of air. Optimize the gas flows and probe position of the SCD for maximum sensitivity. 

Procedure (See Chromatoghaphy, Appendix IIA.) Inject about 5 xL of the Mixed Standard Solution into a suitable gas chromatograph equipped with a flame-ionization detector and a 1.8-m x 3.2-mm stainless steel column, or equivalent, packed with 80- to 100-mesh Porapak QS, or equivalent. Maintain the column at 165°. Set the temperature of both the injection port and the detector to 200°. Use helium as the carrier gas, flowing at 80 mL/min. The retention time of isopropyl alcohol is about 2 min, and that of ferf-butyl alcohol is about 3 min. 

Procedure The conditions for the gas chromatograph, or equivalent, are the same as for the Vapor Partitioning Method, except that the column temperature is set at 120°, and the carrier-gas flow is set to 21 mL/min. Accurately transfer 90 mL of sample and 10 mL of perchloroethylene (free of interfer-... 

Using helium as the carrier gas, set the chromatograph gas flow at 2.0 psi constant flow. Use a 0.5-p.L injection volume. Set the injection temperature in track mode at 3° above oven temperature, and set the oven temperature to 40° for 6 min and raising it to 280° in 15°-per-minute increments over 5 min. Set the detector temperature at 380°. 

Procedure Inject 1-pL aliquots of the Standard Preparation and the Sample Preparation into a gas chromatograph equipped with a split injector, a flame-ionization detector, and a 25 -m fused silica capillary column coated with a 2-pm film of 7% cyanopropyl-7% phenyl-85% methyl-1% vinylpolysiloxane (CP-Sil 19 CB, Chrompack Middelburg, or equivalent). Maintain the column at 100°, raising the temperature at 8°/min to a final temperature of 300°. Set the injector temperature to 270°and the detector temperature to 270°. Use a mixture of helium and methane, at a split ratio of 1 100, as the carrier gas, flowing at 120 mL/ min. Run the chromatogram for 27 min. 

Interesting as the qualitative analysis of heteroatoms by the AFD may be, it is not necessarily applicable to complex mixtures. To distinguish heteroatoms by their direction of response may necessitate detector settings which do not represent the best sensitivity obtainable, and are consequently more susceptible to background interference. Furthermore, since the carrier gas flow may be dictated by the detector, the column dimensions have to be adjusted accordingly. 

The absorption at the mercury wavelength (2537 A) occurs in the gas cell 12 sec after the heating cycle is initiated. The absorption is recorded in arbitrary imits, and the maximum height is noted. After the response has retiumed to the initial base line (60 sec), the heating is stopped, and the column is cooled in air for 30 sec. The U-tube is then returned to the liquid nitrogen bath. The carrier gas flow is set to 0.5 l./min, and the system is ready for the mercury spike additions. Total time for this operation is 9 min. Except for the stannous chloride addition, the system blank is established in a manner identical to the sample determination. 

A two stage regulator is used at the outlet of the carrier gas cylinder to set an appropriate cyhnder outlet pressure to the GC system and to monitor the residual pressure in the cylinder. Most GC instruments are equipped with a flow controller and flow meter. The flow controller is used to ensure obtaining a constant flow despite changes in pressm-e and pressure drops through the GC colmnn. The flow meter is used to set a carrier gas flow rate to a desirable level and to monitor the stabihty of the carrier gas flow. 

The effects of the type of carrier gas and its flow rate upon a capillary column performance can be significant. The flow rate has to be accurately set, measured, and reproduced to fully experience the high efficiencies available with capillary columns. Unlike packed columns, small variations in the carrier gas flow rate can have significant effects on the separations obtained with capillary columns. Even the type of carrier gas can affect separation quality [2]. 

The carrier gas flow setting of the GC also can show effect on the position of the mass calibration. Ion sources with small volumes and also ion trap instruments with internal ionization show a significant drift of several tenths of a mass unit if the carrier gas flow rate is significantly changed by a temperature program. 

Set up the chromatograph in accordance with the manu cturer s recommendations. Install the analytical column and adjust the carrier gas flow and column temperature so that the components will elute within the time desired for the analysis. 

As a matter of practicality, it may be easier to first set an approximately correct flow rate, using methane gas injections. To do this, adjust the carrier gas flow (or column... 

The liquid precursor solution was atomised at a constant pressure of 2x10 Pa and injected into the carrier gas flow before entering the plasma zone. The carrier gas composition, which was set by mass flow controllers, was a N2/O2 (97 3, vol.%) mixture with a total gas flow of 20 standard litres per minute. The gas mixture containing the precursor aerosol was injected into the plasma through a slit between the two top electrodes. The experiments were carried out at atmospheric pressure and ambient temperature. 

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