Capillary Columns and Inlets

Hydrolytic Kinetic Resolution (HKR) of epichlorohydrin. The HKR reaction was performed by the standard procedure as reported by us earlier (17, 22). After the completion of the HKR reaction, all of the reaction products were removed by evacuation (epoxide was removed at room temperature ( 300 K) and diol was removed at a temperature of 323-329 K). The recovered catalyst was then recycled up to three times in the HKR reaction. For flow experiments, a mixture of racemic epichlorohydrin (600 mmol), water (0.7 eq., 7.56 ml) and chlorobenzene (7.2 ml) in isopropyl alcohol (600 mmol) as the co-solvent was pumped across a 12 cm long stainless steel fixed bed reactor containing SBA-15 Co-OAc salen catalyst (B) bed ( 297 mg) via syringe pump at a flow rate of 35 p,l/min. Approximately 10 cm of the reactor inlet was filled with glass beads and a 2 pm stainless steel frit was installed at the outlet of the reactor. Reaction products were analyzed by gas chromatography using ChiralDex GTA capillary column and an FID detector. 

In direct injection, all injected material is carried onto the column by the mobile phase. This eliminates the possibility of sample discrimination in the inlet, but can overload capillary columns and so is more commonly used with packed columns and wide-bore capillary columns. 

GC Analyses, The n-alkanes in the fuel samples were determined with a 100 meter OV-101 wall coated glass capillary column. The inlet split ratio was 50 1, the column oven was temperature programmed from 80 to 240°C, and the inlet temperature was 310°C. The internal standard procedure was used for quantitation. 

The second type of inlet, called a splitter (Fig. 22.7), again uses a syringe to inject the sample. The sample is vaporized and effectively mixed with carrier gas in the mixing tube, after which the vapor passes over a tapered hollow needle. Because of the difference between the inside diameter of the mixing tube outlet and that of the tapered needle, a fraction of the total sample is introduced into the column as a narrow zone. The splitting ratio is a function of the pressure in the inlet, and can be varied. Such a sample splitter is used with capillary columns and high-resolution columns because of their relatively low capacity. 

The optimum linear velocity for a capillary column depends on the pressure in the column because Wopt is proportional to the average diffusion coefficient, which varies inversely with pressure. Operation of a short wide-bore column at vacuum outlet conditions results in a significantly faster analysis than would occur if the same column was used under atmospheric outlet pressures. Mass spectrometry (MS) has made vacuum GC very easy to implement, since the mass spectrometer provides both detection and a source of vacuum. Vacuum GC can be achieved practically by incorporating a restriction at the inlet end of a wide-bore capillary column, and interfacing the terminal end of the column directly into the MS. The function of the restriction is to deliver an optimal helium flow for the mass spectrometer, and it can be as simple as a short section of 20 pm i.d. capillary (or a longer section of 100-150 pm i.d. capillary). An optimal carrier gas velocity of 90-100cms can be expected for a 10 m x 50 pm column with a restriction at the inlet, and a speed gain of a factor of 3-5 times can easily be obtained. 

The catalytic activity of the prepared catalysts for methane combustion was tested in a flow reactor unit. Bottled methane (99.995 % purity from AGA, Sweden) and air were fed to the system using mass flow controllers, giving a methane concentration of 2 vol%. The space velocity in all experiments was 50,000 h". The catalysts were placed in a vertical tubular Inconel reactor situated in a tubular furnace. The exiting gases were analyzed by gas chromatography using a Packard model 427 GC, equiped with a Poraplot Q fused silica capillary column and a thermal conductivity detector. The temperature in the furnace was controlled to give a linear temperature ramp of 2 °C/min in all experiments. Hence, the conversion of methane to carbon dioxide and water was determined as a function of the gas inlet temperature. 

Minimal system requirements are a GC equipped with electronic pneumatic control (EPC), off-the-shelf capillary columns, split/splitless or on-column inlets, standard detectors optimized for capillary columns, and a fast acquisition data system. At any time, users can switch from fast GC back to the original method without major difficulties, or optimize new methods to meet new analytical demands. 

Splitter. A fitting attached to the injection port or column exit to divert a portion of the flow. It is used on the inlet side to permit the introduction of very small samples to a capillary column and on the outlet side to permit introduction of a very small sample of the column effluent to the detector, to permit introduction of effluent to two detectors simultaneously or to collect part of a peak from a destructive detector. 

A programmed-temperature vaporization (PTV) inlet is a hybrid of the techniques described above. It is a spUt/spUtless inlet that has been modified to allow cold injection and rapid temperature programming. Similar to on-column injection, the injection occurs while the inlet is cold. In contrast, the injection is performed into a chamber, similar to the spUt and spUtless techniques. This chamber is then rapidly heated to desorb the sample into the capillary column. This inlet also allows for the injection of up to hundreds of microUters of sample. There are numerous modes in which a PTV inlet can be operated, making it perhaps the most versatile of all available inlets. 

The main advantage of the packed column inlet is that the entire sample that exits the syringe enters the column, making packed column injection highly reproducible. The pneumatics are also very simple and inexpensive. Method development is also very straightforward with only the inlet temperature as an easily adjustable variable. Further, packed columns and inlets typically operate at lower temperatures than capillary inlets, allowing the use of less expensive septa. 

Apart from ES and APCI being excellent ion sources/inlet systems for polar, thermally unstable, high-molecular-mass substances eluting from an LC or a CE column, they can also be used for stand-alone solutions of substances of high to low molecular mass. In these cases, a solution of the sample substance is placed in a short length of capillary tubing and is then sprayed from there into the mass spectrometer. 

Oxyfluorfen column, fused-silica capillary column coated with cross-linked methyl silicone (25 m x 0.3-mm i.d., 0.52- am film thickness) temperature, column 200 °C (1 min), 10°Cmin to 250 °C (5 min), inlet and detector 250 and 300 °C, respectively gas flow rates, N2 carrier gas 30mLmin , N2 makeup gas 30mLmin H2 3.5mLmin" air llOmLmin injection volume, 2 p.L. ... 

Fast chromatography involves the use of narrow-bore columns (typically 0.1-mm i.d.) that will require higher inlet pressures compared with the conventional wide-bore capillary columns. These columns require detectors and computing systems capable of fast data acquisition. The main disadvantage is a much-reduced sample loading capacity. Advances in GC column technology, along with many of the GC-related techniques discussed below, were recently reviewed by Eiceman et... 

The simplest type of two-dimensional gas diromatography for heart cutting or trace enrichment using a packed precolumn and a capillary column is shown in Figure 8.16 [205]. Almost any modem gas chromatograph could be converted into a similar unit with the addition of a few auxiliary components. Preliminary separation t2dces place on the packed column, the effluent from which is directed either to a vent or to the capillary inlet by the Deans switch, nie effluent reaching the capillary inlet is split three ways. One portion passes to a detector used to monitor the preseparation, a second portion enters the cap Bry column and is... 

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