Injection Port Septa

Describe in detail how the loop injector works and tell how it overcomes the problems that would be encountered with an injection port-septum system. 

The thermal desorption tube has a needle at one end and i.s threaded at the other for connection to the SPTD system. When an injection is made, the thermal desorption tube needle pierces the GC injection port septum. Heating blocks close around the thermal desorption tube and provide rapid heating. Helium passes through the desorption tube at 1 mL/min. and the volatile components are desorbed onto a DB-I capiliaiy column held at -20 C with dry ice (see Figure 5). 

The injection port septum is an unsolved weak point in GC. It is arguably the most important but often overlooked functional part of the injector. The septum is used to seal the injection port, and withstands the inlet pressure and provides smooth and repeated access for the syringe needle to inject the sample. The syringe needle punches the septum repeatedly in a small area causing only gradual small leaks, which are detectable with an electronic leak detector already before a noticeable analyte response drop occurs. The risk of leaks increases with use, older septa take longer to re-seal than new ones (Hinshaw, 2008). 

The sample enters the GC at the injection port (Fig. 107). You use a microliter syringe to pierce the rubber septum and inject the sample onto the column. Don t stab yourself or anyone else with the needle. Remember, this is not dart night at the pub. Don t throw the syringe at the septum. There is a way to do this. 

Bring the syringe to the level of the injection port, straight on. No angles. Then let the needle touch the septum at the center. 

Now quickly and smoothly push on the plunger to inject the sample, and pull the syringe needle out of the septum and injection port. 

This is the equivalent of the injection port for the GC technique. With GC you could inject through a rubber septum directly onto the column. With HPLC it s very difficult to inject against a liquid stream moving at possibly 1000 psig. That s why they invented injection port valves for HPLC you put your sample into an injection loop on the valve that is not in the liquid stream, then turn the valve, and voila., your sample is in the stream, headed for the column. 

This is usually caused by changes in the carrier gas flow rate or column temperature. Flow rate changes can be caused by leaks in the system upstream from the column inlet, such as in the injection port (e.g., the septum) by low pressure in the system due to an empty or nearly empty carrier supply or by faulty hardware,... 

In both systems the flow of helium carrier gas through the columns was 0.7-0.8 ml min-1, with a septum purge of 0.5 ml min-1 and a split valve flow of 4-4.5 ml min-1. The injection ports were maintained at 260°C and the detector ovens at 240° C. The detector employed was either a flame ionisation or a nitrogen-specific NPD-40 thermionic detector (Erba Science (UK) Ltd) and the output was recorded on a HP 3390 integrator (Hewlett Packard Ltd, Wokingham, UK). 

Fig. 5.18. Reservior inlet of a JEOL JMS-700 sector instrument with the septum injection port opened. The operation valve switches between evacuation, isolation and admission of the sample a needle valve allows regulation of the sample flow. The GC transfer line crosses in the upper background from the GC (left) to the ion source housing (upper right).

Durham, NC). Under these conditions the temperature of the air stream midway through Section B was 50 C and had dropped to 30°C upon reaching the filter (Section C). Aliquots of the air stream were withdrawn through the septum in Section B and analyzed by glc/ecd in order to determine the air stream concentration. After termination of air flow, the filter, glass wool plug and sorbent, if any, were extracted with acetone and the extracts analyzed by glc/ecd. The apparatus from the injection port to the filters was rinsed with acetone and analyzed to determine the TRIS which was surface absorbed. 

A 600 mL Parr hydrogenation vessel equipped with an injection port with a rubber septum for the addition of the solvent via syringe, a pressure gauge, a tightly fitting removable internal glass liner, thermocouple and an overhead mechanical stirrer was assembled and pressure tested to 14 bar with N2 over... 

Here is a student procedure to measure nicotine in urine. A 1.00-mL sample of biological fluid was placed in a 12-mL vial containing 0.7 g Na2CO , powder. After 5.00 pig of the internal standard 5-aminoquinoline were injected, the vial was capped with a Teflon-coated silicone rubber septum. The vial was heated to 80°C for 20 min and then a solid-phase microextraction needle was passed through the septum and left in the headspace for 5.00 min. The fiber was retracted and inserted into a gas chromatograph. Volatile substances were desorbed from the fiber at 250°C for 9.5 min in the injection port while the column was at 60°C. The column temperature was then raised to 260°C at 25°C/min and eluate was monitored by electron ionization mass spectrometry with selected ion monitoring at m/z 84 for nicotine and m/z 144 for internal standard. Calibration data from replicate... 

Injection port. Closure column on one side and a septum inlet on the other through which the sample is introduced into system. 

Figure 6.12. Blank run using a wel1-conditioned silicone column and a white septum. A new septum was inserted 30 min before each programmed run was made. Programmed from 30 to 200°C at 8°C/min. Detector temperature 350 C. Injection port 400°C. FID range 10. Attenuation 16.

The essential components of a gas chromatography system are shown in Figure 3.4. The mobile phase (called the carrier gas) is inert, usually helium, nitrogen, or argon. The gas is directed past an injection port, the entry point of the sample. The sample, dissolved in a solvent, is injected with a syringe through a rubber septum into the injection port. The column, injection port, and detector are in individual ovens maintained at elevated temperatures so that the sample components remain vaporized throughout their residence time in the system. 

New accessories are continually made available. This unit discusses the basics necessary to conduct SPME analysis for flavor analysis. An automated sampling and injection system is available from Varian. Supelco offers a manual sampling stand setup. Injection liners are available that reduce the injection port volume to presumably produce sharper peaks. Predrilled septa for the GC are available to reduce septum coring. 

A gas chromatograph is an apparatus consisting of an injection port connected to a column that has a detector at its outlet end. The column is contained in an oven that is electrically heated, either isothermally or at a programmed rate. A stream of inert carrier gas, usually helium, is introduced into the injection port and flows through the column and detector. The injection port is a heated region that is sealed from the outside environment by a silicone rubber septum through which the sample is injected using a hypodermic... 

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