Injector valve, modes operation

An injector valve operates in two modes— the fixed-loop mode or the partial-loop mode. In the fixed-loop mode, a sample is overfilled into the loop at 2-4 times the loop volume and the entire loop content is injected. In the partial-loop fill mode, a variable sample aliquot, measured precisely by a syringe at <50% of the loop volume, is injected. Note that the sample slug is introduced into the end of the sample loop and is back flushed onto the column to minimize band dispersion by the sample loop (Figure 9). Due to the emphasis on productivity, manual injectors are seldom used in the pharmaceutical laboratory except for preparative applications. 

Desorption time is generally in the 10-100 s range, but it needs to be optimized. To ensure high sensitivity, the injector is usually operated in splitless mode— this is possible as no solvent is used in SPME. A frequent practical problem using SPME is that GC septa are easily damaged with the wide (24-gauge) SPME needles. To avoid septum coring, predrilled GC septa or septum-less injector valves may be used. 

We use a GC Top 8000 gas chromatograph coupled with a PolarisQ ion-trap mass spectrometer and equipped with an AI3000S autosampler (Thermofinnigan www. thermo.com). The steroids are separated on a DB-1 crosslinked methyl-silicone column, 15 mx 0.25 mm i.d., film thickness 0.25 pm (J W Scientific marketed by Agilent). Helium is used as a carrier gas at a constant pressure of about 35 kPa. A 1-pl aliquot of the final derivatized extract is injected into the system operated in splitless mode (valve opened at 2 min). The GC temperature program is the same described before for the quadrupole GC-MS system. The injector and transfer lines are kept at 260°C and 280°C, respectively. The ion source temperature is 225°C. A damping gas flow of helium is applied to the ion trap. 

Column and detector, 1-ft At-Porasil, RI detector solvent, n-heptane (1 mL/ min) injector, Valeo 6-port valve with IO-aiL sample loop mode, backflush operation for aromatics. 

Sample Introduction. The small internal diameters of SFC capillary columns place stringent requirements on saoiple introduction in order to avoid band-broadening due to too large of an injector volume. These requirements have been met using a 0.2- jL internal volume high pressure valve operated in a splitting mode (12). Split ratios of 3 1 to 5 1 are usual. Recently, valves designed specifically for use with capillaries have become available (Valeo Instruments Co., Houston, Texas). These valves offer internal volumes as low as 60-nL and show promise for use in capillary SFC. 

A 30 m x 0.25 mm id x 0.25 i.m film thickness HP-5ms capillary column was used, with temperature programming from an initial temperature held at 90°C for 2 min before commencing a 7°Cmin-1 rise to 285°C, with a final time of 20 min. The split/splitless injector was held at 280°C and operated in the splitless mode, with the split valve closed for 1 min following sample injection. The split flow was set at 40 ml min-1, and the mass spectrometer transfer line was maintained at 280°C. Electron impact ionization at 70 eV, with the electron multiplier voltage set at 1500 V, was used, while operating in the single-ion monitoring (SIM) mode. 

The operating modes of the injectors are the following. When the electronic control unit (ECU) energizes the control valve solenoid, the control valve leaves its seat and the fuel pressure in the needle control chamber drops suddenly. As the fuel pressure at the nozzle seat is the same as the rail pressure, the needle opens and the injection starts. When the current in the solenoid valve is zero, the solenoid spring force returns the control valve to its seat [80]. 

Today, the most common sample valves operate in the loop filling mode and are called high-pressure sample injectors. This means that the sample is transferred at atmospheric pressure from a syringe into a sample loop. By means of valving action the sample loop is then connected to the high-pressure mobile phase stream, which carries the sample onto the column (Fig. 6). The valving action can be carried out either manually or automatically by electric or pneumatic actuators. 

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