Injector valve, modes

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. 

However, in most reported FI-AAS systems using on-line liquid-liquid extraction, the delivery mode b described in Sec. 3.4.6. involving a preliminary collection of the separated extractant in the loop of an injector valve is preferred. The collected fraction may then be presented to the nebulizer under optimum flow conditions either using a pumped aqueous carrier stream or simply by freely aspirated air-flow. The latter produced higher sensitivities, [14] probably owing to a further enhancement in the aspiration rate. 

Figure 2.4 (a) Diagram of a six-port loop injector being filled by injection of the analyte solution from a syringe. In this mode the loop is bypassed and the mobile phase flows directly from pump to column through the injector valve via ports 2 and 3. In order to inject the sample in the loop onto the column the valve is rotated so that internal connections are now made between ports 1 and 2, and between ports 3 and 4. Reproduced from literature of Rheodyne Corporation, Technical Note 5 (2001), Achieving Accuracy and Precision with Rheodyne Manual Sample Injectors, with permission (www.rheodyne.com). 

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. 

Fig. 1.4. Miniature boiler fittings mode from brass a water-level gauge, a steam valve, a pressure gauge, and a feed-water injector. Brass is so easy to machine that it is good for intricate ports like these.

Figure 14.17 Schematic diagram of the on-line coupled LC-GC system VI, valve foi switcliing the LC column outlet to the GC injector V2, valve for switching the LC column to back-flush mode V3, LC injection valve RI, refractive index monitor detector UV, ulti avio-let monitor detector FID, flame-ionization detector.

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. 

When working with capillary columns, the splitless mode is used for very dilute samples. In this mode, the injection is made very slowly, leaving valve no. 2 in the closed position (Fig. 2.5) for approximately 0.5 to 1 min. This allows vaporisation of the compounds and solvent in the first decimetre of the column by a complex mechanism of dissolution in the stationary phase, which is saturated with solvent. Compound discrimination is very weak using this method. The proper use of this injection mode, which demands some experience, requires a temperature program that starts with a colder temperature so that the solvent can precede the analytes in the column. This mode is typically used for trace analyses. The opening of valve no. 2 eliminates, from the injector, compounds which are less volatile and that can interfere with the analyses. 

Figure 2.5—Injectors, a) A split/splitless injector (the split is regulated by valve 2). The exit labelled 1 is called the septum purge, b) A cold on-column injector. A typical feature of a chromatogram obtained in the splitless mode is the interference of the solvent with the analytes. This can be avoided using a selective detector.

Figure 20.4—Static mode of headspace sample analysis. The sampling phial is pressurised with the carrier gas of the chromatograph. After equilibrium, a small volume of the gas containing the volatile compounds is inserted into a sample loop. Rotation of the six-way valve allows introduction of the sample into the injector of the chromatograph. Consequently, this set-up combines sample preparation with sample introduction into the chromatographic column. (Reproduced by permission of Tekmar.)...

Two types of injectors are frequently employed. For packed column SFC, a standard six port rotary valve with an external sample loop of 1-10 pL has proven to be quite reliable. For capillary column SFC, a similar rotary valve with an internal "loop of 0.2 to 0.5 pL is typically employed. Frequently the rotor is pneumatically actuated in a very rapid fashion to allow only a small fraction of sample to be introduced ("time-split ) this is done to avoid column overload. Alternatively, the flow from the injector is split off in the same fashion as in GC. A disadvantage of the latter mode is the potential for sample discrimination. 

In theory, combining two HPLC modes sequentially would provide an online LC/LC/MS/MS and speed the analytical procedure. Bands from the first separations could be detected and collected with an automated loop-and-valve injector, and then individual bands could be passed to the second LC for... 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

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. 

An alternative approach is to use a splitless injection system. If the valve in Fig. 1 is closed, then all the sample passes into the column and there is no split ipso facto, the device is a splitless injector. When used in the splitless mode, however, it is usual to employ a somewhat wider capillary column, which will allow the penetration of a small-diameter injection syringe and thus permit on-column injection. Under these circumstances, there can be no differential sampling of the form described. This procedure, however, introduces other injection problems that can affect both resolution and quantitative accuracy that need to be addressed (See the entries Retention Gap Injection Method and Solute Focusing Injector Method). 

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. 

When working with capillary columns, this type of injector is also used for very dilutes samples in the splitless mode. In this mode a smaller volume of solution is injected very slowly from the micro-syringe during which bleeding valve 2 (Figure 2.5) is maintained in a closed position for 0.5 to 1 minute in order that the vaporized mixture of compounds and carrier solvent are concentrated in the first decimetre of the column. The proper use of this mode of injection, which demands some experience, requires a program that starts with a colder temperature in order that the solvent precedes the compounds onto the column. The re-opening of valve 2 provides an outlet for an excess of solvent-diluted sample. Some less volatile compounds are eliminated and that can interfere with the results of the analyses. 

Figure 2.5 Injectors, (a) Above left, injection chamber. The carrier gas enters the chamber and can leave by three routes (when the injector is in split mode). A proportion of carrier gas (1) flows upward and purges the septum, another (2) exits through the split outlet (a needle valve regulates the split) and finally a proportion passes onto the column, (b) Above right, cold injection onto the column, (c) Below, a typical chromatogram obtained in splitless mode. For solvent peaks which are superimposed upon those of the compounds, a selective detector which does not see the solvent is recommended.

Injection with elimination of solvent the sample is introduced into the cold injector after which the vent valve is opened. Vent flow rate is very high and can attain 1000 mL/min to eliminate all of the solvent. The injector is then heated to permit transfer of the less volatiles compounds onto the column, the bleed valve now being closed (splitless mode). In this way it is possible to inject up to 50p,L in a single injection or up to 500 p,L of sample solution, over several injections. This method eliminates the step of the preliminary concentration of sample prior to injection. 

Co., Palo Alto, CA.) One rL of each extract was injected (splitless mode 30 s valve delay 200 0 injector temperature) into a capillary column (DB-wax or DB-Sms. 60 m length x 0.25 mm i.d. x 0.25 pm film thickness (d,) J W Scientific, Poison, CA). Helium was u.scd as carrier gas at a constant How rate of 0.96 mI7min. Oven temperature was programmed from 40 C to 200 0 at a rate of 3 Omin with initial and final hold times of 5 and 60 min, re.speclively. MSD conditions were as follows capillary direct interface temperature, 280 C ionization energy, 70 eV mass range, 33-350 a.in.u. BM voltage, 1956 (Atune + 200V) scan rate, 2.2 scans/s. Bach SDB or DE extract was analyzed in duplicate. 

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