Valve injectors

For higher pressures, stopped flow injection has been used in which a valve releases the pressure in the column to atmosphere, at which point syringe injection at the top of the column (without septum) takes place. There has, however, been a trend in recent years to use valve injectors because of their generally high pressure capabilities and their ability to deliver large... 

Figure 6.11 Schenatic diagran of a split/splitless valve injector (A) and a timed-split injector (B) for open tubular column SFC.

Figure A2.1 Waters ProMonix On-Line HPLC analyzer. The upper compartment door contains a keypad for programming and operation of the analyzer. The upper window allows viewing of indicator lights and a liquid crystal display that provides the operator with analyzer interface, programmed parameters, and instrument status results. The lower chamber contains the pumps, valves, injector, and detector(s) required for the chromatographic separation. The sample conditioning plate for online process monitoring is to the right of the analyzer. This is a typical process HPLC. (From Cotter, R.L. and Li, J.B., Lab Rob Autom., 1, 251,1989. With permission of VCH Publishers.)...

Gums which form in aviation gasoline could produce undesirable deposits on induction manifolds, carburetors, valves, injectors, etc. Strict limitations exist for the gum content of aviation gasoline. 

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... 

Loop and Valve Injector—Device for placing sample onto the column head. Modern design consists of a loop, partially or overfilled at atmospheric pressure, that is rotated into the flowing stream from pump to column. Sample is back-filled from the end of the loop closest to the column, described as last in, first out filling. 

The sample is introduced into the chromatograph via a sample injector. To minimize dispersion and the broadening of peaks, the sample must be injected as a sharp plug. Valve injectors are usually used in commercial instrumentation, because the sample is introduced with minimal interruption of flow. The injection devices are the basis for both manual and automatic injection because of their ease of use, reliability, and ease of automation.6 A typical injection valve, the six-port valve, is illustrated in Figure 3.14. 

Fig. 1.2 Sample injectors (a) split valve injector (b) timed-split and direct valve injector...

Sample injector The sample injector is located between the solvent pump and the HPLC column, and serves to deliver or inject the sample dissolved in solvent onto the column. The most common form of sample injector is the so-called loop-and-valve injector. Sample is injected from a specialized syringe (manually) or delivered by means of an autosampler (automatically) into a small diameter loop. The loop-and-valve injector allows switching by means of a valve between initial delivery of solvent alone to the column (so-called column equilibration), to subsequent delivery of the sample onto the column. 

Sample injection in hplc is a more critical operation than in gc. Samples may be injected either by syringe or with a valve injector although the former is now rarely used. Valves, which can be used at pressures up to about 7 000 psi (500 bar), give very reproducible results for replicate injections (<0.2% relative precision) and are therefore ideal for quantitative work (p. 129). They consist of a stainless steel body and rotating central block into which are cut grooves to channel the mobile phase. from the pump to the column (figure 4.27). The sample is loaded into a stainless steel loop incorporated... 

Figure 4.27. Valve injector with external loop...

Even with all these chromatographic parameters optimised, sensitivity may not be enough. The simplest way to increase sensitivity further is to inject more sample. Valve injectors are usually supplied with 20-pl loops but there are few assays for which 100-200 pi of sample cannot be injected simply by changing the loop size. A further way is to use minibore (1 mm or 2mm i.d.) or even microbore (< 1 mm) columns. Such columns give increases in sensitivity due to the narrower peaks eluted but this may reduce column loading volumes and with some equipment the extra sensitivity gained may be lost due to extra column band broadening. 

The sample is injected into the system via a valve injector, as schematically shown in Fig. 1-2. A three-way valve is required, with two ports being connected to the sample loop. The sample loading is carried out at atmospheric pressure. After switching the injection valve, the sample is transported to the separator by the mobile phase. Typical injection volumes are between 10 pL and 100 pL. 

Samples can be introduced manually into the valve with a syringe to fill the sample loop. Automated sampling valves are routinely used today in which samples are taken from an autosampler for unattended operation. The major limitation of valve injectors is that the sample size is fixed, and the loop must be changed in order to vary the injected sample size. There are automated motor-driven adjustable syringes that provide enough pressure to inject the sample past a check valve that prevents backflow. 

A central problem of the approach described above certainly lies in applying it to a whole flare system instead of the combustion process alone. Doing so means that TAB becomes a "lumped parameter" incorporating all effects that have an impact on the acoustical behavior of the flare (i.e., noise emissions from valves, injectors, smoke suppression devices etc.) as well as any noise control measures installed. Other... 

The valve injectors available for liquid chromatographs give very reproducible... 

J. M. Harnly and G. R. Beecher, Two-Valve Injector to Minimize Nebulizer Memory for Flow Injection Atomic Absorption Spectrometry. Anal. Chem.y 57 (1985) 2015. 

For a preparative instrument some parts of the analytical type of chromatograph have to be modified and additional accessories are needed. The flow-rate of the pump must be larger than in analytical work. On the other hand pulse-dampening is less important, Only valve injectors with sample loops up to several hundreds of milliliters are used as sampling devices. As an additional unit a fraction collector is necessary, which should be freely controlled according to peak or time and should allow automatic operation in combination with a suitable sampling system. 

---