Sample Injectors:

• How to Choose a Sample Injector
• Types and Capabilities
• Scale, Sample Volume, and Loop Size
• Liquid Contact Materials
• Make-Before-Break (MBB™)
• ChromTRAC™ Mapping

High Pressure Switching Valves:

• Column Selection
• Column Switching

Sample Injectors

How to Choose a Sample Injector
Table 1 below compares the characteristics of Rheodyne manual sample injectors and will help you choose the most suitable model. The following discussion details the information in the table.

Types and Capabilities
Models ending in -25 are dual mode injectors. Dual mode injectors can use both the partial-filling and the complete-filling method for loading the sample loop (Tech Tip 6). They are variable volume injectors because they allow the loading of various sample volumes. These dual mode injectors, also called front-loading injectors, have a needle port for loading sample built into the handle. The unique Rheodyne injection port design allows the tip of the needle to connect directly to the sample loop for no sample loss during loading.

Models ending in -10 are single mode injectors. Single mode injectors use only the complete-filling method to load the sample loop. They are called fixed loop injectors because the sample loop size determines the sample volume. These injectors require a Loop Filler Port accessory because a needle port is not built into the valve handle. There is not a direct connection between the syringe and the sample loop. Therefore, an excess of sample must be used to overfill the loop filler port and completely fill the sample loop.

Models with an "i" suffix are identical to the models with the same numbers, but the "i" designates a built-in position sensing switch. The switch provides the chromatograph with a reproducible start signal to mark the injection time in the data system.

The reproducibility of manual sample injectors depends on operator skill, syringe calibration, and the loading method. Partial-filling method is typically reproducible to 1.0% relative standard deviation (RSD). Complete-filling method is reproducible to 0.1% RSD for loops >5 µL.

Scale, Sample Volume, and Loop Size
Analytical scale models are for conventional columns with samples from 1.0 µL to 5.0 mL. Micro-scale models are for 1.0 mm and 2.0 mm inner diameter columns. Model 8125 has a sample range of 0.1 µL to 500 µL, and can be used for both analytical and micro columns. Preparative scale models are for columns with diameters from 1 to 10 cm, and operate at high flow rates with samples from 100 µL to 20 mL.

Liquid Contact Materials
All models have a polymeric rotor seal of Vespel® (pH 0 to 10 tolerance), Tefzel®, or PEEK (both pH 0 to 14). Stators are 316 stainless steel, titanium, or PEEK. Most models have an inert ceramic stator face assembly.

Make-Before-Break (MBB™)
Models incorporating Rheodyne's patented MBB architecture design provide uninterrupted flow when switching between LOAD and INJECT positions. MBB greatly reduces transient pressure shocks and is beneficial for flow-sensitive detectors, fragile columns, and pumps. Models 7725, 9725, 3725, 3710, and "i" versions contain the MBB design.

1. LOAD: Flow from pump port to column port is via parallel paths: the rotor seal groove and the MBB passage (dashed line).

Intermediate 1: Parallel flow stops. Flow is only in the MBB passage, MBB hole, and the leading part of the rotor seal groove. This is the mode during most of the 60° rotation.

Intermediate 2: Flow starts through the loop just as it stops through the MBB passage. Flow is not interrupted. Back pressure increases slightly (typically less than 10 psi) at the instant of transition because of the small area of overlap between stator face holes and rotor seal grooves.

2. INJECT: All flow is now via the loop. Note that the sample is never exposed to the MBB passage.

Many sample injector models can be fitted with a pneumatic actuator for automation. There are also models manufactured in titanium. Consult your authorized Rheodyne distributor for these options.

ChromTRAC™ Mapping
Selected Rheodyne manual valves contain the industry standard ChromTRAC Mapping to color-code your fluid connections. You can identify each port by its colored number, which designate the ChromTRAC color for each system component. Simply coordinate the ChromTRAC colored fittings with the port color. All valves are ChromTRAC Ready. The convenience of ChromTRAC color-coding knobs comes with the RheFlex® Fittings you use for all your connections.

High Pressure Switching Valves
Rheodyne offers high pressure manual switching valves to simplify procedures and improve the speed, resolution, and sensitivity of HPLC analysis. The switching valves are available in 316 stainless steel and PEEK, with a choice of 1.6 mm (1/16") or 3.2 mm (1/8") ports.

Column Selection
The six-position switching valves are used in column selection. These valves substitute one column for another without the need to manually disconnect the plumbing. This makes it easy to designate a separate column for each analysis. Designated columns eliminate equilibration delays, reduce interference, and prolong column life. Turning the valve handle selects the column desired for a particular analysis. The columns switched off-line are automatically sealed at both ends.

Column Switching
The two-position switching valves are used to re-route mobile phase during the chromatographic run without changing separation techniques or to perform sequential separations with different columns and/or mobile phases. Although the Model 7000 is the most commonly used and versatile switching valve, other models have specific uses such as for three-way or four-way switching patterns.

Many models have flow passages available in both standard bore and large bore, designated with an "L" suffix. L Models use 1/16" fittings and tubing but have larger flow passage diameters than non-L models. L models can accommodate higher flow rates. Large bore tubing can be used when the pressure drop must be limited. Large bore valves have a lower pressure drop than standard bore valves when both valve sizes accommodate the same flow rate.

Many high pressure switching valve models can be fitted with a pneumatic actuator and mounted in tandem for automation. There are also models manufactured in titanium. Consult your authorized Rheodyne Distributor for these options.

Figure 1. Pressure drop vs. flow rate for Model 7000 and Model 7000L (large-bore) valves; water at 20°C. Experimental measurements: The flow channel is one stator inlet port, one rotor seal groove, one stator outlet port, and two connecting tubes. Solid squares = (1.0 mm 7000L valve) + (two 1.0 mm x 5.0 cm tubes). Open squares = (0.6 mm 7000 valve) + (two 1.0 mm x 5.0 cm tubes). Cross mark = (0.6 mm 7000 valve) + (two 0.5 mm x 5.0 cm tubes). Solid lines are theoretical values for 10 cm long tubes of 1.0 mm and 0.5 mm ID. Pressure drop is in units of psi. 1.0 MPa = 10 bar = 145 psi.

Figure 2. The effect of tubing on analytical and micro-scale analyses can be significant. Since dispersion caused by tubing is proportional to the fourth power of diameter, large bore tubing should be avoided when performing analytical scale or micro-scale analyses. A size < 0.25 mm (0.010") is recommended. Consider a system with a Rheodyne injector and column switching valves, and analytical columns with small-bore connecting tubing. The chromatograms to the right made by using a typical analytical chromatograph, show these effects. Scheme A is the control (injector 'column' detector) with no valves in the system. In Schemes B and C, two Model 7060 Six-Position Switching Valves were placed side by side (injector 'valve #1' column 'valve #2' detector).

The injector and detector were connected to these valves by the same tubing used in the control. The extra tubing pieces required to connect the valves to the column were a 10 cm length for valve #1-to-column and a 35 cm length for column-to-valve #2. The diameters of these tubes are indicated in the figure caption.