6-port injection valve explained: positions, rotor seals and pressure

A 6-port, 2-position injection valve has a fixed stator with six ports and a rotating seal with three grooves, each joining two neighbouring ports. Turning the rotor 60° switches between LOAD, where the pump feeds the column directly and the sample loop is filled at low pressure, and INJECT, where the pump flow is routed through the loop and carries the sample onto the column. Sealing happens between a polymer rotor seal and a ceramic or metal stator, and that pair sets the valve's pressure rating, pH range and solvent limits.

Key points

• In LOAD, pump and column are joined directly; in INJECT, the loop sits between them. Whether the loop empties forwards or backwards depends only on how the pump and column lines are connected.
• The rotor seal material sets the chemistry limits: Vespel to pH 10, PEEK and Tefzel to pH 14, Tefzel only to 50 °C.
• Ratings on the market run from 5000 psi for HPLC valves to 15,000-20,000 psi for UHPLC valves, and higher ratings can come with a narrower pH range or shorter seal life.
• A scratched rotor seal causes cross-port leakage: leaks at the vents, smaller and more variable peaks, and peaks that shrink when injection is delayed.
• Abrasive particles and buffer crystals cause most early seal failures. Position sensing confirms that each switch actually happened.

How is a 6-port injection valve built?

IDEX describes the core of its manual injectors simply: high-pressure switching and sealing take place between a ceramic stator face and a polymeric rotor seal, and the rotor seal is usually the only part that wears (IDEX injectors troubleshooting guide).

• Stator. The fixed part. Six ports sit on a circle, 60° apart, and the tubing connects here.
• Rotor seal. A polymer disc pressed against the stator, with three engraved grooves. VICI's Cheminert valves use a flat engraved rotor; its Valco valves use a tapered rotor held by a preload assembly (VICI rotor materials).
• Sealing force. A spring or adjusting screw sets how hard the rotor presses on the stator, and so the pressure the valve holds.
• Drive. A handle on manual injectors, a motor on autosamplers and switching valves.

Each groove spans two neighbouring ports. Turning the rotor 60° moves every groove along by one port, so the pairs 1-6, 2-3 and 4-5 become 1-2, 3-4 and 5-6.

Load and inject positions, port by port

This is VICI's numbering for its 6-port external-loop injectors (VICI Technical Note 808):

Traced as flow paths:

LOAD    pump 4 -> 5 column
        sample 1 -> 6 -> loop -> 3 -> 2 waste

INJECT  pump 4 -> 3 -> loop -> 6 -> 5 column
        sample line 1 -> 2 waste   (the needle line can be rinsed during the run)

Port numbering is not universal. On Rheodyne front-loading injectors such as the 7725, the syringe needle connects directly to the loop end at port 4 through a needle seal in the rotor, and the vent tubes sit on ports 5 and 6 (IDEX guide). Always work from the diagram for your own valve.

Backflush or forward flush: which way does the loop empty?

In the table, sample enters the loop at the port 6 end during LOAD. In INJECT, pump flow enters at the port 3 end and pushes the sample back out through port 6: the sample leaves through the end it came in. That is backflush.

Swap the pump and column lines (pump on port 5, column on port 4) and LOAD still joins pump to column, but in INJECT the pump now enters at the port 6 end and pushes the sample forward through the whole loop and out of port 3. That is forward flush.

For full-loop injections the direction hardly matters. For partial fills it does: with backflush, a 10 µL plug in a 100 µL loop goes straight to the column; with forward flush it first travels through the other 90 µL of loop and spreads out on the way. The details are in full-loop vs partial-loop injection, and the whole injection cycle in how an HPLC autosampler works.

Rotor seal and stator materials

Sources: IDEX pH ratings (rotor seal materials), temperature and torque (Vespel vs Tefzel vs PEEK), PEEK and Tefzel chemistry (IDEX fluidics FAQ), Rheodyne ratings (Technical Note 5), VICI materials (rotor materials).

IDEX adds that its three standard seal materials have about the same lifetime and wear at about the same rate. Stators come in ceramic (IDEX manual injectors), PAEK, Nitronic 60 or titanium (VICI's 5000 psi Cheminert valves), and with a coating for VICI's 20,000 psi models. Pressure and chemistry can pull in opposite directions: IDEX lists its MX Series II UltraLife stainless steel valve at 15,000 psi with a pH range of 2-10, and its DuraLife stainless steel valve at 6000 psi with full pH range (IDEX MX Series II).

Pressure ratings you will see

Conversions: 1 MPa = 10 bar = 145 psi.

Sources: VICI UHPLC valves, VICI HPLC valves, IDEX MX Series II, IDEX guide, Rheodyne TN5.

Three things to know when reading these numbers. A rating holds at a stated temperature, and VICI's figures are for two-position valves; multiposition selectors are rated lower. Sealing pressure may fall as the seal wears, and IDEX suggests readjusting if leaks appear above about 27.6 MPa (4000 psi). And running near the limit costs seal life, as VICI's Valcon H figures show. Pick a valve rated well above the highest pressure your system reaches at the moment of injection; more on units and margins in HPLC pressure ratings.

For scale: Dolan describes conventional LC as running below about 6000 psi (400 bar) and UHPLC above it (LCGC). The Elutrolab AS-48 valve, with its 5000 psi (345 bar) design rating*, is an HPLC valve, not a UHPLC one. Values marked * are design targets still to be confirmed by testing.

Switching speed and pressure pulses

A valve does not switch instantly, and on a standard valve the flow is briefly interrupted mid-turn. IDEX lists the resulting pressure disturbance as a cause of a spurious peak at time zero in an oversensitive detector, and tells manual users to turn the handle rapidly to INJECT. Rheodyne's make-before-break (MBB) design keeps flow uninterrupted between LOAD and INJECT, which, in Rheodyne's words, greatly reduces transient pressure shocks (Technical Note 5). Motorised valves need enough torque for their seal; VICI sells its universal actuator in high-speed, medium-torque and high-torque versions to cover the range of valve turning torques.

The second pulse comes from the loop itself. A loop filled at near atmospheric pressure is suddenly connected to the column pressure, and its contents compress. A worked example with the textbook compressibility of water, about 4.6 × 10⁻⁵ per bar:

240 µL loop, 1 bar -> 200 bar
water:          240 µL × 4.6e-5 /bar × 200 bar ≈ 2.2 µL
organic-rich:   2-3 times more compressible  ≈ 4-6 µL
at 1 mL/min (16.7 µL/s): 0.13-0.35 s of pump flow to make up

The pump has to supply that volume before flow at the column recovers, so the pressure dips briefly at injection. Vendors work on this: Thermo Fisher says its SmartInject technology limits the disturbance of eluent flow when the loop is switched into the flow path and so reduces run-to-run retention time variation, especially at the start of the gradient (Thermo Fisher SP74151). The reverse happens when the valve returns to LOAD: the compressed loop contents expand into the sample side, which therefore needs a free path to waste.

Cross-port leakage and other wear symptoms

Source for all rows: IDEX injectors troubleshooting guide and IDEX fluidics FAQ. Carryover from worn seal grooves is covered in autosampler carryover, and other injector faults in autosampler troubleshooting.

How often should you maintain the valve?

No vendor gives a universal interval. IDEX says a rotor seal lasts many thousands of injections in normal use and that the most common cause of early failure is surface scratches from abrasive particles: sample debris, mobile phase particles, buffer salt crystals and burrs from tubing. VICI agrees that particles scratching the sealing surfaces are the most frequent cause of premature valve failure and names uncleaned tubing as their most common source (VICI TN801). IDEX's practices for maximum seal life:

1. Check tubing for burrs and flush it before connecting it.
2. Filter mobile phases, and filter samples that are not clean.
3. Fit a filter between the pump and the injector.
4. Flush the valve with water after buffers, especially before shutdown.

When you replace a seal, inspect the stator. IDEX warns that a chip or crack on a ceramic stator face will damage a new rotor seal in a single turn, and VICI says to replace rotor or stator if scratches are visible to the naked eye. Tighten stator screws alternately in quarter turns, and do not overtighten. A practical schedule: log injections, note the count at which leaks or precision drift first appeared, and replace the seal well before that count next time.

How do you know the valve actually switched?

A switch that fails or stops short gives no injection, a partial injection or a leak across ports, and the chromatogram alone rarely tells you which. Valves report their position in different ways:

• Position-sensing switches on manual injectors. Rheodyne models with an "i" suffix, such as the 7725i, have one built in, typically used to start the data system at injection.
• Position feedback on motorised valves. IDEX lists valve-position feedback among the advantages of automated valves, VICI sells position feedback accessories, and VICI's two-position microelectric actuators use stall sensing to cope with misalignment.
• Absolute angle sensing. An end-of-travel switch only says the drive reached a stop. An angle sensor reports where the rotor actually is, so it can catch an incomplete turn, a slipping coupling or a valve that never left its position.

The AS-48 uses an absolute angle sensor to check every valve switch, and its power-up self-check includes valve position. It does not provide a contact-closure inject-start output. For instrument builders, valve feedback is one of the items in our OEM integration checklist.