Two position valves

Since the most common method of sample injection utilizes a 6 port valve with an external sample loop, 6 port valves are often referred to as "injectors". However, as the applications show, 6 port valves can do more than inject sample, and 8 and 10 port valves can be sample injectors at the same time they’re also being backflushers or column switchers.

One more variation is the 4 port internal sample injector, which is used when the sample size must be smaller than the smallest available loop. The internal sample "loop" is actually an engraved connecting slot on the rotor which is sized to contain a specified amount of sample.

Further reference

Sample loops
Two position valve applications

Valco injectors and valves for GC

Internal sample injectors
Sampling and switching valves
   Medium temp
   High temp

Valco injectors and valves for HPLC

Internal sample injectors
Injectors and switching valves
   Analytical
   Semi-preparative
   Preparative