In a previous post (see Merlin Microseal or Septa… should they really “SEAL”?) the topic of leaking with the Merlin Microseal was discussed. We recently had a customer use a Restek Leak Detector to test for helium at the top of a Merlin Microseal and this sensitive device did indeed detect helium coming from the top of the Merlin nut. Does this mean the Merlin Microseal is defective? Is this “leak” a problem?
First a bit about the Merlin Microseal: It is a micro-valve alternative to the conventional silicone rubber septa used in GC injectors. The primary seal of the Merlin Microseal when a syringe needle isn’t in place is a “duckbill” held in the closed position by a stainless steel spring. This duckbill valve seals the injection port during analysis, but allows the needle to slide through without damage. O-rings seal around the outer surface of the syringe needle during sample injection.
With proper use, including using the correct syringe needle outer diameter (23-gauge, 0.63mm) and needle termination (truncated cone or blunt tip), the life of a Merlin Microseal can be thousands of injections. Sharp edged or sharp pointed needles can pierce and damage the duckbill seal. The 23-26 gauge tapered needles also will not work properly because the 26 gauge part of the needle opens the duckbill before the 23 gauge part can seal with the O-ring. More details on the use and design of this product can be found in the Microseal User Manual.
Now, back to the “leak.” The duckbill can allow a very small amount of carrier gas through this primary seal. When a sensitive leak detector is used to sniff around the top of the Merlin nut it will detect helium (or hydrogen) carrier gas. With continued use it is possible for this carrier gas leak to increase due to wear or to small particles getting deposited in the duckbill. However, as Jaap de Zeeuw noted in the previous post, this small leak is at the top of the injector and is analogous to septum purge flow. Since the syringe needle deposits the sample in the inlet liner several inches below the Merlin Microseal, there is no effect on the injected sample.