what's the function of inlet in GCMS , and what's the advantages of Multi mode inlet over other types ?
The inlet is actually a very critical part of any GC System. It is responsible for the transfer of your sample onto the column. For liquid injection I give you a quick overview of the big 3:
SSL Inlet
The Split/Splitless Injector - the liquid is injected into a glass chamber(liner) that has a constant temperature. Ideally the whole sample should be vaporize and you can even split the gas flow to protect the column or improve resolution.
Pros:
- most used inlet
- Very rugged
Cons:
- constant temperature needs to be high can damage thermolabile substances
- The expansion Volumne of your sample can limit the amount of sample you can inject
- Can discriminate high molecular weight components
On column injector(OCI)
The on column injector is the most straight forward approach. The syringe enters directly the GC column and the sample is only vaporized when the GC oven reaches the necessary temperature.
Pro:
- the whole sample is transferred therefore no discrimination
Con:
- the whole sample with all its shit and non-volatile components are on the column. Those components will stay there for ever... also easy to overload a column
- Often the most expensive
PTV Inlet
The Programed Temperature Vaporization Inlet (Agilent sometimes calls it multimode I think) is the most versatile of all inlets. It is basically a SSL where the temperature can be varied during the run...hencefore can protect thermolabile substances. With adapters it can also do On-Column and if you feel really fancy you go for things like solvent venting. Basically that means you inject a large volumne of sample heat it to a point where only the solvent vaporizes and flush it out through the split line, then you close the splitvalve and transfer your sample onto the column.... my favorite
Pros:
- extremely versatile
- can do almost anything the other inlets can do
- Can handle large volumne injection
- Can handle thermolabile substances
Con:
- expensive
- Needs adapters and therefore additional connections for on-column
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