Can I use metal ferrules with the OPTIC inlet?

Yes, metal ferrules are available for OPTIC-3 and OPTIC-4. Please take care that you use a different nut, the standard supplied nut should not be used. 

2406-1005 Metal Ferrule, Column ID 0.25mm, pk/10
2406-1006 Metal Ferrule, Column ID 0.32mm, pk/10
2406-1007 Metal Ferrule, Column ID 0.53mm, pk/10
2406-2049 Inlet Bottom Nut for Metal Ferrule, pk/5
standard supplied with the OPTIC:
2406-2047 OPTIC Stainless Steel Inlet Bottom Nut, pk/5