GC gas leaks fall into two distinct categories: large leaks that prevent the instrument from functioning and smaller leaks that allow the system to operate, but negatively impact chromatography. Large leaks typically prevent a system from reaching a ready state, leading to an electronic pressure control (EPC) safety shutdown. These types of leaks can result from a column not being installed in the expected inlet, a column not being
connected to the expected detector, a broken column, broken or loose fittings, broken ferrules, cored septa, or tubing blockage, to call out a few possibilities. The cause of these symptoms typically can be rooted out quickly by visual inspection or review of the method settings.
Identifying smaller leaks that allow the system to continue to operate can be more involved. Symptoms of smaller leaks can include constant cycling of actual pressure readings (oscillations greater than 0.02 psi), poor retention time reproducibility, higher than typical background, higher than typical bleed (particularly at temperatures greater than 230 °C), baseline drift, higher than usual inlet activity, tailing peaks, the need for more frequent inlet maintenance, and poor area reproducibility.
Figure 1 shows the elution of US-EPA 8081 pesticides on an Agilent J&W DB-1701 phase before and after exposure to 1,000 μL/L oxygen in helium carrier gas. After just 10 injections, column bleed increased significantly and a shift to shorter peak retention times was apparent
- No O2 exposure
2. Increased bleed shift to shorter retention after O2 exposure
Figure 1. US-EPA 8081 pesticides before and after exposure to 1,000 μL/L oxygen in helium
Carrier Gas Considerations
High quality carrier and detector gases of known purity are essential for obtaining optimal results in gas-phase analysis. Agilent specifies carrier and detector gas purity of at least 99.9995% (5.5 nines). Zero-grade air is recommended for flame detectors . Inline indicating gas traps are highly recommended to remove hydrocarbon, moisture, and oxygen. Gas certification testing and product descriptions vary by supplier and so obtaining a certificate of analysis (COA) for the gases in use is essential to understand gas quality. On the COA, key items to look for include tests conducted, specification for contaminants, and indications of whether testing was done on individual (preferred) or representative cylinders from a batch
Selecting an appropriate ferrule for the column tubing size and particular fitting being used are critical for minimizing potential leaks and keeping the flow path free of contamination. Agilent J&W columns require one size ferrule for 0.1 to 0.25 mm id columns, while 0.32 mm and 0.53 mm id columns each require ferrules with larger diameter holes to accommodate the wider outside diameter of these columns. Ferrule material choice is also important for achieving the desired results for specific applications. Graphite ferrules are a popular choice for general-purpose and high-temperature applications (above 350 °C) but, typically, are not as contaminant-free as polyimide/graphite or metal ferrules. Further, graphite is a porous material and slightly permeable to gases, creating a very small continuous leak. Graphite also has a tendency to flake off, becoming a source of contamination. Pure polyimide ferrules are recommended for use outside of heated temperature zones only as they shrink dramatically with exposure to heat cycling.
Polyimide/graphite ferrules are a good choice for GC/MS and trace-level analysis, but they also have a tendency to shrink with repeated heat cycles, forcing the operator to snug the fitting repeatedly to avoid leaks. Ferrule shrinkage results in a tendency for analysts to over tighten fittings using polyimide/graphite ferrules.
Flexible metal ferrules are recommended for use with Capillary Flow Technology (CFT) devices as they are specifically designed for the fittings in these devices . Analysts are finding flexible metal ferrules an attractive alternative to other ferrules for standard column connection, such as the split/splitless inlet.
Table 1 identifies some common benefits offered by various capillary column ferrules
Table 1. Ferrule material selection attributes
Avoid Over Tightening
Be aware that over tightening can break the column or permanently damage fittings and actually produce leaks. The Agilent UltiMetal Plus Flexible Metal ferrule was designed to reduce column breakage by ompressing around the column. With a deactivated surface, these stainless-steel ferrules provide a robust and inert leak-free connection. Over tightening of Swagelok, SilTite or UltiMetal Plus Flexible Metal ferrules can damage fitting threads, making it impossible to obtain a seal, and resulting in costly instrument repairs. Carefully read and follow manufacturer’s instructions on fitting installation and use to avoid chronic leaks from
damaged fittings. Proper installation of graphite, polyimide/graphite, inlet seals, O-rings, and septa is also critical to maintaining leak free connections. Just tight enough (JTE) is the goal for proper installation of these somewhat pliable components. If tight is good, tighter is not better (TNB), as these pliable materials and can easily be crushed beyond their design specifications, causing them to leak sooner and more often. In extreme cases with repeated over tightening of brass mass spec transfer-line nuts, the nuts themselves can crack and potentially cause permanent damage to the mass transfer line. The Agilent septum nut has a C-shaped clip at the top that should not be turned more than 3/4 of a turn past where it begins turning with the nut assembly when a septum is being installed. Over tightening of the septum nut will cause premature septum coring with repeated injections, which in turn causes the septum to leak during a run. The septum nut is another fitting that needs to be JTE.
TO BE CONTINUED
Best Practice for Identifying Leaks in GC and GCMS: Technical Note, Agilent Technologies
Written by Muyiwa Adebola
[email protected], www.aasnig.com