No Interference

Managing the Inertness of the GC Flow-path Protects Your Instrument, Your Column and Your Results

As samples become increasingly active and more complex, labs simply cannot afford interferences caused by flow path activity in their GCs. A non-inert flow path can cause peak tailing and signal loss. It can also hide parts of samples, so users would never know what was missing. In addition, the need to repeat or verify suspect analyses wastes resources, hinders productivity and hurts the bottom line. Most important, unreliable results can be catastrophic in terms of environmental safety, food quality and inaccurate drug abuse accusations.

Users need the most inert flow path possible to achieve the lower detection limits demanded by increasingly tough regulatory obligations and to confidently quantify active analytes.

This article discusses the importance of GC flow-path inertness and includes five top inertness tips so users can be confident that nothing has been lost from samples, even at trace levels.

Figure 1. Are you building the most inert flow path?

Where’s the problem?

Every stage of the flow path can degrade your results, from the inlet liner to the ion source. Figure 1 shows the different components of the flow path where lack of inertness can impact your results.

What’s the solution?

Here are five top tips for GC flow path inertness that can help users be confident that nothing has been lost from samples, even at trace levels, and that productivity is maintained.

1. Maintain the inlet

Inlet cleanliness is critical to reliable, repeatable GC results. The choice of consumables, including septa and liner O-rings, affects the speed and quality of routine inlet maintenance. This is particularly important during work with trace, ECD and MSD analyses when out-gassing or silicone residue can be a problem. Inlet cleanliness is thus a major concern. The best option is to use septa and O-rings made from the purest materials, manufactured in clean facilities and housed in packs that maintain cleanliness and prevent contamination during shipping and handling.

Silcone components in the heated inlet are known to stick to hot metal surfaces. Adherent residues force unscheduled inlet cleaning, reducing productivity. Users can avoid this problem by selecting treated O-rings and septa that stop them from sticking to the metal surface of the inlet. The contaminant-free material prevents adhesion and unnecessary inlet cleaning, saving downtime and expense.

Preventive maintenance helps ensure peak instrument performance and productivity. Inspect and replace worn or dirty flow-path supplies, such as syringe needles, septa, ferrules and inlet seals, on a regular basis to eliminate leaks and minimize downtime. Record any changes in your lab book. Using certified vials, caps, septa, ferrules and gold inlet seals also extends the inert GC flow path.

Gold seals are made from stainless steel, electro-polished and gold-plated. The smooth exterior provides an inert surface that reduces breakdown of active compounds and reduces the risk of leaks.

You can also use high performance (HP) septa that are manufactured from a material lined with a very robust PTFE. These septa significantly reduce the amount of siloxanes that leach out of the material and offer favorable chemical compatibility. Because they provide dramatically cleaner backgrounds under very stringent operating conditions, HP septa are your best option for reliable and efficient headspace analysishigh- temperature GC.

Figure 2 shows a comparison of high-performance and non-high-performance septa. The contaminant-free cleanliness of the chromatogram produced using HP septa is clearly evident.

Figure 3 is an expanded view, showing how high-performance septa provide industry-leading chromatographic purity at 300°C.

Figure 2. GC-MS chromatogram comparison of vial blank with different PTFE/silicone headspace septum and highperformance septum. Vials were equilibrated at 300°C for 30 minutes. Using a high-performance septum delivers a chromatogram free of contamination.

Figure 3. A high-performance septum provides significantly cleaner blank background at high-temperature headspace testing. Even with an expanded abundance scale, the 300°C vial blank chromatogram with an HP septum shows few siloxane peaks with very low abundance.

2. Prevent sample loss at injection

Inlet liners are critical links in the sample flow path, and they can be a source of activity and analyte loss. Liner design and chemistry impact the transfer of compounds into the column because active sites in the liner and the glass wool can cause loss of analyte; therefore, users should always use a reliably deactivated liner suited to the injection technique in use and change the liner as needed. This will maximize sample transfer and minimize sample loss.

Inlet liners with glass wool are widely used because the wool promotes homogenous sample mixing and better quantitation. Using a liner with wool seems like the obvious solution to trap high-boiling-point matrix interference and prevent “junk” from contaminating GC or GC-MS systems.

However, glass wool liners can have drawbacks. The active sites on the wool surface can trap sensitive analytes, preventing these compounds from being delivered to the column for separation and analysis, and therefore causing significant loss of system sensitivity.

Users can now inject heavy matrix samples and retain sensitivity by using ultra-inert liners with wool for trace-level analyses such as semi-volatiles, pesticides and even drugs of abuse. As well as protecting the inlet and column, and ultimately the MS source, the highly deactivated surfaces of these liners and wool are so inert that the negative impact of surface activity is significantly reduced, as shown in Figure 4.

Peaks
1. 2,4-Dinitrophenol
2. 4-Nitropphenol
3. 4,6-Dinitro-2-methylphenol
4. 4-Aminobiphenyl
5. Pentachlorophenyl
IS1. Acenaphthene-d10
IS2. Phenanthrene-d10

Figure 4. Using an ultra-inert liner preserves analyte integrity (above) compared to a liner without ultra-inert capability (below).

With environmental samples or samples extracted from plasma or urine, users will be dealing with heavy matrix samples that can have a marked effect on instrument condition every day. Modern ultra-inert liners provide excellent consistency, even with heavy matrix samples. The high level of inertness permits use of glass wool to trap the nonvolatiles in the matrix, extending lifetime and protecting the column and the detector.

3. Use a column featuring low activity

Peak shape and response can also be adversely affected by what happens to analytes in the column, so high inertness is important here too. High column inertness minimizes compound loss and degradation for more accurate quantitation of active analytes, especially at trace levels of acids, bases and other active compounds. To ensure consistent column inertness, choose a column that has been tested with a rigorous test probe mixture for in-depth evaluation of column deactivation. Check that the chemical species in the probe are known to adsorb onto active surfaces so you can trust that the results provide a searching test of column inertness.

When installing the column, start with high-quality ferrules and examine column ends under magnification for chips and burrs. Make sure the column is positioned at the recommended depth into the inlet and detector. Inert columns can be routinely used for applications in the environmental, forensics, foods/flavors/ fragrances, pharmaceutical and special chemical industries.

Figure 5. Potential cost savings when using gas filters and 4.6 grade helium rather than 6.0 grade helium.

4. Remember the detector

To ensure accurate quantification and high sensitivity, the entire flow path must be highly inert, including detector surfaces. This is especially true of mass spectrometers, where an inert ion source is necessary to prevent active compounds from attaching to metal surfaces. Analyte breakdown products can indicate active sites in the detector that can compromise your data. The best inert sources are constructed of a solid inert material, as opposed to an inert coating that can wear away over time.

5. Purify the gases

Ensuring gas hygiene is one of the most important steps you can take to optimize GC system performance. Impure gases can introduce contaminants and cause installation delays, premature instrument failure and flawed results. In addition, the inefficient use of increasingly expensive and rare gas can go right to the bottom line.

Users need filters for oxygen, hydrocarbons and moisture to avoid loss of sensitivity and accuracy of the GC or damage to the system. Impurities in gases can activate glass wool in liners and accelerate septum degradation. The results are high background signals and ghost peaks. These lead to time-consuming troubleshooting. Inserting gas filters in the gas line immediately in front of the GC inlet greatly reduces the level of impurities, thus improving trace analysis. Contaminants entering the GC column will also be reduced, which is critical for high-temperature analysis and essential for longer column life. Gas filters also ensure clean gas delivery, provide fast stabilization and reduce helium gas consumption.

A good gas clean-filter system enables use of 99.996 percent (4.6) pure helium and gets high-quality analytical results. This is preferable to the more expensive 99.999 percent (5.0) or 99.9999 percent (6.0) grade gas. Figure 5 compares the costs of filtered and nonfiltered carrier gas using helium grades 4.6 and 5.0. The expected cost saving is approximately 30 percent.

Keep your GC clean – and working

With a busy schedule, it can be too easy to lose sight of the need for an inert flow path. However, if you don’t manage the inertness of your system, you risk jeopardizing your instrument, your column and your results, with a potentially serious impact on economy and productivity

To get a poster detailing the components of an inert GC flow path please visit www.agilent.com/chem/UIorder