Clean Gas Yields Clean Chromatography

Wed, 09/05/2012 - 12:05pm
Agilent Technologies

The need for good gas hygiene is important to avoid risking the integrity of instruments, as well as to produce better, more productive chromatography.

Figure 1: Moisture in the carrier gas causes a marked rise in bleed profile.

Ensuring gas hygiene is one of the most important steps researchers can take to optimize the performance of GC or ICP systems. Impure gases can cause installation delays, premature instrument failure and flawed results. What’s more, the inefficient use of increasingly expensive and rare gas can go right to the bottom line. With a busy schedule, it can be too easy to lose sight of the need for good gas hygiene, but if not managed, gas quality can jeopardize instruments, columns and results.

Contaminants such as moisture, oxygen and hydrocarbons contribute to loss of sensitivity, undermine instrument accuracy and ultimately damage GC columns. For example, impurities in the gas-activate glass wool in liners can accelerate septa degradation, causing high background signals and ghost peaks, which lead to time-consuming troubleshooting. Oxygen in supply gas for ICP-OES or ICP-MS can cause plasma shutdown and loss of sensitivity. Carbon dioxide in supply gas for total organic carbon analyzers leads to elevated baselines and loss of sensitivity and accuracy. 

Supply gases can pick up contaminants from every part of the gas line, so researchers need a gas filter system even if the supply gas is of the highest quality. It is not economical to buy expensive, high-purity gases if the quality is downgraded by impurities in the gas line. 

Inserting a gas clean filter system in the gas line immediately before the instrument 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 lifetime. With a gas clean filter system, researchers can ensure clean gas delivery; enjoy fast stabilization; reduce helium gas consumption; benefit from highly sensitive indicators for maximum instrument protection; and minimize downtime with easy filter replacement.

The effect of poor gas quality

Figure 3: A charcoal filter greatly reduces the effects of hydrocarbons in the gas stream (nitrogen 4.5 ppm, hydrocarbons <5 ppm).Figure 1 shows the difference in bleed levels of two GC columns because of moisture exposure, with and without a filter, when running a temperature program of 50 to 350 C at 20 C/min. When no filter is used, an extreme rise in the bleed profile is clearly visible because of moisture in the carrier gas. A good gas clean filter removes all moisture in the carrier gas to provide a normal bleed profile.

The lower chromatogram in Figure 2 is a normal SimDist high-temperature analysis. The upper chromatogram shows the first injection on a new column. Here, the baseline does not return to zero. The higher signal at the end of the chromatogram is caused by the high bleed from the column—because of a leak after the column was installed in the GC—that allowed oxygen to enter the system.

Hydrocarbons contribute to loss of sensitivity and undermine the accuracy of the GC, but fitting a charcoal filter greatly reduces the effects of hydrocarbons in the gas stream, as shown in Figure 3. GC and ICP applications benefit from the use of gas filters, whether the instrument is connected to an FID or MS, or any other type of detector such as flame photometric, thermal conductivity, electron capture, nitrogen-phosphorous or thermionic. 

Filter constructionFigure 4: Using a GCMS filter to save on gas, and a moisture filter to improve productivity.

Gas clean filter systems usually comprise two key parts—the connecting unit and the filters. The connecting unit has inlet and outlet connectors for the gas lines, and the system can be wall mounted or fixed to a bench. Connecting units hold up to four filters and are available for 1/4 or 1/8” gas lines. The connecting unit should allow the instrument to remain under pressure during filter changeover and prevent air from entering the system. 

High-flow connection units are available that handle flow rates up to 20 L/min for collision gas applications, supply gas for ICP and ICP-MS, or any application where high flows are needed. For operations requiring flows above 10 L/min, researchers can save money by using cheaper gas and/or eliminating contaminants.

The filter material should be self-indicating to show when it has become exhausted. This is usually revealed by a color change. For example, in an oxygen filter, the color changes from green to gray, and in a carbon dioxide filter, from white to violet. It is important to change gas filters when they are exhausted.

Newer GCMS filters deliver faster stabilization times (Figure 4) so researchers can use less gas. These single combination filters are optimized to remove oxygen, moisture and hydrocarbons from GCMS carrier gas, so benefits are obtained from a single filter. 

A good gas clean filter system lets researchers use 99.996% (4.6) pure helium instead of the more expensive 99.999% (5.0) or 99.9999% (6.0) grade, while still yielding high quality analytical results. Figure 5 compares the costs of carrier gas with the use of helium 4.6 and 6.0. The expected cost savings are around 30%.

The filter system, in combination with 4.6-grade helium, delivers at least the same quality gas to the instrument as using 6.0-grade helium, with respect to oxygen and water. This provides considerable cost savings over the use of higher quality but more expensive helium.

Renewable gas purification systems are now available that not only trap large quantities of contaminants and last a long time, but are also recyclable. With average use, researchers only have to purchase a replacement cartridge once per year or after approximately 20 cylinders’ worth of purification. When a replacement is needed, the option to purchase a new or recycled cartridge is always open. Recycled cartridges are refilled and should be certified by the manufacturer to the full specification of new cartridges. 

Figure 5: Comparison of costs using a good gas clean filter and 4.6-grade helium rather than 6.0-grade helium.

Tips for gas success

There are some simple steps researchers can take to keep on top of gas purity: determine the gas purity level needed and choose the appropriate filter; keep the number of fittings in the gas line to a minimum; install gas filters in a convenient location close to the instrument; use gas filter log books to determine maintenance and filter replacement schedules; and use indicating traps closest to the instrument so it’s obvious when to change the traps that are upstream.

Agilent offers a comprehensive range of gas management systems to keep GC and ICP instruments operating at peak performance—including the Low Gas Alarm System, Renewable Gas Purification system and leak detectors and flow meters, along with other accessories, such as tubing, brackets and fittings. 

The need for good gas hygiene is evidently important to avoid risking the integrity of instruments, GC columns and results. Fitting gas clean filters and managing their use is a simple and cost-effective step to produce better and more productive chromatography.

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