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A careful examination of the source and remedy for potential contaminants that can plague chromatographers

Noisy base line, inconsistent results, shortened column life and ghost peaks. Every day, chromatographers face these frustrating problems that not only cost money, but also tie up equipment and hamper productivity. An easy target for the blame? The carrier gas.

As long as it is specified correctly, carrier gas is rarely the reason for a chromatographer’s woes. Most problems in the lab tend to arise from different contaminants that enter the system during cylinder change-out. Often, one or more of the many parts of a gas management system — from regulators, tubing, valves or traps — will introduce contaminants thatalter the gas purity, as well.

To better comprehend this concept, try to visualize carrier gas as a glass of drinking water and the gas delivery system as a straw. If you drink water that tastes bad, you might immediately assume the water is contaminated — without ever considering or realizing that the straw you are using is dirty. In this scenario, the problem lies within the straw and not the water. The same is true with gas. If the delivery system is contaminated, so too, will be the gas.

Common Contaminants
The most common and consistent contaminants that chromatographers encounter are moisture, oxygen and hydrocarbons. All three are present in the atmosphere and can cause a variety of issues — from reduced column life to ghost peaks. In some cases there are other contaminants that appear on a case-by-case basis, which often relate to one’s individualized process. This is why, when specifying the gas and grade required for your process, you must know which impurities will affect the system most severely. This knowledge will help you make sure the gas specification meets your requirements and minimizes the risk of dangerous contaminants.

When correctly specifying the gas, be sure that the analyzed limits for specific contaminants are within acceptable levels. Although there exist many different terms that gas companies use to indicate purity, it is also known that no standard exists for analyzing or labeling these gases. Therefore, it is critical that the user specifies the correct limits. Simple math indicates that if the gas is 99.9999% pure, there can only be one rogue molecule per million.

Oxygen levels in carrier gas should never exceed the following: 1ppm (H₂0 less than 10 ppm) for peak performance and minimum baseline noise. Naturally, there will always be some noise or “column bleed.” However, the column manufacturers’performance criteria allow for only 1ppm O₂ and 10 ppm H₂0 for consistent and optimum peak in both baseline ratios and normal column life. Although column bleed always occurs, higher than acceptable levels of O₂ and H₂O will increase the bleed and raise baseline noise.

Three Main Causes of Gas Stream Contamination
There are three primary areas to examine for contaminants:

• The cylinder,
• Cylinder change-out and
• System components

Comparatively speaking, cylinder change-out poses the greater risk for contamination. When the cylinder is disconnected from a regulator, the system drops to atmospheric pressure, allowing air to enter the system and quickly contaminate it. The first line of defense from contaminants is to ensure that the CGA connection has a high-purity, non-lubricated check valve built into the end of its nipple to keep the system pressurized during cylinder change-out. This will minimize the amount of air that can enter — only a very small amount; this is the volume between the nose of the CGA nipple and the back of the cylinder valve. A block-and-bleed valve removes contaminants that enter.

Contaminants also find ways to enter gas management systems. One source is through cylinders that have been hydrostatically tested. Cylinders have an infinite life and gas companies are only required by the DOT to recertify the cylinders every 7 to 10 years. Some cylinders have been in service for 40 or more years. In this instance, a cylinder is pressurized up to five thirds of its working pressure using H₂0; if the cylinder passes this test it is then dried out and put back into service as a recertified cylinder; however, moisture and oxygen become trapped in the inclusions of the cylinder wall. The cylinder is then refilled and is tested and certified at full cylinder pressure. The contamination occurs when a working cylinder’s pressure drops to 700 psi. The contaminants trapped in the inclusions escape from the cylinder wall, enter the gas stream and ultimately contaminate the product (Figure 1). Contamination can be avoided if cylinders are ultrasonically tested rather than hydrostatically. Another method of eliminating the contamination is to have a cylinder with a built-in purifier that eliminates contamination as the gas discharges through the purifier ensuring purity.[1]

Gas management system components can also contribute to being emitted into the gas stream unpredictably. The gas stream consists of the gas source, regulators, tubing, fittings, purifiers, traps, joints and valves, which are all potential sources of leaks and contamination. Therefore, it is imperative that the proper component is used for each system. Choosing the right gas grade, regulator, tubing, flexible pigtails, and valves, as well as correctly installing cylinder regulators and using proper joint techniques will help to eliminate the threat of contamination. Avoid flexible pigtails that have Teflon® cores, never use regulators that have lubricants, and only use purifiers as a backup.

Secondary Sources of Contamination
Chromatographers commonly believe the system purifier will resolve any and all contamination issues with the carrier gas. In reality, purifier systems are designed to safeguard against contamination and are not meant to be the sole method for cleaning gas. In fact, there are several ways that purifiers can actually be the source of contamination.

When the purifier medium is installed, an opportunity for contamination is created. The medium is packed into the tube under a gas purge, typically of argon. The purity of the purged gas may have contaminants that your gas source does not: the purifier manufacturer may not have been careful enough when selecting a gas supplier and did not check the levels of certain contaminants in the gas being purchased. The gas you flow through it may actually purge out these contaminants. Since the purged gas remains in the purifier when it is sealed after it is manufactured, your process flow will work to remove these contaminants and your base line will suffer until the contaminants are completely removed from the purifier.

Contamination is also dependent on the type of media being used. Purifier media are either surface-absorbent or chemically-absorbent. Surface-absorbent purifiers are most prone to produce inconsistent results. Surface-absorbent types catch contaminants on the outside like a Velcro® ball, as they pass over the media. As more contaminants attach, not only does the volume of contaminants grow but also the velocity increases through the purifier. This increased velocity can and will sweep off the contaminants when the purifier is at 40% to 60% of its saturation point. The released contaminants will go straight into the column — resulting in a noisy base line, ghost peaks, and inconsistent results. Plus, if the purifier becomes 100% saturated, it releases all the contaminants. The result is an extremely lengthy and costly cleaning process or can even result in the replacement of the column itself.

Three Tips for Avoiding Contamination
When purchasing regulators, make sure they are of bar stock body. Bar stock bodies have direct gas paths that do not allow contaminants to hide. Using a regulator with a forged body (even if it has been cleaned for oxygen service) with a stainless steel diaphragm will lead to inconsistent results. It is important to note that large-cavity regulators allow contaminants to emit unpredictably.

Tubing should be copper or stainless steel and chromatographically cleaned. Never use PTFE hoses as they are permeable and can introduce contaminants into the gas stream from the Teflon® used during the PTFE hose manufacturing process.

Valves should be packless diaphragm or non-lubricated needle valves — avoid ball valves unless you know the lubricant is not poisonous to your system. contaminants attach, not only does the volume of contaminants grow but also the velocity increases through the purifier. This increased velocity can and will sweep off the contaminants when the purifier is at 40% to 60% of its saturation point. The released contaminants will go straight into the column — resulting in a noisy base line, ghost peaks, and inconsistent results. Plus, if the purifier becomes 100% saturated, it releases all the contaminants. The result is an extremely lengthy and costly cleaning process or can even result in the replacement of the column itself.

Chromatographers should use purifiers with chemically-absorbent media as it absorbs any contaminant for which it is designed. Chemically-absorbent media cannot release the contaminant, and unlike surface-absorbent media, the collected contaminants cannot ever become dislodged. In most cases, chemically-absorbent media indicate which contaminants are present as well as how much is in the gas stream by the rate of its color change.

Dangers of Contaminants
Every chromatographer should consider two key questions: 1) What effects do contaminants have; and 2) What problems will contaminants cause? The two primary devices affected by contaminants are the column and the detector. In the column, the stationary phase starts to break down and elutes from the column. The detector registers these changes and shows it as bleed or noise on the baseline. The Supelco bulletin 848D illustrates these effects on 19 the various columns. Figure 2 shows the correct purifier being added to a system to ensure the purity of the gas before it enters the GC. Figure 3 demonstrates the deterioration of the column when an air leak occurs.

Packed Columns
At elevated temperatures the stationary phases, such as polyesters, (DEGS, EGS, etc.) polyglycols, (Carbowax®, UCON, FAP, SP™-1000) and polyamides hydrolysis. Silicone phases also react with oxygen when the operating temperature is above 250°C. The columns, when the oxygen and H₂0 are below the manufacturer’s threshold of 1 ppm oxygen and 10 ppm H₂O, will operate for months without issue. Yet, once these levels are exceeded, the columns quickly degrade. The first indication of degradation is when the packing at the front end of the column turns brown. The column loses its efficiency, thus shifting retention times (Figure 4).

Capillary Columns
These columns do not show deterioration with a color change but are still susceptible to O₂ and H₂0 contamination. Capillary columns containing bonded, cross-linked phases can handle short-term exposure to these contaminants, but extended exposure will damage the stationary phase.

Detectors
Specific detectors react differently to elevated levels of O₂ and H₂O:

• Electron Capture Detectors and Ionization Detectors are affected when the O₂ and H₂O contaminants compete for free electrons
• Hall detectors show elevated levels of O₂ and H₂O as base line noise.
• In thermal conductivity detectors, the filaments become oxidized creating a noisy base line, low sensitivity and shorter detector life.
• Mass Spectrometers see a reduced sensitivity and a shorter life of the MS filament due to elevated levels of O₂ and H₂O.

To achieve the performance levels stated by the manufacturers of columns and detectors, it is imperative to maintain and remove all contaminants that are in the gas stream. Specifying the correct levels of impurities in the carrier gas and selecting the right components between the carrier gas cylinder and the GC are a critical user responsibility. Maintaining low levels of contamination can be easily achieved by correct gas and component selection. In taking these necessary steps, one can ensure reliable, repeatable consistency and extended column and detector life.

1. Air Products’ technology BIP® (built-in-purifier) eliminates contamination as the gas discharges.

Frank Kandl is National Technical Manager, specialty gas equipment, at Airgas, Inc.,6990a SnowdriftRoad, Allentown, PA 18106. Kandl has been designing gas handling systems for 28 years for analytical and other high purity applications. he can be reached at 610-336-4522 or frank.kandl@airgas.com.

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