The Best Approaches to Gas Purification

By: Robert F. Wallace, Reporter US Volume 26.5

Surpass Instrument/Column Manufacturers’ Recommendations

Robert F. Wallace

bob.wallace@sial.com

Introduction

Even with the use of ultra-high purity gases, contaminants may be present that will shorten column life and/or adversely affect chromatographic performance. To guard against this, gas management/delivery systems must contain components designed to eliminate these damaging contaminants. The installation of gas purifiers is the only sure way to guarantee that every gas stream is contaminant free.

With so many products available, how does one go about determining which ones are appropriate for each gas stream? Let’s take a closer look at the best purifier choices for several gas streams, including GC carrier gas, make-up gas, FID fuel gas (both compressed air and hydrogen), and the protection needed for LC-MS units.

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Sources of Contaminants

Sources of gas contamination can include the gas cylinder, the cylinder changing process, fittings, and regulators. Of these, the primary source is the gas cylinder. All grades of cylinder gas contain contaminants. The differences in grades are in the level and types of contaminants measured. There is always a chance that even the highest purity gas contains contaminants that can adversely affect your application.

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GC Carrier Gas

Selecting the proper purifiers for GC carrier gas begins with determining the contaminants to be removed from the gas, the levels to which the contaminants must be reduced, the flow and pressure needs of the system versus the flow maximums for effective operation of various purifiers, and the desired frequency of changing purifiers. There is always a possibility that a gas cylinder, gas line, or valve can contain residual hydrocarbons. Hydrocarbons and other contaminants in the carrier gas can appear as peaks with commonly used thermal conductivity detectors (TCD) and flame ionization detectors (FID). In addition, it is known that GC columns can degrade when exposed to moisture and oxygen in the carrier gas. Therefore, carrier gas purification should start with purifiers for trapping hydrocarbons, moisture, and oxygen. With compound-specific detectors (sulfur, nitrogen/phosphorus, electron capture), these three contaminants may be unseen, but they still can hinder quantification of analytes and shorten column life. Therefore, they must still be removed.

In carrier gas purification, the best purifier system includes multiple purifiers that help protect each other from surges of contaminants (as when room air enters the system during the process of changing cylinders) while they are protecting columns and detectors. The first step in deciding which purifiers to use is to determine, based on the number and types of chromatographs in your system, what the gas flow through the purifiers will be. Exceeding the flow rate that a purifier is designed for will reduce the contact time between the gas and the medium in the purifier, and the purifier may not reduce the contaminants in the gas to the advertised levels of purity. If the total flow in your system is 1 liter/minute or less, smaller purifiers (typically containing 100-120 cc of adsorbent material) can be used. If the flow is between 1 liter/ minute and 10 liters/minute, a large capacity purifier of each type (typically 750 cc of adsorbent material) can be used.


Table 1. Purifiers and the Contaminants They Remove



As shown in Table 1, for maximum system protection and longest purifier lifetimes, we recommend a carrier gas purification scheme consisting of a Supelcarb HC Hydrocarbon Trap, a Molecular Sieve 5A Water Vapor Trap to remove moisture, a High Capacity Gas Purifier for the removal of moisture and oxygen, and an OMI (Oxygen Moisture Indicator) Purifier. This system will remove hydrocarbons first, followed by moisture, and then oxygen. Figure 1 illustrates the above-mentioned sequence of installation for carrier gas purifiers.


Figure 1. Recommended Configuration for Carrier Gas Purifiers



The primary function of a hydrocarbon trap is to trap most hydrocarbons. Some hydrocarbon traps, like our Supelcarb HC Hydrocarbon Trap, contain a carbon molecular sieve that also restricts the elution of moisture and oxygen. Surges of moisture and oxygen that enter the gas line during the installation of a new gas cylinder enter the Supelcarb HC Hydrocarbon Trap and are released slowly, diluted by the clean carrier gas from the new cylinder to levels that are more effectively dealt with by the Molecular Sieve 5A Water Vapor Trap and/or the High Capacity Gas Purifier moisture/oxygen trap downstream. (Note that hydrocarbon traps do not trap methane under normal conditions.)

The Molecular Sieve 5A Water Vapor Trap, which contains a zeolite molecular sieve, removes most of the moisture and slows the movement of oxygen. Gas reaching the very efficient High Capacity Gas Purifier contains significantly reduced concentrations of contaminants. This conserves the purifying material in the trap, minimizing the frequency of replacement, and ensures the trap will reduce moisture and oxygen levels to less than 1 ppm, the desired level of carrier gas purity to ensure best column performance and long column lifetimes. This 1 ppm level meets the demands of GC columns.

As a final safeguard, we recommend installing an OMI Purifier just before the carrier gas line enters the GC. This protects against contaminants that may have entered through fittings in the gas lines. The OMI Purifier also gives a visible indication of contaminants. If it changes color, the GC system has leaks or the purifiers installed in front of it have expired and need replaced.

It should be noted that contaminants start to break through a purifier after about 75% of the theoretical capacity (Figure 2) is expended. This is because contaminants react with the material along the center of the purifier tube before they react with the material along the periphery of the tube.


Figure 2. Breakthrough Begins at About 75% of Apparent Purifier Capacity


It is not required to purchase costly certified gas in an effort to avoid critical carrier gas impurities that can damage your GC columns and elicit detector noise. These sequentially connected purifiers remove impurities from inexpensive (99.995% pure) carrier gas. Supelco researchers routinely purify low carrier gas grades with this purification system. As shown in Table 2, after purification, our carrier gas actually contains lower concentrations of chromatographically critical impurities than most certified grades.


Impurity Levels (ppm)2

Table 2. Quality1 and Cost Benefits of Proper Purification


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GC Carrier Gas (Helium Specific)

For helium carrier gas protection, the combination of a Supelco Helium Purifier and an OMI Purifier is the best choice. The Supelco Helium Purifier incorporates multiple beds of highly effective, high capacity adsorbent material to remove hydrocarbons, moisture, oxygen, carbon monoxide, and carbon dioxide from the helium stream. It is ideal for GC or GC-MS applications where high purity helium is essential. The output purity will be less than 100 ppb total of all contaminants (less than 30 ppb hydrocarbons [as methane], less than 20 ppb moisture, less than 2 ppb oxygen, less than 20 ppb carbon monoxide, and less than 20 ppb carbon dioxide).

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Make-Up Gas

To increase the total flow entering the detector and to help minimize dead volume, make-up gas may be added at the exit end of the column. Gases such as nitrogen, helium, and argon/methane are typical make-up gases. Make-up gas should be free of contaminants to prevent baseline interference and excessive noise within the detector. If hydrocarbon contamination is suspected, install a Supelcarb HC Hydrocarbon Trap upstream of moisture and oxygen traps. Because moisture reacts with most oxygen traps, install a Molecular Sieve 5A Water Vapor Trap in front of a Supelpure-O Oxygen Purifier. Installing an OMI Purifier last on the make-up gas line will give the final polishing of the gas before it enters the system.

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FID Fuel Gas (Compressed Air)

Air used as fuel for a flame-type detector does not require the same degree of purification as carrier gas. Impurities in fuel gases used with flame ionization detectors (FID) affect the performance of the detector, particularly when operating at high sensitivity. Effective air purification consists of removing the hydrocarbons and reducing the water level to 50 ppm or less. A Supelcarb HC Hydrocarbon Trap and a Molecular Sieve 5A Water Vapor Trap will assist in the removal of contaminants and ensure the proper performance of the FID.

The approach to air purification differs, however, depending on whether the source of the air is cylinders, an air compressor, or an air generator. As an alternative to air cylinders or compressors, a modern, low-maintenance zero air generator will provide air at a purity exceeding the quality demands of your chromatograph.

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FID Fuel Gas (Hydrogen)

For hydrogen used as fuel for a flame-type detector, the same purification system (removal of hydrocarbons and moisture) is used as for air from cylinders. We do not recommend using plastic bodied purifiers to purify fuel hydrogen because they are permeable to moisture and oxygen in the atmosphere. The Supelcarb HC Hydrocarbon Trap and Molecular Sieve 5A Water Vapor Trap are the best options for protecting fuel hydrogen gas.

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LC-MS Units

Removing contaminants from a nitrogen feed line is critical to the performance of an LC-MS unit. A two-cartridge base-plate gas purification system specifically designed to meet the high flow demands of the LC-MS while removing hydrocarbons, allows the nitrogen supplying the system to reach a purity greater than 6.0 gas (99.9999%) quality. Independent of the original gas quality, this purifier can reduce contaminants down below the ppm level or levels lower than the limit of the analytical detection. To allow longer contact with the gas stream the purifiers are connected in parallel and the gas stream is split equally between the cartridges and rejoined after the purification and prior to leaving the base-plate. This LC-MS purifier is a glass/metal cartridge making it completely inert with the ability to withstand pressures up to 150 psi/11bar.

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Conclusion

Gas purifiers to remove contaminants are needed regardless of the gas purity being used. As discussed, sources of contaminants include the gas cylinder, the cylinder changing process, fittings, and regulators. Adhering to Supelco’s purifier recommendations and installing them in the proper sequence is the only sure way to guarantee the highest quality gas, resulting in maximum column life and ultimate performance.

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