Frequently Asked Questions 

What are the structure and bulk properties of Nafion in tubing form?

Nafion^® is a copolymer of perfluoro-3,6-dioxa-4-methyl-7octene-sulfonic acid and tetrafluoroethylene (Teflon^®).  In simpler terms Nafion is a Teflon backbone with occasional side chains of another fluorocarbon. The side chain terminates in a sulfonic acid (-SO₃H).

With the exception of the sulfonic acid group, Nafion is a fluorocarbon polymer. Like most fluoropolymers, it is extremely resistant to chemical attack (corrosion resistant). The sulfonic acid group is immobilized within the bulk fluorocarbon matrix and cannot be removed, but unlike the fluorocarbon matrix the sulfonic acid groups do participate in chemical reactions. The presence of the sulfonic acid adds three important properties to Nafion:

1. Nafion functions as an acid catalyst due to the strongly acid properties of the sulfonic acid group.
2. Nafion functions as an ion exchange resin when exposed to solutions.
3. Nafion very readily absorbs water, from the vapor phase or from the liquid phase. Each sulfonic acid group will absorb up to 13 molecules of water. The sulfonic acid groups form ionic channels through the bulk hydrophobic polymer, and water is very readily transported through these channels. Nafion functions like a very selective, semi-permeable membrane to water vapor.

The physical properties of Nafion are similar to those of other fluoropolymers. It is translucent plastic. When used as an ion exchange membrane, it is specified by its manufacturer DuPont to operate at temperatures up to 190°C. When used with gases as a dryer, it is specified by Perma Pure to operate at temperature up to 150°C. The burst pressure of  Nafion tubing is generally greater than 200 psig (over 13 bar) but it varies with the diameter and wall thickness. An unusual property of Nafion is its propensity to change in physical size. As Nafion absorbs water, it swells by up to 22%. When exposed to alcohols it increases in size by up to 88%.

How stable is Nafion?

Nafion is made from Tetrafluoroethylene (Teflon),  the most chemical-resistant polymer available. Nafion has a strong acid group which reacts with some materials. These products are nominally trace organic materials present in air which are the result in incomplete combustion or chemical leakage of active products. The appearance of a slight color on the tubing from these organics does not indicate it has lost its drying properties. The dryers should be stored in polyethylene bags to prevent the color change.

Nafion is very stable and will not undergo chemical changes unless exposed to water solutions containing salts. The product is thermally stable up to a temperature of 160°C (329°F). Nafion will not react with any of the usual gases and vapors present in monitoring applications.

What are the current uses for Nafion?

Nafion is used primarily as the ion exchange membrane in chlor-alkali production, where salt solutions are separated electrolytically into chlorine and sodium hydroxide. In this environment, Nafion must tolerate elevated temperatures, high electrical currents, and extremely corrosive compounds. Nafion was developed specifically for this application.

Perma Pure offers a wide variety of gas dryers and humidifiers based on the selective water transport properties of Nafion. Nafion is also used as the active membrane in fuel cells. Its acid properties are exploited to drive acid-catalyzed reactions. Its ion exchange properties are used in numerous applications in scientific instrumentation. Its conductive properties make it suitable for use as a coating for the tip of pacemaker electrodes, where it resists overgrowth of surrounding tissues while remaining conductive.

I can't find a Perma Pure product that precisely meets my needs. Do I have any options?

Over 60% of Perma Pure's business is done with OEMs (Original equipment manufacturers) who require custom products to meet their design specifications. All tubing production, molding of fittings and assembly is done in house. Typical modifications include custom dryer lengths, proprietary fittings, and custom molded housings.

How is Perma Pure involved in Nafion production?

Perma Pure is the sole manufacturer of Nafion tubing under exclusive license from DuPont. Perma Pure purchases Nafion resin from DuPont, extrudes tubing, and performs a complex chemical treatment to activate it. Perma Pure exploits the water transport properties of Nafion to produce dryers and humidifiers in a wide range of sizes, from very small research models to large, process versions. Perma Pure also supplies Nafion tubing for applications other than drying.

How are Nafion dryers different from traditional gas dryers?

Drying is usually accomplished with one of four devices: condensers, desiccant dryers, permeation dryers, or Nafion dryers.

Condensers function by cooling a gas stream until water and other liquids coalesce, then collecting the condensate and draining it away. Condensers are simple to operate. Unfortunately, they are very non-specific; not only do they remove whatever gases condense at lower temperature, but also at least a portion of whatever gases dissolve in the condensate. Condenser systems are designed to minimize the contact of the gas stream with the condensate to limit this deficiency, but water-soluble gases are always lost to varying degrees depending upon the solubility of the gas in question. Large amounts of gases such as sulfur dioxide are lost by condensers, and condensers are entirely inappropriate for dry gas streams containing hydrogen chloride or chlorine (unless its removal is desired).

Desiccant dryers function by binding water to an absorbent. The absorbent may be a solid (such as silica gel) or a liquid (such as sulfuric acid) that binds water to its chemical structure as water-of-hydration. Desiccants are very simple to operate. Unfortunately, like condensers, they are very non-specific, and remove many compounds other than water. Unlike with condensers, water cannot be removed from desiccants by simply draining it away. While in operation, desiccants become progressively loaded with water, and must periodically be regenerated by replacement of the desiccant or by driving off the water. Continuous operation desiccant dryers use either a drastic change in surrounding pressure (pressure-swing heatless desiccant dryers) or a drastic change in surrounding temperature (temperature-swing desiccant dryers) to remove water from one chamber of desiccant while a second chamber is used, and the chambers alternate operation and regeneration.

Permeation dryers function on a principle of selection on the basis of molecular size. Permeation dryers are a micro porous material. When forced under pressure across the surface of the micro porous material, large molecules tend to remain in the gas stream while small molecules tend to move through the micro porous material and are removed. Permeation dryers are very simple to operate but are primarily suitable as air dryers. Nitrogen and oxygen are larger molecules than water,  so air can be dried by this method. Permeation dryers are too non-specific to dry complex gas sample streams.

Nafion dryers function on a principle of selection on the basis of affinity for the sulfonic acid group. Although water passage through Nafion is described as permeation, Nafion dryers do not operate on the same principles as permeation dryers. Nafion is not a micro porous material, separating compounds on the basis of their molecular size. For example, Nafion dryers can remove water from a hydrogen stream, even though the hydrogen molecule is much smaller than water. Pressure is not required to drive the process; the driving force for the reaction is the partial pressure of water vapor. Unlike competing methods, Nafion dryers are highly selective in the compounds they remove.

What is the chemical process whereby Nafion tubing dries or humidifies a gas stream?

Nafion dryers contain one or more strands of Nafion tubing. Most of the Nafion tubing wall is inert fluorocarbon polymer, and does not participate in the process. Since sulfonic acid is ionic in character and the bulk material is not, the sulfonic acid group within the Nafion tend to clump together. The activation process for Nafion reorients its sulfonic acid groups together into ionic channels extending from one side of the tubing wall to the opposite side.

When water strikes an exposed sulfonic acid group on the surface of the tubing, the water is initially bound by the surface group. Additional sulfonic acid groups deeper in the wall have less water attached to them, and consequently a higher affinity for water. Water molecules absorbed onto the surface of the tubing are therefore quickly passed on to  underlying sulfonic acid groups, until the water reaches the opposite side. The water molecule then perevaporates into the surrounding medium. This process continues until the water vapor pressure gradient across the tubing wall is eliminated. If a very low water vapor pressure is maintained outside the tubing wall, water will stream across the tubing wall very quickly.

This is a First Order Kinetic reaction, and it proceeds very rapidly. Water is removed from a gas stream directly from the vapor phase, and is released into the surrounding environment directly to the vapor phase. There is no net phase change, and energy is thus not consumed by the process.

What compounds other than water are removed by Nafion? By what mechanism?

When used in contact with solutions (in the liquid phase) Nafion in the form used in Perma Pure dryers functions as an cationic exchange resin, passing not only water but also positively charged ions (cations) from the solution, while resisting the passage of negatively charged ions (anions).

When used in contact with the gas phase, Nafion is much more selective. Ionic compounds do not dissociate into positive and negative ions in the gas phase at the operating temperatures of the dryers, so free negative ions are not available to migrate across the Nafion membrane (tubing wall).

Migration occurs as the result of a different mechanism. Compounds that present an exposed hydroxyl group (-OH) are essentially the only compounds known to migrate through Nafion in the gas phase. This is apparently due to hydrogen bonding with the sulfonic acid groups that are surrounded by the fluorocarbon matrix within Nafion. Most hydroxides are high-boiling solids (sodium hydroxide, calcium hydroxide, etc.) that are not present as gases within the operating temperature range of the dryers. Only three compounds or classes of compounds are normally removed directly by Nafion dryers:

1. Water (H-OH)
2. Ammonia (when water is present, NH3 reacts to form NH3-OH)
3. Alcohols (R-OH, where R is any organic group)

In addition to these three, certain other organic compounds may also be removed if they can be converted into alcohols. Aldehydes (R-H-C=O) and ketones (R1-R2-C=O) can both undergo a process called enolization (conversion to alcohol or "enol"). The carbonyl group within aldehydes and ketones can be acid catalyzed to react with water to form an alcohol in the following reversible reaction: C=O + H2O <-> HO-C-OH.

Nafion is strongly acidic due to the presence of the sulfonic acid groups.

How is it possible that Nafion tubing can function either as a dryer or as a humidifier?

Nafion functions essentially as a highly selective, semi-permeable membrane to water vapor. If gases inside Nafion tubing are wetter than gases surrounding the tubing, drying will occur. If the surrounding gases are wetter, humidification will occur.

In the simplest case, a strand of Nafion is suspended in ambient air. If the sample stream inside is much wetter than ambient air (such as breath samples), the sample falls to ambient humidity. If the sample stream inside is much drier than ambient air (such as calibration cylinder gases), the sample rises to ambient humidity.

To dry the sample to lower humidity, the surrounding air must be dried. For simple, portable applications, the Nafion tubing is packed in desiccant. The desiccant provides a very dry surrounding purge environment, while the Nafion tubing provides selectivity in drying. The desiccant gradually saturates with water and periodically must be regenerated or replaced.

For continuous operation, one or more strands of Nafion tubing are suspended within a housing that is purged with a dry gas. For humidification, the housing is filled with water to create a highly humid purge environment.

What are the effects of pressure on Nafion dryers and humidifiers?

Aside from the purely physical effects of pressure on Nafion tubing, total pressure has essentially no effect on Nafion dryers.

Nafion tubing is relatively tough but quite flexible. The tubing has a relatively high burst point when subjected to a positive pressure. Positive pressure inside the tubing causes it to swell slightly, exposing the maximum surface area and slightly improving performance.

Because the tubing is flexible, negative pressure inside the tubing can cause it to collapse like a soda straw. This collapse will prevent sample flow and cause dryer failure. Negative sample pressure should be limited to 5 inches of water or less if the dryer is heated. Greater negative pressures will constrict the tubing, reducing active surface area and thus reducing performance, or will totally collapse the tubing.

Although total pressure has only physical effects on Nafion performance, the fundamental driving force of the process is the water vapor pressure gradient. Functioning essentially as a semi-permeable membrane to water vapor, Nafion equilibrates the water vapor pressure across the tubing wall. Since doubling the pressure of a sample doubles the partial pressure of the water vapor component of that sample, increased pressure on the sample side of the tubing wall or decreased pressure on the purge side of the sample wall can be used to stimulate the process.

What are the effects of temperature on Nafion dryers and humidifiers?

The effects of temperature on Nafion function are much more complex than the effects of pressure. There are two major effects.

The Primary Effect is a purely kinetic one. Water absorption and transport by Nafion is a First Order Kinetic reaction. As such the rate of reaction is a logarithmic function of temperature.

Within the normal operating temperature range for Nafion dryers/humidifiers, the rate of water absorption roughly doubles for every 10°C rise in operating temperature. Thus, at higher temperatures, the water vapor pressure inside the tubing comes to equilibrium with the outside water vapor pressure faster which means that gases dry or humidify faster.

The Secondary Effect of temperature on Nafion function relates to the final equilibrium point. For drying or humidification to occur, there must be a water vapor pressure gradient across the tubing wall. Drying/humidification stops when there is no longer a gradient; at this point equilibrium has been reached. It might seem that if the water vapor pressure outside the tubing were zero, the water vapor pressure of the sample inside the tubing would eventually fall to zero also. This is unfortunately not the case.

The wall of the tubing always retains some residual water because the sulfonic acid groups within the Nafion polymer never give up all of their water. This residual water is temperature dependent. At higher temperatures more water is retained within the wall and cannot be removed. This water concentration within the wall corresponds to some water vapor pressure level outside the wall. When the water vapor pressure of the sample falls to a level matching the residual water level within the wall, there is no longer a gradient, equilibrium is reached, and drying stops. This residual water level within the tubing wall determines the lowest water level (dew point) of the sample achievable by a dryer.

At room temperature (20°C) the residual water in the tubing wall corresponds to a final achievable dew point of about -40°C (about 75 ppm of water). For every one degree C rise in operating temperature above room temperature, the final equilibrium dew point also rises about one degree (C).

The combination of these two effects means that at higher operating temperatures, Nafion dryers initially remove water faster but stop drying (come to equilibrium) at a higher final dew point. For best performance, a temperature gradient should exist down the length of the dryer. The sample inlet end should be hot to keep water in the vapor phase and to initially remove water very quickly, removing the bulk of the water in a wet sample. As the sample passes down the length of the dryer, the temperature should be reduced, because the sample contains less water and its dew point is lower so the sample temperature will still remain above its dew point. the sample outlet end should be cool, at  room temperature (or lower, if a cooling system is employed) so that the final equilibrium dew point when the sample exits the dryer is as low as possible.

What are the limitations and the most common causes of dryer failure?

Nafion is extremely corrosion resistant. No compounds that exist in the vapor phase within the operational temperature range of Nafion are known to attack it. Even hydrofluoric acid or other concentrated acids are tolerated by Nafion. The corrosion limitations of Nafion dryers and humidifiers are due to the materials used for housings and gas connections. Perma Pure offers product configurations that will tolerate almost any sample matrix.

Pressure limitations of the dryers and humidifiers are also due to the housings and gas connections. Versions are available that will tolerate up to 150 psig (10 bar), depending upon design.

Although Nafion will tolerate temperatures as high as 190°C, a maximum operating temperature of 150°C for Nafion dryers and humidifiers is recommended. Nafion is a strong acid catalyst and, as operating temperatures rise above 110°C, unwanted chemical reactions may be stimulated within the sample gas matrix. For this reason, most dryer installations operate at 100°C or less.

There is no initial water content limitation imposed by Nafion dryers. The final performance of the dryer depends upon the initial water content, the sample flow rate, and the operating temperature. When sizing a dryer always use the wet gas flow rate, not the flow rate required by the analyzer.

As mentioned previously, ammonia, alcohols, and some other organic compounds that can be converted to alcohols are removed by Nafion dryers. Other gases can be dried without loss of the gas of interest.

To function effectively, both the external and internal surfaces of Nafion tubing must be clear of obstruction. Films of oil or other deposits will degrade dryer or humidifier performance. Over time, deposits will accumulate if the purge air is contaminated with oil, if the sample is inadequately filtered, or if unforeseen chemical reactions occur within the sample that deposit residues within the dryer. Generally, these processes will cause a gradual decline in performance over a period of many months or years and may be reversed by periodic cleaning.

There are two common causes of unexpected Nafion dryer failure, collapse of the Nafion tubing and the introduction of liquid water into the dryer.

1. Collapse of the tubing is caused by negative pressure inside the tubing, commonly caused by pulling the sample stream through the dryer with a pump while pushing the purge gas through the purge housing. To avoid  this problem, the pump for the purge gas is placed after the dryer in the sample stream. Also see previous FAQ, "What are the effects of pressure on Nafion dryers and humidifiers".
2. Introduction of liquid water into the dryer causes failure by an unexpected mechanism. Ordinarily Nafion dryers remove water vapor from the sample and perevaporate it into the surrounding medium. There is no net phase change, and no energy is consumed. If liquid water enters the dryer, it is still absorbed then perevaporated as water vapor. Since energy is thereby consumed, the dryer begins to cool. As it cools, it condenses more water, causing more cooling. There is a cascade failure in which the dryer becomes progressively colder and wetter until it is completely soaked and nonfunctional.

In most instances, when the dryer becomes physically wet, the process can be reversed by simply discontinuing sample flow and permitting the purge gas to dry out the device. The dryer then recovers its normal performance. Unfortunately, in some instances the sample may contain ionic compounds in the gas phase. If present, these ionic compounds will dissolve in the liquid water accumulating within the dryer. Once present in solution, the ions can participate in ion exchange with the Nafion tubing, converting the tubing to another form that is much less water absorptive. Should this occur, it will be necessary to regenerate the Nafion tubing by treatment with acid before it fully recovers its normal performance.

If reasonable care is exercised to keep the sample and dryer sufficiently hot to prevent liquid water from entering the dryer, and if excessive negative sample pressure is avoided, the dryer will function indefinitely.

Medical FAQs

What is an ME dryer?

An ME dryer is a short length of Nafion tubing with a protective outer braiding.

How does the ME dryer function?

As water vapor in breath travels through an ME Dryer, it is absorbed into the tubing and evaporates into the surrounding air. Conversely, this product will also function as a humidifier and add ambient moisture to dry cylinder gases.

The amount of water vapor removed by or added to the tubing will depend on the relative humidity outside the tubing. Also see the previous FAQ, "How is it possible that Nafion tubing can function either as a dryer or humidifier".

What are the benefits of using an ME dryer?

ME dryers prevent downtime of breath gas analyzers since the most common source of analyzer problems is condensable water vapor in the sample. Accuracy and precision of the analyzers also improve when water interferences are eliminated.

Where should the ME dryer be placed in the sample line?

When the dryer is located near the patient, the breath sample is still warm and will not have a chance to condense in the sample line. This placement is beneficial to the ME's performance, as the water vapor transfer rate through the tubing increases at higher temperatures.

Should the ME Dryer be replaced after every procedure?

Since it is part of the sampling line, it should be treated as such. Some customers treat these dryers as disposable for hygiene reasons. The dryers are very durable and can theoretically be used for months without replacement.  Each hospital, however, has its own guidelines and we defer to individual institutional policies.

What are the advantages of an ME Dryer compared to a water trap or HME?

Simplicity - A water trap collects liquids that must be drained periodically. Since the ME dryer is removing water vapor, no condensate is formed, and the potential for liquid carryover is eliminated. Further there is absolutely no operator interaction or maintenance required.

Performance - The breath sample at the outlet of a water trap is still saturated (100% RH), whereas the ME will reduce the sample humidity to room levels (generally 30-50%RH).

Analytical Accuracy - Using a water trap for moisture removal in the presence of water soluble compounds such as NO2 causes sample integrity problems, since they can dissolve out in the condensate. An ME Dryer will selectively remove water vapor without affecting the gases being measured.

Is a water trap or HME still needed?

No water trap is required to remove water. A filter to remove sputum is still desirable.

Can an ME dryer be sterilized?

The preferred method of sterilization is gamma radiation. Our recommendation is to utilize the dryer in the unsterilized condition and change it after each operation. If sterilization is desired, gamma radiation is the preferred method.

What is the size of an ME dryer?

The length of Nafion tubing required is determined by the sample flow rate and ambient humidity to which it is exposed.  As a general rule of thumb, a 6" dryer can be used for a sample flow rate of 75 cc/min and a 12" dryer can be used for flows of 150 cc/min or less. Once the sample RH reaches ambient levels, moisture exchange ceases.

How cost effective is the ME Dryer?

Significant cost savings can be realized by the elimination of repairs and downtime of equipment caused by wet samples. No one can place a dollar value on preventing a gas monitor failure at a critical juncture during a medical procedure.

What is the proper disposal method for a used dryer?

The dryer can be disposed of using the same procedures as for any vinyl plastic components used in the hospital;  land fill or special incineration. The product is very stable and will not decompose.