Custom Sensor Solutions, Inc

What Is A "Zero Filter"?

Question: I have often run across the term "zero filter". What does it mean? Would it be useful in my application? Can I make them?

A zero filter is a way of removing reactive gases from a sample or stream of air. A typical zero filter might consist of a tube filled with activated carbon, which will absorb most organic compounds and many inorganic gases from a stream of air. The two main uses of the zero filter are (a) routine zeroing of gas sensors; (b) real-time baseline corrections of sensor outputs; and (b) a cheap and ready supply of dilution air for dynamic gas diluters, such as our Model 1010 Precision Gas Diluter.

In most types of sensor, the most important limitation on sensitivity is the gradual drift in its baseline, which is the sensor's output when no sample gas is present. One way of improving the effective sensitivity of a sensor is to periodically measure its baseline output. This baseline can be subtracted from the routine measurements to yield a net signal which more faithfully represents the sample.

In an instrument meant to be used away from the laboratory, there are very few practical ways to measure the baseline. Zero filters are one solution. They create a stream of clean air which is then passed to the sensor. In an instrument containing a sample pump, the zero filter is placed on the inlet, and the air is cleaned as it is drawn in. Zero filters are less useful with sensors operating in diffusion mode; they create a diffusion barrier which can drastically delay the response of the sensor to changes in sample concentration.

A 1995 report from our laboratory [1] dramatically illustrates the increase in sensitivity that can be achieved by the use of a zero filter. The TSI, Inc., nitrogen dioxide sensor will respond to rapid changes in ozone concentration as little as 1 PPB (part per billion). Unfortunately, the sensor baseline drifts over a range as great as 200 PPB over a 24-hour period. In most applications, this would place a practical limit on its sensitivity of 400 - 1000 PPB -- it would usually be impossible to distinguish changes in sample concentration from random baseline drift. Our study showed that by measuring the sensor baseline every 30 minutes, the full potential of the sensor could be achieved.

Zero filters for pumped systems are easy to construct. Any chemically-inert tube can be packed with a sorbent which is capable of absorbing gases that may react on the sensor. The sorbent should be in pellets 3 to 6 mm in size; loose glass wool plugs can be used to prevent the pellets from blocking the outlet of the tube. A standard 12 x 150 mm drying tube filled with 3 mm sorbent pellets will offer almost no resistance to airflow. This is important in zero filters used with dynamic dilution systems. The more uniform the size of the pellets, the less flow resistance there will be.

Our favorite sorbents are activated carbon, Purafil IITM, and platinum/alumina. Not all of these are needed for every application. In most cases, carbon alone is sufficient. To remove reducing gases such as hydrogen sulfide, ethylene, and other alkenes, Purafil alone may suffice.

Activated Carbon removes larger hydrocarbons and many chemically active gases and vapors. It will not remove carbon monoxide, methane, ethane, or propane. Some gases, such as arsine, will be absorbed but may desorb later under different conditions. It comes in a wide variety of chemical and physical forms. 3 mm or 6 mm pellets can be purchased from most chemical companies, such as Aldrich. It should be activated in a vacuum oven at 110 øC for 2 hours and stored in a sealed container. (Note: Acid-washed charcoal often emits traces of hydrogen chloride!)

Purafil (tm) is made of potassium permanganate deposited on alumina pellets. It destroys many reactive gases, including arsine and hydrogen sulfide, but not carbon monoxide. It also destroys unsaturated hydrocarbons such as ethylene. Purafil is self-indicating; it is violet when supplied, and turns brown or black as it is consumed. It needs no preparation before use, except to remove the violet dust that forms during shipping, by using a sieve. Purafil is toxic, and must be handled and disposed of properly. It is not available from chemical supply houses. Call Purafil, Inc., 800-222-6367, for the names of local dealers.

Platinum (1%) on 3 mm alumina pellets will destroy low levels of carbon monoxide. Since it is so expensive, it is best placed in line after other sorbents such as PurafilTM or charcoal. Unless poisoned, it is not consumed in use, and can often be removed and used in other filters. One linear inch of pellets in the filter is generally enough. We use Aldrich P/N 23,211-4. Heat at 200 øC before re-use.

Zero filters, once made, can be used for quite a long time, as long as six months. The exact lifetime depends on the nature of the application. Also, the composition of the filter will depend on the type of gas being sampled. For alkanes, aromatics, and very dirty samples, charcoal is best. Purafil is better with reactive gases like alkenes and oxidizable inorganic gases such as hydrogen sulfide.

Reference

[1] W.R. Penrose, L. Pan, J.R. Stetter, and W.M. Ollison, 1995. Sensitive measurement of ozone using amperometric gas sensors. Anal. Chim. Acta 313, 209-219.

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