High-purity gas is essential for the best GC results. Many point-of-use gas purifiers are available on the market to clean lower grades of gas to carrier grade gas. The point-of-use purifiers come in all kinds of sizes, prices and (claimed) capacities. Are you getting your money’s worth with the purifier you’re using?
We’ll focus on oxygen-removal capacity in this blog, because oxygen is usually the most damaging common contaminant for stationary phases. So a purifier’s capacity for oxygen is a good place to start when judging its value (cc’s oxygen removed per dollar could even be a way to produce a numerical comparison). Since we make gas purifiers at CRS, we test our own to make sure they meet product claims, and we occasionally review competitor’s purifiers. Amazingly, we’ve often found that actual capacities of name-brand purifiers can come nowhere close to their claimed capacity.
Let’s take a look.
How to determine O2-removal capacity
It’s easy to measure a gas purifier’s capacity for oxygen when it has a built-in indicator. Simply flow a gas standard through it with a known concentration of oxygen, then wait for the indicator to change color. It’s more expensive to test purifiers that don’t have indicators, but still easy with appropriate equipment. We use a Delta-F Nanotrace oxygen analyzer downstream of the purifier to measure efficiency of removal to sub-ppb levels, and simultaneously capacity.
The output of the oxygen analyzer with an ultra-high-capacity filter typically looks something like this (at least in an ideal world):
This example is for a filter that scrubbed 4 liters of oxygen from the gas stream at a low flow rate before the oxygen content of the effluent gas rose above 1 ppb.
We’ll show results of tests we’ve run on three competitor’s purifiers, described as “Test Filters” 1, 2 and 3, and two purifiers made here at CRS, the AFS I and the ZPure O2. The photos are approximately to scale; two of the purifiers are really big.
Test Filter 1 has an interesting history for oxygen claims. The manufacturer used to claim 500 cc adsorption for oxygen, along with removal of water and hydrocarbons. More recently the O2 claim is published as 75 cc, which is getting closer to the actual capacity…
The manufacturer of Test Filter 2 doesn’t say how much O2 it will remove, but does say that it has “Two oxygen scavenging materials for both high capacity and high efficiency O2 removal”. It should maybe last for at least 1 cylinder of helium (300 ft3 or 8400 L) with 1 ppm O2 contamination. That equals 8.4 cc O2 capacity. Sounds easy, right?
Test Filter 3 is huge. It’s about 21” long and 2 3/8” wide (about 53 cm x 6 cm in more recognizable units). That should have plenty of room to pack in oxygen adsorbent and in fact the manufacturer claims 2 L capacity. Well, almost...
And finally, here at CRS we claim for the AFS I and ZPure O2, 850 cc and 2.5 L capacity, respectively. Do we walk the talk?
Standard gas flows for these tests are less than 1 L/minute except for Test Filter 3 at 2 L/minute, because it’s so big. Some filters were tested more than once because we couldn’t believe our initial results.
Without further ado, here they are:
Except for the 2nd example, Test Filter 2, the output purity was generally good while the filters were functioning. However, the capacity claims could not be substantiated for Test Filters 1, 2 or 3.
Check it Yourself
We realize most labs operate on a budget and you might not have extra cash to burn through a test filter. But in case you want to try...
You can test your oxygen filters quickly and easily if they include a visual indicator. Purchase a calibrated gas mixture of 0.1% O2 in N2 and expose the filter to a measured flow of the standard. Keep the flow fairly low, below 5 purifier-body-volumes per minute, to avoid overwhelming the filter.
You can figure out in advance about how long it will take for the oxygen to break through the adsorbent and oxidize the indicator based on the capacity spec. Just make sure you have your eye on it when it breaks through in case it should happen early. The total capacity of your filter is Time x Flow rate x Concentration plus a little bit for the capacity of the indicator and any adsorbent after the indicator.