I am surprised we have not seen this argument applied also to Auschwitz. If you go to the trouble of building airtight chambers in which to kill people, why would you bother with the complication, danger and expense of using a slow-release cyanide compound, especially an unsuitable one optimised for a different use. You could simply wait for the people to suffocate.
Of course, this procedure is slow, but it has been shown that the Zyklon-B process would also have been slow, because the cyanide gas is released slowly, and must be fully exhausted and evacuated before the disposal teams could enter. Zyklon-B is also dangerous to the personnel inserting it, and to the surrounding area when vented. Suffocation does not have this particular disadvantage.
As well as being slow, suffocation is also painful and distressing, and while this need not have concerned the executioners in itself, the prolonged screaming might disturb their card games. There is however a method which is simple, cheap, quick, humane, safe to all except the victims and leaves no forensic traces in the bodies: asphyxiation with nitrogen. The air is almost 80% nitrogen, so industrial nitrogen is readily and cheaply available in cylinders, or a nitrogen generator can create it on demand. It can safely be vented to the surrounding air, and provided sufficient fresh air has entered the chamber, disposal staff can safely enter at an early stage.
Wikipedia has this to say on nitrogen asphyxiation:
Nitrogen asphyxiation is an occasional cause of accidental death and a theoretical method of capital punishment advocated in a National Review article, "Killing with kindness – capital punishment by nitrogen asphyxiation" (Creque 1995). The painful experience of suffocation is not caused by lack of oxygen, but rather because of a buildup of carbon dioxide in the bloodstream which is exhaled under normal circumstances.
When a human breathes in pure nitrogen, they exhale carbon dioxide without resupplying oxygen. Nitrogen is a colorless, odorless and tasteless gas that comprises approximately 78 percent of the Earth's atmosphere. As such, the subject would detect no abnormal sensation. This leads to asphyxiation without the painful and traumatic feeling of suffocation. Because of this property, nitrogen in German is called "Stickstoff" ("suffocation matter").
You can read more here: http://en.wikipedia.org/wiki/Nitrogen_asphyxiation
Until I began researching this post I was unaware of the German for nitrogen, but this seems an additional reason why the Germans would not have been unaware of this option.
In the Panorama programme referred to in the Wikipedia article, John Portillo experiments with inhaling nitrogen from a mask. He is laughing, and enjoying the experience so much that the mask has to be taken away from him, rather than him removing it voluntarily.
I invite comments both on the validity of what I have written and on whether and how it might be exploited.