Im sorry for late reply!
It looks like I haven't bookmarked this page I refereed to.
Oxygen mask they used didnt provide sealing good enough to stop air from getting in. This was the main reason It took that long.
Why not N? I dont remember if it was mentioned. Maybe medical issues against N? My memory is like sieve these days.
Update.
https://www.researchgate.net/public...h_helium_at_a_Swiss_right-to-die_organisation
Case 4 (Female)
The member exhaled prior to placing the mask in the working position and after 30 seconds she
appeared conscious. At 33 seconds she nodded "yes" to an attendant's query whether she was
breathing. Immediately afterwards the member's eyelids blinked rapidly. It is estimated that
consciousness was lost 55 seconds after the mask was put in place. At 1:11 her eyeballs rolled,
and there were tremors in both hands. The tremors continued to 2:06 and then the body appeared
relaxed. At 2:09 the breathing rate quickened for about 6 seconds. At 3:03 there was a slow
extension and contraction of both arms, which then relaxed at the member's sides at 3:26.
At 3:58 breathing began to accelerate, pausing occasionally, and then accelerating again. From
5:36 to 10:12 there was intermittent moaning. During this same period the eyelids were open
and the eyeballs were moving, but without appearance of control. Between 10:13 to 38:16,
intermittent patterns of accelerated breathing, relaxed breathing, and moaning continued. During
this period a number of movements occurred: at 26:03 the head tilted back; at 30:41 the
shoulders shrugged and left arm contracted; at 34:55 the left shoulder shrugged; at 37:06 both
arms contracted for 10 seconds after which the member appeared quite inert.
At 38:16 the camera was turned off, to replace the video tape. The time elapsed for this is not
known. The duration of the second tape is 26:57. At 0:49 of part 2 the member let out a deep
gasp and the head tilted back to 0:57. At 1:31 the tongue extended slightly and withdrew. This
tongue movement continued at 15 – 20 second intervals until 3:45, after which no further signs
of life were apparent. The camera continued to run from 3:45 to 26:57, but the member appeared
dead.
The recorded time from the start of the procedure to cessation of all signs of life was
approximate 42 minutes. The actual time from start to finish is not known due to the change of
video tape. The changes in breathing patterns, moaning, and longer dying time appeared to
concern and confuse the Dignitas attendants.