Evaluation of long sampling tubes for remote monitoring by mass spectrometry.

The long sampling tubes required for remote mass spectrometry alter the sampling system's performance characterized by sample flow, residence time, and 10 to 90% response time. We searched for an easy-to-handle tube with (1) a length of 30 m, (2) sample flow less than 50 ml.min-1, and (3) residence and response times approaching those predicted by our mathematical model. We tested tubes of various geometries and various commercially available materials by using them as inlet catheters for a quadrupole mass spectrometer (Centronic 200 MGA, Centronic Ltd, Craydon, UK). We measured their responses at 0 to 10% (on transients) and 10 to 0% (off transients) step changes in gas concentration for nitrogen, argon, nitrous oxide, oxygen, and carbon dioxide and 0 to 3% and 3 to 0% for halothane, enflurane, and isoflurane. With 5 polyethylene tubes, halothane response times were up to 38 times longer than predicted. One 30-m polyethylene tube combined a 158-ms response time for nitrogen and argon with a 2,205-ms response time for halothane. Teflon, polyvinyl chloride, and stainless steel also proved to be unsuitable because of unacceptable signal distortion: the carbon dioxide response time for a 30-m Teflon tube was 2,600 ms. A glass tube showed the least signal distortion but was hard to handle. Our requirements were fulfilled by a 29.77-m tube made from nylon with a 1.00-mm inside diameter to which a 0.23-m length of nylon with a 0.25-mm inside diameter was added at the patient end. It offers (1) sample flow equals 46 ml.min-1, (2) residence time equals 11.1 seconds, and (3) response times approaching our theoretical predictions, that is, 159, 164, 180, 159, 188, 302, 298, and 300 ms (means of on and off transients) for nitrogen, argon, nitrous oxide, oxygen, carbon dioxide, halothane, enflurane, and isoflurane, respectively. This tube allows the accurate monitoring of breathing frequencies up to 25 and 50 breaths/min for volatile agents and gases, respectively.