In a breathing mixture containing 50% helium and 50% oxygen with isoflurane present, how does helium affect the mass spectrometer reading of isoflurane concentration?

In a breathing mixture containing 50% helium and 50% oxygen with isoflurane present, how does helium affect the mass spectrometer reading of isoflurane concentration?

• A. The reading will be slightly increased falsely
• B. The reading will be approximately double the actual isoflurane concentration
• C. The reading will be the correct value
• D. The reading will be half the actual concentration

Answer: (B)

Mass spectrometry readings for anesthetics are influenced by the gas matrix in which the analyte is carried. The instrument is usually calibrated with a typical carrier gas (like air/nitrogen). When the breathing mix contains a large amount of helium, the gas environment changes the ionization and transmission of isoflurane signals, effectively boosting the detected signal for a given actual concentration. In this helium-rich mix, the response of the mass spectrometer to isoflurane tends to increase, so the reported concentration appears higher than reality. With equal parts helium and oxygen, this effect is about a factor of two, so the reading is approximately double the true concentration.

Mass spectrometry readings for anesthetics are influenced by the gas matrix in which the analyte is carried. The instrument is usually calibrated with a typical carrier gas (like air/nitrogen). When the breathing mix contains a large amount of helium, the gas environment changes the ionization and transmission of isoflurane signals, effectively boosting the detected signal for a given actual concentration. In this helium-rich mix, the response of the mass spectrometer to isoflurane tends to increase, so the reported concentration appears higher than reality. With equal parts helium and oxygen, this effect is about a factor of two, so the reading is approximately double the true concentration.