Figure 1.

Typical mass spectrometer tracing far CO₂ gas (mass 44) obtained from the combustion of NAP–leucine and NAP–isoleucine isolated from human vastus lateralis muscle protein hydrolysate. Six reference gas (CO₂) square-wave pulses are also shown. Assuming muscle tissue is 25% protein and 8% of this protein is leucine, the capillary GC/combustion IRMS instrument is capable of performing a ¹³CO₂/¹²CO₂ determination on 12–24 μg of NAP–leucine, which is equivalent to 1–2 μmol to CO₂.