Abstract
A technique is described for analysing small discrete samples of gas (about 100 μl) by injecting them into a stream of carrier gas being continuously sampled by a ‘respiratory’ mass spectrometer. This involves interrupting the normal respiratory monitoring use of the mass spectrometer for only 20s per sample. The theory for calculating the composition is given for the case when the carrier gas is totally different from the discrete sample, and for the case when it is air and the discrete sample may contain air. Allowance is made for difference of viscosity between sample and carrier and for different response times to different components of the sample. The method was developed for the analysis of gas bubbles equilibrated with blood. When tested on a mixture of 1·2% halothane and 5% CO2 in 50/50 N2O/O2, with air as the carrier, the standard deviation between repeat determinations was about 0·5% of the actual concentration of each component.
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Abbreviations
- F ic :
-
fractional concentration of ith component of sample in carrier
- F iD :
-
in discrete sample
- F iM :
-
in mixture of the two
- F ijc :
-
fractional concentration ofith component of sample, injth time element of peak, in carrier
- F ijD :
-
in discrete sample
- F ijM :
-
in mixture of the two
- F jc :
-
fractional concentration, at molecular leak injth time element of peak, of carrier
- F jD :
-
of discrete sample
- l :
-
number of time elements in peak
- m :
-
in capillary delay time
- n :
-
number of components of discrete sample
- V D :
-
volume (expressed at s.t.p) of discrete sample
- \(\dot V_C \) :
-
volume flowrate (expressed at s.t.p) of carrier in absence of sample
- \(\dot V_{jM} \) :
-
of mixture of sample and carrier injth time element of peak
- \(\dot V_M \) :
-
of mixture of sample and carrier assumed constant throughout peak
- w :
-
weighting function, see eqn. 7
- δF i :
-
F iM-F iC
- δt :
-
time element (interval between a.d.c. readings)
- δV D :
-
volume (expressed at s.t.p.) of discrete sample passing molecular leak in δt
- δV jD :
-
injth time element of peak
- ηC :
-
viscosity of carrier
- ηD :
-
viscosity of discrete sample
- ηjM :
-
viscosity of mixture of the two at molecular leak injth time element of peak
- \(\bar \eta _{jM} \) :
-
mean viscosity of mixture of the two along length of capillary injth time element of peak
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Mapleson, W.W., Chilcoat, R.T. & Willis, B.A. Analysis of small discrete samples of gas with a respiratory mass spectrometer. Med. Biol. Eng. Comput. 18, 701–708 (1980). https://doi.org/10.1007/BF02441894
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DOI: https://doi.org/10.1007/BF02441894