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Need help with this question please!

The thermal conductivities of some gases are given in the table below. Which gas would be most suitable for use as a carrier gas for a GC with a TCD if CO2 and O2 are to be analysed?why?

Thermal Conductivities of common gases

Gas                               1J/(Kxmxs)
H2                                 0.1700
He                                 0.1410
N2                                 0.0243
O2                                 0.0246
Ar                                  0.0162
CO2                               0.0144
CL2                                0.0076
C2H4                              0.0170         
C3H8                              0.0151   



Also:
What are the three highest intensity peaks that are likely to be seen in the mass spectrum of acetone (CH3COCH3). I dentify the ions associated with these peaks.


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