How much minimum ppm value can be measured by a TCD type GC?

Generally TCD is used to measure Percentage levels, however the MDL can vary between manufactures. The PPM measurement through TCD is acceptable since TCD is a very Linear Detector and the key performer in TCD PPM detection is the Sample Load. The minimum a good TCD Can measure, which i have witnessed is 10~15 ppm.

TCD is used to measure the Percentage level, However the minimum level of accuracy depends to its noise ratio

TCD is can be used to measure upto 1-10 ppm for gases such as Argon or Oxygen, CH4, Carbon monoxide, Hydrogen, Nitrogen etc.

FID is used for measuring Hydrocarbons and usually can have range from 1-10 ppm as well. 

TCD are used to measure Higher concentrations of hydro carbons in %. FPD and FID are used in PPM level depends upon the process.