  We present an analysis of the dust continuum emission at 870 um in order to
investigate the mass distribution of clumps within infrared dark clouds
(IRDCs). We map six IRDCs with the Large APEX BOlometer CAmera (LABOCA) at
APEX, reaching an rms noise level of 28-44 mJy/beam. The dust continuum
emission coming from these IRDCs was decomposed by using two automated
algorithms, Gaussclumps and Clumpfind. Moreover, we carried out single-pointing
observations of the N_2H^+ (3-2) line toward selected positions to obtain
kinematic information. The mapped IRDCs are located in the range of kinematic
distances of 2.7-3.2 kpc. We identify 510 and 352 sources with Gaussclumps and
Clumpfind, respectively, and estimate masses and other physical properties
assuming a uniform dust temperature. The mass ranges are 6-2692 Msun
(Gaussclumps) and 7-4254 Msun (Clumpfind) and the ranges in effective radius
are around 0.10-0.74 pc (Gaussclumps) and 0.16-0.99 pc (Clumpfind). The mass
distribution, independent of the decomposition method used, is fitted by a
power law, dN/dM propto M^alpha, with an index of -1.60 +/- 0.06, consistent
with the CO mass distribution and other high-mass star-forming regions.
