Here some more serious information about the sampling.
Read the full text here
Atmospheric sampling missions flown by the Air Force's specialized WC-135 Constant Phoenix, the last Cold War era "sniffer" plane still in service,
were begun by the United States shortly after the test. On 13 October it was reported that traces of radioactivity had been detected and on 16 October
National Intelligence Director John Negroponte's office released a statement confirming that samples collected on 11 October showed that the test was
indeed a nuclear blast, laying to rest some initial speculation that the low yield explosion might in fact be simply a very large conventional
Subsequent analysis of samples has shown that the fissile material used in the test was plutonium. Since different fissile materials produce different
proportions of various radionuclides, measuring these ratios (such as the ratios of Xe-133, Xe-133m and Xe-135) can unambiguously determine the
fissile material used. Leaks of additional isotopes which can occur would make the determination even easier.
Garwin and von Hippel report that if radioxenon leaked into the atmosphere at a rate of only 0.1 percent a day, a concentration of 10,000 atoms per
cubic meter of air would be detectable downwind three days after the test, one hundred times the detectability threshold. If the isotope ratio of
about 8000 radioxenon atoms could be measured, then the identity of the fissile parent could be established as being plutonium with 95 percent
Fizzle.. or Low Yield ?
The North Koreans have high grade plutonium (content of neutron emitting Pu-240 measured at 2.44% by the IAEA in the July 1992, compared to 6% for
U.S. weapons plutonium), so problems with predetonation are almost certainly not the cause.([May 2001])
The low yield, almost certainly less than a quarter of its reported planned yield, indicates a partial failure of the device. The most likely cause is
poor implosion performance (that is, poor compression), though late initiation is also a possibility.
Regarding the possibility of poor compression, it should be observed that they are likely trying to develop a relatively sophisticated light system
suitable for missiles, in the range of 500-1000 kg, not the 3500 kg design of the WWII Fat Man, which proved very reliable. Failure might be due to
problems perfecting the design, or simply some test-related technical fault in an otherwise sound design.
The relatively low yield announced prior to the test was possibly to conserve plutonium of which North Korea has a fairly limited supply.
[edit on 26-10-2006 by Canopus]