Why was another agent, BZ, identified in some samples? On 14 April, Russian Foreign Minister Lavrov was reported in press reports in TASS and RT as suggesting that the OPCW designated laboratory in Switzerland (the Spiez Laboratory) identified BZ (a chemical weapons agent that is not in the Novichok family) in samples provided by the OPCW.
When a sample, such as one from Salisbury, is sent to an OPCW designated lab, it isn't sent on its own. There are "negative control samples" (made of a material similar to that in the authentic sample that has no detectable chemical weapons agent in it) and "positive control samples" (made of a material similar to that in the authentic sample that has been spiked to contain a chemical weapons agent or other relevant chemical in it). For an active investigation of alleged use the samples are numbered and not otherwise identified so that each laboratory cannot tell them apart. In simple terms, the negative control samples protect against incorrect results from contaminated lab equipment; the positive control samples raise confidence in the lab work to identify a specific agent.
The spike has to be of a high quality and purity, and to a known standard. As there are no standards written for the
Novichoks, another material would have to be used for the spike. A good candidate for this would be BZ. [Editorial note: when I suggested to someone familiar with the classified report that the spike might be BZ in this case I was not told I was wrong -- RG].
Thus, in an ideal world, each laboratory would report nothing detected in the negative control samples, report the spike chemical in the positive control samples and report the Novichok in the actual samples from Salisbury -- an overall detection of BZ and of the Novichok. Therefore, such a report from the laboratory does not constitute any suggestion that the Skripals were exposed to BZ.
[Update 13 July 2018] It later became known that the control sample contained a BZ precursor, not BZ itself. The OPCW Director General told the Executive Council, in his opening remarks to its meeting on 18 April: "The precursor of BZ that is referred to in the public statements, commonly known as 3Q, was contained in the control sample prepared by the OPCW Lab in accordance with the existing quality control procedures. Otherwise it has nothing to do with the samples collected by the OPCW Team in Salisbury. This chemical was reported back to the OPCW by the two designated labs and the findings are duly reflected in the report."