Both types of contamination tests presented here should be conducted when drilling for samples that will be used for microbiological studies. The PFT analysis is very rapid and can be used to screen samples for potential contamination before time and supplies are expended on samples. It is important to note that the absence of a tracer in the interior of a core has little meaning if the successful delivery of the tracer is not confirmed. Therefore, it is necessary to confirm the successful delivery of the tracer with each sample as outlined above.
We were able to detect as little as 6 × 10-12 g of PFT. Higher sensitivity may possibly be achieved by using a smaller bore column on the GC. This should increase resolution and improve the signal-to-noise ratio. The use of a less volatile PFT may also improve the reliability of the method as an indicator of microbial contamination.
Results from the PFT experiments during Leg 185 suggest that the intrusion of drilling fluid is on the order of nanoliters per gram of core material (Smith et al., in press). Bacterial abundance in surface seawater is typically in the range of 0.1-1.0 × 109/L. Our experience with different coring and formations suggests that the intrusion of drilling fluid may account for at most, 1-10 bacteria/g of core material. Microspheres were never detected in the interior of APC-cored unconsolidated sediments or RCB-cored consolidated sediment or igneous rock. Together, these results indicate that cores recovered on the JOIDES Resolution are suited for deep-biosphere research. Microspheres were seen in the interiors of thin sections prepared from igneous samples. This indicates that postrecovery processing can be a source of contamination.