What two items are sent to the MSL with the samples themselves?

The correct choice involves sending a chain of custody and a letter of request along with the samples. The chain of custody is a critical document that ensures the integrity of the samples during their transportation and processing. It provides a record of who collected, handled, and transferred the samples, which is essential for maintaining their validity for analysis or any legal purposes.

The letter of request typically explains the context of the samples being sent, such as the objectives of the analysis, any specific tests required, and the sender's contact information. This helps the laboratory understand the priority and necessary actions relating to the samples.

In contrast, other options, such as invoices, field notes, or preliminary findings, do not serve the dual purpose of ensuring proper handling and contextual understanding of the samples during shipment. Each of these elements holds importance in their respective roles, but they do not fulfill the critical needs addressed by the chain of custody and a letter of request.