When can I use a custom field type instead of a custom entity?
Custom entities and custom fields are not mutually exclusive, you can use them together.

You can build a custom field type with more than one property. See for example the Password field which defines three properties:

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Field%21Plugin%21Field%21FieldType%21PasswordItem.php/function/PasswordItem%3A%3ApropertyDefinitions/8.2.x

You can also extend the entity references field. See for example the File field:

https://api.drupal.org/api/drupal/core%21modules%21file%21src%21Plugin%21Field%21FieldType%21FileItem.php/function/FileItem%3A%3ApropertyDefinitions/8.2.x

The File field adds two properties to the reference field. In your case you would add the properties chemical name and concentration to a reference field which points at the analysis entity.