What are bottom up and top down approaches of sample analysis? what are the differences?

In the bottom up approach, intact proteins are digested into peptides prior to introduction into the mass spectrometer. this approach is very user friendly, generates a high throughput and gives more information about proteins with 'extreme' properties. but posttranslational modifications and isoform information is lost. in the top down approach, intact proteins are introduced into the mass spectrometer. this keeps posttranslational modifications and protein isoforms intact, but has limited sensitivity and throughput and quite pure samples are required and expensive instrumentation is needed.