Klentaq LA is for Long and Accurate PCR. It is almost the original mixture of enzymes that was the invention of Long PCR (Barnes, 1994). Klentaq LA also has inhibition-resistance properties, tolerating up to 10% whole blood or serum in a PCR reaction.
8-20 kb should be so easy and reliable with Klentaq LA that we will refund a tube of your purchase if it doesn't work for you, if you show us the data. 35 kb is attainable with more practice, at least from phage or cosmid template. Longer PCR amplicons require more excellent and intact preparation of the template DNA. Klentaq LA gives more reliable and efficient PCR at all sizes of amplicon, down even below 1 kb. Our original-strength enzyme is twice the concentration of licensed Klentaq LA from other providers, but theirs works well if you put in twice as much, and use our buffer.
100 ul of Klentaq LA is sufficient for 2000 25 ul reactions of 1 kb length. For each kb in amplicon length, add 0.05 ul enzyme, not to exceed 0.325 ul per 25 ul reaction. For instance, for a 4 kb product, add 0.2 ul enzyme for each 50 ul reaction. For a 30 kb product, add 0.325 ul per 25 ul reaction.
To accurately pipet 0.05 ul for a typical 25 ul PCR of 1 kb or less, make up 500 ul of master mix and add 1 ul of Klentaq LA. This master mix will allow you to test several variables, such as template concentration, annealing temperature, PCR enhancers, etc.