Hello Samuel,

The sample with a ratio of 1.6 may be contaminated with protein; the one with a ratio of 3.57 is contaminated with something that is neither DNA, nor RNA (the A260/A280 ratio off RNA is about 2, not > 3). No need thus to perform an additional RNase treatment. I would rather recommend that you peruse the instruction manual of the kit to find out what may possibly have gone wrong. I am not familiar with the Invitrogen PureLink™ kit, having used mostly the Qiagen and Macherey Nagel kits; however, these kits are usually very reliable when used as recommended by the manufacturers.

You mentioned that you have no resource to perform 260/230? Then how did you measured the A260/A280 ratio?

We usually prefer Qubit results than nanodrop. Anyway, do you have the gel image of your extracted DNA sample?

I think you should use a filter to be suitable for dna binding and discarding to all wastes. I mean that it will be the best way to purify DNA by a spin column kit

Do not bother about 260/280 ratio for minipreps; they are dirty anyway. (contaminating RNA and nucleotides absorb a lot at 260 and proteins are present in minipreps: as long as you used E. Coli deficient in nuclease this will not be a problem) I If you follow the protocol of the kit you should be able to proceed with your experiments (digestion, sequencing, ...) If you need to do more sophisticated experiments (eukaryotic cells transfection for example) then you will need to better purify your DNA. If digestion or sequencing is a problem it is proobably because you did something wrong or because you work with a low copy plasmid