That's interesting they are using Zn and NaOH solutions together, this type of medium forms ZnO, Have you looked at the reference they are referencing regarding the Zn and NaOH precipitation? (http://onlinelibrary.wiley.com/doi/10.1002/bit.260480405/abstract)

Alternative you could just use acetonitrile, it works almost equally good, also stated by Polson et al.. During my master thesis, I develop a method, where I precipitated the proteins with acetonitrile in a ratio of 1:7, which showed to give less matrix effect than 1:3. Again this will depend on your sample material

So it is very strange because i have been using ACN like prepipitating reagent in ratios of 1:1 and no backpressure problem appeelars during 2 years of work with my LC equipement.My column is in perfect conditions of pressure, retentions and reproducibility.

Dear Carlos Ezquer

Zinc sulfate and sodium hydroxide reagents (Somogyi's method) are a good deproteinizing tool for several biological fluids. The accuracy of their concentrations is less important than the requirement that the alkali must neutralize the zinc sulfate solution precisely, volume for volume, when titration is performed with phenolphthalein as indicator. For example a check with 0.5 N sodium hydroxide against and 0.7 M zinc sulphate heptahydrate, is that 10 ml of 0.7 M zinc sulphate, diluted to about 50 ml with water in which a few drops of phenolphthalein indicator are added, and 0.5 N sodium hydroxide solution is added, to zinc sulfate diluted solution, from a burette drop by drop, and about 10.8 - 11.2 ml are required to produce a permanent pink colour. Utilizing same solutions, we obtained a good protein precipitation (100%) with Somogyi's procedure when protein concentration of the biological fluid is about 0.8 - 0.4 g/dl and 1.0 ml of diluted sample is mixed with 125 µl of 0.7 M ZnSO4 heptahydrate, vortexed for 60 sec. and then 125 µl of 0.5 N NaOH are added and the mixture vortexed again for 60 sec. and then centrifuged at 4 °C at 3000 rpm for 15 min. Protein precipitation using zinc sulfate/NaOH, is very effective, for absolute absence of protein, assayed with BCA method, in supernatant. Then, such as you written, the mixture of the solutions cannot be made because when zinc salts react with alkali metal hydroxides, a jelly like white precipitate of zinc hydroxide is formed, but if you introduce the two reagents in different moment of the process a precipitating protein occur. The high efficacy of zinc sulphate-sodium hydroxide precipitating agent, procedure quick and easy, is demonstrated by formula below described:

% Protein precipitation efficiency = sample protein - protein in supernatant x 100

sample protein