Hello!
I work with a Zorbax SB-C18 150x3.0 mm ID, 3.5 um. Temperature 37 C, flow: 1 ml/min, mobile phase start (for 6 min) 31 % MeCN and 69% phosphate 40 mM, pH 2.3, then wash with 70% MeCN and 25% phosphate for 1 min, reechilibrate 3 min. The wash phase is for elimination of late eluting peaks. The operating pressure is 208 bar at start and, when %MeCN is 70%, fall to 150 bar (208 bar----1 min----150 bar----1 min----208 bar)
Injection volume 100 ul (after reextraction of 200 ul sol H3PO4 0.25% from 7 ml hexane with 1% 2-buthanol). The extract is clear.
Until now I have to change three guard columns (SB-C18, 12.5x4.6 mm ID, 5 um), each after 40-60 injections (!!!), due to severe peak broadening and loss of resolution. Now I left the guard column and I use an inline filter with a 2 um frit.
A few month ago I used that extraction procedure, with the same solvent, but the column was SB-C18 250x4.6 mm ID and there was no gradient and no pressure surges. The guard column I used has not any problem for at least 300 injections.
The question is:
1. what is the reason for short life of guard column? Pressure surges or another?
2. Is a good solution to use the inline filter instead of guard column?

Thank you

PS phosphate does not precipitate at 70% MeCN, only at 82-85% MeCN.

You're doing liquid-liquid reextraction of what? Blood? Urine? Plants? Meat? Ground Mice?

Guard columns are always better column protectors than filters, since they work with more than one principle.

Extraction from 1 ml human plasma, at pH 13, with 7 ml hexane containing 1 % 2-buthanol

1 ml human plasma at pH13 is extracted with 7 ml hexane containing 1% 2-butanol

A good bet is that you are carrying over fats, i.e triglycerides, and that these stick to the guard column. You could try to fix this by using the 70% acetonitrile wash without buffer, followed by a short 100% wash with acetonitrile.
Another possibility is that the quality of the guard columns is variable, but I have my doubts about this, since three of them worked the same way. It is not impossible that the wash procedure that you are currently using does not work well any more, possibly due to some instrument failure (your variable pressure).

I forgot something important to say:
After a certain number of injections, the guard column fail suddenly , without any variation in system pressure. You can see some representative chromatograms to following location:

http://www.geocities.com/vlaselaur

More important to say, three days ago, I removed the guard column and, before analytical columns it remains only inline filter. With the same chromatographic conditions, from then, 3x24 hours, the system works well, without any variation of baseline pressure! And when the system contained only guard column OR both inline filter (after injector) and guard column, the guard column has the problem I said before.
So, I think that the cause for guard column failure it is not plugging.
Any suggestions ?????

Thank you

To follow anonymous # 2, it is possible with dirty samples to have lots of hydrophobic material stick to your guard column, and this can cause lost performance without physical plugging. I recommend you disconnect your guard column, backflush it with mobile phase with buffer removed, then backflush it with 100% ACN, then again with mobile phase with buffer removed. This has worked for me with samples containing lipids.

I did not think it was plugging, but the guard column is supposed to catch things that would otherwise accumulate on the column. A change in backpressure is not needed, it may or may not happen, depending on the contamination that the guard column is catching.