how short is too short for a guard column?

[jf]

were running astm d6584 method for analysis of glycerides in biodiesel.

the method recommends a 2-5m guard to extend column life. weve been starting with a 2m guard and clipping it down to say, 1/2 meter before discarding

triglycerides are notorious for gunking up a column, and were using an expensive hi-temp metal column. I dont want to push it and risk column damage by using too short of a guard.

any recommendations? Is there any disadvantage to starting with a longer 5m guard? i imagine since its deactivated the added heat cycling wont really affect anything, all the contamination tends to occur in the first few inches anyway, which gets progressively clipped

thanks!

[AZBiodiesel]

i am also interested in the answer since i am doing the free and total glycerin test as well. I have a 5m guard column but it is fused silica not metal. It would be nice to be able to cut it into 1m lengths, but i think what i will end up doing is just cut two 2m lengths and try out the 1m length and see if there is any difference.

[Bruce Hamilton]

In my experience, there is no rule, because it depends on the feedstock and process.

If you have a clean feedstock (very little of other lipids, relatively low concentrations of polyunsaturates ), then a guard column may be unnecessary, however it's good insurance. The question is, how confident are you in the consistency and cleanliness of your samples?.

Bruce Hamilton