Chromatography Forum

We change our columns monthly and trim in between. Is it critical to input into Chemstation the removed length trimmed? Also when a new column is installed to run a unretained compound and calculate the length for Chemstation use?
Every six months we calculate linear velocity to verify it's within a range we've deemed acceptable. Does anyone continue to do this on a scheduled basis? Again couldn't this be input into Chemstation software by adjusting length removed and or running an unretained compound?

Also quarterly we use an independent digital thermomenter to verify the GC oven ramp rates at their respective levels. The specifications Agilent has for their GC's 5890, 6890. Does anyone perform this check? Almost always we are exceeding the capability of the machine due to method development exceeding the heat ramp rate at all levels. I've never found a problem with the GC meeting it's stated ramp rates.
I'd like to do away with both of these procedures and have a reason.
Thank you in advance.

How critical any of this is depends on what you need the data for.

Checking linear velocity by injecting some air takes about 2 min. I do it every time I change the plumbing, as well as check for leaks with a leak seeker.

This focus on gas flows and column length suggests that you use retention time a lot. If you use retention indices instead, with a couple of internal standards as markers, your results will be robust to quite wide variations in conditions. Agilent has some fancy software that "locks" retention time by adjusting gas flows and programme rates, but I doubt that saving 2 min every six months will justify the cost.

Peter

Regarding inputting the length of removed column, since the software is attempting to maintain a specific linear velocity, for the calculations to work properly, it needs to know the pressure. All the system really knows regarding what's happening with your capillary column is the amount of pressure applied, and what you tell it (column dimensions).

The benefit of inputting the column length removed is that you will maintain your separation longer (since the software is taking into account the column length). The reality though, is that if you recall a Van Dempter plot, your separation does deteriorate to higher flows, but overall not by much (particularly in comparison to having the flow set too slow).

So, where this leaves you is that you'll get slightly better separation if you take the few moments to enter in the change in column length. Does it make a "difference" is relative to your analysis.

Sincerely,
Greg