Clipping of a GC Column

[NCjazzhands]

If you have a 30m column installed in your GCMS, how much of the column can you clip cumulatively before the column is no longer good? My colleagues and I installed a new column in our Agilent 6975 yesterday. The column we removed had been installed in May and only had 12 meters left of the original 30 meters. 12 meters!! We were shocked that over half the column had been removed! Our supervisor is the one who is clip happy - he'll clip the column 1 -3 times a week 9We run probably 60-80 specimens fairly clean specimens a week). He didn't seem to agree with us that the instrument needed anew column, nor was he suprised that over half of the column had been removed. My collegeaues and I were thinking somewhere around five meters might be where we would draw the line and remove the column. Any thoughts, comments or suggestions in this matter would be greatly appreciated.

[Don_Hilton]

Of the column will separate the compounds of interest and allow for the degree of quantitation and confirmation desired, it is a sufficient column. And if 12 meters of column was doing the job - 30 my be overkill.

Some analyses require more frequent removal of the front end of th column. In this case, it may be more effective to use a pre-column connected by a union, which would allow for a 2-3 m secton to be removed and replaces on a regular basis. Press fits work well, but should be removed from the column when the guard column is replaced - necesitating trimming the front end of the active column (by no more than a centimeter) to get a fresh end of the column for seating the new press fit. There are a number of unions available that could work well.

You might find out what criteria your supervisor uses to determine that the column needs to be trimmed. If it is peak tailing, it may be worth taking a look at the liner being used. Injection on a liner that remains a bit cooler than the maximum oven temperature used on the column with frequent liner changes can prolong the column life.

[Yama001]

It is pretty routine in some applications to trim the column every day - anywhere from a few inches to a few feet.

12 meters left is pretty short - once a 0.25 mm ID column gets too short, if it is hooked to mass spec, the actual flow through the column is more than the operator realizes (unless careful to update the EPC parameters - often not worth the effort). The higher flow eventually can cause ionization or pumping problems. These are not always show stoppers, but at some point a column does indeed get too short.

I generally get worried about "too short' if I think I have trimmed the length down to 20 meters or less on a 30 meter column. Normally the need to change the column comes earlier than that for other reasons. Needing to trim a lot of the column usually means it is getting damaged too much by samples anyhow, so the end point will be reached sooner for other reasons. I will gleefully push a column as long as I can though.

Turning an abused column around can get some more life out of it, though if it has been trimmed aggressively, its better to put a new one in, unless you need to squeak one may day or so out of it. The environmental industry injects things no instrument should take, but thats the nature of that work.

[Alexandre]

We buy long columns and chop them to 2-3 pieces all the time. In your case 10 m may be enough.
If you do not like to adjust Rt of analytes after clipping, use precolumn via connector or there are columns with molded precolumns.

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"If your experiment needs statistics, you ought to have done a better experiment." Rutherford

[Johnny Rod]

I'm with Don, constant trimming of the colum head either means you don't know what your supervisor is doing it for, or they don't. Would be interested to know the answer!

[PaulDDodson]

If you have a 30m column installed in your GCMS, how much of the column can you clip cumulatively before the column is no longer good? My colleagues and I installed a new column in our Agilent 6975 yesterday. The column we removed had been installed in May and only had 12 meters left of the original 30 meters. 12 meters!! We were shocked that over half the column had been removed! Our supervisor is the one who is clip happy - he'll clip the column 1 -3 times a week 9We run probably 60-80 specimens fairly clean specimens a week). He didn't seem to agree with us that the instrument needed anew column, nor was he suprised that over half of the column had been removed. My collegeaues and I were thinking somewhere around five meters might be where we would draw the line and remove the column. Any thoughts, comments or suggestions in this matter would be greatly appreciated.

Dear NC Jazzhands, When you clip a column down from it's original 30m to 12 you had better change the head pressure. Otherwise the vacuum from your MS roughing pump will justy be sucking the neutral molecules through so fast that they won't have time to interact with the electrons to ionize.-Paul