at the moment I am reading a lot of paper in order to understand the advantage and disadvantage of a coated GC and packed GC. Perhaps somebody is able to give me a short overview of the advantages and disadvantages.
Thank you very much
By Rodney on Wednesday, October 29, 2003 - 05:01 am:
Packed columns have the advantage of durability, sample capacity, phase selectivity options, and COST. They are capable of doing many 250ppm and above analyses for most applications. They are particularly useful in the fixed gas and low molecular weight analyte area. The disadvantages are larger amounts of bleed, smaller number of theoretical plates (separating power for similar compounds), and particle loss into the detector. They require larger amounts of carrier gas TO THE DETECTOR but actually can use less gas than capillary wall coated columns. In micropacked versions they can perform separations much faster than wall coated capillary columns while dealing with much higher sample loads.
Open tubular columns (wall coated) have the advantage of larger theoretical plate numbers and inertness. They excell at low ppm to ppt analyte analyses. They are limited in phase selectivity options but their low carrier gas use TO THE DETECTOR and low bleed characteristics allows Mass Spectroscopy and other sensitive detectors to be used effectively. The use of bonded phases permits higher oven temperatures to be used in applications.
I have not exhausted the differences or the advantages and disadvantages of packed versus capillary columns, but I hope this gives you a starting point for discussion.
By Anonymous on Wednesday, October 29, 2003 - 06:47 am:
One of the other big advantages that Rodney omitted is that even though there may be less phases to shoose from in capillary columns (and even then the choices continue to expand) is that since the capillary columns have so much more efficiency that separations that might not work on a packed column might work on a capillary column of the same phase type just because of the greater number of thoeretical plates. This means that you may not need as large a selection of phases as you would when working with packed columns alone.
By jstuermann on Wednesday, October 29, 2003 - 09:19 am:
Dear Rodney and Mark,
thank you very much for your quick answer. But do you know somethink about the temperature dependence, because i want to reduce the separation time whereby i heat the column during the separation. I realize a packed gas chromatographic column in silicon with a heating element. Is it better to use a coated or a packed column?
Thank you very much
By Rodney on Wednesday, October 29, 2003 - 09:55 am:
It is not entirely clear to me what you are trying to do. But if you want fast heating of the column then lowering the mass of the column is the best answer. Capillary columns can be heated quickly and cooled quickly, but most importantly for good GC separations, heated and cooled EVENLY.
Packed columns would not work as well in my opinion.
There are manufacturers who use microwave energy to heat columns. Other use special heating elements which coil around the column, be it FSOT or metal packed.
Many analysts use microcapillary columns, 0.10 mm ID, for example, for faster chromatography.
My company, Supelco, does manufacture these columns with several different phases, but there are others.
I hope I have been helpful.
By Anonymous on Thursday, October 30, 2003 - 04:11 am:
Not entirely relevant, but I once put a thermocouple inside a packed glass column and monitored the difference between the packing and oven temeratures during heating and cooling. Surprisingly (to me), on cooling back to the start temperature, it took over 5 minutes for the packing to cool to that of the oven. I had usually only allowed 2 to 3mins for equilibration up until then.
Is your query a work related one or a college homework one?
By Rodney on Thursday, October 30, 2003 - 05:59 am:
The packed particles have vapor gaps and do not conduct heat quickly to the core of the column.
Even the glass column itself or even the oven walls do not cool as quickly as might be supposed compared to the air inside the oven.
That is why fast heating oven GCs use capillary columns as a rule.
Microwave heating is a new technique for fast chromatography and since the capillary column itself is the only mass heated (oven walls and the air are not heated) very fast heating and cooling rates are possible.
Has anyone in the group used this technique? Can anyone offer additional comments?
By jstuermann on Thursday, October 30, 2003 - 06:50 am:
Dear Ralph and Rodney
I am phd student at the university of bremen and I realized a ultra short packed gas chromatographic column with silicon. Now I want to modifiy the heating element in order to optimize the separation time.
The bulk silicon is reduced thus the mass of the column is very low.
Furthermore i want to know if the gas flow in a packed GC could be higher than in a coated GC?
By Anonymous on Friday, October 31, 2003 - 09:03 am:
The linear velocity of the carrier gas through the packed column and the capillary columns should be roughly the same. Since there is more volume in the packed column the flow RATE will be higher than in the capillary column.
By Rodney on Monday, November 3, 2003 - 05:07 am:
Mark is correct. While less than 1cc/min is common for capillary FSOT columns and 2-10cc/min for 0.5mm ID FSOT, packed columns may be used at 10-100cc/min. Even Micropacked varieties can be used effectively at 2.0-50.0cc/min.
It depends upon the nature of the analytes involved.
You can even pack a 0.5mm ID FSOT tube with coated supports and use it for separations.
[ Good luck with the glass wool, however. ]
Don't use glass wool!
Use unions with frits or screens to secure the packing.