Hi Chris,
Agilent, or Hewlett Packard as they then were, used to distribute to
their GC customers (on request) a wee software package for
calculating required column head pressure setting for desired flow
rate depending on column dimensions.
However, there is an easy of arriving at the same net result by
using a syringe and some lighter gas (butane, i-propane mix;
methane will do even better) on any system with an FID or like
yours with a parallel MS. I am quite pleased to learn that some
people appreciate the benefits of the GC-(MS/IRMS) setup [ref: W.
Meier-Augenstein, W. Brand, G. F. Hoffmann, and D. Rating:
"Bridging the Information Gap between Isotope Ratio Mass
Spectrometry and Conventional Mass Spectrometry", Biol. Mass
Spectrom., 23, 376-378 (1994); W. Meier-Augenstein: "On-line
Recording of 13C/12C Ratios and Mass Spectra in one Gas
Chromatographic Analysis", J. High Resolut. Chromatogr., 18, 28-
32 (1995)].
As Hinrich rightly pointed out, you can measure the flow and adjust
the column head pressure until you end up with the desired flow
rate. Of course, on our machines this is easier said than done. A
simple way of achieving the same end is to inject (at a GC oven
temperature of 40 C ) a few microlitres of lighter gas and clock the
time until you see the signal at the FID or the Ion Trap MS.
Based on your message, I am guessing your current column has
an ID of 0.25 mm as in this case 17psi would roughly give you an
average flow rate of 1.2 ml/min (which sounds about right for GC/C-
IRMS work) and a linear carrier gas velocity of 38 cm/s. In other
words, injected lighter gas (no significant retention on the column
at 40 C) will arrive at your MS after about 65 seconds. For a 60 m
column this would translate into approx. 158 seconds.
Incidentally, I trust you are aware of the "downside" of a 60 m
column? Unless you are prepared to run very slow temperature
gradients (1.5 C/min and less thus increasing your analysis time
considerably) you are not going to exploit the additional length and,
hence, translate it into improved separation.
Hope this helps.
Best wishes,
Wolfram
PS: The answer is no, you cannot expect a strictly linear
relationship between column head pressure and column length,
chiefly because the column ID enters the equation as the square of
the column radius.
> At 04:25 PM 12/5/01 -0500, you wrote:
> >Hello all,
> >
> >We have a GC-IRMS (system specs: Varian GC with 90% flow going to
> >Combustion Interface & Delta S IRMS, 10 % going to Finnigan Magnum ion trap
> >MS). The GC has a regulator to set the head pressure of the He flow
> >(currently at 17 psi for a 25 m column), and as I measure the flow at the
> >backflush vent, I measure 4.5 ml/min. The system has no flow controller
> >other than the pressure regulator. As I understand the flow requirements,
> >this is the appropriate flow rate to ensure that the sample is able to
> >transit the GC column and then make it to the IRMS.
> >
> >My question concerns the installation of a new column that is 60 m
> >long. Am I correct that a longer column will necessitate a higher head
> >pressure to maintain the 4.5 ml/min at the backflush vent? Does anyone
> >know if I can expect a linear relationship between column length and head
> >pressure?
> >
> >Thanks,
> >Chris
> >
> >***********************
> >Christopher L. Osburn, Ph. D
> >NRC Research Associate
> >Code 6115
> >US Naval Research Laboratory
> >4555 Overlook Ave. SW
> >Washington, DC 20375
> >202-767-1700 (office)
> >202-404-8515 (fax)
> >202-404-3374 (lab)
> >***********************
***********************************
Dr. W. Meier-Augenstein, CChem, MRSC
Senior Research Fellow
University of Dundee
School of Life Sciences
Division of Molecular Physiology
Old Medical School, Small's Wynd
DUNDEE DD1 4HN
Tel.: +44-(0)1382-34/5124, /4574, /4968
Fax: +44-(0)1382-34/5514
e-mail: [log in to unmask]
URL1: http://www.dundee.ac.uk/biocentre/SLSBDIV6wma.htm
URL2: http://www.dundee.ac.uk/anatphys/wma/meieraug.htm
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