Separating 500ppm methane from a similar concentration of CO2 should be a
relatively simple task on that column (N2/O2 can be more challenging). Is
the interfering peak before or after methane? Does it have a good peak
shape, or is it a mess? Try running pure air through, see where the N2/O2
come out as well as CO2. Then run diluted methane in helium. Where is that
peak? Dilute the same methane in air - how does that look? The same as your
sample, or does the sample have something else showing up?
If N2/O2 is the problem, what is your column temperature? Drop it to as
close to room temp as possible. You know you have good N2/O2 separation from
CH4 when you can get the same isotopic value for methane when diluted in
either air or helium. Other ways to improve the separation: 1) Focus the
methane on the first few inches of the column using LN2. Tightens up the
peak, also lets some of the air pass straight through. 2) Put a second PLOT
column in series with the first (ie make a 50m column).
A few other questions: how are you injecting? A six port valve with sample
loop, or split/splitless injector? What sample volume are you using?
Land and Water Management Group
Alberta Innovates - Technology Futures
Vancouver Island Technology Park
3 - 4476 Markham St., Victoria, BC, Canada V8Z 7X8
Phone: 250-483-3290 | Fax: 250-483-1989
[log in to unmask]
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of lijun Zhang
Sent: July 19, 2012 1:54 AM
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] Low Concenntraion Methane analyses on IRMS
if you could use GC/GC/IRMS to purge out the N2 and O2 and cut methane peak
to the second column, you would get good separation and even remove the
interference of CO2 coelution. The first column is molecular sieve, and the
second column is probably Hayesep Q/R. I hope this it helpful.
M. Sc. Lijun Zhang
Instrumental Analytical Chemistry
Universitštsstr. 5, S05 V02 E35
> The peak that is interfering is most probably CO2. Air contains about
> 400 ppm of it.
> Best regards, Rob
> Rob Kreulen
> ISOLAB B.V.
> 1e Tieflaarsestraat 23
> 4182 PC Neerijnen, The Netherlands
> tel: +31 (0)345 620 120
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
> Behalf Of Harish Pande
> Sent: Thursday, July 19, 2012 6:45 AM
> To: [log in to unmask]
> Subject: Re: Low Concenntraion Methane analyses on IRMS
> Dear Peter
> Thanks for your reply on the above subject.
> We are using Varian make Plot Fused Silica 25Mx0.32 MMx coated
> poraplot Q GC column and our sample is approximately 500-1000 ppm
> methane and still lower concentration of ethane and propane,but rest
> of the volume would be air.
> I shall try if I could access to the references you have cited.
> Harish C Pande
> superintending chemist
> Stable Isotope Laboratory
> Geochemistry Group
> KDMIPE,ONGC,Dehradun,India 248 195
> Ph:+91 135 279 5713
> fax: :+91 135 2755265
> Mob:+91 9411114146
> e mail [log in to unmask]
>> Date: Wed, 18 Jul 2012 13:41:33 +0200
>> From: [log in to unmask]
>> Subject: [ISOGEOCHEM]
>> To: [log in to unmask]
>> Dear Harish,
>> It would be helpful to know more about your setup (type of GC column)
>> and about the composition of your samples in order to get a better idea.
>> I am rather new to the field of GC-IRMS and there is probably a lot
>> more knowledge out there, but this could be a start. I came across
>> two options that could produce that extra peak, assuming you use a
>> 1) N2 and O2 elute just before CH4 and can form N2O in the combustion
>> furnace. The N2O would then enter the IRMS just before the
>> CO2 peak and therefore appeares on m/z 44, 45 and 46. A finely tuned
>> preconcentration step can eliminate this effect. The N2O production
>> is described by Bruce Vaughn et al. and the tuning of the
>> pre-concentration device in Umezawa et al.
>> 2) Another candidate could be CO which also elutes just before CH4 in
>> our GC column. This would be combusted and measured along with the
>> CH4-induced CO2.
>> --> Bruce Vaughn et al., Handbook of Stable Isotope Analytical
>> Techniques, edited by Groot, P. A., vol. 1, pp. 272-304, Elsevier
>> B.V., 2004.....
>> --> Umezawa et al. (2009), A High-precision Measurement System for
>> Carbon and Hydrogen Isotopic Ratios of Atmospheric Methane and Its
>> Application to Air Samples Collected in the Western Pacific Region,
>> Journal of the Meteorological Society of Japan, Vol. 87, pp. 365-379
>> All the best,
>> Peter Sperlich
>> PhD Student
>> Niels Bohr Institute, Centre for Ice and Climate University of
>> Copenhagen Juliane Maries Vej 30 2100 KÝbenhavn ō Denmark
>> +45 353-20609
>> On 18.07.2012 12:55, Harish Pande wrote:
>> > Hi all
>> > We are trying C-Isotopic analyses of low methane concentration (~
>> > 500ppm) in desorbed gas samples,rest of the volume in our sample is
>> > air/nitrogen.The methane peak seems to have
>> > camouflaged with some unknown peak close to its retention time.
>> > We have delta V plus IRMS but no concentrator device.
>> > Can some one suggest the analyses protocol or any reference.
>> > Regards
>> > //
>> > /Harish C Pande/
>> > /superintending chemist /
>> > /Stable Isotope Laboratory/
>> > /Geochemistry Group/
>> > /KDMIPE,ONGC,Dehradun,India 248 195/
>> > /Ph:+91 135 279 5713/
>> > /fax: :+91 135 2755265/
>> > /Mob:+91 9411114146/
>> > /e mail [log in to unmask] <mailto:[log in to unmask]> /
>> > //