Paul-
My mistake on the present flow rate of oxygen I listed in my last note, it
is the same as my O2 purge flow rate or 25ml/min.
As for my analytical times, yes they are long with a He flow rate of
60ml/min, but it is not recommended to have a pressure much above 120 kPa
(pers com w/ Costech) on the CE NA1500 using a 4m column. I have selected
60ml/min (100kPa) and 60 deg on the column because of the separation time I
wanted between the N and C peaks.
It is possible for us to go to a 5 ml O2 loop, but we have not gone that
route at this time, but is in our short term plans. At the moment with our
present column configuration a get approx 700 to 800 samples/copper column
change. With a smaller mesh copper, which will switch to when my present
supply runs out, we will get even more runs per column change. How does
this compare to others out there using similar equipment?
The idea of going from a He purge to an O2 purge was more an act of
convenience than anything else. As I mentioned we have recently changed
from an AS200 to an AS128 carousel. The amount of He flow needed to purge
the larger volume AS128 was approx 60ml/min. The fixed constrictor that
came with our instrument in 1989, is that, fixed and cannot be changed
without replacement. Which of course can be done although getting one with
enough flow rate would solve the problem but would still have a fixed flow
rate. It would be better to have some flexibility with the purge flow rate
in the event I change the present EA configurations. Second, I could
replace the fixed constrictor with a metering valve as I also mentioned and
still use He as a purge. This would be a good option and give me
flexibility wrt purge flow . However, since the O2 on the CE1500 instrument
is always flowing (in my case at 25ml/min) I decided to put it to use as a
purge. With my present configurations on the EA I do not use any more
oxygen using the O2 as a purge or to just fill the O2 loop. In addition, by
using the oxygen on the NA1500 I can use the O2 metering valve and/or
regulator on the EA to regulate the purge flow a feature that I believe now
comes on the CE2500 model. Perhaps other labs running these CE instruments
can add some information on analytical precision vs O2 loops, flow rates,
etc. The C precision on our 1500 using a 10ml O2 loop at less than 20ml/min
is not as good as it is with our present settings. At this time I am not
sure if we will continue to use oxygen as our purge for various reasons we
may change back to He as a purge in the future, but for now it provides us
with a solution and with information that might be useful to the community.
Paul, perhaps you might want to contact the CE people directly and debate
some of the other issues you mentioned in you msg or maybe Guido Giazzi and
some others will join in following this msg. My intent with my last note
was to inform those who are using similar equipment that our lab had
switched from using the AS200 with a He purge and was now an AS128 using
the O2 as a purge on a CE NA1500 connected to a Delta plus with good
results. Perhaps some would find this information of some use.
As for peak centering between N and C, we have had some problems with
magnet drift on our new Delta plus since its installation in Aug 98. We
feel that we have fixed this problem with the latest modifications Finnigan
made to the magnet controller (late Jan 99) and now the tuning on the
instrument is stable with only about a +/-1v change in HV on C, N is
stable. So now it is probably not necessary to peak center between N and C,
however, as a precaution we will continue to run the instrument with the
peak center between N and C until we convince ourselves that we will not
sacrifice precision for throughput.
Cheers,
Dave
At 06:50 AM 5/25/99 -0700, you wrote:
>Dave, Why do you have to run your Oxygen line at 30-40 mL min-1 to purge a
>10 mL loop? You don't give analysis time but with a 60 mL min-1 flow rate,
>60 C oven and 4 meter column I am guessing that your analysis time is over
>8 minutes. Wouldn't 10 ml min-1 of O2 for 8 minutes purge the O2 loop 8
>times in 8 minutes and then use He to purge the samples? Would it be
>possible to simply use a 10 mL min-1 O2 purge plus a 25 mL min-1 helium
>purge, and tee them together into the sampler purge?
>
>For that matter, does anyone out there know why CE instruments C/N
>analyzers come with such a high O2 purge through the O2 injection loop?
>
>At the same time I might ask as to why the ones I have seen from the
>factory come with a 25 mL loop, which in our initial experience went
>through copper columns every 200 samples? Why not ship them with 10 mL
>loops?
>
>Also, I am curious as to why it would be necessary to do a peak center on C
>when jumping from N to C. My Europa Sci. 2020 manages to go days without
>any retuning of the source and continually jumps back and forth between N
>and C tuning. Is it really necessary on a delta plus to peak center on
>every sample?
>
>Paul.
>
>
>
>At 06:53 PM 5/24/99 -0700, you wrote:
>>For what it's worth.... Our CE NA1500 Series 2 (EA) running on a Delta plus
>>is now setup using the oxygen as the purge. Originally when we were using
>>the AS200 carousel on the EA the helium was used as the purge as per the
>>original instrument configuration. The flow rate through the fixed
>>constrictor on the purge using helium was 25 ml/min. Plenty of flow to
>>purge the small volume of the AS200 carousel.
>>
>>We are now using the AS128 carousel on the EA. The helium flow rate on the
>>fixed constrictor (25ml/min) was not enough to purge the larger volume
>>carousel. We had two options, by-pass the constrictor using a metering
>>valve or use the oxygen as the purge. After running some tests, I found
>>that using the oxygen as a purge would not consume any additional gas.
>>Baselines and blanks are excellent.
>>
>>Original configuration using AS200:
>>He flow rate 60-65ml/min at 100kPa
>>Oven temp 60 deg C
>>Oxygen flow 30 - 40 ml/min @ 100kPa, 10ml sample loop
>>Helium purge = 25ml/min
>>4 meter column
>>
>>Present configuration using AS128:
>>He flow rate 60-65ml/min at 100kPa
>>Oven temp 60 deg C
>>Oxygen flow 25-30ml/min @ 100kPa, 10ml sample loop
>>Oxygen purge = 25-30ml/min
>>4 meter column
>>
>>I should note that the flow rates and temps I am using are setup so we can
>>do a slow peak jump between the nitrogen and carbon analyses. Slow peak
>>jump means we do a peak center on C during the jump. Time between N and C
>>is approx 88 seconds. The slow peak jump takes approx 50 sec.
>>
>>Dave
>>
>>At 09:43 AM 5/24/99 +0200, you wrote:
>>>Isogeochemist and especially Dr William J. Showers of North Carolina State
>>>University
>>>
>>>
>>>on our CEInstruments Elemental analyzer we used Helium and Oxygen as
>>>utilities.
>>>
>>>In the EA1108 the Oxygen is used for Flash Combustion while Helium is used
>>>as carrier gas and purge gas.
>>>The function of purge is to eliminate the presence of air , therefore N,
>>>over the tin container before dropping in the oxidation reactor.
>>>
>>>In the CE 2500, Helium is carrier gas while Oxygen has double function :
>>>oxidative gas and purge gas.
>>>Using Oxygen as Purge gas, the blank value is reduced (10 time less): this
>>>factor was very important for trace elemental analysis
>>>Consequently in CE2500 configuration, also the He consume , using only for
>>>carrier , is riduced.
>>>
>>>According to the autosampler used the Oxygen flows for purge in CE 2500
>>>are the following:
>>>
>>>AS200 40 - 50 ml/min
>>>AS128 80 ml/min
>>>
>>>while for EA1108, the Oxygen use for Oxidation gas was 10 - 20 ml/min :
>>>this is the reason of the different Oxygen gas consume in EA1108 and
>CE2500.
>>>
>>>Best regards
>>>
>>>
>>>
>>>Guido Giazzi
>>>Product Manager
>>>CE Instruments - Rodano Italy
>>>phone ++39 02 95059336
>>>fax ++39 02 95059309
>>>
>>>
****************************************************************************
David A. Mucciarone
Laboratory Manager
Stanford University
Department of Geological and Environmental Sciences, 320
Stanford, CA 94305-2115
650-723-0817 (office) 650-725-0979 (fax)
Office - Green Earth Sciences Rm. 365
Stanford Stable Isotope Laboratory - Green Earth Sciences Rm. 332 & 334.
e-mail: [log in to unmask]
WWW: http://pangea.stanford.edu/isotope/dam
****************************************************************************
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