Hi Carolyn,
Just to add my tuppence worth; as all previous respondents have already
pointed out, the main contributors to m/z 30 are either NO (14N16O) or CO
(12C18O) plus HCO (H13C16O).
In my experience, the NO formation shouldn't affect the d15N measurement
from a pure presence point of view because we all measure solely the 29/28
ratio for d15N. However, its formation will srew up d15N measurement if (a)
there isn't much N in the samle to go 'round in the first place and when
(b) not all the N in one' sample is converted to N2. The main reason for
this is incomplete reduction which is points towards an exhausted reduction
tube. Even a fresh reduction tube can let one down if there's a problem
with the O2 valve that pulses O2 into the oxidation reactor for flash
combustion.
So, one other thing to look at would be the O2 line going into the
oxidation reactor, i.e. checking for leaks and for proper functioning of
the valve.
The other option is the CO / HCO which will influence d15N measurement
because 12C16O causes an isobaric interference with 14N2 and H12C16O as
well as 13C16O will caused problems with 14N15N. Since CO will coelute with
N2 one doesn't see its contribution to m/z 28 and 29 but because of the 300
times higher amplification on cup 3 (compared to cup 1) one can see the m/z
30 contribution.
In this case, changing the water trap as already suggested by others is
part of the solution. The other is to give the GC column is good bake-out.
The fact that your m/z 30 background goes down after a few hours with no
samples being run suggests the column is almost saturated with nasty stuff
(that in an ideal world shouldn't be there in the first place but just
can't be completely avoided) and only freshly added gunge from the reactor
side causes an increase in background.
Just out of interest, have you run a magnet background scan while the m/z
30 background was high? It may be interesting to see if and what other
masses are present during the period of high m/z 30 background.
Best wishes,
Wolfram
On Aug 2 2006, Gerard Olack wrote:
> Hi Carolyn--
>
> We had a similiar problem with our Costech ECS. It has a "T" off the He
> line that is not used in our setup, and that would eventually be a
> source of an air leak (or a virtual leak after changing He tanks) even
> though it was well capped. We ended up removing the cap and putting on
> a fine capillary pig-tail so there is always a little bleed out of the
> line (<1mL/min).
>
> The Zero-Blank autosampler works well, but bits of ash or dust on the
> seals or misalignment of the purge valve o-ring can cause some
> problems--which I'm sure you know. Rarely we've had to clean the seal
> between the carousel housing and the tube to the reactor.
>
> We always see a little N2 blank from our O2. We've stopped using the
> high purity O2 because the N2 contaminant was hard to pick up, yet was
> running at 200+ per mil so it still threw off our N2 readings,
> especially when the signal was small. With standard O2, we tend to see
> a peak >5mV with ~100+ per mil value.
>
> One other thing, occassionally we'll see a "virtual" leak from a
> sample--with trapped air slowly bleeding out of the tin. Once the
> offending tin is run, there is a sudden drop in the baseline N2.
>
> take care
>
> gerry
>
> p.s. A bad He tank can also lead to rising N2 or Ar levels over time,
> though that doesn't seem to be the case here. The reactor tube can also
> get a fine fissure in it, but you've already replaced those.
>
> Carolyn L K Colonero wrote:
>
> >Hi,
> >
> > We have been running dC and dN on our EA-IRMS (an NA1500 w/zero blank
> > to a Delta plus XP) for about two months. The first month went great;
> > this month has been nothing but headache.
> >
> > The first month's samples were dC and dN on whole rocks (HCl/HF
> > cleaned) and bitumens and fractions thereof (sats, aros, etc). We took
> > what dN results we could get, but there wasn't much N to be had in
> > these. Some of the whole rock sample sizes went as high as 20mg. Key
> > here though is that through all of these very large samples, the
> > background mass 30 remained low, and the standard deviations for both
> > the dC and the dN results were good.
> >
> > This month's samples are kerogens in whole rocks, also HCl/HF cleaned,
> > also with low N concentrations, and many with some amount of pyrite.
> > These samples tend to be smaller in size, in the <10 mg range, but as
> > we run a carousel we see the mass 30 rise until halfway through the
> > batch, the dN becomes nonsense. Worth noting is that the dC for all of
> > these samples is good, both accurate and precise.
> >
> > Standards run in a batch without unknowns will stay good. Our blanks
> > are truly blank. Initially I was sure that our Cu reactor was spent,
> > but when I removed it, the Cu was only half used. After another 100 or
> > 200 samples I changed both reactors again with no improvement in the
> > conditions. I have also replaced the H2O trap, and baked out the packed
> > column overnight. I have tried adjusting the O2 (timing and amount),
> > but we were already seeing a good flash combustion. Except for the
> > increasing mass 30 background, our peaks are really quite lovely
> > (symmetrical and tight).
> >
> > Any ideas what might be causing the increasing mass 30 background? It
> > invariably arrives after about 10 or so unknowns, making the remaining
> > samples garbage. The high 30 background will go down after a few hours,
> > but standards run at this point are not as good, and any unknowns will
> > cause mass 30 to rise rapidly. The only recourse seems to be changing
> > the reactors again, but the reagents are not used up. Is the pyrite
> > poisoning our reactors?
> >
> >Any thoughts or advice?
> >Carolyn
> >_______________________________
> >Carolyn Colonero
> >Lab Manager
> >Massachusetts Institute of Technology
> >Summons? Geobiology Lab
> >E34-510
> >42-44 Carleton St.
> >Cambridge, MA 02142
> >Office: 617-253-7850
> >Lab: 617-324-4002
> >
> >
> >
>
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