O analyses of various sample matrices with δ the vario PYRO cube Introduction

Application note: 050
δ18O analyses of various sample matrices with
the vario PYRO cube
Introduction
We have used a low-cost, high–precision and rapid method of oxygen isotope analysis applied to various oxygen bearing matrices,
organic and inorganic (sulfates, nitrates and phosphates), previously measured for δ18O. It was first successfully applied to δ18O analyses
of natural and synthetic phosphate samples. The technique utilises a newly developed pyrolysis method based upon high-temperature
EA–Pyrolysis combined with patented “purge & trap” chromatography interfaced to an IsoPrime100 IRMS.
Using this pyrolysis method we have been able to generate a single calibration line for samples showing pyrolysis efficiencies independent on the type of matrix pyrolysed. This could be a very important step forward as silver phosphate is a very stable material, it is weakly
hygroscopic and easily synthesised with predictable δ18O values. It could then be considered as a good candidate to become a reference
material for the determination of δ18O by EA-pyrolysis-IRMS.
Technical Setup
The configuration follows standard high temperature pyrolysis methods using a glassy carbon tube filled with glassy carbon chips and
carbon black held inside a ceramic furnace tube heated to 1450°C. However, the vario PYRO cube also offers a number of advantages
over other EA systems.
Autosampler
‘Column Flush’
He Flow
Analytical
He Flow
Analytical
He Vent
Sheath
He Vent
TCD
Dilutor
He Flow
Sample
Dilutor
IsoPrime100
Reference
He Flow
P
Sheath
He Flow
CO Column
CO
Reference Gas
Injector
Figure 1. During column flushing the source of the He carrier gas is switched from ‘Analytical He’ to ‘Flush He’ when CO is desorbed
from the ‘purge and trap’ column.
Purge & trap chromatography
At the heart of the vario PYRO cube EA is a patented ‘purge & trap’ separation device which is used in place of the traditional packed GC
column. This system traps CO from pyrolysis at 40°C and rapidly releases the sample CO by heating to 150°C. The TCD output determines
when the trap is heated. This results in excellent chromatography which has obvious benefits for the integration of the sample peak
Patented column backflushing to improve separation of N2 from CO when organic materials are pyrolised
During sample pyrolysis the carrier He flow is set to “straight” mode. N2 is not trapped on the column and flows to the IRMS whereas CO
is trapped. Once pyrolysis is complete, the He flow is set to “column flush” mode to divert slowly decomposing N2 away from the IRMS
and to use a clean He flow to move the desorbing CO into the IRMS. The ‘purge & trap’ column is then heated to 150°C, the sample CO is
desorbed and enters the Thermal Conductivity Detector (TCD) where the signal is detected as a focused peak. This patented column flushing technique offers excellent advantages for this complex technique (See Technical Note 005).
Two independent He flows through the furnace tubes
The analytical He travels in a standard way, through the centre of the reaction tube carrying sample CO through the EA to the IRMS. However, a second independent He flow passes between the inner glassy carbon tube and the outer ceramic tube (furnace purge helium).
This flow purges any ambient air which has diffused through the ceramic tube at high temperature which can lead to high “blank” peaks
in other systems.
Removable ash finger made from glassy carbon
For simple maintenance, an ash finger made of glassy carbon is used which can be removed easily.
Heated sample tray
The sample tray can be heated at a user definable temperature to prevent any interference on the results by atmospheric
moisture.
Isoprime Ltd
Isoprime House
Earl Road
Cheadle Hulme
SK8 6PT, U.K.
www.isoprime.co.uk
application@isoprime.co.uk
A member of elementargroup
Application note: 050
Results
Chromatography
4
2
0.15
CO Ref. Peak
6
CO Sample Peak
N2 Ref. Peak
8
N2 Sample Peak
10
A significant advantage of the vario
PYRO cube system is the quality of the
CO peak shapes compared with
conventional systems using packed GC
separation column. This has a direct
influence on the quality of the isotopic
data integration. Also, there is no
deterioration with time of the peak
quality or the N2 - CO separation as has
been shown with packed GC systems.
0.20
Mass 28
Mass 29
Mass 30
0.10
0.05
Signal Minor (nA)
Signal Major (nA)
12
0.00
0
0
100
200
300
400
500
600
700
Time (Seconds)
Results
The instrumental setup was derived from Sieper et al. (2010) and details are included in Technical Note 005. First of all we investigated the
treatment of the background signal; we conclude that it is necessary to use a blank subtraction routine in order to correct the data, considering that in any conventional system using GC column separation, any blank entering the system will be taken into account as a constant
background dealt with during peak integration.
Standard
(Measured)
SD
n
40
(Ref. Values)
NBS120c
21.70
0.10
5
21.70
Sucrose
35.55
0.21
4
36.40
Saccharose
27.96
0.04
5
27.90
9.01
0.22
4
AgPO4
30
Sucrose
25
Vanilin
Vanilin
y = 1.0022x + 0.1397
R² = 0.9977
35
9.50
20
15
BaSO4
10
AgNO3
BaSO4
11.14
0.42
4
12.00
AgNO 3
20.34
0.28
4
19.60
EP02
3.29
0.24
4
3.36
EP05
8.83
0.29
6
8.91
Saccharose
5
0
0
10
20
EP011
15.79
0.23
6
30
!
"
40
#
15.84
Different pyrolysis and desorption temperatures have been investigated to optimize the analysis of silver phosphate samples. The best
results were obtained by using a pyrolysis temperature of 1450°C and a CO desorption temperature of 150°C. Using these parameters we
conducted an experiment mixing various sample matrices within the same batch using calibrated material measured with conventional
techniques. Sucrose, vanillin, BaSO4, AgNO3 were then pyrolysed alongside our silver phosphate samples (Table 1; Figure 3). Once again, the
quality of the best fit line (R2 = 0.997) obtained between measured values and reference values (3‰ < δ18O < 35‰) emphasises the performance of this pyrolysis method. A key parameter is the slope of the regression line (1.0022) which is very close to unity and an advantage
for this technique compared to conventional EA-pyrolysis-IRMS.
Conclusions
The data obtained through this study confirm that the EA–pyrolysis method has the potential to be reliably used for the high precision
determination of δ18O not only of phosphates, but also various oxygen bearing organic and inorganic compounds.
This new procedure, using patented ‘purge & trap’ technology on the vario PYRO cube elemental analyser, is able to generate robust data
in terms of accuracy and precision, taking into account that the various caveats related to the chromatographic separation of gases, as
described by Farquhar et al (1997) are avoided.
The lack of universal calibration material has been a major drawback to the development of oxygen pyrolysis techniques.
Silver phosphate is a chemical compound very stable with time, weakly hygroscopic and thus could be considered as a top candidate to
become a reference material for the determination of δ18O by high–temperature pyrolysis.
Isoprime Ltd
IsoPrime100 Applications
Isoprime House
Earl Road
Cheadle Hulme
SK8 6PT, U.K.
Phone: +44 161 488 3660
Fax: +44 161 488 3699
Email: application@isoprime.co.uk
A member of elementargroup