deletions | additions
diff --git a/untitled.tex b/untitled.tex
index 989bd0b..e5a297d 100644
--- a/untitled.tex
+++ b/untitled.tex
...
Two sections of the vein were cut parallel to the main growth axis and, using a Dremel drill with a dental bit, samples for isotopic analysis drilled at 5mm intervals in the direction of growth, Figure 2. Care was taken to avoid using undue pressure so as to minimize any frictional heating of the sample. Analyte CO_{2} is produced by reacting 6-8mg of sample powder with 102\% ortho-phosphoric acid in vacuo at 25$^{\circ}$C and for a period of 12 hours. The evolved CO_{2} is then dried, collected by cryo-distillation into a calibrated volume manometer to check reaction yields and then stripped of potential hydrocarbon contaminants before collection in a Louwers-Hapert valved glass gas tube. The drying stage involves freezing the CO_{2} into a glass spiral trap at liquid nitrogen temperatures before sublimation at -120$^{\circ}$C, passing the gas through a second trap at -120$^{\circ}$C whilst freezing with liquid nitrogen into the manometer. We strip any potential hydrocarbon contaminants from the CO_{2} by cryo-distillation into the valved gas tube via a 20cm x 4mm i.d. glass tube packed with porapak Q ion exchange resin and held at a temperature of -20$^{\circ}$C.
The sample gases were analysed for their clumped isotope values, $\delta$^{45} - $\delta$^{49} on the UEA MIRA dual-inlet isotope ratio mass spectrometer (Dennis, 2015). All analyses are made at a major beam (m/z=44) intensity of 7.5×10^{-8}A
and with
simultaneously simultaneous data acquisition for each cardinal mass of the CO_{2} molecule (m/z = 44 - 49). Each measurement consists of 4 acquisitions, each of 20 reference-sample gas pairs. Before analysis and between each acquisition the sample and reference gas volumes and signal strengths are balanced to within 1\%. A sample or reference cycle consists of a 10s ‘dead time’ after switching of the changeover valve followed by a 20s integration period. The total measurement time, including sample and reference gas balancing is approximately 90 minutes. The
total integration time is 1600s each for the sample and reference gas. Internal precisions ($\pm$1$\sigma$σ) for $\delta$^{45} and $\delta$^{46} are better than 0.001‰, for $\delta$^{47} better than 0.008‰, for $\delta$^{48} better than 0.03‰ and for $\delta$^{49} better than 10‰. The reference gas used in MIRA is
CO2 CO_{2} produced by reaction of BDH marble chips with
85% 85\% ortho-phosphoric acid and subsequently equilibrated with water at
20°C 20$^{\circ}$C for a period of 1 month. This is to ensure that the
Δ47 $\Delta$_{47} value of the reference gas is in equilibrium at the laboratory temperature. The nominal composition of the reference gas is:
δ13C $\delta$^{13}C =
2.007‰VPDB; δ18O 2.007‰_{VPDB}; $\delta$^{18}O =
34.899‰VSMOW, 34.899‰_{VSMOW}, and
Δ47 $\Delta$_{47} =
0.94URF. 0.94$\permille$_{URF}. To ensure a robust calibration of scale compression and transfer function between the local reference frame for
Δ47 $\Delta$_{47} and the universal reference frame (URF) both
1000°C 1000$^{\circ}$C heated and
20°C 20$^{\circ}$C water equilibrated reference gas samples are measured on a daily basis (Dennis et al., 2011). Data quality and long term stability of measured values is monitored by daily measurement of two laboratory standards that bracket the range of
Δ47 $\Delta$_{47} values for samples in this study: UEACMST
(Δ47 ($\Delta$_{47} = 0.39±0.01, n= ) and UEAHTC
(Δ47 ($\Delta$_{47} = 0.56±0.01, n=).
MIRA is inherently linear with no variation in the calculated Δ47 and Δ48 values of samples as a function of their bulk isotopic composition as represented by the δ47 and δ48 values of samples (Huntington et al., 2010). Not-with-standing this we regularly check for linearity by measurement of 1000°C heated cylinder CO2 (BOC) that is depleted in δ47 with respect to the reference gas by approximately 65‰. We have not recorded any change in the linear behaviour of MIRA over a four year period since commissioning.
(ii) Data handling and calculation of Δ values.
The clumped isotope Δi value of a sample is defined as:
