Headspace solid phase microextraction – GC/C-IRMS for δ13CVPDB measurements of mono-aromatic hydrocarbons using EA-IRMS calibration
This work aims at comparing the δ13CVPDB of mono-aromatic hydrocarbons benzene, toluene, ethylbenzene and xylene isomers (BTEX) measured by elemental analyser (EA)-isotope ratio mass spectrometer (IRMS) with the δ13CVPDB measured on the same compounds by headspace solid phase microextraction – GC/C-IRMS (hSPME – GC/C-IRMS) with the final goal of using these compounds as internal standards on the latter system. The EA-IRMS measurements were done using calcium and lithium carbonate isotopic reference materials: NBS19 and L-SVEC for establishing the δ13CVPDB scale. The EA-IRMS measurements with helium dilution of a set of five reference materials (USGS40, USGS41, IAEA-CH-6, IAEA-CH-3 and IAEA-601) show systematic bias of 1‰ relative to their assigned values. This bias due to the dilution mechanism in the used ConfloII interface device could not be avoided. As the selected hydrocarbons: BTEX could not be analysed by EA-IRMS without helium dilution, their δ13CVPDB must be corrected from this observed bias using an external calibration. The CO2 gas calibrated using EA-IRMS without helium dilution, was used as an in-house reference for the δ13CVPDB measurements of the BTEX by the hSPME – GC/C-IRMS system. The comparison made between the δ13CVPDB measured on the same BTEX compounds by EA-IRMS (with external calibration) and by hSPME – GC/C-IRMS techniques showed good agreement.
Two new organic reference materials for delta13C and delta15N measurements and a new value for the delta13C of NBS 22 oil.
Analytical grade L-glutamic acid is chemically stable and has a C/N mole ratio of 5, which is close to that of many of natural biological materials, such as blood and animal tissue. Two L-glutamic acid reference materials with substantially different 13C and 15N abundances have been prepared for use as organic reference materials for C and N isotopic measurements. USGS40 is analytical grade L-glutamic acid and has a delta13C value of -26.24 per thousand relative to VPDB and a delta15N value of -4.52 per thousand relative to N2 in air. USGS41 was prepared by dissolving analytical grade L-glutamic acid with L-glutamic acid enriched in 13C and 15N. USGS41 has a delta13C value of +37.76 per thousand and a delta15N value of +47.57 per thousand. The delta13C and delta15N values of both materials were measured against the international reference materials NBS 19 calcium carbonate (delta13C=+1.95 per thousand ), L-SVEC lithium carbonate (delta13C=-46.48 per thousand ), IAEA-N-1 ammonium sulfate (delta15N=0.43 per thousand ), and USGS32 potassium nitrate (delta15N=180 per thousand ) by on-line combustion continuous-flow and off-line dual-inlet isotope-ratio mass spectrometry. Both USGS40 and USGS41 are isotopically homogeneous; reproducibility of delta13C is better than 0.13 per thousand, and that of delta15N is better than 0.13 per thousand in 100-microg amounts. These two isotopic reference materials can be used for (i) calibrating local laboratory reference materials, and (ii) quantifying drift with time, mass-dependent fractionations, and isotope-ratio-scale contraction in the isotopic analysis of various biological materials. Isotopic results presented in this paper yield a delta13C value for NBS 22 oil of -29.91 per thousand, in contrast to the commonly accepted value of -29.78 per thousand for which off-line blank corrections probably have not been quantified satisfactorily.
Cryoflow: Cryofocusing Nanomole Amounts of CO2, N2, and SO2 from an Elemental Analyzer for Stable Isotopic Analysis
A manual "cryoflow' system is described for measuring C, N and S isotopic compositions of small gas samples in the 10–1000 nmol range. Using a continuous flow system with helium as the carrier gas, sample gases are injected or produced via combustion in an elemental analyzer, purified, separated and collected in separate traps with cryogens, then degassed into a slowly flowing helium stream (1–4 mL/min) for measurement with an isotope rato mass spectrometer. Blanks associated with combustion of organics in an elemental analyzer were 5–20 nmol for N and C, and not detectable for S, so that samples of >50 nmol C, N or S could be routinely analyzed with a precision of better than 0.3‰ after blank correction. In some cases, slow degassing of sample gases from traps over several minutes gave broad, low-amplitude peaks that yielded high-precision measurements (e.g., <0.05‰ δ13C differences between replicates). Work with this manual prototype showed that it is possible to routinely obtain high precision isotope measurements for C, N and S isotopes from the same small sample.