Minimization of sample requirement for delta18O in benzoic acid

Natal origin of the invasive biosecurity pest, brown marmorated stink bug (Halyomorpha halys: Penatomidae), determined by dual-element stable isotope-ratio mass spectrometry

BACKGROUND

Post-border detection of a single brown marmorated stink bug (BMSB) in New Zealand warranted a biosecurity response, the nature of which would be influenced by its status as part of an established population or as a new arrival. Stable isotope analysis has the potential to determine natal origins, but is difficult to achieve for samples as small as a single insect. Here an analytical modification to measure small samples was successfully trialled as a means to supply evidence as to the local or exotic natal origin of the intercepted BMSB specimen.

RESULTS

Sufficient analytical sensitivity was achieved using a modified isotope ratio mass spectrometry method, involving thermolysis and carbon monoxide cryofocusing, to enable the simultaneous analysis of δ² H and δ¹⁸ O from wings of the post-border BMSB sample. The values were much lower than those of the New Zealand green vegetable bug, used as a local reference. However, they fell within the range of those for BMSB of Northern Hemisphere origin intercepted at the New Zealand border over the same time period, specifically overlapping with the USA and Italy, but not China.

CONCLUSION

The isotope signature of the post-border detected BMSB suggested a significantly cooler climate than the North Island of New Zealand, indicating that it was a new arrival and did not represent an established population. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Extending the Limit of Measurement for Dual H and O Isotope Ratios Using Thermolysis

Hydrogen and oxygen isotope ratios are of use to determine the origin of matter. Thermolysis is used to convert matter to H₂ and CO gases, which are the respective substrates for measurement of these two isotope ratios, using isotope ratio mass spectrometry (IRMS). This work was done in response to the need for analysis of small invasive insects, requiring a decrease in the limit of measurement for isotope ratiometry of hydrogen and oxygen, while determining both isotope ratios on the same sample. Miniaturization of a thermolysis reactor using commercially available components is presented that results in improvement in the limit of measurement for both hydrogen and oxygen isotope ratios. δ²H was determined on 0.4 μg of H and δ¹⁸O determined on 5 μg of O with precisions of 3 mUr and 0.7 mUr, respectively. To extend the usable sample size range or increase the resolution of sampling gives obvious advantages in forensic and environmental sciences. The technique has been applied to determining the natural origin of Tephritidae fruit flies for which only the wing is suitable for analysis and provides just 60 μg of material for analysis.

Stable carbon isotope analyses of nanogram quantities of particulate organic carbon (pollen) with laser ablation nano combustion gas chromatography/isotope ratio mass spectrometry

RATIONALE

Analyses of stable carbon isotope ratios (δ13 C values) of organic and inorganic matter remains have been instrumental for much of our understanding of present and past environmental and biological processes. Until recently, the analytical window of such analyses has been limited to samples containing at least several μg of carbon.

METHODS

Here we present a setup combining laser ablation, nano combustion gas chromatography and isotope ratio mass spectrometry (LA/nC/GC/IRMS). A deep UV (193 nm) laser is used for optimal fragmentation of organic matter with minimum fractionation effects and an exceptionally small ablation chamber and combustion oven are used to reduce the minimum sample mass requirement compared with previous studies.

RESULTS

Analyses of the international IAEA CH-7 polyethylene standard show optimal accuracy, and precision better than 0.5‰, when measuring at least 42 ng C. Application to untreated modern Eucalyptus globulus (C3 plant) and Zea mays (C4 plant) pollen grains shows a ~ 16‰ offset between these species. Within each single Z. mays pollen grain, replicate analyses show almost identical δ13 C values.

CONCLUSIONS

Isotopic offsets between individual pollen grains exceed analytical uncertainties, therefore probably reflecting interspecimen variability of ~0.5-0.9‰. These promising results set the stage for investigating both δ13 C values and natural carbon isotopic variability between single specimens of a single population of all kinds of organic particles yielding tens of nanograms of carbon. © 2016 The Authors. Rapid Communications in Mass Spectrometry Published by John Wiley & Sons Ltd.

Effects of GC temperature and carrier gas flow rate on on-line oxygen isotope measurement as studied by on-column CO injection

Although deemed important to δ¹⁸ O measurement by on-line high-temperature conversion techniques, how the GC conditions affect δ¹⁸ O measurement is rarely examined adequately. We therefore directly injected different volumes of CO or CO-N₂ mix onto the GC column by a six-port valve and examined the CO yield, CO peak shape, CO-N₂ separation, and δ¹⁸ O value under different GC temperatures and carrier gas flow rates. The results show the CO peak area decreases when the carrier gas flow rate increases. The GC temperature has no effect on peak area. The peak width increases with the increase of CO injection volume but decreases with the increase of GC temperature and carrier gas flow rate. The peak intensity increases with the increase of GC temperature and CO injection volume but decreases with the increase of carrier gas flow rate. The peak separation time between N₂ and CO decreases with an increase of GC temperature and carrier gas flow rate. δ¹⁸ O value decreases with the increase of CO injection volume (when half m/z 28 intensity is <3 V) and GC temperature but is insensitive to carrier gas flow rate. On average, the δ¹⁸ O value of the injected CO is about 1‰ higher than that of identical reference CO. The δ¹⁸ O distribution pattern of the injected CO is probably a combined result of ion source nonlinearity and preferential loss of C¹⁶ O or oxygen isotopic exchange between zeolite and CO. For practical application, a lower carrier gas flow rate is therefore recommended as it has the combined advantages of higher CO yield, better N₂ -CO separation, lower He consumption, and insignificant effect on δ¹⁸ O value, while a higher-than-60 °C GC temperature and a larger-than-100 µl CO volume is also recommended. When no N₂ peak is expected, a higher GC temperature is recommended, and vice versa. Copyright © 2015 John Wiley & Sons, Ltd.

Elimination of nitrogen interference during online oxygen isotope analysis of nitrogen-doped organics using the "NiCat" nickel reduction system

RATIONALE

Accurate online analysis of the δ(18)O values of nitrogen-bearing organic compounds is of interest to several emergent fields, including ecology, forensics and paleontology. During online analysis, high-temperature conversion (HTC) of nitrogen-bearing organics produces N(2) gas which creates isobaric interference with the isotopic measurement. Specifically, N(2) reacts with trace amounts of oxygen in the mass spectrometer source to form (14)N(16)O (m/z 30), which prevents accurate evaluation of the sample (12)C(18)O peak (m/z 30).

METHODS

We present an alternative system to the conventional HTC, which uses a nickel-catalyzed ("NiCat") reduction furnace to convert HTC-produced CO into CO(2), allowing for δ(18)O measurement using signal intensities at m/z 44 and 46.

RESULTS

This system yields identical δ(18)O values for nitrogen-doped and undoped sucrose and cellulose compounds up to molar yield ratios of N(2):CO  =  0.22. In contrast, our conventional HTC system configured to factory recommendations with the stock gas chromatography (GC) column produced a discrepancy of ~5‰ between nitrogen-doped and undoped samples.

CONCLUSIONS

Because of its ability to eliminate isobaric interference, the NiCat system is a viable alternative to conventional HTC for δ(18)O measurement, and can be constructed from relatively inexpensive and readily available materials. As an additional advantage, the CO(2) analyte produced by NiCat may be cryofocused, to allow for oxygen-isotope determinations on very small amounts of sample substrate.