Membrane permeation continuous‐flow isotope ratio mass spectrometry for on‐line carbon isotope ratio determination

Gaseous membrane permeation (MP) technologies have been combined with continuous‐flow isotope ratio mass spectrometry for on‐line δ¹³C measurements. The experimental setup of membrane permeation‐gas chromatography/combustion/isotope ratio mass spectrometry (MP‐GC/C/IRMS) quantitatively traps gas streams in membrane permeation experiments under steady‐state conditions and performs on‐line gas transfer into a GC/C/IRMS system. A commercial polydimethylsiloxane (PDMS) membrane sheet was used for the experiments. Laboratory tests using CO₂ demonstrate that the whole process does not fractionate the C isotopes of CO₂. Moreover, the δ¹³C values of CO₂ permeated on‐line give the same isotopic results as off‐line static dual‐inlet IRMS δ¹³C measurements. Formaldehyde generated from aqueous formaldehyde solutions has also been used as the feed gas for permeation experiments and on‐line δ¹³C determination. The feed‐formaldehyde δ¹³C value was pre‐determined by sampling the headspace of the thermostated aqueous formaldehyde solution. Comparison of the results obtained by headspace with those from direct aqueous formaldehyde injection confirms that the headspace sampling does not generate isotopic fractionation, but the permeated formaldehyde analyzed on‐line yields a ¹³C enrichment relative to the feed δ¹³C value, the isotopic fractionation being 1.0026 ± 0.0003. The δ¹³C values have been normalized using an adapted two‐point isotopic calibration for δ¹³C values ranging from ?42 to ?10‰. The MP‐GC/C/IRMS system allows the δ¹³C determination of formaldehyde without chemical derivatization or additional analytical imprecision. Copyright © 2009 John Wiley & Sons, Ltd.     

Validation of deuterium and oxygen18 in urine and saliva samples from children using on‐line continuous‐flow isotope ratio mass spectrometry

The doubly labelled water method is valuable for measuring energy expenditure in humans. It usually involves blood or urine sampling, which might be difficult in neonates and children with cerebral palsy or other disabilities. We therefore aimed to validate a method making use of saliva samples analyzed by automated thermal conversion elemental analyzer in combination with isotope ratio mass spectrometry (TC‐EA/IRMS). The subjects received labelled water orally and urine and saliva samples were collected and analyzed. Deuterium as well as oxygen¹⁸ was measured in one single run using a peak jump method. Excellent linearity was found for measurement of enrichments of deuterium (R² = 0.9999) and oxygen¹⁸ (R² = 0.9999). The intra‐assay precision and the inter‐assay precision of the measurement of two standards were good for both deuterium and oxygen¹⁸. The variation between urine and saliva samples was small (4.83% for deuterium and 2.33% for oxygen¹⁸ n = 40). Saliva sampling is to be preferred, therefore, as it can be easily collected and is non‐invasive. Moreover, its time of production is almost exactly known. The TC‐EA/IRMS method is a good alternative to the more laborious off‐line IRMS measurements. Copyright © 2009 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 δ¹⁸O measurement by on‐line high‐temperature conversion techniques, how the GC conditions affect δ¹⁸O 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 δ¹⁸O 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. δ¹⁸O 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 δ¹⁸O value of the injected CO is about 1‰ higher than that of identical reference CO. The δ¹⁸O distribution pattern of the injected CO is probably a combined result of ion source nonlinearity and preferential loss of C¹⁶O 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 δ¹⁸O 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.     

N2: a potential pitfall for bulk 2H isotope analysis of explosives and other nitrogen‐rich compounds by continuous‐flow isotope‐ratio mass spectrometry

Observations made during the ¹³C isotope analysis of gaseous CO₂ in the simultaneous presence of argon in the ion source of the isotope ratio mass spectrometer prompted us to investigate what influence the simultaneous presence of nitrogen would have on both accuracy and precision of bulk ²H isotope analysis of nitrogen‐rich organic compounds. Initially an international reference material, IAEA‐CH7, was mixed with silver nitrate in various ratios to assess the impact that N₂ evolved from the pyrolysis of nitrogen‐rich organic compounds would have on measured δ²H‐values of IAEA‐CH7. In a subsequent experiment, benzoic acid was mixed with silver nitrate to mimic the N:H ratio of organic‐rich nitrogen compounds such as cellulose nitrate and RDX. The results of both experiments showed a significant deterioration of both accuracy and precision for the expected δ²H values for IAEA‐CH7 and benzoic acid when model mixtures were converted into hydrogen and nitrogen, and subsequently separated by gas chromatography using standard experimental conditions, namely a 60 cm packed column with molecular sieve 5 Å as stationary phase held at a temperature of 85°C. It was found that bulk ²H stable isotope analysis of nitrogen‐rich organic compounds employing published standard conditions can result in a loss of accuracy and precision yielding δ²H values that are 5 to 25‰ too negative, thus suggesting, for example, that tree‐ring ²H isotope data based on cellulose nitrate may have to be revised. Copyright © 2009 John Wiley & Sons, Ltd.     

Sample vial influences on the accuracy and precision of carbon and oxygen isotope ratio analysis in continuous flow mass spectrometric applications

Due to the small amounts of sample gas involved in continuous flow mass spectrometric analysis, care should be taken to evaluate the influence of sample containers on the carbon and oxygen isotope ratios of samples. Data indicate that Na-glass and borosilicate glass vials, equipped with butyl rubber septa, can cause significant changes in the isotopic composition of CO(2) gas, even where sample gases are stored within the vial for less than one day. The magnitude of these changes varies from vial to vial. Given the leverage that contamination can potentially exert on small gas samples, each researcher should carefully evaluate the effect of sample vials in order to eliminate unknown and unwanted changes in the composition of samples. Copyright 2000 John Wiley & Sons, Ltd.     

Analysis of low‐concentration gas samples with continuous‐flow isotope ratio mass spectrometry: eliminating sources of contamination to achieve high precision

Developments in continuous‐flow isotope ratio mass spectrometry have made possible the rapid analysis of δ¹³C in CO₂ of small‐volume gas samples with precisions of ≤0.1‰. Prior research has validated the integrity of septum‐capped vials for collection and short‐term storage of gas samples. However, there has been little investigation into the sources of contamination during the preparation and analysis of low‐concentration gas samples. In this study we determined (1) sources of contamination on a Gasbench II, (2) developed an analytical procedure to reduce contamination, and (3) identified an efficient, precise method for introducing sample gas into vials. We investigated three vial‐filling procedures: (1) automated flush‐fill (AFF), (2) vacuum back‐fill (VBF), and (3) hand‐fill (HF). Treatments were evaluated based on the time required for preparation, observed contamination, and multi‐vial precision. The worst‐case observed contamination was 4.5% of sample volume. Our empirical estimate showed that this level of contamination results in an error of 1.7‰ for samples with near‐ambient CO₂ concentrations and isotopic values that followed a high‐concentration carbonate reference with an isotope ratio of ?47‰ (IAEA‐CO‐9). This carry‐over contamination on the Gasbench can be reduced by placing a helium‐filled vial between the standard and the succeeding sample or by ignoring the first two of five sample peaks generated by each analysis. High‐precision (SD ≤0.1‰) results with no detectable room‐air contamination were observed for AFF and VBF treatments. In contrast, the precision of HF treatments was lower (SD ≥0.2‰). VBF was optimal for the preparation of gas samples, as it yielded faster throughput at similar precision to AFF. Copyright © 2009 John Wiley & Sons, Ltd.     

Can we use the CO2 concentrations determined by continuous‐flow isotope ratio mass spectrometry from small samples for the Keeling plot approach?

A common method to estimate the carbon isotopic composition of soil‐respired air is to use Keeling plots (δ¹³C versus 1/CO₂ concentration). This approach requires the precise determination of both CO₂ concentration ([CO₂]), usually measured with an infrared gas analyser (IRGA) in the field, and the analysis of δ¹³C by isotope ratio mass spectrometry (IRMS) in the laboratory. We measured [CO₂] with an IRGA in the field (n = 637) and simultaneously collected air samples in 12 mL vials for analysis of the ¹³C values and the [CO₂] using a continuous‐flow isotope ratio mass spectrometer. In this study we tested if measurements by the IRGA and IRMS yielded the same results for [CO₂], and also investigated the effects of different sample vial preparation methods on the [CO₂] measurement and the thereby obtained Keeling plot results. Our results show that IRMS measurements of the [CO₂] (during the isotope analysis) were lower than when the [CO₂] was measured in the field with the IRGA. This is especially evident when the sample vials were not treated in the same way as the standard vials. From the three different vial preparation methods, the one using N₂‐filled and overpressurised vials resulted in the best agreement between the IRGA and IRMS [CO₂] values. There was no effect on the ¹³C‐values from the different methods. The Keeling plot results confirmed that the overpressurised vials performed best. We conclude that in the cases where the ranges of [CO₂] are large (>300 ppm; in our case it ranged between 70 and 1500 ppm) reliable estimation of the [CO₂] with small samples using IRMS is possible for Keeling plot application. We also suggest some guidelines for sample handling in order to achieve proper results. Copyright © 2008 John Wiley & Sons, Ltd.     

Adaptation of continuous‐flow cavity ring‐down spectroscopy for batch analysis of δ13C of CO2 and comparison with isotope ratio mass spectrometry

Measurements of δ¹³C in CO₂ have traditionally relied on samples stored in sealed vessels and subsequently analyzed using magnetic sector isotope ratio mass spectrometry (IRMS), an accurate but expensive and high‐maintenance analytical method. Recent developments in optical spectroscopy have yielded instruments that can measure δ¹³CO₂ in continuous streams of air with precision and accuracy approaching those of IRMS, but at a fraction of the cost. However, continuous sampling is unsuited for certain applications, creating a need for conversion of these instruments for batch operation. Here, we present a flask (syringe) adaptor that allows the collection and storage of small aliquots (20–30 mL air) for injection into the cavity ring‐down spectroscopy (CRDS) instrument. We demonstrate that the adaptor's precision is similar to that of traditional IRMS (standard deviation of 0.3‰ for 385 ppm CO₂ standard gas). In addition, the concentration precision (±0.3% of sample concentration) was higher for CRDS than for IRMS (±7% of sample concentration). Using the adaptor in conjunction with CRDS, we sampled soil chambers and found that soil‐respired δ¹³C varied between two different locations in a piñon‐juniper woodland. In a second experiment, we found no significant discrimination between the respiration of a small beetle (~5 mm) and its diet. Our work shows that the CRDS system is flexible enough to be used for the analysis of batch samples as well as for continuous sampling. This flexibility broadens the range of applications for which CRDS has the potential to replace magnetic sector IRMS. Copyright © 2011 John Wiley & Sons, Ltd.     

Nicotine,acetanilide and urea multi‐level 2H‐, 13C‐ and 15N‐abundance reference materials for continuous‐flow isotope ratio mass spectrometry

Accurate determinations of stable isotope ratios require a calibration using at least two reference materials with different isotopic compositions to anchor the isotopic scale and compensate for differences in machine slope. Ideally, the δ values of these reference materials should bracket the isotopic range of samples with unknown δ values. While the practice of analyzing two isotopically distinct reference materials is common for water (VSMOW‐SLAP) and carbonates (NBS 19 and L‐SVEC), the lack of widely available organic reference materials with distinct isotopic composition has hindered the practice when analyzing organic materials by elemental analysis/isotope ratio mass spectrometry (EA‐IRMS). At present only L‐glutamic acids USGS40 and USGS41 satisfy these requirements for δ¹³C and δ¹⁵N, with the limitation that L‐glutamic acid is not suitable for analysis by gas chromatography (GC). We describe the development and quality testing of (i) four nicotine laboratory reference materials for on‐line (i.e. continuous flow) hydrogen reductive gas chromatography‐isotope ratio mass‐spectrometry (GC‐IRMS), (ii) five nicotines for oxidative C, N gas chromatography‐combustion‐isotope ratio mass‐spectrometry (GC‐C‐IRMS, or GC‐IRMS), and (iii) also three acetanilide and three urea reference materials for on‐line oxidative EA‐IRMS for C and N. Isotopic off‐line calibration against international stable isotope measurement standards at Indiana University adhered to the ‘principle of identical treatment’. The new reference materials cover the following isotopic ranges: δ²H_nicotine ?162 to ?45‰, δ¹³C_nicotine ?30.05 to +7.72‰, δ¹⁵N_nicotine ?6.03 to +33.62‰; δ¹⁵N_acetanilide +1.18 to +40.57‰; δ¹³C_urea ?34.13 to +11.71‰, δ¹⁵N_urea +0.26 to +40.61‰ (recommended δ values refer to calibration with NBS 19, L‐SVEC, IAEA‐N‐1, and IAEA‐N‐2). Nicotines fill a gap as the first organic nitrogen stable isotope reference materials for GC‐IRMS that are available with different δ¹⁵N values. Comparative δ¹³C and δ¹⁵N on‐line EA‐IRMS data from 14 volunteering laboratories document the usefulness and reliability of acetanilides and ureas as EA‐IRMS reference materials. Published in 2009 by John Wiley & Sons, Ltd.