Methods to reduce interference effects in thermal conversion elemental analyzer/continuous flow isotope ratio mass spectrometry δ¹⁸O measurements of nitrogen‐containing compounds

Abstract

On-line determination of the oxygen isotopic composition (delta(18)O value) in organic and inorganic samples is commonly performed using a thermal conversion elemental analyzer (TC-EA) linked to a continuous flow isotope ratio mass spectrometry (IRMS) system. Accurate delta(18)O analysis of N-containing compounds (like nitrates) by TC-EA-IRMS may be complicated because of interference of the N(2) peak on the m/z 30 signal of the CO peak. In this study we evaluated the effectiveness of two methods to overcome this interference which do not require any hardware modifications of standard TC-EA-IRMS systems. These methods were (1) reducing the amount of N(2) introduced into the ion source through He dilution of the N(2) peak and (2) an improved background correction on the CO m/z 30 sample peak integration. Our results show that He dilution is as effective as diverting the N(2) peak in order to eliminate this interference. We conclude that the He-dilution technique is a viable method for the delta(18)O analysis of nitrates and other N-containing samples (which are not routinely measured using He dilution) using TC-EA-IRMS, since it can easily be programmed in the standard software of IRMS systems. With the He-dilution technique delta(18)O values of the nitrate isotope standards USGS34, IAEA-N3 and USGS35 were measured using the shortest possible traceability chain to the VSMOW-SLAP scale, and the results were -28.1 +/- 0.1 per thousand, +25.5 +/- 0.1 per thousand and +57.5 +/- 0.2 per thousand, respectively. An improved background correction was also an effective method, but required manual correction of the raw data.

Water isotopic ratios from a continuously melted ice core sample

“…More recently, in combination with the use of continuous flow mass spectrometers, conversion of water to CO and H 2 is performed in a pyrolysis furnice (Begley and Scrimgeour, 1997) and allows simultaneous δD and δ 18 O measurement, but still on a single discrete sample. One of the drawbacks of this technique is the interference of NO , formed at the ion source by the reaction of N 2 and O 2 with the CO signal at m/z = 30 (Accoe, 2008). Nowadays, commercial IR spectrometers are available with a precision comparable to IRMS systems (Lis et al, 2008;Brand et al, 2009).…”

Stable isotope analyses of oxygen (18O:17O:16O) and chlorine (37Cl:35Cl) in perchlorate: reference materials, calibrations, methods, and interferences

“…As a final step in the calibration process, the O2‐DIIRMS model values were normalized to be consistent with our previous determination of NO 3 − isotopic reference values . This selection is supported by another independent calibration study for NO 3 − , it has been adopted for high‐precision analytical developments and applications for NO 3 − at USGS and elsewhere, and it is consistent with normalization schemes used for ClO 4 − isotope data reported previously . As illustrated in Fig.…”

“…For H 2 O, we used VSMOW and SLAP with their accepted δ 18 O values, plus another H 2 O reference with a higher δ 18 O value from an independent O isotope calibration study . For NO 3 − , we used USGS34 and USGS35 with three different sets of reported δ 18 O values . The data in Table indicate that the differences among the six different normalization schemes were not much larger than the reproducibilities for individual compounds in single batches.…”

“…Upper left block gives mean measured values of δ 18 O with respect to (wrt) the machine reference gas (rg) analyzed on four different days (batches 1–4); n = number of individual aliquots analyzed in each batch. Upper right block gives mean values of δ 18 O after daily normalization ( n = 4 batches) using six alternative pairs of H 2 O and NO 3 − isotopic reference materials and calibration values, as indicated by bold italic entries: VSMOW =0; SLAP = −55.5; W89262 = +78.89; USGS34 = −27.93 [37] or −27.78 [35] or −28.12; USGS35 = +57.5 [37] or +56.81 [35] or +57.42 . Two columns on the right (bold entries) give the overall unweighted composite mean values based on the preceding columns ( n = 6 alternative normalized means).…”

Gas Source Isotope Ratio Mass Spectrometry (IRMS)

“…Compared to plain oxidation with O 2 , the high-temperature reduction technique is chemically less well constrained. [100][101][102] The reactions of sample material containing nitrogen are not well understood and postconversion reactions like recombination between CO and H 2 may occur. Numerous gaseous byproducts (e.g.…”