RESUMEN
RATIONALE: We report modifications to a commercial elemental analyzer-isotope ratio mass spectrometer that permit high-precision isotopic analysis of nanomoles of carbon (C), nitrogen (N), and sulfur (S) on a single sample without chemical or cryogenic trapping of gases. The sample size required for measurement by our system is about two orders of magnitude less than that for conventional analyses. METHODS: Our system builds on the analytical advancements offered by the EA IsoLink IRMS System and employs simple modifications to reduce the diameter of the flow path (reactors, water trap, and transfer lines), enhance peak separation (gas chromatography capillary column), and improve sample transfer to the ion source of the mass spectrometer (reduced flow rates). RESULTS: Conventional precision (<0.2) can be achieved down to c. 500 nmol C, N, and S for samples analyzed without modification of the commercial system. Further reduction in sample size (<50 nmol C, N, and S) was achieved with minor modifications. There is a significant carbon blank and a small nitrogen blank that can be measured directly and a sulfur blank that can be calculated using regression. Only 30 nmol of N, 22 nmol of C, and 12 nmol of S are needed to achieve better than 1 precision (1σ) from a single measurement. Larger samples and more replicate measurements provide better precision. CONCLUSIONS: The nano-EA method described here reduces sample size requirements by two orders of magnitude compared to traditional approaches and improves the accuracy and precision of isotope measurements on sample sizes less than 1 µmol. These advancements simplify the analytical technique and broaden the range and type of samples amenable to EA analysis.
RESUMEN
RATIONALE: We report modifications to compound-specific isotope analyses (CSIA) to enable high-precision isotopic analyses of picomoles of carbon for intact organic molecules. This sample size is two orders of magnitude below the amounts required for commercial systems. The greatly enhanced sensitivity of this system expands molecular isotope studies and applications previously prohibited by low concentrations and small samples. METHODS: We utilize the resolving power and low volumetric flow rates of narrow-bore capillary gas chromatography to improve sample transfer efficiency while maintaining narrow peak widths. Post-column peak broadening is minimized using a micro-fluidic valve for solvent diversion, capillary combustion reactor, narrow-bore capillary transfer lines, and cryogenic water trap. The mass spectrometer was fitted with collector amplifiers configured to 25 ms response times and a data logger board with firmware capable of rapid data acquisition. Carbon dioxide gas was introduced directly into the ion source to evaluate the dynamic range of the system and accuracy and precision of carbon isotope ratio (δ13 C value) measurements. The accuracy and precision for combusted compounds were evaluated using a suite of n-alkanes. RESULTS: For ≥30 pmol carbon introduced directly into the ion source, the mean difference between the measured and expected δ13 C values is 0.03 (1σ, n = 57) and the standard deviation of replicate measurements is 0.11 (1σ). The CO2 peak widths generated by the exponential dilution flask were 250 ms and the peak widths produced by combusting n-alkanes were ca 500 ms, less than 25% the width of conventional gas chromatography peaks. For a mixture of 15 n-alkanes (n-C16 to n-C30 ), the accuracy is 0.3 (1σ) and precision is 0.9 (1σ) for replicate δ13 C measurements with 100 pmol carbon per compound on column. CONCLUSIONS: The pico-CSIA method described here offers improved chromatographic resolution and reduces sample size requirements by two orders of magnitude. These advances significantly broaden the available analytical window for CSIA in research areas frequently hindered by sample size limitations, such as forensics, paleoclimate, astrobiology, and biochemistry.
