Mass Spectrometers

Finnigan MAT 252

State-of-the-art ThermoFinnigan MAT isotope ratio mass spectrometers support NIWA research and commercial analyses.

ThermoFinnigan MAT 252

NIWA operates a ThermoFinnigan MAT 252 isotope ratio mass spectrometer in a purpose-built laboratory at Greta Point, Wellington. This instrument is fitted with both dual inlet and continuous flow inlet systems and has been highly modified to provide tailor-made solutions to various aspects of the environmental research undertaken by NIWA and its clients. In particular, modifications have been made to enable high-precision determinations of δ ¹³C in the atmospheric greenhouse gases, methane and carbon dioxide. These data have been widely used in studies of the source structure of the gases as well as models examining global climate change. (See Greenhouse Gases and Climate Change research programme.)

Other research on the instrument includes both marine and freshwater studies as well as investigations of marine carbonates. For these studies the mass spectrometer is equipped with a Kiel device, which provides automated high-precision determinations of δ ¹³C and δ ¹⁸O in individual foraminifera and other carbonate samples.

Contacts – MAT 252 IRMS

Dave Lowe [ d.lowe@niwa.co.nz ]

Gordon Brailsford [ g.brailsford@niwa.co.nz ]

ThermoFinnigan Delta^Plus

The ThermoFinnigan Delta^Plus is a fully automated continuous flow isotope ratio mass spectrometer (CF-IRMS) that is set up to detect concentrations and stable isotope ratios of carbon, nitrogen or sulphur.

Samples are introduced to the Delta^Plus via a Carlo-Erba NA1500 CHN elemental analyser (EA) which processes solid and liquid samples into gas samples before analysis on the mass spectrometer.

Computer software integrates the mass spectrometer output data and calculates % weight, atom % and delta values for each element. Sample throughput for dual C and N analysis is 6 samples per hour enabling analysis of up to 100 samples per day. The Delta^Plus has high sensitivity and excellent linearity, resulting in extremely good reproducibility of measured isotope ratios. Precision on 20 repeat reference gas injections is always better than 0.07 ‰ (Table 1).

Table 1: Precision of 20 repeat reference gas injections of N₂ and CO₂ (Zero Tests).

Repeat analyses of standards, including urea, acetanalide, flour, DL-leucine, and several international standards demonstrate the excellent accuracy and precision of the Delta^Plus. A selection of results is given in Table 2, with some comparisons against commercial analyses. Even for extremely low sample amounts (20 μg N, and 50 μg C) precision was always better than 0.2‰ and usually less than 0.1‰ for both δ¹⁵N and δ¹³C. Six repeat analyses of barium sulphate (30–60 μg S) gave a δ³⁴S precision of 0.06‰, and 10 repeat analyses of sulphanilamide (20–50 μg S) gave a precision of 0.14‰. These precision data compare extremely favourably with published literature values.

Table 2: Delta^Plus precision of δ¹⁵N and δ¹³C analyses of standards run consecutively in a batch analysis.

Research Applications within NIWA

The Delta^Plus mass spectrometer has wide-ranging applications within NIWA’s environmental research programmes and also has many commercial applications. Carbon, nitrogen and sulphur natural abundance variations provide insight into freshwater, marine, and terrestrial ecosystem research as well as atmospheric and agricultural research.

In ecological studies, stable isotopes are an extremely useful determinant of the trophic status of organisms and an indicator of their diet. There is a slight variation in C, N and S stable isotope values between an organism and its food source, which has resulted in the statement “You are what you eat, plus or minus a few per mil”. Nitrogen isotopes are the strongest indicators of trophic status, showing a step-wise enrichment in ¹⁵N of about 3–4‰ at each successive trophic level. By combining C, N and S isotopic values of a consumer, it is possible to deduce its likely dietary intake and resolve complex food web structures.

In the field of atmospheric research, sulphur isotopes are being used to trace atmospheric emissions. Similarly, δ¹⁵N values can be utilised to identify different sources of nitrogen and indicate pollutant pathways. Several natural processes fractionate nitrogen, e.g. inorganic nitrogen assimilation, nitrogen fixation, nitrification, denitrification, ammonification, and other processes of nitrogen metabolism.

A further interesting application of the Delta^Plus is that of forensic sample characterisation. C, N and S isotopic ratios can be utilised to validate the authenticity of substances such as drugs (cocaine, heroin), explosives (TNT) and traces of paint.

A valuable application of stable isotopes is that of tracer or enrichment studies. Such studies enable rates of processes to be directly measured by the incorporation of an artificially-enriched stable isotope. This is an extremely powerful experimental method in environmental studies.

Some examples of specific stable isotope research that NIWA is currently involved in include:

• food web structure in Antarctic freshwater lakes (Antarctic Aquatic Ecosystems)
• crayfish ecology, aquatic insect ecology and stream food webs (River Ecosystems and Land-Use Interactions)
• nutrient dynamics in lakes, riparian systems and estuaries.
• nutrient pollution pathways.
• ecosystem structure across oceanographic frontal zones.
• phytoplankton research linked to ocean-atmosphere flux studies (Drivers and Mitigation of Global Change)
• seabird diet interpretation.
• enrichment studies in marine, freshwater and terrestrial environments.

Contact – Delta^Plus Facility

We provide competitive rates for commercial analysis of stable isotope samples and are enthusiastic about research collaboration. For information, including enquiries relating to collaborative research or commercial analyses, contact:

Sarah Bury [ s.bury@niwa.co.nz ] or phone +64 4 386 0347.

Delta^Plus Publications

A selected list of publications and presentations illustrates NIWA’s recent research using the Delta^Plus mass spectrometer.