Measuring d13C to High Precision within Large Concentrations

Bulk stable isotope analysis of a wide variety of organic and inorganic materials for C, N, H, and S & O is generally carried out over a varied assortment of applications. For multi-element analysis from a single sample, the dynamic range of the instrumentation must be great, as organic substances usually have large C/N and C/S ratios.

With a mixture of Isoprime's experience within isotope ratio alongside the forward-thinking technology of Elementar's elemental analyzers, the isoprime visION and isoprime precisION devices offer an outstanding opportunity to examine samples with high elemental ratios.

Elementar’s elemental analyzers are the only systems able to execute truly multi-elemental isotope analysis from just one sample. The exclusive Advanced Purge and Trap technique (APT) in combination with their high dynamic range inimitably offer easy stable isotope analysis of CHNS of substances with large differences in element concentration at the same time.

It is simple to increase the sample weight to examine substances with a very low S or N content. The result of this larger sample weight is that the more plentiful elements reach a greater concentration. To achieve this, the isoprime visION and isoprime precisION devices boast an amplifier with twice the range of any other instrument along with a fully automatic sample gas dilution system.

This piece will establish how the centrION dilution system permits the simple analysis of a wide variety of elemental concentrations without conceding performance, ensuring great accuracy even in the most challenging analytical trials.

Advanced Purge and Trap (APT) Technology

Elementar’s exclusive APT technology is a pioneering chromatographic method for the quantitative separation of combustion gases. It makes use of up to three gas-selective trapping columns that adsorb CO2, H2O, and SO2 in particular while allowing clearance to other combustion gases. Once the column is heated, the adsorbed gas is let go.

The quick pace of heating for the gas desorption guarantees sharp peaks and exceptional signal-to-noise ratios, leading to revolutionary levels of precision and sensitivity (Figure 1). Unlike with isothermal GC-based analyzers, a total distinction of peaks is always attained, even at greater ratios of elemental content and elemental concentration.

Example of the peak focusing effect of the APT technology for δ34S analysis of a 0.5 mg (GC) and 0.3 mg (APT) barium sulfate sample.

Figure 1. Example of the peak focusing effect of the APT technology for δ34S analysis of a 0.5 mg (GC) and 0.3 mg (APT) barium sulfate sample.

The APT technology has been demonstrated to be clear of isotopic fractionation and is when combined with mass spectrometric detection, able to resolve C/N and C/S ratios of up to 10,000: 1. Moreover, in comparison to GC columns, APT columns can be loaded up to 250 times higher, leading to exceptional sample flexibility.

The higher quality peak-shapes and greater sensitivity response of the APT technology requires an amplifier with a powerful range and automated dilution of the sample gas.

CentrION CF Interface System

CentrION is a continuous flow (CF) interface system which manages sample gases transported by the elemental analyzer automatically. With the application of high precision mass flow controllers under totally automated control, the currents of helium and monitoring gases can be managed with a great degree of precision.

CentrION is made up of two key elements:

  • The monitoring gas injection structure conveys up to six monitoring gases into the ion source for performance examination.
  • The sample dilution function condenses high concentration sample gases from the elemental analyzer using designated sample types.

Stable isotope ratio analysis makes use of the delicate deviations in isotope abundance stemming from chemical, biological or physical procedures. It is vital that the analytical equipment itself does not yield any fractionation, as this can prejudice the stable isotope results.

CentrION has been developed with great care to guarantee that the dilution function adds absolutely no isotope fractionation through its use of a wide variety of dilution systems.

Performance of the Dilution System

A carefully regulated glutamic acid standard of known isotopic composition was primed over a 20 fold range in sample sizes and examined. The sample gas was kept undiluted for the smallest sample weight (dilution rate = 0), while for larger sample weights, glutamic acid was examined over a range of dilution regimes, so that a comparable signal for the isotope ratio studies was attained.

Dilution of the sample gas was only carried out for the δ13C analyses. The findings of the δ13C studies are illustrated in Figure 2 and Table 1. The results demonstrate that the δ13C ratio of glutamic acid could be determined with a high degree of accuracy.

δ13C analyses of glutamic acid samples with different sample weights and dilution rates. The red markers depict the individual analyses, the solid line the mean value of all analyses and the dotted line the standard 2-sigma expected precision of δ13C isotope ratio analyses. The analyses show an exceptional precision over a very large range in sample sizes.

Figure 2. δ13C analyses of glutamic acid samples with different sample weights and dilution rates. The red markers depict the individual analyses, the solid line the mean value of all analyses and the dotted line the standard 2-sigma expected precision of δ13C isotope ratio analyses. The analyses show an exceptional precision over a very large range in sample sizes.

Table 1. δ13C analyses of glutamic acid samples with different sample weights and dilution rates.

No variances were detected between the δ13C isotope ratio without dilution of the sample gas and the ratios for altered dilution rates, indicating that the diluter does not fractionate the sample gas.

The minimal standard deviation of the whole series of glutamic acid analyses (0.04), demonstrates the outstanding performance of the centrION system for a wide variety of sample weights. For the sample weights illustrated in Table 1, the capacity of the APT column for CO2 is far from exhausted.

To illustrate, for the examination of δ34S in wood with a sulfur content down to 50 ppm, 40 mg wood was studied, matching with an absolute carbon content of around 20 mg. Despite a great rate of dilution, these studies will automatically lead to comparatively large CO2 peaks, which cause no issues for the 100 V amplifier integrated into isoprime visION and isoprime precisION.

Dependable Dilution

Isotope ratios of samples with extremely high C/N or C/S ratios can easily be studied in a single analysis run, with confidence, thanks to the isoprime visION and isoprime precisION devices, with a built-in centrION dilution system.

A great sample weight must be conveyed into the instrument to allow for a reliable signal to noise ratio for the element with the lowest concentration. For elemental analyzers which use isothermal chromatographic gas separation, as opposed to the cutting-edge purge and trap technology of the isoprime visION and isoprime precision, this would cause issues.

To avoid overburdening the amplifier during isotope ratio measurements of, e.g., high carbon concentrations, the sample gas for the δ13C analyses can be diluted without any impact on the isotope ratio results, allowing the operator a greater degree of freedom in how they opt to carry out their analysis.

The outstanding performance of the centrION dilution system ensures that the studies of samples with great variations in element concentrations can be carried out in a single analysis run. Using varied dilution rates for diverse compounds, up to four stable isotope ratios (δ13C, δ15N, δ34S, and δ2 H) can be examined from a single sample.

This makes sample preparation and analysis far less time-consuming, resulting in considerably lower running costs and enhanced instrument output.

Further Reading

For more information, please explore these technical notes on the advanced purge and trap technique and the large dynamic range amplifier:

  • TN considerations of dynamic range in stable isotope analysis - part 1: intrinsic isotope abundance
  • TN Advanced Purge and Trap Technology

This information has been sourced, reviewed and adapted from materials provided by Elementar Analysensysteme GmbH.

For more information on this source, please visit Elementar Analysensysteme GmbH.

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