Advanced Purge and Trap (APT) Technology

Advanced Purge and Trap (APT) Technology for CHNS Analysis — Technical Note Summary

Importance of the topic

The simultaneous, accurate determination of carbon, hydrogen, nitrogen and sulfur (CHNS) in organic matrices is a cornerstone measurement in environmental, materials, energy, agricultural and quality-control laboratories. Reliable elemental results are essential for stoichiometry, impurity tracking, regulatory compliance and material characterization. Methods that expand sample types, improve sensitivity (especially for sulfur), and increase automation and throughput therefore deliver direct operational and scientific value.

Objectives and overview of the technical note

This technical note presents Elementar’s proprietary Advanced Purge and Trap (APT) chromatographic approach implemented in the vario EL cube, vario MACRO cube and vario MAX cube elemental analyzers. The document outlines the APT principle, instrument configurations, analytical performance (sensitivity, peak separation, sample weight range) and practical advantages compared to conventional gas chromatography (GC)-based gas separation for high-temperature combustion (HTC) CHNS analysis.

Methodology and analytical principle

- Combustion workflow:

• Sample combustion occurs at furnace temperatures >1000 °C in oxygen, converting C to CO2, H to H2O, S to SO2 and generating nitrogen oxides that are reduced to N2 downstream.
• Carrier gases: historically helium, with argon increasingly used due to helium scarcity.

- APT gas separation:

• Three gas-selective trapping columns separate CO2, H2O and SO2 by selective adsorption; N2 is non-adsorbed and elutes first to the thermal conductivity detector (TCD).
• Each trap is heated sequentially only after completion and recognition of the previous peak by the detector, enforcing baseline-separated peaks independent of concentration ratios.
• Fast desorption heating rates produce sharp peaks and high signal-to-noise ratios, mitigating peak broadening typical of long-retention GC separations.

- Detection strategies:

• Thermal conductivity detector (TCD) for quantitative detection (common approach).
• Optional gas-specific infrared (IR) detector for SO2 to achieve enhanced sulfur detection limits, and a trace SN cube configuration for ultra-trace sulfur down to <10 ppb.

Used instrumentation

Elementar instruments employing APT: vario EL cube, vario MACRO cube and vario MAX cube. Key features by model (summarized):

- vario EL cube: optimized for small, highly homogeneous samples (e.g., chemicals, pharmaceuticals, polymers); typical workable sample masses around tens of mg; suitable for CHN and CHNS modes.
- vario MACRO cube: designed for moderate sample masses (∼150 mg maximum C content) and medium homogeneity matrices such as coal and biomass; balances representative sampling and sensitivity.
- vario MAX cube: targets large, inhomogeneous samples (up to ∼500 mg C content, samples up to several grams) and includes automated sample introduction and ash removal with a robotic arm for high robustness and throughput.

Main results and discussion

- Baseline separation and dynamic range:

• APT provides complete control over sequential desorption so each combustion gas is baseline-separated regardless of elemental ratios; experimentally demonstrated C:N ratios up to 12,000:1 were quantified without interference.

- Sulfur sensitivity and peak focusing:

• The SO2-selective trap and rapid desorption minimize peak broadening typically seen in long GC retention; this produces sharp SO2 peaks and improves detection limits.
• Using an IR detector, absolute sulfur masses as low as ~2 µg were reported (relative LOD ≈ 2 ppm); trace analyzer variants reach <10 ppb S.

- Sample mass flexibility:

• APT trap capacities and fast desorption enable analysis of samples ranging from low mg to grams without gas splitting or dilution, preserving sensitivity and minimizing contaminant effects.

- Robustness and longevity:

• APT trapping columns are optimized for quantitative adsorption and engineered for longevity compared with conventional GC columns; Elementar offers extended warranties on furnace and TCD components.

Benefits and practical applications

- High accuracy and automated throughput: reliable baseline separation facilitates straightforward, automated data processing for high-volume labs and 24/7 operation.
- Wide matrix compatibility: solids, viscous liquids, volatiles and gases can be analyzed with minimal prep and without compromising sensitivity.
- Enhanced sulfur analysis: focused SO2 peaks and IR detection support trace sulfur quantification relevant to environmental monitoring, fuel analysis, materials and contaminant tracking.
- Flexible sampling strategy: instruments tuned for low to very large sample masses allow representative analysis of homogeneous laboratory samples to heterogeneous environmental specimens.

Future trends and potential applications

- Broader adoption of argon as carrier gas in response to helium supply constraints, with implications for detector response optimization and method validation.
- Combining APT with additional gas-specific detectors or mass-spectrometric coupling to expand specificity (e.g., isotope analysis, species-resolved sulfur/nitrogen characterization).
- Increased automation and robotics for sample handling and ash removal in high-throughput environments, extending applicability to large-scale environmental and industrial screening.
- Miniaturization and modular trap designs to tailor capacity and sensitivity for niche trace analyses or portable field-deployable systems.

Conclusion

Elementar’s APT technology represents a distinct improvement over conventional GC separation for CHNS analysis by delivering baseline-separated, high-resolution peaks, extended dynamic range, enhanced sulfur sensitivity and broad sample mass flexibility. These features translate into higher laboratory efficiency, reliable quantification across challenging elemental ratios, and practical benefits for diverse analytical laboratories.

References

- Elementar technical note: Advanced Purge and Trap (APT) Technology, Art.-No. 05 005 069, 01/2019 A
- Elementar instrument family: vario EL cube, vario MACRO cube, vario MAX cube, trace SN cube