An Introduction to Direct Temperature Programmed Desorption (Direct TPD)

An important parameter which is required in manifold applications is the determination of the carbon, hydrogen, nitrogen and sulfur content in organic matrices. The optimum analysis technique for quantifying these four elements at the same time is high temperature combustion (HTC) in an oxygenated atmosphere.

Throughout the combustion process, carbon is oxidized to carbon dioxide, sulfur to sulfur dioxide, and hydrogen to water. Subsequently, the nitrogen oxides which are created during the combustion process are reduced to elemental nitrogen gas in a downstream reduction furnace. Helium has been the most popular inert carrier gas chosen historically, but because of possible helium shortages argon has also become more common in recent years.

After the combustion and reduction steps, the detection of the analysis gases produced (N2, SO2, CO2, and H2O) can be done in two different ways:

  • A series of separate infrared and thermal conductivity detectors for a gas-specific quantification
  • Quantitative separation of the combustion gases followed by quantification with a thermal conductivity detector (the most common approach)

The high temperature combustion analysis technique allows for outstanding flexibility regarding the types of matrices that can be analyzed and requires minimal sample preparation efforts. Any organic matrix is achievable in any form (solid, liquid, gaseous or viscous). Unlike other methodologies, an additional advantage is that a high degree of automation is attainable in HTC analysis, permitting unattended 24/7 operation and high sample throughput.

Efficient Gas Separation

Elementar has invented the patented direct Temperature Programmed Desorption (direct TPD) technology. It is a high performance gas separation method which combines the simplicity of gas-chromatographic separation with the advantages of gas-specific adsorption. This results in a unique chromatographic technique with best limit of accuracy and detection for CHNS+O analysis in the industry and an extraordinary dynamic measurement range.

Figure 1 shows a schematic setup of an elemental analyzer equipped with the direct TPD technology. The direct TPD technology utilizes a high capacity trapping column that selectively adsorbs SO2 CO2, and H2O. N2 reaches the thermal conductivity detector first and is not adsorbed (Figure 2a). The peak is identified and the software recognizes the return to baseline as the start signal for the identification of the next species of gas.

The direct TPD adsorption column is signaled to heat up to the first out of three temperature ramps, releasing the CO2 at a set temperature which is high enough to desorb all CO2, but low enough to still keep the combustion gases that remain (Figure 2b). In the same way, the direct TPD column consecutively releases H2O and lastly SO2 when the second and third temperature ramps are prompted (Figure 2c and 2d).

Functional scheme of UNICUBE.

Figure 1. Functional scheme of UNICUBE.
* Temperature Programmed Desorption
** Thermoconductivity Detector

Peak graph of CHNS analysis of sulfanilic acid with UNICUBE.

Figure 2. Peak graph of CHNS analysis of sulfanilic acid with UNICUBE.

Sample Throughput and Capacity

Through measurement of the gas temperature directly inside the gas stream the temperature control of the direct TPD column is realized and so responds quickly to start and stop signals from the peak integration. The outcome of this is that the highly optimized direct TPD adsorption column can adsorb up to 14 mg of carbon in CHNS mode and up to 50 mg of carbon in CN mode, which is around four times the capacity of general gas chromatography (GC) columns.

Additionally, the direct temperature control makes the analytical sequence faster and so it is possible to complete a simultaneous CHNS determination in under seven minutes (Figure 2). Thanks to system uptime and robustness when utilizing equipment from Elementar, sample throughput has always been high, but the maximum possible daily sample throughput is now increasing in CHNS mode to an impressive amount of 200 samples.

Photograph of UNICUBE`s direct TPD gas adsorption column

Figure 3. Photograph of UNICUBE`s direct TPD gas adsorption column.

Best Peak Shapes

As a beneficial side effect, the accelerated desorption assures sharp peaks thanks to the fast heating rate of the direct TPD column, higher baseline separation and peak height, which results in excellent signal-to noise ratios and exceptionally low limits of quantification.

This is particularly important for the detection of sulfur and hydrogen, which are influenced by substantial peak tailing and peak broadening when GC-based separation is utilized in CHNS mode. In contrast, even for extreme elemental ratios, the direct TPD technology permits peak separation. Even challenging C/S and C/N elemental ratios of up to 12,000:1 can consequently be quantified easily (Figure 4).

The distinct peak separation assures absolutely reliable and trouble-free data acquisition. The analysis run can therefore easily be automated for larger sample amounts while maintaining utmost analytical performance. As a result, laboratory efficiency can be improved substantially.

Figure 4. The distinct peak separation assures absolutely reliable and trouble-free data acquisition. The analysis run can therefore easily be automated for larger sample amounts while maintaining utmost analytical performance. As a result, laboratory efficiency can be improved substantially.

Improved LOD Through Peak Focusing

On the direct TPD adsorption column the trapping of all analysis gases and the consecutive release of each analysis gas by temperature-programmed desorption is extremely effective in concentrating small amounts of gas at a specified time for a pulsed release.

The concentration of the gas on the direct TPD adsorption column will allow focusing of the peak when the target element is only present in small concentration, resulting in more accurate and reproducible detection. Using this method sulfur concentrations as low as 2 ppm can be detected when an optional IR detector is employed.

The analysis of a carbonaceous soil sample with a sulfur concentration of 7 ppm can be observed in Figure 5. This analysis shows the unique sensitivity of direct TPD technology in sulfur analysis.

Determination of 7 ppm sulfur in a subsoil sample analyzed on UNICUBE with optional IR detector.

Figure 5. Determination of 7 ppm sulfur in a subsoil sample analyzed on UNICUBE with optional IR detector.

Summary

Direct TPD technology is a powerful way to separate gases for thermal conductivity detection – the standard type of detection in combustion analysis of the organic elements.

Several benefits are a result of gas specific adsorption of each target gas species:

  • Controlled release of each target gas species results in guaranteed baseline separation and so accurate and reliable detection, even for samples with extreme C:N or C:S elemental ratios of up to 12,000:1
  • High adsorption capacity of the dedicated filling material of the direct TPD column results in a wide dynamic measurement range, enabling analysis of large sample amounts, for example up to 50 mg of organic material or up to 1 g of soil
  • Fast desorption of the target gas species results in increased peak height, enhancing the limit of detection
  • Direct temperature control of the gas stream speeds up the analytical sequence, increasing the sample throughput per hour

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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