Factors Affecting C,H,N Micro-Analytical Performance Part 2

Following from “Factors Affecting C,H,N Micro-Analytical Performance Part 1,” we now look at the experimental consequences of how the design of a commercial horizontal furnace elemental analyzer affects data that can be achieved in the laboratory.

A series of experiments were carried out to test a commercial horizontal furnace elemental analyzer, in this case an Exeter Analytical CE440, in the key areas that provide operational benefit to the micro-analytical user.
• Accuracy and precision
• Instrument stability
• Demanding sample types
• Ease of use

Accuracy and Precision

The most important criteria for C,H,N analysis in the majority of analytical laboratories are for optimal accuracy and precision across a wide range of sample types. With constant pressure to increase laboratory productivity, analysts do not want to set up their analyzer with different operational parameters for every different sample type they come across.

In the U.K., the independent Royal Society of Chemistry, MicroAnalytical Group regularly runs programs to study achievable data accuracy and precision on currently available micro-analytical instruments in working laboratories. Table 1 shows a set of data, run on an Exeter Analytical CE440, used by the MicroAnalytical Group as the benchmark for accuracy and precision testing.

The test data in Table 1 can be clearly seen to satisfy one standard deviation from the theoretical values of the test compound accepted as true values. The routine achievability of producing such data quality highlights the accuracy and precision available from a horizontal furnace design.

A horizontal furnace design elemental analyzer has been number one in terms of accuracy and precision in the last three independent Royal Society of Chemistry, MicroAnalytical Group tests.

Nearly all commercially available microanalysis instruments can be demonstrated to give acceptable data accuracy and precision on selected samples. However, a truer reflection of the real data quality an analyst can expect in working laboratory conditions may be demonstrated in a longer-term stability test. Straying outside acceptable limits of data accuracy and precision can not only cause loss of data quality but also markedly increase the time spent in sample re-runs and recalibration.

To test the longer-term stability of the Exeter Analytical CE440 in real laboratory conditions, a run of samples was placed in the system and the calibration constants calculated. The calibration constant relates the number of microvolts detected to each microgram of element. A run of 60 samples (including 11 standards) was analyzed under normal conditions. To maintain an accuracy of 0.3% absolute, the maximum deviations of the calibration constants allowed should be approximately C=0.08, H=2.75, N=0.22. The data in Table 2 run on the Exeter Analytical CE440 shows that all the calibration constants easily meet these criteria.

Demanding Sample Types

Considerable variance in instrument performance can also be seen with more demanding samples. Time dependent combustible samples gave an example of how horizontal systems provide considerably better results with demanding samples. Another example of the benefits a horizontal system can offer is the ability to use rigid wall tin containers for the analysis of volatile liquids. With horizontal furnace analyzers, sample residue is removed between samples, eliminating the chance of residue buildup from the heavier rigid wall containers that are necessary for volatile liquid analysis. The use of rigid wall tin containers allows samples to be sealed with a cold weld device that enables routine encapsulation of even the most sensitive samples, such as volatile liquids and air sensitive materials. The results in Table 3 were derived from analysis of volatile fuel samplesthe samples were sealed using a cold weld sealing device. The sample was analyzed ten times.

Ease of Use

Ease of use is a commonly made claim from almost all microanalysis instrument suppliers. It has many contributory factors, including simple attainment of accurate and precise results, ability to cope with wide-ranging sample types and design features removing the need for constant system reoptimization.

The Exeter Analytical Model CE440 elemental analyzer with its horizontal furnace design is a fully automated CHN/O/S elemental analyzer. The system and its Windows-based operating software have been designed to reduce human error. It incorporates extensive automation and diagnostic processes, along with comprehensive reporting facilities, enabling easy integration into other software packages.

Conclusion

Elemental microanalysis to produce accurate C,H,N composition data on unknown samples is a broadly applicable and widely used technique. However this article demonstrates that results from C,H,N analyzers of different design can vary considerably.

The Exeter Analytical Model CE440 has been demonstrated to produce accurate and precise data routinely and simply over long periods of time without system reoptimization, thus saving time and expense to the micro-analyst. The examples above show how a horizontal furnace design for elemental microanalysis gives accurate and precise data without the inherent problems associated with vertical furnace design systems.

For additional information on the company discussed in this article, visit www.exeteranalytical.co.uk or contact Exeter Analytical at support@eai1.com or via phone at 978-251-1411. The author, Paul Hemming, general manager of Exeter Analytical Ltd., U.K., may be contacted directly at info@exeter-analytical.co.uk.