Factors affecting C,H,N micro-analytical performance--Part 1: it is in the detection and measurement of a sample's combustion products that micro-analytical systems differ most markedly.
During the ensuing forty years, the development of new global synthetic chemical industries created an escalating demand for elemental microanalysis. As demand to analyze greater numbers of samples rose, the need to automate the micro-analytical procedures while maintaining the accuracy and precision of the classical techniques became apparent. The 1960s (3) saw the introduction of the first automated elemental analyzers, which quickly gained acceptance in laboratories throughout the world. Today the requirements for high data quality, reliability, system productivity and ease of use have placed demands on analysts and instrument designers alike. Only certain micro-analytical instrument designs appear able to deliver these goals.
Sample Preparation and Combustion Chemistry
The sample preparation methodology and combustion chemistries (3,4) used by different instrument designs are generally similar. To analyze a sample, a weighed (1 to 2 mg) quantity is introduced into a high temperature furnace, and the sample is combusted in oxygen. Typically, the sample is weighed into a tin container, which gives the advantage of strong exothermic combustion, ensuring complete sample oxidation at approximately 1800 C. The resulting combustion products pass through specialized oxidation reagents, to produce from the elemental carbon, hydrogen, and nitrogen, Carbon dioxide (C[O.sub.2]), water ([H.sub.2]O), nitrogen ([N.sub.2]) and N oxides respectively. These gases are then passed over copper to remove excess oxygen and reduce the oxides of nitrogen to elemental nitrogen (2). Helium is used as the carrier gas. Other elements present are removed by the use of specialized combustion reagents (4).
Detection of Combustion Products
It is in the detection and measurement of a sample's combustion products that microanalytical systems differ most markedly. C,H,N micro-analytical systems broadly fall into two categories--static and dynamic.
Static System--In a static system (Figure 1), the sample is introduced via a ladle into the combustion tube into a pure oxygen environment. After combustion has occurred the sample residue is removed from the combustion tube. The mixture of combustion products (C[O.sub.2], [H.sub.2]O and [N.sub.2]) is pulsed into a mixing chamber to ensure a homogeneous mixture at constant temperature and pressure. The procedure of pulsing the combustion products into the mixing chamber speeds up the formation of a homogenous mixture, which contributes to faster analysis times. The pressure of the mixture is typically monitored by a pressure transducer and a known volume of the product mixture released when a pre-set pressure is reached, i.e. 1500 mm Hg. This known volume of combustion mixture then passes through a series of traps where [H.sub.2]O and C[O.sub.2] are absorbed completely with high precision thermal conductivity detector filaments located before and after each absorption trap (Figure 2). The difference between the output of each set of detectors before and after absorption can be seen to be proportional to the trapped component and, hence, the quantity of carbon and hydrogen in the original sample can be determined. The remaining component of the combustion products, i.e. nitrogen, is measured with reference to pure helium carrier gas, the difference in thermal conductivity being proportional to nitrogen content. Detection is in the steady state and, thus, highly accurate and precise. Static systems have been proven to be highly reliable in thousands of installations worldwide.
[FIGURES 1-2 OMITTED]
Dynamic System--In a dynamic system, the sample is dropped via gravity into the combustion tube at a predetermined time to meet with an oxygen enriched atmosphere. The mixture of combustion products (C[O.sub.2] [H.sub.2]O and [N.sub.2]) is then passed through a gas chromatographic column to separate the components, resulting in a gas chromatogram of three peaks eluting in the order of [N.sub.2], C[O.sub.2] and [H.sub.2]O (Figure 1). In dynamic systems, measurement is of the integrated area under an eluting gas chromatogram peak. The subsequent signals are measured and referenced against compounds of known C,H,N content. Loss of precision can occur, however, in dynamic systems for reasons including: Samples with high hydrogen content often lead to tailing of the [H.sub.2]O peak; larger sample weights can give poor chromatographic separation (unresolved peaks); and gas chromatographic column efficiency decreases over time. By comparison, in a static system the C[O.sub.2], [H.sub.2]O and [N.sub.2] are determined by a simple linear voltage measurement of a steady signal that provides constant high precision.
The Advantages and Disadvantages of Different Instrument Designs
Today analysts are increasingly looking for improved data accuracy, precision and long-term stability when determining C,H,N content to comply with more stringent quality practices. Currently, there are three types of elemental analyzers on the market--dynamic, hybrid and static measurement systems-all of which can be demonstrated to produce accurate and precise data. However, the consequences of these design differences start to show when the analyzers are run in real laboratory environments.
Combustion Orientation--A notable design difference between the three types of systems is the orientation of the combustion furnace. While the static design has a horizontal combustion furnace, the others operate with a vertical arrangement. The horizontal furnace arrangement enables the sample to be introduced into the combustion tube on a quartz ladle, which critically enables the removal of all sample residues after combustion. In a vertical furnace arrangement, the samples are combusted on top of previously combusted samples. This difference is a major factor contributing to the advantages of a static system with a horizontal furnace over both the dynamic and hybrid designs.
When a laboratory undertakes to carry out a C,H,N analysis on a sample, the purpose of the analysis is to acquire a set of analytical data representative of that sample. There is no analytical justification to combust a sample on top of previously combusted samples as this can lead to inferior analytical data. The build up of sample residue in the combustion zone of vertical furnace systems considerably increases the potential for poor analytical data. Consequently, long-term stability is compromised, and spurious results are likely to be generated due to memory effects from certain sample types. As the residue collects in the combustion tube (Figure 3), the flow characteristics of the combustion tube change. This change is particularly important when applying such changes to dynamic type systems as these systems depend on constant gas flow. Any changes in gas flow have a direct effect on calibration characteristics and stability.
[FIGURE 3 OMITTED]
The build up of sample residue is a particularly important consideration where a sample combusts slowly or where a large volume of residue is produced by each sample, such as is found in filter analysis. A common example of a slowly combustible material is the carbon fibers found in many modern day composite materials. The combustion of carbon fibers is a function not only of temperature but also of time. Vertical furnace analyzers typically do not combust refractory type materials, such as carbon fibers, very well because insufficient time exists in the dynamic process for complete combustion. This is easily demonstrated if a blank is run directly after a refractory sample, such as carbon fibers. In such instances, it is found in vertical systems that the blank values are elevated due to the sample that was left in the residue from the initial combustion and now combusting along with the next sample, in this case a blank. The consequences for effects on sample data are obvious. In a horizontal furnace design, memory effects such as the carbon fibers example above do not occur.
Firstly, the residue is removed between the analyses of samples, thus preventing memory effects. Secondly, in horizontal systems, such as the CE440 from Exeter Analytical, complete control over the combustion process enables the analyst to extend combustion time and oxygen flow to ensure total sample combustion.
AT A GLANCE
* Sample preparation methodology and combustion chemistries used by different instrument designs are generally similar
* C,H,N micro-analytical systems fall into two categories--static and dynamic
* Analysts are looking for improved data accuracy, precision and long-term stability when determining C,H,N content
* Design differences of dynamic, hybrid and static elemental analyzers are evident in real lab environments
In Part 2 of "Factors Affecting C, H, N Micro-Analytical Performance," we will look at the experimental consequences of how the design of a commercial horizontal furnace elemental analyzer affects data that can be achieved in a laboratory environment.
For more information, Exeter Analytical may be contacted at firstname.lastname@example.org or via phone at 978-251-1411. The author may be contacted directly at info@exeter-analytical, co.uk.
(1.) Belcher, R. 1976. The elements of organic analysis. Proc. Anal. Div. Chem. Soc. 13: 153-164.
(2.) Grant, J. 1945. Quantitative organic microanalysis, based on the methods of Fritz Pregl. London: J. & A. Churchill.
(3.) Belcher, R., ed. 1977. Instrumental organic elemental analysis. London: Academic Press.
(4.) Hemming, P. E. 1995. Micro-analysis: improved combustion reagents for determination of elemental. Composition. Exeter Analytical (UK) Ltd.
Paul Hemming, general manager of Exeter Analytical Ltd., U.K., has been involved with elemental microanalysis for more than 25 years. Also, he is a past chairman of the Royal Society of Chemistry, Micro Analytical Group.
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|Title Annotation:||Instrumentation, Systems & Equipment|
|Author:||Hemming, Paul E.|
|Date:||Jul 1, 2007|
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