We have maintained an Elemental Microanalysis facility for over 25 years, which provides for the research needs of the School, and also a contract service for the wider research community.
The service is able to obtain accurate, reproducible data for a wide range of samples. Samples are analysed using the Exeter Analytical CE440 Elemental Analyser with a Sartorious SC2 balance used for weighing.
CHN analysis provides a quick and inexpensive method to find sample purity, and in conjunction with mass spectroscopy and NMR data can be used to characterise a compound.
Benefits of using the School's CHN Elemental Microanalysis Service:
- Analyses completed according to workload - please enquire
- Results made available immediately on completion via a web interface
- Access to our analytical team via email or by arranged visit
- Monitored quality control system
- Archived results available from 1.1.2008
Principle of the Method:
The analytical method is based on the complete and instantaneous oxidation of the sample by “flash combustion”, which converts all organic and inorganic substances into combustion products. The resulting combustion gases pass through a reduction furnace and are swept into the chromatographic column by the carrier gas (helium), where they are separated and detected by a thermal conductivity detector (TCD), which gives an output signal proportional to the concentration of the individual components of the mixture.
Determination of Carbon, Nitrogen and Hydrogen (CHN):
The technique used for the determination of CHN is based on the quantitative “dynamic flash combustion” method. The samples are held in a tin container, placed inside the autosampler drum where they are purged with a continous flow of helium and then dropped at present intervals into a vertical quartz tube maintained at 900ºC (combustion reactor). When the samples are dropped inside the furnace, the helium stream is temporarily enriched with pure oxygen and the sample and its container melt and the tin promotes a violent reaction (flash combustion) in a temporary enriched atmosphere of oxygen. Under these favourable conditions even thermally resistant substances are completely oxidised.
Quantitative combustion is then achieved by passing the mixture of gases over a catalyst layer. The mixture plug of combustion gases is then passed over copper to remove the excess of oxygen and to reduce the nitrogen oxides to elemental nitrogen.
The resulting mixture is directed to the chromatographic column (porapak PQS) where the individual components are separated and eluted as Nitrogen (N2), Carbon dioxide (CO2), Water (H2O) with the help of a Thermal Conductivity Detector whose signal feeds the automatic workstation known as Eager 200. The instrument is calibrated with the analysis of standard compounds. All results for elemental analyses are calculated based on a known value of a standard by using the K value factors calculation. This K value is determined by analysing an organic standard of a known elemental composition.
The standards used are all traceable back to NIST primary standards and the instrument is checked with NIST primary standards on a regular basis to assure day-to-day accuracy of results. The instrument is standardised with acetanilide.
Exeter Analytical CE440 Elemental Analyser and a Sartorius SC2 balance. This instrument automatically determines carbon, hydrogen and nitrogen. Combustion of the sample, separation of the combustion gases and measurement by thermal conductivity are all carried out in the dynamic mode.
The method is used for analysis of pure organic compounds as well as for industrial and agricultural raw materials and intermediate and final products such as oils, pharmaceuticals. The high temperature created by the combustion of the tin capsules ensures a complete decomposition of metal salts and other refractory materials.
New users will be added to a database by using this application form (Word). This will allow subsequent sample submission and retrieval via a web interface. The web application must contain
- The chemical formula of your sample
- Its proposed structure and molecular weight
- The theoretical percentage of C H N elements in your sample
- A reference number, which must be the same as on the sample vial
- Your comments on the physical state of the compound at submission (e.g. colours, crystal, liquid, melting point)
- Hazard information
The determination of the mass percentage of CHN elements in your sample is based upon the direct weight of the material sampled. Therefore, it is very important that samples are dry, free of foreign substances such as dust, rust, hair, aluminum foil, parafilm, and paper filter fibers (the most common contaminant). The lab needs to know if and what metals, halides, or other interferences are within the sample.
- Although approximately 2 x 1.5mg of organic/pharmaceutical material is needed for duplicate analyses, please supply at least 5mg of solid material to facilitate handling (10mg if possible)
- Please use a flat bottomed vial with an opening of at least a 7mm
- Unused sample will be returned on request.
Elemental Analysis serves two purposes:
- To provide for pure materials elemental composition (C H and N) of pure materials for which empirical and/or molecular formulas can be derived in conjunction with MS data
- To assess the purity of pharmaceutical preparations and compounds. The results of an elemental analysis for carbon, hydrogen and nitrogen have traditionally been regarded as acceptable, if the accuracy of the results is within 0.3% of the theoretical value. Also, that the precision of sample duplicates is within ± 0.2%, a variation primarily due to the variability in sampling, provided that the sample is pure and homogeneous.
What causes CHN results to be inaccurate?
The sample must have the correct structure and must also be very pure. If there are any solvents or moisture trapped in the sample, this will affect the accuracy of the results. Also, if the sample is not homogeneous, duplicate runs will not agree to within 0.2%. If a sample is extremely volatile, it may loose mass due to evaporation after it has been weighed out, even if it is crimp-sealed in a special volatile sample pan. This can also cause the results to be inaccurate.
Some compounds are inherently difficult to combust completely. Incomplete combustion can also cause inaccurate results. In this case, the sample could be re-run under different conditions, with an added oxygen boost, or with the addition of chemical combustion aid such as vanadium pentoxide.
Cost of Service:
University (other than UCL) rates for C, H and N microanalysis
|Routine analyses||Fees *|
|Single sample C, H, N analysis in duplicate||£20.00|
Industry and non-University C, H, and N microanalysis, subject to Contract
*All charges are exclusive of VAT.
All samples submitted must be accompanied by an official order number.
Sample Drop-Off Service:
Samples should be brought or sent to:
CHN Microanalysis Service
Department of Pharmaceutical and Biological Chemistry
UCL School of Pharmacy
29/39 Brunswick Square
London WC1N 1AX
Tel: 020 7753 5805/4834