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CHN Elemental Microanalysis
The School has maintained an Elemental Microanalysis facility for over 25 years. In addition to providing for the research needs of the School of Pharmacy's own academic staff, the microanalysis facility also provides a contract service for the wider research community within the UK and overseas.
The service is able to obtain accurate, reproducible data for a wide range of samples. Samples are analysed using the Carlo-Erba EA 1108 with PC based data system, Eager 200 for Windows and a Sartorious Ultra Micro Balance, 4504MP8.
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:
- Quick turnaround, in most cases with results available on the same day
- We email or fax instantly any data requested
- Immediate access to our analytical experts
- Monitored quality control system
- Fast retrieval of archived results
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 continuos flow of helium and then dropped at present intervals into a vertical quartz tube maintained at 10300C (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 rection (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 pasing 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.
Elemental Analyser, Model 1108 (Carlo-Erba, Milan, Italy) with PC based data system, Eager 200 for WindowsTM and a Sartorious Ultra Micro Balance, 4504MP8.
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.
Submission forms can be obtained from CHN Microanalysis Service at the Pharmaceutical and Biological Department, or you can download a form here.
The CHN submission forms must be accompanied with each sample submitted. The submission card must be labeled the same as on sample vial. The copy stays with the lab and the original returns with results.
*Important Information on Submissions*
You must include on the submission form the following information for each sample:
- Your name and telephone number or e-mail
- 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
- sample date
- your comments on the physical state of the compound at submission (e.g. colours, crystal, liquid, melting point).
Typically 2 mg to 5mg will be enough material for a duplicate run. In cases where you have very little sample available, it is possible to do one CHN run with as little as 1 mg for organic samples.
|Organic / Pharmaceutical samples||Minimum 1.5 mg|
|Solid samples with a particle size between 0.2 to 0.5 mm diameter||About 3 to 15 mg|
The weight required is the amount that must be removed from the sample vial - not the amount put into the sample vial. We will happily return any unused sample to you.
Molecular formula, theoretical mass percentage composition, COSHH
As the calibration of instrument requires either the molecular weight and formula of the compound or the approximate theoretical percentage of each element, it is important to have listed at least a range or approximate value of the expected mass percentage composition for each element in the compound.
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. It is also require specifying
the physical state of the compound at submission (e.g. color, crystal,
liquid, melting point and molecular weight).
Containers should be clean and all surfaces inert to your sample. CHN analyses of overfilled capsules could lead to erroneous results.
|solids, powders||flat-bottomed (1) dram vial||<0.75" diameter|
Elemental Analysis serves two purposes:
- To provide for pure materials elemental composition for which an empirical and/or molecular formulas can be derived in conjuction with MS data.
- To assess the purity of pharmaceutical preparations, chemicals, and high quality solvents.
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 % absolute of the theoretical value. And precision of sample duplicates is within ± or 0.2 % due primarily 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
Experienced and committed analytical chemists at the School Microanalysis Laboratory will do their best to bring you the accuracy you need in elemental analysis with carefully documented results, in most cases within the same day.
- Turnaround time averages less than three working days for routine single samples.
- An express service is available for results within hours with an advance appointment.
Results will be reported as a percentage value or/and heat value as requested, and carried out in duplicate unless otherwise instructed.
If you have requested same day results, we will e-mail, fax or send your results. Otherwise, results will be sent via UK Mail or e-mail. Please specify clearly who should receive the results for each sample. Your samples will be returned upon request.
University rates for C, H and N micro-analysis
|Routine analyses||Fees *|
|Single sample C, H, N analysis in duplicate||£20.00|
Industrial Standard rates for C, H, and N micro-analysis
|Routine analyses||Fees *|
|Single sample C, H, N analysis in duplicate||£40.00|
*All charges are exclusive of VAT.
All samples submitted must be accompanied by an official order number.
Sample Drop-Off Service
Samples should be taken to the Centre of Pharmaceutical Analysis, Room 444 and placed in the CHN service tray.
CHN Microanalysis Service
Department of Pharmaceutical and Biological Chemistry
The School of Pharmacy
University of London
29-39 Brunswick Square
Tel: +44 (0)20 7753 5805 or 4882
Page last modified on 18 jul 13 11:03