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CHNSO Elemental Analysis – Sample Preparation

Why Weighing is Crucial to Quantify Organic Elements in a Sample

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CHNSO Organic Elemental Analysis - Sample Preparation

CHNSO Elemental Analysis

CHNSO Elemental Analysis

CHNSO Elemental Analysis Workflow

Know-How: Sample Preparation and Weighing of Organic Material for CHNS/O Elemental Analysis

Organic elemental analysis is used in various industries for several different purposes and applications:

  • Pharmaceuticals industry
    Determination of composition, structure and purity of organic chemicals in pharmaceutical products both in R&D and in manufacturing.

  • Material characterization industry
    Determination of organic compounds in a wide variety of materials to assure the characteristics and behavior of the final product e.g. rubber, plastic, paper, metals.

  • Environmental industry
    Examination of organic compounds in water, waste water, compost and particulate matter emissions.

  • Agriculture industry
    Scientific analysis of plant and soil samples (land and water-based applications).

  • Food and animal feed industry
    Determination of protein content as a measure of quality and for price setting. Also used to ensure compliance with food composition declarations.

  • Energy / Petrochemicals industry
    Quality assurance and quality control of petrochemical products including fuels and lubricants

1.    Sample Preparation and Weighing

The following steps are presented as a guide to preparing samples for CHNSO analysis. Other methods of preparing such samples exist. Please refer to your SOP to ensure correct procedures. If no SOP exists, it is recommended that one is implemented to ensure consistency across your analyses.

  • Prepare the required tools: a spatula, and two pairs of tweezers (straight and curved). You will also need a flat surface to work on.
  • All equipment and surfaces to be used must be clean and dry. Ethanol is useful for cleaning, and can be left to dry naturally or wiped dry with a clean cloth.
  • Prepare the tare containers. Depending on the sample type and the application, these may be very small tin or silver crucibles, or capsules. The tare containers must also be pre-cleaned and be thoroughly dry to avoid trace contamination.
  • Place the empty tare container on an accurate microbalance and tare.
  • Using the spatula, add the sample to the container until the pre-defined target weight range is reached.
  • Using the tweezers, remove the container from the balance. Using the second pair of tweezers, pinch the top of the container to close it and fold over the edge twice to ensure the sample is securely sealed inside. Flatten the container and compress gently to make a ball or cube. Ensure that no sample is leaking.
  • The sample container can now be transferred to the elemental analyzer. Ensure each container can be clearly identified. Microtitre plates with 96 wells are useful when handling multiple samples.

2.    Analysis in the Elemental Analyzer

Multiple samples may be introduced into the elemental analyzer in an automated process or manually introduced individually.

3.    Results and Reporting

The elemental analyzer software calculates the amounts of the resultant gases as a percentage of the initial sample weight. The results, showing %C, %H, %N and %S, are given in a report.

Sample preparation, homogenization and drying

Samples must be dry and free from residual solvents to provide reliable results. Samples are usually freeze-dried or vacuum oven dried to constant weight, crushed, and homogenized using a mortar and pestle or electric mill.
Preparation of samples is one of the most critical steps in the CHNSO analysis process. Homogeneity of the substance under investigation is an important consideration. For substances with high homogeneity, small sample amounts can be used, as this is sufficient to provide a true representation of the substance. A substance with low homogeneity requires larger samples to represent its true nature. Preparing the smallest possible sample amounts for CHNS/O analysis minimizes the total amount of substance required, reduces the amount of oxygen needed to combust the samples, and decreases waste.

Sample sizes, safe weighing range and minimum weight

Organic elemental analysis is a highly sensitive procedure and therefore requires that samples are accurately weighed in order to determine the precise percentages of the elements C, H, N, S and O in the original sample. Sample sizes vary from application to application; the ideal sample weight is dependent on a variety of different factors including the type of sample, the detection limits of the elemental analyzer and the balance used to weigh the sample. In many cases, it is prudent to run several experiments to establish the range of the anticipated amounts of %C, %H, %N and %S prior to performing the actual analysis to help define the size of the initial sample.

The table below provides an indication of sample size for various different elemental analyses.

Elemental analysis

Sample size (dependent on sample characteristics)


 0.01-100 mg

 N in Lubricant

 5-20 mg

 N and C in Soil

 1 mg to 1 g


 10 mg to 1 g

With very small sample sizes, it is crucial to choose a microbalance that can deliver accurate and reproducible results. This is even more important when the resultant percentages of C, H, N, or S in the original sample are expected to be low. When the sample size and the volumes of the resultant gases are low, measurement error in the resultant percentages can become significant. Hence this is why weighing your initial sample accurately is vital. The minimum weight of the balance is important as this specifies the smallest possible sample size that can be accurately weighed on that specific balance. Weighing samples below the minimum weight of the balance cannot be deemed to be accurate or reliable. However, it is also imperative to consider the safe weighing range of the balance, which is determined by applying a safety factor to the minimum weight. For a more detailed explanation of this topic please download the free guide “Safe Weighing Range Ensures Accurate Results”.

How to select the correct weighing device

METTLER TOLEDO's Good Weighing Practice™ (GWP®) helps you ensure you use the right balance to meet the accuracy needs of your application. GWP® is a standardized scientific methodology for the secure selection, calibration and operation of weighing equipment. Our free GWP® Recommendation service supports you in selecting the right weighing instrument for your measurement needs. This ensures that the equipment you choose will be “fit-for-purpose”. The GWP® Recommendation report includes minimum weight and safety factor calculations to confirm that you have selected the right instrument for your process and takes into account:

  • Maximum amount weighed
  • Smallest net sample weighed
  • Weighing tolerance
  • Safety Factor
  • Standards or regulations that must be adhered to

CHNSO Elemental Analysis Expert

Challanges of weighing in CHNSO Analysis

Preparing Samples for CHNS(O) Analysis


Efficient Elemental Analysis (CHNSO)


Laboratory Software for CHNSO Analysis

Related Products

FAQ — Weighing for CHNSO Analysis

Can accuracy be improved by increasing the sample size? Wouldn't the larger sample produce bigger signals in the elemental analyzer?

It is possible to increase the size of the sample but there are restrictions because the supply of oxygen for the combustion process is limited. For samples with a low carbon content (30-40%) that combust easily, the sample size can be increased to 10 mg. In CHNS analyses for quantifying unknown substances, it is advisable to use less than 3 mg. When it is not known how combustible the sample is, it is good practice to run two analyses at different weights (e.g. 1 mg and 3 mg). If combustion difficulties are encountered, the combustion conditions must be optimized. Macro elemental analyzers can take 1 g samples but this requires greater amounts of oxygen and larger reduction tubes. The amount of ash generated is also increased.

Which balance is the best for CHNS analysis?

Since the elements may only be present at levels of 10% or less, it is essential to use an accurate balance to correctly quantify the ratio of the elements in the sample. Typical sample sizes are 2-5 mg. Furthermore, the ideal balance depends on the nature of the sample, the accuracy required and the elemental analyzer used. It's also worth noting that samples which are too large may have a negative impact on your elemental analyzer. METTLER TOLEDO's free GWP® Recommendation service can help you select the right balance according to your specific process accuracy requirements. The service can also be used to determine if your existing balance is fit for purpose.

How can I find out what the safe weighing range of my balance for organic elemental analysis is?

The safe weighing range of your balance defines an upper and lower limit for weighing in which the weighing results meet the accuracy requirements of your application. Weighing results can only be accurate and reliable if you are weighing in the safe weighing range. While the upper limit is the capacity of the balance, the lower limit is determined from the balance minimum weight and the application of a safety factor in accordance with your accuracy requirements (process tolerances). Our free guide “Safe Weighing Range Ensures Accurate Results” explains the science behind safe weighing and describes how you can determine the safe weighing range of your balance in three easy steps.

I do elemental analysis for our pharmaceutical products. Traceability is very important. How can I ensure it?

XPR microbalances feature the unique results notepad. At the touch of a button, results are saved on the notepad and can be printed out or transferred to a PC. Connect your XPR microbalance to LabX® software for the ultimate in data security. LabX saves all results and process information automatically in a secure, centralized database. Results are easily accessed at any time to print reports or for further analysis.

How can I speed up the sample preparation process?

Automated results recording and documentation significantly reduces the time taken to prepare samples. Consider connecting your balance to a printer or a PC to record the results electronically rather than writing results down by hand. Electronic data transfer eliminates transcription errors. By connecting your balance to LabX®, all data and calculations will be taken care of automatically in the background. Step-by-step user guidance on the balance display will ensure you always know where you are in the sample preparation process.

XPR microbalances have Active Temperature Control (ATC) which provides a stable environment for the weighing cell, meaning you get results delivered faster. Automated draft shield doors avoid the need to keep opening and closing the doors during weighing processes; this saves time and is much more ergonomic.

It takes so long to write down all the weight values. Can you suggest a better solution?

Recording results manually is time intensive and prone to error, especially when results may be 6 or 7 digits long. Connect your balance to a PC to eliminate writing results down by hand. By connecting your balance to LabX Balance software, all weighing results are collected, stored and processed electronically.

I think I have an issue with electrostatic charging as my samples get scattered all over my balance. What can I do?

Electrostatic charging is one of the largest hidden sources of weighing errors. When powders scatter over the balance, this is a strong indication that there is an issue with electrostatic charging. Other indications may be a balance doesn't settle or a results value that slowly drifts. To resolve the issue, place an ionizing unit next to your microbalance. Pass your containers and sample through the ionizer as you place them on the balance.

Accessories for Elimination of Electrostatic Charge