Have you ever found yourself in an awkward situation where the fiber content analysis results between control samples in the same batch differ significantly? This is a common occurrence in many animal feed laboratories today. When the deviation is large, the RSD index will skyrocket beyond acceptable limits. The consequence is that you are forced to discard the entire batch and waste a lot of time starting over.
Analysis of crude fiber, NDF, or ADF is an extremely sensitive micronutrient weighing test. Traditional manual concentration methods using glass beakers make it difficult to control temperature and solvent flow uniformity. Therefore, random errors appear continuously, reducing the reliability of the measurement. This directly puts a lot of pressure on laboratory technicians.

In this article, we will explore solutions to control and bring the RSD index down to an ideal level. You will receive detailed guidance, from standardizing SOPs for sample preparation and blank sample processing techniques to the application of advanced automated filter bag fiber extraction technology. Let’s explore these technological secrets together.

1. Definition of RSD Index and the Role of Lab Verification

1.1 Mathematical Concept of Relative Standard Deviation

In analytical chemistry, the RSD (Relative Standard Deviation) index, also known as the relative standard deviation, is an essential mathematical tool. The formula for calculating this index is as follows: RSD = (SD / (X average)) . 100%. Where SD represents the standard deviation value and (X average) is the arithmetic mean of the sample batch.
We need to clearly distinguish the nature of this quantity from the ordinary standard deviation SD. While SD only measures the absolute dispersion of the data, this index expresses relative dispersion as a percentage. Therefore, you can easily compare the precision between sample batches with completely different background masses.
To optimize working time, you should set up the STDEV function combined with scaling in Excel software. When importing raw weight data into the spreadsheet, the software will automatically and quickly output the final percentage value for the sample batch. Good control of this data helps us detect mechanical anomalies early in the lab.

1.2 Significance of RSD for Measurements Meeting ISO/IEC 17025 Standards

The national standard TCVN ISO/IEC 17025:2017, in section 7.7, clearly specifies the requirements for ensuring the usability of results. Accordingly, testing laboratories are required to continuously monitor the precision and repeatability of each measurement. Maintaining the deviation index within the permissible limits is a leading criterion for evaluating technical competence.
For accredited animal feed testing centers, this error value for the conventional crude fiber test must be strictly controlled at a level below 2% to 5%. The specific percentage fluctuation threshold will depend directly on the range of fiber content in the actual sample. If the data exceeds this threshold, the analysis results will have no legal or regulatory validity.
It is reiterated that a stable indicator within a safe range is the most objective evidence for expert evaluation teams. It demonstrates the technician’s precise operational capability. At the same time, it is also a measure of the reliability of the entire system of equipment that the laboratory is operating.

The RSD value in fiber analysis must meet the standards of ISO 17025

2. Technical Causes of RSD Error Explosion in Fiber Analysis

2.1 Errors in Sample Preparation and Solvent Handling

First, we need to recognize common technical errors right from the experimental sample grinding stage. If you use non-standard sieves, the particle size of the sample will not achieve optimal uniformity. According to the Sonnen technical document, sample particles must pass through a sieve with a sieve size of 0.5 – 1.0 mm to ensure hydrolysis efficiency.
In addition, high fat content in specific samples such as soybeans (>5%) is also a dangerous factor. If the technician does not perform preliminary defatting with petroleum ether, a hydrophobic fat film will surround the fiber. This film prevents acids or alkalis from contacting the core, leading to extremely large variations in results.
Finally, mechanical errors during weighing of the input micro-weight (0.2 – 5 g) also increase systematic errors. Weighing the sample before the beaker or filter bag has completely cooled in the desiccator will alter the true value of the weighing. Therefore, the homogeneity of the initial data is completely broken.

2.2 Local Temperature Unevenness and Mechanical Loss

When applying the independent heating method using a traditional electric stove system, there is always a significant mechanical difference in temperature at each glass beaker. Different boiling rates cause the efficiency of the acid (H2SO4) and alkali (NaOH) hydrolysis reactions to be inaccurate. This is the reason why the control results between beakers in the same batch are scattered.

Furthermore, sample clumping and sticking to the glass beaker walls also frequently occur. In this case, the chemical solvent cannot fully penetrate 360 ​​degrees into the structure of the sample. This situation prevents some fibers from being completely released, directly increasing the RSD value.

Furthermore, the manual process of concentrating and transferring the fibrous residue through a porous glass funnel carries a significant risk of mechanical loss. Fine fibers can easily be left behind in the instrument or slip through the coarse filter membrane. This accidental loss seriously distorts the dry residue mass after drying, leading to inaccurate measurements.

Do not concentrate and transfer fibers through a glass funnel as this will affect the RSD

3. Legal Regulations and National Standards for Fiber Analysis

3.1 Crude Fiber Standardization Procedure according to TCVN and ISO

To ensure the legality of the measurements, we need to strictly adhere to TCVN 4329:2007 (ISO 6865:2000). This is the national standard applied to the determination of crude fiber content in animal feed by the intermediate concentration method. This standard sets out stringent requirements regarding technical conditions.

Specifically, the concentrations of the chemical solutions must be absolutely precise. The sulfuric acid (H2SO4) solution must be at a concentration of 0.13 mol/L and the sodium hydroxide (NaOH) solution must be exactly 0.31 mol/L. Any dilution or deviation in concentration will change the rate of organic hydrolysis reaction.
In terms of data control, the regulatory document clearly specifies the maximum deviation between two parallel test results on the same matrix sample. If the difference exceeds the allowable error limit of ISO, that batch of analysis is completely rejected. Therefore, monitoring repeatability is mandatory for the technician.

3.2 Standards for Determining Advanced NDF and ADF

Besides crude fiber, the determination of advanced fiber components also needs to be based on clear legal frameworks. You need to apply TCVN 9590:2013 for the neutral fiber (NDF) analysis procedure. This standard emphasizes the use of active alpha-amylase enzyme to remove adhering starch, as research by David R. Mertens published in the prestigious journal AOAC International (2002).

In parallel, the test for determining acid fiber (ADF) and lignin (ADL) is strictly regulated in TCVN 9589:2013 (ISO 13906:2008). This method requires maximum uniformity in sample processing using specialized acid-based detergents. The aim is to preserve the pure cellulose and lignin structure after sample degradation.

We can quickly summarize the standard parameters through the technical reference table below:

TCVN Standards	Analysis Subject	Main Chemical/Solvent	Purpose of Control
TCVN 4329:2007	Crude Fiber	H2SO4 0.13 M & NaOH 0.31 M	Measurement of insoluble residue
TCVN 9590:2013	Neutral Fiber (NDF)	Neutral bleaching agent + Alpha-Amylase	Removal of starch, retaining cell membrane
TCVN 9589:2013	Acid Fiber (ADF)	Cetyltrimethylammonium bromide in H2SO4	Determination of cellulose and lignin

4. Specialized Filter Bag Technology – A Solution to Eliminate Sample Clumping

4.1 The Mechanical Structure of 25-Micron Porous Membrane Filter Bags

To completely overcome the mechanical disadvantages of the boiling method, intelligent membrane filtration technology has emerged. This advanced solution utilizes the high-end FB25 Sonnen fiber extraction bag product line. The product boasts a very compact, standard geometric size of 52 mm x 60 mm.
The core of this technology lies in its 3D network structure with an absolutely precise porosity of 25 microns. This micropore size allows acidic or alkaline solvents to freely circulate in and out of the sample surface. However, all the fine, insoluble fibers are completely retained inside the filter bag.
The polymer material making up the membrane bag has extremely superior chemical resistance. They do not deform, tear, or change pore size even when continuously boiled in strong H2SO4 acid. This stable physicochemical property helps protect the trace sample mass safely throughout the digestive cycle.

Use Sonnen FB25 fat-filling pouches to ensure accurate RSD (Reduced Sugar Content) readings

4.2 Effective Sample Accuracy Control Through Clumping Elimination

The completely enclosed design of the filter bag provides exceptionally effective clumping control. Under the pressure of the circulating chemical flow, the sample is continuously and evenly mixed inside the bag. Localized sample clumping in mechanical dead corners is completely eliminated.
When acid and alkali molecules are in uniform 360-degree contact with the entire sample surface, the hydrolysis reaction rate is synchronized. This absolute uniformity helps to narrow the deviation gap between parallel samples. Therefore, you can easily control repeatability and bring the RSD value down to an ideal level (<1.5%).

Furthermore, this solution completely eliminates the intermediate washing step through a manual porous glass funnel. Technicians no longer face the risk of fiber loss due to incomplete rinsing. This is a breakthrough that eliminates random human errors, elevating the quality of fiber analysis.

5. Automated temperature control process using advanced fiber extraction equipment

5.1 Synchronous heating mechanism of the semi-automatic fiber extraction machine

To optimally realize filter bag technology, synchronization of operating equipment is essential. We can apply the F20 semi-automatic fiber extraction machine using the filter bag method, researched and manufactured by Sonnen. This breakthrough device is currently officially distributed in Vietnam by Duc Duong Company.
The heart of the F20 system is the intelligent digestion chamber with a synchronized heating mechanism. The operating temperature range from 0 – 105 degrees Celsius is controlled by a touch screen and evenly distributed to all tray positions. This mechanism completely eliminates local temperature difference errors of the manual heating method.
The machine boasts exceptional operational efficiency, processing up to 30 samples simultaneously per batch. All samples undergo a precise digestion time of 60 minutes and a drain time of 2 minutes. This standardized automated process eliminates time variables, keeping the RSD (Reference Storage Size) at a minimum.

5.2 Specific Fat Sample Processing Procedure Before Fiber Extraction

For samples containing high lipid content (>5%) such as soybeans, the preparation process requires strict steps. Based on the technical guidance document from the Sino-Sonnen Product Technical Data system, we need to perform the preliminary defatting process according to the following steps:
Place the weighed sample filter bags into a 500 ml wide-mouth glass bottle.
Pour petroleum ether solvent into the bottle so that the surface of the sample bags is completely submerged, and seal the protective cap.
Shake thoroughly 10 times and soak the sample for exactly 10 minutes to dissolve the fat phase.
For specific raw soybean products, the soaking time must be extended to exactly 12 hours before drying.
Strict adherence to these steps helps to completely remove the lipid matrix adhering to the fiber. Therefore, the acidic solvent in the subsequent cycle will attack the experimental sample mass evenly. This is a prerequisite for protecting the accuracy of the final error calculation.

>> Optimal RSD precision solution for your lab:
Are you struggling with skewed fiber analysis results and RSD exceeding the permissible limit? Let Duc Duong’s team of technical experts accompany you. We provide a synchronized Sonnen filter bag technology solution, support SOP standardization, and direct technology transfer at your lab.

>> CONTACT DUC DUONG FOR CONSULTATION

6. Guide to calculating the white bag correction factor and processing data in Excel

6.1 Parallel Blank Bag Run Technique

In micro-weight weighing tests, determining the systematic error of consumable materials is extremely important. The polymer material of the filter bag, despite its high chemical resistance, can still suffer microscopic weight loss when interacting with boiling acid. Therefore, running a blank filter bag is mandatory.

This blank bag must be placed in the same digestion batch and undergo the same physical and mechanical processes as the real sample. The purpose is to accurately measure the natural weight fluctuation of the bag shell. After the process, the blank bag is stably dried at a standard temperature of 102°C ± 2°C for 2 hours.

The data obtained from the blank bag serves as a constant compensation for background material noise. If this technical buffering step is omitted, the natural weight loss of the bag shell will be included in the fiber weight. This systematic error will distort the control data and undermine efforts to control the fineness of the measurement.

6.2 Calculation of Correction Factor C1 and Excel Data Processing

To refine the raw data before calculating the RSD value, you must apply the formula for calculating the white bag correction factor: C1 = M3 / M1. In this mathematical expression, M1 represents the initial weight of the white bag before running, and M3 represents the weight of the dry residue white bag after drying or ash incineration.
Before weighing the final residue, the technician needs to perform a small trick with an organic solvent. Take the filter bags out after washing them with warm water, gently squeeze them, and completely immerse them in a cup containing acetone for 3-5 minutes. Acetone will replace all the adhering water molecules, helping the polymer film dry naturally and evenly.
Once you have all the stable weighing values ​​and the correction factor C1, we proceed to import the data series into an Excel spreadsheet. You apply standard statistical functions to automatically output the final percentage value. This scientific data processing helps to demonstrate the absolute reliability of the experiment.

Conclusion

Strict control of repeatability and RSD in crude fiber, NDF, or ADF analysis processes requires a comprehensive standardization solution. We need synchronization from the preparation of 0.5-1.0 mm sample grinding sieves, defatting by soaking in petroleum ether solvent, to the calculation of the white bag coefficient C1 in Excel.
Besides human intervention, the most fundamental solution is to shift technology from manual beaker heating to an automated system with synchronized heating, such as the Sonnen F20 machine combined with specialized 25-micron foam bags. This advanced technology completely eliminates mechanical variables, ensuring that laboratory data always meets ISO/IEC 17025 standards.
Customers who need to upgrade their laboratory equipment, optimize error control processes, or purchase genuine consumables from the Sonnen brand are welcome to contact Duc Duong directly for the best service from our team of dedicated engineers.

 

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Tel: (028) 3762 8042 – 3762 8043 – 3750 8514 – 3750 8793

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Email: ducduong@ducduongco.com

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FAQ (Frequently Asked Questions in the Lab)

1. What RSD value is acceptable for fiber analysis results?

In the acceptance testing of animal feed results according to international standards, the acceptable error value is clearly defined. For parallel control batches, the target RSD value must be controlled at 2.0% for samples with high fiber content. For samples with low microfiber content, the allowable error range can be relaxed but must not exceed 5.0%.

2. Why is the RSD value of my sample batch still above the limit even after running the automated machine?

Data discrepancies during automated machine operation largely stem from the mechanical preparation before sample digestion. You need to check if the particle size after grinding has uniformly passed through a standard 0.5 – 1.0 mm sieve. Additionally, please note whether the sample batch rich in fat (>5%) has been thoroughly rinsed with petroleum ether to remove the lipid film that hinders the hydrolysis reaction.

3. Can the Sonnen FB25 filter bag be reused for the next fiber analysis batch to save costs?

Absolutely NOT. The FB25 filter bag product line is specifically designed by the manufacturer for single-use cycles. Continuous drying at 102°C along with the incineration process will severely alter the geometric structure of the micropolymer. Attempting to reuse will tear the filter membrane, causing fiber loss and completely skewing the measurement results of the next batch.

4. Is the white bag correction factor C1 mandatory for each batch run?

This is MANDATORY if you want to build an ISO/IEC 17025 compliant lab. Even the most durable membrane bag materials will have a certain percentage of mass loss when interacting with high-intensity boiling chemicals. Placing at least one blank bag (without sample) running in parallel helps us obtain the C1 coefficient, thereby accurately compensating for the fictitious weight loss in the mathematical formula.

5. How to handle the issue of filter bags sticking to the water solvent, which prolongs drying time?

After completing the warm water rinsing cycles on the F20 system, the technician needs to remove the sample tray and gently squeeze the bag shells to remove excess biological water phase. Next, immerse all the sample bags in a beaker containing pure acetone solvent for 3 to 5 minutes. Acetone’s rapid evaporation properties completely replace the clinging water molecules, allowing the sample bags to dry naturally and evenly before being placed in the drying oven.