Elemental determination of dog food samples using lithium borate fusion and XRF analysis

It is essential to quantify nutrients such as vitamins and minerals in animal feed products, including dog food, to avoid legal and financial consequences related to misbranding. Dog food samples are difficult to analyse since they have a high volatile organic content. To overcome this challenge, special attention must be paid to the pre-treatment of samples prior to the lithium borate fusion step. In fact, special thermal pre-treatments are necessary to remove the organic materials and keep volatile elements of interest. An oxidizer is also needed to complete the oxidation of the partly reduced material during the fusion process. 

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Introduction

It is essential to quantify nutrients such as vitamins and minerals in animal feed products, including dog food, to avoid legal and financial consequences related to misbranding. Dog food samples are difficult to analyse since they have a high volatile organic content. To overcome this challenge, special attention must be paid to the pre-treatment of samples prior to the lithium borate fusion step. In fact, special thermal pre-treatments are necessary to remove the organic materials and keep volatile elements of interest. An oxidizer is also needed to complete the oxidation of the partly reduced material during the fusion process.

The purpose of this project is to demonstrate that fusion using TheOx® Advanced instrument is a well-suited technique to prepare highly organic samples and obtain good XRF analytical results. Unlike wet chemistry that is often used to prepare these kinds of samples, fusion is fast, it does not require the use of dangerous acids and it leads to a great return on investment (ROI).

Challenges

• Retention and concentration of volatile elements of interest using adequate thermal pre-treatments
• Excellent precision of mineral nutrients analysis in highly organic dog food samples

Benefits
• Easy preparation of challenging samples
• Fusion can be used to prepare a wide range of minerals for XRF analysis
• Avoids damages to your platinumware
• Automated (the operator can then perform more valuable tasks in the laboratory)
• Safer and faster than wet chemistry
• Quick ROIGlobal Sample Preparation 

Apparatus and Instrumental Conditions

An automatic Claisse® TheOx Advanced fusion instrument was used to generate 32 mm lithium borate glass disks. Its resistance-based electric system, excellent insulation properties and preset fusion programs allow a uniform heating, thus providing repeatable and reproducible fusion conditions for its six (6) positions.

Samples containing Al, Ca, Cu, Fe, K, Mn, Mg, Na, P, S and Zn were fused, resulting in glass disks to analyse with the XRF spectrometer. Measurements were performed using a PANalytical 4 kW MagiX PRO sequential WDXRF spectrometer with a 27 mm collimator mask.

Pre-Treatment Method

As mentioned in the introduction, dog food samples needed specific thermal treatments prior to the fusion step to ensure great XRF analytical results. The Claisse Expertise team highlights the importance of these specific treatments of dog food samples to avoid damage to the platinumware.

Global Sample PreparationMethod

Dog food samples were prepared using a 1:10 sample-to-flux ratio with a pre-fused lithium borate flux with specific composition, pure grade (99.98+%). The use of the proper oxidizer (NH4NO3) is also required.

The sample and then the flux were weighed directly into the Pt/Au crucible. A fully automatic TheOx Advanced instrument was used to fuse the samples.

The validated fusion program for dog food materials was used for all samples. The fusion method includes an oxidation step to put all nutrients in appropriate chemical state and ensure optimal fusion method as well as good XRF results. The sample was oxidized and diluted in the melted borate flux. The mixture was then automatically poured into a 32 mm Pt/Au mold. The whole cold-to-cold fusion process took about 20 minutes.

Results

The performance of TheOx Advanced fusion instrument has been verified by validating this methodology with a certified reference material (CRM) and six (6) samples available on the market. Eleven (11) compounds were analysed as they were of importance in the sample type.

Table 1 shows the used range of concentration (in %) obtained from the tested samples and the lower limit of detection (LLD) determined by the XRF software SuperQ. Table 2 contains the certified composition of the CRM-DF-A and the relative standard deviation (RSD) for the ten (10) replicates.

Table 1: Concentration Ranges and LLD
CompoundsConcentration range
(%)
Lower limit of detection 
(ppm)
Al0.00-0.030.34
Ca2.50-28.500.43
Cu0.002-4.770.01
Fe0.002-0.290.38
K0.0-0.81.2
Mn0.006-11.80
0.41
Mg0.21-6.010.51
Na0.49-18.052.4
P0.02-8.700.31
S0.22-1.561.4
Zn0.015-11.900.45
Table 2: CRM Sample and RSD (in %)
CompoundsCRM-DF-A composition (%)RSD (%)
Al0.0271.91
Ca2.50.39
Cu0.0021.50
Fe0.021.90
K0.830.80
Mn0.0061.27
Mg0.2110.39
Na0.490.54
P1.8440.18
S
0.221.04
Zn0.0150.60

Conclusion

The results presented in the previous tables indicate that including pre-treatment of samples prior to lithium borate fusion is effective to prepare mineral nutrients in dog food samples with high organic content for XRF analysis.

The method showed good precision for compounds of interest and an excellent retention of eleven (11) minerals, thus proving the quality of the method, leading to reliable XRF results and ensuring the proper labelling of the product. Although the samples were tricky to prepare, the use of TheOx Advanced fusion instrument was proven efficient in the subsequent determination of true compositions.

Do not hesitate to contact our experts if you experience any difficulty in preparing samples containing highly organic compounds and volatile elements for XRF analysis, or if you need support in the thermal pretreatment of your samples. They will be happy to collaborate with you.

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