Analyzing Mineral and Trace Element Content in Livestock Feed

This article examines the capabilities of Epsilon 3XLE — a benchtop energy dispersive X-ray fluorescence spectrometer — as a tool for analyzing Pb, Zn, Cu, Fe, Mn, Ca, K, Cl, S, P, Mg, and Na in a range of livestock feeds.

Application Background

The XRF technique is an interesting analytical method for the feed industry, due to its simple sample preparation together with fast and simultaneous measurement of elements. It enables rapid analysis close to production lines.

Instrumentation

Measurements were carried out using PANalytical Epsilon 3XLE EDXRF spectrometers, fitted with a high-resolution silicon drift detector (SDD10), a 50 kV silver anode X-ray tube, a helium purge facility, 6 beam filters, a 10-position removable sample changer and a sample spinner.

Sample Preparation

Initially, milling was carried out on complete feeds, including turkey meal, chicken feed, sow feed, soybean meal, dry pulp and piglet meal to reduce particle size prior to mixing with a binder and pressing with the help of a manual press. Five grams of the sample was pressed by applying a force of 100 kN for 30 seconds. Five pellets were prepared for each sample.

Measurement Procedure

To calibrate the instrument, a range of organic certified reference materials were used and these were prepared using the same method as that for the feed samples. For all the feed samples, a single calibration was prepared. Five different measurement conditions were employed, each one optimizing the excitation of a group of elements as shown in Table 1. Either a helium or an air atmosphere was used to perform the analyses, and the total measurement time per sample was 15 minutes. Figure 1 shows an example spectrum for the measuring condition .

Table 1. Measurement conditions

Condition kV µA Measurement time (s) Medium Filter Elements
<F-Si> 4.5 2500 360 Helium None Na, Mg
<P-Cl> 10 509 120 Helium Ti P, S, Cl
<K-V> 12 900 120 Air Al-thin K, Ca
<Cr-Co> 20 350 150 Air Al-thick Mn, Fe
<Ni-Mo> 50 160 150 Air Ag Cu, Zn, Pb

The spectrum obtained using condition <Cr-Co, for Fe and Mn in chicken feed." src="/images/Article_Images/ImageForArticle_14028(3).jpg" style="border-width: 0px; border-style: solid;">

Figure 1. The spectrum obtained using condition <Cr-Co>, for Fe and Mn in chicken feed.

Calibration Results

The calibration plot for Zn in livestock feed, applying the conditions listed in Table 1, is shown in Figure 2. The calibration data is summarized in Table 2. The lower limit of detection (LLD) for each element was computed using the equation:

Where,
s = sensitivity (cps/mg/kg)
rb = background count rate (cps)
tb = live counting time (s)

The LLD can be lowered in many cases by increasing the measurement time.

Calibration graph for Zn in livestock feed pellets.

Figure 2. Calibration graph for Zn in livestock feed pellets.

Table 2. Calibration details (* RMS: The more accurate calibrations have the smaller RMS values).

Element Concentration range (mg/kg) RMS* (mg/kg) Correlation coefficient LLD (mg/kg)
Na 200 - 19600 543 0.9977 165
Mg 878 - 5520 195 0.9901 31
P 1000 - 11100 314 0.9931 16
S 1720 - 9600 383 0.9873 10
Cl 65 - 19200 347 0.9977 6
K 1120 - 47800 1360 0.9928 15
Ca 140 - 59700 759 0.9985 8
Mn 0.2 - 246 7 0.9950 2
Fe 2.3 - 540 17 0.9951 1
Cu 0.6 - 63.3 1 0.9978 1
Zn 12.5 - 190 2 0.9983 1
Pb 0 - 47 1 0.9986 1

Accuracy

The feed samples were tested as unknown samples and the results acquired were compared with those reported by the suppliers, determined using alternate techniques. This was done to test the accuracy of the calibration. Five pellets of each sample were prepared and the results were presented as average ± standard deviation (SD). The results for sow feed and chicken feed are shown in Table 3.

Table 3. Accuracy results of chicken feed (top) and sow feed (bottom) samples.

Element Chicken feed Average concentration ±SD (mg/kg) Reported concentration (mg/kg) Relative difference (%)
Na 1472 ± 130 1500 1.9
Mg 1586 ± 28 1600 0.9
P 5381 ± 59 5500 2.2
S 2760 ± 268 N/A N/A
Cl 1537 ± 21 N/A N/A
K 7519 ± 42 7500 0.3
Ca 8257 ± 102 8400 1.7
Mn 83 ± 3 89 6.7
Fe 211 ± 3 192 9.9
Cu 14 ± 1 15 6.7
Zn 83 ± 5 84 0.8
Pb 2.8 ± 0.1 N/A N/A

Element Sow feed Average concentration ±SD (mg/kg) Reported concentration (mg/kg) Relative difference (%)
Na 2372 ± 97 2500 5.1
Mg 1780 ± 20 2000 11
P 4485 ± 77 5000 10.3
S 2084 ± 21 N/A N/A
Cl 3447 ± 50 6700 48.6
K 6919 ± 48 6800 1.8
Ca 9456 ± 135 9400 0.6
Mn 81 ± 4 85 4.3
Fe 259 ± 10 273 5
Cu 15 ±0.3 16 7
Zn 108 ± 4 109 1.1
Pb 2.2 ± 0.2 N/A N/A

Precision

One pressed sample was measured five times consecutively to test the repeatability. Table 4 shows the average concentration, RMS (1 sigma standard deviation) and the relative RMS of the repeated measurements of a turkey meal sample. The results show excellent precision.

Table 4. Results of the repeatability test (n=5) of a turkey meal sample prepared as pressed pellet.

Element Average concentration (mg/kg) RMS (mg/kg) Relative RMS (%)
Na 1727 72 4.2
Mg 2265 19 0.8
P 7294 7 0.1
S 3161 11 0.3
Cl 2495 6 0.2
K 10659 67 0.6
Ca 9751 37 0.4
Mn 94 1 1.1
Fe 149 1 0.7
Cu 29 1 3.4
Zn 91 1 1.1
Pb 1.8 0.2 11

Conclusion

The above data clearly shows that an Epsilon 3XLE EDXRF spectrometer is suitable for studying a wide range of minerals and trace elements in many livestock feeds with analysis using a single calibration and taking only 15 minutes. The repeatability results show the robustness and stability of the Epsilon 3XLE. An excellent detector resolution coupled with high sensitivity and powerful software deconvolution models contributes to the precision and accuracy of the results.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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