Analysis of Trace Elements in Polymers Using XRF

Most polymerisation processes used to create modern plastic materials require catalytic substances containing elements such as aluminium, titanium or zinc. Yet, it is imperative to minimize the presence of catalysts in the finished product in order to maintain its desirable properties. In addition to catalysts, a number of other charges and additives - plasticisers, lubricants, stabilizing agents, neutralisers, antioxidants, pigments - are utilized in the production process. These products contain elements such as magnesium (Mg), iron (Fe), aluminium (Al), phosphorous (P), chlorine (Cl), calcium (Ca), chromium (Cr) or titanium (Ti).

It is therefore essential to conduct elemental analyses on the finished polymer to optimize the control over the process. Plastics are generally electrical insulators, acid-resistant and hence difficult to put into solution. This makes X-ray fluorescence the best method to obtain rapid and precise analyses for all the elements of the periodic table, starting from elements of atomic number Z as low as boron (Z = 5).

Instrument

Tests were performed on a series of polypropylene (PP) standard samples, to determine the performance of the Thermo Scientific ARL PERFORM'X series spectrometer (Fig. 1) for the analysis of trace elements in polymers.

Figure 1. Thermo Scientific ARL PERFORM’X series.

Results

Better performances are achieved on the ARL PERFORM'X series through optimized coupling between X-ray tube and sample, as well as with the high-power tube and generator. This allows high-sensitivity and lower limits of detection. The comprehensive analytical configuration enables better analysis across the periodic table. Operating the system is extremely simple thanks to the state-of-the art Thermo Scientific OXSAS software and its Analytical Assistant.

Chemical analyses were offered and used for the calibration of the instrument. Table 1 shows the limits of detection achieved for various elements together with the measurement conditions.

Table 1. Elements and Analytical conditions

ELEMENT 4200 W (3 SIGMA) [PPM] 2500 W (3 SIGMA) [PPM]
Mg 0.77 1.00
Al 0.21 0.27
P 0.12 0.19
Cl 0.27 0.35
Ca 0.13 0.16
Ti 0.09 0.12
Cr 0.10 0.13
Fe 0.06 0.08

Precision tests at 4200 W on a polymer sample for trace element. The analysis has been carried out by running the sample for ten consecutive investigations with a counting time of 20 seconds per element. The results are summarized in Tables 2 and 3.

Table 2. Repeatability for trace analysis in polymers.

RUN Al [PPM] Ca [PPM] Ce [PPM]
1 71.3 104.4 24.2
2 72.0 104.2 24.0
3 71.6 104.1 23.0
4 71.5 105.1 23.5
5 72.5 104.9 23.2
6 73.5 105.2 23.7
7 72.7 105.6 23.6
8 73.4 105.8 22.4
Avg. 72.3 104.9 23.5
SD 1.4 1.0 1.3

Table 3. Repeatability for trace analysis in polymers (4200 W).

RUN Fe [PPM] Mg [PPM] Ti [PPM] P [PPM]
1 11.4 60.1 2.1 15.6
2 11.6 59.9 2.0 15.6
3 11.6 60.8 2.0 15.6
4 11.4 60.6 2.1 15.7
5 11.6 61.3 2.0 15.6
6 11.4 61.5 2.0 15.9
7 11.6 61.7 2.2 16.1
8 11.6 60.5 2.1 16.2
Avg. 11.5 60.8 2.0 15.8
SD 0.1 0.28 0.1 0.4

Conclusion

Analysis of these elements at low concentration levels require high sensitivity and accuracy. These results show that the ARL PERFORM'X series is suitable for obtaining high-precision results for the detection of Mg, Al, P, Cl, Fe, Ca and Ti in polymer samples. Other elements could have been equally identified, but were not present in the standard samples used for this test. Due to an optimum power management, the ARL PERFORM'X spectrometers can operate at 2500 W without requiring external water cooling. Therefore neither tap water, nor a water cooler is required in these cases. At higher power levels (4.2 kW), energy savings and reduced stress on the X-ray tube are obtained thanks to intelligent management of the X-ray tube power. Furthermore, the state-of-the-art OXSAS analytical software under Windows 7 offers detailed functions and ease of use.

About Thermo Fisher Scientific- Elemental Analysis

For over 75 years, Thermo Fisher Scientific has been a worldwide supplier of spectrochemical instrumentation to such major industries as steel, transportation, cement, construction, food, pharmaceuticals, chemicals, academic research, petroleum and electronics. They offer unsurpassed capabilities in the areas of optical emission (OE), X-ray fluorescence (XRF), X-ray diffraction (XRD) and automation of spectrometers.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific- Elemental Analysis.

For more information on this source, please visit Thermo Fisher Scientific- Elemental Analysis.

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