Zetium Petro edition
The Petro edition of the Zetium X-ray fluorescence spectrometer is capable of the determination of lead in gasoline according to the latest ASTM D5059 standard test method C (low-lead method), using bismuth as internal standard.
Lead, in the form of tetraethyl lead (TEL) has for many years been an additive to automotive fuel, to improve the ignition and combustion properties of the fuel in vehicle engines. However, because of the harmful environmental effects of lead, international regulations introduced after 1995 have limited the amounts of this element contained in automotive fuel. The recommended method for lead determination using X-ray spectrometry is described in ASTM 5059.
This study was performed using the wavelength dispersive optics of the Petro edition of the Zetium spectrometer. The Zetium spectrometer is configured with a 1 kW Rh anode SST R tube with optional CHI-BLUE tube protection coating. A fully integrated system with an X-Y sample handler and state-of-the-art SuperQ software, featuring an advanced Fundamental Parameters algorithm, the Zetium spectrometer is engineered for excellence in terms of both analytical accuracy, precision and operational performance.
Sample preparation and measurement details
A series of commercially available standards and bismuth solutions from VHG Labs Inc. (US), were used to set up the calibration according to ASTM D5059 test method C (concentrations of lead between 0.010 and 0.50 g/US gallon). According to this method, 20 ml of sample are first mixed thoroughly with 2 ml of Bi solution. From this mixture,
10 g are poured into a 38 mm disposable liquid cup and assembled with a 3.6 µm Mylar film. The specimen is measured in less than 4 minutes, Table 1.
Table 1. Measurement times
Accurate calibration results
The accuracy of the Pb calibration is shown by the data in Table 2.
Table 2. Calibration results
ASTM 5059 test method C, mandates plotting the Pb concentration against the corrected ratio, Rc, Figure 1. This way of plotting is due to historical reasons, as it is common practice to plot intensities against concentrations. However, SuperQ’s flexibility is perfectly capable of catering for this.
Figure 1. Calibration graph of Pb according to D5059 test method C plotting requirements
Precision and instrument stability
Repeatability was tested by consecutive measurement of 20 freshly prepared gasoline samples. The maximum allowed difference between two successive measurements is calculated using the D5059 test method C formula: 0.007 + 0.14X, where X is the lead concentration in g/US gal. The results of these tests are given in Table 3.
Additional triplicate measurements were performed on 5 different days spread out over 72 days. When these measurements were treated as consecutive measurements, the maximum differences between these measurements were also well within requirements set by the ASTM D5059, Table 4. During this period, no monitor corrections were required. The graphical representation of this data is given in Figure 2. The dotted lines represent the repeatability limits set by D5059 test method C.
Table 3. Precision results
Figure 2. Long-term stability (> 2 months)
The lower limit of detection (LLD) for lead in gasoline fuel is given in Table 5.
Table 5. Lower limit of detection
Low detection limits are possible even for relatively short measurement times, for example LLDs of 3.78 mg/US gal
Table 4. Precision results : short term and long term ( > 2 months)
The Petro edition of the Zetium spectrometer is very capable of analyzing low lead concentrations in gasoline according to the latest ASTM D5059 test method C. Excellent calibration accuracy, together with outstanding stability, both on short and long term, make the Zetium spectrometer ideal for high-precision measurements and low-maintenance analysis of lead in gasoline.