Analyzing Pb Levels in Gasoline According to ASTM D5059 Norm

Goal

A calibration curve has been set up according to the ASTM D5059 norm to demonstrate the analysis of lead (Pb) in gasoline.

Fuel pump

Instrumentation

Thermo Scientific ARL PERFORM’X series spectrometer with a 4200 W system was used in this analysis. This system is configured with a 5GN+ Rh X-ray tube, helium purge, up to nine crystals, 4 collimators, 6 primary beam filters and 2 detectors for best performance from ultra-light to heaviest elements using its 50 µm Be window. Fixed with a low current filament, this novel X-ray tube ensures an unequalled analytical stability month after month.

The ARL PERFORM’X provides the ultimate in sample analysis safety and performance. The system’s unique LoadSafe design comprises a series of features that prevent damage during sample loading. Liquid cassette recognition prevents exposure of any liquid sample to a vacuum even by mistake. If X-ray exposure time is very long, then over exposure safety automatically ejects a liquid sample.

The primary chamber is protected by the Secutainer system by collecting any drops in a uniquely designed container that can be easily removed and cleaned by an operator. To protect the spectral chamber, the ARL PERFORM’X uses a helium shutter that is designed to provide complete protection for the goniometer during liquid analysis under helium operation. A special X-ray tube shield offers total protection against liquid cell rupture in the “LoadSafe Ultra” optional configuration.

Lead Analysis in Gasoline

The ASTM D5059 norm - test method C - has been selected for low Pb levels. In accordance with the norm, five standard samples were prepared to construct a calibration curve. This norm recommends the use of bismuth (Bi) as an internal standard.

The following are instrument settings:

Tube volt. KV Tube curr. mA Detector Collimator Crystal
Bi Lα1 60 50 Scintillator Fine LiF200
Pb Lα1 60 50 Scintillator Fine LiF200
Pb background 60 50 Scintillator Fine LiF200

A 3 kW power is used to prevent overheating of the supporting mylar film that would lead to sagging of the film as well as instability of analysis.

The intensity ratios of Pb have been determined using the following formula:

     Pb ratio = (Pb Lα1 – Pb background) / Bi La1

The calibration curve shown in Figure 1 is obtained by correlating the standard intensities with the Pb content in ppm. The numerical results including the absolute disparity between calculated and nominal concentrations, as well as the Standard Error of Estimate are shown Table 1.

Calibration results for five standard samples with low Pb content.

Figure 1. Pb calibration curve.

Table 1. Calibration results for five standard samples with low Pb content.

Sample # Pb ratio Nom. conc. ppm Calc. conc. ppm Absol. diff. ppm
00 437 0.1 0.0 - 0.1
02 588 1.9 1.7 - 0.2
05 950 4.9 5.5 0.6
10 1’304 9.7 9.4 - 0.3
19 2’240 19.4 19.4 0.0
Standard error of estimate (ppm): 0.4

Repeatability Test

A repeatability test for Pb analysis has been performed using two samples with different Pb concentration: A and B.

Three liquid cells of every sample (A1, A2, A3, B1, B2 and B3) were prepared and analyzed. The results obtained are shown below:

Table 2. Repeatability test on sample A.

Sample Pb concentration in ppm
A1 3.7
A2 3.7
A3 3.6
Average 3.7
SD 0.02

Table 3. Repeatability test on sample B.

Sample Pb concentration in ppm
B1 10.4
B2 10.4
B3 10.1
Average 10.3
SD 0.2

Conclusion

By using the appropriate ASTM standard method, the ARL PERFORM’X XRF can help obtain good calibration curves for Pb determination in gasoline.

Due to the reproducibility of loading and the helium shutter protecting the goniometer chamber from the helium environment, high repeatability of analysis can be demonstrated for Pb analysis.

The results obtained demonstrate that excellent accuracy and precision can be acquired using the ARL PERFORM’X sequential XRF instrument, which is suitable for the analysis of Pb in petrochemical products.

ARL PERFORM’X sequential XRF instrument,

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

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

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