Measurement and Testing
Petra MAX is powered by High Definition X-ray Fluorescence (HDXRF) technology: an elemental analysis technique offering significantly enhanced detection performance over traditional XRF technology. This technique applies state-of-the-art monochromating and focusing optics, enabling higher signal- to-background ratio compared to traditional polychromatic XRF. HDXRF does not require sample conversion, equating to no consumable gasses, little to no sample preparation, and delivers results in minutes.
Figure 3 shows the basic configuration of HDXRF and its use of focused monochromatic excitation. In this system, the diffraction-based doubly curved crystal optics capture a wide angle of X-rays from the source and focus a narrow energy band (monochromatic) of X-rays to a small spot on a measurement cell. The monochromatic beam excites the sample and secondary characteristic fluorescence X-rays are emitted. A detector processes the secondary X-rays and the instrument reports elemental composition of the sample.
digested with sulfuric acid and heat, using both an infrared lamp and a hotplate. The sample is stirred continuously with a glass rod during digestion and carefully monitored to ensure that no sample is lost due to frothing and spattering. Once the foaming has finished, the heat is increased until the sample has been reduced to a carbonaceous ash. This process is completed inside a well- ventilated fume hood using gloves and a face shield for protection from strong oxidizing fumes generated during digestion. This ash is then carefully transferred to a muffle furnace and all of the carbon is burned away. The remaining material is then reconstituted with nitric acid and returned to a steam bath. This beaker is then transferred to a hotplate where it remains until the liquid has fully evaporated. Once again, nitric acid is added and the sample is transferred to a volumetric flask and brought to a known volume with additional nitric acid. The sample is then ready for analysis. Users report that this process of sample preparation to results takes anywhere from 8 to 12 hours to complete.
HDXRF Sample Preparation Notes Figure 3: HDXRF Technology
Figure 4 compares the detector signal of polychromatic (competitor) with monochromatic (XOS) XRF to demonstrate how monochromatic excitation reduces background noise and improves signal definition, delivering lower limits of detection and dramatically better precision.
Samples prepared for analysis by Petra MAX are first shaken to homogenize the sample. Using a disposable pipette, 7 to 10 mL of sample is transferred to a disposable plastic sample cup designed for XRF analysis. An X-ray transparent film is then placed over the opening of the cup and affixed using a snap-on ring. Finally, the sample is placed in the instrument and ready for analysis. This process from sample preparation to results takes about 6 minutes.
HDXRF VS ICP Correlation
A simple way to show correlation between techniques is to measure a sample set spanning a range using two different techniques. Using a spreadsheet, scatter plot the results of the study with each technique on a separate axis. Next, plot the trend line with the R-squared value, also known as the coefficient of determination. This is a value between 0 and 1. The better the correlation, the closer this value will be to 1. If the correlation between the two techniques is good, the plotted data points will be on or near the trend line, and the R-squared value will be close to 1. If the correlation is poor, the data points will not be near the trend line, and the R-squared value will be much less than 1. Figure 5 depicts an example of good correlation, and Figure 6 depicts an example of poor correlation. Figure 7 shows the correlation between Petra MAX and ICP as a result of this study. The plotted points are near the trend lines for both nickel and vanadium, and the R-squared value for both elements is 0.99. This indicates that there is good correlation between Petra MAX and ICP for nickel and vanadium in crude oil.
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Figure 7: ICP vs Petra MAX Correlation Petra MAX VS ICP Precision
Precision is an important characteristic of measurement technologies. Because a single measurement is typically used to represent an important quality parameter like sulfur, nickel, or vanadium, it is important to understand how much variability is associated with the measurement value. The more precise a measurement technique is, the less likely that undesirable results will occur. In the case of crude oil, a buyer and seller are less likely to dispute whether quality specifications have been satisfied, and the refiner can be sure they are taking the proper considerations during processing. ASTM and ISO standard test methods evaluate the precision of a test method in terms of repeatability and reproducibility.
Repeatability (r) is defined as the difference between repetitive results obtained by the same operator in a given laboratory, applying the same test method with the same apparatus, under constant operating conditions, on identical test material and within short intervals of time, would in the long run and in the normal and correct operation of the test method, exceed the value calculated only once in 20 measurements (5% of the time). Or more simply put, repeatability is the maximum expected difference (at 95% confidence) between two measurement results run on the same material using the same apparatus, test method, and operator.
Figure 4: Superior Signal-to-Noise Ratio HDXRF VS ICP Study
A study was conducted to compare the sample preparation process and precision using Petra MAX, powered by HDXRF, and ICP to measure nickel and vanadium in crude oil. Four crude oil samples were obtained for the comparison study:
A. Custom doped crude oil standard from VHG Labs B. Sour crude oil retain from Intertek C. Medium sour crude retain from
Crudemonitor.ca D. Heavy sour crude retain from
Crudemonitor.ca
Three independent laboratories analyzed the sample set using ASTM D5708B (ICP) and Petra MAX (HDXRF). Each participant received two randomized sample sets packaged in blind duplicate for analysis. The resulting raw data sample means can be seen in Table 1.
ICP Sample Preparation Notes
ICP performed by ASTM method D5708B requires extensive sample preparation when analyzing crude oil. Crude oil is first transferred into a glass beaker and weighed. The sample is
Figure 6: Poor Correlation
Reproducibility (R) is the difference between two single independent results obtained by different operators, applying the same test method in different laboratories, using different apparatus on identical test material, would in the long run and in the normal and correct operation of the test method, exceed the value calculated only once in 20 measurements (5% of the time). Or, reproducibility is the maximum expected difference (at 95% confidence) between two measurements taken on the same material using the same test method by two different laboratories each using a different apparatus and operator.
Precision is often dependent on the concentration level of the material being tested, and in these cases will be expressed as equation. Precision equations for elemental analysis test methods are generally linear or exponential as in the following examples:
• Linear precision example: (r) or (R) = X * 0.1234
• Exponential precision example: (r) or (R) = X0.123 Figure 5: Good Correlation
In these precision examples, X is the mean value or concentration of interest. To improve visualization, precision statements are often graphed with the concentration (X) on the x-axis and the corresponding repeatability or reproducibility on the y-axis. The lower the value on the y-axis for a given concentration, the better the precision. Figure 8 depicts an example of good precision, and Figure 9 depicts an example of poor precision (as compared to the good precision plot).
* 4.567
Figure 8: Good Precision OCTOBER / NOVEMBER •
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