ASTM D4294 for sulfur in petroleum and petroleum products
ASTM D4294 standard test method for sulfur in petroleum and petroleum products by energy-dispersive X-ray fluorescence spectrometry
3. Summary of Test Method
3.1 The sample is placed in the beam emitted from an X-ray source. The resultant excited characteristic X radiation is measured, and the accumulated count is compared with counts from previously prepared calibration standards that bracket the sample concentration range of interest to obtain the sulfur concentration in mass %.
4. Significance and Use
4.1 This test method provides rapid and precise measurement of total sulfur in petroleum products with a minimum of sample preparation. A typical analysis time is 2 to 4 min per sample.
4.2 The quality of many petroleum products is related to the amount of sulfur present. Knowledge of sulfur concentration is necessary for processing purposes. There are also regulations promulgated in federal, state, and local agencies that restrict the amount of sulfur present in some fuels.
4.3 This test method provides a means of compliance with specifications or limits set by regulations for sulfur content of petroleum products.
4.4 If this test method is applied to petroleum matrices with significantly different composition than the white oil calibration materials specified in this test method, the cautions and recommendations in Section 5 should be observed when interpreting the results.
4.5 Compared to other test methods for sulfur determination, Test Method D4294 has high throughput, minimal sample preparation, good precision, and is capable of determining sulfur over a wide range of concentrations. The equipment specified is in most cases less costly than that required for alternative methods. Consult the ASTM website or ASTM Subject Index for names of alternative test methods.
5. Interferences
5.1 Spectral interferences result when some sample component element or elements emit X-rays that the detector cannot resolve from sulfur X-ray emission. As a result, the lines produce spectral peaks that overlap with each other. Spectral interferences may arise from samples containing water, lead alkyls, silicon, phosphorus, calcium, potassium, and halides if present at concentrations greater than one tenth of the measured concentration of sulfur, or more than a few hundred milligrams/kilogram. Follow the manufacturer's operating-guide to compensate for the interferences.
5.2 Matrix effects are caused by concentration variations of the elements in a sample. These variations directly influence X-ray absorption and change the measured intensity of each element. For example, performance enhancing additives, such as oxygenates in gasoline, can affect the apparent sulfur reading. These types of interferences are always present in X-ray fluorescence analysis and are completely unrelated to spectral interferences.
5.3 Both types of interferences are compensated for in contemporary instruments with the use of built-in software. It is recommended that these interferences be checked from time to time and that the software corrections offered by the manufacturer not be accepted at face value. Corrections should be verified for new formulations.
5.4 M-85 and M-100 are fuels containing 85 and 100 % methanol, respectively. As such, they have a high oxygen content, hence, absorption of sulfur Ka radiation. Such fuels can, however, be analyzed using this test method provided that the calibration standards are prepared to match the matrix of the sample. There may be a loss of sensitivity and precision. The repeatability, reproducibility, and bias obtained in this test method did not include M-85 and M-100 samples.
5.5 In general, petroleum materials with compositions that vary from white oils as specified in 9.1 may be analyzed with standards made from base materials that are of the same, or similar, composition. Thus, a gasoline may be simulated by mixing isooctane and toluene in a ratio that approximates the true aromatic content of the samples to be analyzed. Standards made from this simulated gasoline will produce results that are more accurate than results obtained using white oils.
NOTE 1 - In the case of petroleum materials that contain suspended water, it is recommended that the water be removed before testing or that the sample be thoroughly homogenized and immediately tested. The interference is greatest if the water creates a layer over the transparent film as it will attenuate the X-ray intensity for sulfur. One such method to accomplish the removal of water is to centrifuge the sample first under ambient sealed conditions, taking care that the sample integrity is not compromised.
