Analytical Techniques and Instrumentation

X-Ray Techniques:

X-ray techniques are essentially used for elemental analysis. With properly built x-ray equipment, almost every element in periodic table (with a few exceptions) can be analyzed by x-rays. There are generally two types of x-ray techniques:

  • X-Ray Fluorescence
  • X-Ray Diffraction

I will only discuss x-ray fluorescence because x-ray diffraction is beyond the scope of our analytical capability. Also in our business x-ray fluorescence use is limited to the analysis of sulfur and now chlorine as well. The equipment is called x-ray spectrometer. There are several in our laboratories. X-ray equipment can be:

  • Energy Dispersive
  • Wavelength Dispersive

ASTM D4294 is the typical example of testing sulfur with good precision to about 0.02% wt and up to 5% wt. Uses energy dispersive x-ray.

ASTM D2622 is the typical example of testing sulfur with good precision to about 10ppm (0.0001% wt) and up to 5% wt. Uses wavelength dispersive x-ray.

Essentially, wavelength dispersive x-ray methodology has very low minimum detection limit. Sulfur when present above 0.02% wt can be best done by either of the two methods but sulfur levels below 0.02% wt, the more reliable x-ray method is ASTM d 2622.

Here it is very important to understand that before running sulfur by any of the above two methods, the pertinent equipment must first be calibrated using reliable sets of standards and then verifying the calibrations by applicable procedure given in each of the two methods. Equipment’s reliability of test results rests with technician preparing and verifying the equipment before use. X-ray techniques described above can also be used for running other elements like vanadium, lead etc. However, it requires additional hardware in the equipment. We have therefore limited our discussion to testing of sulfur and chlorine/chloride.

ASTM D7536 is also used in our labs to detect the presence and quantity of total, inorganic & organic chlorides by x-ray method. X-ray chloral is available in our labs for chlorides testing. This test, when used for organic chlorides, requires careful sample preparation before analyzing chlorides.

Some of the newly added equipment (sindies) can also be used with better precisions based on manufacturer’s claims.

In summary, x-ray testing must be done with proper sample and equipment preparation. The technician should strictly follow both ASTM method as well as manufacturer’s recommendations for insuring the reliability of his test results.

Low levels of sulfur in LSD and regulated gasolines are best tested by ASTM D5453 which is not x-ray but combustion and detector based technique. Sulfur can be analyzed down to 1 ppm level by this method with good precision.