Helium or Vacuum: What's Best for Analyzing Light Elements?

Characteristic X-ray radiation from light elements like sodium, magnesium, aluminum, and silicon is significantly absorbed by air, posing a challenge for analyzing these elements using desktop X-ray fluorescence (XRF) spectrometers. To enhance the analysis of light elements, XRF spectrometer manufacturers resort to either vacuuming the X-ray optical system or purging it with gaseous helium. But what's the difference between these two methods, and what are their pros and cons?

Air gaps between the sample and the X-ray detector, even at the smallest possible distance, absorb up to 97% of sodium's radiation intensity, 87% of magnesium's, and 72% of aluminum's. Radiation from lighter elements is almost entirely absorbed. That's why XRF analyzers operating in air can analyze elements starting from magnesium (atomic number 12 in the Periodic Table). To avoid absorption and expand the range of analyzable elements, it's necessary to remove air from the space between the sample and the detector.

The most straightforward method to remove air is vacuum pumping. The main (and perhaps only) advantage of this method is the absence of consumables (excluding periodic vacuum oil replacement in the pump). However, there are several disadvantages. First, pumps are noisy, requiring placement outside the laboratory and additional soundproofing for comfortable operation. Second, the degree of vacuum achieved affects absorption in the residual air, potentially impacting analysis accuracy. Moreover, liquids can't be analyzed in a vacuum as they instantly boil and evaporate. Analyzing powdery samples is also problematic since reintroducing air into the spectrometer chamber at the end of the analysis can scatter the sample throughout the chamber and even contaminate the detector window and sensitive electronics.

An alternative method to air removal is replacing it with gaseous helium. Helium, the lightest inert gas, has an extremely low absorption coefficient for X-ray radiation, even for light elements. In a helium atmosphere (compared to a vacuum), detectors capture 99.2% of sodium's characteristic radiation, 99.6% of magnesium's, and 99.8% of aluminum's. Analytically, helium nearly matches the performance of a vacuum, allowing analysis of samples in any state: solid, liquid, or powdery. The only drawback is helium consumption and the need for periodic cylinder replacement. For Elvatech spectrometers, this is a minor issue since purging is automatically activated only during the analysis of light elements, minimizing helium usage. Even with intense operation, a standard 40-liter cylinder lasts for a year.

Might vacuum be used to extend the range of elements analyzed towards those lighter than sodium, given its 100% transmission at any quantum energy, while helium's transmission decreases with lowering energy? This could only be answered affirmatively if no other absorbers were present between the sample and the detector. However, for desktop XRF spectrometers, this isn't the case. Firstly, the sample is placed on the spectrometer's measuring window, covered by a special ultra-thin X-ray transparent film. This film prevents foreign objects, dust, dirt, samples, and their fragments from reaching the fragile windows of the X-ray tube and detector. The absorption at the sodium line even for the best and thinnest films is 40 – 60% and sharply increases with the decrease in the atomic number of the analyzed element. Secondly, the X-ray detector is equipped with its protective window, made of beryllium with a thickness of 8 – 12 micrometers or graphene with a thickness of 1 micrometer in new-generation detectors. The beryllium window transmits 77% of aluminum's radiation, 60% of magnesium's, and only 38% of sodium's. The graphene window is slightly more X-ray transparent, transmitting 87%, 76%, and 64% for Al, Mg, and Na, respectively. Together, these two construction elements make it practically impossible or inefficient to register radiation from elements with atomic numbers below 11, even in a vacuum. Additionally, the low-power X-ray tubes used in desktop spectrometers cannot excite a significant intensity of radiation from these elements due to the sharp decrease in fluorescence yield with the reduction of the element's atomic number.

Therefore, using a vacuum in desktop laboratory spectrometers offers no advantages over helium purging, limits the device's functional application, and is impractical.

But when is a vacuum in the spectrometer truly irreplaceable? Vacuuming does allow for expanding the analysis range up to boron (atomic number 5) in stationary wave-dispersive XRF spectrometers equipped with several-kilowatt X-ray tubes, where protective films are not used. But that's a different class of X-ray spectral equipment.