Elemental Analysis Goes Nuclear

Elemental oil analysis typically consists of techniques that use readily available, stand-alone equipment. Consequently, inductively coupled plasma mass spectro-metry (ICP) and rotating disc electrode (RDE) spectroscopy are used often in most commercial and in-house laboratories.

While these techniques offer convenient ways to provide routine elemental analysis, they are limited. Problems include particle size limitations, incomplete sample digestion and nonhomogeneity of the sample.

In addition, the instruments can be difficult to calibrate, become easily contaminated and often must be operated by an experienced analyst to obtain quality results. Also, many elements of potential interest, like halogens, cannot easily be detected by these techniques.

However, complete, quality elemental analysis is possible by combining a common method with a unique multielement technique.

An Introduction to INAA

This multielement trace level analysis technique is Instrumental Neutron Activation Analysis (INAA). Due to its inherent simplicity, sensitivity and accuracy, INAA (discovered in 1936)¹ has been traditionally used in a research capacity.

For example, because INAA can detect many of the same elements as conventional methods and is a completely independent method, it is used as a reference/referee technique for other analytical methods. Also, because the technique is based on nuclear, not chemical properties, and is matrix independent, INAA can be applied to a wide range of disciplines.

Fields that use the INAA technique include environmental sciences, nutritional and health-related studies, geological and geochemical sciences, material sciences, archaeological studies, forensics and others.

However, INAA has evolved into more than a high-precision research tool for academia. It can be configured to provide a high-volume, low-cost elemental analysis that is practical for the demands of used oil analysis. Why then isn’t neutron activation a mainstream analytical technique? Two words - nuclear reactor.

The major stumbling block for most commercial labs is the lack of a local, low-power nuclear reactor (let alone one that is configured for industrial INAA applications). As a result, there is an insufficient awareness within most industries regarding the opportunities of this technique. INAA laboratories do exist, yet they remain an untapped analytical resource.

This article is aimed at making those in the used oil analysis industry aware of an alternate, and in some cases complementary in technique to ICP and RDE.

Theory of INAA

Neutron activation may be nuclear science, but one of INAA’s biggest advantages is its simplicity. Because the technology and its concepts are not widely known, some technical background is presented here. If a more thorough lesson full of equations, diagrams and Greek letters is desired, an Internet search will yield numerous university Web sites containing detailed accounts.

In elemental analysis, samples (oil, grease, sludge, polymer, etc.) must first be exposed to an excitation source. For ICP, this source is a high-energy plasma². For INAA, the high-energy source of choice is neutrons (symbolized by “n”). In every operating nuclear reactor there is a sea of neutrons. These neutrons are the by-products of a nuclear chain reaction found within a reactor core (Figure 1).

Figure 1. Nulcear Reactor Core

Neutrons are penetrating and interact with the nuclei of the sample’s atoms to form measurable radioactive isotopes. INAA is a multielement technique because activation occurs simultaneously for each isotope present in the sample. The following example, which uses calcium, explains the basic INAA concept (Formula 1).

Formula 1

From the stable Ca-48 isotope, a measurable activation product is produced. It is distinguished from all other activation products produced from the sample by its characteristic high-energy gamma rays and decay rate (represented by the half-life, T½). The gamma ray energy spectrum is obtained from a spectrometer over a predetermined acquisition time, from seconds to hours.

This produces a nuclear fingerprint of all the activated elements in the sample. Because numerous elements have multiple radioisotopes of varying half-lives, there is significant flexibility in what isotope is used to determine an elemental concentration. This characteristic of INAA can be especially beneficial when trying to overcome the spectral interferences that can hinder traditional techniques.

Using INAA in Practice

As mentioned earlier, the strength of INAA is its simplicity - encapsulate, irradiate, count. Because radioisotopes decay at different rates, varying the irradiation conditions and/or counting after different lengths of decay allows for optimization of the elements most desired (Figure 2).

Figure 2. Sample Gamma Ray Spectrum

Elemental groupings include:

1. Short-lived elements, such as fluorine, selenium and silver can be detected if the reactor is equipped with an automated pneumatic sample transport system. These elements are sensitive and require only brief (seconds) irradiation, decay periods and count times.

2. Elements that have a half-life of less than a few hours can normally be detected in a single count but the count must be performed at the reactor facility. Longer (minutes) irradiation, decay periods and count times than those of the elements previously mentioned are typically used for the most common elements found in lubricants. Aluminum, barium, calcium, magnesium, manganese, tin, vanadium and the halogens (chlorine, bromine and iodine) can be detected simultaneously.
   
   With a slight variation in irradiation conditions, silicon and molybdenum can also be detected. When samples irradiated for any of the above elements are allowed to decay overnight, the short-lived elements will decay away. This lowers the overall activity allowing for copper, sodium, potassium and zinc to be detected (and with less sensitivity antimony, arsenic, tungsten and chromium). With a coordinated setup, these analyses can be completed for next-day service - thus dispelling one misconception that INAA has unacceptably long turnaround times.

3. Samples that require the identification of elements with long half-lives (iron, chromium, nickel, molybdenum, and more than 30 others) are packaged separately and irradiated for 10 minutes or more. Counting is performed after the decay of short-lived isotopes, and the count length can be hours if extreme sensitivity is needed.

4. Using a variation of INAA known as prompt gamma neutron activation analysis (PGNAA), boron and gadolinium can also be detected. In this scenario, no decay period is necessary - the spectra are obtained during irradiation. Low parts per million (ppm) detection limits can normally be achieved.

Figure 3. A Gamma Detector in a Liquid Nitrogen Dewar.
The upper part of the detector is surrounded with lead to keep
natural radiation from raising the spectral background.

Incorporating INAA Into Your Analysis Protocol

INAA has a number of advantages that solve the shortcomings of common techniques. Simplicity and large sample sizes are two examples. More comparisons between INAA and ICP are listed in Table 1.

Click Here to See Table 1.

In June 1998, the International Atomic Energy Agency organized a meeting on the “Enhancement of Research Reactor Utilization for Neutron Activation” and summed up INAA’s role succinctly. “Each analytical technique has its own particular advantages and disadvantages that make each suitable (or unsuitable) for a given application. INAA is unique in several important aspects, such as being largely independent of matrix effects, being suitable for analysis of materials that are difficult to dissolve, being relatively insensitive to sample contamination and having specific means of detection.”³

INAA is not meant to replace other multielement methods. In fact, in most instances, INAA and the other methods complement each other. For example, INAA can be used to determine concentrations of elements that other methods cannot practically obtain. The halogens (F, Cl, Br, I) are a specific example. INAA’s multielement feature that analyzes for only short-lived or long-lived groups of elements as demonstrated in Table 2 can be advantageous.

Click Here to See Table 2.

These groupings provide additional elements as well as significant elemental overlap. Finally, a complete INAA analysis (at about $100 per sample) of 40 or more elements combined with another technique can provide a total elemental characterization. With a significant number of elemental concentrations being identified by totally different and independent methods, accuracy will be ensured in the same way reference materials are certified.

References

1. Alfassi, Z. (1990). Activation Analysis (Volume 1). Boca Raton, Fla. CRC Press.
2. (2002, January-February). Elemental analysis. Practicing Oil Analysis. pp. 28-32.
3. Bode, P. (2001). Use of research reactors for neutron activation analysis. International Atomic Energy Agency (IAEA) - Tecdoc 1215.

Editor’s Note
Becquerel Laboratories Inc. is a privately owned and operated, ISO17025 accredited laboratory. It has been offering trace level, inorganic analysis via neutron activation commercially to industry and laboratories for more than 20 years. Blake Barber is a chemist at Becquerel who has specialized in neutron activation for the last eight years. He can be reached via e-mail at blakeb@becquerellabs.com or by phone at 1-877-726-3080 (toll free).