Shelf Life

Dr. Bruce Scruggs, XRF Product Manager, EDAX

Recently, we had a customer request to see a demonstration on the Orbis micro-XRF system. As we talked about what they would like to see, he mentioned that he had made some test XRF measurements on table salt, and he couldn’t measure the iodine content. I agreed to measure the iodine content in table salt. Initially, I thought this would be a very straightforward exercise, as table salt is just NaCl with some iodine added, but this was anything but straightforward.

The iodization of salt in the United States began about a century ago. Iodine is an important micronutrient for thyroid gland health. Certain portions of the American population had diets deficient in iodine and the iodization of table salt was chosen as a method to increase the level of iodine in the average American diet. The salt iodization process was inexpensive; salt does not spoil and estimates of table salt consumption were available.

Some weeks before the customer demo, I bought some iodized table salt from the local grocery store. The ingredients list showed iodine in the form of potassium iodide at about 45 ppm iodine. This concentration was consistent with my web searches. I pressed a pile of salt grains onto a piece of carbon tape and measured it with the Orbis system using a 2 mm spot size (the system was equipped to measure down to a 30 μm spot size, small enough for individual grains, but I wanted to avoid any potential issues with grain to grain variations). It was easy enough and I could measure the I(L) lines with I(Lα) at 3.937 keV (Figure 1).

(A): Salt spectrum with peak deconvolution, not including I(L) series; Fig 1(B): The same salt spectrum as in (A) with peak deconvolution including I(L) series.

Figure 1. (A) Salt spectrum with peak deconvolution, not including I(L) series. (B) The same salt spectrum as in (A) with peak deconvolution including I(L) series.

Some weeks later, during the actual customer demonstration, we measured a variety of customer supplied samples and the customer asked to measure table salt near the end of the demo. I put my table salt sample into the Orbis and was astonished to find that the iodine signal disappeared (Figure 2). Peak fitting and quantification results showed no detectable iodine. After a discussion with the customer, I began to suspect that the salt iodization level was not stable, given that solid I2 is known to undergo sublimation at room temperature. I spoke to the customer again and in his previous attempts, he measured table salt (from shakers) in the company cafeteria. I often wonder how long that salt has been in the shaker!

The same salt sample, as Figure 1, measured on the Orbis a few weeks later without the presence of iodine.

Figure 2. The same salt sample, as Figure 1, measured on the Orbis a few weeks later without the presence of iodine.

Further web searches indicated that indeed, the iodization level of salt has a certain shelf life depending on many factors, including temperature, humidity, impurities in the salt, the chemical form of the iodine bearing additives, and product packaging. For example, potassium iodide is oxidized by contact with oxygen and atmospheric moisture and the resulting iodine then undergoes sublimation. In various regions of the world, iodized table salt is formulated to improve its shelf life with regard to iodine retention based on the characteristics of the table salt and the general environment, e.g., desert, tropical. Based on this loss mechanism, I suspect that there must also be a significant loss of iodine during cooking depending on whether salt is added while cooking or directly applied before consuming.

In my case, the iodine level had dropped below detectable limits in about three weeks of being left out on the table. The grains of salt ranged in size from about 100 – 500 μm in characteristic dimensions, and I was curious to what characteristic depth XRF was measuring. Was there possibly any iodine left in the largest crystals? This depth can be estimated based on the fluorescent signal energy as the exciting X-ray energy always has to be greater than the fluoresced photons (The physics are a bit different for electron excitation where the answer is determined by electron penetration depth into the sample).

XRF measurement depth can be estimated from the Beer-Lambert equation for the absorption and transmission of light:

Equation 1

Equation 1.

The mass absorption coefficient (MAC) describes how readily the I(Lα) signal line at 3.937 keV will be absorbed by the NaCl matrix. It can be described as follows:

Equation 2

Equation 2.

For NaCl, we have two MACs describing how Na and Cl each absorb the 3.937 keV photon. The easiest way to get the full matrix MAC is to back-calculate it from the Beer-Lambert equation and any web-based calculator describing X-ray absorption/transmission characteristics modeling the fluoresced photon traversing the sample matrix to the detector. I prefer the website, http://henke.lbl.gov/optical_constants/filter2.html. By inputting the sample matrix formula (including trace elements if desired), and an arbitrary path length, one can get the calculated result for I/Io and then rearrange Equation 1 to solve for the NaCl matrix MAC by inputting the previously used path length and the known density of table salt. The result is: μNaCl(3.937 keV) ~ 540 cm2/g.

Rearranging Equation 1, one can solve for the signal path length through the sample traversed by the fluoresced photon to the detector as a function of I/Io:

Equation 3

Equation 3.

The XRF Emission Depth, D, would typically be defined as normal to the sample surface, and you should also consider the take-off angle (TOA) of the detector defined from the sample surface, as shown in Equation 4.

Equation 4

Equation 4.

Table 1 shows the XRF Emission Depth as a function I/Io with a nominal detector TOA of 50ᵒ.

I/Io [%] Path Length, x [μm] Emission Depth, D [μm]
10 20 15
1 39 30
0.1 59 45

Table 1. XRF Emission Depth as a function of the signal transmission ratio, I/Io.

The definition of the characteristic XRF path length and emission depth is somewhat arbitrary, as it depends on the value assigned to the signal transmission ratio, I/Io. Typically, the characteristic path length is defined as the length over which 99% of the signal is absorbed. Hence:

Equation 5

Equation 5.

It is interesting to note from Table 1, that at 50% of the critical emission depth, the XRF signal is undergoing 90% absorption.

Coming back to the original analysis, it is possible that iodine was still present at the core of the larger 500 μm grains of salt. Further analyses could be done on cross-sectioned grains or pulverized grains to make that determination. It would be possible to measure cross-sectioned grains of NaCl using the 30 μm spot size on the Orbis to study how readily iodine is lost as a function of depth into the NaCl grain, but that is a study for another day.

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