Strategies for Optimizing Elemental Analyses

By Atlantic Microlab

Elemental Analysis, sometimes referred to as Combustion Analysis, is an incredibly useful tool for researchers in the fields of medicinal chemistry, pharmaceutical sciences, material sciences, and the biotech industry. Recently, demands for precise percent-by-weight assays in the fields of Agronomy, Botany, and Plant Sciences have increased, especially for Total Nitrogen determinations in plant material and soil.

Determining the percentages of elements like Carbon, Hydrogen, and Nitrogen have been in great demand for many decades. In the late 1960s, automatic analyzers were introduced, revolutionizing the industry and allowing for speedier and more efficient, accurate results. Researchers began to rely heavily on the data produced by these instruments to help characterize their compounds and determine their purity. While other tools have become available in recent times,  helping chemists understand the nature of the compounds produced in their laboratories, elemental analysis for CHN as well as Sulfur and Halides have continued to be in strong demand.

In this article we are specifically looking at strategies to optimize the use of elemental analysis. First, let’s consider synthetic organic research compounds, the products made by pharmaceutical and biotech companies used to manufacture life-saving and life-altering drugs. These compounds require strict purity and along with other data derived from High Resolution Mass Spectroscopy, Carbon-13 Nuclear Magnetic Resonance Spectroscopy, and the classic Proton NMR, researchers can put all of the pieces of the puzzle together to characterize their synthetic compounds. Many chemists consider combustion elemental analysis to be the ultimate check for purity. And while compounds don’t always meet specs, the data is quite revealing to researchers in determining the subsequent steps needed to purify or even reconstruct their compounds.

So how can a chemist get the most out of their data? Well, it’s a two-way-street. A chemical analysis cannot be any better than the sample supplied, so taking the steps to properly synthesize and purify a compound in the lab is an important beginning. But let’s look at other factors involved in submitting a sample to an analytical facility like Atlantic Microlab! Before shipping a sample, the compound must be of sufficient weight, homogenous, reasonably dry, and placed in a flat, glass-bottomed vial free of dust, debris, and any other contaminants. Remember, anything with mass will affect a percent-by-weight determination, so even a tiny eyelash, a speck of filter paper or dust, a shard of glass, or a sliver of cardboard flaked off from dividers in boxed vials could compromise an analysis if unnoticed…and yes, in the 5 decades we’ve been doing this work, we’ve seen it all. Using plastic bags, or wax paper to ship your sample is never a good idea. Even the conical plastic vials present a problem. Technicians remove sample with a spatula, and precious material can be difficult to extract when pressed into the bottom of the cone. A 1 dram vial is always the best option, but a similar glass container will suffice.

Elemental Analysis doesn’t require a huge amount of material for organic research compounds normally, but we recommend a minimum of 5 milligrams of transferrable sample for a single CHN run. Analyses can be performed with smaller weights, but there is invariably a little product lost in the transfer, especially if the sample is electrostatic, so adhering to the 5 milligram rule is always a good idea. A small piece of parafilm is advisable to secure the cap and to protect the sample from picking up moisture or preventing sample oxidation. Scotch tape dries out and cracks, creating an environment for sample contamination if the tape flakes around the rim of the vial, so its use should be avoided. Also worth noting, a material like vermiculite for packing is often problematic. The vermiculite becomes embedded under the vial cap, sticks to the glass threads of the vial, and can easily contaminate the sample. So to reiterate, a minimum of 5 mg of transferrable sample placed in a 1 dram vial with a piece of parafilm around the cap wrapped in a soft, cushiony material is ideal.

Nothing is more frustrating to a submitter or staff on our end than when a sample arrives with the vial shattered and the sample unsalvageable. Time is wasted along with the material that may be in short supply, not to mention the wasted shipping costs. It is advisable to place the sample in bubble wrap, tissue, or even a paper towel, and then secure it with a piece of tape or two—there is no need to use excessive taping if the vial is properly wrapped. When shipping, it’s always a good idea to shake the package a bit to make sure the packing is secure and nothing is rattling. Placing a glass vial inside another glass vial is folly. About half of the samples shipped this way arrive with the outer vial broken and sometimes the sample-containing vial as well.

When sending a sample to a contract lab, the sample must be accompanied by a submittal form—one form for each sample. These forms can be obtained from the analytical laboratory, usually by downloading them from the company’s website. The submittal forms are the instructions for handling the samples. The more information provided, the better. Leaving out critical information is like going to the doctor and telling him/her you are ill but declining to give specifics. The doctor might be able to assist but it will take longer and likely create limitations to how helpful the physician can be. So, providing everything requested on the form is essential to creating an environment for the analyst to provide the best possible results. Information is key to providing optimal service, so avoid leaving any of the sections blank if at all possible. Let’s start with sample number. As an example, say the sample number written on the form is ATL-4-11-65, then the sample vial should match it exactly. Next, fill in the area requesting your name, company or school name, along with address and contact information. A clear, legible email address will be needed when it’s time for results to be sent.

Now for the really critical part. List the elements in the compound in the “Elements Present” section—all of them, not just the elements being tested. This will aid the technician on how your sample will be treated. Next, clarify in the “Analyze For” section all the elements required for analysis. Does the sample need a little touchup drying to remove moisture picked up in transit? Mark this clearly on the form and specify temperature and vacuum. A properly prepared sample with fewer than 20 milligrams submitted shouldn’t need more than an hour drying time once received in an analytical lab. Finally, provide theories or complete molecular formulas. Why is this important? Labs use a variety of calibration standards and the analyst wants to select the best possible standard for the sample. A sample with a 3% Hydrogen will analyze much better with a standard at 4% Hydrogen than a standard at 9%. Otherwise, the sample will require being recalibrated with an appropriate standard or re-analyzed. This slows down turnaround time and there may be associated charges as well.

Finally, we turn to analyzing plants and soils. This is a slightly different animal and some of the suggestions above won’t always apply. Generally, clients in the Botany and Agronomy fields are looking for Nitrogen in plant material—or sometimes Carbon/Nitrogen ratios in soil. Since molecular formulas are not associated with these materials, an exact theory is impossible and ranges are perfectly acceptable. Other criteria is also relaxed. Though we still recommend sending in glass vials, scientists in these areas of study may require testing of larger sample sizes and therefore a variety of containers are acceptable. But to receive the best possible data, it’s recommended that samples be as homogenous as reasonably possible and dried—remember, moisture has mass and will affect results if not dried properly.

A lot to absorb here and we realize perfect compliance is unlikely—we don’t mean to be the ogre school librarian smacking hands with a splintered ruler. We strive to be helpful and useful in aiding scientists to get the most out of their analytical results. So, adhering to the criteria listed in this article will almost certainly produce better outcomes and better data. As mentioned before, optimal elemental analyses is a two-way street!