trace elemental analysisAs summer draws to a close, I’m being inundated with back-to-school advertisements, and I find myself reminiscing fondly about my days as a chemistry graduate student. I looked forward to spending hours in the lab, tackling the next trace element (link to trace metal community page) research project. As nerdy as it may sound, I enjoyed each application and the challenges it presented, the biggest of which was usually the data collection itself.

How To Develop A Well-Designed Trace Analytical Method

Many questions would roll around in my brain as I prepared samples for analysis: Which instrument should I use? Which conditions should I use to run the instrument and what should my instrument method look like? How do I verify that my instrument method will produce accurate results? How do I fix my method if it produces incorrect data? I thoroughly enjoyed the thought process and looked forward to the satisfaction of having a well-designed, optimized analytical method that completely satisfied the needs of the application.

 

Start By Knowing Trace Elemental Techniques and Units of Measure

In my post-graduate career experiences, I’ve come to realize that not everyone approaches this type of analytical challenge with the same enthusiasm that I do. Instead, many people view it as a daunting task, with a multitude of opportunities to make mistakes and produce poor-quality data. Most likely, that anxiety stems from a lack of reliable information to help people navigate this analytical journey and troubleshoot hurdles along the way. To that end, I’ve written a two-part blog series to address some of the questions that must be asked and properly answered when developing an analytical method for an elemental analysis application. This first blog will outline key terms, which will allow us to build a foundation for a subsequent discussion on instrument selection and method optimization.

 

Trace Elemental Techniques

Atomic Absorption Spectroscopy (AAS)

A technique involving the introduction of liquid samples that are desolvated, vaporized and atomized with either a flame or a graphite furnace. The gaseous atoms are then passed through a beam of light that is emitted from a radiation source at a single wavelength. The element for that wavelength absorbs an amount of light directly related to the concentration of the element in the sample. The process is repeated for each element.

 

Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES)

A technique involving the introduction of liquid samples that are desolvated, vaporized, atomized (and/or ionized) and excited in an argon plasma. When the excited atoms and ions relax back to their ground state, they emit characteristic wavelengths of light. A solid state detector collects all the emitted wavelengths of light simultaneously. For each wavelength, the amount of emitted light is directly proportional to the concentration of the element in the sample.

 

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)

A technique involving the introduction of liquid samples that are desolvated, vaporized, atomized, and ionized in an argon plasma. Each element has a different mass and, once ionized, forms its own characteristic mass spectrum. An ion counting detector counts the number of ions at each mass-to-charge ratio, which is directly proportional to the concentration of each element in the sample.

 

Speciation

A technique (link to ion chromatography page) for separating different chemical species of an element within a sample. Chemical species are specific forms of an element that include varying oxidation states, charges and molecular species.

 

Hydride Generation

A technique (link to ICP-OES consumables page) for separating hydride-forming elements (As, Bi, Ge, Pb, Sb, Se, Sn, Te) from their sample matrix. In this technique, the sample solution is acidified and mixed with a reductant (such as sodium borohydride) to convert select elements to their gaseous hydride form. The hydrides evolve out of the sample solution and are swept into an analytical instrument for quantitation. This technique helps reduce spectral interferences and improve detection limits as compared to conventional analysis methods.

 

Cold Vapor Generation A technique (link to flow vapor generator page) for generating elemental mercury vapor from a sample solution. All mercury present is converted to Hg(2+) using a mixture of acids, then reduced to elemental mercury via exposure to tin chloride (SnCl2). Gaseous mercury evolves from the sample and is swept into an analytical instrument for quantitation.

 

 

Trace Elemental Figures of Merit

 

Signal

 

Spectral radiation produced from the analytes (atoms and ions) of interest.

 

Background

 

Spectral radiation produced from sources other than the analytes of interest. These sources include: the sample matrix, the atom source, the instrument’s detector (dark current) and noise from the electronics during the readout of the signal.

Accuracy A measurement of how close the measured values are to the known or accepted values. Accuracy is independent of precision.

 

Precision

 

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A measurement of how close repeated measurements are to one another. Precision is independent of accuracy.

 

RSD

 

(Relative Standard Deviation); a measurement of the error within a set of repeated measurements. Expressed as a percentage, the RSD of a measured result is based on the quotient of its standard deviation and its mean value.

 

Sensitivity

 

A reflection of the slope of the calibration curve, sensitivity is based on the change in an analyte’s response per unit of change in its concentration.

 

Signal-to-Noise Ratio (SNR)

 

Reflects the sensitivity of an analytical measurement. This ratio compares the magnitude of the analyte signal to its accompanying background noise.

 

Background Equivalent Concentration (BEC)

 

This is a reflection of the signal-to-noise ratio. The BEC is the concentration of an analyte required to produce a signal equal to the background signal at that wavelength.

 

Limit of Detection (LOD)

 

While this can be calculated using several different formulae, the LOD for a given analyte is the smallest concentration of that analyte that can be detected.

 

Limit of Quantitation (LOQ)

 

The LOQ for an analyte is the lowest concentration at which it can be reliably measured and reported with calculated statistics. The LOQ can be calculated using different formulae; however, it is often based on a multiple of the detection limit.

 

 

Now that we have established a basic vocabulary relevant to elemental analysis, we are ready to discuss instrument selection, method optimization, and tips and tricks for tackling challenging sample matrices. If you have terms and topics you would like discussed in future posts, please provide me with comments.

 

Additional Resources

  • Do check out our trace elemental analysis page, as well as some of our community pages, which contain resources such as application notes, on-demand webinars, videos and podcasts for reference and guidance.