Lithium Ion Quantitation Methodologies

Lithium ions are commonly detected through both colorimetric or fluorometric methodologies and lithium quantification is routinely used in the environmental research field, in the manufacturing of a wide variety of batteries, in biopharmaceuticals and medical analysis. Both older and more recent methods have their pros and cons, and researchers may select a particular technique for a variety of reasons, including sensitivity, type of sample, amount of sample available, possibility of sample destruction, etc.

There are several methods commonly used to quantify lithium ions, including flame photometry, ion-selective electrodes (ISE), atomic absorption spectroscopy, and fluorescence spectrophotometry. Flame photometry and atomic absorption spectroscopy require the inflammation of the samples. They are tedious to use and require expensive and sophisticated instrumentation. Ion-selective electrodes require large volumes of samples and often have low selectivity.

Example Protocol for a Fluorescent Lithium Ion Assay

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The following protocol only provides a guideline and should be modified according to specific experimental needs.

Prepare a Lithium Standard by diluting known lithium stock solutions, either in-house preparations or one purchased or supplied by assay manufacturer. The typical requirement will be to perform 1:2 serial dilutions to get serially diluted Lithium Standard (LS2-LS7).

Layout of Lithium Standards and Test Samples in a Solid Black 96-well Microplate (left) & Reagent Composition for Each Well (right)

BL	BL	TS	TS
LS1	LS1	...	...
LS2	LS2	...	...
LS3	LS3	...	...
LS4	LS4	...	...
LS5	LS5	...	...
LS6	LS6	...	...
LS7	LS7	...	...

Well	Volume	Reagent
LS1-LS7	50 µL	Serial dilutions (300 to 4.69 mM)
BL	50 µL	Assay Buffer
TS	50 µL	Sample

LS=Lithium Standards (LS1 - LS7, 300 to 4.69 mM, 2X dilutions); BL=Blank Control; TS=Test Samples

Protocol

1. Prepare Lithium Standards (LS), blank controls (BL), and test samples (TS) according to the layout provided in the above tables. For a 384-well plate, use 25 µL of reagent per well instead of 50 µL.
2. Add 50 µL of lithium indicator working solution (in this case, Lithiumighty™ 520) to each well of Lithium Standards, blank control, and test samples to make the assay volume of 100 µL/well. For a 384-well plate, add 25 µL into each well instead, for a total volume of 50 µL/well.
3. Incubate the reaction at room temperature for 5 to 10 minutes, protected from light.
4. Monitor the fluorescence increase with a fluorescence microplate reader at Ex/Em = 490/525 nm (cut off at 515 nm).

Other Recent Methods of Lithium Ion Quantification

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In Situ Colorimetry for Lithium Quantification

It is well known that graphites, and other intercalation materials used in lithium-ion batteries, change their color upon electrochemical insertion of lithium ions. In situ colorimetry has been developed as a simple method to measure the local state of charge of lithium-ion battery electrodes. The lithium distribution in anodes of aged lithium-ion batteries was found to be highly heterogeneous. Colorimetric techniques are popular primarily due to miniaturization innovations such as paper-based analytical devices that mainly use colorimetric reactions, and by the advances and popularity of image capture instruments.

Typically, methods based on colorimetric reactions have been widely used in analyses of different matrices because of their simplicity, fast response, and the need for low reagent volumes. The methods based on colorimetric assays analyze the changes in absorbance or reflectance generated by the products formed between analyte-reagent. Generally, colorimetry-based technology involves device miniaturization, reduced operating costs, and in-situ analysis. The development and enhancement of different modes of color detection to meet the demands of making qualitative, semi-quantitative, and fully quantitative analyses of multiple analytes are now available through the use of cameras, scanners and smartphones. These devices have become suitable alternatives for different approaches to colorimetric analysis.

Colorimetric LEDs Analysis Technique for Lithium

Another colorimetric analysis technique is based on the use of LEDs (light-emitting diodes). Generally, a LED is a solid-state semiconductor with a current flow resulting in light emission. The mechanism that leads to emission is related to electron-hole recombination within the n-p junction of the semiconductor. Depending on the energy bandgap of the semiconductor (i.e. the energy difference between valence and conduction bands) the LED will emit radiation at a certain wavelength; a change in the semiconductor material will result in a typical color that can be analyzed and used for quantification.

Under similar conditions, these two methods proved superior to the X-ray fluorescence method. A one pot analysis of lithium ions using an Eppendorf microtube modified for performing reaction, filtration and detection can also be used. This method is simple and very convenient for didactic and field assays.

Lithium Ion Quantification based on Arrays of chemoresponsive colorants

Lithium ion quantification can also be done through the use of arrays of chemoresponsive colorants to provide high-dimensional data from the color or fluorescence changes of the dyes in these arrays as they are exposed to analytes. This provides chemical sensing with a very high sensitivity (often at parts per billion), impressive discrimination among very similar analytes, and exceptional identification of extremely similar mixtures over a wide range of analyte types, in both the gas and liquid phases. The variety of applications for the use of arrays of chemoresponsive colorants includes:

• Personal dosimetry of toxic industrial chemicals
• Detection of explosives or accelerants
• Quality control of foods and beverages
• Biosensing intracellularly
• Identification of bacteria and fungi, and
• Detection of cancer and disease biomarkers

Overcoming lithium analysis difficulties with a simple colorimetric or spectrophotometric method

Often the analytical determination of lithium ions is performed by atomic absorption and X-ray fluorescence methods. Chemical analysis based on polyfluoroporphyrin chromogenic methods is also being employed, especially for biological samples. The problem, however, is that all existing methods are expensive and not suitable for routine work or field assays. The simple alternative method is based on the formation of a LiKFe(IO6) compound which is converted into a tris(1,10-phenanthroline)iron(ii) complex and monitored by spectrophotometric or colorimetric methods, the latter using a smartphone app.

References

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Colorimetric Determination of Lithium Content in Electrodes of Lithium-Ion Batteries
The Optoelectronic Nose: Colorimetric and Fluorometric Sensor Arrays
Novel approaches for colorimetric measurements in analytical chemistry - A review
Overcoming the lithium analysis difficulties with a simple colorimetric/spectrophotometric method