Determination of Electrolytes in Sports Drinks by Microwave Plasma Atomic Emission Spectroscopy (MP-AES)

Sports drinks, also known as electrolyte drinks, are a popular beverage choice among consumers and come in a wide variety of flavors and electrolyte compositions. They may vary substantially in their additives, which typically include sweeteners, coloring agents, flavoring agents, and additional vitamins and nutrients. The determination of electrolyte elements is important for accurate product labeling and quality control; however, conventional instrumentation, such as inductively coupled plasma optical emission spectroscopy (ICP-OES), may be costly to both acquire and operate, especially given its relatively high consumption of argon gas. Here we present a rapid and cost-effective method using microwave plasma atomic emission spectroscopy (MP-AES), which utilizes a nitrogen plasma that may be supplied with either a conventional gas source (dewar or cylinder gas) or a nitrogen generator on-site. Using a simple “dilute and shoot” method, electrolyte elements can be determined without prior sample digestion with good reproducibility and excellent limits of detection across a variety of sample matrices. Thus, MP-AES offers a simple multi-element alternative to ICP-OES for sports drink analysis without costly argon consumption.

Sports drinks are a common beverage choice, especially for those who engage in regular physical activity. While their primary purpose is to assist active people with rehydration and to replenish electrolytes lost through sweat, their use has also expanded to the general consumer beverage market. For athletes, sports drinks potentially serve a clear function. During intense or prolonged exercise, the body loses water and essential elements such as sodium, potassium, calcium, and magnesium that are crucial for maintaining muscle function and hydration. Drinking a sports beverage during or after a workout may help restore this balance, though their overall effectiveness is a matter of debate, and which formulations may be more effective is an area of active study.

Many sports drinks also contain sugar and, in some cases, caffeine, which both offer a quick energy boost. This is potentially attractive to people participating in long-distance events or high-intensity sports, where energy stores deplete quickly. By replenishing carbohydrates during the activity, athletes could potentially improve their endurance and reduce fatigue. Alternatively, many sports drink formulations contain artificial sweeteners to appeal to consumers concerned about excessive sugar consumption, leading to a wide variety of product formulations available on the consumer market. To meet product labeling requirements and provide quality control in manufacturing, there is a need for robust, low-cost methods for electrolyte element quantification that provide accurate results across a variety of product formulations.

Microwave plasma atomic emission spectrometry (MP-AES) is an emerging atomic emission technique that utilizes collisions in plasma to promote some atoms to excited electronic states from which they can emit photons as they return to lower energy states. The MP-AES instrument utilizes a microwave-induced nitrogen plasma instead of combustible gases or costly argon, allowing analysis at a fraction of the cost of traditional analytical techniques (1). During the last decade, this technique has shown promising performance as a quantitative analytical technique, with applications in fields such as geological, environmental, food, health, energy, agricultural, pharmaceuticals, and waste electrical and electronic equipment regulation (WEEE)/restriction of hazardous substances (RoHS) compliance (2–7). Thus, MP-AES offers an alternative to both atomic absorption spectroscopy (AAS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) techniques that had several advantages including improved lab safety relative to flame AAS, relatively inexpensive operating costs, multi-element capability, good speed, and good detection power; making it an efficient cost-effective analytical tool compared to previous techniques (1). The MP-AES models record emission signals sequentially. Therefore, analytical conditions can be optimized for each specific element within the same experiment, which maximizes efficiency and may minimize potential interferences (2).

The objectives of this study were to evaluate the suitability of MP-AES for the analysis of electrolyte elements in sport drinks and to develop a simple, low-cost method for routine analysis. While traditional methods often require complicated digestions before sample analysis, this study utilizes a “dilute and shoot” method for elemental analysis of Na, K, Ca, and Mg, which may be present in concentrations as high as several hundred mg/L if used in electrolyte mixtures in sports drinks.

Methods

Sample Preparation

Samples of sports drinks from five different brands in multiple varieties were obtained from a major local retailer (Walmart, Sault Sainte Marie, MI). Varieties were selected to represent different brands, sweeteners, and flavoring/coloring agents (Table I). Samples were prepared by diluting each beverage 1:500 with 2% Nitric Acid prepared from trace element certified concentrated acid (Aristar) diluted with ultrapure water. Additionally, a second sample of each beverage was prepared identically and spiked with 2mg/L of each element from a National Institute of Standards and Technology (NIST) traceable stock element standard (Inorganic Ventures).

Analysis Conditions

Samples were analyzed with an Agilent 4200 Microwave Plasma Atomic Emission Spectrometer equipped with a OneNeb Series 2 nebulizer (Agilent Technologies) and a cyclonic spray chamber, and a standard torch. The Agilent 4200 is equipped with a Czerny-Turner monochromator with 600 mm focal length and covers a spectral range of 178-780 nm. For all experiments, the nitrogen plasma was supplied from high-pressure cylinders, though dewar nitrogen or a nitrogen generator could also be utilized. Samples were introduced manually, though an autosampler can also be equipped. Additional method parameters are described in Table II.

Calibration standards were prepared in 2% nitric acid from stock element solutions covering the expected elemental range of 0.05–10 mg/L in the diluted samples. Standards were analyzed at the beginning of the analysis, with a 2 mg/l standard measured every 10 samples to monitor method performance. The built-in three-channel peristaltic pump was also utilized to introduce an internal standard solution via an online mixing tee. The solution contained rhodium (10 mg/L) to monitor for sensitivity drift and to verify the absence of matrix interferences due to variations in sample viscosity, which can negatively affect accuracy and precision. Additionally, the internal standard solution contained an excess of cesium (500 mg/L) to act as an ionization buffer and suppress easily ionizable element effects.

Results

Calibration Results

As instrument calibration is fundamental to producing accurate results, we carefully examined the calibration curves for each emission line during method development. The sensitivity of MP-AES is excellent for Group I (Na, K) and Group II elements (Ca, Mg), so the calibration standards were designed to bracket the samples and matrix spikes within the linear response range of the instrument. Following analysis of calibration standards, calibration curves were visually examined for linearity to determine the best calibration model. While a linear fit to the calibration curves resulted in an r² value of more than 0.999 for all emission lines, a slight curvilinearity could be visually detected, so the rational fit calibration model in the MP Expert software was selected for all lines, resulting in slight improvements to the r² values. The resulting calibration curves (Figure 1) were subsequently used for the determination of element concentrations.

Evaluation of Matrix Effects

A variety of possible matrix effects may negatively affect results by MP-AES, so several steps were taken to minimize and evaluate any possible effects on the accuracy of results. The significant sample dilution (1:500) was expected to eliminate any effects of viscosity variation on nebulizer efficiency, but to confirm this, we monitored the response of the Rh internal standard (10 mg/l) at the 343.489 and 437.480 emission lines. Across the analytical run, only minimal variation in response for both emission lines was observed, demonstrating that viscosity variations were minimal (Figure 2).

As the Agilent 4200 is an axial view design, easily ionizable element (EIE) effects are a potential concern, even with high sample dilution. The EIE effect can result when ionization in the plasma produces excess free electrons, resulting in reduced ionization of low first ionization potential elements (such as Na and K). This subsequently results in signal enhancement for these elements, which can negatively affect accuracy.

A common strategy for reducing EIE effects is to add an excess of a low first ionization potential element, usually cesium. Excess Cs acts as an ionization buffer, ideally by providing a constant supply of excess free electrons to the plasma. In the method presented here, Cs was added at a final concentration of 500 mg/L. Variations in ionization conditions in the plasma were monitored by analysis of the Cs 455.528 and 672.328 emission lines over the course of the analytical run (Figure 3). Signal intensity for Cs lines was relatively constant (variation <10% in all samples and standards), suggesting that EIE effects were likely to be minimal.

We also evaluated each sample individually for matrix interferences by monitoring the recovery of a 2 mg/L element spike. The spike recoveries demonstrated good recovery, suggesting matrix interferences generally have minimal effect on the accuracy of the results (Table III). Sample 6 did show an unacceptably low recovery for the Ca spike. That sample contained the highest concentration of Ca (Table V), which may have impacted the spike recovery by resulting in an insufficient difference in concentration between the sample and spiked sample for accurate recovery determination.

Method Detection Limits

Method detection limits were assessed by analysis of 10 replicates of the blank solution (2% nitric acid; Table IV). Determined MDLs, even with the significant degree of dilution utilized in this method, are sufficiently low to determine electrolyte additives in sports drinks, which are often in the range of several hundred mg/l or higher.

Results from Beverage Analysis

Results of the electrolyte elements determination in the sports drinks studied here are presented in Table V. Sodium is the main electrolyte element added to most of the drink formulations studied, though significant quantities of potassium and lesser quantities of Ca and Mg are sometimes present. As sodium is the primary cation lost in sweat, it is unsurprising that most of the studied formulations utilize sodium as the primary electrolyte. The observation of alternative formulations, however (such as those with significant K), demonstrates the utility of using a multielement technique such as MP-AES, which can detect all four elements (Na, K, Ca, and Mg) when present as significant components of electrolyte mixtures.

Conclusions

MP-AES is a fast, low-cost, multielement method suitable for determining electrolyte elements in sports drinks utilizing a simple “dilute and shoot technique.” Use of an internal standard and ionization buffer provides adequate control of matrix interferences in diluted samples, and the excellent sensitivity of Group I and II elements such as Na, K, Ca, and Mg allows sufficient sensitivity even with significant dilution. This makes MP-AES an attractive choice, especially for smaller labs that with lower sample throughput for which ICP-AES may prove too costly, but FAAS may be too slow (or have concerns about the safety of combustible gases).

Acknowledgments

Funding for this work was provided by the Lake Superior State University College of Arts and Sciences.

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Direct correspondence regarding this article to dwright1@lssu.edu