Determination of calcium and zinc in gluconates oral solution and blood samples by liquid cathode glow discharge-atomic emission spectrometry

Highlights

• A novel flowing LCGD-AES technique was established.

• The electron temperature (Te) and electron number density (Ne) were calculated.

• The stability and the optimized analytical conditions were investigated.

• The Ca and Zn in gluconates oral solution and blood samples were simultaneously determinate.

• The measurement results of the LCGD-AES were consistent with the ICP-AES and the reference values.

Abstract

A novel flowing liquid cathode glow discharge (LCGD) was developed as an excitation source of the atomic emission spectrometry (AES) for the determination of Ca and Zn in digested calcium and zinc gluconates oral solution and blood samples, in which the glow discharge is produced between the electrolyte (as cathode) overflowing from a quartz capillary and the needle-like Pt anode. The electron temperature and electron density of LCGD were calculated at different discharge voltages. The discharge stability and parameters affecting the LCGD were investigated in detail. In addition, the measured results of real samples using LCGD-AES were verified by ICP-AES. The results showed that the optimized analytical conditions are pH = 1 HNO₃ as supporting electrolyte, 4.5 mL min^–1 solution flow rate. The power consumption of LCGD is 43.5–66.0 W. The R² and the RSD ranged from 630 to 680 V are 0.9942–0.9995 and 0.49%–2.43%, respectively. The limits of detections (LODs) for Zn and Ca are 0.014–0.033 and 0.011–0.097 mg L^–1, respectively, which are in good agreement with the closed-type electrolyte cathode atmospheric glow discharge (ELCAD). The obtained results of Ca and Zn in real samples by LCGD-AES are basically consistent with the ICP-AES and reference value. The results suggested that LCGD-AES can provide an alternative analytical method for the detection of metal elements in biological and medical samples.

Introduction

Calcium is an essential mineral for the human body, mostly required as structural element for strengthening the bones and teeth tissues as hydroxyapatite [1]. Getting enough calcium throughout our lifetime can help to prevent osteoporosis. In addition, zinc is an important trace element and has a number of functions in the human body, such as promoting healthy growth during childhood, properly synthesizing DNA, healthy immune system, and the senses of taste and smell [2]. So calcium and zinc deficiencies in human body can lead to various diseases. When we do a physical examination to a hospital, contents of some trace elements as a conventional project are often detected in blood. If you have calcium and zinc deficiency, doctors usually suggest you taking calcium and zinc gluconates oral solution if necessary.

The well-known atomic spectrometry techniques, such as atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS), have been widely used for the determination of trace metal elements in a variety of real samples. However, these techniques are generally performed at laboratories, requiring high power input, high temperatures, high vacuum, and even as inert/special gases [3], [4]. To meet the trend of miniaturization in analytical instrumentation and the requirements of on-site, real-time and on-line monitoring for metal elements, it is highly desirable to develop portable and low-cost instruments [5], [6]. Over the past 20 years, a novel electrolyte cathode atmospheric glow discharge (ELCAD) has been successfully developed as an important tool in atomic spectrum analysis for monitoring the metal elements. It is considered as one of the most promising alternative miniaturized excitation which possesses potential advantages over traditional plasma sources (i.e., ICP) due to its more compact and portable instrument, lower power consumption (< 75 W), no inert gas requirement and operation in atmospheric pressure air [3], [4], [5]. Also, ELCAD needs no special sample introduction system like a spray chamber and a nebulizer to transport the analytes to the analytical zone [7]. All these characteristics make it more attractive as a field deployment instrument.

ELCAD was reported for the first time in 1993 by Cserfalvi and co-workers [8], [9]. The main apparatuses of the early ELCAD is a very simple excitation source, that is, the sample solution is served as cathode, which overflows with typical flow of 8–10 mL min⁻¹ from a capillary into an approximately 35 mL reservoir filled with electrolyte solution, and a counter-electrode (mostly W or Pt rod) above it (2–4 mm) is the anode. The capillary is emerged upwards about 1–3 mm from the reservoir containing a grounded graphite electrode to make it electrically conductive [3], [5]. At atmospheric air pressure, a glow discharge is produced between the electrolyte overflowing from the capillary and the metal anode. The atomic lines of metals dissolved in the solution appear immediately in the spectrum emitted by the ELCAD, in this way the content of metals in a sample can be directly determined [3]. However, a shortcoming of the earlier ELCAD source is that very high solution flow rates and volumes are typically required as the capillary surface must be covered with solution for maintaining the stable discharge [10]. Moreover, their higher limits of detections (LODs) do not meet the requirements for the determination of trace metals in environmental and biological samples.

In order to reduce the consumption of solution sample, improve the detectability, enhance discharge stability and emission efficiency, and achieve more compact and portable plasma sources, many improvements for excitation source of ELCAD have been carried out based on the original design by Cserfalvi and co-workers [3], [4], [5], [7]. For example, Webb et al. developed a solution-cathode glow discharge (SCGD) which exhibited better detection limits (0.06 μg L^–1 for Li and 270 μg L^–1 for Hg) and operated at low flow rate (2.5 mL min^–1) [10], [11]. Shekhar et al. designed a new V-groove ELCAD device in which a stable plasma could be obtained even at a flow rate of 0.96 mL min^–1 [12], [13]. Marcus and co-workers constructed a liquid sampling-atmospheric pressure glow discharge (LS-APGD) for direct analysis of electrolytic solutions at low flow rates (0.3–1.0 mL min⁻¹) and permitted direct coupling with flow injection [14], [15]. Greda et al. developed a novel miniaturized APGD to enhance the intensities of the atomic emission lines and reduce the LODs [16], [17]. Zhu et al. developed an alternating current-electrolyte atmospheric liquid discharge (ac-EALD) in which plasma could be sustained at low flow rate (0.2 mL min⁻¹) and low discharge power (≤ 18 W) supply [18]. In addition, liquid film dielectric barrier discharge (LF-DBD) was also constructed by Zhu et al. to reduce the LODs at low flow rates (0.1–0.8 mL min⁻¹) [19]. All these improvement for device indicated that miniaturized and/or portable apparatus may be the most fruitful applications of ELCAD.

At present, ELCAD-type has been widely applied for the analysis of many real samples, such as tuna fish [13], soils and spruce needles [16], brass as well as mineral water and tea leaves [17], human hair and stream sediment [20], [21], brines [22], colloidal silica [23], and so on. Although ELCAD-type has made considerable development, to the best of our knowledge, it is rarely used for the determination of metal elements in medical and biological samples, perhaps due to their complex matrix.

Recently, we successfully developed a novel flowing liquid cathode glow discharge-atomic emission spectrometry (LCGD-AES) for the simultaneously determination of K, Na, Ca, Mg and Zn in water samples (tap water, mineral water, Yellow river water and waste water) [24] and Cu and Pb in ores samples [25]. This LCGD device can offer several advantages over conventional ELCAD. For example, sealed Pt wire into a quartz tube can form a Pt tip discharge, which can further reduce the energy consumption (< 66 W) and improve excitation efficiency. In addition, several knots in peristaltic-pump tubing can increase the stability of discharge plasma. Moreover, inserted the quartz capillary into graphite tube is excluded the reservoir of ELCAD.

Herein, the method's feasibility of LCGD-AES was continued to evaluate for the simultaneous detection of Ca and Zn in digested calcium and zinc gluconates oral solution and blood samples. The electron temperature (T_e) and electron number density (N_e) were calculated at different discharge voltage. The energy consumption and stability of LCGD, and effects of working conditions such as supporting electrolyte, solution pH, solution flow rate and interfering substance on emission intensity, were systematically investigated. In addition, the LODs of calcium and zinc were compared with those measured by the closed-type ELCAD techniques. Moreover, the analytical result of the calcium and zinc in gluconates oral solution and blood samples by LCGD-AES were compared with ICP-AES and reference values.