Rapid multi-element characterization of microgreens via total-reflection X-ray fluorescence (TXRF) spectrometry

Highlights

• Microgreens are a relevant source of mineral macro and micronutrients.

• Microgreens can be analysed by TXRF without sample digestion.

• Simultaneous multi-elemental analysis can be performed by TXRF.

• TXRF detection limits vary from 0.1 to 42 mg kg⁻¹, depending on the element being analysed.

• The method is cheaper and faster than ICP-AES for most elements.

Abstract

Microgreens are an emerging class of vegetables, which have become increasingly important in the agri-food market in recent years, and contain a number of macro- and micro-nutrients. This paper presents a rapid method for the elemental analysis of microgreens based on total reflection X-ray fluorescence (TXRF) spectroscopy, without preliminary sample digestion.

The following elements were detected and quantified simultaneously for six microgreen genotypes, belonging to Asteraceae and Brassicaceae: P, S, K, Ca, Cl, Mn, Fe, Ni, Cu, Zn, Br, Rb, Sr. The limit of detection (LOD) varied depending on the element and ranged between 0.1 mg kg⁻¹ for Sr and 42 mg kg⁻¹ for P.

The method was validated using certified standards, and results compared with those obtained using a conventional ICP-AES method requiring sample digestion. The paper also presents the advantages and disadvantages of the two techniques.

Introduction

Microgreens are an emerging food consisting of young edible vegetables and herbs, which are harvested when cotyledonary leaves have fully developed and the first true leaves have emerged (usually 7–21 days after germination). The production of microgreens differs from sprouts and common freshly cut leafy vegetables, as microgreens are marketed together with their growing medium, which extends their shelf-life (Kyriacou et al., 2016). Recent studies have revealed that microgreens are richer than mature greens in some vitamins, sugars and antioxidants, including carotenoids (Kyriacou et al., 2019, Mir et al., 2017, Sun et al., 2013, Xiao et al., 2012, Xiao et al., 2014, Xiao et al., 2019). Their consumption also appears to be associated with nutraceutical effects, i.e. a reduced risk of cardiovascular disease, possibly due to prevention of hypercholesterolemia (Huang et al., 2016), and also provides protection against inflammatory processes, oxidative stress and chronic diseases (Choe, Yu, & Wang, 2018). Few studies have investigated the mineral contents of microgreens, but these suggest that microgreens could provide an important supply of K, Ca, Fe and Zn (Pinto et al., 2015, Xiao et al., 2016).

To date, multi-elemental characterization of microgreens has been carried out using inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Xiao et al., 2016) and inductively coupled plasma mass spectroscopy (ICP-MS) (Pinto et al., 2015). Both methods require a complex and hazardous process of sample preparation based on acidic or alkaline digestion. These digestion procedures often require special heating systems and apparatuses to prevent the loss of volatile elements, so that sample processing makes these analyses time-consuming and relatively expensive.

Analytical techniques to perform elemental analysis without the need for sample digestion would be extremely useful to speed up sample preparation procedures, thereby reducing the cost of analysis and the risks involved in using chemicals.

X-ray fluorescence spectroscopy (XRF) is potentially a good alternative to ICP-AES, and usually requires very simple preparation of samples (e.g. fine grinding and pellet pressing). In conventional XRF, a primary X-ray beam is focused on the sample to expel electrons from the inner valence shells, causing the emission of secondary X-ray radiation, which is characteristic for each element in the sample. Both qualitative and quantitative elemental analysis can be performed according to the secondary X-ray beam energy (or wavelength) and intensity. Unfortunately, the high detection limits of conventional XRF (ranging from 10 s to 100 s mg kg⁻¹, depending on the element) make this technique less suitable for the elemental analysis of vegetables, in particular for micronutrients, whose concentrations usually range from 0.1 to 100 mg kg⁻¹ dry weight.

It is possible to overcome the limits of conventional XRF by using a particular type of XRF, named total-reflection X-ray fluorescence spectroscopy (TXRF). In TXRF, the primary X-ray beam strikes the sample at an angle lower than the critical angle, making it possible to reduce sample self-absorption, thus increasing the signal-to-noise ratio and lowering detection limits compared with conventional XRF (Klockenkämper & von Bohlen, 2015). For this reason, TXRF has proven in recent years to be a useful and reliable analytical technique also for the analysis of trace elements in organic samples (Allegretta et al., 2017, De La Calle et al., 2012, Stosnach, 2010), including vegetable foodstuffs (Dalipi et al., 2018, Dalipi et al., 2017, De La Calle et al., 2013). However, to the best of our knowledge, TXRF has never been used for the elemental analysis of microgreens.

In this paper, we propose a dedicated TXRF analytical method to study these innovative agricultural products, whose importance on the worldwide agrifood-market is expected to increase in the next few years. The proposed method has been developed on two certified reference standards and applied to six different genotypes of microgreens belonging to Asteraceae and Brassicaceae, analysed using TXRF and ICP-AES, as the reference method. The results obtained with the two techniques were compared, and the advantages and disadvantages of the two techniques are presented below.