Applications in Food and Related Items
The quality and safety of food samples always require strict control. From an analytical perspective, many techniques can be used for this type of monitoring, one of which involves the application of laser pulses in LA-ICP-MS. One of the difficulties and challenges associated with this technique involves quantitative analyses because of problems in the process of ablation, plasma formation, and/or matrix effects. Calibration strategies combining internal standards, standard addition methods, and chemometric tools can help improve the results. Food samples are among the most variable samples in terms of their matrix and composition, which can further complicate analysis by laser ablation and require the development of ablation strategies. The study of dos Santos Augusto et al. describes the use of calibration strategies and the determination of Ca, Cd, Cr, Cu, Fe, Mg, Pb, and Zn in food samples, including liquid (i.e., orange juice) and solid (i.e., dietary supplements) samples (9). The calibration curve for the solid samples was constructed using microcrystalline cellulose and a proportional mixture of solid samples chosen according to their higher and lower concentrations of metals. The liquid samples were immobilized in a polymer film with the help of polyvinyl alcohol (PVA). Using these calibration strategies, it was possible to obtain accuracy values between 60 and 120% for almost all samples, except for Ca, Mg, and Zn. These exceptions could be a reflection of unresolved matrix interferences. Carbon was used as the internal standard but did not show promising results.
A protocol was validated for the elemental analysis of plant and food materials using LA-ICP-MS and pressed pellets in compliance with the Green Chemistry principles, i.e., avoiding the use of hazardous reagents (10). The protocol included sample pre-treatment, preparation of pellets, and LA-ICP-MS optimization for the food sample matrix using leaf certified reference materials. Sample homogenization and particle size reduction (-10 pm) for samples with larger initial particle sizes proved crucial in improving signal stability and analytical results, whereas the raster mode was preferred over the spot mode for greater accuracy. The validation of the technique was successful for determination of Mn, Ni, Cu, Zn, As, Se, Cd, and Pb (г-score values <2).
Coptis chinensis Franch., is a widely used medicinal plant in China. This plant is often contaminated by Cd. Understanding the distribution of Cd and its chemical forms is important to evaluate accumulation of the metal and its detoxification mechanisms in this plant. LA-ICP-MS was used to spatially locate Cd in rhizome cross-sections, and ICP-MS was used to analyze the Cd subcellular distribution and the chemical forms of Cd in different tissues (11). Rhizome bioimaging results showed that Cd was distributed predominantly within the periderm, cortex, pith, and root trace vascular bundle. The LA-ICP-MS results suggested that Ca2* channels might be a pathway for Cd entry into the plant. Subcellular distribution data indicated that most of the Cd was associated with the cell wall (41.8-77.1%) and the soluble fraction (14.4-52.7%) in all tissues. Analysis of chemical forms revealed that the majority of the Cd existed in less mobile and less toxic forms in all tissues, and P could convert to insoluble phosphate with Cd to moderate Cd toxicity.
A LA-ICP-MS method was developed to determine Cu, Zn, Cd, Hg, and Pb and metal distribution in longitudinal tissue sections of the marine snail Nassarius reticulatus (Gastropoda, Prosobranchia) (12). Snails were sampled in northern Brittany (France) at three stations with different contamination levels. The quantification of metal distribution (imaging or mapping) in a thin slice of the snail tissue was carried out using different strategies: by one-point calibration and via matrix-matched laboratory standards using different biological materials (BCR 278, snail tissue, and rat brain). Together with the imaging of metals the distribution of two non-metals (carbon and sulfur) was analyzed. The imaging LA-ICP-MS analysis yielded an inhomogeneous distribution for all elements investigated. The detection limits for the distribution analysis of Cu, Zn, Cd, Hg, and Pb measured by LA-ICP-MS were in the low pg g-1 range.
LA-ICP-MS was used for the quantitative imaging of nutrient elements (such as K, Mg, Mn, Cu, P, S, and B) in the leaves of Elsboltzia splendens (13). The plant leaves were scanned directly with a focused Nd:YAG laser in the laser ablation chamber. The ablated material was transported with argon as the carrier gas to a quadrupole-based ICP-MS (ICP-QMS), and the ion intensities of ',9K% 24Mg% ssMn+, 63Gr, 31P% 34S% and UB+ were measured by ICP-QMS to study the distribution of the elements of interest. The imaging technique using LA-ICP-MS on plant leaves does not require any sample preparation. Carbon (13C+) was used as an internal standard element to compensate for the difference in the amount of material ablated.
Baba et al. (14) used LA-ICP-MS to determine Cd in rice flour, employing the responses from Cd-doped NIES rice flour standards 10a, 10b, and 10c to construct calibration curves. Good linearity was observed in the range 0.023-1.82 mg g_1. Internal standardization was unnecessary, and no significant interferences were observed. Such studies show the typical advantages of LA-ICP-MS of no sample preparation and rapid sample throughput, which might be more widely employed in quality control of foodstuffs during or after processing.
The provenance identification of selected types of food and drink was undertaken from data obtained using both solution and laser ablation-based inductively coupled plasma mass spectrometry (ICP-MS) and solution-based inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis of selected food and drink materials (pork, wine, tea, coffee, and olive oil) (15). Quantitative determination of up to 55 elements was undertaken using solution-based ICP-MS and AES while counts-per-second data for 49 elements were used for samples analyzed using LA-ICP-MS. ICP-AES was used to determine the concentrations of Na, K, Ca, Mg, and Fe where appropriate. A quantitative analytical methodology for LA-ICP-MS analysis of olive oil was also developed and preliminary trials undertaken. Additional information, to establish more detailed geographic resolution in provenance determination, especially for tea and coffee (plantation of origin), was provided by the incorporation of data for light-stable isotope distribution patterns (2 H (D), 13C, and 15N). LA-ICP-MS was used for the direct analysis of olive oil and coffee beans. The use of laser ablation for the analysis of olive oil has the potential to overcome problems associated with both acid dissolution and thermal oxidation of this material, which are the traditional methods of sample dissolution and which either introduce significant contamination or cause the loss of volatile organo- metallic compounds that may be indicative for provenance determination. Results from the overall study indicated that it was possible not only to establish country of origin of the study materials but also in some cases to improve resolution of provenance to state of origin (wine and pork), and even plantation of origin (tea and coffee).
A method has been developed for a rapid and precise location of selenium-containing proteins in large two-dimensional (2D) electrophoresis gels (16). A sample was divided into four aliquots which were analyzed in parallel by ID isoelectric focusing electrophoresis (IEF)-laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS), ID sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE)-LA-ICP-MS, and, in duplicate, by 2D IEF-PAGE. On the basis of the ID elec- tropherograms obtained, areas supposed to contain the largest concentrations of Se were subjected to LA-ICP-MS imaging to locate precisely the position of Se-containing proteins which were then identified in the parallel 2D gel by electrospray Orbitrap MS/ MS. The method was applied to the identification and semiquantitative determination of selenium storage proteins in wheat.
Cadmium is one of the most widespread metals which is highly toxic, and can enter the human food chain directly via accumulation in nutritional plants. To better understand the transport and protein-binding capabilities of heavy metals cadmium uptake in Spinacia oleracea L., a typical nutritional plant with a high cadmium uptake susceptibility was investigated (17). A new strategy for the screening of cadmium-containing proteins was elaborated which consists of the extraction of proteins by application of an ultrasonic homogenizer and separation of the metal-binding proteins by use of native PAGE. For comparison, denaturing PAGE has been investigated, too. LA was utilized as a sample introduction system for ICP-MS detection of the metals in the proteins after PAGE separation and blotting onto membranes.
Concentrations of 12 elements (Mg, Al, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd) were determined in six herbal supplements (18), Korean Panax ginseng (Panax ginseng), goldenseal (Hydrastis canadensis), ginger root (Zingiber officinale), St. John’s wort (Hypericum perforatum), green tea (Camellia sinensis), and valerian root (Valeriana officinalis), by both LA-ICP-MS and conventional closed-vessel digestion solution nebu- lization-ICP-MS (SN-ICP-MS). For LA-ICP-MS, powder from supplement capsules and leaf reference materials were pressed into pellets, the latter being used for calibration and quality assurance. Laser ablation was performed using line scans with a scan rate of 30 pm min-', a frequency of 20 Hz, and a spot size of 100 pm; 13C served as the internal standard. For LA it was found that low resolution (m/Дт и 400) yielded good recoveries for the reference materials and results comparable to SN-ICP-MS for most elements, except for Ca, which was better determined in medium resolution (m/Am « 4000).
With rice being the main staple crop in Asian countries such as China, Korea, and Japan, the detection of arsenic (As), an element known to be carcinogenic to humans, has been the topic of high public interest. The total arsenic content in 200 white and 104 brown rice samples collected in Korea was analyzed using a quadrupole inductively coupled plasma-mass spectrometer (ICP-MS) (19). One of the rice grain samples was polished with 3, 5, 7, 9, and 11 degrees of milling, and arsenic concentration variance from the surface to the inner core region was investigated. Furthermore, spatial distribution of arsenic over the cross-section of a brown rice grain was obtained using femtosecond laser ablation-inductively coupled plasma-mass spectrometry (fs LA-ICP-MS). For the total arsenic content analysis, 91.7 ± 28.1 and 101 ± 33.6 pg-1 kg-1 of arsenic were measured in the white and the brown rice, respectively. The fs LA-ICP-MS mapping image explains that the higher arsenic concentration in the brown rice is due to high arsenic distribution in the rice husk (protective covering of rice). Consequently, some degree of rice milling may be effective in the reduction of arsenic intake.