Spectrochemical plasmas for clinical and biochemical analysis
Plasma-based analytical strategies have been used in the context of elemental detection (total elemental determination) in biochemical and clinical samples for a number of years. But the applicability of these detection methods to more sophisticated analytical problems associated with the use, for instance, of new metallodrugs as therapeutic agents in different kinds of diseases or the interest in the fate of metallic nanoparticles in biochemical environments has reactivated the use of plasma-based methods in the bioworld. Thus, the state of the art of metal-derived information obtained in biological and clinical samples using plasma-based strategies has been the driving force for this special issue.
In this regard, from examination of the contributions to this issue, the first conclusion that can be drawn is that although element-specific detection can be conducted using a plethora of plasma techniques (a review on dielectric barrier discharge is included in this issue), inductively coupled plasma (ICP) mass spectrometry (MS) is still the most widely used detection method for bioapplications. The analytical advantages of ICP-MS over other elemental detection methods based on photon measurement (e.g., atomic absorption spectrometry or ICP optical emission spectrometry) are well known, and are mainly ascribed to the extremely high sensitivity of the technique together with its capability to measure isotopes and isotopic ratios, making possible screening and speciation of very low levels of heteroatoms in biomolecules or biological tissues. This fact has been exploited in some of the work reported in this issue for biosamples in combination with different separation methods necessary in order to achieve the desired species purity or directly on the tissues by applying laser ablation techniques.
Secondly, an interesting feature observed in the articles of this issue is the “quantitative” dimension of the work realized. Most articles contain quantitative elemental data and try to correlate them with species information (in the case of having these heteroelements as part of metabolites or proteins) or element/drug distribution in tissues (imaging) by sorting with different quantification strategies. In this context, it is possible to address that plasma-based analysis becomes quantitative by providing a variety of strategies for absolute quantifications of low levels of the target heteroatoms in biological samples. In addition, it is noteworthy that the trend in some articles to combine the elemental information (provided by the corresponding plasma-based strategy) with specific biological information—for example, protein activity or DNA–protein interaction—has been rarely addressed so far using ICP-MS. It may be that novel future uses of plasma technology go in this direction.
Lastly, regarding the topics of research chosen, it is possible to observe the increasing use of plasma-based strategies to address the fate of heteroelement-containing drugs in different biological samples or their consequences. It seems clear that the excellent analytical features provided by ICP-MS, in particular for monitoring platinum-containing drugs of extended use in chemotherapy, have now been favorably used also for monitoring gadolinium contrast agents or boron-based therapies. Additionally, the monitoring of “nontraditional” plasma elements such as phosphorus has also been shown to play an important role, in particular for the monitoring of DNA-related samples, for which these techniques could play an important role soon. In this sense, the availability of the ICP-QQQ instrument, with considerably enhanced detection limits for the bioelements phosphorus and sulfur as compared with commoner ICP-MS instruments, could provide a substantial thrust for ICP-MS use in the genomics and proteomics fields.
In brief, we hope that this topical collection provides a flavor regarding the opportunities that plasma-based techniques are offering today and heralds their use in other possible challenging fields of research for clinical and biochemical analysis.