Landmark Literature: Part I

Love Is in the Air

By Victoria F. Samanidou, Laboratory of Analytical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki, Greece.

No matter what matrix is analyzed, what analyte is determined, or which analytical technique is applied, sample preparation is a crucial step. My choice for landmark publication of 2017 brings the benefits of fabric phase sorptive extraction (FPSE) to gaseous samples.

FPSE is a sample preparation approach that was introduced in 2014 by Kabir and Furton (1). When it first came to my attention, I was impressed by its simplicity and efficiency – it requires no matrix modification or clean-up and still achieves great performance, effectively assimilating most of the benefits of solid-phase microextraction while avoiding most of the shortcomings of conventional sample preparation techniques. FPSE’s versatility makes it a suitable technology for a wide range of applications and allows it to resolve diverse analytical problems.

There are already hundreds of sol-gel coatings that can be readily used as sorbents for FPSE, with unique selectivity and affinity towards an extensive range of target analytes. In our laboratory we have used the approach in the analysis of milk and biological fluids for the determination of several compounds, such as endocrine disrupters, antibiotics and antidepressants by HPLC (2)(3)(4)(5)(6), and metals by flame atomic absorption spectrometry (7).

However, gaseous samples have not previously been analyzed using FPSE, which is why my choice for landmark publication captured my interest. The paper is the outcome from joint research by two expert groups that combined their experience in FPSE and environmental analytical chemistry to develop an integrated sampling and analysis unit. For the first time, they applied FPSE principles to air sampling and pre-concentration for the determination of sexual pheromones in environmental air by headspace-gas chromatography–mass spectrometry.

The authors introduce a novel laboratory-built unit that incorporates sol-gel hybrid inorganic-organic polymeric coatings on fiber glass substrate surface as sorbent traps in air analysis. The unit is simple and cheap and it can be directly hyphenated with a multipurpose GC autosampler, without any instrumental modification. As the same unit can work under sampling and analysis mode, any need for sorptive phase manipulation prior to instrumental analysis is eliminated. In the sampling mode, the unit is connected to a sampling pump to pass the air through the sorptive phase at a controlled flow-rate, while in the analysis mode it is placed in the gas chromatograph autosampler without any instrumental modification, thus diminishing the risk of cross-contamination between sampling and analysis.

The applicability of the approach has been evaluated using the major components of the Tuta absoluta (tomato leafminer moth) sexual pheromone as solute probes. The performance of the integrated unit was evaluated by analyzing environmental air sampled in tomato crops.

I was impressed by the critical summary, presented as a SWOT (strengths, weakness, opportunities and threats) analysis, in which the authors give an honest and unbiased account of the strengths and weaknesses of their proposed method, and identify key points for further investigation.

The results presented in my landmark paper open a door to further FPSE applications, enabling the determination of volatile and semi-volatile compounds in air – and once again prove that imagination in sample preparation and sample handling has no limits.

Good Vibrations

By Karen Faulds, Professor of Chemistry, University of Strathclyde, UK.

Enhanced-Raman spectroscopy is an excellent tool for probing biochemical processes – the molecular information that can be gained from the vibrational fingerprint spectra produced is invaluable. Raman has led to great advances in non-destructive analysis in recent years, particularly in the biomedical field, where it can be used to non-invasively probe systems, such as single cells, to gather information-rich spectra. However, though the technique can be used to gain molecular information at sub-cellular resolution, sensitivity is limited by the intrinsically weak signals obtained from Raman scattering. In addition, it can be challenging to track specific or labeled molecules within complex biological matrices, because of the complexity of the multi-component spectra obtained.

Therefore, it was with great interest that I read a recent Nature paper from Columbia University researchers on the use of stimulated Raman scattering (SRS). SRS uses non-linear Raman effects to enhance the Raman signal, and Wei and colleagues reported using pre-resonance electronic SRS (epr-SRS) to enhance signals by up to 1000 times compared with non-resonant SRS. It allowed the authors to use commercially available fluorescent dyes, as well as synthesizing their own suite of dyes containing alkyne bands (C to C triple bonds), with a view to carrying out multiplex imaging. Multiplexing – the detection of multiple analytes simultaneously – is highly desirable in imaging, as it allows tracking of multiple species with subcellular resolution. However, multiplex imaging has traditionally been carried out using fluorescence, which is limited in the number of species that can be detected using one excitation wavelength. In addition, the broad nature of fluorescence bands can make deconvolution of the data challenging. Therefore, using vibrational techniques with narrower peak widths for multiplexing could bring major benefits, and this work demonstrates the potential for 24 different resolvable codes.

The use of alkyne labels – first reported by Sodeoka and colleagues (8) for imaging of alkyne-tagged EdU in cells using normal Raman – is particularly attractive, as the alkyne band falls in the so-called cellular silent region (1800−2800 cm−1) of the spectrum – a region where there are no vibrational bands from the cell components. Wei’s group used the imaging approach to observe metabolic activity in the form of DNA replication and protein synthesis in hippocampal neuronal cultures and cerebellar tissue. The increased sensitivity and rapid imaging achieved by epr-SRS in combination with multiplexed dyes make this a very powerful technique for cellular imaging and provides a compelling argument for using SRS over fluorescence in future studies.

Closing the Gap

By Martin Giera, Head Metabolomics Group, Leiden University Medical Center (LUMC), Leiden, the Netherlands.

Asked to put forward my landmark paper of 2017, I didn’t hesitate; for me, it has to be an article from Huan et al. in the Siuzdak group. The paper fills an important gap – for me personally, but also for the metabolomics community.

The story starts in the 2000s, when I applied for a postdoc at the Scripps metabolomics center… and totally blew the interview. Looking back, untargeted metabolomics involved too much programming for a wet-lab scientist like me – I wasn’t ready for it, or perhaps it wasn’t ready for me!

Ten years later, and I recently joined the group as a visiting professor. I had a wonderful time, with many intriguing scientific discussions; even when we could not agree on certain concepts or ideas, the general idea of combining systems biology and (untargeted) metabolomics always prevailed. There is no doubt about the value of integrating multi-omics data sets to obtain a better systems-wide understanding of biology – and eventually manufacture tools shaping biological phenomena to our needs. However, there are several gaps in our current capabilities. One is that statistically relevant information needs to be extracted from different types of data and integrated to give a detailed mechanistic understanding of molecular biology – and that’s not just about the numbers, it’s also about people; scientists from different fields need to understand each other and work together hand in hand.

Though some software tools, such as MetaboAnalyst, allow for integrated pathway analysis using different data sets, in its 2017 paper the Siuzdak group for the first time described a completely integrated approach, using raw LC-MS data in the XCMS workflow. The work is a first step and many bottlenecks remain, but I am convinced that such approaches are the future when it comes to the generation of meaningful biological information from complex data sets. Ultimately, they will boost our success in the search for new diagnostics and therapeutics. Imagine an integrated platform like XCMS, with tens of thousands of users, bringing together people from different fields and stimulating their interaction. This landmark paper closes the gap between untargeted metabolomics and systems biology, and demonstrates that untargeted metabolomics is ready for deeper biological meaning.

Manipulating the Matrix

By Jhanis J Gonzalez, Lawrence Berkeley National Laboratory, Environmental Energy Technologies Division, Laser Technologies Group, Berkeley, California, USA.

The paper I have chosen addresses a major roadblock in advancing laser ablation for routine chemical analysis – sample preparation and matrix manipulation. The authors present a new approach to solid matrix modification ¬– ammonium bifluoride (NH₄HF₂) digestion – which helps eliminate matrix effects and opens the door for wider implementation of laser ablation-based techniques into bulk analysis.

As described in the article, digesting silicate rock samples with ammonium bifluoride causes complete breakdown of silicate minerals, to create an ultrafine powder with a very consistent size and shape of grains. Compared with powders achieved by milling, there is a reduced risk of sample contamination, and the method is faster and more eco-friendly than traditional acid digestion. Essentially, the technique converts varied solid matrixes into the same matrix, thus eliminating matrix match requirements and matrix effects between samples during chemical analysis by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS). The work is a game changer – not only can the approach be used to modify solid sample matrices, but it also serves as a new way to prepare solid standards, and so on. In the next few years, I hope to see a surge of applications based on this approach across a number of fields.

The New Old-Fashioned Way

By Hans-Gerd Janssen, Science Leader Analytical Sciences, University of Amsterdam and Unilever Research, Amsterdam, the Netherlands.

“Whole column detection (WCD) is as old as chromatography itself.”

“In Tsvet’s first experiment on chromatography of plant pigments, he necessarily carried out visual whole column detection (WCD).”

These are just two quotes from an excellent article that showed us once again that the mature technique of chromatography still has tricks up its sleeve! In regular chromatography, the criterion for peak identification is solely the time of appearance of the analytes at the column exit. Unfortunately, many compounds have the same retention time. If we could track the location of a compound as it travels along the column we could use the entire place/time plot for compound assignment, rather than just using the time required to travel to the detector. In gradient operation, especially, this would significantly increase our ability to distinguish compounds. Note I use the word “distinguish” instead of “separate” – compounds might be distinguishable even if they have the same elution time. Non-separated compounds, or “co-eluting compounds” in classical terminology, might have different place/time plots on the column in gradient operation. With WCD, chromatography becomes a method to distinguish compounds rather than to separate molecules. Other applications of WCD include column length reduction. Very often compounds will be separated long before they are seen by the end-column detector, and with WCD we can detect them as soon as they are separated and before further dispersion. Finally, if we see separation develop we can change conditions on-the-go. Imagine if we combine this with flow switching in microfluidic systems – using segmented columns with flow-switching options in between two segments we could stop the separation once it is there. It is another proof that there are endless options for further improving chromatography, a technique many non-chromatographers may see as dull and not requiring further investigations.

The tricky aspect of WCD is the detection. Like early pioneer Mikhail Tsvet, who used light to detect compounds, virtually all other reports on WCD use optical detection. With recent trends towards smaller particles we can now use shorter columns, making arrays of optical detection devices more feasible. My landmark article uses “admittance detection” – basically, contactless conductivity detection, in which the column contents are sensed through the column wall without galvanic contact with the mobile phase. Read-out is mainly determined by the mobile ions, with little contribution of the stationary phase-bound ones. A new, double-quadrupole admittance detector was developed to improve signal to noise at high scanning rates, with good sensitivities obtained at scan rates of up to 3 cm/s. Since columns longer than 15 cm are becoming a rarity, an image reflecting the position of all compounds can be obtained every five seconds, or even faster for shorter columns. The drawings and (even better!) videos in the article are impressive. In one gradient elution video, a trailing analyte band can be seen “overtaking” one that was initially ahead – something that would never have seen by an end-column detector. Could WCD be the (unexpected) next hero of chromatography?