Aston Labs - Purdue University

Purspec technologies (https://www.purspec.com/) has developed many generations of portable mass spectrometers (pMS) making them a strong company in this field. Spun out of Graham Cook's Aston labs (link) at Purdue University they have been building portable MS systems for over 20 years. So, to try and understand more about portable MS I aim to understand more about this research group. What projects do they work on? What type of subject expertise do they have? What equipment and techniques have they developed? (Unfortunately non Purdue alumni aren’t able to freely access dissertations via the website page)

The Aston lab disclaims their research aims as stated:

“A central current interest is in ambient ionisation mass spectrometry and its application in drug discovery, tumour resection guidance, therapeutic monitoring in biofluids, and microorganism biotyping. Each of these experiments involves minimal sample workup. Another strong interest is in preparative mass spectrometry, both by soft landing ions onto surfaces in vacuum to generate new materials and also by collecting the products of accelerated reactions occurring in microdroplets. This accelerated chemistry also provides unique insights into the prebiotic origin of biomolecules and chirality. In situ analysis using miniature mass spectrometers coupled to ambient ion sources for environmental, clinical and public safety applications is of interest. So, too, is online reaction monitoring and high-throughput experimentation.”

Accelerated Reactions

The rates of many reactions can be dramatically increased in microdroplets and thin films compared with the bulk phase. It was observed that reactions which typically took minutes to hours in bulk solution occurred in the few seconds between ion generation and ion capture by a mass spectrometers. Accelerating it by factors of 10 to 10⁶. This is thought to occur through increased reagent concentration due to desolvation, pH extrema, Le Chatelier’s principle, as well as decreased reaction activation energy at the partially desolvated droplet / thin film surface. Reminder of Le Chatelier’s principle, if a dynamic equilibrium is disturbed by changing conditions the position of the equilibrium shifts to counteract that change and restore a new equilibrium.

To greater understand the droplets, fluorescence microscopy was developed to determine droplet sizes below the diffraction limit. Pushing this discovery further, the Aston labs are able to deposit very thin films from reactions on surfaces.

Homochirogensis (symmetry breaking, chiral enrichment and chiral transmission). The Aston lab have looked at Ser₈ as a stable clust in a formation that is highly chirally selective and relates prebiotic chemistry.

Brain Cancer Diagnostics

DESI-MS (Desorption electrospray ionisation) is being used to profile normal and cancerous tissues and methodologies for the rapid intraoperative diagnosis of cancer from surgical tissue biopsies. DESI-MS imaging used a 1:1 DMF-MeCN solvent system to look at heterogenous brain and glioma tissue section. Using bioinformatics principal component analysis (PCA) to correlate the MS with the tissue pathology, permitting the creation of statistical models based on linear discriminant analysis (LDA) used to classify tissue sample as normal or tumour.

They modified a linear ion trap mass spectrometer (LTQ-MS) for use in the operating room. It contained a computer, high purity compressed gas cylinders, power converters, moving stage controller and can be manoeuvred by a single person in the operating room.

MRM-Profiling

Multiple reaction monitoring (MRM) is one of the four basic operations of a triple quadrupole mass spectrometer. Product ion scan (all fragment ions of precursor scan), precursor ion scan (all precursor ions with the same m/z fragment ion), neutral loss scan (all pairs of ions with the same m/z difference) and multiple reaction monitoring (A precursor/product ion pair being monitored). As small molecules, including metabolites and lipids play significant roles in disease and especially in cancer. Diagnosis, prognosis and therapy of certain diseases have been designed or improved with increasing knowledge from pathway analysis.

MRM uses different scan methods for various types of functional groups. 2D MS/MS data points are plotted of Product ion m/z vs precursor ion m/z. Data analysis by univariate and multivariate statistical methods is used to isolate the informative MRMs, which are explored further in structural studies and validated as biomarkers. MRM profiling means smaller number of functional groups, compared to individual metabolites, meaning much smaller data sets acquired for analysis and shorter time needed for data collection. This has been used in profiling human plasma coronary artery disease, Parkinson’s disease, human polycystic ovarian syndrome, atopic dermatitis, diet compliance, animal fertility…

High Throughput Synthesis and Analysis

For high throughput screening of organic reactions, a liquid handling robot and DESI was used for fast reaction mixture analysis. The pin tool transfers 50 nL onto the PTFE surface allowing 384-well plates, which the location of the pinning slightly offset allowing for 6,144 unique reaction conditions on one plate. This combination results in a screening system capable of analysing thousands of organic reactions per hour.

As we’ve seen before, DESI allows for accelerated reaction synthesis, allowing for measurement of accelerated organic reactions. Custom software has also been developed to allow chemists to quickly analyse the data from the experiment. Using the system they’ve screened amine alkylation, acylation, sulfonylations, Suzuki cross couplings and sonogashira couplings.

Ion Trap Developments

As quadrupole ion traps are common mass analysers due to their ability to perform tandem mass spectrometry and low vacuum requirements. Mass selective instability allows for the removal of specific ions allowing them to be measured. By raising the radio frequency trapping voltage, ion are ejected in increasing m/z order. The motion of ions in a quadrupole is dictated by the Mathieu equation.

z is the number of charges, e is the elementary charge, U is the DC potential on the rods, V_rf is the zero-to-peak rf potential, Ω, is the angular frequency (2πf is the rf frequency), r_ois the half distance between rods, m is the mass of the ion in kilograms.

Ions trapped in a quadrupole field will oscillate with a specific frequency. This fundamental frequency is related to another parameter, β.  β is approximated by specific a and q values. An ion will resonant with an applied ac matching their fundamental frequency, this increases the amplitude of the ion’s trajectory until it is unstable and ejected. This was coupled to the rf ramp so that ions will successively be raised to a specific q value and resonant with the ejection frequency.

What’s Cooks lab has been able to do is implement a frequency sweep at a constant trapping voltage to allow ions to be ejected at their secular frequency. The sweep is created such that m/z excited is linearly correlated to time allowing for far easier m/z calibration.

Scanned frequency resonance experiment allows only the ions of interest to be activated / ejected as they will be individually activated by a waveform containing their fundamental frequency. (A specialised article is required to fully explain what they are trying to do)

Ambient Pressure Ion Manipulation and Ion Mobility Spectrometry

As ambient ionisation techniques requires ions being generated in the open air. Work has been done on understanding how ions behave at high pressures. Using an ellipsoidal electrode operating at high pressure they focused a beam of nanosprayed ions which resulted in increased transfer efficiency of ions to the mass spectrometer.

Using a 3D printed IMS from carbon nanotube doped plastic filament which allows for high voltage they were able to use it for ion separation, ion-molecule reactions and ion focusing. As well as developing ion mobility spectrometers using only ambient air as the separation gas. The aim is to better understand how environmental conditions influence ion mobility spectra and achieving chiral selectivity in ambient ion mobility.

Origins of Life

Following on from the microdroplet chemistry, reactions deemed impossible under bulk conditions become feasible. The microdroplet environment accelerates the formation of peptides from amino acids, offering a glimpse into how life’s building blocks might have assembled in prebiotic times.

This group has many active areas of investigation with some more detailed articles needed to explore, particularly diagnosis, MRM profiling, ion trap and focusing developments.