Determination of Noncovalent Binding Using a Continuous Stirred Tank Reactor as a Flow Injection Device Coupled to Electrospray Ionization Mass Spectrometry

Use of a Continuous Stirred Tank Reactor for the Determination of Electrospray Response Factors and Its Application to Underivatized Sugars Under Various Solvent Parameters

The relationship between the electrospray ionization (ESI)-mass spectrometric (MS) response of an analyte and its concentration has been well studied for permanently charged and basic analytes in the positive ionization mode, but there has been a lack of research effort for other analytes, and for the negative ionization mode, in general. In this study, this relationship was investigated for various adducts and deprotonated species of glucose, sucrose, and raffinose using a continuous stirred tank reactor (CSTR) coupled with ESI-tandem MS to obtain a continuum of response factors across a wide concentration range in both the positive and negative ionization modes with a single injection under 18 different combinations of solvents and additives. Profiles of response factors vs. concentrations varied widely and were dependent upon the analyte and solvent parameters. The use of ammonium trifluoroacetate resulted in the highest response factors for methanol-based and acetonitrile-based solvents in the positive and negative ionization modes, respectively. Ammonium acetate, ammonium formate, and sodium chloride in 80:20 acetonitrile:water in the negative ionization mode resulted in good linearities, useful for quantitative analysis. In the positive ionization mode, response factors tended to increase with an increase in the molecular weight of the analyte, and acetonitrile was generally found to decrease response factors. We have also demonstrated the ability of CSTR-ESI-MS to visualize ionization suppression in the presence of co-analytes. These data should be useful for liquid chromatography-ESI-MS method development for sugar analysis, to help guide the choice of mobile phase that will result in high sensitivity and linearity. Graphical Abstract.

Emerging flow injection mass spectrometry methods for high-throughput quantitative analysis

Where does flow injection analysis mass spectrometry (FIA-MS) stand relative to ambient mass spectrometry (MS) and chromatography-MS? Improvements in FIA-MS methods have resulted in fast-expanding uses of this technique. Key advantages of FIA-MS over chromatography-MS are fast analysis (typical run time <60 s) and method simplicity, and FIA-MS offers high-throughput without compromising sensitivity, precision and accuracy as much as ambient MS techniques. Consequently, FIA-MS is increasingly becoming recognized as a suitable technique for applications where quantitative screening of chemicals needs to be performed rapidly and reliably. The FIA-MS methods discussed herein have demonstrated quantitation of diverse analytes, including pharmaceuticals, pesticides, environmental contaminants, and endogenous compounds, at levels ranging from parts-per-billion (ppb) to parts-per-million (ppm) in very complex matrices (such as blood, urine, and a variety of foods of plant and animal origin), allowing successful applications of the technique in clinical diagnostics, metabolomics, environmental sciences, toxicology, and detection of adulterated/counterfeited goods. The recent boom in applications of FIA-MS for high-throughput quantitative analysis has been driven in part by (1) the continuous improvements in sensitivity and selectivity of MS instrumentation, (2) the introduction of novel sample preparation procedures compatible with standalone mass spectrometric analysis such as salting out assisted liquid-liquid extraction (SALLE) with volatile solutes and NH4(+) QuEChERS, and (3) the need to improve efficiency of laboratories to satisfy increasing analytical demand while lowering operational cost. The advantages and drawbacks of quantitative analysis by FIA-MS are discussed in comparison to chromatography-MS and ambient MS (e.g., DESI, LAESI, DART). Generally, FIA-MS sits 'in the middle' between ambient MS and chromatography-MS, offering a balance between analytical capability and sample analysis throughput suitable for broad applications in life sciences, agricultural chemistry, consumer safety, and beyond.