Analysis of Michigan hop varieties and easy and direct typification by paper spray ionization mass spectrometry and principal component analysis

Technical Session 02: Analytical I Session
Andre R Venter, Western Michigan University
Co-author(s): Kari Blain, Western Michigan University, MI, USA

ABSTRACT: Paper spray ionization is a novel method of mass spectrometric analysis that allows for rapid, easy, and accurate direct chemical analysis of plant materials and extracts. Paper spray is an ambient ionization method related to desorption electrospray ionization (DESI) and direct analysis in real time (DART). With papers pray a small isosceles triangle made from paper is used directly as the ion source. An extension of this technique (known as leaf spray) uses plant material directly. A small drop of aqueous solvent, typically 5–20 µL, is spotted onto a leaf to which 3–5 kV is applied by alligator clip. Ions are then produced from compounds in the leaf or on the surface of the leaf when these are soluble in the spray solvent. These ions are sampled into a mass spectrometer for analysis. In this presentation we demonstrate direct analysis of hops by leaf spray. A single bract is separated from a hop cone and analyzed directly. The entire analysis takes less than 30 sec per run, so a representative analysis can be obtained by analyzing multiple bracts from a sample. Rich spectra are obtained and the alpha- and beta-acids can be quantified relatively, so, for example, cohumulone ratios can be calculated. In addition various classes of lipids and polyphenolic compounds are also observed, allowing for accurate typification of hops varieties by fingerprint matching or principle components analysis. Hop is a relatively new agricultural, but rapidly expanding, crop in Michigan, with around 150 acres currently under hop yards. This presentation also highlights some differences between Michigan and other U.S. hop-growing regions in the ratios of alpha- and beta-acids and other flavor compounds.

Andre Venter completed both his bachelor’s and master’s (cum laude) degrees at the University of Pretoria, South Africa. He received his Ph.D. degree in 2003 from the University of Pretoria, where he developed a comprehensive multidimensional supercritical fluid and gas chromatography (SFC×GC) method for petrochemical and natural product analyses. He worked with R. Graham Cooks as a post-doctoral researcher at Purdue University, West Lafayette, IN. There he investigated the fundamentals of desorption electrospray ionization mass spectrometry and other ionization methods. Since 2008 he has been an assistant professor in the Department of Chemistry at Western Michigan University, where his research in ambient desorption ionization continues. His research focuses on ambient surface analysis and ionization mechanisms involving the solvent-air interface and further development of ambient ionization and mass spectrometry technology. Applications and method development in agricultural product, food chemistry, industrial, environmental, and occupational health monitoring are being pursued.