Instrumental Analysis ACS Exam Questions With 100% Correct Answers

Instrumental Analysis ACS Exam Questions With 100% Correct Answers How to calculate signal to noise - answerS/N = mean/STDEV -inverse of RSD -STDEV: 5 (#STDEV gaussian curve) -STDEV: peak-to-peak noise/5 S/N relationships - answer-increase S/N, increase run time -decrease noise, increase S/N Beer's Law - answer Beer's Law Calculations - answer Hard ionization vs soft ionization - answer-hard: no M+ peak (or small) -soft: M+ peak is visible MS: hard ionization sources - answer-impart high quantities of residual energy in the subject molecule invoking large degrees of fragmentation MS: hard ionization sources examples - answer-electron impact Electron Impact (EI) - answer-Uses electron impact to ionize a molecule -gas phase -energetic electrons -possibility of over fragmentation -hard -most common EI advantages - answer-good stability -universal ionization -reproducible spectra EI disadvantages - answer-MW can be hard to determine -compatible only with volatiles (GCMS) -Hard to make negative ions MS: soft ionization sources - answer-processes which impart little residual energy onto the subject molecule and as such result in little fragmentation MS: soft ionization sources examples - answer-CI -ESI -MALDI Chemical ionization (CI) - answer-First ionizes a molecular gas which in turn ionizes the molecule of interest. A "gentler" method of ionization. -soft -electron beam creates ions -proton transfer between reagent gas and the analyte -gives MW and MW+1 or MW-1 fragmentation in CI - answer-occurs if enough excess energy after the ionization of the analyte CI advantages - answer-MW info more readily available -fewer fragments -control specificity CI disadvantages - answer-fragments not very reproducible -compatible only with volatiles (GCMS) Electrospray ionization (ESI) - answer-multiple charged ions -can make positive or negative ions by taking on hydrogens or removing hydrogens MALDI abreviation - answerMatrix assisted-laser desorption ionization MALDI Theory - answer-non-volatile ionization -don't want fragmentation