Instrumental Analysis ACS Exam

How to calculate signal to noise

S/N = mean/STDEV

-inverse of RSD

-STDEV: 5 (#STDEV gaussian curve)

-STDEV: peak-to-peak noise/5

S/N relationships

-increase S/N, increase run time

-decrease noise, increase S/N

Beer's Law

Beer's Law Calculations

Hard ionization vs soft ionization

-hard: no M+ peak (or small)

-soft: M+ peak is visible

MS: hard ionization sources

-impart high quantities of residual energy in the subject molecule invoking large degrees of fragmentation

MS: hard ionization sources examples

-electron impact

Electron Impact (EI)

-Uses electron impact to ionize a molecule

-gas phase

-energetic electrons

-possibility of over fragmentation

-hard

-most common

EI advantages

-good stability

-universal ionization

-reproducible spectra

EI disadvantages

-MW can be hard to determine

-compatible only with volatiles (GCMS)

-Hard to make negative ions

MS: soft ionization sources

-processes which impart little residual energy onto the subject molecule and as such result in little fragmentation

MS: soft ionization sources examples

-CI

-ESI

-MALDI

Chemical ionization (CI)

-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

-occurs if enough excess energy after the ionization of the analyte

CI advantages

-MW info more readily available

-fewer fragments

-control specificity

CI disadvantages

-fragments not very reproducible

-compatible only with volatiles (GCMS)

Electrospray ionization (ESI)

-multiple charged ions

-can make positive or negative ions by taking on hydrogens or removing hydrogens

MALDI abreviation

Matrix assisted-laser desorption ionization

MALDI Theory

-non-volatile ionization

-don't want fragmentation

-TOF

-absorption of laser beam by matrix

-transfer energy form matrix to analyte

-desorption of both matrix and analyte

-transfer of proton to analyte from matrix

-large MW

MS Mass Analyzers

TOF, quadrupoles and traps

TOF

-fastest

-highest m/z range

-uses electric field to accelerate ions through the same potential, and then measures the time they take to reach the detector

-lower mass reaches the detector first

-assumes that all ions have same voltage, velocity, position, and angle (not always true)

Quadrupole mass analyzer

-oscillating electrical fields to selectively stabilize or destabilize the paths of ions passing through the field created b/w 4 parallel rods

-mass selective filter

photomultiplier tube

-An apparatus that converts a photon of visible light into an electrical pulse

-most sensitive photodetector

CCD

-is a silicon-based multichannel array detector of UV, visible and near-infra light

-sensitive to light

Beer's Law extension to mixtures of compounds

-can extend to sample containing several absorbing components if there are no interactions

-individual absorbances are additive

-low concentration analytes

-assumes one monochromatic wavelength

-

Sensitivity of absorbance methods

-low sensitivity at low sample concentrations

Sensitivity of Fluorescence methods

-sensitivity is x1000 greater than absorption

-leads to more accurate and precise results

-more expensive because of this

Ion Selective Electrodes (ISE)

-transducer that converts the activity of a specific ion dissolved in a solution into an electrical potential.

Electrochemical detector (HPLC)

-analyte gets either oxidized or reduced at the working electrode

-only responds to compound that can partake in redox reactions

-measures current when an electroactive solute emerges from the column and passes over the working electrode.

Potentiometry

the use of electrodes to measure voltages that provide chemical information

Coulometry

a technique in which the quantity of analyte is determined by measuring the number of coulombs needed for complete electrolysis

Voltammetry

an analytical method in which the relation between current and voltage is observed during an electrochemical reaction

How does changing flow rate affect retention time? (HPLC)

-a high flow rate reduces retention times

How does solvent polarity affect retention time? (HPLC)

-if polarity of stationary phase and compound are similar, retention time increases because the compound interacts stronger with the stationary phase.

-therefore more polar compounds have longer retention times w/ polar stationary, and shorter with nonpolar.

GC split injection

- high concentration

-most common

-high resolution

-best injection due to narrow width of beaks

GC splitless injection

-trace analysis

-dilute sample

-high resolution

-requires solvent trappnig

Is split or splitless injection preferred? (GC)

split injection

General Elution Problem

to overcome this general elution problem and obtain good separation of all compounds, conditions are started at a low temperature to allow compounds with low BP to separate, then the temperature is gradually increased by increments set by the experimenter to allow for the separation of compounds with high BPs. This allows for a shorter run time and better overall separation.

GC Detectors

Thermal conductivity, Flame ionization, photo ionization

Thermal Conductivity Detector

- not as popular

-packed columns

-less sensitive than other detectors

-all analytes

-measures a change in voltage

Flame Ionization Detector

-analyte burned in air and H2

-carrier gas: He

-flammable gas: H

-Increase current, increase ionization

-ions produced are measured by the change in voltage when the ion hits the collector

-great detector for hydrocarbons

-combust at low temperature flame

-if you increase combustion temperature, no ions will be produced, and nothing will reat with O2

Electron Capture Detector

-particularly sensitive to compounds with halogen atoms, nitro groups, and other groups with high electron affinity

-GC detector

Advantages of tubular columns

-faster

-shorter retention times

-more inert

-longer life

-less bleed

-higher efficiencies

-greater reproducibility

-higher resolution

Advantages of packed columns

-durability

-greater sample capacity

-phase selectivity

-cost

-broader peaks

-longer retention times

-less resolution

-non destructive

Effect of packing material diameter on resolution

-smaller particle sizes have been shown to offer higher peak efficiencies and increased resolution

when to use quartz cuvette

-used to measure wavelengths under 320 nm

Flame furnace

-gets rid of excess solvent

-breaks particles into aerosol

Graphite furnace

-Atomization through electrical heating

-sample stays in optical path longer than flame

-uses less sample

-sample injection is tricky, precision is worse

-one wavelength, one element

How to calculate the resolution of two peaks

R = (RT1 - RT2) / [0.5 * (W1 + W2)]

Grating monochromator

-disperse ultraviolet, visible, and infrared radiation typically using replica gratings,