Mass Spectrometry Inlet system Ion Source Mass Analyzer Ion Detector

Mass Analyzers Which one is best for a given application? Depends on resolution (separating power of mass spectrometer) For two peaks m and m+Dm resolution is: m/Dm MW of C16H22O2 and C17H26O ? Resolution required?

Types of Mass Analyzers Focuses based on momentum Magnetic Sector: Single Focusing: A magnetic field is used to focus ions based on their momentum as they are ejected from an ion source at high energy. Resolution <2000

Magnetic sector double focusing: An electrostatic analyzer is used serially with a magnet to select monoenergetic ions. High resolution >10,000 mass up to 100,000Da

Quadrupole: electronic mass filter that selects ions of a single m/z by superimposing a radio frequency and a direct current potential Unit mass resolution mass range to approximately 2000 most common detector coupled to GC systems

Time of Flight Measures all m/z simultaneously

Ion Trap RF fields yield m/z band of stability • 3D RF Fields • Detect those ions that are selectively ejected due to destabilized trajectory Quadrupole First developed as a detector for GC/MS but its application has expanded consists of a doughnut shaped ring electrode and a pair of endcapped electrodes A variable RF voltage is applied to the ring electrodes while the end caps are grounded

Ions with an appropriate m/z ratio circulate in a stable orbit As the RF increases the orbits of heavier atoms are stabilized while those of lighter atoms are destabilized and collide with the electrode Typical commercial devices are able to resolve peaks that differ in 1 mass unit w/ MW 500-1000 Da This kind of device is like a monopole (as opposed to a quadropole) Ions can be maintained in the trap for a long time ( on order of minutes) allowing you to do ion chemistry

FT-ICR (Fourier Transform Ion Cyclotron Resonance) Advantages of FT: Improved S/N Greater speed per spectrum Higher sensitivity Greater resolution The heart of the FT-ICR technique is an ion trap just as the heart of FT-IR was the Michelsen interferometer

FT-ICR When ions drift into magnetic field their motion becomes circular as we saw with ion traps An ion in a circular path is capable of absorbing energy from an ac field. The absorbed energy increasing the velocity (and thus the radius of curvature) without affecting it. Application of the ac signal sets all the particles into coherent motion that have the same w but does not affect those with different w (different m/z). Ions are selectively destabilized and the spectral signals recorded. Time domain signals are transformed into frequency domain then converted into mass domain. Resolution is limited by precision and frequency measurements rather than slits or field measurements. Frequency measurements can be made with high precision therefore high resolution is possible < 10^6 $400K

FT-ICR Animation: http://info.med.yale.edu/wmkeck/prochem/fticr/FTICR.mpg

Mass Spectrometry Inlet system Ion Source Mass Analyzer Ion Detector

Inlet Systems Purpose is to introduce a very small amount of sample into the mass spectrometer so that its components may be converted into gas ions Batch inlet pictured here is simplest, sample is volatilized outside and allowed to slowly leak into ionization region Want minimal loss of vacuum

Direct Probe Inlet Good for solid and non-volatile liquids Sample is on surface, probe positioned near ionization source and slit leading to spectrometer Good for low concentrations or thermally unstable compounds Chromatographic inlets we will discuss in chromatography section

Mass Spectrometry Inlet system Ion Source Mass Analyzer Ion Detector

Detectors for MS Need to be very sensitive Typical gains are 10^5 up to 10^8! Provides high current with nanosecond response time Ionization efficiency is only 1 in 10^6, then substance fragments into many pieces some are at low abundances so detector needs to be very sensitive What is our most sensitive detector in optical spectroscopy? PMT detector in MS is similar, continuous dynode Continuous Dynode

Mass Spectrometry Summary MS is the ultimate in a qualitative analysis tool! MS by itself is most useful for analyzing pure compounds Analyte in mixture requires MS to be hyphenated to another technique For ex. GC-MS, LC-MS, MS-MS, or CE-MS

Advantages of MS Detection limits up to 3 orders of magnitude better than optical methods Remarkably simple spectra that are usually unique and easily interpretable

Disadvantages of MS Instrument costs 2-3x higher than optical atomic instruments Instrument drift can be as high as 5-10% per hour Subject to interferences

TONS of applications! Regiospecific Analysis of Diricinoleoylacylglycerols in Castor (Ricinus communis L.) Oil by Electrospray Ionization-Mass Spectrometry Characterization of Puff-by-Puff Resolved Cigarette Mainstream Smoke by Single Photon Ionization-Time-of-Flight Mass Spectrometry and Principal Component Analysis Electrospray Ionization Mass Spectrometry Fingerprinting of Brazilian Artisan Cachaça Aged in Different Wood Casks Determination of Nitrofuran Residues in Milk of Dairy Cows Using Liquid Chromatography-Tandem Mass Spectrometry 110 articles that had MS in the title published in ACS journals from January to March of 2007