DOC PREVIEW
CU-Boulder CHEM 5181 - ThermoFinnigan LCQ

This preview shows page 1 out of 4 pages.

Save
View full document
View full document
Premium Document
Do you want full access? Go Premium and unlock all 4 pages.
Access to all documents
Download any document
Ad free experience
Premium Document
Do you want full access? Go Premium and unlock all 4 pages.
Access to all documents
Download any document
Ad free experience

Unformatted text preview:

10-29-02 The ThermoFinnigan LCQ™ Classic offers analyte separation via liquid chromatography, coupled with electrospray ionization (ESI) quadrupole ion-trap mass spectrometry. The instrument is best suited for analysis of proteins, peptides, and other large non-volatile polar molecules, and has the advantage of automated MSn (n = 1 to 10) scanning of fragment peaks for more detailed molecular structure information. ThermoFinnigan LCQ™ Classic, Quadrupole Ion-Trap Mass Spectrometer Application Areas The development of electrospray ionization (ESI) helped broaden the list of compounds suitable for mass analysis by enabling ionization of compounds with high molecular weight and very low volatility. Any polar compound that can readily form ions (including adducts) in solution when exposed to an applied voltage can be analyzed with ESI. The most common applications for this instrument include protein, peptide, and drug analysis, but the LCQ is very versatile and is able to analyze smaller molecules as well. Other application areas include: pharmaceuticals and their metabolites, environmental samples, industrial and biological polymers, pesticides for soil and agriculture, impurities in synthetic compounds and drugs. ESI is usually chosen when a molecule of low volatility cannot be analyzed by other techniques. The LCQ in the CU Mass Spectrometry Facility is most commonly used for protein and peptide analysis, and so the instrument is most often optimized for samples of this type. Small molecule analysis, including DNA and RNA, do not achieve spectra of high quality under normal operating situations on the instrument in this facility. Sample Preparation and Analyte Requirements Sample preparation time varies widely between applications, and it is very important for ionization and analysis. Samples must be in liquid solution form, and in general must be thoroughly de-salted before ESI can be performed. The liquid chromatograph can achieve this, and so samples need not be prepared without salt before LC analysis. Solutions containing glycerol and detergents will hinder the ionization process of the analyte, and should not be used. Samples must also be prepared in appropriate concentration.10-29-02 Instrument Design and Operation Sample Introduction Liquid samples can be introduced by way of the external liquid chromatograph, or by direct infusion through a syringe pump. Direct infusion may be used if chromatographic separation is not necessary, the number of compounds to be analyzed is small, and the solution has been adequately de-salted and prepared. The LCQ instrument is coupled with an Agilent 1100 Series Capillary LC system. This instrument allows for very low flow rates necessary for the MS analysis. Typically 200 μL/min is injected from the stock solution, but only 3-20 μL/min will flow through the column. The columns are all reversed-phase with an inner diameter of between 50 and 500 μm, and are all hand-packed on site. The LC system is equipped with an auto-sampler, vacuum degasser, and UV-detector. Ion Source Ions are produced via electrospray ionization (ESI). Liquid sample introduced from the LC or syringe pump then enters the ESI needle, to which a high voltage is applied (+4 kV). The needle sprays the sample solution into very fine droplets that retain electrical charges at their surface. As the droplets fly through a region of dry gas at atmospheric pressure, solvent from the droplets evaporates, thereby concentrating the number of charges on a smaller area. After a flight path of approximately a millimeter the solvent has completely evaporated, leaving a multiply charged analyte molecules. The mist of charged molecules then hits a heated capillary skimmer that leads to the ion optics and mass analyzer. The instrument is capable of detecting both positive and negative ion polarities, but it most often achieves optimum spectra for positive ions. The LCQ instrument at the facility is equipped with the ESI interface as well as a nanospray ionization (NSI) interface. This allows for even lower flows when only small sample sizes can be used. ESI is a very inefficient, and only ionizes approximately 1% of the analyte molecules, whereas NSI ionizes approximately 10%. Other problems can occur due to such small sample volumes used, but the NSI interface can be very useful. Both ESI and NSI are very soft ionization techniques, which cause very little initial fragmentation. The LCQ can also be equipped with atmospheric pressure chemical ionization (APCI), but this feature is not available in this facility.10-29-02 Mass Analyzer The quadruple ion trap mass analyzer serves to store, fragment, and select ions for detection. In between the ion source and the mass analyzer is a sequence of ion lenses and two octapoles that serve to focus the ejected ions on their way to the mass analyzer. The ion trap consists of three stainless steel electrodes: the entrance end-cap; the exit end-cap; and the ring electrode circling the center of the trap. A radio-frequency (RF) ac voltage is applied to the ring electrode, which produces a three-dimensional quadrupole field. This time-varying field determines the motion of the ions within the trap. During the trapping sequence, the voltages are set such that all ions will keep a stable trajectory. When ready to scan the ions present, the voltages of the electrodes are changed to eject certain m/z ratios one at a time. The LCQ can be run in a variety of modes, depending on what kind of information is desired. The entire spectrum of ions can be scanned in order to determine molecular weight information on pure compounds in a mixture. Alternatively, the user can select a single ion to detect through one, or more stages of fragmentation. After selecting an ion the introduction of He-gas allows for collision-induced dissociation (CID) that creates fragments of the original ions. The LCQ offers MS/MS capability, and up to 10 stages of MS fragmentation in order to get more detailed structural information through each fragmentation event. The instrument is fully automated, which allows the MSn aspect of the instrument to be very user-friendly. Detector The LCQ is equipped with an off-axis detection system. The ions are accelerated from the ion trap to a 15-kV conversion dynode, and from there to a channel electron multiplier. When an ion strikes the conversion dynode, one or


View Full Document

CU-Boulder CHEM 5181 - ThermoFinnigan LCQ

Download ThermoFinnigan LCQ
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view ThermoFinnigan LCQ and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view ThermoFinnigan LCQ 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?