Time lag focusing

Figure 2.10. Schematic of a MALDI-TOF mass spectrometer with reflector and time-lag focusing. Reprinted from A. Westman-Brinkmalm and G. Brinkmalm (2002). In Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, J. Silberring and R. Ekman (eds.) New York John Wiley Sons, 47-105. With permission of John Wiley Sons, Inc.

Time Focusing Devices. The resolution of the TOF analyzer is limited by the initial velocity spread of the ions. However, there are powerful devices that can compensate for this velocity distribution, and the most widespread techniques at present are the electrostatic ion reflector (electrostatic mirror) and time-lag focusing (delayed extraction). 

The mass accuracy is highly dependent on the mode the instrument is operating in. In the reflector mode, with time-lag focusing, the best MALDI-TOF and oa-TOF instruments are capable of achieving <5 ppm with internal standards, provided that the isotopes are resolved. In many cases it is not possible to add internal calibrants, and then the error in mass accuracy is often increased to 50-100 ppm. Operation of an instalment in a linear mode will typically decrease the mass accuracy. 

R. M. Whittal, L. M. Russon, S. R. Weinberger, and L. Li. Functional Wave Time-Lag Focusing Matrix-Assisted Laser Desorption/Ionization in a Linear Time-of-Flight Mass Spectrometer Improved Mass Accuracy. Anal Chem., 69(1997) 2147-2153. 

Note Unfortunately, establishing patents and trademarks has caused redundant names for almost the same thing Time Lag Focusing (TLF, Micromass) Delayed Extraction (DE, Applied Biosystems) Pulsed Ion Extraction (PIE, Bru-ker Daltonik). 

Partial mass spectrum of cow s milk (containing 2% milk fat) observed by MALDI/time-of-flight mass spectrometry. [From R. M. Whlttal and L Li, "Time-Lag Focusing MALDI-TOF Mass Spectrometry," Am. Lab. December 1997, p. 30.]... 

Naven T, Harvey D, Brown J, Critchley G (1997) Fragmentation of complex carbohydrates following ionization by matrix-assisted laser desorption with an instrument fitted with time-lag focusing. Rapid Commun Mass Spectrom 11 1681-1686... 

To reduce the kinetic energy spread among ions with the same m/z ratio leaving the source, a time lag or delay between ion formation and extraction can be introduced. The ions are first allowed to expand into a field-free region in the source and after a certain delay (hundreds of nanoseconds to several microseconds) a voltage pulse is applied to extract the ions outside the source. This mode of operation is referred to as delayed pulsed extraction to differentiate it from continuous extraction used in conventional instruments. Delayed pulsed extraction, also known as pulsed ion extraction, pulsed extraction or dynamic extraction, is a revival of time-lag focusing, which was initially developed by Wiley and McLaren in the 1950s, shortly after the appearance of the first commercial TOF instrument. 

Time lag focusing presumes that (a) ionization occurs within the source and (b) the ionization mechanism imparts upon the ions a distribution of transient energies and velocity vectors. An ion source such as that used in MALDI can greatly benefit from time lag focusing, and this technology is becoming standard on commercial MALDI/TOF instruments. 

Figure 13.2 (a) Time-lag focusing applied to ions formed on a surface to focus ions... 

Today, the modern version of time-lag focusing, or delayed extraction,14 16 is used in almost all MALDI-TOF mass spectrometers. It is in fact a special case of the Wiley and McLaren method, in which the initial spatial distribution (usually a very thin sample and matrix mixture dried on a stainless steel plate) is assumed to be zero. Therefore, after the delay time the velocity and spatial distributions are correlated, i.e. ions of highest velocity have moved the greatest distance. This space velocity correlated focusing has been described by Colby and Riley,17 and provides considerably better focusing than can be obtained from two independent and uncorrelated initial distributions in space and velocity/energy that comprise the general Wiley-McLaren case. 

Figure 6. Peptide mass profiling of a silver stained protein spot from a narrow range, pH 4-7, 2DE separation of human heart (ventricle) proteins. A MALDI-TOF mass spectrum of tryptic peptides is shown, analyzed using a Micromass Tofspec 2E spectrometer (Manchester, UK) operated in the positive ion reflectron mode at 20 kV accelerating voltage with time-lag focusing enabled. The protein spot of interest was identified as human vimentin, (courtesy of J.A.Westbrook and R. Wait) (unpublished data). 2DE gels were silver stained using a modified Amersham Biosciences kit.

Whittal, R. M., Schriemer, D. C., and Li, L., Time-lag focusing MALDI time-of-flight mass spectrometry for polymer characterization oligomer resolution, mass accuracy, and average weight information. Anal. Chem., 69, 2734, 1997. 

A significant improvement in mass resolution, respect to the above described "continuous extraction" procedure, was obtained by the introduction of "time lag focusing" or "delayed extraction" (DE). This principle can be used in both linear and reflection mode yielding an improvement in resolution by a factor of above ten in both cases. 

Jackson, A. T., Yates, H. T., Lindsay, C. I., Didier, Y., Segal, J. A., Scrivens, J. H., and Brown, J., Utilizing Time-lag Focusing Matrix-assisted Laser Desorption/ Ionization Mass Sp>ectrometry for the End-Group Analysis of Synthetic Polymers, Rapid Comm. Mass. Spectrom., 11, 520,1997. 

Figure 3.11. Principle of time-lag focusing. Ions with initial velocity dispersion drift to a different extent in the field-free environment of the extraction region the slower-moving ions lag behind the faster-moving ions. After a short delay, when an acceleration pulse is applied, the slower ions are subjected to a higher field than the faste ions and thus are accelerated to a higher velocity than the faster ions. In the FFR region, aU isomass ions will get closer to each other, and ultimately will arrive at the detector simultaneously.

Enumerate the principle of time-lag focusing in TOF mass analyzers. 

The initial velocity spread is converted to a spatial spread by allowing a delay between ionization and acceleration. Ions separate to create a correlation between position and velocity. Spatial focusing can be applied to correct this. This method was also developed by Wiley and McLaren and the conditions (length of delay) must be adjusted for each miz. Time-lag focusing does not work well when a significant spatial and velocity spread are present simultaneously. As will be discussed later, time-lag focusing has recently been adapted to greatly increase the resolution in MALDI TOF-MS. 

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