Matrix-assisted laser sample introduction

In direct introduction the sample can be introduced via a sample probe or plate through a vacuum lock, and can subsequently be ionized via El, Cl or matrix-assisted laser desorption ionization (MALDI see Section 2.4). Alternatively, the sample can be introduced as a liquid stream into an ion source at atmospheric pressure, after which it is subjected to electrospray ionization (ESI see Section 2.3). Direct injection does not offer any form of sample separation. 

FAB and PD have been replaced by electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) in the analytical mass spectrometry laboratory, because both of these newer techniques have a wider mass range of analysis and have lower detection limits. ESI and MALDI have become invaluable ionization techniques for nonvolatile components. This is particularly true for a wide range of biological molecules including proteins, peptides, nucleic acids, etc. Samples can be analyzed by ESI using either direct injection or introduction through liquid chromatography. 

A variety of MS formats are widely accepted and applied in the pharmaceutical industry. The specific MS application is often defined by the sample introduction technique. The pharmaceutical applications highlighted in this article feature two types of sample introduction techniques dynamic and static. Dynamic sample introduction involves the use of high-performance liquid chromatography (HPLC) on-line with MS. The resulting liquid chromatography/mass spectrometry (LC/MS) format provides unique and enabling capabilities for pharmaceutical analysis. The electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) modes are the most widely used. Static sample introduction techniques primarily use matrix-assisted laser desorption/ionization (MALDI). ... 

Another important development in the field of biopolymer analysis is the introduction of matrix-assisted laser desorption ionization (MALDl), which is a rather recent soft ionization technique that produces molecular ions of large organic molecules. In combination with time-of-flight (TOP) mass spectrometry, it was proposed as a valuable tool for the detection and characterization of biopolymers, such as proteins, peptides, and oligosaccharides, in many types of samples.The use of these recently developed techniques has not decreased the use of chromatography in determinations of biopolymers. Some efforts on the adaptation of the separation abilities of HPLC to the high potential of MALDl-TOF for the sensitive determination of additives in biocomposites are currently being carried out. 

Mass Spectrometric Detection. The very small volumetric flow rates of less than 1 pi,/min from electrophoresis capillaries make it feasible to couple the effluent directly to the Ionization source of a mass spectrometer. The most common sample-introduction and ionization interface for this purpose is currently electrospray (Section 20B-4), although fast atom bombardment, matrix-assisted laser desorption-ionization (MALDI) spectrometry, and inductively coupled plasma mass spectrometry (ICPMS) have also been used. Because the liquid sample must be vaporized before entering the mass spectrometry (MS) system. 

Parallel to this progress, new ionization methods have been developed that are based on the direct desorpHon of ions from polymer surfaces. With the introduction of "desorption/ionization" techniques, it has become possible to eject large molecules into the gas phase directly from the sample surface, and thereby mass spectra of intact polymer molecules have been produced. Much progress to date has been made using matrix-assisted laser desorp-tion/ionization (MALDl-MS), which is capable of generating quasimolecular ions in the range of 10 Daltons (Da) and beyond. 

A mass spectrometer analyzes ions that are created from molecules. The overall working scheme can be viewed as a combination of three major processes ionization, sorting, and detection. (a) The first step is introduction of sample into the instrument. Samples can be introduced as either a solid, liquid, or vapor into a vacuum chamber through an inlet. Depending on the type of inlet and ionization techniques used, the sample may already exist as preformed ions or it may be imiized in the ion source. Several ionization modes [1] are commercially available with electrospray ionization (ESI) [2] and matrix-assisted laser desorption/ionization (MALDl) [3, 4] being the most widely used nowadays. ESI uses an electrical field to produce a droplet spray the sample in this case is typically in a volatile solvent. Irrespective of the original... 

Mass spectrometry is an analytical technique that can determine precisely the atomic or the molecular weight of atoms or molecules once they have been ionized. There are four key elements in mass spectrometer the sample introduction, the source where ionization occurs, the mass analyzer, and the detector. Mass spectrometry can analyze many different types of samples that range from solid, liquid, or gases. First, the molecules have to be ionized either under vacuum or at atmospheric pressure. Depending on the ionization technique, either molecular ions (M ) with an odd electron number or protonated ions ([M + H] with an even electron number are formed in the positive mode and M , M or ([M - H] in the negative mode. Ionization techniques are often classified into soft ionization, where little or no fragmentation occurs, and hard ionization, where fragmentation is extensive. Electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are... 

Ambient MS is another advance in the field. It allows the analysis of samples with little or no sample preparation. Following the introduction of desorption electrospray ionization (DESI) [108,109], direct analysis in real time (DART) [110], and desorption atmospheric pressure chemical ionization (DAPCI) [111, 112], a number of ambient ionization methods have been introduced. They include electrospray-assisted laser desorption/ionization (ELDI) [113], matrix-assisted laser desorption electrospray ionization (MALDESI) [114], atmospheric solids analysis probe (ASAP) [115], jet desorption ionization (JeDI) [116], desorption sonic spray ionization (DeSSI) [117], field-induced droplet ionization (FIDI) [118], desorption atmospheric pressure photoionization (DAPPI) [119], plasma-assisted desorption ionization (PADI) [120], dielectric barrier discharge ionization (DBDI) [121], and the liquid microjunction surface sampling probe method (LMJ-SSP) [122], etc. All these techniques have shown that ambient MS can be used as a rapid tool to provide efficient desorption and ionization and hence to allow mass spectrometric characterization of target compounds. 

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