Electron ionization sample introduction

Some analyte fragmentation can be induced with APCI-MS and ESI-MS by collision-induced dissociation (CID) on octapole, hexapole, or cone devices at the input of the mass spectrometer. The newly developed direct electron ionization interface (DEI) involves the direct introduction of a nano-LC system working with a mobile phase flow rate of between 0.3 and 1.5 /rl/min into a mass spectrometer equipped with an electron ionization interface. It has been used to determine and identify several OPPs in water samples. Electron ionization generates spectra that can be interpreted using commercially available documentation (Wiley or NIST). 

A direct insertion probe is used for introduction of liquids with high boiling points and solids with sufficiently high vapor pressure. The sample is put into a glass capillary that fits into the tip of the probe shown in Fig. 9.5. The probe is inserted into the ionization source of the mass spectrometer and is heated electrically, vaporizing sample into the electron beam where ionization occurs. A problem with this type of sample introduction is that the mass spectrometer can be contaminated because of the volume of sample ionized. 

Since electron bombardment occurs in the gaseous phase, the volatility of the sample becomes a critical factor in mass spectrometry. This feature was mainly responsible for the slow development of mass spectrometry in organic chemistry, and more specifically in natural products chemistry. In 1955 the technique of direct sample introduction through a vacuum lock (13) into the ionizing chamber was applied (14-17). This modification allowed the study of samples of relatively low volatility and those which are thermally unstable. Polyfunctional compounds of low volatility can be rendered more volatile by a suitable selection of substituents or by chemical modification. 

Electron ionization (previously called electron impact or electron bombardment) has a long history in MS, as it was the hrst widely nsed ionization method. El sonrces, located inside the instrument s vacnnm chamber, consist of a box (stainless steel, 1 ml, also called the ion volume), with a series of openings that allow the introduction of both the sample and the ionizing electrons and the ejection of the resulting ions into the analyzer (Eignre 2.9). El sonrces are held at 200-250 °C to maintain the analyte(s) in the vapor phase and to prevent their deposition on the walls. The vapor pressure of the samples in the sonrce mnst be in the lO -lO Torr range. A heated tungsten or rhenium hlament is used to produce an electron beam thermionic... 

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... 

Most LC-MS sample introduction techniques, such as thermospray, electrospray, and APCI, do not employ an electron beam to induce ionization, and lead to the formation of predominantly molecular ion species. This leads to the formation of higher mass fragments that may be monitored by SIM or SRM. Such techniques are frequently used in biomedical applications, and find particular application in the quantitative analysis of labile or polar compounds such as biomolecules from complex biological matrices. 

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