Ion in-source

MALDI has also been used to analyze several molecular forms of phospholipids (GPC, GPE, GPS, GPI, GPG, phosphatidic acid, and cardiolipin) [29]. In the positive-ion mode, all phospholipids yield signals due to the [M - - H]+, [M -h Na]+, and [M - - K]+ ions. In-source fragmentation and PSD provide useful structure information [30]. With the exception of GPC, all phospholipids can also be analyzed in the negative-ion MALDI mode, where the molecular ion region is relatively noise-free and only the [M — H] signal is produced. 

The principles of operation of quadnipole mass spectrometers were first described in the late 1950s by Wolfgang Paul who shared the 1989 Nobel Prize in Physics for this development. The equations governing the motion of an ion in a quadnipole field are quite complex and it is not the scope of the present article to provide the reader with a complete treatment. Rather, the basic principles of operation will be described, the reader being referred to several excellent sources for more complete infonnation [13, H and 15]. 

Another type of ion is formed almost uniquely by the electrospray inlet/ion source which makes this technique so valuable for examining substances such as proteins that have large relative molecular mass. Measurement of m/z ratios usually gives a direct measure of mass for most mass spectrometry because z = 1 and so m/z = m/1 = m. Values of z greater than one are unusual. However, for electrospray, values of z greater than one (often much greater), are quite coimnonplace. For example, instead of the [M + H]+ ions common in simple Cl, ions in electrospray can be [M + n-H]- where n can be anything from 1 to about 30. 

An ion beam mainly comprises normal ions, all having the same kinetic energy gained on acceleration from the ion source, but there are also some ions in the beam with much less than the full kinetic energy these are called metastable ions. 

A major advantage of the TOF mass spectrometer is its fast response time and its applicability to ionization methods that produce ions in pulses. As discussed earlier, because all ions follow the same path, all ions need to leave the ion source at the same time if there is to be no overlap between m/z values at the detector. In turn, if ions are produced continuously as in a typical electron ionization source, then samples of these ions must be utihzed in pulses by switching the ion extraction field on and off very quickly (Figure 26.4). 

Ions formed in the ion source are considered to be the normal ions in a mass spectrum. For example, as illustrated in Figure 33.2, some of the ions (m/) dissociate to smaller mass ions (m," ) and neutrals (Hq) any unchanged ions (mC) and the fragments (mj" ) are drawn out of the source by an electric potential of V volts. 

Formation of normal ions in an ion source. In this example, some initially formed ions (m,+) dissociate (fragment) to give smaller ions (mj ) and a neutral particle (iio). Unchanged ions (m +) and the fragment ions (mj" ) are drawn out of the source as beams moving with velocities v, V2, respectively. 

Norma ions (m, or formed in the ion source will pass (filter) through the first, second, and third quadrupoles (Ql, 2, 3) if these are set correctly. If Ql is set to pass only m, ions, then normal mj ions cannot reach the detector, and if Q3 is set to pass only mj ions, then m + ions cannot reach the detector. Any mj ions that reach the detector must have been formed (metastable or induced by collision) by dissociation of m,+ ions in Q2. 

An example of linked scanning on a triple quadrupole instrument. A normal ion spectrum of all the ions in the ion source is obtained with no collision gas in Q2 all ions scanned by Q1 are simultaneously scanned by Q3 to give a total mass spectrum (a). With a collision gas in Q2 and with Q1 set to pass only m+ ions in this example, fragment ions (f, fj ) are produced and detected by Q3 to give the spectrum (b). This CID spectrum indicates that both f, and fj are formed directly from m+. 

A product ion scan. Source ions (mT, f,, . .., fs ) are selected by setting Ql, in this case, to pass only m,. Collisional activation of these ions in Q2 induces dissociation to give fragment ions (f,, fj, f, ), which are detected by scanning Q3. The symbolism for this process is shown. 

Quadrupole, Q3, is set to pass only the product ions under investigation. All the ions from the ion source are scanned by Ql and passed successively into Q2, where collisional activation occurs. Those ions that fragment to give product ions of interest are revealed by the appearance of the product ions in Q3 (Figure 33.7). 

A precursor ion scan. Source ions f,. . .., f, ) are all passed successively by Q1 into the collision cell, Q2, where a selected fragment (i ) is produced and detected by Q3. Only the ions (m, f,. fj) give f, fragment ions in this example. 

Quadmpoles or hexapoles are used as transmission guides for both slow and fast ions. In both cases, the objective is to ensure that as many ions as possible are guided from the entrance of the device to its exit. The ions are usually in transit in a straight line between an ion source and a mass analyzer. Any ions within the transmission guides that are deflected from the desired trajectory are pushed or pulled back on course by the action of the inhomogeneous RF fields applied to the poles of the guides. 

Substances are converted into species having positive or negative charges (ions) in the ion source. 

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