Ion Traps with Internal Ionization

Figure 2.169 Methanol as reagent gas (ion trap with internal ionization).

The change of ion volnme is problematic. Most of the time, it is necessary to break the vacnnm at the level of the source or even thronghout the total mass spectrometer device with all the consequences that this implies (see Chapter 2). Quadrupole nsers generally make little use of Cl for practical reasons. A few recent devices allow the nse of semi-open ion volumes, a compromise between open and closed ion volnmes. The main advantage is to avoid intervention at the source level when one wants to pass from one ionization mode to another. This type of ion volume is useful for working mainly on structural analyses bnt does not supply the best results for quantification. We will see below that the ion trap with internal ionization is the most practical device (and often the most efficient) when switching between ionization modes is required. 

Performing Positive Cl Using an Ion Trap with Internal Ionization... 

Ion traps with internal ionization allow use of a liquid reagent placed in a small flask next to the mass spectrometer. Under the effect of the depression in the spectrometer, the molecules constituting the vapour pressure at the surface of the liquid are sucked into the ion trap via a small tube. The pressure is easily adjusted by a needle valve. This procedure is both practical and economic. It presents no need for a cylinder of gas under pressure, a two-stage regulator, or a manometer. A few milliliters of liquid are sufficient to carry out Cl for several days to weeks. " Methanol and acetonitrile are the two most common liquid reagents they provide excellent results. Their proton affinities are, respectively, 760.3 and 787.5 kJ/mol. ... 

For the reasons specified in the chapter dedicated to analyzers (Section 4.4), it is impossible to proceed to analyses in negative chemical ionization with an ion trap using internal ionization because the thermalized electrons are instantly ejected from the trap. 

The quadrupolar field is homogeneous only in the center of the ion trap. Ions should not approach the electrodes or their trajectories will become unstable and they will not be stored. To prevent this situation, one must introduce helium at a partial pressure of around 10 torr into the analyzer. The ions undergo multiple collisions with helium atoms. The collisions reduce the amphtude of their trajectories and confine them to the center of the analyzer. With ion traps using internal ionization, the carrier gas eluting from the chromatograph also plays the role of cooling gas. In any case, special attention must be taken to ensure that very high purity helium (and no other carrier gas) must be used. 

Recently, ion traps with hybrid ionization have appeared on the market. They offer the possibility of working under internal ionization, external ionization, or a combination of both. The changes in configuration are simple. This kind of ion trap combines the advantages of both systems it is very efficient in terms of detection limits and very robust because its performances resist the presence of interfering compounds from the matrix. 

Ion trap with an RF voltage applied to the ring electrode, providing the fundamental frequency v and its associated variable amplitude V. Instead of injecting ions, electrons may be injected for internal ionization. Variable RF voltage can be applied to the end caps for ion excitation or ion ejection. 

FIGURE 15.4 A schematic diagram showing typical internal El ionization in the ion trap with an off-axis detector. In the internal ionization mode, analytes are eluted into the ion trap through a transfer Une. Electrons are injected into the trap and ionize the analytes. 

Methanol Because of its low vapour pressure, methanol is ideal for Cl in ion trap instruments with internal ionization. Neither pressure regulators nor cylinders or a long tubing system are required. The connection of a glass flask or a closed tube containing methanol directly on to the Cl inlet is sufficient. In addition, every laboratory has methanol available. Also, for ion traps using external ionization and quadrupole instruments, liquid Cl devices are commercially available. 

When water is used as the reagent gas (Figure 2.170), the intensity of the HjO ion should be as high as possible. With ion trap instruments with internal ionization, no additional equipment is required. However, a short tube length should be used for good adjustment. 

Ion Trap Instruments Ion trap mass spectrometers with internal ionization can be used for Cl without hardware conversion. Because of their mode of operation as storage mass spectrometers, only a very low reagent gas pressure is necessary for instruments with internal ionization. The pressure is adjusted by means of a special needle valve which is operated at low leak rates and maintains a partial pressure of only about 10 Torr in the analyser. The overall pressure of the ion trap analyser of about 10 -10 Torr remains unaffected by it. Cl conditions thus set up give rise to the term low pressure CL Compared to the conventional ion source used in high pressure Cl, in protonation reactions, for example, a clear dependence of the Cl reaction on the proton affinities of the reaction partners is observed. Collision stabilization of the products formed does not occur with low pressure Cl. This explains why high pressure Cl-typical adduct ions are not formed here, which would confirm the identification of the (quasi)molecular ion (e.g., with methane besides (M + H), also M + 29 and M +41 are expected). The determination of ECD-active substances by electron capture (NCI) is not possible with low pressure Cl (Yost, 1988). 

Switching between El and Cl modes in an ion trap analyser with internal ionization takes place with a keyboard command or through the scheduled data acquisition sequence in automatic operations. All mechanical devices necessary in beam instruments are dispensed with completely. The ion trap analyser is switched to a Cl scan function internally without effecting mechanical changes to the analyser itself. 

Figure 2.171 Switching between the El and Cl scan functions in the case of an ion trap analyser with internal ionization (Finnigan).

The carrier gas flow setting of the GC also can show effect on the position of the mass calibration. Ion sources with small volumes and also ion trap instruments with internal ionization show a significant drift of several tenths of a mass unit if the carrier gas flow rate is significantly changed by a temperature program. 

The EPA method 521 by Munch and Bassett from 2004 provided at that time a suitable GC-MS method based on chemical ionization (Cl) using an ion trap mass spectrometer with internal ionization (Munch and Bassett, 2004 March and Hughes, 1989), in contrast to standard quadrupole or ion trap mass spectrometers... 

Analyzer type Ion trap MS with internal ionization (with MS/MS waveboard)... 

The injector is a heated zone where the sample solution is introduced via a syringe to be then vaporized and mixed with the carrier gas which is the mobile phase. Except for rare applications, the carrier gas used in GC-MS is helium. If nitrogen and hydrogen are often used when a chromatograph is equipped with an FID, ECD, or NPD detector, these gases are rarely used with a mass spectrometer. They are not compatible with the use of an ion trap analyzer with internal ionization where the carrier gas also serves as cooling gas in the mass spectrometer (see Chapter 4). The inert carrier gas has no other function but to allow the elution of the compounds in the analytical column. 

Two types of ion traps can be used as analyzers in the context of GC-MS coupling. When the ions are produced in a source equivalent to that of a quadrupole before introduction into the trap, we speak of an ion trap with an external source. When the ions are produced directly in the heart of the trap (the compounds are eluted in the trap as they exit the chromatographic column), the trap plays in turn the role of source and analyzer and acts as an internal ionization ion trap. Recent ion traps can function in either internal or external mode and also in a hybrid ionization mode whose operation principle is described next. 

Ion traps with an external source appeared on the analytical chemistry market a few years after their internal ionization homologues. This type of device separates ionization and detection, as in the case of a quadrupole. The ions are extracted from the source, accelerated, and focalized by an extraction system analogous to those desaibed for quadrupoles. An ion gate permits the sequential introduction of ions into the trap. 

Figure 6.1 compares two electron ionization mass spectra of diazepam, one recorded with a qnadrupole, the other with an ion trap operated with internal ionization. The mass spectra are very similar bnt the base peak is at m/z 256 in external source operation and at m/z 257 in internal sonrce. In an internal ionization ion trap, the danghter ions F + (m/z 256) react with the molecule M to produce the FH+ ion (m/z 257). The radical (M-H)- issued from the grasping of a hydrogen radical from the molecule does not, of course, appear because it is electrically neutral. 

Ion traps with hybrid sources are not presented in the table. As long as they allow working alternatively in internal ionization and external ionization, they combine the advantages and possibilities of both systems. Rare artifacts can nevertheless complicate the interpretation of mass spectra in hybrid mode. Ghost ions can indeed appear in the chemical ionization mass spectra of certain compounds. ... 

Derivatization was conducted by the addition of a 10% H-ethyl-diiso-propylethylamine solution and a-bromo-2,3,4,5,6-pentafluorotoluene. Sample obtained from the derivatization procedure were dissolved in ethyl acetate prior to injection in splitless mode using a DB-1 capillary column. Helium was used as the mobile phase, and the injector temperature was set at 290 °C with a transfer line temperature of 270 °C. Sample detection used ion trap MS for detection, with the detector being set at negative chemical ionization with m/z = 262 (for CCA) and m/z = 286 (for the internal standard). The limit of quantitation was 5 ng/ml, and the average recovery ranged from 92.0% to 114%. In addition, the extraction efficiency ranged from 48.2% to 55.6% for concentrations of 5, 50, and 250 ng/ml. Samples were reported to be stable for up to 6 months when stored at 18 °C. 

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