Product ion scan mode

A Q-TOF spectrometer is similar to a triple quadrupole but Q3 is replaced by an orthogonal TOF mass spectrometer. Using a Q-TOF instrument only the product ion scan mode can be collected, but because of its high resolving power, accurate masses for both the precursor ion and product ions can be obtained. (See the section below on accurate mass measurements.)... 

The product ion scan mode is probably the most used in mass spectrometry for strucmral elucidation. 

No examples for quantification in the product ion scan mode were found in the literature even though data processing would allow extraction of selected ions, integration of related signal areas, and summation for quantification. This procedure has been used by John et al. for the determination of the human haemoglobin derived peptide hHEM-y 130-146 in plasma [102], However, quantification especially of small molecule analytes is best performed in the MRM mode that is addressed below. 

Figure 11 Product ion scanning mode of the triple-quadrupole mass spectrometer.

Its basic structure is reported in Fig. 3.12 and can operate in TOF or in product ion scan mode. In the former, both Q1 and Q2 operate the rf only mode in other words they transmit all the ions from the ion source to the ion pusher (IP). Once IP is reached, the ions are pulsed by the application of a suitable electrical field (typical voltage applied is on the order of 104V) for 100 ns every 100 ps, in the TOF analyzer, in a direction orthogonal to the original pathway. By this experimental setup, the mass spectrum of all the ions generated into the ion source can be obtained, with resolution on the order of 15,000-20,000 and accuracy in the parts per million range. 

In the product ion scan, mode Qx is used to select the ionic species of interest by applying the suitable U, V voltages on the rods. Where Q2 is used as a collision cell and operating in the rf only mode, the selected ions collide with the target gas and the product ions are analyzed by TOF, thus obtaining accurate mass values. 

The precursor ion scan mode scheme is scan-disso-ciate-select (o-> in Kondrat symbols, and the opposite of the product ion scan mode). This mode is used in instruments containing two or more analyzers in series, but is unavailable in ion storage devices. The first analyzer scans the ion stream over a particular mass range and ions of increasing miz value are passed to the collision cell and fragmented. The second analyzer is fixed to select product ions of a specific miz ratio. Therefore, ions passing through... 

Decision 2002/657/EC introduces criteria for confirmatory analysis, based on the so called identification points (IPs). Then, for confirmation of Group A substances, a minimum of four IPs are required, whereas for compounds listed in Group B the minimum number of IPs is set to three for a satisfactory confirmation of a compounds identity. Although the cited document still accepts detection techniques like diode-array (DAD) and fluorimetric detection (ELD) as possible confirmatory techniques, the confirmation of veterinary residues in food is performed nowadays by LC coupled to different MS detection systems. The system of IPs relies on the identification power of the different mass analyzers. For instance, a low resolution mass spectrometer (e.g., triple quadru-pole, QqQ or ion trap IT), provides 1.0 IP for the precursor ion and 1.5 IPs for each product ion. By contrast, high resolution mass spectrometers (HRMS resolution >20,000 fwhm, full width at half maximum) provide 2.0 IPs for the precursor ion and 2.5 IPs for each product ion, which means that (2 + 2.5n) IPs can be acquired when working in the product ion scan mode. In addition to IPs, the retention time of the suspected peak has to correspond to the measured retention time of the relative standard, and the area ratio between the selected ion traces has to be equal in the sample and in the standard [12]. 

Figure 4.150 Chromatogram of a hair sample extracted by SFE (200 bar), taken in product ion scan mode (MS/MS).

In addition to the product-ion scan, most MS-MS instruments allow other scan modes, e.g. neutral-loss and precursor-ion scan modes. The modes are not only useful in the elucidation of the fragmentation pattern of a particular compound, but also in the screening for a series of structurally-related compounds in complex samples. In the product-ion scan mode, the first mass analyser selects a particular precursor ion, while the product ions obtained by CID of this precursor are analysed in the second mass analyser. In the precursor-ion scan mode, this process is virtually reversed the first mass analyser transmits all ions in a preset mIz window to the collision cell, while the second analyser selects only the ions of one particular miz, e.g. a particular structure informative fragment for a series of ions or compounds. An example of the use of the precursor-ion scan mode is the monitoring of phthalate plasticisers by means of the common fragment ion at miz 149 due to protonated phthalic anhydride. In the neutral-loss scan modes, both mass analysers are... 

In both the quadrupole ion traps and the FT-ICR instruments, MS-MS in the product-ion scan mode is feasible, but other scan modes such as neutral-loss and precursor-ion scans are not possible. 

MS-MS in the product-ion scan mode is generally quite successful in structure elucidation of unknowns. There are ample examples available in the literature. However, the interpretation of the production mass spectrum is not always straightforward. When a protonated molecule is selected as precursor ion, the fragmentation rules significantly differ from the well-known fragmentation rules valid for molecular ions generated by electron ionization. Upon fragmentation, protonated molecules have a... 

A powerful tool to enhance the analytical potential of combined chromatography-MS especially in LC-MS, is tandem MS (MS/MS). MS/MS was first introduced as a way to induce fragmentation of ions generated in the ion source, e.g. by soft ionization methods, and after their mass selection. In such an experiment, the first mass spectrometer selects a particular precursor ion, which is (collisionally) dissociated, and the product ions are analysed with the second mass spectrometer. This is the product-ion scan mode which is primarily used to elucidate the structures of ions. 

For the analytical use of MS/MS, other scan modes can be useful as well as the product-ion scan mode. The various modes are explained and summarized in Table 1. The precursor-ion and neutral-loss scan modes are particularly useful in screening. They are part of a powerful analytical strategy applicable in impurity, degradation product and/or metabolite profiling, as well as other applications where searching for structurally related compounds is important. After an appropriate ionization mode is found, a product-ion mass spectrum is obtained for the parent compound. In this spectrum, possible common precursor ions and/or common neutral losses have to be identified. 

---