Full-mass-range scanning

The electrospray stability of this device was studied with two different samples 1 pg mL 1 imipramine- 3 directly dissolved in 75% methanol, 25% water and 0.1% formic acid and extracted urine sample containing only methylphenidate-d3, which corresponds to the blank calibration standards. Figure 6.13A shows the total ion current of full mass range scan (m/z 250 to 350) of the imipramine-J3 sample over 15 min with a scan speed of 1.16 s per scan while Figure 6.13B shows the extracted ion current of the base peak (m/z 284.2) from Figure 6.13A. The RSD for the total ion current over the 15 min periods was 10.7% and the protonated molecule ion was even more stable with an RSD of 5.7%. Figure 6.13C shows the total selected ion current for the SRM data (m/z 234.2 > m/z 84.1, m/z 234.2 > m/z 84.1) from 150ngmL 1 methylphe-nidate-fi 3 sample. The SRM mode was also very stable with an RSD of 3.05% over a period of 5 min. 

The use of MS detection in electron ionization (El) mode increases selectivity with respect to ECD and enhances analyte identification potential conjointly with the GC retention time information. It can be used either in the full scan mode (observation of the full mass range) from which specific ions can be extracted, or in the SIM mode (observation of a few selected ions). 

Another approach to imaging dosed compounds involves using MS scans alone (14, 15). This was demonstrated for both an injected P-peptide (15) and a small molecule with its metabolites (14) in whole-body rats and mice. The advantage to this approach is mostly speed of acquisition, as all signals obtained in the full mass range can be imaged in one experiment. However, it lacks the unambiguous identification of compounds that is typically obtained from an MS/MS experiment. 

TOF analyzers are especially compatible with MALDI ion sources and hence are frequently coupled in aMALDI-TOF configuration. Nevertheless, many commercial mass spectrometers combine ESI with TOF with great success. For proteomics applications, the quadrupole TOF (QqTOF) hybrid instruments with their superior mass accuracy, mass range, and mass resolution are of much greater utility than simple TOF instruments.21,22 Moreover, TOF instruments feature high sensitivity because they can generate full scan data without the necessity for scanning that causes ion loss and decreased sensitivity. Linear mode TOF instruments cannot perform tandem mass spectrometry. This problem is addressed by hybrid instruments that incorporate analyzers with mass selective capability (e.g., QqTOF) in front of a TOF instrument. 

When operated as a specific detector the ion-trap detector is more sensitive still but not to the extent that would be expected from the performance of other mass spectrometers operated in this mode in view of the large number of ions monitored in full scan mode there is little more sensitivity to be gained by spending a little extra time scanning a narrow mass range, and the detection limit in this mode is in the region of l-2pg. 

True profile analysis requires scanning over the whole mass range for the acquisition of all data on excreted compounds. Quantitation has been more challenging on a quadrupole instrument because total ion current peaks are seldom a single component and extracted-ion chromatograms (EICs) when recovered from scanned data are of poor quality due to the lower sensitivity of scanning GC-MS. Thus, we developed profile analysis based on SIM of selected analytes but tried to ensure the components of every steroid class of interest were included. For ion traps the fundamental form of data collection (in non-MS/MS mode must be full -scans). Thus, the quantitative data produced are EICs obtained from scanned data. The EICs are of the same ions used for SIM in quadrupole instruments and the calibration external standards are the same. 

The reason for the disparity between the MS/MS spectra from the two detectors originates from a time-of-flight effect, which is also observed in a full-scan MS mode and is a consequence of the spatial separation between the two mass detectors. The result is a discrimination against the lower mass region when scanning a relatively wide mass range (e.g., 85-850 Da) and is a feature on most hybrid FTMS... 

The low resolution mass spectrometers used in EPA Methods 8260 and 8270 are not as sensitive as some of the selective chromatographic detectors (for example, the ECD) and for this reason are not capable of reaching the low detection limits that may be required for some DQOs. The mass spectrometer scans a large number of ion masses in a short period of time (for example, in EPA Method 8270, a mass range of 35-500 is scanned in 1 second) and dwells only briefly on each detected mass. In such full scan mode, the sensitivity of detection is traded for a wide range of detected ions. It is also affected by the background spectra (an equivalent of the electrical signal noise). 

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