Positive-negative polarity switching mode

One of the challenges in the determination of lovastatin and its P-hydroxy acid in mouse and rat plasma was the small amount (0.1 ml) of plasma available [46], The analytes were isolated from plasma by SPE. The reconstituted extracts were separated using a 5Qx2-mm-lD Cig column (5 pm) and 1 mmol/1 aqueous AmOAc (pH 4.0)-acetonitrile gradient. They were analysed by ESl-MS in SRM mode with polarity switching, i.e., in positive-ion mode for lovastatin and in negative-ion mode for its P-hydroxy acid. Simvastatin and its P-hydroxy acid were used as ANIS. An LOQ of 0.5 ng/ml was achieved with intra-day and inter-day precision better than 7%. The method was vahdated. 

The simultaneous determination of 3TC and AZT in human serum and seminal plasma was reported [41-42]. The sample pretreatment consisted of an automated ultrafiltration step. For isocratic LC, a 150x2-mm-ID Cig colunrn was used and 15% acetonitrile in water at 0.3 ml/min. The eluate of the first 2 min after injection were directed to waste. ESI-MS in SRM mode was apphed in the positive-ion mode for 3TC and negative-ion mode for AZT. Polarity switching was done in between the two peaks. The LOQ was 2.5 ng/ml. 

In the analyses, isotope-dependent MS/MS acquisition methods with or without polarity switching are employed to record MS/MS spectra of analytes that display isotope patterns similar to those predefined. Then, NL filtering of 129 and 307 Da in the positive ion mode, or product ion filtering of m/z 272 in the negative ion mode, are carried out to identify GSH adducts [161,162],... 

A recent trend in MS devices is to attempt to perform detection in both the positive and negative ionization modes within the same run (37,38). For this purpose, it has become possible to alternate the polarity (-1-/—) at a speed between 15 and 20 ms for the most recently developed instruments. This polarity switching is very fast and can be employed to increase productivity, particularly when dealing with the simultaneous analysis of both acidic and basic compounds. However, because the MS instrument works sequentially and the peak widths in UHPLC are very small, the use of this feature is not recommended as it would compromise the quantitative performance because the number of data points across the peak would be too small. 

After all four positive scans are completed (typically within 3 s), the polarity is switched and the fifth scan event records a negative-ion full-scan MS. If the expected protonated molecule is not detected in the positive mode, the second, third and fourth scan events are skipped and the fifth (negative-ion-mode) scan event is triggered. Similar to the positive-ion mode, if the expected [M-H]- ion is detected in the full-scan MS, IT data-dependent MS/MS (sixth), FT accurate MS (seventh), and FT MS/MS (eighth) scan events are acquired. Clearly demonstrated here is the ability of the LTQ-FT to handle multiple experiments on a chromatographic time scale. One might question the need for such an elaborate data-dependent scheme when apparently all that is needed is an accurate mass determination followed by a data-dependent accurate mass MS/MS spectmm. Apart from the fact that using the... 

TABLE 5.3. Accurate Mass Measurements for a set of 30 Pharmaceutical Compounds Collected in Positive Ion Mode (Experimental Error in ppm Shown in Column 4), Negative Ion Mode (Experimental Error in ppm Shown in Column 5), and with Pos/Neg Switching (Data Shown in Columns 6 and 7, Respectively) Using a Lock Mass (LM) for Each Polarity, Reserpine, m/z = 609 for Positive Mode and Hippuric Acid, m/z 178 for Negative Mode... 

The thermodynamic convention of cell potential defined in Eq. 5 is not practical for most electrochemical work. For a battery switching from discharge to charge, cathode and anode interchange, but the electrode polarities remain the same. It is not practical to switch the leads of the voltmeter when the battery switches to the charge mode. Rather, the voltmeter stays connected during the switch with the positive terminal attached to the positive terminal of the battery and likewise for the negative terminal. The practical convention for ceU potential is... 

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