Comprehensive Two‐Dimensional Liquid Chromatography Combined with Mass Spectrometry

A comparison of one-dimensional and microscale two-dimensional liquid chromatographic approaches coupled to high resolution mass spectrometry for the analysis of complex samples

“…Furthermore, very fast cycle times of less than 1 min are necessary for the second dimension separation which oen results in high ow rates up to 5 mL min À1 . [11][12][13] Such high ow rates are not compatible with electrospray ionization (ESI) MS detection. Therefore, ow splitters are used to minimize the solvent load that is introduced into the ESI source.…”

Online and Splitless NanoLC × CapillaryLC with Quadrupole/Time-of-Flight Mass Spectrometric Detection for Comprehensive Screening Analysis of Complex Samples

“…9 Hyphenation of LC × LC to mass spectrometry (MS) was recently reviewed in a devoted book edited by Mondello. 11,12 In principle, all possible effects and problems known from the hyphenation of one-dimensional liquid chromatography (1D-LC) to mass spectrometry can as well arise for the LC × LC-MS combination, as the MS is still connected to only one liquid chromatographic dimension, which is the second dimension (D2). Depending on the LC × LC mode in use, however, the operating conditions of the D2 can be dictated by the Murphy−Schure−Foley (M−S−F) criterion that every peak of the first dimension (D1) has to be sampled about three to four times for a low-loss conservation of the D1 separation.…”

“…In the past decade, comprehensive two-dimensional liquid chromatography (LC × LC) has gained much interest, as can be seen from several reviews. − This boost in publications on LC × LC surely coincides with the commercialization of promising hardware such as UHPLC systems or the sub-3-μm superficially porous particles . Hyphenation of LC × LC to mass spectrometry (MS) was recently reviewed in a devoted book edited by Mondello. , In principle, all possible effects and problems known from the hyphenation of one-dimensional liquid chromatography (1D-LC) to mass spectrometry can as well arise for the LC × LC-MS combination, as the MS is still connected to only one liquid chromatographic dimension, which is the second dimension (D2). Depending on the LC × LC mode in use, however, the operating conditions of the D2 can be dictated by the Murphy–Schure–Foley (M–S–F) criterion that every peak of the first dimension (D1) has to be sampled about three to four times for a low-loss conservation of the D1 separation. , This rule does not necessarily affect D2 conditions of offline-operated LC × LC systems, where there is no time dependence between both LC dimensions. − Nevertheless, it means a major constraint to systems used in the online mode, especially in the case of a fast single-column D2 system.…”

“…Thus, to fulfill the M–S–F criterion, D2 speed has to be very fast, otherwise resolution is decreased by undersampling . Therefore, much attention is given to shorten D2 run times that are typically around 20 to 120 s. Speed is usually gained by the application of flow rates as high as 1 to 5 mL min –1 on short D2 columns with diameters of 1 mm and above. − Apart from the economically inefficient solvent consumption, a flow splitter is necessary to avoid overloading the ion source of the mass spectrometer. Upper tolerable flow rate limits of common electrospray ionization (ESI) sources are approximately 1 mL min –1 , and for atmospheric pressure chemical ionization (APCI), they are 2 mL min –1 ; however, these values are already far above the optimum.…”

“…Besides the possible interface overload, the practitioner has to pay attention to the data acquisition rate of the respective mass spectrometer, especially if older quadrupole-based MS systems are used, as resulting peak widths for short D2 runs can be 1–2 s and below . Although fast-scanning systems of the latest generation may be capable of measuring the minimum-needed 6 to 10 data points per peak under certain conditions (e.g., reduced mass-to-charge ratio ( m / z ) range), it is recommended in the literature to switch to time-of-flight (TOF) type MS as the high data acquisition rates are more than sufficient and independent of the selected m / z range. , The application of multiple D2 systems operated in parallel allows decreasing the critical D2 speed; − however, besides increased complexity, additional hardware such as columns, valves, and costly MS detectors would be necessary. The alternate use of stop-flow methods also allows for a sufficient number of D1 eluate samples, but overall analysis time is drastically increased because the D1 has to be stopped after each eluate sample to allow the D2 separation to be performed. − …”