Orthogonal chromatograph

Where RPLC often fails in separating polar compounds from the injection peak, ordinary CE mostly succeeds efficiently. As a consequence, it can be very useful to add at least one CE method to an orthogonal chromatographic set. ... 

The full-scan mode is needed to achieve completely the full potential of fast GC/MS. Software programs, such as the automated mass deconvolution and identification system (AMDIS), have been developed to utilize the orthogonal nature of GC and MS separations to provide automatically chromatographic peaks with background-subtracted mass spectra despite an incomplete separation of a complex mixture. Such programs in combination with fast MS data acquisition rates have led to very fast GC/MS analyses. 

Venkatramani, C.J., Zelechonok, Y. (2003). An automated orthogonal two-dimensional liquid chromatograph. Anal. Chem. 75, 3484-3494. 

The aim of this chapter is to evaluate the orthogonality of selected 2DLC systems for the separation of peptides. The orthogonality of different chromatographic modes was quantitatively characterized using a novel geometric approach. Practical peak capacity was calculated from the theoretical peak capacity and the knowledge of... 

This concept assumes that each fraction (peak) collected in the first dimension further separates in the second dimension with regular spacing and that the entire 2D separation space is evenly covered by eluting peaks. More realistically, the peaks would be distributed randomly over the 2D separation space some peaks are likely to coelute, while some area will remain vacant of peaks. Therefore, Equationl2.1 represents an idealized peak capacity estimate although the real number of resolved peaks is lower. Most importantly, the peak capacity proposed by Equation 12.1 is achievable when the chromatographic modes used for separation are completely orthogonal. 

The proposed estimate has several limitations. When taking into account the limited orthogonality of investigated 2DLC modes, the practical peak capacity is reduced approximately to half. It needs to be also emphasized that a full separation power of the first LC dimension is realized only when the number of collected fractions exceeds its peak capacity (Murphy et al., 1998). If the number of fractions analyzed is low, the achievable chromatographic peak capacity suffers. 

While chromatographic peak capacity is not adequate to resolve hundreds of thousands of components, many researchers argue that MS itself is an additional separation dimension with an orthogonal selectivity (separation is based on mass-to-charge ratio). Therefore, the combined resolution of LC and MS is greater than the chromatographically defined peak capacity. The question therefore stands What is the achievable peak capacity of the 2DLC-MS/MS system ... 

---