Resolution multidimensional systems

A multidimensional system using capillary SEC-GC-MS was used for the rapid identification of various polymer additives, including antioxidants, plasticizers, lubricants, flame retardants, waxes and UV stabilizers (12). This technique could be used for additives having broad functionalities and wide volatility ranges. The determination of the additives in polymers was carried out without performing any extensive manual sample pretreatment. In the first step, microcolumn SEC excludes the polymer matrix from the smaller-molecular-size additives. There is a minimal introduction of the polymer into the capillary GC column. Optimization of the pore sizes of the SEC packings was used to enhance the resolution between the polymer and its additives, and smaller pore sizes could be used to exclude more of the polymer... 

Multidimensional LC-LC, using two high-resolution eolumns with orthogonal separation meehanisms, has only a few applieations in environmental analysis. The limitations that sueh a multidimensional system has with regard to seleetivity must... 

Coupled column (multidimensional) systems in chromatography have also been developed to improve resolution. The coupled column procedure, as noted in Section 6.4, requires two or more columns of different kinds having different retention mechanisms. 

There is often a misconception that multidimensional systems are difficult and costly to set up and implement as a routine tool for analysis. It is true that in some instances additional equipment such as pumps and switching valves for multidimensional LC methods will need to be purchased, but once correctly installed, the costs and additional system maintenance required is insignificant when the improvements in resolution are considered, especially when most online multidimensional techniques proceed in the same time it would take to achieve a typical one-dimensional separation. Currently, some comprehensive techniques, such as GC x GC, are overshadowed by a lack of computer software for integration purposes and data reduction. However, it is only a matter of time before user-friendly multidimensional data presentation packages are developed and are commercially available. 

Sometimes, the resolving power attainable with a single chromatographic system is still insufficient for the analysis of complex matrices. An approach commonly used to obtain greater resolution is multidimensional chromatography. 

J. P. E. M. Rijks and J. A. Rijks, Programmed cold sample intr oduction and multidimensional preparative capillary gas cliromatogr aphy. Part I introduction, design and operation of a new mass flow contr olled multidimensional GC system , J. High Resolut. Chromatogr. 13 261 -266 (1990). 

S. Nitz, B. Weinreich and F. Draweit, Multidimensional gas cliromatography-isotope ratio mass spectrometry (MDGC-IRMS). Part A system description and technical requirements , 7. High Resolut. Chromatogr. 15 387-391 (1992). 

Unquestionably, most practical planar chromatographic (PC) analytical problems can be solved by the use of a single thin-layer chromatographic (TLC) plate and for most analytical applications it would be impractical to apply two-dimensional (2-D) TLC. One-dimensional chromatographic systems, however, often have an inadequate capability for the clean resolution of the compounds present in complex biological samples, and because this failure becomes increasingly pronounced as the number of compounds increases (1), multidimensional (MD) separation procedures become especially important for such samples. 

Multidimensional (or eoupled) eolumn ehromatography is a teehnique in whieh frae-tions from a separation system are seleetively transferred to one or more seeondary separating systems to inerease resolution and sensitivity, and/or to reduee analysis time. The applieation of seeondary eolumns is illustrated sehematieally in Figure 8.1. The smaller the At, value applied, then the greater is the resolution and number of runs needed to eheek a eertain portion of the sample (5). 

Figure 10.4 Schematic representation of the multidimensional GC-IRMS system developed by Nitz et al. (27) PRl and PR2, pressure regulators SV1-SV4, solenoid valves NV— and NV-I-, needle valves FID1-FID3, flame-ionization detectors. Reprinted from Journal of High Resolution Chromatography, 15, S. Nitz et al, Multidimensional gas cliro-matography-isotope ratio mass specti ometiy, (MDGC-IRMS). Pait A system description and teclinical requirements , pp. 387-391, 1992, with permission from Wiley-VCFI.

Figure 12.19 Schematic diagram of the interface system used for supercritical fluid cliromatography-gas chromatography. Reprinted from Journal of High Resolution Chromatography, 10, J. M. Levy et al., On-line multidimensional supercritical fluid clrromatogi a-phy/capillary gas chromatography , pp. 337-341, 1987, with permission from Wiley-VCH.

Figure 15.11 (a) Total ion clnomatogram of a Grob test mixture obtained on an Rtx-1701 column, and (b) re-injection of the entire clnomatogram on to an Rtx-5 column. Peak identification is as follows a, 2,3-butanediol b, decane c, undecane d, 1-octanol e, nonanal f, 2,6-dimethylphenol g, 2-ethylhexanoic acid h, 2,6-dimethylaniline i, decanoic acid methyl ester ], dicyclohexylamine k, undecanoic acid, methyl ester 1, dodecanoic acid, methyl ester. Adapted from Journal of High Resolution Chromatography, 21, M. J. Tomlinson and C. L. Wilkins, Evaluation of a semi-automated multidimensional gas chromatography-infrared-mass specti ometry system for initant analysis , pp. 347-354, 1998, with permission from Wiley-VCH. 

M. J. Tomlinson and C. L. Wilkins, Evaluation of a semi-automated multidimensional gas chromatography-infared-mass spectrometry system for initant analysis , ]. High Resolut. Chromatogr. 21 347-354 (1998). 

Multidimensional methods thus involve a combination of single mechanisms and systems. In any multidimensional (usually 2D) approach, it is desirable that each dimension be as pure as possible in terms of selectivity of the separation mechanism. In comprehensive 2D separations, the precision (or chromatographic resolution) becomes a limiting factor and is ultimately determined by the quality of the separation in both dimensions. 

Using MS detection relaxes the constraints on LC resolution, because additional separation occurs in the mass domain. In principle, LC-MS may yield a complete 2D distribution of a polymer according to chemical composition and molar mass. If MS detection is employed, the efficient cleaning in the LC step makes it possible to use total ion monitoring and even to identify unknown compounds from the sample. As extracts often contain interfering compounds, mass spectrometry in selective ion mode is a practical detector. Fully automated multidimensional LC-MS-MS-MS systems are available. 

Another study (Bedani et al., 2006) starts from the multidimensional sampling theory (Murphy et al., 1998a), which is discussed in Chapter 2. This sampling theory states that one needs to sample the first dimension separation system at least three to four times per peak width for maximum resolution. Bedani et al. then equate the second-dimension total analysis time to the first-dimension narrowest peak standard deviation. This defines the second-dimension operational parameters. All other parameters can be derived from this balance and Bedani s study goes through this and discusses how the rest of these variables are obtained. 

At least two driving forces have contributed to the recent increased use and development of multidimensional liquid chromatography (MDLC). These include the high resolution and peak capacity needed for proteomics studies and the independent size and chemical structure selectivity for resolving industrial polymers. In this regard, separation science focuses on a system approach to separation as individual columns can contribute only part of the separation task and must be incorporated into a larger separation system for a more in-depth analytical scheme. 

Multidimensional GC with time of flight MS (ToF-MS) is increasingly being employed to examine complex mixtures. In this technique non-polar and polar GC stationary phases connected by a thermal modulator enable increased resolution of GC peaks. The fast scanning made possible by the ToF measuring system leads to many data points across peaks and the possibility of deconvolution of complex overlapping peaks. 

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