Peak capacity separation measure

In order to talk about fast gradient separations, we need to review the principles involved in measuring the performance of a gradient separation. We can use the concept of peak capacity to measure the separation power of a particular gradient on a given column. The peak capacity (P) is defined as follows ... 

In LC-LC coupling (2D system), the peak capacity is the product of the peak capacities of its component one-dimensional (ID) processes (9). The power of the separation measured by the LC-LC peak capacity is given by the following ... 

An automated procedure to measure peak widths for peak capacity measurements has been proposed.35 Since peak width varies through the separation, the peak capacity as conventionally measured depends on the sampling procedure. The integral of reciprocal base peak width vs. retention time provides a peak capacity independent of retention time, but requires an accurate calculation of peak width. Peak overlap complicates automation of calculation. Use of the second derivative in the magnitude-concavity method gives an accurate value of the standard deviation of the peak, from which the base peak width can be calculated. 

This chapter examines another measure of the space used by 2D separations subject to correlation. Some researchers use the words, peak capacity, to express the maximum number of zones separable under specific experimental conditions, regardless of what fraction of the space is used. By definition, however, the peak capacity is the maximum number of separable zones in the entire space. No substantive reason exists to change this definition. The ability to use the space, however, depends on correlation. In deference to previous researchers (Liu et al., 1995 Gilar et al., 2005b), the author adopts the term, practical peak capacity, to describe the used space. The practical peak capacity is the peak capacity, when the separation mechanisms are orthogonal, but is less than the peak capacity when they are not. The subsequent discussion is based on practical peak capacity. 

The author anticipates that many readers will find the results reported here to be commonplace. If so, then why do we so often report the individual peak capacities of the two dimensions and their product as the 2D peak capacity One answer—the conservative one—is that the latter is indeed the maximum number of peaks that can be separated, in agreement with the definition. A more realistic answer is that it is easy to do and appears more impressive than it really is—especially to those who fund our work. In fact, as a practical metric it is often nonsense. Because orthogonality is so difficult to achieve, especially in 2DLC, the peak capacity is a measure of only instrumental potential, not of separation potential, and consideration of... 

First, we need to establish how we measure the quality of a gradient separation. Theoretically, the best-way to do this is to calculate the peak capacity.Peak capacity describes the number of peaks with a fixed resolution that can be resolved in a gradient. It is also something that can easily be calculated and measured for a particular gradient ... 

A convenient measure of the performance of a chromatographic system for the separation of complex samples is the peak capacity P, which is defined as the upper limit of the number of fully... 

The equations of this section show that resolution and peak capacity are inversely proportional to a and w (usually reflected in H and N). These equations illustrate how the capacity for separation is diminished, using any reasonable measure, by increases in zone width. This conclusion reemphasizes our deep concern with zone spreading phenomena and the fundamental transport processes that underlie them. 

We should add that while resolution and peak capacity are excellent criteria of merit for the separation of multicomponent mixtures into discrete zones, other criteria exist, some very general, for judging the efficacy of separation and purification in any separative operation (see Section 1.4). Various terms such as impurity ratio and purity index abound. Rony has developed a criterion termed the extent of separation [22]. Stewart, as well as de Clerk and Cloete, have shown that entropy can be formulated as a very general measure of separation power, as we might expect from the discussion of Section 1.6 [23,24]. An excellent discussion of separation indices, with an emphasis on non-Gaussian zones (below), is found in Dose and Guiochon [25]. 

The peak capacity nc, as usual, is the number of zones that can be crowded into the available separation space. However, in two dimensions space is measured by area, not length. We can thus estimate ne as bed area L,L2 over spot area A... 

Since there is no selectivity (a) involved in GPC, the ability to separate a pair of compounds depends upon the calibration curve and the efficiency of the column(s). One measure of the ability of the GPC column(s) to separate is the peak capacity of the column, which is defined as the number of resolvable peaks per chromatogram, n. The peak capacity (16) for a GPC column used in the analysis of small molecules is related to the number of theoretical plates (N) according to the equation... 

For separations in the ng/mL-to-pg/mL range and solutes that absorb in the UV/visible portion of the spectrum, optimize the detector wavelength. The optimal wavelength is usually in the low-UV portion of the spectrum. Using LC, the UV cutoff of the mobile phase often prevents such low wavelengths from being employed. Limits of detection usually approach 10"6 M without heroic measures. The downside of low-UV detection is a loss of selectivity, since more solutes will absorb there. This is countered in part by the high peak capacity of CE. In some cases, appropriate sample preparation may be required for selectivity. 

Coupled-column separations or multidimensional chromatography can be considered as a sample preparation form, as one column is used to derive fractions for the second column. It provides a two dimensional separation in which sample substances are distributed over a retention plane formed by the operation of two independent columns. This type of two dimensional based separation method is more powerful than a single dimensional based one. A retention plane has more peak capacity than a retention line and so can accommodate much more complex mixtures. Component identification is more reliable because each substance has two identifying retention measures rather than one. These type of combinations offer high selectivity and high sensitivity, and could be used with less expensive and more robust detectors (e.g., flame ionization). ... 

The number of theoretical plates N, included in Equation (8), is a measure of column efficiency, which can be individually applied to the two columns of a GCxGC set. But in comprehensive GCxGC, the use of two columns having phases with different characteristics results in the redefinition of peak capacity, a 1 D GC efficiency concept. It also results in the introduction of two new concepts related to the separation behaviour, orthogonality and chromatographic structure, which are specific to GCxGC. These three concepts are discussed in Sections 4.2, 4.3, and 4.4, respectively. 

Figure 4 Orthogonality and peak capacity, p is estimated from orthogonality measures. Practical peak capacity available for separation is represented by the area marked A . Based on reference [48].

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