Capillary column ideal

A gas chromatograph with a capillary column coupled to a mass spectrometer is an ideal analytical partnership. Effluent from the column has an elevated temperature and the molecules of interest are in a vapor state and ready to enter the ion source. This eliminates the need for desolvation that is required in high-performance liquid chromatography (HPLC)-MS. 

It follows that retention measurements on silica based stationary phases for the purpose of obtaining thermodynamic data is fraught with difficulties. Data from solutes of different molecular size cannot be compared or related to other Interacting variables ideally, thermodynamic measurements should be made on columns that contain stationary phases that exhibit no exclusion properties. However, the only column system that might meet this requirement is the capillary column which, unfortunately introduces other complications wmcn will be discussed later. 

Figure 6.22. Ideal configuration for metal capillary columns (500-foot coil).

Many reports (78-84) investigated the differences in packed and capillary supercritical fluid chromatography. Unfortunately, the rift between packed and capillary column users of SFC impeded the development of the science. This rift is a likely cause of the current low interest in SFC. Ideally, the unique features of the mobile phase is the area of scientific exploration that should be exploited. Choice of column size or type should be dependent upon the analytical problem to be solved. 

In GC we have a real choice between packed columns (dp = 100-200 pm 150-65 mesh) and open columns (dc= 50-500 pm). Capillary columns have the advantage of enhanced speed of analysis (eqn.7.6). In order to exploit this advantage, narrow-bore capillaries (dp< 100 pm) should ideally be used. However, such columns require relatively high inlet pressures (especially for high plate counts) and considerable experimental modifications and have a very low sample capacity [702],... 

The ideal capillary column is a tube of circular cross section coated with a uniform stationary film of thickness d (see Figure 12.1). Its uniformity and symmetry greatly simplify its theoretical description. Accordingly, Eq. 12.1 reduces to a much more tractable form and the parameters (y, q, A, and a) become calculable. 

For both packed and ideal capillary columns, we have simplified the plate height expressions by using coefficients A, A B> Cm, and Cs to replace groups of more basic parameters. In Table 12.1 we summarize the expressions for these coefficients. 

TABLE 12.1 Summary of Plate Height Coefficients for Packed and Ideal Capillary Columns a... 

The coupling of an MS with CEC or PEC provides several advantages. With the capillary columns of 100 mm inner-diameter (i.d.), flow rates of 1-2 L/min are obtained, which are ideal for electrospray MS [4]. No interface like a liquid sheath flow is required and the sintered silica gel frits allow direct coupling of the packed capillary columns without additional transfer capillaries. The spray is therefore formed directly at the outlet side of the column. Verheij et al. carried out the first coupling of a pseudoelectrochromatography system to a fast-atom bombardment (FAB)-MS in 1991 [6]. However, this required transfer capillaries that caused a loss in efficiency, which was also a problem with other experimentations with this technique. 

Clearly, further improvements in the reliability and accuracy of the IGC method depend on the development of more suitable columns to support the stationary phase. Several authors have speculated that the use of capillary columns, or open tube columns would eliminate some of the concerns cited above, and would be advantageous for IGC applications (33-351. The principal attraction of a capillary column is the possibility of achieving more uniform dispersal of the polymeric phase. Ideally, the polymer would cover the wall as a uniform annular film. Such a geometrical configuration would simplify modelling of the transport processes within the column, and improve the inherent reliability and accuracy of IGC measurements. 

The ideal agents are selective and non-toxic, have very small retention times, high efficiency and no interferences. E.g. the derivative process of chlorophenols into their pentaflorinebenzoles gives a higher sensitivity and selectivity for GC on capillary columns with electrons capture detectors (Patnaik, 1996). 

When using a support with sufficiently large pores and/or a completely nonporous support, or in the case that the pores of a microporous support are completely filled with the appUed Uquid sorbent, the term al C ) can be omitted. When the liquid sorbent forms a completely continuous film on the support (a situation which may occur in an ideal case when using a macroporous support or when using a capillary column), then q in the term al(Cg)) has a value of 2/3, whereas in the case of a microporous support with the pores filled completely with the liquid sorbent, q in the term o (Cs,) equals l/SOyg and df = dp, where is the obstructive factor for diffusion of the solute in the liquid sorbent inside the pores. 

Ideally a sample should be introduced onto the first few plates of the column. However, capillary columns have a very low gas volume, in the order of 10 il per plate, which means that equally small liquid samples are needed... 

ESI Interface for CapiUary-LC and Nano-LC Columns Currently, the applications of capillary and nano-LC are on the upswing especially for many biochemical studies, where the sample amounts and volumes are both limited. For such samples, packed capillary columns of 50 to 300 xm i.d. are the ideal solutions. As pointed out above, the combined use of small-i.d. columns with an ES ion source has the advantage of optimal detection sensitivity because of its concentration-dependent response. Because these columns operate in the flow range nanoUters to microliters per minute, an ideal LC/MS system is realized when these columns are connected directly to nanospray or microspray sources [42,43]. The coupling of these columns to a conventional ES ion source can also be accomplished if an additional sheath liquid is added to increase the flow to a range that is acceptable by the source. 

GC is an ideal separation technique for small thermally stable volatile molecules. Capillary columns provide high-resolution separation of complex mixtures. These columns, owing to their low gas flow requirements, can be coupled directly to mass spectrometry. A variety of interfaces, such as an open-split interface, a jet-separator interface, and a molecular-effusion interface, are used to handle larger carrier gas flows of packed GC columns. 

Estimates can be made of ideal cell volume requirements, since the width of a peak can be expressed in volume units (the base width, 4cr, where the x-axis is in mL units). A narrow peak from a capillary column might have a width as small as 1 second, which represents a volume of 0.017 mL (17 fiL) at a flow rate of 1 mL/min. If the detector volume were the same or larger, the entire peak could be contained in it at one time and the peak would be very broad. An ideal detector for this situation should have a significantly smaller volume, say 2 piL. When this is not possible, make-up gas can be added to the column effluent to sweep the sample through the detector more quickly. This remedy will be helpful for mass flow rate detectors but less so for concentration detectors. In the latter case, the make-up gas dilutes the sample, lowering the concentration as well as the... 

Ideally, the sample is injected instantaneously onto the column, but in practice this is impossible and a more realistic goal is to introduce it as a sharp symmetrical band. The difficulty keeping the sample sharp and narrow can be appreciated by considering the vaporization of a 1.0 microliter sample of benzene. Upon injection, the benzene vaporizes to 600 pL of vapor. In the case of a capillary column (at a flow rate of 1 mL/min), 36 seconds would be required to carry it onto the column. This would be so slow that an initial broad band would result and produce very poor column performance (low N). Clearly, sampling is a very important part of the chromatographic process and the size of the sample is critical. 

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