Transport, detectors

Transport detectors are a unique type of solute property detector. In most solute property detectors, the sensing system monitors some property of the solute that is not shared by the solvent or that the solvent has to markedly less extent. It follows that such detecting systems are,to some extent, selective in their detecting capabilities and further restricts the choice of solvents to those that do not possess the property being measured. Thus, the choice of mobile phase is limited, and this can be particularly disadvantageous when employing gradient elution development. The transport type of detector was developed to overcome these limitations. 

A transport detector consists of a carrier that can be for example, a metal chain, wire or disc that continuously passes through the column eluent taking a sample with it as a thin film of mobile phase adhering to its surface. The mobile phase is then 

The first wire transport detector was the detecting system originally developed by James et al. (12), in 1964 and subsequently was manufactured by Pye Unicam and became commercially available. 

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P. R. Boshoff, B. J. Hopkins and V. Pretorius, Thin-layer chromatographic transport detector for high-performance liquid chromatography , J. Chromatogr. 126 35-41 (1976). 

The only detector in Table 14 that is not based on an established analytical technique is the transport detector, which uses one of the GC detectors—FID, ECD, PID, or TID—as the measuring device. It consists of a wire, chain, or belt used to deliver the column effluent to the GC detector, removing the volatile mobile phase enroute. Obviously, the samples run on this system must not be volatile, and for them the FID is a universal detector, which is one reason for its use. 

This classification is satisfactory for GC detectors but as there is only one LC detector that is mass sensitive [8] (the transport detector), which at the time of writing this book is also not commercially available, this manner of classification is of little use for LC detectors. 

Estimation of the minimum detectable mass estimated from this chromatogram was again made to be about 10 ng of solute. To some extent, this detector provides a replacement for the transport detector as it detects all substances irrespective of their optical or electrical properties. 

Transport detectors are a unique type of solute property detector in that the signal from the sensor is entirely independent of the solvent that is used as the mobile phase. Various forms of transport detectors have been commercially available over the years past but, due to certain deficiencies in the early models, they did not become popular and (to the author s knowledge) none are currently being manufactured. Nevertheless, the transport detector has the potential qualities that are inherent in the ideal detector, i.e. universal detection, high sensitivity,... 

About the same time as the development of the wire transport detector Haahti and Nikkari [11] described a similar device, more simple in design, that employed a chain loop in place of the wire transport system. A diagram of their apparatus is shown in figure 9. 

The sensitivity of the original wire transport detector, besides being degraded by the high noise level, was also determined by the quantity of pyrolysis product that could find its way to the FID. Excluding synthetic polymers, which often quantitatively produce monomers on pyrolysis, many compounds yield only a few percent of volatile compounds and the higher boiling components of these often condense in the conduits and never actually reach the FID. Thus the FID may only sense a very small fraction of the products from the solute deposited on the wire. 

Stolyhwo et al. [17] attempted to improve the sensitivity of the detector by using metal spirals wound on wire and stranded wire to increase the surface area of the carrier and thus increase the proportion of the column eluent taken into the detector. The authors claimed a minimum detectable mass of 100 ng of triolein. However, again the exact volume of mobile phase in which the mass of solute was contained was not clear from the publication. If the solute was eluted in a peak 1 ml wide at the base, the concentration at the peak maximum would be twice the average concentration /.e., 2 x g/ml, which, for a transport detector, would be a greatly improved sensitivity. If, however, the same mass was eluted as an early peak in the chromatogram with a band width of only 50 pi, then the sensitivity would be 4 x 10 g/ml, which would be no better than the previously developed transport detectors. This confusion emphasizes the importance of specifying sensitivity in terms of concentration, which allows the direct comparison of one detector with another. 

As already mentioned under transport detectors, Dugger [6] modified the moving wire detector to detect tritium and carbon. To detect carbon, the solute coated on the wire after evaporation of the solvent was oxidized to carbon dioxide and water. The radioactive carbon dioxide was passed to a Geiger counter and detected in the same manner as that described by James and Piper [7] which was discussed under GC radioactivity detectors in an earlier chapter. Tritium could be detected by passing the water vapor from the oxidation process over heated iron to reduce it to hydrogen and tritium, which was then also passed through a Geiger counter. 

Another instrument called the transport detector, used for detection of lipids, proteins or carbohydrates, requires the transport of the column eluent by a moving wire disc, chain or helix. The solvent is evaporated in a furnace and the nonvolatile sample passes into a flame ionization detector (FID) which is detailed later under gas chromatography (GC) wherein FID counts amongst the major detectors. 

Desolvationjtransport detectors. The principle of transport detectors, typified by the moving wire detector (Figure 6.29), was based on the concept of physically separating the solvent, which is necessarily volatile, from the involatile solute. The transport wire is passed through a coating block where eluant from the column is applied. The solvent is then evaporated, and the wire plus solute then passes to a pyrolysis or combustion... 

The transport detector is ideal for most gradient elution applications, the major limitation being those which use involatile buffers. The full potential of the FID cannot be realised due to the deficiencies of the transport system. The overall detector response is dependent on temperature stability and on the coating procedure which is related to both viscosity and surface tension of the solvent and sample. Due to these drawbacks particularly the lack of sensitivity these detectors were soon withdrawn. 

A.1. Scott et al. used the already known principle of the wire transport detector and adapted it for mass spectrometry. As shown in Fig, 1, a steel wire... 

A.2. The work described by McFadden and co-workers is an extension of the wire transport detector (section Z.A.l). Because the wire will transport only about It of the total effluent into the vacuum locks, sample utilization is low. Therefore, the wire was replaced with a stainless-steel ribbon (3.2 mm wide, 0.05 mm thick) such that effluent was carried into the vacuum lock at a rate of 1 ml/min, thus achieving efficient sample transport to the ion source region (Fig. 2). Depending on the nature of the sample, the efficiency of sample vaporization and other operational processes, sample utilization in the range 30-50% was achieved. The detection limit for the system was less than 1 ng for carbaryl,... 

Mass detector. The liquid chromatographer s demand for a universal detector which overcomes some of the problems encountered with the RI detector, (such as poor sensitivity and temperature instability) led to the development about ten years ago of the mass detector described here. The transport detectors of the 1960s detected the solute by means of a flame ionization detector after removal of the solvent from the eluent stream. They were abandoned, owing to lack of sensitivity and mechanical problems associated with the moving belt or wire. The new mass detector is similar in principle, but here the eluent leaves the column and is pumped into a nebulizer, assisted by an air supply. The atomized liquid is passed into a heated evaporation column where all the solutes less volatile than the solvent are carried down the column as a cloud of fine particles. A light source and photomultiplier arranged at the bottom of the column, perpendicular to the flow, detect the cloud of particles. The output from the photomultiplier, which is proportional to the concentration, can be amplified and directed to a recorder or data system. 

The transport system for LC detection was developed to render the detector independent of the choice of mobile phase and allow any solvent to be used without compromise. The column eluent flows over the transporter, which may be a moving wire, chain or disc which takes up all, or a portion of the column eluent. The solvent is then evaporated from the transporter, usually by heating, and the solute is left as a coating on the surface. The transporter then carries the solute into a detection area, where it is sensed by suitable means, such as pyrolysis and subsequently detected by passing the pyrolysis products to a flame ionization detector. The transport detectors, by and large, are not very... 

A diagram of the overall system is shown in Figure 16. The wire employed was that supplied for the wire transport detector, 0.005 in. O.D. and made of stainless steel. The wire from the drive system passes over an electrically insulated pulley, over a coating block (where the column eluent wets the wire), into the left hand interface of the mass spectrometer and thence through the ion source. It then exits through another identical interface, round another pulley and back to the drive system. In the lower portion of Figure. 16 is shown the location of the interface with respect to the ion source it is seen that the wire leaves the interface about 2 mm from the ion source and less than a centimeter from the electron beam. A potential is applied across the two pulleys causing a current of about 200 mA to pass... 

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