LCIR Transport Interfaces

The first transport system to be used as a liquid chromatograph-spectrometer interface was introduced by Scott et al. for a liquid chromatogra-phy/mass spectrometer tandem system. Eventually, the transport concept was extended to LC/IR tandem instruments and the most effective LC/FTIR interface commercially available incorporated a solvent transport interface. One of the first LC/IR transport systems was developed by Kuehl and Griffiths. Initially, moving ribbon devices were used in a similar manner to that of Scott et al., but were eventually discarded in favour of a rather crude, but effective, rotating disc transport system. Their final model consisted of a cup carousel containing potassium chloride that actually acted as a fraction collector and thus was hardly an in-line interface. Depending on the speed of the transport process, many transport interfaces could be considered as automated off-line monitoring devices. Fast moving transport systems such as a wire or belt transport however, give the impression of being in-line devices although in principle, they are not. The LC/IR carousel had 32 cups fitted with a fine mesh screen and filled with potassium chloride powder. Carousel position was controlled automatically in three positions, where specific sampling activities took place. In position (1) the eluent passed onto the potassium chloride until the halide powder was saturated with mobile phase. In position (2) a stream of air was drawn through the packing to evaporate the solvent. In position (3) infrared light was directed through the dry halide, and the spectrum was taken. The carousel interface concentrated the solute and increased the sensitivity of the LC/IR combination. Unfortunately, with modern LC columns, many peaks can be eluted in a few seconds and so intermittent sample collection is unsuitable. The carousel interface primarily acts as a chromatographic 'memory'; all the eluted solutes are stored as a 'physical' chromatogram as localized masses, deposited on the transport medium. The first chromatographic memory was introduced by Kar-men, who used a wire transport detector to accumulate each eluted solute onto the wire surface, which was then stored on a reel. Subsequently, the wire was passed continuously through the flame of an FID, to produce a record of the separation. The most effective LC/IR interfaces are directly or indirectly based on this principle.

Jino and Fujimoto employed a potassium bromide plate as the transport system which was used as the sample holder for the IR measurement. Plate rotation was actuated by the detector signal and, at the start of a peak, the plate was moved to a new collection position. The disc moved on when elution was complete so that the sample was isolated at a specific position on the plate perimeter. A small bore column was used (flow rate 5 |L min"1) so the eluent fell onto the plate and the solvent either evaporated under ambient conditions or with the aid of an infrared heater. After the separation was complete a spectrum was taken by measuring the light transmitted through the dry deposit in the usual manner. Obviously, because of the solubility of the halide in water, aqueous solvents could not be used.

Gagel and Bieman employed an aluminium disc, on top of which was cemented a circular glass mirror to form a transporter with a reflective surface; it was used in conjunction with a simple nebulizer that deposited the sample on the surface. Their basic apparatus is shown diagramatically in Figure 3.

The disc rotated continuously during separation, leaving a spiral trail of solid deposits on the surface of the reflective plate. Evaporation was accomplished by the nebulizer. The column eluent passed into a T, one limb of which carried a flow of nitrogen gas. The gas and eluent passed out via a narrow nozzle in the third limb, which directed the spray onto the disc surface. After separation, the disc was placed in a modified total reflectance IR accessory. The disc was rotated, the surface scanned by the IR spectrometer, and the reflectance-absorbence spectra continuously collected. This LC/FTIR interface appeared successful, and functioned without significant peak dispersion or loss of chromatographic resolution. The minimum mass needed to provide a satisfactory spectrum varied with the characteristic absorbence of the substances being monitored. However, it was shown that between 50 and 100 ng of sample could provide a recognizable spectrum.

Gagel and Bieman modified the nebulizer to improve the deposition, to make it amenable to aqueous solvents by reducing spreading, and to concentrate the material into a smaller area. The modified jet design involved the use of two nitrogen streams. The column eluent was mixed in a high pressure mixing T with nitrogen under pressure and directed through a syringe needle to the deposition surface. The needle

Figure 3 The layout of the transport LC/FTIR apparatus developed by Gagel and Bieman. (Reproduced with permission from Gagel and Bieman, 1986.)
Figure 4 Results from the modified LC/FTIR interface demonstrating the overall sensitivity of the tandem instrument. (Reproduced with permission from Gagel JJ and Bieman K (1987) Anal. Chem., 59(9): 1267.)

was situated inside another nozzle through which heated nitrogen was flowing. The new arrangement functioned well with aqueous solvent mixtures and the overall sensitivity of the apparatus was significantly increased. The sensitivity of the modified interface is demonstrated in Figure 4.

The peaks from the injection of different masses of phenanthraquinone are shown on the left. The peaks are curves relating the IR absorbance at 1678 cmto scan number for samples deposited from 29% water in methanol. The ultimate sensitivity, defined as the mass of solute that would provide a signal to noise ratio of 2, was about 16 ng.

Solvent elimination is relatively easy with nonaqueous mobile phases but the majority of LC separations employ reversed phase columns and require mobile phases with a high water content. Poor volatility of such solvent mixtures, causes the deposits to be smeared into one another. This seriously impairs the separation. Water in the mobile phase also restricts the choice of the transport medium as it must be water resistant. A considerable amount of work has been carried out on nebulizer design to improve solute deposition and focus the material onto a smaller spot. Techniques that have been tried include thermospray and hydrodynamic focusing that employs a concentric gas flow to reduce the jet diameter by the Bernoulli effect. Different transport media have also been explored, including potassium chloride layers on the surface of a zinc-selenium metallic stage using diffuse transmission spectroscopy to obtain the spectrum of the deposited material. The deposition of the eluent from a narrow bore reversed phase column, onto the surface of a linearly moving substrate, using a jet spray assembly as an interface, has also been developed. The immobilized chromatogram (actually a chromatographic memory) is analysed by moving the substrate linearly under an FT-IR microscope while collecting the spectra. Zinc selenide was found to be preferable to an aluminized reflective surface as a disc transport. An example of the disc system used to display a reversed-phase separation of some polynuclear hydrocarbons is shown in three-dimensional form in Figure 5. The sensitivity to pyrene at a signal to noise ratio of 2 was 13 ng.

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