Microcolumns limitations

Open tubular microcolumns also have been developed, with internal diameters of 1-50 pm and lengths of approximately 1 m. These columns, which contain no packing material, may be capable of obtaining column efficiencies of up to 1 million theoretical plates.The development of open tubular columns, however, has been limited by the difficulty of preparing columns with internal diameters less than 10 pm. 

The usual means of identifying and quantifying the level of these additives in polymer samples is performed by dissolution of the polymer in a solvent, followed by precipitation of the material. The additives in turn remain in the Supernatant liquid. The different solubilites of the additives, high reactivity, low stability, low concentrations and possible co-precipitation with the polymer may pose problems and lead to inconclusive results. Another sample pretreatment method is the use of Soxhlet extraction and reconcentration before analysis, although this method is very time consuming, and is still limited by solubility dependence. Other approaches include the use of supercritical fluids to extract the additives from the polymer and Subsequent analysis of the extracts by microcolumn LC (2). 

The low flow rate in the microbore column ensures sample volumes compatible with the secondary conventional column and permits the injection of a small volume onto the secondary column, making the transfer of incompatible solvents possible without peak shape deterioration or resolution losses [63], The possible disadvantage could be the lower sample capacity of microbore LC columns. However, in LCxLC, a sensitivity enhancement can be obtained if the formation of compressed solute bands at the head of the secondary column is achieved during the transfer from the first to the second dimension. Moreover, a larger volume can be injected into the first-dimension microcolumn, used as a highly efficient pre-separation step, and a limited decrease in efficiency due to a large injection volume can be tolerated. 

Both Cr111 and Cr concentrations in natural water samples were measured by flame AAS after pre-concentrations of the chromium species on microcolumns packed with activated alumina (acidic form) (Sperling et al., 1992). An FI manifold was used in this work to obtain conditions for species-selective sorption and subsequent elution of the chromium species directly to the nebuliser of the spectrometer. In this procedure, water samples were maintained at a safe pH of 4 prior to analysis. Analytical conditions of pH 2 and 7 were attained by adding buffers on-line only fractions of a second before the corresponding chromium species was sorbed into the column. In this manner, any risk of losses of analytes and/or shifts in equilibria between the species at pH 2 and 7 were minimised. The detection limits were 1.0 and O.Smgdm 3 for Cr111 and Cr, respectively. 

The focus of the present chapter is limited to review major accomplishments with partially integrated microcolumn separation systems that have been achieved in the last five years. Partial integration here refers in most cases to the fluidic part of the system which consists for example of a network of interconnected microchannels. The examples chosen have all been developed at least to the level of functional models and demonstrate principal feasibility. Many aspects of great importance in this context, such as chip fabrication, detection issues, higher levels of functional integration, etc., will be discussed in chap. 1 and 2 of this volume. 

An often-cited advantage of microcolumn LC is that of enhanced detection. However, careful examination of these claims reveals that most often comparisons are made between micro- and traditional LC columns with a fixed sample injection volume. Here there will certainly be enhanced sensitivity with the small-diameter column, as sample dilution is proportional to the square of the inside diameter of the column. However, if the injection volumes onto different columns of different diameters are scaled proportionally to the square of their diameters, then the dilution of the two samples will be equivalent (29). This means that microcolumn LC will only offer enhanced detection sensitivity when the available sample volume is limited. A critical comparison of micro- and standard LC columns in terms of sample detectability using UV absorbance detectors has been made by Cooke et al. (30). 

This technique has increased rapidly in popularity over the past several years. In certain situations an electrochemical detector can offer picogram limits of detection. Furthermore, it is one of the few detectors that is easily adaptable for use with microcolumns. White et al. have shown the feasibility of using a single carbon fiber as the working electrode inserted into the end of a 15-p.m-i.d. capillary column (62,63). Slais has reviewed the use of electrochemical detectors with low-dispersion (microbore) columns (64). 

The on-line microcolumn technique allowing improvement of the detection limit of atomic absorption and atomic emission spectroscopy by... 

Munaf et al. (1990) utilized microcolumn technique for preconcentration and LC separation of mercury compounds in waste water samples, using cystein-acetic acid as the mobile phase. After separation, the mercury compounds were digested on-line" with peroxodisulphate at room temperature, using Cu(ll) as a catalyst. The mercury was then reduced by alkaline Sn(ll) and determined by CV-AAS. The detection limit was calculated at 5 ng/L. 

Amperometric detectors are easily miniaturized with preservation of performance, since their operation is based on reactions at the electrode surface. Using a single carbon fiber or microelectrode as a working electrode allows detector cells of very small volume and in-column detectors to be constructed for use in open tubular and packed capillary column liquid chromatography [189-192]. These microcolumn separation techniques combined with amperometric detection are exploited for the quantitative analysis of volume-limited samples such as the contents of single cells [193,194]. 

Gaspari M, Gucek M, Walhagen K, Vreeken RJ, Verheij ER, TJaden UR, van der Greef J (2001) Ion trap mass spectrometry as detector for capillary electrochromatography of peptides possibilities and limitations. J Microcolumn Sep 13 243-249... 

Flow-injection online preconcentration and separation with ion exchange or sorbent extraction in packed microcolumns and/or precipitation and collection in knotted reactors has proved to extend the capabilities of ET-AAS by allowing relative detection limits to be lowered by two to three orders of magnitude and troublesome matrices to be removed. 

---