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A piezoelectric flow‐through microdispenser interfacing capillary liquid chromatography (LC) with matrix‐assisted laser desorption/ionization time‐of‐fight mass spectrometry (MALDI‐TOF MS) was developed for the identification of biomolecules. The MALDI target plate was placed on a computer controlled high‐resolution x–y stage, on to which the column effluent was deposited as discrete spots, which thereby facilitated tracing of the chromatographic separation. The entire target plate was sprayed with a homogeneous layer of α‐cyano‐4‐cinnamic acid mixed with nitrocellulose by using an air‐brush. Hence the tedious manual handling of a micropipetter applying matrix solution on top of each fraction collected spot was avoided. The pre‐made target plates were stable for at least 3 weeks if kept in darkness at room temperature, which easily allowed re‐analysis of dispensed sample spots. The integrated microsystem was characterized and optimized by means of fluidics, dispersion, operational stability and sensitivity parameters. The dispensing unit was developed specifically to match high‐resolution capillary LC separations using a dispenser with an internal volume from inlet to the ejecting nozzle of 250 nl. Minimizing dead volumes was crucial in order to maintain the chromatographic resolution. The volume of the ejected droplets was of the order of 60 pl. Successful separations of seven immunoregulating peptides were made: ACTH 1–17, bradykinin, enkephalin, angiotensin III, angiotensin II, angiotensin I and ACTH 18–39. On‐line sample dispensing on the target plate in combination with trace enrichment followed by automated MALDI‐TOF MS identification is demonstrated, reaching a sensitivity of 100 amol. Copyright © 2000 John Wiley & Sons, Ltd.

A hollow fiber miniaturized supported liquid membrane (SLM) device for sample preparation is connected on-line with capillary electrophoresis and used for determination of a basic drug, bambuterol, in human plasma. The analyte is extracted from the outside of the hollow fiber (donor) through the liquid membrane (pores of the fiber impregnated with organic solvent) into the acceptor solution in the fiber lumen. The process is driven by differences in pH between the donor and acceptor solution. The whole volume of the acceptor solution can then be injected into the CZE capillary by using the double-stacking procedure for large volume-injection. Very clean extracts of low ionic strength are obtained from the SLM treatment, making this sample pretreatment method compatible with the CZE double-stacking procedure, which in turn makes it possible to inject large volumes of sample onto the separation capillary. Good performance of the whole procedure is demonstrated, and detection limits in the low nanomolar range were obtained in spite of the relatively weak UV absorbance of bambuterol. Extractions through the miniaturized SLM unit can be performed for 5-6 h without regenerating the fiber. The regeneration procedure was tested, and no relevant changes in the performance of the extraction could be found after seven regenerations, allowing the same fiber to be used for a week.

The methodology for ready-made matrix-assisted laser desorption/ionization (MALDI) target plates covered with an optimized thin layer consisting of matrix and nitrocellulose has been developed. Piezoelectric microdispensing enabled sample depositions in a high-density array format of 2000 sample depositions on a conventionally sized target plate (45 x 47 mm). The sample depositions were made reproducibly in a fully automated mode by using an in-house developed computer-controlled piezoelectric flow-through microdispenser. Additionally, the piezoelectric technique facilitated significant analyte enrichment that increased the detection sensitivity. The MS signal was obtained rapidly, generally within ten laser pulses. An airbrush device was used to generate a fine spray of matrix and nitrocellulose dissolved in acetone. The acetone evaporated instantly when reaching the target plate leaving the entire surface with a thin and uniform matrix/nitrocellulose coating consisting of very small crystals of matrix embedded in the nitrocellulose. These crystals acted as a seed-layer on subsequent analyte depositions, rendering homogeneous sample spots when using alpha-cyano-4-hydroxycinnamic acid (CHCA) as matrix. The relative standard deviation of the signal intensity between spots was (20-30)% (n = 30). The detection sensitivity was improved by restricting the sample spot diameter to 300 microm. The spot size was affected by the deposition rate and the evaporation rate of the dispensed sample volume. Mass spectra of a 25-amol peptide mixture deposition were successfully recorded.

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