Micro packing materials

Micro-injections in micro-flow and nano-flow systems are done with injectors in which the external sample loop is replaced with the internal fixed volume within the injector body. HPLC-on-a-chip systems also build the column into the injector body. The internal path within the injector body is abladed with a laser, packed with micro-packing material, and this serves as the separating media. The injector inlet is connected to the pumping system and the outlet to the detector. Sample is loaded into an internal loop in the load position, then injected onto the chip HPLC by turning the injector. Obviously, in a system like this sample size is very limited and the detector is usually a highly sensitive mass spectrometer. 

Adamski, RP Anderson, JL, Configurational Effects on Polystyrene Rejection from Micro-porosou Membranes, Journal of Polymer Science Part B Polymer Physics 25, 765, 1987. Adler, PM, Porous Media, Geometry and Transports Butterworth-Heinemann Boston, 1992. Afeyan, NB Fulton, SP Regnier, FE, Perfusion Chromatography Packing Materials for Proteins and Peptides, Journal of Chromatography 544, 267, 1991. 

Micro- and nano-HPLC systems (Fig. 15.11) rely on small-diameter and capillary columns packed with high-efficiency packing materials and very slow flow rates to produce concentrated solutions and sharp chromatography peaks to feed electrospray interfaces for mass spectrometers. 

Trefz, P., Kischkel, S., Hein, D., James, E.S., Schubert, J.K., Miekisch, W. Needle trap micro-extraction for VOC analysis effects of packing materials and desorption parameters. J. Chromatogr. A 1219, 29-38 (2012)... 

For the same outer diameter, the inner diameter and, therefore, the capacity of the tube can be increased. Thus more catalyst can be accomodated within a tube of a given outer diameter and, particularly when an optimized catalyst is used with improved packing and heat transfer properties, throughput is increased at (possibly) lower pressure drop than before retubing. Figure 36, demonstrates this by comparing HK 40 with Micro Alloy material. 

A 5-20-pm-lD micro-ESl needle was described by Eimnett and Caprioli [70]. The flow-rates used were 0.3-6.4 pEmin. Similar needles were successfully made by other as well. Robins and Guido [71] reported an integrated packed 150-250-pm-lD microcapillary LC coluitm-micro-ESl device. A Teflon frit retains the coluttm packing material. The last part of the fused-silica colirnm tubing is drawn into a sharp tip to act as an ESI emitter. 

The authors have recently employed fused-silica tubing ais a column material aind found that micro packed fused-silica columns possess high efficiencies(3-7), owing to the smooth auid inert surface of fused-silica tubing. In previous work(4-6), 20-50 cm columns were connected in series in order to attain laurger theoretical plate ntmbers. Theoretical plates auround or in excess of 100,000 could be produced on 1.5-4 m micro packed fused-silica columns in the reversed-phatse ( ) and size-exclusion chrcmiato-graphy(4,6). 

Small consxamption of expensive packing materials is one of adveui-tages in micro HPIG. A micro column of uround 10 cm X 0.25 mm... 

A variety of micropellicular packing materials has been developed for the analysis of both small and large molecules by various HPLC modes, including ion exchange (lEC), metal interaction (MIC), reversed-phase (RPC), and affinity chromatography (AC). Besides analytical applications, other possible utilization of micro-pelhcular stationary phases includes fundamental kinetic and thermodynamic studies of the retention mechanisms on a well-defined surface. Nevertheless, a relatively limited variety of micropellicular columns are commercially available. They are mainly restricted to ion-exchange and reversed-phase stationary phases. This may reflect certain practical disadvantages of micropelhcular sorbents. 

Micronization in the field of column hardware and packing materials led to the development of microbore and short columns the latter allow separation times sometimes to be shortened by a factor of 10 (83, 385). The advent of microbore columns created a whole new methodology in HPLC with detection limits lowered into the femto-mole range (224, 260, 291). Special micro-metering pumps give pulseless flow rates down to pl/min, and allow direct interfacing of the LC apparatus to a mass or infrared spectrometer. They also open the field of fused silica columns for HPLC use, and it might be not too optimistic to expect theoretical plate numbers of >500000 per LC column for the near future. 

In our latest contribution to the analysis of iso-alpha acids using Micro-LC (15), we have furthermore shown that not only the trace metal in the packing material, but also the column metal and the shelf-lifetime of the columns (slowly dissolving metal of the column or other instrument hardware ) have a negative influence. Stainless steel columns showing no metal trace activity can be produced, but keeping the columns in that state is impossible. It is evident that this trace metal activity will always be a critical... 

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