CPSA Digest 2001

Proceedings -Tuesday, October 9, 2001

TuOC3

MUX for Parallel LC/MS

Premise:
Micromass' Quattro Ultima™ with MUX-technology™ was customized by this laboratory at Glaxo SmithKline in collaboration with Micromass to allow for the interface of 8 LC columns in parallel to one tandem quadrupole (illustrated below). A specific effort was made to improve the LC pumping system which was hindering performance. A Gilson 215 liquid handling injector system (8-probes) was used as the autosampler.

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The LC pumping systems available to date have been a limitation to the proliferation of the MUX technology. The problem is caused by use of a single pump, in which the flow is split eight ways. In order to obtain equal flows down each line, the LC columns and tubing must be precisely matched to provide equal pressure drops. Back-pressure differences in one or more paths during use can seriously misalign peak retention times. An initial choice made was to use electrical switching instead of a physical 'shielding' device to maintain LC flow integrity. It was desirable that all liquid flow be maintained without extra moving parts in the liquid phase.

In the MUX approach, the conventional electrospray probe and outer source assembly are replaced with a new source housing containing an array of 8 miniaturized, pneumatically assisted electrosprays. The position of the sampling rotor is monitored in real-time enabling the eight liquid inlets to be indexed, as shown below.

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The original specifications outlined for this system were:

• 70% sensitivity of single Z spray system
• Relative sensitivity of between +/- 10 to 15% between sprays
• 0.8 s cycle time for four way system
• 1.2 s cycle time for eight way system
• Analogue sampling sufficient for very fast chromatography (20 Hz for 2.1 min methods)
• Interchannel crosstalk - 1% from adjacent spraying channels

Some issues identified by this laboratory with the original MUX interface were listed as:

• MS hardware/software was generally robust
• Chromatographic performance was limited by pre-injector splitter configuration
• no control or monitoring of individual flow streams
• tendency for blockages on Gilson 215/889
• excessive dead volume of Waters 600 pump leading to long inject to inject times
• poorer chromatographic efficiency than equivalent single, serial HT LC/MS e.g. Micromass Diversity with Agilent 1100 LC

LC pumps found to give good performance in this configuration were the Jasco PAR 1500 Series. The Jasco pumps deliver an equal gradient flow to eight individual flow channels and were superior to a conventional system in which the flow is split eight ways after the pump.

Value of the Technology
The throughput of this system (MUX-8 system with Jasco pumps) was described as:

• "A complete combinatorial synthesis plate (96-samples) was able to be analyzed in 40 minutes"
• "50 plates analyzed in 48 hr"
• "78 plates analyzed in 3 days"

This MUX system in a 4-channel configuration was applied to preparative LC, as well. This application was described as follows.

• High throughput chemistry demands high throughput purification paradigms
• Historically the parallel LC approach (e.g. Biotage Parallex 4 way system) was faster than serial LC/UV (Gilson) or MS-directed LC prep (PE Sciex/others)
• Each approach has its pros and cons
• The major drawback with the Parallex is mapping back to input plate from plethora of wells collected
• Software solutions developed in house by GSK (Winnow) aid this process but we would also like mass confirmed UV prep LC

Links
Micromass with MUX technology