Parallel sample introduction

ESI and APCI techniques are limited by the rate of sample introduction into the ion source. Autosampler technology is the primary rate-determining factor but in addition to this, fluid transfer rates and chromatographic separation can also be bottlenecks in the process. TWo approaches, generally categorized as parallel sample introduction and fast serial sample introduction, have been taken to increase the throughput. 

The //PLC system described in this chapter is equipped with 24 parallel columns for liquid chromatography, each with its own sample introduction port and exit port for connection to detectors of choice (UV absorbance and/ or fluorescence). Flow from a binary solvent delivery system is divided evenly across 24 channels and results in 1/24 of the programmed pump flow rate through each column (i.e., total flow of 300 /zL/ min will produce a flow of 12.5 /zL/ min in each column). Samples are introduced to the columns by a multichannel autosampler configured to sample from either 96-or 384-well SBS standard plates. Figure 6.2 depicts a general view of the system. 

For continuous sample introduction, gated injection was adopted. With EK flow, the analyte continually flowed in parallel with a separation buffer to the analyte waste reservoir (see Figure 4.15). Injection of the sample analyte was achieved by interrupting the flow of the buffer for a short time (known as the injection time) so that the analyte stream was injected. This scheme was achieved by four reservoirs (without considering the reagent reservoir) and two power supplies [317], Gated injection has also been achieved using one power supply and three solution reservoirs [564]. 

Xu, H., Roddy, T.P., Lapos, J.A., Ewing, A.G., Parallel analysis with optically gated sample introduction on a multichannel microchip. Anal. Chem. 2002,74(21), 5517-5522. 

Synthesis control has two tasks directly associated with it. These are to identify or verify the identity of a combinatorial component and to determine the purity of the synthetic product. When characterizing a parallel library it is a relatively easy task to obtain a molecular weight from a small amount (femto-mole) of compound and thereby obtain a crude identification of the product. This circumvents the need to perform more difficult NMR or IR spectral interpretation and sample introduction maybe performed by a simple flow-injection atmospheric pressure ionization (API)/MS system. Purity assessment is typically based on area percentage normalization of the total ion chromatogram, assuming equivalent ionization of impurities and parent compounds, or a secondary detector, such as UY... 

Xu, H. et al. Parallel separations of oligonucleotides with optically gated sample introduction on multichannel microchips. J. Sep. Scl 27, 7, 2004. 

In biological imaging, confocal laser scanning microscopy (CLSM) has in the last decade significantly extended our ability to visualize highly complex samples as multidimensional datasets (space, time, colors). In parallel, the introduction of fluorescent protein variants as in vivo tags of structures of interest has opened up new ways to observe cellular processes inside the living cell or tissue (for review see Miyawaki et al., this issue). 

In the case of parallel production in processing operations, the precautionary measure may be an adequate cleansing process that has been demonstrated to be effective. In the case of the importers, a measure to be taken may be the introduction of an internal plan to take samples to be tested for possible traces of pesticides. 

Table 1.2 gives some of the reasons for the LGC setting up its automation team. The primary motivation was economic. LGC was often subject to constraints on staffing in parallel with large increases in analytical commitments. The introduction of cost-effective analyses, using mechanical or automatic instruments, reduces staff involvement and allows well qualified people to be released for the development of new analytical requirements. The analysis of beer samples by multi-channel continuous flow analyser [S, 6, 7] and the introduction of a mechanical solvent extraction and identification system to analyse and measure levels of quinizarin in gas oil, both for duty purposes, were prime examples [8], Both systems involved commercially available components and/or instruments integrated with modules designed and built in-house. 

The introduction of commercial instrumentation in this automated area has been too slow and too disappointing to meet the need for routine analysis of numerous samples. The options have been the extraction of one sample at a time or individual samples in parallel. Either of these options make the repetitive analysis of the same sample or the sequential analysis of different samples exceptionally time consuming. Parallel analysis, proposed by one manufacturer, is susceptible to cross-contamination and across the board sample loss with clogging of one extraction vessel. In order to move supercritical fluid extraction into the realm of routine operations for residue analysis, rapid analysis of multiple samples needed to be addressed. 

The total extraction time for these samples was 12 minutes per extraction. However, the procedure was conducted twice for aged samples to ensure complete extraction of the desired materials. Therefore, the overall analysis time was 24 minutes per sample. This time estimate can be misleading. The design of our system is such that the extractions are conducted in a combination of serial and parallel fashion, reducing overall analysis time. Illustrated in Table I is the extraction program for Diuron from Tama soil. Prior to the start of this method program, modifier has been introduced to each extraction vessel in a stepwise fashion. This procedure is carried out to insure that there is no pressure build-up where modifier introduction... 

The S/N of individual spectra is optimized when each sample is contained within a size-matched RF coil. As mentioned in the introduction to this chapter, this implies that the coils must be small in order for a number of them to fit inside the homogeneous region of the magnet. The first implementation using multiple coils was by MacNamara and co-workers [12], who connected four coils in parallel to make the circuit shown in Figure 8.2.2. 

EOF has been applied as a pumping or mixing mechanism in microreactors (Fletcher et al., 2002). Mixing concepts include the introduction of two (or more, see Kohlheyer et al., 2005) parallel streams from a T- of Y-junction, where mixing at the low Reynolds numbers achieved occurs principally by interdiffusion of the two streams. This is a relatively slow process which may take tens of seconds to complete. Faster mixing can be achieved by injection of a sample of a specific composition via, for example, a double T-injector into a stream of liquid with a different composition. 

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