Valveless Injection IC

The sample probe is moved between eluent and standard or sample solutions using an autosampler. Some weUs in the autosampler contain samples to be injected. Other wells contain standards of known concentration. StUl others contain one or more eluents of different concentration. 

A peristaltic pump deUvers the eluent and sample to the low-pressure column. Because a peristaltic pump is a flow-through device, the integrity of the sample is maintained as it is passed through the pump. However, because peristaltic pumps can only pump at a maximum of 100 psi, the columns must operate at a low backpressure. Development of a low-backpressure column depends on several factors. First, the ion-exchange resin must be small, uniform and efficient. The column hardware and fluid connections must be well designed and not contain flow paths 

Standard analytical separation columns probably operate at too high a backpressure to be used in this type of system. However, guard columns often contain the same separation material as an analytical column and have much lower backpressure. To a certain extent, the flow rate can be reduced to lower column backpressure. 

One of the advantages of the valveless mode is the ability to perform step-gradient elution. In this case the autosampler vials contain eluents of increasing concentration. An eluent step gradient is formed as the autosampler samples in turn each eluent vial. 

Since ICP-MS detection is used, a nonanalyte element may be spiked into the eluent to provide a known, urtiform signal for instrument optimization. Alternatively, the column may be removed from the sample introduction apparatus to allow direct pumping of optimization solutions to the detector. Another advantage of removing the column is that the total element content of a particular species can be determined and compared to the total amount of the various species. This could be done automatically with a six-port, two-position switching valve where 

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