Determination of low levels of krypton in helium by gas chromatography

Abstract

579maintain an aspiration rate of 1.2 mL min-', and operating all nebulizers a t 103 kPa pressure differential, the Plasma-Therm crossflow blocked after 10 min, the Meinhard nebulizer blocked after 15 min, and the fixed crossflow design showed no signs of blockage after 60 min of continuous operation. Blockage of the other units was indicated by a rapid drop in nebulization gas flow rate. The units were all operated disconnected from the plasma torch, as experience has shown that aspiration of 20% NaCl solution into an operating plasma over time intervals of the order of 10 min leads to encrustation of salt on the sample injection tube tip. The consequent localized heating can lead to devitrification and structural collapse of the injection tube tip. In certain commercial instrumentation, the Meinhard type of nebulizer is able to run with solutions containing 2&30% salt content by use of wetted Ar gas and a computer controlled switching procedure (19). By this means, the gas passageway is rinsed with 50-100 pL of water following each measurement period. With the fixed crossflow nebulizer, this complicated procedure is unnecessary for routine operation with solutions of high dissolved-solid content. Particulate matter in the sample may, of course, still cause blockage problems for the solution uptake capillary, but this type of blockage can be readily removed with a cleaning wire.Cross contamination effects were also investigated with both Type A and Type B fixed crossflow nebulizers. Type B gave wash-out times comparable to the combination of a variable crossflow nebulizer and a Scott chamber. Type A (large droplet recycling design) required longer wash times at higher solution concentrations, as would be anticipated. CONCLUSIONSThe fixed crossflow nebulizer described in this study has most of the characteristics desirable for ICP sample introduction. It produces a spray with a suitably small droplet size distribution, using a low nebulization gas flow, is stable in operation, and does not block readily. However, should blockage occur, the nebulizer is sufficiently robust that it can be cleaned with a wire. All alignments are made during construction, and so no further adjustments are necessary during routine operation. In addition, the large-droplet recycling version should prove very valuable when sample volume is restricted, as in much clinical analysis.