[mandytan]

1)Let's say we want to test a water sample for potassium (i.e. K+) ion concentration. To perform a 100x dilution on the sample, why is it better to use a 100ml plastic volumetric flask or a glass volumetric  flask? It was observed that the first injection from the glass volumetric flask gave a reading of 204ppb but the second injection from the same volumetric flask gave a reading of 187ppb whereas the 2 injections  using the plastic volumetric flask were consistent.

2)Do we need to store glassware used for ion chromatography (IC) in DI water? Is it necessary to pre-rinse the volumetric flask used for dilution in IC with DI water ?( there may be water droplets adhering to the side/neck of the volumetric flask which may affect the accuracy of the dilution???)

3)I was told not to use a filter funnel to transfer deionised (DI) water from a beaker into the volumetric flask or to use a dropper to top up to the mark of the volumetric flask (this method is slower)but instead use DI water directly from the DI system to minimize transfer errors. However, I realise that this method results in water droplets on the sides of the volumetric flask,especially the plastic ones. Is there a good/better way to do dilutions? Would there be possible contamination from glassware (i.e. beaker, glass dropper etc)?

Thanks.

[Arkcon]

1)Let's say we want to test a water sample for potassium (i.e. K+) ion concentration. To perform a 100x dilution on the sample, why is it better to use a 100ml plastic volumetric flask or a glass volumetric  flask? It was observed that the first injection from the glass volumetric flask gave a reading of 204ppb but the second injection from the same volumetric flask gave a reading of 187ppb whereas the 2 injections  using the plastic volumetric flask were consistent.

Plastic may be better sometimes.  But its likely more important to mix carefully. Plastic may have a better nonwetting surface, so mixing is easier.

2)Do we need to store glassware used for ion chromatography (IC) in DI water? Is it necessary to pre-rinse the volumetric flask used for dilution in IC with DI water ?( there may be water droplets adhering to the side/neck of the volumetric flask which may affect the accuracy of the dilution???)

That is an old crutch that comes in handy sometimes -- insuring the glassware stays clean, free from contamination, etc.  If the glassware is carefully cleaned and rinsed free of previous samples, and any detergents used in cleaning, the droplets shouldn't be there.  You can try stoppering, inverting, before you top up to the line, to insure those droplets are inside the sample.

3)I was told not to use a filter funnel to transfer deionised (DI) water from a beaker into the volumetric flask or to use a dropper to top up to the mark of the volumetric flask (this method is slower)but instead use DI water directly from the DI system to minimize transfer errors. However, I realise that this method results in water droplets on the sides of the volumetric flask,especially the plastic ones. Is there a good/better way to do dilutions? Would there be possible contamination from glassware (i.e. beaker, glass dropper etc)?

Probably a contamination issue.  If you can get good enough to work rapidly and accurately straight from the DI system, then that's cool.  But I don't see the accuracy problem with a beaker and a pipette (perhaps a wrapped disposable one,) if you keep all glassware as clean as you can.