Direct analysis of sea water using two dimensional ion chromatography

1
Direct analysis of sea water using two
dimensional ion chromatography

• Aine Moyna
• Supervisors Brett Paul and Mirek Macka

2
Conferences

• May 2008 - 5th Biennial Conference on Analytical
  Science in Ireland, Waterford Institute of
  Technology, Ireland.
• July 2008 - Analytical Research Forum, Hull, UK.
• September 2008 Poster - International Ion
  Chromatography Symposium, Portland, Oregon, USA.

3
Overview

• Ion chromatography
• Instrumentation
• Columns
• Effects of eluent pH
• Effects of sample salinity
• Direct analysis of sea water
• Standard additions
• 3D surface graph
• Conclusions and future work

4
Ion Chromatography

• The reversible interchange of ions of like charge
  between a solution and a solid, insoluble
  material in contact with it. 1
• Separations are preformed on a column packed with
  a solid ion-exchange resin.
• The stationary phase surface displays ionic
  functional groups that interact with analyte ions
  of opposite charge.
• 1 P.R. Haddad and P.E. Jackson, Ion
  Chromatography- Principles and Applictions.
  Journal of Chromatography Library Series Volume
  46, 1990.

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Instrumentation
Figure 1 Schematic of instrumental setup for the
first and second dimension
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Monolithic Column

• Onyx 100 x 2 mm monolith column.
• Zwitterionic surfactant - (N-dodecyl-N,N-dimethyla
  mmonio)undecanoate (DDMAU).
• Method of coating
• 10 mM DDMAU in 5 (v/v) MeOHH2O.
• Solution was passed through the column at flow
  rate of 0.1 ml/min for 8 hrs.
• Column was reversed and coated for a further 2
  hrs.
• Column was washed with ultra pure water for 2hrs.
• Conditioned with eluent until a steady baseline
  was achieved.
• results in a charge double layer

Figure 2 Chemical structure of DDMAU
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IonPac AS17

• Dimensions 2 x 250 mm
• Microporous polyvinylbenzyl ammonium polymer
  cross linked with divinylbenzene
• Designed for analysis of inorganic ions.
• Used with a hydroxide gradient.

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Effect of eluent pH

• Standards of 10 mg/L Nitrate were injected using
  a 10mM phosphate buffer at pH 2 and pH 7.
• The dissociation of COOH acts to shield the
  inner anionic exchange sites
• At lower pH there is a reduced no. of dissociated
  sites which increases anion retention

Figure 2 Injection of a 10 mg/L nitrate standard
separated on the 2nd dimension surfactant-coated
monolith with a phosphate buffer eluent at (a) pH
7 and (b) pH 2.
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Linearity studies for nitrate
Figure 4 Overlay of nitrate standard peaks
Figure 3 Linearity curve for nitrate in the 2nd
dimension

• Standards were prepared using 2-40 mg/L nitrate
  for n7 concentrations.
• Standards were injected onto the 2nd dimension.
• Flow rate used was 0.3 ml/min.
• Eluent 10 mM Phosphate buffer at pH 2.

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Effect of sample salinity on selectivity of the
coated monolith for nitrate and chloride

• Standards of 10 mg/L nitrate were made up in
  various concentrations of salt solutions.
• 0-30,000 mg/L salt concentrations n6
• Decrease in retention.
• Figure 5. Plot of retention vs
  concentration of
  chloride
  

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Direct analysis of sea water

• Seawater was injected onto the 1st dimension.
• 2 detectors used.
• Using the heart cutting technique, eluate was
  collected from 7.5 mins 10.5 mins.
• Eluate injected onto 2nd dimension
• Figure 6. Injection of neat seawater on
  the 1st dimension using UV detection at 214nm
  and conductivity detection (inset).

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Heart cutting technique

• A technique of isolating a pair of unresolved
  solutes and placing the mixture on another column
  which does resolve them.
• Figure 7. Injection of sea water in the 1st
  dimension using UV detection

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Gradient Elution

• Figure 7 Gradient used in the direct analysis of
  sea water in the 1st dimension.

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Standard additions

• The sea water samples were spiked with nitrate
  standards from 0-30 mg/L.
• Eluate collected and injected onto 2nd dimension.
• Plot of peak area vs nitrate concentration
  yielded a correlation coefficient of R20.99.
• Figure 8 Overlay of spiked nitrate sea water
  samples

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Comprehensive 2D separations

• Eluate was collected from the 1st dimension every
  30 seconds.
• Injected onto 2nd dimension
• Shows nitrate eluting at approx 8 mins.
• Gradient elution to achieve resolved 2D
  chromatograms.
• Figure 9 Offline 2D separation of sea water
  sample spiked with 10 mg/L NO3-. Eluate collected
  from t1 min in 0.5 min intervals

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Conclusion and future work

• Proof of concept.
• Heart cutting technique was successfully applied.
• Interference from chloride in the sea water was
  minimised.
• Future work will include investigation into
  online two dimensional IC.
• Include analysis of more complex mixtures.
• Vary loop size.
• Gradient elution

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Acknowledgements

• Dr. Leon Barron
• Prof Brett Paull
• Dr. Mirek Macka