Methods: Lithium carbonate (0-1.1 mmol/L) was dissolved in distilled water, buffered to pH 7.4 and used to spike saliva samples collected from a single donor. Potassium was added to some samples to exclude interference. Predicted lithium concentration was measured using colorimetric assay following reaction of the lithium with a substituted porphyrin (Infinity Lithium Liquid Stable Reagent, Thermo-Scientific, USA). Two aqueous solutions (A, ‘water’ and B, ‘polymer’) were tagged with 1mmol/L lithium carbonate and buffered to pH 7.4. Solution B contained 1%w/v Sodium carboxymethyl cellulose (CMC). A randomised, crossover study was conducted with a ten-minute washout period. Participants (n=20) rinsed their mouths with water before swilling 10mL solution A or B in their mouth for 30 seconds then expectorating into a cup. Mass expectorated was determined and samples centrifuged (4°C, 4000rpm, 10min). Lithium concentration in the supernatant was analysed by spectrophotometry and fraction retained calculated.
Results: Measured lithium correlated with predicted lithium over the range 0.04-1.05 mmol/L (R2>0.999) and no potassium interference was observed. The mean lithium concentration recovered from participants following oral rinsing with solution A and B were 0.91±0.03 mmol/L and 0.81±0.04 mmol/L respectively, corresponding with a calculated oral retention of 10.4±4.7% for A and 15.3±4.1% for B, thus 4.9% more solution A was retained in the oral cavity compared to B (p<0.01, Student’s t-test).
Conclusion: The fraction of two aqueous solutions retained after oral rinsing were determined and addition of 1%w/v CMC appeared to increase retention. Lithium is a potential marker for the retention of aqueous liquids and this technique may be useful for complex fluids including mouth rinses as well as medication for delivery to the oral cavity.