Demonstration of a memory calibration method in water isotope measurement by laser spectroscopy

Rationale Measuring delta O-18 and delta H-2 values in water using wavelength-scanned cavity ring down spectroscopy (WS-CRDS) requires multiple injections of up to six (and sometimes eight or more) of one sample to remove the memory effect, which decreases the sample throughput and increases the consumables cost. Thus, improved methods for removing the memory effect are required. Methods We calculated the memory coefficients by sequential WS-CRDS measurement of two lab standard waters with isotopic differences, and used them to establish calibration equations. We then used these equations to correct the measured delta O-18 and delta H-2 values by removing the memory effect, instead of using multiple injections in the routine daily measurements. Results By using this method, the number of injections per sample was reduced to one. The reproducibility (one standard deviation) of the delta O-18 and delta H-2 values obtained for quality control sample was less than 0.05 parts per thousand and 0.5 parts per thousand for an annual average, respectively. Conclusions By measuring the memory coefficients and establishing the calibration equations, a highly effective method was developed for determining the delta O-18 and delta H-2 values of water, which could significantly improve sample throughput for liquid water dual isotope measurement without sacrificing the precision.