Removing the memory effect from water stable isotope analysis

Over the past decade or so, laser spectrometric instruments have revolutionized the field of isotope analysis of water samples. These instruments do not require complex lab facilities, are easy to use and can provide hydrogen and oxygen isotope data at high precision and high throughput.

One well-known shortcoming of these laser spectrometric analyzers is that individual measurements display significant sample-to-sample memory effects. Particularly at larger isotopic differences between samples, isotopic contamination by the previous sample can off-set the following measurements even after multiple injections. Therefore, it is common in many laboratories to run 7 or more replicate analyses of each sample, and discard the first 4 or so, to come to an accurate isotope value of that sample.

Because the single-shot precision of these instruments is rather good, the sample replication is not so much necessary for obtaining better precision, but indeed mostly needed to flush out the memory effect on the isotope values. Therefore, any technical adaptation that decreases the memory effect of these analyzers, and thus reduces the number of replicate analyses required to come to an accurate isotope ratio, would greatly improve the sample throughput of these instruments.

We here present an adapted injection interface system, coupled to a Picarro L2140i analyzer, that practically removes sample to sample memory effects. This effectively leads to accurate and high-precision isotope analysis of single-shot sample injections, even at large sample-to-sample isotope differences. Key to the removal of the memory effect is that the analyzer runs on a moisturized carrier gas, providing a constant water background upon which the injected samples are analyzed (De Graaf et al., 2021). We will present results of series of standard waters and natural samples (including seawaters) and discuss protocols that we developed for data calculation and quality control.

 

Reference:

de Graaf, S., Vonhof, H.B., Levy, E.J., Markowska, M., Haug, G.H., 2021. Isotope ratio infrared spectroscopy analysis of water samples without memory effects. Rapid Communications in Mass Spectrometry 35.