Discussion/Major Findings

Interesting/Key Findings:

Optimal Zn measurement mode– We found that Zn-66 and Zn-68 measurements had better precision when they were operated in no-gas mode compared to helium mode. This was true for all of the samples run. Zn-66 and Zn-68 in no-gas mode were slightly less accurate than the other modes and isotopes for the QC standards, so it’s hard to definitively say which mode is better across the board.

Limits of Quantification for Zn and As– Based on our spike results, the limit of quantification for Zn and As should be higher than the limit of detection that is calculated based on the blank data. In our run, the three spikes had values lower than the limit of detection for zinc and near the limit of detection for arsenic, but the known concentrations in the spikes were higher than that. This means that the calculated detection limits aren’t always representative of the lowest concentration that can be measured repeatedly. Based on our results, the limit of quantification for zinc and arsenic should be greater than 0.21 ppb Zn and 0.11 ppb As.

What didn’t work:

Zn-70 Interferences- Zn-70 concentration values from this run were consistently high compared to expected concentrations, as seen in tables 8-10 in the results, indicating the possible presence of interferences. The most likely interference present is ⁴⁰Ar¹⁴N¹⁶O which may account for the differences between the expected and observed. 

Negative Concentrations- Additionally concentrations of all zinc isotopes, excluding Zn-70, were measured as negative in each of the spikes and in the blanks. This is because zinc counts per second were lower than the calibration blank counts per second in both the spikes and the blanks. Blank concentrations are based on the counts per second of the calibration blank, which for our run was higher than the counts per second of subsequent blanks and spikes, making the spikes negative with respect to the calibration blank. Figure 4 shows this for Zn-66 in helium mode.

Future 

One unplanned obstacle we encountered in this project was troubleshooting our arsenic concentrations. The results initially appeared to be 10 times higher than expected in the NIST 1643 f QC standard, leading us to suspect a transcription error or a mistake in determining the dilution factors. Upon reviewing the constants used to calculate our calibration range, we identified a transcription error in the recorded stock concentration of arsenic. After correcting the reference value and rerunning the calibration, the results aligned more closely with our expectations. This goes to show that human error is possible and should be considered when assessing the reliability of data produced. 

In the future, we aim to transition to using the triple quadrupole, which should provide better sensitivity due to it being newer. Additionally, we need to evaluate zinc spikes at higher concentrations to accurately determine the defined point of quantification, as the spikes in this project were too low to estimate effectively.