Results & Discussion

Data Quality & Tune Comparison

The high oxide tune (HOT) produced good QC recoveries, while the low oxide tune (LOT) showed high (poor) recoveries (Table 1). This discrepancy may stem from error in the calibration curve. Ideally, the calculated calibration standard concentrations should match their known values. However, the calculated concentrations were higher than the actual values at low concentrations. This overestimation likely extended to the QC standards, resulting in concentrations that were higher than expected. Additionally, the coefficient of determination (R²) for the LOT calibration curves was generally lower than that of the HOT, suggesting a weaker correlation and contributing to the poorer QC recoveries observed in the LOT data (Table 2). Finally, the calculated sample concentrations were all higher for the LOT than the HOT, indicating error in the calibration curve (Table 3). These findings may help explain why HOT yielded better overall performance in QC assessments.

The LOT generally yielded a higher oxide production ratio than the HOT, which was opposite from expected (Table 4). The percent intensity of interfering MO/MOH species was thus generally higher for the LOT, however, the most interfered isotope, Gd, has the greatest percent interference on the HOT (up to 22.5%; Tables 5 & 6). These results might indicate that there is no significant difference between tuning methods, however, because the LOT results had poor data quality, the experiment may need to be completed again for a more accurate tune comparison.

The LOT generally had a limit of detection (LOD) that was near or much higher than the HOT (Table 7). The LOD was generally lower than the concentrations of the REEs, and therefore did not impair the results significantly. The method blank concentrations were also much lower than those of the samples, indicating that contamination during lab processes was also not an issue in this study (Table 8).

REE Analysis

The REE patterns display an unusual zig-zag trend that does not align with the expected distribution when normalized to the NASC (Figures 1 & 2). Carbonates (Cow Creek and Lower Glen Rose) show a slightly enriched HREE pattern, which is consistent with seawater (and therefore carbonate) REE patterns. A possible cerium (Ce) anomaly is present in the carbonates, but it cannot be clearly distinguished from the irregular zig-zag pattern. Carbonates also show significantly higher REE concentrations than anticipated. The REE patterns for the Hammett Shale and the Sycamore and Hensel sandstones are somewhat flatter but still show somewhat of the same zig zag pattern that the carbonates have. The flatter pattern matches expectations for the shale and sandstones because the data is normalized to a shale (NASC). The shales and sandstones had higher REE concentrations overall compared to the carbonates, which aligns with expected results.

Figure 2: NASC normalized LOT REE concentrations, showing abnormal zig zag pattern.

The unusual zig-zag pattern may partly result from oxide interference removal (which will affect the measured intensity); however, the zig-zag is also observed on REEs that have no interferences (or interference equations), suggesting other contributing factors. Sample contamination prior to analysis may also explain the irregular pattern. The samples were collected from a core, which was taken from the ground via air-rotary drilling. The outer part of the samples was not removed prior to crushing, meaning some of the metal from the drill may have contaminated the sample. Drill contamination would explain the high REE concentrations found in the carbonates, and may also explain the similarities between the samples of different composition. Because many people touched the samples with bare hands, contamination may have also occurred during the handling of the samples. Otherwise, the REE patterns found are not what was expected, and do not seem to match the results from other papers that analyzed similar rocks.

The REE patterns between each tune are nearly identical, except for at Eu. The HOT indicates a negative Eu anomaly, whereas the LOT shows no trend for Eu. When calculating the calibration curves for each tune, the Eu intensities for each standard were much higher for the HOT than the LOT, meaning that when Eu concentration was calculated from the calibration, the the HOT would indicate a lower concentration for the same intensity as the LOT. The differences in these calibrations may have caused these alternate patterns.

Key Takeaways

The LOT produced generally poorer results as compared to the HOT, which might be attributed to the calibration curve calculation. Therefore, it is difficult to accurately compare the effectiveness of having a tune with low vs high oxide production. Also, the observed REE patterns were not comprable to other studies, making it difficult to make conclusions about each rock. Overall, the results of this study were not what was expected, and the experiment may need to be completed again to try to create more reliable results, or to find what part of the method may have created error in the results.