Data Quality

The Calibration curves have rhos of 1.0000 which suggest very strong linearity. The LOD for Na is relatively high compared to the Ca at ~10 ppb. For the Ca isotopes, Ca 44 has a higher LOD at ~4 ppb while Ca 40 and 43 have LOD’s below 1 ppb. The mid calibration for QC1 and QC2 show excellent recoveries at or over 98%. The NIST 1643e external standard shows good recoveries at or above 95%. These results suggest that these calibration curves are strongly linear and reliable, however the high Na LOD presents problems.

Table 3: Analyte recoveries and detection limits on quality control (QC) standards

QC1 = mid-calibration range standard (same stock as calibration stds)

QC2 = mid-calibration range standard (stock independent of calibration stds)

QC3 = NIST 1643e (trace elements in water)

The Unknown samples fall within the concentration range of the external standards. For the diluted feed samples, the Ca median, minimum, and maximum values all fall within L4 and L5. Falling within this midpoint of the calibration curve provides helps generate the highest quality data. For the digested samples, the Ca median, minimum, and maximum values all fall within L3 and L4.  While this is less ideal than falling within L4 and L5, some quality measurements can be extracted from this set. The signal to noise for the Ca 40 and 43 is acceptable at around 2,000 for the feed samples and 450 and 350 for the digested samples. The signal to noise for the Ca 44 measurements are not as impressive as the other Ca isotopes but are still acceptable. The Ca concentration for QC1, QC2, and QC3 are 1,800-2,000 ppb which are close to the median concentration for the feed samples, but higher than the Ca concentration in the digested samples.

The Na data for both diluted and digested samples falls between L1 and L3 with the median for both sets falling close to L2. This is not an ideal concentration for this standard curve The signal to noise for both the feed and digested sodium samples are below 4 which is not an acceptable range. The QC1, QC2, and QC3 concentrations for Na and much higher than the Na concentration observed in both sets of samples and are not appropriate for these experiments.

Table 4. Standards, and Diluted Sample Statistics

The Quantified Relative Standard Deviation (QRSD) for Na is higher than the Ca isotopes, most likely due to where these samples fall on the standard curve. The Ca 40 isotope has the lowest QRSD at close to 1% for both the feed and digested samples while the Ca 43 and 44 both have close to 7% and 9% QRSD for the feed and digested samples respectively. The Ca 40 isotope has the most accuracy, but all Ca isotopes are more accurate than the Na measurement. This could be due to using an inappropriate calibration range for Na.

Good spike recoveries should be within a few percentage points of 100%. For the Ca spike recoveries, Ca40 has the best feed recovery at ~74% and Ca 44 has the best digested recovery at ~77%. These isotopes should give the most accurate results for the feed and digested samples respectively, but it should be noted that these recoveries are not acceptable. The Na spike recoveries for the feed and digested samples are acceptable and between 95-105%.

Table 5. Sample QRSD and Spike Recoveries

While this data set produced usable primary data, future work will be needed to generate a better method. The standard curve was acceptable and the quality control concentrations were close to the Ca concentrations in the unknowns for Ca, but further dilution could reduce matrix effects and achieve appropriate recoveries. If this does not help then employing the method of standard addition could overcome these matrix effects. The Na concentration is on the low end of the calibration curve and further dilution would lower this below the detection limits of the instrument. Na had good spike recoveries but needs a standard curve at a lower concentration range to achieve acceptable results. A set of two dilutions for each sample, one for Ca and one for Na, could overcome the issues surrounding both of these analytes.