Sample Analysis Design Internal Standard

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3 Internal Standard Advantages: Fluctuations are monitored in each sample/ calibration / blank Disadvantages: Assume that behavior of IS is the same as the analyte

4 Instrumental Drift Correct for instrument drift with: Internal standardization is a common procedure Use of drift corrector solutions (DCS)

5 Instrumental Drift Drift Corrector Solutions (DCS): Measure the same solution intermittently throughout the course of the analytical session Change in ion signal is assumed to be linear between each DCS measurement

6 Instrumental Drift The DCS should contain all elements of interest and can be matrix matched to samples Example: use standard reference materials (SRMs) for DCS

7 Instrumental Drift Apply a linear correction to samples between DCS solutions DCS 1 + ((DCS 2 - DCS 1 )*F) F = position dependent fraction

8 Instrumental Drift Advantages of DCS correction: all analytes are monitored for drift nothing added to sample solutions Disadvantages of DCS correction: assume change is linear cannot easily monitor matrix effects

9 Background & blanks Standard blank - blank used to monitor polyatomic ion interferences, gas peaks, and contamination from reagents; used for background subtraction Procedural blank - blank used to monitor contamination acquired during all stages of sample preparation; grinding, digestion, acidification, powdering, etc

10 Background & blanks Use of blanks during an analytical session: ALWAYS begin an analytical session with at least one standard blank Analyze standard blanks periodically throughout the course of the session in particular to monitor memory effects Process and analyze at least one procedural blank at some point during your research study; for its analysis, it s preferable to measure it early in order to avoid any potential memory effects

11 Background & blanks The more standard blanks that are run during an analytical session, the more information you will have with regards to monitoring change(s) in background levels throughout the entire session

12 Background & blanks How to determine the background : 1. just use the first standard blank 2. average all standard blanks 3. take median of all standard blanks 4. apply statistical analysis to standard blanks and select some of them

13 Background & blanks Outlier tests: 1. I know the truth 2. Looks different 3. Statistical proof

14 Background & blanks Option 1 should be avoided - unscientific and invalid Option 2 is better but only if the measurement is repeated Option 3 is the best approach, but needs to be carried out carefully in order to avoid false negatives and positives

15 Background & blanks Huber Outlier Test take median of all values calculate absolute deviation x i - x m take mean of absolute deviations (MAD) multiply MAD by coefficient (k = 3-5) anything higher than k*mad is rejected as outlier

16 Background & blanks Calculation of Limit of Detection (DL) and Limit of Quantification (QL) Easy way: LOD = 3*STDEV blank ; LOQ = 10*STDEV blank

17 SUMMARY A good analytical method will: 1. provide the means to calculate an accurate background level 2. allow for correction of instrument drift 3. use Internal standardization to monitor matrix effects 4. provide some method for monitoring/ correcting interferences 5. Use a proper calibration strategy

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21 Sample Analysis Design PART II

22 Sample Analysis Design Generating high quality, validated results is the primary goal of elemental abundance determinations It is absolutely critical to plan an ICP-MS analysis carefully from sample gathering to final analysis on the ICPMS Always create and follow an analysis design that shall permit you to follow the procedure for future samples, AND know what you did when looking at old data

23 Sample Analysis Design Goal: high quality, validated, quantitative determination of elemental concentrations/isotope ratios This goal can be achieved by: Specificity - only detecting isotope(s) of interest, not interferences, OR making sure that matrix effects do not play a role Sensitivity - can we differentiate the isotope of interest from the background signal? Accuracy - does the analysis represent the real value? Precision - how repeatable is the value?

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25 Sample Analysis Design Sensitivity and Precision: generally defined by how well the instrument is tuned (assuming dilution factor is appropriate) Specificity and Accuracy: generally governed by how well you prepare your sample and set up your analysis method/parameters

26 Sample Analysis Design Spectroscopic interferences Matrix effects

27 Sample Analysis Design Interference analytical artifact which causes the ion signal in a sample to vary from an idealized (true) signal Interferences cause inaccurate analyses Two main types: 1. Spectroscopic 2. Nonspectroscopic

28 Sample Analysis Design Spectroscopic Interferences in ICP-MS 1. Isobaric overlap 2. Polyatomic ions 3. Refractory oxides 4. Doubly charged ions

29 Sample Analysis Design Isobaric Overlap Two different elements having the same nominal mass E.g., 40 Ca (96.9%) and 40 Ar (99.6%) Monoisotopic elements: B, Na, Al, Sc, Mn, As, Nb, Y, Rh, I, Cs, Pr, Tb, Ho, Tm, Au, Th None have isobaric interferences

30 Isobaric Overlap A few multi-isotope elements have no interference free isotopes: E.g., In In 113 Cd, and 115 In 115 Sn Usually possible to choose an interference free isotope or one that is unlikely to be interfered upon strongly (either due to very low natural abundance of interfering element (isotope) or the interfering element (isotope) is not present in sample)

31 Isobaric Overlap IF an isobaric interference is unavoidable, you can correct for it by measuring the counts on a non-interfering isotope of the interfering element CPS analyte = CPS total -CPS interferent *(X a /X b ) X a = abundance of interfering isotope X b = abundance of non-interfering isotope