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8. Internal standards

8. Internal standards. Adv. Chromatography. Instrument variations. problems caused by instrument or sample introduction: standard addition or matrix-matched standards will not help often intermittent examples injection volumes for GC & HPLC

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8. Internal standards

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  1. 8. Internal standards Adv. Chromatography

  2. Instrument variations • problems caused by instrument or sample introduction: • standard addition or matrix-matched standards will not help • often intermittent • examples • injection volumes for GC & HPLC • flame/plasma temperature for flame photometry and ICP • nebuliser uptake rates for flame photometry and ICP • need a checking mechanism in solution that indicates when variations occur • known as internal standard

  3. Example 8.1 • A sample with an internal standard included is measured for both species twice • If these were two different samples, and the internal standard wasn’t used, you would conclude that #2 was 10% lower in concentration than #1 • IS used in calcsto correct for variations

  4. Choosing a suitable internal standard • internal standard is NOT the analyte, but a totally different species • must be chosen carefully • The chosen compound cannot be present in the sample. • The chosen compound should have similar physical and chemical properties to the analytes. • The chosen compound should be readily available. Exercise 8.1: Why are these important? • It would add to the IS response, but not be due to instrument variation • The two species need to behave similarly with instrument variation • Convenience and practicality

  5. what constitutes similar properties depends on the technique: • emission spectro: chemical => same periodic table group • chromatography: physical => similar retention times (more in Ch 6) Exercise 8.2 How would you determine whether the chosen IS was present in the sample? • analyse the sample for the proposed IS

  6. Adding the IS • The IS is added to all solutions – standards, samples and recovery checks. • The same concentration of IS is added to all solutions. • The response of the IS should be similar to that of the main analyte. Exercise 8.3: Why are these important? • Instrument variation can occur at any time in any solution • A change in the IS response is supposed to indicate instrument variation, not differences in concentration • The instrument response at very different intensities could be different anyway

  7. In calculations • any variation in IS response must mean something has happened in the process • this will have affected the analyte response as well • if the IS response is down by 10%, then so presumably is the analyte • ratio should be unaffected • - NOT the other way • the ratio becomes the response (y-axis) value

  8. Example 8.2

  9. Exercise 8.4 • Answer from graph: 0.478 %w/v • Original conc: x 5 = 2.390% • in 100 mL = 0.239g • %w/w = 1.94

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