Chemistry 155 Introduction to Instrumental Analytical Chemistry

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Chem 155 Unit 1 Page 24 of 3131.7 Vocabulary: Properties of MeasurementsSensitivitySelectivitySpecimenSampleAnalyteCalibration CurveInterferantDetection Limit C MIN orC MLimit of Quantitation(LOQ)Limit of Linearity (LOL)Linear Dynamic Range(LDR)A detector or instrument that responds to only asmall change in analyte concentration issensitive. Numerically, the sensitivity of aninstrument is the slope of the calibration curve inuntis of: signal / unit concentration – often givenas ‘m’. Sometimes the detection limit (see below)is called the instrument sensitivity – but this is notcorrect.The ratio of the sensitivity of an instrument to ananalyte to that of an interferant.The material removed for analysis – e.g. a giventablet from an assembly line.The mixture that contains the chemical to bemeasured – e.g. the blood sample that we wantto measure iron in.The chemical that is being measured – e.g. theiron in the blood sample.A linear or non-linear function relating instrumentresponse (signal) to analyte concentration.Another chemical in the sample that either affectsthe instrument’s sensitivity to the sample or givesa signal of it’s own that may be indistinguishablefrom the analyte signal.The minimum detectable concentration ofanalyte. Usually defined as that concentrationthat gives a signal of magnitude equal to threetimes the standard deviation in the blank signal.The minimum concentration of analyte for whichan accurate determination of concentration canbe made. The LOQ is typically that concentrationfor which the signal is 10x the standard deviationof the blank signal.The largest concentration for which a calibrationcurve remains linear.The range of concentrations (or signal strengths)between the LOQ and the LOL. An instrument ismost useful within its’ LDR.Page 24 of 313
Chem 155 Unit 1 Page 25 of 3131.8 Detection LimitThe detection limit is denoted “C M ”C M is the minimum concentration that can be“detected,” or distinguished confidently from a blank.Let us define the minimum detectable signal changeas: ΔS MTherefore: ΔS M = mC M must be a multiple (n) of thenoise level in the blank: (σ b ).ΔS M ≡ 3σ b = mC MMinimumDetectable SignalSignal due toblankMinimumdetectableconcentrationBy convention, m=3.Derive a formula for C M based on σ B and m:mC M = 3σ bsoC M = 3σ b /mDerive a formula for C M based on γ :γ = m/σ so 1/γ = σ/mC M = 3σ/m = 3/γPage 25 of 313
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Chemistry 155 Introduction to Instrumental Analytical Chemistry

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