WEEK 1

• The Science of Chemical Measurements

  ANSWER : ANALYTICAL CHEMISTRY
• The compound or chemical species to be measured, separated or studied

  ANSWER : ANALYTE
• Classical Methods (Earliest Techniques)

    a.) Separations: precipitation, extraction, distillation

    b.) Qualitative: boiling points, melting points, refractive index, color, odor, solubilities

    c.) Quantitative: titrations, gravimetric analysis
• Instrumental Methods (~post-1930’s)

  a). separations: chromatography, electrophoresis, etc.

  b). Qualitative or Quantitative: spectroscopy, electrochemical methods,

    mass spectrometry, NMR,radiochemical methods, etc
• Measurements invariably involve errors and uncertainties.
• it is impossible to perform a chemical analysis that is totally free of errors or uncertainties

  
  

  We can only hope to minimize errors and estimate their size with acceptable accuracy
• Errors are caused by faulty calibrations or standardizations or by random variations and uncertainties in results
• Frequent calibrations, standardizations, and analyses of known samples can sometimes be used to lessen all but the random errors and uncertainties.
• error refers to the difference between a measured value and the “true” or “known” value.
• error often denotes the estimated uncertainty in a measurement or experiment.
• The degree to which an experimental result   approaches the true or accepted answer.

  ANSWER : ACCURACY
• Absolute Error = (X – μ)
• Relative Error (%) = 100(X – μ)/μ

  where:    X = The experimental result

    μ = The true result
• All Methods, except counting, contain errors – don’t know “true” value
• Two types of error: random or systematic
• Random Error :   results in a scatter of results centered on the true value   for repeated measurements on a single sample.
• Systematic Error : results in all measurements exhibiting a definite difference from the true value 
• Precision :   The reproducibility of results. The degree to which an   experimental result varies from one determination to   the next.
• Precision is related to random error and Accuracy is related to systematic error.
• To improve the reliability and to obtain information about the variability of results, two to five portions (replicates) of a sample are usually carried through an entire analytical procedure.

  
  

  Replicates are samples of about the same size that are carried through an analysis in exactly the same way
• •Individual results from a set of measurements are seldom the same

  
  

  •Usually, the “best” estimate is considered to be the central value for the set.

  
  

  •The central value of a set should be more reliable than any of the individual results.

  
  

  •Usually, the mean or the median is used as the central value for a set of  replicate measurements.
• The mean, also called the arithmetic mean or the average, is obtained by dividing the sum of replicate measurements by the number of measurements in the set
• The mean, also called the arithmetic mean or the average, is obtained by dividing the sum of replicate measurements by the number of measurements in the set
• The median is the middle value in a set of data that has been arranged in numerical order.
• The median is used advantageously when a set of data contain an outlier. An outlier is a result that differs significantly from others in the set.
• An outlier can have a significant effect on the mean of the set but has no effect on the median.
• Ways to Describe Precision: RESPONSE
• Range :    the high to low values measured in a repeat series of experiments
• Standard Deviation : describes the distribution of the measured results about       the mean or average value.
• Response :   The way in which the result or signal of a method   varies with the amount of compound or property being   measured
• Calibration Curve:  A plot of the result or signal vs. the known amount of a known   compound or property (standard) being measured.
• Parameters used to Describe a Calibration Curve:

  
  

  SENSITIVITY

  SELECTIVITY

  LIMITS OF DETECTION

  DYNAMIC RANGE
• SENSITIVITY : ability to discriminate between small differences in analyte concentration.
• SELECTIVITY : degree to which the method is free from interference by other species in the sample
• LIMITS OF DETECTION : minimum/maximum concentration or mass of analyte that can be   detected at a known confidence level.
• DYNAMIC RANGE : linear region of calibration curve where the lower limit is ten times   the standard deviation of the blank.
• plot of the number of occurrences or population of each measurement (Gaussian curve) 
• Noise : random variation in signal or background

   
• Signal : net response recorded by a method for a sample
• LOQ - limit of quantitation