Quantitative Chemical Analysis

Introduction to Quantitative Analysis

• Analysis, in chemistry, involves breaking down a system to identify, study, and measure specific parameters or components
• This involves using a method, which includes reagents that cause chemical changes and specific procedures with steps in the correct sequence
• Analytes are the components of the system that are studied and measured
• Each analysis method will have it's own analyte(s)
• Matrix is the term for the other components of the system that are not the analyte

Qualitative vs. Quantitative Analysis

• Qualitative analysis identifies analytes and determines the presence of toxic substances
• Qualitative analysis is preliminary and shows "There is" vs. "There isn't", not providing the amount of analyte
• Quantitative analysis measures analytes and determines the amount of toxic substances
• Quantitative analysis helps determine, “Is the amount enough to be toxic?" and can only follow qualitative analysis

Chemical Analysis Steps

• Choose a method
• Acquire a sample
• Prepare the sample
• Isolate the analyte
• Measure the amount of the analyte
• Determine if the method produces reliable results for the specific sample

Sampling

• Testing is cost-effective on smaller material amounts
• Representative samples of 50g are taken from 50-ton shipments
• A sample has to be representative to have the highest probability of representing all bulk material parts
• The representative sample must be homogenized

Analysis Types (Based on Sampling)

• Macro Analysis: Samples greater than 0.1g
• Semimicro Analysis: Samples range 0.01g - 0.1g
• Micro Analysis: Samples range 0.1mg - 0.1g
• Ultramicro Analysis: Samples smaller than 0.1mg

Sampling Methods

• Chemical analyses use probability sampling to obtain a representative sample
• Probability Sampling include; Simple Random, Clustered, Systematic and Stratified Random methods

Coning and Quartering

• Is a process involving repeatedly creating a cone shaped pile of mixed material, then dividing into quarters and discarding two opposing quarters
• The remaining material is then remixed for further use

AQL Sampling

• Also known as Acceptance Quality Limit sampling
• It can be coupled with probability sampling methods

Sample Preparation

• Analyte preparation depends on the procedure of each method
• Considerations include physical state (solid, liquid, gas, solution)
• Consider where the analyte is located in the sample (tissues, cells, crystal structure)
• The stability of the analyte needs to be considered
• It is important to note if method of analysis is destructive
• Analyst must decide if the analyte needs transformation for detection

Elimination of Interferences

• The addition of reagents occur for two reasons
• To remove interfering chemicals, which interfere with analyte detection
• To react with the analyte, to make it more detectable
• When reagents react with an analyte and shows a visible reaction they are called indicators
• Examples of elimination; determining metals in organic materials and acetic acid in vinegar
• Other techniques include dilution and matrix matching

Detection and Quantification

• Detecting and quantifying involves various methods
• Types of detection:
  • Isolation which is removing the analyte in any form and quantifying it
  • Simple detection using indicators, which triggers at the analysis' end, leading to the quantification of the reagent added
  • Instrumental detection measuring the instrumental signal of the analyte and the quantity is calculated

Types of Constituents/Analytes (Based on Quantity)

• Major Constituent: Anything greater than 0.1% of the sample
• Minor Constituent: from 0.0001% (1 ppm) to 0.1%
• Trace Constituent: from 0.0000001% (1 ppb) to 0.0001%
• Ultratrace Constituent: Anything lower than 0.0000001%

Standardization

• Added reagents quantify the sample directly, therefore the purity/concentration must be precisely determined
• The reagent is reacted with a stable compound/mixture of unchanging or known purity
• The stable compound is a standard
  • Primary standard: mixture/compound with unchanging purity/concentration
  • Secondary standard: is a reagent and a primary standard determine concentration/purity
• Stoichiometry is how the true reagent concentration is calculated
• Process of true reagent calculation is standardization

Calibration

• During detection, known analyte concentration samples must be analyzed
• Before analyzing an unknown analyte the relationship between analyte and signal detected is established
• This process is called calibration
• Calibration includes external-standard, internal-standard calibration and standard addition
• NIST standard reference materials (SRMs) ensure traceability, and are for quality management in the measurement system

Figures of Merit (Excluding Accuracy and Precision)

• Sensitivity measures the change in measured quantity, per unit change in analyte concentration
  • Calibration sensitivity is the slope of an external calibration curve
  • Analytical Sensitivity is the ratio of the calibration sensitivity, to the standard deviation of the analytical signal at a concentration
• Procedure for Analytical Sensitivity:
  • Calibration sensitivity is derived from taking the slope of the calibration curve
  • Take the standard deviation of the signals over the range of calibration
  • Divide calibration sensitivity by the standard deviation of the signals

Detection Limit

• Detection limit (DL or Limit of Detection/LOD) is the smallest concentration which can be reported with confidence
  • Each technique will have its own detection limits
  • Blank sample sensitivity (m) uses a calibration deviation s(b) when tested repeatedly
  • The DL at a confidence factor k is: DL = k.sb / m
  • k = 2 (92.1 % confidence) or 3 (98.3% confidence)

Linear Dynamic Range

• Linear Dynamic Range: The concentration in which the calibration curve is linear
  • Calibration curves are a line, with a linear equation
  • Linear equations apply within the range
  • Outside the liner range, the linear equation does not apply
  • Best determined by what signals are closed to the line from calibration standards