Calibration Curves
Calibration curves are often affected by the limitations of the instrumentation. Data can become biased by calibration points, the instrument's limits of detection, quantitation and linearity, and by the response of the system versus its baseline (signal-to-noise)
- • Limit of detection (LOD): lower limit of a method or system which the target can be detected as different from a blank with a high confidence level (usually over three standard deviations from the blank response).
- • Limit of quantitation (LOQ): lower limit of a method or system in which the target can be reasonably calculated where two distinct values between the target and blank can be observed (usually over ten standard deviations from the blank response) (Figure 22.2.)
- • Signal-to-noise (s/и): response of an element measured on an instrument as a ratio of that response to the baseline variation (noise) of the system. Limits of detection are often recognized as target responses which have three times the response of baseline noise or sin > 3. Limits of quantitation are recognized as target responses which have ten times the response of baseline noise or sin > 10.
- • Limits of linearity (LOL): upper limits of a system or calibration curve w'here the linearity of the calibration curve starts to be skewed creating a loss of linearity (Figure 22.3). This loss of linearity can be a sign that the instrumental detection source is approaching saturation.
- • Dynamic Range: an array of data values between the LOQ and the LOL is where the greatest potential for accurate measurements will occur.
The understanding of a system’s dynamic range, the accurate bracketing of calibration curves within the range and around the target element concentration increases the accuracy of the measurements. If a calibration curve is created that does not potentially bracket all the possible target data points
FIGURE 22.2 Limits of detection and quantitation.
FIGURE 22.3 Calibration curve limits and range.
then the calibration curve can be biased to artificially increase or decrease the results and create error. To create calibration curves and working standards, there must be an accurate process of converting units, calculating dilution targets and preparing dilutions.
Dynamic Range, Concentration & Error
The first step in creating standards, working solutions, and dilutions is to understand the dynamic range your analysis is targeting - is it ppb, ppm, percent?
If you are looking for a major component of the sample, then your standards have to be in percent levels or the samples must be diluted down to the correct concentrations. If target elements are trace elements or trace contaminants, then standards and calibration curves have to be diluted down to match the target within the dynamic range of the instrument technique.
Tables 22.3 and 22.4 show the basic conversions between different concentrations based on mass or volume.
Laboratory Sources of Error & Contamination
Calibration curves are created by diluting standards into several target points along the dynamic range to cover the possible target results. Proper dilution of standards and samples is based on the understanding of basic dilution and volumetric procedures and dilution factors. Volumetric measurement is a commonly repeated daily activity in most analytical laboratories. Many processes in the laboratory from sample preparation to standards calculation depend on accurate and contamination-free volumetric measurements. Unfortunately, laboratory volumetric labware, syringes and pipettes are some of the most common sources of contamination, carryover and error in the laboratory.
Weight to Weight Concentrations
Parts per thousand 5
Parts per million
Parts per billion
Parts per trillion
5 Parts per thousand and parts per trillion both can use the ppt abbreviation; so use care to select appropriate unit of measure.
Weight to Volume Concentrations
Parts per thousand 5
Parts per million
Parts per billion
Parts per trillion
5 Parts per thousand and parts per trillion both can use the ppt abbreviation; so use care to select appropriate unit of measure.
The second type of improper use and incorrect choice can be seen in the selection of pipettes and syringes for analytical measurements. Many syringe manufacturers recommend a minimum dispensing volume of approximately 10% of the total volume of the syringe or pipette. A study by SPEX CertiPrep showed that dispensing such a small percentage of the syringe’s total volume created a large amount of error. The largest rates of error were seen in the smaller syringes of 10 and 25 pL. Dispensing 20% of the 10 pL syringe created a 23% error. Error only dropped down to below 5% as the volume dispensed approached 100%. In the larger syringes, measurements over 25% were able to see error in and around 1%. The larger syringes were able to get closer to the 10% manufacturer’s dispensing minimum without a large amount of error, but the error did drop as the dispensed volume approached 100%. 6
The third “I” of volumetric error is inadequate cleaning. Many volumetric containers can be subject to memory effects and carryover. In critical laboratory experiments, labware sometimes needs to be separated by purpose and use. Labware subject to high levels of organic compounds or persistent inorganic compounds can develop chemical interactions and memory effects. It is sometimes difficult to eliminate carryover from labware and syringes even when using a manufacturer’s stated instructions. For example, many syringes are cleaned by several repeated solvent rinses prior to use. A study of syringe carryover by SPEX CertiPrep showed that some syringes are subject to high levels of chemical carryover despite repeated rinses.
The final source of error is infrequent calibration. Many laboratories have schedules of maintenance for equipment, such as balances and automatic pipettes, but often overlook calibration of reusable burettes, pipettes, syringes, and labware. Under most normal use, labware often does not need frequent calibration but there are some instances where a schedule of recalibration should be employed. Any glassware or labware in continuous use for years should be checked for calibration. Glass manufacturers suggest that any glassware used or cleaned at high temperatures, used for corrosive chemicals or autoclaved should be recalibrated more frequently.
It is also suggested that under normal conditions soda-lime glass be checked or recalibrated every five years and borosilicate glass after it has been in use for ten years. The error associated with the use of volumetric containers can be greatly reduced by choosing the correct volumetric for the task, using the tool properly and making sure the volumetric containers are properly cleaned and calibrated before use.
Inorganic analysts know that glassware is a source of contamination. Even clean glassware can contaminate samples with elements such as boron, silicon, and sodium. If glassware, such as pipettes and beakers, are reused, the potential for contamination escalates. At SPEX CertiPrep, a study was conducted of residual contamination of our pipettes after being manually and automatically cleaned using a pipette washer. 6 - 9
An aliquot of 5% nitric acid was drawn through a 5mL pipette after the pipette was manually cleaned according to standard procedures. The aliquots were analyzed by ICP-MS. The results showed significant residual contamination still persisted in the pipettes despite a thorough manual cleaning procedure.
The experiment was repeated using a pipette washer especially made for use in parts-per-trillion analysis. The pipette washer repeated forced deionized water through the pipettes for a set time period. The pipettes were cleaned in the pipette washer then the same aliquot of 5% nitric acid was drawn through the 5 mL pipettes. The aliquot was analyzed by ICP-MS. The automated washer reduced the contamination significantly as compared to the manual cleaning of the pipettes. The reduction of contamination by moving from manual cleaning to an automated cleaning process was clear. High levels of contamination of sodium and calcium (almost 20 ppb) dropped to 7 Trace inorganic analysis are best performed in polymer or high purity quartz vessels or
Major Elemental Impurities Found in Laboratory Container Materials 7
tt Elements
Fluorinated ethylene propylene (FEP)
High-density PE (HDPE)
Changes in Element Concentration after Storage (ppb)
fluorinated ethylene propylene (FEP), to minimize contact with borosilicate glass. Metals, such as Pb and Cr, are highly absorbed by glass but not by plastics. On the other hand, samples containing low levels of Hg (ppb levels) must be stored in glass or fluoropolymer because Hg vapors diffuse through polyethylene bottles.
It is always important to know the best conditions for storage for a standard. The expiration dates on most standards reflect the manufacturer’s confidence level of continued accuracy at proper storage conditions for the shelf life listed on the product. If a standard is not stored properly, it can affect the quality and accuracy of the standard. Elements within the packaging can slowly be leached from the packaging over time and change the value of the samples or standards. Some of the common packaging elements were examined in samples at the time of manufacturing and bottling and again after one year to determine if the packaging contributed to the contamination of the samples over time. After one year, the amounts of common elements, such as aluminum, calcium, iron, magnesium, sodium, and silica, more than doubled in the solutions. Table 22.6 shows the changes of elemental concentrations in six common elements after a year’s storage.
- [1] The root of these errors is based on the four “I” errors of volumetric container:
- [2] Improper use: measuring tool is not used correctly 2. Incorrect choice: measurement tool is not appropriate for the volume or type ofmeasurement 3. Inadequate cleaning: carryover causes contamination 4. Infrequent calibration: measuring tool is not calibrated for use These four “I’s” can lead to error and contamination which negate all intent of careful measurementprocesses. Many errors can be avoided by understanding the markings displayed on the volumetric containers and choosing the proper tool for the job. There is a lot of information displayed on volumetriclabware. Most labware, especially glassware, is designated as either Class A (analytical or quantitative) or Class В (general use) labware. If a critical measurement process is needed, then only ClassA glassware should be used for measurement. Other information that can be found on labware is the name of the manufacturer, country oforigin, tolerance or uncertainty of the measurement of the labware, and a series of descriptors as tohow the glassware should be used. Labware can be marked with letters which designate the purposeof the container. If a volumetric is designed to contain liquid, it will be marked by either the lettersTC or IN. Labware which is designated to deliver liquid will be marked by either the letters TDor EX. Sometimes there are additional designations such as wait time or delivery time inscribedon the labware. The delivery time refers to a period of time required for the meniscus to flow fromthe upper volume mark to the lower volume mark. The wait time refers to the time needed for themeniscus to come to rest after the residual liquid has finished flowing down from the wall of thepipette or vessel.
- The Importance of Testing Cannabis: An Overview of the Analytical Techniques Used
- How is Cannabis Being Used?
- Cannabis or Marijuana?
- The Role of Analytical Chemistry
- Sampling Protocol for Cannabis in the Field
- Sample Preparation of Cannabis Plant Materials and Cannabis-Derived Products
- Heavy Metals and ICP-MS
- Organic Compounds in Cannabis
- Pesticides
- Potency: Cannabinoids
- Terpenes
- Microbiology
- Mycotoxins
- Residual Solvents
- Determination of Origin of Growth of Cannabis Using IRMS
- Biological Sample Analysis: Forensic Toxicology
- The Determination of THC in the Breath of Motorists
- Final Thoughts
- Acknowledgments
- Further Reading
- Main Factors for Metal Uptake from Soil
- Lead
- Mercury
- Cadmium
- Arsenic
- Cobalt
- Nickel
- Chromium
- Vanadium
- Manganese
- Other Elements of Concern
- Elemental Species/Metalloids
- Metallic Nanoparticles
- State-Based Heavy Metal Limits
- Potential of “Real-World” Sources of Elemental Contaminants in Cannabis
- Outdoor Growing Sources
- Indoor Growing Sources
- Manufacturing/Processing Sources
- The Smoking/Inhaling of Cannabis
- Testing Procedures
- Laboratory Testing Protocols
- A Word about Hemp Regulations
- Final Thoughts
- Further Reading
- Elemental Impurities in Pharmaceuticals: A Historical Perspective
- The Process for Change
- USP Implementation Process
- Validation Procedures
- Toxicity Classification
- How are PDEs Calculated?
- Cadmium PDE—Oral Exposure
- Cadmium PDE—Inhalation Exposure
- Potential Sources of Elemental Impurities in Drug Compounds
- Final Thoughts
- Further Reading
- Principles of Operation
- Ion Formation
- Natural Isotopes
- Aerosol Generation
- Droplet Selection
- Nebulizers
- Concentric Design
- Cross-Flow Design
- Microflow Design
- Spray Chambers
- Double-Pass Spray Chamber
- Cyclonic Spray Chamber
- Aerosol Dilution
- Final Thoughts
- Further Reading
- The Plasma Torch
- Formation of an ICP Discharge
- The Function of the RF Generator
- Ionization of the Sample
- Further Reading
- Capacitive Coupling
- Ion Kinetic Energy
- Benefits of a Well-Designed Interface
- Final Thoughts
- Further Reading
- Role of the Ion Optics
- Dynamics of Ion Flow
- Commercial Ion Optic Designs
- Further Reading
- Quadrupole Technology
- Basic Principles of Operation
- Quadrupole Performance Criteria
- Resolution
- Abundance Sensitivity
- Benefit of Good Abundance Sensitivity
- Magnetic Sector Mass Spectroscopy: A Historical Perspective
- Use of Magnetic Sector Technology for ICP-MS
- Principles of Operation of Magnetic Sector Technology
- Resolving Power
- Basic Principles of TOF Technology
- Commercial Designs
- Differences between Orthogonal and On-Axis TOF
- Benefits of TOF Technology for ICP-MS
- Rapid Transient Peak Analysis
- Improved Precision
- Rapid Data Acquisition
- Basic Principles of Collision/Reaction Cells
- Different Collision/Reaction Cell Approaches
- Collisional Mechanisms Using Nonreactive Gases and Kinetic Energy Discrimination
- Reaction Mechanisms with Highly Reactive Gases and Discrimination by Selective Bandpass Mass Filtering
- Dynamic Reaction Cell
- Low Mass Cut-Off Collision/Reaction Cell
- “Triple Quadrupole” Collision/Reaction Cell
- M/S Mode. MS/MS Mode
- On-Mass MS/MS Mode
- Mass Shift MS/MS Mode
- Channel Electron Multiplier
- Faraday Cup
- Discrete Dynode Electron Multiplier
- Extending the Dynamic Range
- Filtering the Ion Beam
- Using Two Detectors
- Using Two Scans with One Detector
- Using One Scan with One Detector
- Measurement Variables
- Measurement Protocol
- Optimization of Measurement Protocol
- Multielement Data Quality Objectives
- Data Quality Objectives for Single-Particle ICP-MS Studies
- Final Thoughts
- Further Reading
- Quantitative Analysis
- External Standardization
- Standard Additions
- Addition Calibration
- Spectral Interferences
- Oxides, Hydroxides, Hydrides, and Doubly Charged Species
- Isobaric Interferences
- Ways to Compensate for Spectral Interferences
- Mathematical Correction Equations
- Cool/Cold Plasma Technology
- Collision/Reaction Cells
- High-Resolution Mass Analyzers
- Matrix Interferences
- Compensation Using Internal Standardization
- Space Charge-Induced Matrix Interferences
- Further Reading
- Sample Introduction System
- Peristaltic Pump Tubing
- Nebulizers
- Spray Chamber
- Plasma Torch
- Interface Region
- Ion Optics
- Roughing Pumps
- Air Filters
- Other Components to Be Periodically Checked
- The Detector
- Turbomolecular Pumps
- Mass Analyzer and Collision/Reaction Cell
- Final Thoughts
- Further Reading
- Sample Preparation Procedures as Described in USP Chapter
- Grinding Solid Samples
- Cryogenic Grinding
- Collecting the Sample
- Typical Sampling Procedures for Cannabis
- Sample Dissolution
- Reasons for Dissolving Samples
- Digested Sample Weights
- Microwave Digestion Considerations
- Why Use Microwave Digestion
- Choice of Acids
- Commercial Microwave Technology
- Digestion Strategies for Cannabis
- Fundamental Principles of Microwave Digestion Technology
- Sequential Systems
- Rotor-Based Technology
- Single Reaction Chamber Technology
- Nitrogen-Pressurized Caps
- Sampling Procedures for Mercury
- Reagent Blanks
- Final Thoughts
- References
- Laser Ablation
- Commercial Laser Ablation Systems for ICP-MS
- Excimer Lasers
- Benefits of Laser Ablation for ICP-MS
- Optimum Laser Design Based on the Application Requirements
- nm Laser Technology
- Flow Injection Analysis
- Electrothermal Vaporization (ETV)
- Chilled Spray Chambers and Desolvation Devices
- Water-Cooled and Peltier-Cooled Spray Chambers
- Ultrasonic Nebulizers
- Specialized Microflow Nebulizers with Desolvation Techniques
- Direct Injection Nebulizers
- Productivity Enhancing Techniques
- Faster Analysis Times
- Automated In-Line Autodilution and Autocalibration Systems
- Automated Sample Identification and Tracking Systems
- Further Reading
- HPLC Coupled with ICP-MS
- Chromatographic Separation Requirements
- Ion Exchange Chromatography (IEC)
- Reversed-Phase Ion Pair Chromatography (RP-IPC)
- Column Material
- Isocratic or Gradient Elution
- Sample Introduction Requirements
- Optimization of ICP-MS Parameters
- Compatibility with Organic Solvents
- Collision/Reaction Cell or Interface Capability
- Optimization of Peak Measurement Protocol
- Full Software Control and Integration
- Final Thoughts
- Further Reading
- Understanding Data Accuracy and Precision
- Estimating Error
- Types of Errors
- Standards and Reference Materials
- Using Standards and Reference Materials
- Calibration Curves
- Dynamic Range, Concentration & Error
- Laboratory Sources of Error & Contamination
- Sources of Laboratory Contamination & Error
- Water Quality
- Reagents
- Laboratory Environment and Personnel
- General Principles and Practices
- Further Reading
- Commercial Reference Materials
- Alternate Reference Materials
- Quality Assurance Programs
- Final Thoughts
- Further Reading
- Vaping Liquid Solvent: The Nature of the Sample
- Choice of Liquid Solution Containers
- Microwave Digestion of Vaping Oils
- Liquid Sample Containers and Aerosol Collection Materials
- Vaping Machines and Trapping Materials
- Preparing for Analysis
- What Analytes Are Appropriate for Regulatory Purposes?
- ICP-MS Instrumentation
- Introduction Systems and Optimization
- Single Quadrupole-Specific Parameters
- “Triple Quadrupole”-Specific Parameters
- Final Thoughts
- Further Reading
- Basic Definitions
- Principles of Emission
- Atomic and Ionic Emission
- Instrumentation
- Sample Introduction
- Aerosol Generation
- Nebulizers
- Spray Chambers
- Torches
- Fore Optics
- Optical Designs
- Detectors
- Historical Perspective
- Photomultiplier Tubes
- Photodiode Arrays
- Charge Transfer Devices
- Charge-Coupled Devices
- Charge-Injection Devices
- Analytical Performance
- Dependence on Environmental Operating Conditions
- Exhaust Requirements
- Electrical Requirements
- Temperature and Pressure Requirements
- Maintenance
- Dependence on Plasma Operating Conditions
- RF Power
- Plasma Gases
- Pump Settings
- Plasma Viewing Height
- Precision and Accuracy
- Detection Limits
- Limit of Quantitation
- Background Equivalent Concentration
- Sensitivity
- Method Development Considerations
- Analytical Wavelength Considerations
- Interferences
- Physical Interferences
- Chemical Interferences
- Spectral Interferences
- Data Acquisition
- Method Validation
- Final Thoughts
- Further Reading
- Flame AAS
- Advantages of FLAAS
- FLAAS Interferences and Their Control
- Disadvantages of FLAAS
- Graphite Furnace AAS
- GFAAS Interferences and Their Control
- Advantages of GFAAS
- Disadvantages of GFAAS
- Vapor Generation AAS
- Advantages of Cold Vapor AAS
- Disadvantages of Cold Vapor AAS
- Hydride Generation AAS
- Advantages of Hydride Generation AAS
- Disadvantages of Hydride Generation AAS
- Hyphenated Techniques
- Atomic Fluorescence
- Advantages and Disadvantages of AFS
- Final Thoughts
- Further Reading
- X-Ray Fluorescence
- XRF Instrumental Configuration
- Quantitation by XRF
- XRF Detection Limits
- Sample Preparation for XRF
- X-Ray Diffraction
- Laser-Induced Breakdown Spectroscopy
- LIBS Fundamental Principles
- LIBS Capabilities
- LIBS Application Areas
- LIBS Detection Capability
- LIBS on Mars
- Microwave-Induced Plasma Optical Emission Spectroscopy
- Basic Principles of the MP-AES Technology
- Benefits of MP-AES
- Typical Applications of MP-AES
- Laser Ablation Laser Ionization Time-of-Flight Mass Spectrometry
- Basic Principles LALI-TOFMS
- Matrix Effects
- Diffusion and Transport
- Interferences
- Transmission Efficiency
- Inorganic and Organic Analysis
- Operational Use
- User Interface
- Performance Capabilities
- Final Thoughts
- Further Reading
- Flame Atomic Absorption
- Electrothermal Atomization
- Hydride/VAPOR Generation AA
- Atomic Fluorescence
- Radial ICP-OES
- Axial ICP-OES
- ICP-MS
- Comparison Highlights
- Demands of the Cannabis Industry
- Suitability of Technique
- Relationship between LOQ and J-Value
- Final Thoughts
- Further Reading
- Gases
- Electricity
- Consumables
- Cost per Sample
- Running Costs of Atomic Fluorescence
- Final Thoughts
- Further Reading
- Evaluation Objectives
- Analytical Performance
- Detection Capability
- Precision
- Isotope Ratio Precision
- Accuracy
- Dynamic Range
- Interference Reduction
- Reduction of Matrix-Induced Interferences
- Sample Throughput
- Transient Signal Capability
- Single-Particle ICP-MS Transient Signals
- Ease of Use
- Routine Maintenance
- Compatibility with Productivity and Performance Enhancing Tools
- Installation of Instrument
- Technical Support
- Training
- Service Support
- Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Glossary
- Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) Glossary
- Atomic Absorption and Atomic Fluorescence
- Other Atomic Spectroscopy Techniques
- Analytical Performance