ANALYTICAL PROCESS Audrius Maruška [email protected] Vytautas Magnus University Dept. of Chemistry 10 –14 November, 2003
ANALYTICAL PROCESS
Audrius Maruš[email protected]
Vytautas Magnus UniversityDept. of Chemistry
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10 –14 November, 2003
Objectives• Role of analytical chemistry solving the
problems defined by clients• Importance of interaction between client
and analyst• Various steps in the total analytical
process• Basic characteristics of the performance
of analytical methods
Broad scope of samples
PigmentsRestoration/Ancient painting
BloodClinics/Cholesterol content
Metal analysisMining industry/Ores
Tap waterPublic health/Assessment of purity
Fresh water samplesEnvironment/Pollution degree
SoilAgrochemistry/Need for fertilizers
Raw materials and finished products
Chemical industry
SampleArea/Object
: WE NEED A COMMON LANGUAGE!!!
Investigation subject
Investigation subject Bulk matter/Object
Part of the object Sample
Compounds or elements Analytesof interest
Analytes are embedded in Matrix
• The analysis of objects or test samples must be performed with a clear objective
• The problem is formulated by others clients e.g. process engineer, farmer, water control authority, steel production
industry, physician, museum director.
• The problem often is not put in analytical terms. Analyst should recollect the information necessary to reformulate the problem in to analytically manageable dialog with client
• Extensive tool kit of the analysthow to select proper approach?
• The are no “best” methodsindividual problem will ask for a specific approach
• What you can learna feeling for the systematic development of analytical
procedures• What you need?
knowledge of the most important analytical techniques
• How to compare/evaluate different methods?objective way is to compare performance
characteristics
Other important factors1. Availability of equipment 2. Expertise
3. Presence of trained personnel
Scheme of the TOTAL ANALYTICAL PROCESS
Analyst Client9. ReportAnalyst8. ConclusionsAnalyst7. Evaluation of dataAnalyst6. MeasurementAnalyst5. Sample preparationClient Analyst4. SamplingAnalyst3. Selection of procedure
Client Analyst2. Specific analytical statement of problem, Definition of objective
Client1. General statement of problem
ALL STEPS AFFECT THE FINAL RESULT!!!
General definition of problem• Often beyond the field of expertise of the analyst• Often outside the chemical domain• Client often does not appreciate the opportunities
and technical aspects of analysisWhat is needed intensive discussion
before problem solving to make clear the real basis of the analysis
Specific analytical statement of problem
• Client and analyst what is the object of study, i.e.
1. What has to be analyzed?2. What can be analyzed?
Characteristic questions to be solved:1. The nature of the object or sample, functional
groups, molecular composition.2. Quantitative or qualitative analysis, what accuracy (in
case of quantitative analysis) is needed?3. Quantity of material available, approximate
concentration of analyte?4. Matrix composition?5. Single component or multicomponent analysis?6. How much time is available?7. Unique analysis or recurrent supply of samples, if so
with what frequency? Continuous monitoring?8. Should integrity of the sample be preserved? etc.
Selection of procedure• Of crucial importance • Determines the cost
a) instrumental b) personnel
• Also influenced by :a) sample sizeb) time availablec) information extracted out of analytical
investigation
Sampling
• Tendency to underestimate importance of sampling: when dealing with homogeneous samples
• Reliability of analytical result depends on the quality of the sample
• Whole object can be investigated by nondestructive method (antique ring by X-ray)
• Mostly samples have to be taken (ore samples from the carrier ship)
• Important operations:a) Definition of a sampling planb) Taking of the samples as suchNever start an analytical procedure without
knowledge of the sample history: sampling process, storage, preservation, pretreatment etc. and of its representativity to whole object
Avoid any alterations of sample compositionImportant to know type of analyte, concentration
level, composition of matrixSafety precautions
Solid materials
1. Representative2. Not be larger than necessary, size
depends on:a) Precision requiredb) Material heterogeneityc) Grain sizeAmount up to several kg or even several
hundreds kg
Liquid and gases
1. Usually sufficiently homogeneous2. Small samples therefore sufficient3. Adsorption may be risky due to large
surface–to–volume ratio in the sample vessels:
Deactivate or equilibrate the walls of vessels
Sampling tools are outside of the scope of this presentation
Transport and storage
• Avoid sample changes due to reactions induced by vigorous shaking
• Time between sampling and sample investigation is important
• Adsorption of trace compounds on the container walls should be avoided
Sample preparation
• Important: adaption of the state of the sample to that acceptable by the analytical technique
• Consider that the concentration range in the adapted sample is optimum: use dilution or enrichment
• Suppress the contribution of unwanted compounds: “masking” procedure
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Measurements/Determination
• Typical instrumental techniques are based on signal comparison of unknown composition sample and (series of) standard(s) of exactly known compositions: calibrate the instrument
Important: estimate unwanted fluctuations in signal-noise
Signal- to- noise ratio is optimized through data acquisition and data processing
Evaluation of data
• Most of the modern instruments are computerized mathematical processing is used to transfer data to analytically relevant (concentration, structural information etc.)
• Analyst can not check and does not have access the raw data: software should be validated
• Use “ library search” procedures for data evaluation
Conclusions and report
• Analyst is responsible for the analytical results presented in the report
• To warrant the quality:a) Test procedures on certified samples (various
methods in various laboratories)b) Compare the results obtained with proposed
procedure to the results obtained with other procedures
The conclusions to be drawn on the basis of the analytical results (by client) is responsibility of the analyst
Performance characteristics• Analytical method can be characterized
on the criteria based on the quality of the result and criteria of economic origin
•Costs of investment• Duration of analysis with regard to personnel costs•Special safety precautions•Costs of installation, housing•Degree of training of operator because of salary•Sample throughput•Costs of reagents
•Sensitivity•Precision•Bias•Accuracy•Limit of detection•Limit of determination•Selectivity•Dynamic range
Economic aspectsQuality criteria
• Accuracy – the agreement between the average of the measured values and the accepted reference value
• Precision – is the scatter of the measured values around the average value
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Accurate and precise
Inaccurate but precise
Accurate but not precise
Inaccurate and not precise
• Sensitivity – signal change dependence on change of concentration
S = dY/dc• Limit of detection – equal to k times the
standard deviation of the background noise. Typically k = 3.
• Limit of determination – used for quantitative analysis is always higher than the limit of detection
• Selectivity – gives an indication how strongly the result is affected by other sample components
You learned:• Role of analytical chemistry solving the
problems defined by clients• Importance of interaction between client
and analyst• Various steps in the total analytical
process• Basic characteristicsof the performanceof analytical methods