part 5 quantitative analysis Flashcards
DEF qualitative analysis
Analysis in which substances are identified or classified on the basis of their chemical or physical properties, such as chemical reactivity, solubility, molecular weight, melting point, radiative properties (emission, absorption), mass spectra, nuclear half-life, etc.
DEF quantitative analysis
Analyses in which the amount or concentration of an analyte may be determined (estimated) and expressed as a numerical value in appropriate units.
Qualitative analysis may take place without quantitative analysis, but quantitative analysis requires the identification of the analytes which numerical estimates are given.
DEF quantity
The value of a quantity is generally expressed as the product
of a number and a unit
Units
The international system of units is the preferred system of units, the basic language of science. In the SI, there are seven base units from which derived units are constructed: - length in m - mass in kg - amount of substance in mol - ... Derived units: - volume in m^3 (litres is a non-SI units but accepted) - concentration in mol.m^-3 - density in kg.m^-3
Multiples and submultiples of SI
- 10 = deca
- 10^2 = hecto
- 10^3 = kilo
- 10^6 = mega
- 10^9 = giga
- 10^12 = tera
- 10^15 = peta
- 10^-1 = deci
- 10^-2 = centi
- 10^-3 = milli
- 10^-6 = micro
- 10^-9 = nano
- 10^-12 = pico
- 10^-15 = femto
Expression of the concentration
c = mass of substance / mass of sample c = mass of substance / volume of sample c = amount of substance n / volume of sample
Unit n: amount of substance
n (amount of substance) is the number of specified elementary entities. One mole contains exactly 6.022x10^23 elementary entities.
n of a sample of entity = mass of the sample / molar mass of the entity
Result and uncertainty
All the measurements (methods) are affected by error that determine the uncertainty of the result. The results is expressed by an number (often the mean) and the associated uncertainty.
Result => Xmean +- s
The uncertainty determines the number of digits to be used in the expression of the result. Significant figures = digits in a number representing the results know with certainty + the first uncertain digit
Types of errors
- random errors do not have a pattern, unpredictable, unavoidable (also known as “indeterminate” errors); they can be reduced, not eliminated; due to random fluctuations in the replicated experiments, changes in environmental conditions, etc. Basis of the analytical property of precision.
- systematic errors occur in the same direction (over or underestimation); they can be eliminated; examples, offset error in the instrument, consistent loss of analyte in procedural step. In principle can be eliminated. Basis of the analytical property of trueness.
- gross errors: outliers far above/below the other data (from operator carelessness (wrong labels, calculations…), instrumental fault…).
- replication is essential for obtaining reliable estimates of method performance characteristics
Accuracy (qualitative description) depends on
Truesness + precision
DEF precision
Precision is a measure of how close results are to one another. It is expressed by statistical parameters which describe the spread of results obtained from replicate measurements on a given sample.
Statistical parameters describing the precision
- standard deviation & relative standard deviation (RSD)
Xmean = (X1 + X2 + X3 + … + Xn)/n
s = sqrt ( (sum(Xi - Xmean)^2 / (n-1) )
with Xi result of the measurement
RSD = s / Xmean (can be also expressed in %)
To determine if the result is accurate, the difference between the value and the mean value must be low. To determine if the result is precise, the difference between each value and the mean value must be approx the same for all.
- Repeatability: expected to give the smallest variation in results. it is a measure of the variability in results when a measurement is performed by a single analyst using the same equipment over a short timescale.
- Reproducibility: expected to give the largest variation in results. It is a measure of the variability in results between laboratories. Different analysts. Standard deviation is higher but better indication of the precision of the method.
- intermediate precision: gives an estimate of the variation in results when measurements are made in a single laboratory but under conditions that are more variable than repeatability conditions.
Method accuracy
When the expected value is known. It expresses the closeness of a single result to a ref value. Method validation seeks to investigate the accurary of results by assessing both systematic and random effects on single results. Accuracy is normally studied as two components: trueness and precision.
Trueness: how close the mean of an infinite number of results is to a ref value. Cannot be measured since it is not possible to take an infinite number of measures. We can make a partical assessment of the trueness. This assessment is expressed quantitatively as “bias”.
BIAS = Xmean - Xref
(the higher the bias the lower the trueness)
The bias can be determined by the analysis of a ref material or recovery experiments using spiked samples (matrix with a known quantity of pure analyte).
Relative recovery = Xmean / Xspike x 100
DEF calibration
The set of operations which establish, under specified conditions, the relationship between values indicated by the analytical instrument and the corresponding known values of an analyte.
A material of known composition or properties can be used on the analytical instrument for calibration purposes.
Response curve from calibration obtained from calibration solutions of known concentration:
GC peak area = coeff a * analyte concentration + b
DEF sensitivity
The change in instrument response which corresponds to a change in the analyte concentration. It is the gradient of the response curve. It corresponds to the slope a in the relationship of the response curve of calibration.
