Unit 1 - Analytical Concepts and Statistics Part 3 Flashcards
Significant figures
a reflection of a measurement’s magnitude and uncertainty. It consists of the number of digits known plus 1 uncertain digit (uncertain digit is usually the last digit in a number)
ex. measuring on an analytical balance:
1. 2637 +/- 0.0001 g, where the last digit is uncertain and significant
General sigfig rules
the digits to the right of the decimal point in pH is not significant. In 2.45, the 2 is not significant
an exact number has infinite number of sigfigs, like 105 dogs, or stoichiometric coefficients
if the number before rounding is 5, we round up if it will be an even number, and round down in it is an odd number
ex. rounding 12.45 gives us 12.4 while rounding 12.55 rounds to 12.6
Standard solutions
standard solutions are solutions with precisely known concentrations and sample solutions are solutions with unknown concentrations
Least squares method of analysis
minimizes the residuals between the data points and the line of best fit. Assumes there is only error in the y data
Empirical calibration curve assumptions
1) the underlying physical principle that relates signal to concentration is linear
2) any difference in our experimental data and the calculated regression line is the result of indeterminate errors only affecting y
3) indeterminate errors affecting y are normally distributed
4) the indeterminate errors in y are independent of the value of x (unweighted linear regression)
Calibration curve
it is a plot of measured signal versus a known quantity
Sensitivity
the slope (m) of the calibration curve, ie. signal per unit analyte
Dynamic range
the concentration range over which the calibration curve is analytically useful (ex. linear range)
Selectivity
in linear calibration plots, the selectivity of a method for compound 1 and compound 2 is reflected by the ratio of their slopes, m1/m2. Higher slope means it is more sensitive, therefore worse selectivity
selectivity is how good a method is in measuring a specific compound and ignoring the other distractions
Calibration standards
they should closely approximate analytical samples in the concentration of analyte and the composition of non-analyte species
Matrix effect
the combined effect of all non-analyte components in a sample on the quantitative measurements of the analyte. This terminology is used when specific interference is not known
ex.
some components in the sample generates a signal similar to the analyte
some component in the sample has a chemical interaction with the analyte
some component in the sample is co-isolated with the analyte
The standard addition concept
add different concentrations of the wanted analyte to the standards (ex. x0, x0+x1, x0+x2, etc.). The result is the amount of analyte that would need to be “removed” from the sample to get zero signal—that is, the amount of analyte in the original sample
Purpose of standard addition
used to minimize matrix effects that interfere with analyte measurement signals
Standard addition limitations
1) precise results are obtained when the amount of standard added is comparable in magnitude to the original quantity of analyte
2) time consuming
3) prone to dilution error
4) added standards should not overwhelm any interferences
5) may require large quantities of sample
Internal standard
an added sample of known quantity that is not expected to be found in the sample (different from the analyte), but is expected to behave similarly
the int. standard is analyzed with the analyte to account for loses during sample processing or fluctuations in instrument signals. Reference the analyte signal to the int. standard signal
