UV-Vis

Requirements for UV-Vis measurements

• select desired wavelengths (usually 800-250nm) and speed of measurement

• calibrate instrument to ensure accuracy and reliability

• run baseline to establish a fixed reference point

Benefits and pitfalls of UV-Vis

Benefits: provides quick results, relatively easy to perform, nondestructive so don’t need to alter the sample (allows for further analysis). Provides information about concentration of substances

Pitfalls: Can struggle with low concentration samples; noise interference and instrument detection limits can impact measurement accuracy

When to use UV-Vis spectroscopy

• when want to determine color properties of liquids or solutions (qualitative)

• measuring concentrations of known substances (quantitative)

• when want to identify substances based on their absorption spectra

When to use fluorescence spectroscopy

• when wanting to monitor decay rate during photochemical reactions

• when wanting to determine at what concentration inner-filter effects will start to occur

• when wanting measure intensity of analytes with low concentrations

When to use atomic absorption spectroscopy (AA)

• when want to measure metal concentrations

When to use atomic emission spectroscopy (AE)

• when want to measure interactions between electromagnetic radiation and physical atoms and molecules

What is the effect of pH on absorbance? Why would pH change the absorbance

• pH can affect the equilibrium between different chemical forms of an analyte which can shift absorption peaks

• higher pH increases absorbance at higher wavelengths

• lower pH increases absorbance at lower wavelengths

Settings on instrument for Nicotinic acid experiment

slow scan mode, 800-250 nm range

pH used for nicotinic acid experiment

low pH because exhibited much clearer peaks, had much higher molar absorptivity, used pH of 2.84

what is the absorptivity coefficient associated with 0.0725 and 0.1312? What does this slope mean in relation to a 95-100% constant

slope= (m₁/m₂) x 100% = (0.0725/0.1312) x 100% = 55%. This point can be ignored when determining linear dynamic range, it has no use.

if niacin has a ppm pf 5.03, and absorbance of 0.36459, what is the concentration of niacin in solution 1 if the volume is 0.025 mL.

m = A/c = (0.36459/5.03) = 0.0725

LOD/LOQ/SD discussion (including your numbers, conclusions about these figures of merit)

LOD = 3(s_blank)/m, ended up being 4.44ppm, meaning that the method can detect the presence of the analyte at a concentration of 4.44 ppm with 95% probability.

What is SD? What does an SD value of 1 mean

The amount of variance in a set of values. An SD of 1 means that the set of values deviate from the average by 1 unit

What is LOQ

LOQ is the lowest concentration of a substance that can be measured with accuracy and precision.