Analytical II Exam 1

signal

sample dependent signal output

noise

random (or almost) time-dependent signal output (waiting longer to get the desired signal would produce more noise)

baseline

constant average minimum signal output

what defines the quality of the spectrum?

signal to noise ratio

what are the types of noise?

thermal/white/johnson noise, shot noise, 1/f noise/flicker/pink noise, electrical interference

which types of noise can be avoided?

all except for thermal/white/johnson noise

thermal/white/johnson noise

normal random motion of charge carriers due to temperature; would disappear at absolute zero

shot noise

big signal for an instant; random; due to quantized signal carriers, electrons, photons; to reduce it, increase light intensity

1/f noise, flicker/pink noise

arises from using low frequencies because the rate at which we collect data impacts what data looks like; material dependent; the solution is to detect data more frequently

electrical interference

environmental noise, microphonic noise (vibrational); can be lessened by shielding

UV light source

deuterium, xenon, mercury

IR light source

Nernst glower

visible light source

tungsten filament lamps

IR detectors

thermocouple, thermistor, pyroelectric crystal (DTGS, MCT, CCD)

UV-VIs detectors

CCD, phototube, photomultiplier tube

CCD

an array of diodes for multichannel; light passes through the sample onto the array; used in phone cameras; silicon crystal lattice

photomultiplier

single channel, a photomultiplier tube detects single photons by converting them into electrons at a photocathode, which are then amplified through a cascade of dynodes, where each electron knocks loose multiple secondary electrons, resulting in a large electrical signal at the anode, allowing for the detection of very faint light level

fourier transform

converts frequency domain data to time; applies a filter to selectively reduce frequencies; then inverse FFT converts it back from time to frequency; while this can reduce noise, it can also result in lower peaks because there is a faster decay indicating destructive interference

what happens to time domain graphs as peaks become wider?

there is more destructive interference so the waves decay more quickly when peaks are wider; this is seen in FFT which can make peaks wider

how are filters helpful in FFT?

they emphasize more accurate, earlier readings

examples of FFT

audio editing, spectrum analysis

what is aliasing and how to avoid it?

appearance of peaks mirroring real peaks around ½ f due to sampling at less than two times the frequency; avoid sampling at less than two times the frequency

formula for number of spectra and S/N ratio

original S/N ratio times square root of number of scans equals improved S/N ratio

kinetic plot vs calibration plot

kinetic plot has time as the x axis and shows reaction proceeding over time often to find reaction rate; a calibration plot shows the absorbances at different concentrations often to find unknown concentrations at known absorbances

how many nm is in one meter

10 to the ninth

how to convert from nm to joules

1.987×10^-16 over the number of nm = number J