Quiz 3

Question 1

How does an ultrasonic nebulizer work?

Answer 1

Sample is sprayed at crystal, crystal vibrates creating spray, solvent then evaporates at aerosol chamber, solvent then condenses and is removed in condenser, some leaves out drain, remaining solvent vapor diffuses through membrane and is swept by Ar in membrane desolvator, analyte then reaches plasma as solid particles.

Question 2

At what speed does the piezoelectric oscillate at?

Answer 2

1 Mhz

Question 3

What is the benefit of an ultrasonic nebulizer?

Answer 3

conc of analyte needed for adequate signal is reduced by an order of magnitude (DL reduced by an order of magnitude)
Leads to increased mass transfer.

Question 4

What does a heated quartz cell do? ie the hydride technique?

Answer 4

involves heating a quartz cell to temps 850 - 1000 degrees celsius which contains the sample and turn sit inot a volatile hydride by adding hydrogen through Nabh4 to acidify it.

Question 5

Why do we wish to turn samples into volatile hydrides?

Answer 5

Arsenic and groups IV, V, VI form volatile hydides which makes it more stable and gives it a lower DL (easier seperated). elimintaes interference

Question 6

hat do you add to turn samples into volatile hydrides?

Answer 6

we add hydrogen via NABH4 to acidify it and then volatile it through quartz cell heat. Also eliminate sinteferences

Question 7

What two ways does temperature effect atomic spectroscopy?

Answer 7

Temp determines to which degree sample breaks down in (Flame atomic abs spectrom and Gas furnace atomic abs spectrum)
and the extent to which the atom is found in ground/excited/ionized state

Question 8

What does the boltzmann distribution describe?

Answer 8

Describes relative population od different states at thermal equilibrium (no heat transfer anymore)

Question 9

If thermal equilibrium exists what does the relative population of the higher e state and lower e state equal?

Answer 9

it equals exponential ^Energy diff divided by boltzmanns constant (times K=kelvin)

Question 10

Describe the effect of temp on Na atoms?

Answer 10

Increasing temp by 10K hardly effects ground state population and doenst effect atomic absorption signal
However it does effect atomic emission signal as it arises from the intensity proprtional to the population of excited state atoms. In groumd state Na atoms have less than 0.02% in the excited state, however when you increase temp by 10K excited state popln increases by 4% jncreasing emission signalm by 4%

Question 11

Why is it critical in atomic emission spectroscopy that the flame is very stable?

Answer 11

Because if higher temp emission intensity will vary significantly and give wrong results

Question 12

Whys is plasma normally used in emission?

Answer 12

Bcuz it’s so hot it increases atmoic emission signal by increasing excited state popln of atoms/ions

Question 13

What does beers law require?

Answer 13

Beers law requires that linewidth of the radiation source must be narrow than linewidth in atomic sample vapor, otherwise measured abs wont be equal to sample conc

Question 14

How are linewidths measured?

Answer 14

both radiation source linewidth and atomic sample vapor linewidth are measured at half the signal height.

Question 15

Why are atomic linewidths 10^-4 nm? What broadens them?

Answer 15

They are 10^-4 nm because of the short lifetime of the excited states- two mechanisms broaden them- doppler effects and pressure broadening

Question 16

What are doppler effects?

Answer 16

is when an atom moving toward the radiation source experiences lower frequency light which causes linewidth to lengthen

Question 17

hat is pressure broadening?

Answer 17

When atoms collide, the lifetime sof the excited state shorten and therefore lead to broader wavelengths

Question 18

What are hollow cathode lamps used for?

Answer 18

they are lamps used to narrow lines with the correct lamba as monocchromaor cant narrow lines smaller than 10^-3 to 10^-2 nm.

Question 19

what do hollow cathode lamps contain?

Answer 19

They contain a vapor of element that we want to analyze for and Neon or argon at a pressure of 1-5 Torr

Question 20

Describe how hollow cathode lamps work?

Answer 20

When 500 V applied between cathode and anode, it ionizes the gas n it and positive ions ctreated are accelerated towards cathode. After ionization occurs the lamps are maintained at a current of 2-30 mA.
Then cations strike cathode with enough energy to sputter metal atoms from cathode into gas phase.
Then gas atoms hit high e electrons to emit photons.
Atoms in lamp are cooler than atoms emitted by flame which leads to a smaller width of abs lines.

Question 21

What is an ICP-AES?

Answer 21

Stands for Inductively Coupled Plasma - Atomic Emission Spectroscopy

Question 22

What is good about ICP-AES?

Answer 22

Does not require many lamps and cam measure as many as 70 elements simultaneously, here we observe wavelengths from range 100-800 nm covering halogens and SPCN

Question 23

What are the two types of ICP-AES? Describe them?

Answer 23

Have PMT (photomultiplier tube)
- here atomic emission enters the polychromator and is dispersed into its component wavelengths by the grating at the bottom. 1 photo multiplier detector needed for each elements. Is expensive

CID detector
here atomic emisison enters, is reflected by a collimating mirror, then dispersed by a prism, and then dispersed by a grating. This raidation then lands on CID (charged injection device) whihc has 262000 pixels. You read each pixel one at a time- is cheaper.

In both 02 is an interference so we box with analyte with AR ti avoid forming O2 products.

Question 24

What is blooming? Does the Charged injection device do this?

Answer 24

Is when strong signals in pixels spille rover to neighbouring pixels, CID does not do this.

Question 25

Name which techniques have the highest and the lowest detection limits?

Answer 25

Inductively coupled plasma emission, flame atomic absorption, Graphite furnace atomic absorption, Inductively coupled plasma-mass spectrometry.

Question 26

Why is standard solution used in FAAS not suitable for ICP-based techniques and GFAAS?

Answer 26

Because you need need extremely pure water & acids to avoid contamination (trace metal grade acids, Teflon vessels)
- clear room conditions
are advisable for an ICP-
MS to avoid contamina-
tion with dust (you
cannot smoke in room
where Cd measured by
ICP-MS leads to increased
background)
standard solutions have
limited shelf life &
evaporation will change
concentration over time!

Question 27

Define interference?

Answer 27

is any effect that changes signal while analyte conc remains unchanged

Question 28

What are the 4 types of interference?

Answer 28

spectral interference, chemical interference, ionization interference, and isobaric interference

Question 29

describe spectral interference?

Answer 29

Refers to the overlap of analyte signals due to other elements or mlcls in the sample/flame or furnace.
Deal w this by chossing other wavelength for analyte instead of overlapping ones.

Question 30

Describe chemical interference?

Answer 30

chemical rxns decreasing conc of analyte atoms by hindering atomization,
ex: So42- and Po42- hinder atomization of Ca2+ in FAAS, so we increase temp.

Question 31

Describe ionization interference?

Answer 31

is the ionization of analyte atoms which decreases the conc of neutral atoms

Question 32

How and where does ionization interference occur?

Answer 32

This happens when analyzing alkali metals at low temps, this is because alkali metals have low ionization potentials so they are most extensively ionized, when ionized they have different energy levels than neutral atoms which causes a decreased energy signal from neutral atoms.

Question 33

At 2450 K and 0.1 Pa, how much is Na ionized and K ionized?

Answer 33

Na is ionized at 5%, K is ionized at 30%

Question 34

How do we compensate for interferences?

Answer 34

We use the method of standard addition where we add known quantities of analyte to a stable matrix ie water, in order to create a calibration curve in which we can compare the sample with the analyte to determine how much of the signal is from the analyte versus the interference.

Question 35

How does inductively coupled plasma help with interferences?

Answer 35

It eliminates many common interferences as it is 2x as hot as a flame which leads to more complete and intense atomization. There is also no formation of refactory oxides/hydroxides which cause interference bcuz the plasma has argon not oxygen. Also there is no background radiation, and no self absorption (due to uniform temp)

Question 36

Are calibration curves more linear with flame/furnace absorption or plasma emission?

Answer 36

Plasma emission as it produces much more linear curves over 5 orders of magnitude compared to the 2 orders of magnitude from flame/furnace

Question 37

What does ICP-AES stand for?

Answer 37

It stands for Inductively coupled plasma atomic emission spectroscopy

Question 38

What temp does ICP-AES work at? How does it differ from atomic absorption spectroscopy and atomic fluorescence spectroscopy?

Answer 38

Here high temp plasma >6000 K is used to ionize/atomize the sample, the emission intensity of these atoms are used to measure the conc of each element.

They differ cause no lamp is needed

Question 39

What does ICP-MS stand for?

Answer 39

This stands for Inductively Coupled plasma mass spectroscopy

Question 40

What can ICP-MS measure?

Answer 40

This can measure metals and metalloids at very low conc’s and isotope ratios

Question 41

Explain how ICP-MS works?

Answer 41

Works by using AR plasma which has an ionization energy greater than 15.8 eV, greater than all elements except He, Ne, and F.
First the plasma ionizes the analyte by collisons with Ar, excited Ar atoms, or energetic electrons, then it is directed onto a water-cooled Ni sampling cone with a 1 mm diameter orifice (aperture) through which a fraction of plasma passes, the extraction lense after this attracts positive ions from the plasma. These ions then enter the rxn cell which reduces isobaric interference. Ions are then seperated by a Mass spectrometer according to their mass to charge ratio with selected ions deflected into the detector where they are then counted.

Question 42

Why does the ICP-MS have so many vacuum pumps?

Answer 42

to maintain a low pressure as a specific pressure gradient is needed to carry the ions to the mass spectrometer

Question 43

Describe an example using ICP-MS

Answer 43

Hawaiian and cuban coffee beans were extracted with trace metal grade nitric acids and then that aq extract was analyzed using ICP-MS. Shows that cuban coffee has much more Hg equal to the conc of Pb (15 ng/ml) compared to hawaiian.

Question 44

Why are calibration standards made in the same matrix as the unknown?

Answer 44

It’s to avoid differences in signals due to having diff matrixes as this can have a big effect on yield of ions generated in plasma.

Question 45

What is isobaric interference?

Answer 45

Ar is an inert gas but can form problematic ions in plasma such as ArH+, ArC+, ArN+, ArCl+ which can be problematic as they have the same masses as some analytes. This causes isobaric interference: interference by ions of similar mass to charge ratio.

Question 46

What do we do to combat isobaric interference?

Answer 46

We either use a high resolution ICP-MS or used a DRC (dynamic reaction cell)

Question 47

What is a DRC and how does it work?

Answer 47

This can covert argon into a new species quantitatively through ammonium, do this by fusing ammonium into cell which causes ions to bump into ammonium, this seperates ions letting upper and lower masses pass removing any problematic ions from being made.

Question 48

What is the biggest weakness of ICP-MS?

Answer 48

Salt, builds up on sampling cone

Question 49

What is the Geotraces program?

Answer 49

Using big ICP-MS to analyze ocean chemistry as worried about phytoplankton death, focused on characterizing depth distribution of trace metals and their isotopes in the ocean.