Lecture Notes Friday 19th March 2007

Answers to quiz.

Question Answer
What does the abbreviation ICP stand for? Inductively coupled plasma
What is one major advantage of ICP-based techniques compared with AAS? Can determine mutiple elements simultaneously
Does the intensity of atomic emission depend on the temperature of the atoms? YES definitely. See pp 434 and 435
The ICP-OES detection limit for arsenic is 2 ppb.  What is the detection limit in nmol L-1?  At. wt. As = 75 g mol-1 27 nmol L-1
You have a stock 10.00 ppm solution.  You want to make 50 mL of a 20 ppb solution.  What volume (in µL) do you take? 100 µL

 

You need to remember that:

1 ppb is 1 ng per mL or 1 µg per L and that 1 ppm is 1 µg per mL or 1 mg per L.

So 2 ppb of As is 2 x 10-6 g per L. To convert from mass to moles divide by the molar mass.

Remember that:

number of mol = (mass in g) divided by (molar mass)

So 2 ppb is 2 x 10-6 / 75 mol per L. To convert to nmol multiply by 10^9^.

So 2 ppb is 2 x (10-6 / 75) x 10^9^ nmol per L = 27

Also remember that:

new concentration = old concentration x dilution factor

dilution factor = volume taken/final volume.

Concentrations and volumes have to have the same units, respectively.

So 20 = 10,000 x V/50,000

Therefore V = 100 µL.

 

Interference Effects in Atomic Spectrometry.

Two sorts: spectral and chemical.

ICP: Greater freedom from chemical interferences than AAS? Yes, the plasma is very hot and breaks all chemical bonds.

ICP: Greater freedom from spectral interferences than AAS?  No, the hot plasma emits significant background radiation (it functions as a black body) and the noise on the background limits the lowest light intensity from an analyte than can be measured and hence the detection limit. Also, some elements have lots of lines, and at high concentration these lines can be broad enough to swamp a neighboring weak line for another (analyte) element. May need to select another analyte line

AA spectrometers come with background correction to remove effets from absorption of light by stable compounds. "Optional" extra, but is needed if ETAAS is to be used.

Suppression of atomization can be dealt with by calibration: either matrix matchng the calibration standards or the standard additions method may be used.

 

Standard additions method of calibration.

A 25-mL portion of a sample solution is diluted to 50 mL with distilled water.  To a second 25-mL portion is added 5.0 mL of 100 ppm strontium standard solution, and the mixture diluted to 50 mL.

What concentrations of strontium have been added to (a) the first solution, and (b) the second solution?

Answer: 0 ppm and 10 ppm

When sprayed into an atomic absorption spectrometer, the absorbances measured are 0.14 and 0.34.  What is the strontium concentration in the original sample?

Solve the simultaeous equations to find the slope b and intercept a of the plot of absorbance against concentration. The solve for teh concentration when Abs = 0 (- 7 ppm). Therefore the concentration in the original sample is 14 ppm (see Fig 20-17 in the book).

 

 

 

Atomic Fluorescence Spectrometry

Need a bright light source (boosted hollow cathode). Direct line fluorescence used, but limited to As, Hg, and Se by HG (hydride generation) with flame atomizers. Several commercial instruments capable of LOD down to maybe 10 ppt. Instruments are not very expensive and many labs are interested in measureing these elements in clinical and environmental materials