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# calibration techniques - PowerPoint PPT Presentation

Calibration Techniques. 1. Calibration Curve Method 2. Standard Additions Method 3. Internal Standard Method. Calibration Curve Method. Most convenient when a large number of similar samples are to be analyzed. Most common technique. Facilitates calculation of Figures of Merit.

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### Calibration Techniques

1. Calibration Curve Method

3. Internal Standard Method

### Calibration Curve Method

Most convenient when a large number of similar samples are to be analyzed.

Most common technique.

Facilitates calculation of Figures of Merit.

### Calibration Curve Procedure

Prepare a series of standard solutions (analyte solutions with known concentrations).

Plot [analyte] vs. Analytical Signal.

Use signal for unknown to find [analyte].

### Example: Pb in Blood by GFAAS

Results of linear regression:

S = mC + b

m = 5.56 mAbs/ppb

b = 0.93 mAbs

A sample containing an unknown amount of Pb gives a signal of 27.5 mAbs. Calculate the Pb concentration.

S = mC + b

C = (S - b) / m

C = (27.5 mAbs – 0.92 mAbs) / 5.56 mAbs / ppb

C = 4.78 ppb

(3 significant figures)

Calculate the LOD for Pb of 27.5 mAbs. Calculate the Pb concentration.

20 blank measurements gives an average signal

0.92 mAbs

with a standard deviation of

σbl = 0.36 mAbs

LOD = 3 σbl/m = 3 x 0.36 mAbs / 5.56 mAbs/ppb

LOD = 0.2 ppb

(1 significant figure)

Find the LDR for Pb of 27.5 mAbs. Calculate the Pb concentration.

Lower end = LOD = 0.2 ppb

(include this point on the calibration curve)

SLOD = 5.56 x 0.2 + 0.93 = 2.0 mAbs

(0.2 ppb , 2.0 mAbs)

Find the LDR for Pb of 27.5 mAbs. Calculate the Pb concentration.

Upper end = collect points beyond the linear region and estimate the 95% point.

Suppose a standard containing 18.5 ppb gives rise to s signal of 98.52 mAbs

This is approximately 5% below the expected value of 103.71 mAbs

(18.50 ppb , 98.52 mAbs)

Find the LDR for Pb of 27.5 mAbs. Calculate the Pb concentration.

LDR = 0.2 ppb to 18.50 ppb

or

LDR = log(18.5) – log(0.2) = 1.97

2.0 orders of magnitude

or

Find the Linearity of 27.5 mAbs. Calculate the Pb concentration.

Calculate the slope of the log-log plot

Not Linear?? of 27.5 mAbs. Calculate the Pb concentration.

Not Linear?? of 27.5 mAbs. Calculate the Pb concentration.

Remember of 27.5 mAbs. Calculate the Pb concentration.

S = mC + b

log(S) = log (mC + b)

b must be ZERO!!

log(S) = log(m) + log(C)

The original curve did not pass through the origin. We must subtract the blank signal from each point.

Corrected Data of 27.5 mAbs. Calculate the Pb concentration.

Linear! of 27.5 mAbs. Calculate the Pb concentration.

### Standard Addition Method of 27.5 mAbs. Calculate the Pb concentration.

Most convenient when a small number of samples are to be analyzed.

Useful when the analyte is present in a complicated matrix and no ideal blank is available.

### Standard Addition Procedure of 27.5 mAbs. Calculate the Pb concentration.

Add one or more increments of a standard solution to sample aliquots of the same size. Each mixture is then diluted to the same volume.

Prepare a plot of Analytical Signal versus:

volume of standard solution added, or

### Standard Addition Procedure of 27.5 mAbs. Calculate the Pb concentration.

The x-intercept of the standard addition plot corresponds to the amount of analyte that must have been present in the sample (after accounting for dilution).

the curve is linear over the concentration range

the y-intercept of a calibration curve would be 0

### Example: Fe in Drinking Water of 27.5 mAbs. Calculate the Pb concentration.

The concentration of the Fe standard solution is 11.1 ppm

All solutions are diluted to a final volume of 50 mL

-6.08 mL of 27.5 mAbs. Calculate the Pb concentration.

[Fe] = ? of 27.5 mAbs. Calculate the Pb concentration.

x-intercept = -6.08 mL

Therefore, 10 mL of sample diluted to 50 mL would give a signal equivalent to 6.08 mL of standard diluted to 50 mL.

Vsam x [Fe]sam = Vstd x [Fe]std

10.0 mL x [Fe] = 6.08 mL x 11.1 ppm

[Fe] = 6.75 ppm

### Internal Standard Method of 27.5 mAbs. Calculate the Pb concentration.

Most convenient when variations in analytical sample size, position, or matrix limit the precision of a technique.

May correct for certain types of noise.

### Internal Standard Procedure of 27.5 mAbs. Calculate the Pb concentration.

Prepare a set of standard solutions for analyte (A) as with the calibration curve method, but add a constant amount of a second species (B) to each solution.

Prepare a plot of SA/SB versus [A].

### Notes of 27.5 mAbs. Calculate the Pb concentration.

The resulting measurement will be independent of sample size and position.

Species A & B must not produce signals that interfere with each other. Usually they are separated by wavelength or time.

### Example: Pb by ICP Emission of 27.5 mAbs. Calculate the Pb concentration.

Each Pb solution contains 100 ppm Cu.

No Internal Standard Correction of 27.5 mAbs. Calculate the Pb concentration.

Internal Standard Correction of 27.5 mAbs. Calculate the Pb concentration.