Multi components Multi Multi components Multi wavelength calibrationwavelength calibration

Correlation between absorbance and concentration:

In the calibration process the correlation factors between absorbance and concentration are determined. These factors depend on path length and wavelength.

Beer Lambert’s law: The amount of light absorbed is proportional to the number

of absorbing molecules through which the light passes. The result of plotting absorbance Vs. concentration is shown

below. Where A is the absorbance, C concentration b path length

and e the extinction coefficients of the specific component.

A= b c

Fig 1: calibration curve of 5 standards the slope is equal to the extinction coefficient times the path length.

A= b c

Correlation at a specific wavelength: The absorbance at a specific wavelength can be correlated to concentration

the extinction coefficient at each wavelength are different.

A() = () C d

Where : A = absorbance = wavelength C = concentration d = path length () = extinction coefficient

The extinction coefficient, is a characteristic of a given substance under a precisely defined set of conditions of wavelength, solvent, temperature and other parameters. The measured extinction coefficient is also, to some extent, dependent upon the instrumental characteristics. For these reasons predetermined values for the extinction coefficient are not usually used for quantitative analysis. In practice, a calibration or "working" curve for the substance to be analyzed is usually constructed using one or more standard solutions with known concentrations of the analyte.

Single wavelength calibration (filter)

When calibrating a filter type spectrometer only one wavelength range is used, several standards with known concentration values are measured, a graph of the concentration Vs. Absorbance is plotted. Figure 2 and 3 demonstrate a single wavelength range calibration. In this example four standard of SO2 were measured by a photometer the concentration Vs. Absorbance was plotted (fig 3) the slope is the extinction coefficient for SO2 at the 270-295nm range at the specific path length.

Figure 2: absorbance values of four SO2 standards. A filter type photometer will output the absorbance at the marked (270-295nm) range only. SO2 absorbance spectra

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0.05

0.1

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0.25

0.3

0.35

0.4

240 250 260 270 280 290 300 310 320

nm

AU

960 ppm

1850 ppm

2460 ppm

3041 ppm

0.08

0.15

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0.25

Calibration curve using the SO2 standards shown in figure 2.

y = 8E-05x

R2 = 0.9993

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0 1000 2000 3000 4000

Concentration (PPM)

Ab

sorb

ance

(A

U)

Series1

Linear (Series1)

Multi wavelength calibration Now if instead of a photometer you are

using a full high resolution spectrum you can have many such calibration curves. If for example a wavelength range of 220-320nm is used with 1 nm resolution it will result in 100 different calibration curves (1 for each nm). The results are much more accurate since we are now averaging over 100 detectors instead of just 1. If we look at the Beer equation per wavelength:

A() = () C d

if C the concentration is 1 then A (absorbance) is equal the extinction coefficients times the path length. For C=1:

A() = () d

In the multi wavelength calibration the extinction coefficients times the path length at each wavelength are determined (() d).

The Math The sample’s Absorbance

value at specific wavelength  , is the absorptivity coefficient of the sample at that wavelength.

b is the path length C is the concentration. The absorptivity coefficient

for every component is different, but for a given compound at a specific wavelength, this value is a constant.

Calibration

Determine () the absorptivity coefficient

Analysis (one component one wavelength)

C = A / ( * b) K = * b C = A / K

Two components two wavelengths

A a * Ca

A b * Cb

Spectral overlap A a * Ca + A

b * Cb

A a * Ca + A b * Cb

Simultaneous equations A a * Ca + A

b * Cb +E1

A a * Ca + A b * Cb + E2

E1 & E1 are the errors between the theoretical and actual absorbencies

Matrix

2 wavelength 2 components matrix

P wavelengths n components

p>n

How to calibrate H2S RSH application

Start the analyzer in the offline mode by clicking on OMA OFFLINE ver 5_8

Click on Measure, load edit STD

Click on del std to delete the old H2S RSH standards – do not erase the H2S only standards . (standards 1-5)

Repeat del std until all the H2S RSH std are deleted

Click on new blank

Click on auto

The following screen will appear it will switch the nitrogen on for purge, it will take 2 minutes

If the analyzer was not running previously, wait 4 hours

If the analyzer was in the online mode and was just switched to the offline mode for recalibration wait 1 hour

After 1 or 4 hours (see previous slide) click on accept

The absorbance at the 220-240 nm range should be in the +/- 0.004 range

Click on new STD

Insert cal gas into the flow cell and wait 10 minutes

Click accept

Record P cal

Click here to enter the concentration of the standard

Enter the H2S concentration in the bottled gas in ppm/100 see example below

Enter the total RSH concentration in the bottled gas in % see example below

H2S concentration = 260 ppm; enter 2.6 (its later multiplied by 100); RSH = 0.51% enter 0.51

Click new std , wait 5 min and accept Edit and enter concentration Repeat this procedure 5 times so that

you will have a total of five new standards or a total of 10 standards.

Click on Stop

Click on save

Click on Calibrate & store coef.

Click on stop

Click on replace

Click here

Setup pressure Setup pressure compensationcompensation

Click here

Click here

Enter the P cal (see new std) in this example its 1.3

Should be on

The analyzer is now re calibrated

How to Span the H2S RSH application

The analyzer should be running continuously in the online mode for at least 4 hours.

Click on blank

If the analyzer was not running previously, wait 4 hours

If the analyzer was in the online mode for more then 4 hours then wait 5 minutes

Click on accept

The absorbance at the 220-240 nm range should be in the +/- 0.004 range

Insert cal gas into the flow cell Wait 10 minutes

Click on span

If default is on the concentration will be multiplied by 1 ; so there is no span correction, and the raw concentration is equal to the new concentration out

If default is off ; under enter new concentration insert the concentrations of the H2S and RSH as specified on the cal gas bottle. The new conc out will now be the same as the Enter new conc. Press save & exit to save the new span values.