Spectrophotometers have some stray light or detector dark current which causes the calibration to be non linear at high absorbances.

This program corrects for the calibration curvature caused by the presence of stray light.

Absorbance A = log(Io/I) .... ideal

Io is the incident light intensity and I is the transmitted light intensity.

A' = log(Io+Is)/(I+Is) .... measured absorbance

Is is the stray light intensity (or the dark current in the detector)

A = A'-log(k(1-10

where k = Is/Io and 100k = % stray light

k = (10

C1 and C2 are standards and A1' and A2' are the measured absorbances.

k2 = (10

Substituting A1 in equation (1)

A1 = A1'-log(k1(1-10

For ideal absorbances A2 = A1(C2/C1)

A2 = (A1'-log(k1((1-10

Equation (3) can be substituted in equation (2) and an iteration runs until (k1 - k2)*100 = 0 within an error tolerance determined by the FIX, SCI or ENG significant digits setting.

The value for k is now set equal to k1, the last value calculated.

100k = % stray light

You can enter f USER to avoid the f or GSB key during data input.

Input two standard concentration values C1 ENTER C2 GSB A.

Input two corresponding absorbances A1' ENTER A2' GSB B.

The % stray light is displayed. This is stored in register 8.

Input an unknown sample absorbance A' GSB D the resulting concentration C' is displayed and stored in register 9

Standard data: (Concentrations C1, C2 = 5, 10, Absorbances A1', A2' = 0.53, 0.83).

5 ENTER

1 0 GSB A

. 5 3 ENTER

. 8 7 GSB B

Stray light is 9.62%

This curvature is not unusual for Atomic Absorption spectrometry. Spectrophotometry can be much more linear in the near absence of stray light.

Unknown sample absorbance is 0.74

. 7 4 GSB C

The unknown sample concentration is 7.66 units, using the same concentration units as the standards.

Repeat A' GSB C for further samples.

Name | Description | |
---|---|---|

A | C1 Enter C2 GSB A, Enter concentrations | |

B | A1' Enter A2' GSB B, Enter measured absorbances | |

C | Enter sample absorbance GSB A' = concentration C' | |

1 | Start of loop |

Name | Description | Name | Description | ||
---|---|---|---|---|---|

1 | C1 concentration of standard 1 | 8 | % stray light | ||

2 | C2 concentration of standard 2 | 9 | Concentration C' | ||

4 | Stray light fraction | .1 | A1' absorbance of standard 1 | ||

5 | Stray light old | .2 | A2' absorbance of standard 2 | ||

6 | Temp 1 | .3 | A' sample absorbance | ||

7 | Temp 2 | I | Loop counter register, decrement 20 to 0. |

Line | Display | Key Sequence | Line | Display | Key Sequence | Line | Display | Key Sequence | |||
---|---|---|---|---|---|---|---|---|---|---|---|

000 | 027 | 43 13 | g LOG | 054 | 43,30, 0 | g TEST x≠0 | |||||

001 | 42,21,11 | f LBL A | 028 | 30 | − | 055 | 22 1 | GTO 1 | |||

002 | 44 2 | STO 2 | 029 | 44 6 | STO 6 | 056 | 45 4 | RCL 4 | |||

003 | 33 | R⬇ | 030 | 45 2 | RCL 2 | 057 | 26 | EEX | |||

004 | 44 1 | STO 1 | 031 | 45 1 | RCL 1 | 058 | 2 | 2 | |||

005 | 43 32 | g RTN | 032 | 10 | ÷ | 059 | 20 | × | |||

006 | 42,21,12 | f LBL B | 033 | 20 | × | 060 | 44 8 | STO 8 | |||

007 | 44 .2 | STO . 2 | 034 | 44 7 | STO 7 | 061 | 43 32 | g RTN | |||

008 | 33 | R⬇ | 035 | 45 .2 | RCL . 2 | 062 | 42,21,13 | f LBL C | |||

009 | 44 .1 | STO . 1 | 036 | 45 7 | RCL 7 | 063 | 44 .3 | STO . 3 | |||

010 | 2 | 2 | 037 | 30 | − | 064 | 1 | 1 | |||

011 | 0 | 0 | 038 | 13 | 10ˣ | 065 | 45 .3 | RCL . 3 | |||

012 | 44 25 | STO I | 039 | 1 | 1 | 066 | 13 | 10ˣ | |||

013 | 0 | 0 | 040 | 30 | − | 067 | 30 | − | |||

014 | 44 4 | STO 4 | 041 | 1 | 1 | 068 | 45 4 | RCL 4 | |||

015 | 42,21, 1 | f LBL 1 | 042 | 36 | ENTER | 069 | 20 | × | |||

016 | 45 4 | RCL 4 | 043 | 45 .2 | RCL . 2 | 070 | 1 | 1 | |||

017 | 44 5 | STO 5 | 044 | 13 | 10ˣ | 071 | 40 | + | |||

018 | 45 .1 | RCL . 1 | 045 | 30 | − | 072 | 43 13 | g LOG | |||

019 | 1 | 1 | 046 | 10 | ÷ | 073 | 30 | − | |||

020 | 45 .1 | RCL . 1 | 047 | 44 4 | STO 4 | 074 | 45 6 | RCL 6 | |||

021 | 13 | 10ˣ | 048 | 45 5 | RCL 5 | 075 | 10 | ÷ | |||

022 | 30 | − | 049 | 30 | − | 076 | 45 1 | RCL 1 | |||

023 | 45 5 | RCL 5 | 050 | 26 | EEX | 077 | 20 | × | |||

024 | 20 | × | 051 | 2 | 2 | 078 | 44 9 | STO 9 | |||

025 | 1 | 1 | 052 | 20 | × | 079 | 43 32 | g RTN | |||

026 | 40 | + | 053 | 43 34 | g RND |