StrayLightP - Stray light curve correction

Description

J E Patterson - jepspectro.com - version 20190904

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

Absorbance = Log (Incident light Intensity)/(Transmitted light Intensity).

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(1-10A')+1 ....................................................... (1)
where k = Is/Io and 100k = % stray light
k = (10(A'-A)-1)/(1-10A')
C1 and C2 are standards and A1' and A2' are the measured absorbances.
k2 = (10(A2'-A2)-1)/(1-10A2') ............................................... (2)
Substituting A1 in equation (1)
A1 = A1'-log(k1(1-10A1')+1)
For ideal absorbances A2 = A1(C2/C1)
A2 = (A1'-log(k1((1-10A1')+1)C2/C1 ................................ (3)

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

Example 1:
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.

Example 2:
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.

Program Resources

Labels

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

Storage Registers

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.

Program

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