Interference Correction Equation

Concept of Interference Correction Equation

If a sample contains a large amount of matrices such as Cl, polyatomic ions caused by those matrices may interfere with target ions. A small amount of polyatomic ions can be corrected by using a formula. Also when you wish to analyze samples including all isotopes, you can use a correction equation. The correction equation is called an interference correction equation. The following example describes the use of it.

Example)

When As is measured in a Cl matrix sample, Cl bonds with Ar and ArCl polyatomic ions are formed, one of which has the same m/z as As (75). As a result, quantitative analysis can have an error due to ArCl. In this case, the knowledge that ArCl is present at m/z 75 and m/z 77 in the proportion to the isotope ratio of ³⁵Cl : ³⁷Cl, 75.8% : 24.2%, can be used to correct for the interference at m/z 75. The ArCl counts at m/z 75 are calculated based on the m/z 77 ArCl count. By subtracting ArCl from the count at m/z 75, the correct As concentration can be obtained.

However, as Se has an isotope at m/z 77, any Se in the sample will increase the m/z 77 counts accordingly. Fortunately, Se also has an isotope at m/z 82. By measuring the Se at m/z 82, the Se count at m/z 77 can be estimated (see below), and subtracted from the counts at m/z 77 to calculate the counts due to ArCl.

 

Isotope Patterns AS, Se, and ArCl

As the result, the As count is obtained by the following interference equation:

(where⁷⁵C, ⁷⁷C, and ⁸²Care the counts on m/z 75, 77 and 82 respectively)

Therefore, m/z 77 and m/z 82 must be measured as well as m/z 75 to perform quantitative analysis of As by using the interference equation. Be sure to select the additional mass numbers specified in the interference correction equation. These are automatically selected by the software if the interference correction equations are entered at the time masses are selected in the method.

The interference equation can also be used for elements such as Pb, for which the isotope ratio varies for different sample sources. In this case, the sum of the Pb isotopes should be measured and used to calculate the Pb concentration.

1,000 ´ ⁷⁵C3.132 ´ ⁷⁷C0.874 ´ ⁸²C

 

The equations for As and Cd are derived as follows:

• As

  If there is an ArCl interference at m/z 75, it will be corrected for by the following equations:

  As(75)

  = M(75) - 75.8/24.2 * ArCl(77) = M(75) - 3.132 * ArCl(77) --- (1)

  But at m/z 77 there is Se, and ArCl at m/z 77 will be corrected for by the following equation:

  ArCl(77)

  = M(77) - 7.6/8.7 * Se(82) = M(77) - 0.874 * Se(82) --- (2)

  Then this equation should be applied to equation (1):

  As(75)

  = M(75) - 3.132 * (M(77) - 0.874 * Se(82)) = M(75) - 3.132 * M(77) + 2.736 * Se(82) -- (3) considering ArCl and Se

  But there is a Kr interference at m/z 82 in some cases, arising from Kr found in the Ar supply (mainly from bottled Ar) therefore the signal at m/z 82 should be corrected as follows:

  Se(82)

  = M(82) - 11.6/11.5 * Kr(83) = M(82) - 1.009 * Kr(83) --- (4)

  If this equation is applied to the equation (3):

  As(75)

  = M(75) - 3.132 * M(77) + 2.736 * (M(82) - 1.009 * Kr(83)) = M(75) - 3.132 * M(77) + 2.736 * M(82) - 2.760 * Kr(83)--- (5) considering ArCl, Se and Kr

  There can sometimes be an additional, small interference on m/z 77, due to ArArH. It can be compensated for in the following way:

  At first, a blank solution must be analyzed, and m/z 77 and 80 should be measured. The ratio of 77/80 will be calculated and this ratio will be entered into equation (3) with (4).

  As(75)

  = M(75) - 3.132 * (M(77) - 0.874 * M(82) + 1.009 * Kr(83)- ratio of 77/80 * M(80)) = M(75) - 3.132 * M(77) + 2.736 * M(82) - 2.760 * Kr(83) + 3.132 * ratio of 77/80 * M(80) --- (6) considering ArCl, Se, Kr and ArArH

• Cd

  For Cd analysis, interference due to mainly MoO and Sn is possible. There are two equations that are recommended by the EPA:

  When m/z 114 is used (EPA Method 6020), MoO and Sn should be corrected based on m/z 108 and 118.

  Cd(114)

  = M(114) - 24.1/14.8 * MoO(108) - 0.65/24.2 * Sn(118) = M(114) - 1.628 * MoO(108) - 0.0269 * Sn(118) --- (7)

  But, at m/z 108 there is a contribution from Cd and it must be corrected for:

  MoO(108)

  = M(108) - 0.89/1.3 * Cd(106) = M(108) - 0.685 * Cd(106) --- (8)

  When this equation is put into equation (7):

  Cd(114)

  = M(114) - 1.628 * (M(108) - 0.685 * Cd(106)) - 0.0269 *Sn(118) = M(114) - 1.628 * M(108) + 1.115 * Cd(106) - 0.0269 *Sn(118) --- (9) considering MoO and Sn

  If there is a Pd contribution, which interfere with m/z 106 and 108. Therefore m/z 106 on Cd and m/z 108 on Mo must be corrected as follows:

  Cd(106)	= M(106) - 27.3/22.3 * Pd(105) = M(106) - 1.224 * Pd(105) --- (10)
  MoO(108)	= M(108) -0.89/1.3 * Cd(106) - 26.5/22.3 * Pd(105) = M(108) -0.685 * (M(106) - 1.224 * Pd(105)) - 1.188 * Pd(105) = M(108) - 0.685 * M(106) - 0.349 * Pd(105) --- (11)

  When equation (11) is put into equation (7):

  Cd(114) = M(114) - 1.628 * (M(108) - 0.685 * M(106) - 0.349 * Pd(105))- 0.0269 * Sn (118)
  	= M(114) - 1.628 * M(108) + 1.115 * M(106) + 0.568 *Pd(105) - 0.0269 * Sn (118)
  --- (12) considering MoO, Cd and Pd

  When m/z 111 is used (EPA 200.8), MoO should be corrected based on m/z 108. Cd contribution on m/z 106 must also be considered as described in equation (8):

  Cd(111)

  = M(111) - 15.9/14.8 * MoO(108) = M(111) - 1.074 * (M(108) - 0.685 * Cd(106)) = M(111) - 1.074 * M(108) + 0.735 * Cd(106)--- (13) considering MoO

  When there is a Pd contribution, m/z 108 must be corrected using equation (11):

  Cd(111)

  = M(111) - 1.074 * (M(108) - 0.685 * M(106) - 0.349 *Pd(105)) = M(111) - 1.074 * M(108) + 0.736 * M(106) + 0.375 * Pd(105)--- (14) considering MoO and Pd

  The following equations are EPA recommendation.

• EPA Method 200.8

  As (1.000) (75C) - (3.127) [(77C) - (0.815) (82C)]

  Cd (1.000) (111C) - (1.073) [(108C) - (0.712)=(106C)]

  Pb (1.000) (206C) + (1.000) (207C) + (1.000) (208C)

  Mo (1.000) (98C) - (0.146) (99C)

  V (1.000) (51C) - (3.127) [(53C) - (0.113) (52C)]

  In (1.000) (115C) - (0.016) (118C)

• EPA Method 6020 (CLP-M Ver.9)

  As (1.0000) (75C) 15.875 mm- (3.127) (77C) + (1.0177) (78C)

  Cd (1.0000) (114C) - (0.0149) (118C) - (1.6285) (108C)

  Pb (1.0000) (208C) + (1.0000) (207C) + (1.0000) (206C)

  Se (1.0000) (78C) - (0.1869) (76C)

  V (1.0000) (51C) - (3.1081) (53C) + (0.3524) (52C)

  ⁶Li (1.0000) (6C) - (0.0813) (7C

  In (1.0000) (115C) - (0.0149) (118C)

  m C m/z = the total ion count at m/z 'm'

  In the case of EPA methods, the values are different from the theoretical values, because they contain some experimental factors. Moreover, interferences are matrix dependent. They might also vary depending on the instrument and conditions.

 

Interference Correction equations do not apply to polyatomic ion signals that are much higher than the analyte signal. If the signal of a polyatomic ion is 10,000 counts, the standard deviation will be theoretically about 100 counts. This deviation affects an analyte signal. If the analyte signal is 100 counts, it will be impossible to calculate accurately. If several correction factors are used in an equation, errors are also accumulated with each term. In addition, the imprecision from an internal standard affects the measurement when an internal standard correction is applied.

 

Setting Up the Interference Correction Equation

The set up can be performed in [Select Elements] of the Element Selection Pane.

The interference equation must be set up before you select elements in the acquisition parameters. When an element is selected, related masses based on the interference equation are automatically selected. However, there is no direct link between the database of interference equation and acquisition parameters, and the related mass is not selected automatically, only when the interference equation is set.

Always acquire data with the interference equations selected, to ensure that all the appropriate masses are selected.