1. Strategies for Eliminating Interferences in Optical Emission Spectroscopy
Best practices to optimize your method and correct for interferences to produce accurate results
National Environmental Monitoring Conference
Washington DC

2. Anything that causes an inaccurate signal for your analyte of interest
What is an Interferent
Anything that causes an inaccurate signal for your analyte of interest
Know your interferences
Collect information
Frequently from multiple sources
Have specific, identifiable, and correctable causes
Examples: Contaminated Method Blank, Incorrect Standard Concentration.
Quantify your interferences
Obtain information quantitatively or semi-quantitatively
How significantly do the interferences affect your accuracy

3. Accuracy can also be expressed as Percent Relative Error
Interferences may Affect Accuracy or Precision
Accuracy: Usually expressed as error. The difference between a measurement and the True Value is its absolute error (mg/L).
Accuracy can also be expressed as Percent Relative Error
Precision: Simply the degree of reproducibility of a set of replicate measurements.
Precision can expressed by Standard Deviation (SD) or Percent Relative Standard Deviation (%RSD)

4. ICP Interferences that may Affect Accuracy or Precision
Plasma density
Interferences
Physical
Spectral
Matrix
Ionization
Environment
User
Plasma
Lab cleanliness
Unwanted light
Sample introduction
User error

5. PerkinElmer Avio 500 Simultaneous ICP
Instrumentation
PerkinElmer Avio 500 Simultaneous ICP
Vertical Plasma Torch Design
Flat Plate™ Plasma Technology
Dual View
PlasmaShear™ System
Universal Data Acquisition
Color PlasmaCam™
Syngistix™ for ICP Software

6. Spectral Interferences in ICP-OES
The most severe problems:
line rich matrix element(s)
few analytical wavelengths of analyte
Possible solutions:
Improve resolution
Matrix match
Use an alternate i.e. cleaner wavelength if possible
Interelement correction
Multivariate correction (MSF)

7. Indirect Overlaps vs. Indirect Overlaps
Interferent
Emission Line

8. Spectral Interference Correction
Software Based Interference
Correction Techniques
Multicomponent Spectral Fitting (MSF)
Interelement Corrections (IEC)

9. IEC Correction
The IEC correction factor is a measurement of the effect of an interfering element upon the observed concentration of an analyte
The correction factor is simply the correlation between interferent concentration and the apparent or “false” analyte concentration
Relationships established between analyte and interferent
IEC Correction Factor
Ratios determined based on this relationship
Ratios highly dependent on plasma conditions
200 Fe
500 Fe 2 5
Analyte Conc.

10. Analytical Considerations for IEC
IEC standards – Single element standards of all suspected interferences
Interferent Standards must not contain the analyte
Choose analytes and concentrations that are likely to be in the unknown samples
Interferent needs to be in the linear range
Plasma conditions must be constant

11. Interferences on Arsenic
50ppb As in the presence 100ppm Aluminum, Calcium, Iron, and Magnesium

12. Arsenic Blanks in a Matrix of 100ppm Aluminum and Iron
Improper Peak Integration
Proper Peak Integration
Fe on As
-6ppb
Fe on As -525 ppb
Al on As
-110ppb
Al on As
-22ppb

13. Interferences Causing Erratic Baseline and False Concentrations
Aluminum; 100ppm produces an false concentration of -243 ppb
Accuracy
Calcium; 100ppm spectra mimics the blank, but poor baseline
Precision
Iron; 100ppm produces an false concentration of -104 ppb
Accuracy
Magnesium; 100ppm spectra mimics the blank, but poor baseline
Precision

14. Software Generating an IEC Table
IEC Factor = False analyte concentration (ppb) / Apparent concentration of the interferent (ppm)

15. Software Generates an IEC Summary Table
Units: ppb False Analyte / ppm Interfering Element
Set correction factors
Look at values: if numbers are getting large, check background points

16. IEC Review Mathematical ratio between the analyte and the interference
Highly dependent on Plasma conditions
Highly dependent on analyte integration parameters
Table must be generated from analyte free interfering standards
The interferences must be analyzed
The Interferences must be within the linear range.
Alternate wavelengths can be used to extend the linear range of the interferent
Interferent usually quantitated in the radial mode
May add noise to the signal
Can correct for direct spectra overlaps

17. MSF – An Alternate to IEC’s
Multi-Spectral Fitting
Does not depend on plasma conditions
Interferences do not need to be quantitated
Does not correct for direct spectral overlaps

18. MSF Modeling Analyte Spectra 40 ppb As in Combined Matrix
Matrix 200ppm Al, Ca, Fe Mg
Blank Spectra
Analyte Spectra
40 ppb As in Combined Matrix

19. Components of the Matrix
Combined Matrix Spectra
Aluminum Spectra
Calcium Spectra
Magnesium Spectra
Iron Spectra

20. Defining the Components in Syngistix

21. Entering the Model into the Method
MSF is now the peak integration algorithm
Reference the model created in “Examine Spectra” in the method

22. Improving Detection Limits / Precision With MSF
10ppb As in 2% HNO3
Data can be reprocessed with either peak integration methods

23. Detection Limit Calculation
Sample ID ( matrix 200ppm Al, Ca, Fe, and Mg,
10ppb As with Background Points
10ppb As with MSF interference corrections of the Blank
MDL Sample #1
0.005
0.009
MDL Sample #2
0.006
0.008
MDL Sample #3
0.010
MDL Sample #4
MDL Sample #5
MDL Sample #6
0.011
MDL Sample #7
Calculated MDL (3.14* STDEV Sample 1-7)
0.007
0.003

24. 1 ppm Indium in 100ppm Cobalt Matrix
Severe Co spectral interference on all usable Indium Wavelengths
Modeling a Cobalt Standard in MSF can eliminate matrix contribution.
Indium Can Successfully be Analyzed in a Cobalt matrix using MSF In In

25. Indium Spike Recoveries With and Without MSF

26. Review
IEC
MSF
Interferences must be calibrated for and within the Linear range
Once MSF Model is generated interferences do not need to be contained within the Method
Corrects for Direct Spectral overlaps
Interference must not be a direct spectra overlap
Factors are dependent on plasma conditions, power etc.
Model is valid for different plasma conditions
Background Point and Peak Integration is Critical
Automatically integrates Peak

27. Thank You for Your Attendance!
Application Notes

28. Thank You for Your Attendance!
Contact information:
Dan Jones