Background Types

EPMA Probe

Probe Window Details > Acquire!

Background Types

The choice of background measurement either off-peak or MAN can be specified here. Off-peak correction types include linear, average, high only, low only, exponential and polynomial. MAN backgrounds are based on a calibration curve fitted to appropriate standards that do not contain the element of interest (Donovan and Tingle, 1996). The MAN calibration is assigned from the Analytical | MAN Fits menu after the standards to be used in the MAN fit have been acquired.

Note that one can utilize on-peak intensities for MAN background calculations even if the standards were acquired using off-peak background types but selecting the "Use Off-Peak Elements for MAN Fit" menu (under the Analytical menu).

Note that because the background type flags may be specified differently for standards and unknowns, the Off/MAN flags specified here will be saved for only the current sample type. This means that if the current sample type is an unknown, then the MAN flags defined in the Element/Cation Properties dialog will only be applied to unknown samples. To define the MAN flags for both the standards and unknowns, select them from the Acquisition Options window by clicking the Acquisition Options button in the Acquire! window.

Off-Peak Backgrounds

The off-peak background correction measures the background on the sample of interest (i.e., your standard or unknown) with the spectrometer adjusted to a position on each side of the analytical peak.

Probe for EPMA can individually off-peak correct each point analysis for standard and unknown samples. If the element concentration is very small, it may be necessary to use the off-peak corrected sample method. The program is very flexible in that it allows you to use all MAN or all off-peak background corrections or any combination of the two methods on each element of an acquisition.

Note that since the off-peak positions are saved on a sample by sample basis, one must always start a new standard or unknown sample after changing the off-peak positions or adjusting them graphically using the mouse from a wavelength scan from the Plot! window. Probe for EPMA uses a linear slope calculation as the default off-peak background correction. Because of this, unsymmetrically offset background position measurements can be easily handled. One can even position both of the off-peak measurements on the same side of a peak and extrapolate the background correction.

In addition, the user can also specify that the program calculate the off-peak counts using the average of the off-peak counts, or only the high or only the low off-peak counts. Finally, the user can specify an exponential slope calculation for curved backgrounds. If one changes the off-peak background type for an element, it may be necessary to change it for both the unknown samples, as well the standards, depending on the analytical situation. This is easily done in the Model Background dialog which can be reached from the Graph dialog from the Plot! Window Output button, by selecting all samples that you wish to assign the new background type to and clicking Ok.

Multi-Point Backgrounds

Multi-point backgrounds are utilized when performing high accuracy trace element analysis on sample with variable composition, e.g., monazites. In such cases the presence of minor and even trace levels of unexpected elements may cause a significant interference with the normal high and low off-peak positions of the analytical peak in question.

To avoid this situation the multi-point background acquisition was developed to allow the program to automatically acquire a number of off-peak intensities distributed on each side of the analytical peak (which can be specified precisely by the user) so that at least a few of the background measurements will not be affected by the unpredicted presence of various other elements.

This process is automatically performed iteratively when the background correction is calculated by looping on the data starting with the full set of background measurements and one by one throwing out the background points with the highest variances above the fitted background until the specified number of valid background positions is reached. See code here:

' Iterate from NumberofPointsAcquire to NumberofPointsIterate

k% = 1

npts% = MAXMULTI% * 2

nptshi% = MAXMULTI% * 2

nptslo% = MAXMULTI% * 2

Do While nptshi% > sample(1).MultiPointNumberofPointsIterateHi%(j%) Or nptslo% > sample(1).MultiPointNumberofPointsIterateLo%(j%)

' Load data array for fitting

npts% = 0

nptshi% = 0

nptslo% = 0

' Load high side points

For m% = 1 To sample(1).MultiPointNumberofPointsAcquireHi%(j%)

If sample(1).MultiPointProcessManualFlagHi(i%, j%, m%) Or (Not sample(1).MultiPointProcessManualFlagHi(i%, j%, m%) And Not badflaghi(m%)) Then

npts% = npts% + 1

nptshi% = nptshi% + 1

mxdata!(npts%) = sample(1).MultiPointAcquirePositionsHi!(j%, m%)

mydata!(npts%) = sample(1).MultiPointAcquireCountsHi!(i%, j%, m%)

End If

Next m%

' Load low side points

For m% = 1 To sample(1).MultiPointNumberofPointsAcquireLo%(j%)

If sample(1).MultiPointProcessManualFlagLo(i%, j%, m%) Or (Not sample(1).MultiPointProcessManualFlagLo(i%, j%, m%) And Not badflaglo(m%)) Then

npts% = npts% + 1

nptslo% = nptslo% + 1

mxdata!(npts%) = sample(1).MultiPointAcquirePositionsLo!(j%, m%)

mydata!(npts%) = sample(1).MultiPointAcquireCountsLo!(i%, j%, m%)

End If

Next m%

' Re-fit the data using new points

If sample(1).MultiPointBackgroundFitType%(j%) = 1 Then

order% = 2 ' 2nd order polynomial

Else

order% = 1 ' linear

End If

Call LeastSquares(order%, npts%, mxdata!(), mydata!(), acoeff!())

If ierror Then Exit Sub

' Calculate deviations for high side and flag highest positive multi-point background as "bad"

If nptshi% > sample(1).MultiPointNumberofPointsIterateHi%(j%) Then

maxt! = 0#

maxm% = 0

For m% = 1 To sample(1).MultiPointNumberofPointsAcquireHi%(j%)

If Not sample(1).MultiPointProcessManualFlagHi(i%, j%, m%) And Not badflaghi(m%) Then

' Calculate fit and relative distance from fit for each high point

temp! = acoeff!(1) + acoeff!(2) * sample(1).MultiPointAcquirePositionsHi!(j%, m%) + acoeff!(3) * sample(1).MultiPointAcquirePositionsHi!(j%, m%) ^ 2

If temp! <> 0# Then

temp! = (sample(1).MultiPointAcquireCountsHi!(i%, j%, m%) - temp!) / temp!

If temp! > maxt! Then

maxt! = temp!

maxm% = m%

End If

Next m%

' Flag high side points if indicated

If maxt! <> 0# And maxm% <> 0 Then

badflaghi(maxm%) = True

Else

noneflaggedhi = True

End If

' Calculate deviations for low side and flag highest positive multi-point background as "bad"

If nptslo% > sample(1).MultiPointNumberofPointsIterateLo%(j%) Then

maxt! = 0#

maxm% = 0

For m% = 1 To sample(1).MultiPointNumberofPointsAcquireLo%(j%)

If Not sample(1).MultiPointProcessManualFlagLo(i%, j%, m%) And Not badflaglo(m%) Then

' Calculate fit and relative distance from fit for each low point

temp! = acoeff!(1) + acoeff!(2) * sample(1).MultiPointAcquirePositionsLo!(j%, m%) + acoeff!(3) * sample(1).MultiPointAcquirePositionsLo!(j%, m%) ^ 2

If temp! <> 0# Then

temp! = (sample(1).MultiPointAcquireCountsLo!(i%, j%, m%) - temp!) / temp!

If temp! > maxt! Then

maxt! = temp!

maxm% = m%

End If

Next m%

If maxt! <> 0# And maxm% <> 0 Then

badflaglo(maxm%) = True

Else

noneflaggedlo = True

End If

' Check if no points were flagged on the high and low side (could not iterate further possibly due to manual override points flagged)

If noneflaggedhi And noneflaggedlo Then

msg$ = "DataCorrectDataMultiPoint: Unable to iterate to Iterate Points in " & sample(1).Name$ & ", " & sample(1).Elsyms$(j%) & " " & sample(1).Xrsyms$(j%) & ", line " & Format$(sample(1).LineNumber&(i%))

Call IOWriteLogRichText(msg$, vbNullString, Int(LogWindowFontSize%), vbMagenta, Int(FONT_REGULAR%), Int(0))

End If

' Iterate next cycle

k% = k% + 1

If k% > maxiter% Then GoTo DataCorrectDataMultiPointTooManyIterations

Loop

' Iteration complete, calculate background based on last fit coefficients

If acoeff!(1) = 0# And acoeff!(2) = 0# And acoeff!(3) = 0# Then GoTo DataCorrectDataMultiPointNoFitCoefficients

bgdcts! = acoeff!(1) + acoeff!(2) * sample(1).OnPeaks!(j%) + acoeff!(3) * sample(1).OnPeaks!(j%) ^ 2

sample(1).BgdData!(i%, j%) = bgdcts!

The user can alternatively utilize the combined background intensities by selecting one of the normal off-peak background calculations.

Off Peak Background Correction Types