How to set run to measure backgrounds always, not just every "Nth" point

[Dan MacDonald]

Good afternoon, everyone:

I have yet another small question regarding setting a run to always measure backgrounds instead of measuring them every "Nth" point, and I can' locate the option button to set this.  I'm sure that it's right in front of me, but danged if I can find it.  Thanks very much in advance.

Kind regards

Dan MacDonald
Probe Tech

[Gseward]

Dan,

In the Acquire! window, click Acquisition Options.
On the left of the window, see: Nth point Off-Peak Background options.
You can check/uncheck the Global option here. Unchecking 'Use Nth Point Acquisition for Off-Peaks' with turn off the option and an Off-Peak background will be acquired with each acquisition. Typically Nth point is not 'turned on' by default.

Alternatively, if you are wanting to enable/disable Nth point on an element by element basis:
open Acquisition Options
Click the element row of interest in the top pane
Toggle Nth point, and/or adjust Nth point Acquisition Interval.

Gareth

[Dan MacDonald]

Thanks very much, Gareth. I really appreciate it.

Best regards

Dan

[John Donovan]

I couldn't find a topic devoted to Nth point background acquisition, though I'm sure it must be somewhere...

Anyway, Julien Allaz and I were having a bit of a chin wag last week regarding the "on-peak time fraction" option that is utilized for the Nth point background feature.  For those that aren't aware this is a cool option for those that need to measure off-peak backgrounds, but do not need to measure the backgrounds on every point.

This might be because the sample is very homogeneous and so the background isn't changing much, or it might be that one is measuring traces in beam sensitive samples (e.g., monazite) and by the time the background is being measured the sample is damaged enough to affect the background intensity measurement.

Now one could instead utilize the "alternating off/off peak" feature, which by setting the Unknown Count factor to more than 1.0, and checking the Use Alternating On and Off peak Acquire option in the Acquire! Acquisition Options dialog, the program will alternately acquire sub intervals on the peak and off the peak in order to better track the "delta" of the net intensities over time.  See here for more details:

https://probesoftware.com/smf/index.php?topic=812.0

But the On Peak Time Fraction option, seen also in the Acquisition Options dialog, now allows for a value of zero for the On Peak Time Fraction as seen here:



This means that for the Nth point measurement, one can now acquire *no* on-peak intensity data and instead acquire only the off-peak intensity data. Furthermore, the program will not even move to the on-peak position and in fact will even "mark" this acquisition as "B" (for bad) so it will automatically not be included in the analysis as seen here:

On-Peak (off-peak corrected) or EDS (bgd corrected) or MAN On-Peak X-ray Counts (cps/1nA) (and Faraday/Absorbed Currents):
ELEM:    si ka   BEAM1   BEAM2
BGD:       OFF
SPEC:        1
CRYST:     TAP
ORDER:       1
   21B    -.31  30.025  30.017
   22G  230.66  29.982  30.001
   23G  231.54  29.978  29.987
   24B    -.37  29.995  29.994
   25G  231.13  29.992  30.013
   26G  231.41  30.005  30.013
   27B    -.39  29.981  29.999
   28G  232.31  30.009  29.993
   29G  231.30  30.010  29.977
   30B    -.42  29.986  30.002

AVER:   231.39  29.996  29.997
SDEV:      .55    .014    .015
1SIG:      .88
SIGR:      .62
SERR:      .22
%RSD:      .24

Off-Peak (calculated) or EDS (bgd) X-ray Counts (cps/1nA):
ELEM:    si ka
TYPE:   LINEAR
NthPT:       3
   21B     .31
   22G     .31
   23G     .31
   24B     .37
   25G     .37
   26G     .37
   27B     .39
   28G     .39
   29G     .39
   30B     .42

Raw Hi-Peak X-ray Counts (cps/1nA):
ELEM:    si ka
   21B     .28
   22G     .28
   23G     .28
   24B     .30
   25G     .30
   26G     .30
   27B     .32
   28G     .32
   29G     .32
   30B     .41

Raw Lo-Peak X-ray Counts (cps/1nA):
ELEM:    si ka
   21B     .35
   22G     .35
   23G     .35
   24B     .44
   25G     .44
   26G     .44
   27B     .46
   28G     .46
   29G     .46
   30B     .43

In the above acquisition the Nth point was set to 3, and the on-peak time fraction was set to zero.  Note how the same off-peak backgrounds are utilized every N points every Nth point.

So when the sample is analyzed, the "B" points are automatically skipped:

Un    3 Fe, V, C 1A, 7, Results in Elemental Weight Percents
 
ELEM:       Si
TYPE:     ANAL
BGDS:      LIN
TIME:    10.00
BEAM:    30.00

ELEM:       Si   SUM 
    22  98.922  98.922
    23  99.301  99.301
    25  99.122  99.122
    26  99.246  99.246
    28  99.632  99.632
    29  99.196  99.196

AVER:   99.237  99.237
SDEV:     .234    .234
SERR:     .096
%RSD:      .24
STDS:       14

STKF:    .4101
STCT:    95.62

UNKF:    .9924
UNCT:   231.39
UNBG:      .36

ZCOR:   1.0000
KRAW:   2.4199
PKBG:   655.15


Please let me know what you all think of this new feature.

[Julien]

Thanks John! It's exactly what I was thinking, and is already a serious improvement for us! Let's see what Mike J think

Aside of this, a note about the alternate peak & bkg acquisition. It would effectively be wonderful to use this on monazite dating, for instance, and we did indeed tested it several years ago. It works great! HOWEVER, it will add a huge amount of delay time on the measurement, nearly doubling the total acquisition time (from 12-15 min to 25-30 min!). Why? Well... Because we are measuring 6 to 8 backgrounds with the multipoint background acquisition and this add a lot of time for spectrometer motion. Yet, it truly remains more accurate as the background is also corrected for drifting of intensity due to beam damage.

Cheers,

Julien

[John Donovan]

Thanks John! It's exactly what I was thinking, and is already a serious improvement for us! Let's see what Mike J think

Aside of this, a note about the alternate peak & bkg acquisition. It would effectively be wonderful to use this on monazite dating, for instance, and we did indeed tested it several years ago. It works great! HOWEVER, it will add a huge amount of delay time on the measurement, nearly doubling the total acquisition time (from 12-15 min to 25-30 min!). Why? Well... Because we are measuring 6 to 8 backgrounds with the multipoint background acquisition and this add a lot of time for spectrometer motion. Yet, it truly remains more accurate as the background is also corrected for drifting of intensity due to beam damage.

Cheers,

Julien

Hi Julien,
OK, cool.

Yes, adding all that spectrometer motion is one disadvantage of the alternating on/off peak acquisition method. We implemented it mainly because the Australians like to use it for trace elements in garnet (for diamond mineralization) when running 500-1000 nA beam currents!
john