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PLASMACHEM-L  November 1999

PLASMACHEM-L November 1999

Subject:

Re: PQ3 with SIM detector

From:

Phil Shaw <[log in to unmask]>

Reply-To:

Analytical Chemistry using ICP's, DCP's, MIP's.

Date:

Mon, 8 Nov 1999 14:36:04 -0000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (129 lines)

Hello Claudio and other Plasmachemists,

I would like to clarify a couple of matters on cross calibration with a PQ3
or PQExCell with a simultaneous detector. To get a good cross calibration
for each analyte being measured the count-rate must be higher in the
analogue than the noise of the offset analogue baseline (the offset is about
200 Hz) but low enough in the pulse counting that the detector protection
mechanism is not activating the "gate" which will reduce the measured
signal. Our standard practices document used by engineers in the factory
suggests between 1 and 3 million CPS in the pulse counting for this. The
voltages on the two stages of the detector also need to be adjusted such
that the pulse counting stage is plateaued and the analog voltage set to
give the desired cross over factor at a given mass, typically 1000 is chosen
as the cross over value for 115In. This means that the analog stage should
be reading about 1200Hz (1000Hz analyte + 200Hz offset) for about 1M CPS in
Pulse counting.

All of the above processes are now automated in the Windows NT PlasmaLab
software suite. Using the Instrument Setup sample you can specify that the
detector be Plateaued, the Analog voltage set-up and the two modes cross
calibrated simultaneously. If your system has the typical 80-100K cps/ppb
reponse then a 25 ppb mixed element solution will give countrates across the
mass range of 2-2.5M cps for 7Li, 24Mg, 59Co, 115In, 140Ce, 209Bi and 238U
for example. Specify 1000 as the cross cal factor for 115In and then tell
the system to Plateau the pulse counting stage on 9Be, 60Ni and the minor
isotope of 113In (to avoid having too high a count rate on the detector).
With the simultaneous detector, the value set for the analog stage can
effect the plateau point of the pulse counting stage so set the above
automatic procedure to do 2 iterations. Once the voltages are setup the
cross calibration software will take a full mass scan and automatically
choose as many appropriate analytes to calibrate with as it can, you do not
need to specify which ones to use. With one mixed element solution I had a
calibration with over 70 analyte points with measured cross calibration
factors. A curve is constructed to fit the points and then when analog data
is reported, if the analyte mass has a directly measured cross calibration
factor it is used, otherwise the interpolated value from the curve is used.

When analysing solutions later, the software will then determine which of
the two simultaneous detector mode count rates to report based on the
following logic:

Convert the analog counts to equivalent pulse counting cps (using either the
direct factor or the interpolation)
If the value is less than the specified "analog minimum countrate" (default
1M cps) report pulse counting data
If the value is greater than the specified "pulse counting maximum count
rate" (default 4M cps) report analog data
Otherwise compare the two readings and if the pulse counting cps is less
than a defined perentage (default 95%) of the analog report the analog.

This logic will then allow an accurate transition between the two detector
modes as the analog data will always be considerably higher than the noise
and the pulse counting data will be testing for "gating effects" where the
automatic hardware protection has started to suppress the apparent count
rate in the pulse counting mode. The above default values can of course be
changed by the user, even post acquisition, as with a simultaneous detector
both modes are stored in the database for audit purposes.

I hope this has explained some of the "mystery" behind the simultaneous
detector and how with appropriate software automation it can be a very
simple to use and accurate tool for wide dynamic range ICP-MS.

For those people using PlasmaLab I refer you to the user guide sections
2.1.4 (Menus), 3.4 (instrument calibrations), 6.3 (Experiment Instrument
parameters), 6.5 (Experiment Sample List properties) and the Appendix on
calculations. Other Frequently Asked Questions (FAQ) on PlasmaLab are
addressed on our PlasmaLab FTP site. I suggest that people with specific
questions on VG Elemental equipment first direct their queries to VGE's
PlasmaCare Forum. You can access the FTP site and the Forum by registering
as a VG ICP-MS (or GD-MS) user on the Customer Care area of our website.

For those interested in PlasmaLab generally I refer you to our website
WWW.VGElemental.COM.

Dr. Phil Shaw
VG Elemental

-----Original Message-----
From: Claudio Mucchino [mailto:[log in to unmask]]
Sent: 08 November 1999 12:21
To: [log in to unmask]
Subject: Re: PQ3 with SIM detector


> Hello, I am a relatively new PQ3 user who is trying to do multi-element
standard additions analyses using an ETP (simultaneous) detector operating
in the "dual" mode.  For elements at low concentrati>
> But in the PC/Analog crossover region I'm afraid that the some integrals
might be reported as PC data for the unspiked and Analog data for the spiked
samples, respectively.  Does this matter?  If the>
> Can anyone comment on the above and the following:  In the
Configuration/Integration menu, can someone explain how the "Crossover
Tolerance Level" settings work in conjunction with the PC/Analog Swit>


Hello Plasmachemists and Steve, as a PQ3 user I can confirm that
detector calibration with the software provided by VG could be a
difficult issue. First of all you have to assure, by using all the
operative parameters (i.e. lens, X-torch, nebulizer etc) to have a
almost flat response over the whole mass range (or the range of yr
interest) otherwise the interpolation algorithm could lead at
considerable errors. Personally I prefer to do a off-line
intercalibration of the detector for each m/z exporting the raw data
in a speadsheet (excel or equivalent) and using at least 3 points
with reliable data both in analog and in pulse. I hope this will
help.

To VG people: once more I have to disagree with yr point-to point
approach to such general problems. I think that most of VG
plasmachemists will be more than happy to know yr advices on this
and other general  points.
Could I have some feedback from other VG users? I suggest a direct
reply to my email address to avoid flooding the listserver: I will
later let you known, via the list, the result of the poll.
Best Regards
Claudio Mucchino


dr. Claudio Mucchino
Chemistry department
Parma University
v.le delle Scienze, 17/a
43100 PARMA
Italy
Ph:  + 39 0521 905133
fax: + 39 0521 905557
e-mail  [log in to unmask]
alternate: [log in to unmask]

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