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PLASMACHEM-L  July 2012

PLASMACHEM-L July 2012

Subject:

Re: On line internal standard addition

From:

Ed McCurdy <[log in to unmask]>

Reply-To:

PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's).

Date:

Fri, 13 Jul 2012 03:42:20 -0600

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (109 lines)

Hi Craig

Not wishing to scupper your justification for new instrumentation, but for total levels of those elements, ICP-MS analysis after microwave digestion should be perfectly straightforward, I think.  Steer clear of hot block if you want to recover the volatile elements.  Usually organic samples don't pose much of a problem in terms of dissolved solids, as most of the carbon is removed during digestion.  So if you work on the basis of a 500x dilution from the dry sample weight (0.2g digested and then diluted to a final volume of 100ml), you will be far below the standard 0.2% TDS limit quoted for ICP-MS.  In fact you should be able to go with much lower dilution than this (say 200x), if you are pushed on detection limits.

The spectral interferences also shouldn't be too problematic, despite the fact that you will probably want to add HCl as part of the digestion mix (to give a level of about 0.5% HCl in the final solution you run).  The HCl will help a lot with the Hg stability and recovery.  The ArCl and CaCl on As will be removed effectively in He mode on the cell, and the other potential interference (such as MoO on Cd) should also be reduced (this will of course depend on the relative concentrations of Mo and Cd in your seaweed). 

If there is carbon left in your sample digests, you might want to add some extra carbon to all the samples and standards as well, to equalize the ionization enhancement effect on As.  Usually adding 2% of some solvent like IPA, methanol or ethanol gives a consistent effect for all samples.

Best regards

Ed

________________________________________________________________
Ed McCurdy
ICP-MS Product Marketing
Agilent Technologies UK Limited
Phone: +44 (0)1270 878126
VoiceMail: +44 (0)118 927 5373
Fax: +44 (0)161 492 7171
TelNet: 317 5373 (voicemail only) 
E-mail: [mailto:[log in to unmask]] 
Agilent Technologies UK Limited, Registered Office: 610 Wharfedale Road, IQ Winnersh, Wokingham, Berkshire, RG41 5TP - Registered No. 03809903
P SAVE PAPER - Please do not print this e-mail unless absolutely necessary
________________________________________________________________



-----Original Message-----
From: PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's). [mailto:[log in to unmask]] On Behalf Of Musante, Craig
Sent: 12 July 2012 18:38
To: [log in to unmask]
Subject: Re: On line internal standard addition

On a different note,

I have been asked to investigate the analysis of a dried Eucheuma seaweed product for As, Cd, Hg, and Pb.  I have a hot block and/or microwave for digestion and ICP-OES (Thermo iCAP 6500) and/or  ICP-MS (Agilent 7500ce w/ORS) for detection.  It would be ideal if I could use one digest and one instrument (most likely my ICP-MS for As and Hg detection levels).  That being said, I am a little concerned about % TDS and other elemental interferences.

Also, this project may afford me the opportunity to purchase additional instrumentation.  I am curious about people's experiences with current thermal desorption instrumentation for the analysis of Hg
  
Any comments on this analysis would be greatly appreciated

Craig Musante
The Connecticut Agricultural Experiment Station
123 Huntington Street 
New Haven, Connecticut 06504
203-974-8454
[log in to unmask]


-----Original Message-----
From: PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's). [mailto:[log in to unmask]] On Behalf Of Mindak, William R
Sent: Thursday, July 12, 2012 11:52 AM
To: [log in to unmask]
Subject: Re: On line internal standard addition

We use on-line addition all the time. My suggestion is to look at the tune window with blank and IS solution flowing. Monitor your IS elements and make sure you have a smooth signal. Carefully adjust the peri pump tension. Most people do not change the IS pump tube often enough because of the difficulity in changing/connecting such a small diameter tube. However, you will jump for joy when you switch to flared end tubing and will not have an excuse for using an old pump tube, You can buy flared end tubing form several sources like Glass Expansion.

Bill
William Mindak, Chemist
Center for Food Safety & Applied Nutrition US Food & Drug Administration
5100 Paint Branch Pky
College Park, MD 20740
240-402-2005   
[log in to unmask]


-----Original Message-----
From: PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's). [mailto:[log in to unmask]] On Behalf Of Henry Longerich
Sent: Thursday, July 12, 2012 11:04 AM
To: [log in to unmask]
Subject: Re: On line internal standard addition

On 11/07/2012 7:23 PM, Rob Franks wrote:
> Most often we just want to add the IS in continuous flow operation, 
> but we do have an application where we dilute the sample about 10X 
> using discrete sampling (direct analysis of seawater).
>
> We find that at low flow rates (using an ESI PFA-ST nebulizer) 
> sensitivity is linear with both flow rate and sample concentration, so 
> when mixing sample and IS at 130+(60-130) ul/min we get almost the 
> same count rates as running just sample by itself at 130 ul/min (the 
> sample is diluted by 33-50%, but the higher flow rate to the nebulizer 
> makes up for that).  The trick is getting short uptake times, good 
> mixing, and good washout at the low flow rates, I guess.
>
> At higher flow rates the count rate becomes dependent only on sample 
> concentration at the nebulizer, so as you increase the total combined 
> flow to the nebulizer the count rate decreases from the dilution, but 
> sometimes that's actually desired, like our direct seawater application.
>
A change in the liquid sample up take rate effects the apparent plasma temperature.

An increase in the liquid sample up take lowers the apparent plasma temperature. To return the plasma temperature one or more of the ICP parameters (any of the three gas flows, rf power, torch position) must be changed. Most convenient it to lower the nebuliser gas flow which results in an increase residence time and a resulting higher plasma temperature.

The liquid sample up take rate is not an independent parameter.

An increase in liquid up take needs to be compensated with a decrease in the gas flow to maintain the same sensitivity (signal per unit
concentration) and poly atomic ion formation.

Henry Longerich
Earth Sciences
Memorial Universithy of Newfoundland
St. John's, NL  A1B 3X5
Canada



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