Hi Geoff,

I had briefly worked with naptha on an ICPOES equipped with a chilled spray
chamber so I can definitely sympathize with you. Hg was not one of my
analytes.

It is apparent from your message that you anticipate very low Hg conc. in
naptha.  The LODs you mentioned are based on an aqueous matrix and not based
on naptha and we really don't know what these would be. How would one
calibrate for Hg or any other element in naptha? Can one purchase Hg
standards in a naptha matrix or in an organic solvent? An aqueous Hg
standard cannot be added to an organic solvent since the matrices are
immiscible. Plus, I doubt a suitable internal standard can be found. 

The problem is not limited to the physical parameters (density, surface
tension, viscosity) that affect nebulization efficiencies but also the Hg
distribution in the different hydrocarbons. Afterall, according to
Wikipaedia:
"Naphtha is comprised of shorter chain hydrocarbons (c5-c7), can be
aromatic, and can contain sulfuric compounds or other impurities. It most
often consists of paraffins with colorants and a thiol. It has the lowest
boiling point of all the petrols, which gives it the highest vapor pressure.
This means that it will produce flammable vapour. "

"Naptha" is a generic term - the proportion of different hydrocarbons is not
fixed. Also, Naptha has a flash point of -18C so it's always a good idea to
check with the MSDS before mucking about with it.  

Since there are no suitable calibrants in naptha, one could ostensibly
dilute it in xylene and do the same with an organometallic Hg standard. The
latter can be obtained in a base oil, but the dilution will increase the
LOD. 

Digestion is required to convert the naptha into a form where calibrants
would be valid. The calibration problem may be solved but it creates
another. Would the recoveries be adequate and precise? Are there any
reference materials for Hg in naptha? I don't think so. Plus digestion
increases the LOD.

With CVAAS, even with amalgamation, we run into the same problem getting the
sample into a suitable form. That is, the Hg must be released from all
organic bonds and be converted into inorganic Hg (ie. digestion). If the
digestion procedure is adequate, then CVAAS with amalgamation may work. I
anticipate the methods development will not be trivial. I have been able to
obtain a Hg LOD by cold vapour ICPOES of 0.1ppb, which is 10 times higher
than standard CVAAS (no amalgamation). Mind you it was an aqueous matrix. I
did not pursue the study into accuracy of the Hg determinations in real
samples. The LOD by ICPOES is most likely inadequate for this particular
matrix due to the dilutions needed. A USN would put too much naptha into the
plasma, so the naptha has to be diluted or the USN has a membrane
desolvator. I do not know whether the latter would trap Hg the way it traps
P. Best to ask the manufacturer. 

ICPMS that's equipped to do petrochemicals is a plausible alternative. The
option to dilute and shoot would be the most convenient, however, there is
the question of the heterogeneity of Hg distribution in the naptha. A micro
neb would help in increasing the residence time in the plasma and high power
to facilitate energy transfer.  If there are only a few samples to do, it
would be justifiable to send it out to a contract lab that is set up to do
napthas. It would be cheaper than an ICPMS. 

So Geoff, yours is a difficult application. I have no experience running
organics on an ICPMS so let's hope the Plasmachem readership can lend some
assistance. 

Best of Luck,
Pamela Wee
ICPAA Expertise Inc
Canada


-----Original Message-----
From: PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's).
[mailto:[log in to unmask]] On Behalf Of (Geoff Tyler)
Sent: January 4, 2010 11:30 AM
To: [log in to unmask]
Subject: digestion method of naphtha for sub ppb Hg determination (by
hydride) ICP-OES

 

 Hello everyone
I have a puzzle that hopefully someone can help me.

 We have a need to determine sub ppb (ug/L in solution) of Hg in naphtha by
ICP-OES. 
While the peltier cooled Spray chamber on the ICP-OES will get us  to about
5 ppb Hg, we need to think of digestion & pre-concentrate for aqueous
hydride generation Hg determination to achieve the goal of sub ppb LOQ. (so
the LOD needs to be < 0.2ppb).

I have thoughts of conc HNO3 attack on neat naphtha & then careful
pre-concentration. I can see of sorts of problems like:
1       Would Conc HNO3 digest all the naphtha?
2       How to pre-concentrate the digested aqueous solution, without loss
of Hg
          maybe a closed vessel microwave oven will solve that ?
3       How to ensure oxiding state is not too strong to prevent NaBH4 (or
SnCl2) reduction to enable reduction to Hg vapour.

There is insufficient need to justify a specific low level Hg analyser in
petroleum ( like that supplied by PSA )

Any ideas?

cheers
Geoff TYLER
HJY,
Longjumeau, France