focus on Mass Spectrometry &


ICP-MS technology has the sensitivity and multi-element capabilities required, but the nature of organic matrices makes their analysis very difficult. The volatility of organic liquids means the vapours can overload the plasma, which may reduce sensitivity or destabilise, if not extinguish, the plasma. The high carbon content of such solvents causes soot or carbon particles to be deposited on the sampler cone, reducing the aperture and causing signal drift. Excessive carbon can also result in high levels of polyatomic interferences from 12 with 27


C14 Al+ and similarly 40 Ar12 C+ NH+ , which interferes with 52


and 13 Cr+


C14 N+ , which overlap . It is essential that these


interference issues are addressed for ICP-MS to be successfully applied to the analysis of organic matrices.


Preventative measures


Minimising reduced ICP-MS sensitivity and retaining instrument stability when analysing organic solvents is particularly important. Wet ashing organic samples is one procedure that can alleviate the issues caused by high carbon content and sample volatility. This decomposes the organic matrix and ultimately solubilises the elements in an aqueous matrix. However, this method is both time intensive and involves significant sample manipulation, which increases the risk of contamination and element loss. Direct sample analysis is a much more accurate and efficient technique – if matrix issues can be addressed. Chilling the spray chamber lowers the solvent vapour pressure and reduces the vapour loading on the plasma. Carbon build up on the sampler cone, which would result in drift and reduced sensitivity, is removed by the addition of oxygen into the nebuliser stream and the interfering polyatomics can be addressed with the use of reaction gases in a Dynamic Reaction Cell (DRC).


Enabling direct organic analysis


Naphthas are a class of organic liquids produced as a by-product of the crude-oil refining process within the petrochemical industry. The term covers the volatile, middle-distillate hydrocarbon mixtures, falling between light gases and the heavier kerosene. Differentiated by the hydrocarbon fractions, there are many types of naphtha; the lighter ones usually contain higher levels of paraffin, with naphthenes and aromatics forming a higher percentage of the heavier naphthas.


As napthas are used as fuel in combustion engines, any contaminating elements can potentially cause serious problems. Calcium, magnesium, sodium and potassium can form hard deposits and create excessive wear on engine components. There are also potential environmental concerns. Emissions from combustion engines and the refining process can release heavy metals, such as arsenic, mercury and lead into the atmosphere; it is for this reason that monitoring of these elements is extremely important.


The effectiveness of ICP-MS to analyse highly volatile organic mixtures was studied using three different naphthas; petroleum ether and ligroin, representing the light naphthas with boiling-point ranges of 35-60o


C and 60-80o C respectively, and Stoddard solvent, a heavier naphtha with a boiling-point range of 154-202o C.


Herein lies a significant challenge. When it comes to such a wide array of hydrocarbon composition and boiling-point ranges, how can the naphthas be successfully and accurately analysed in one run? Adequate sample-introduction conditions need to be developed to effectively handle the wide range of mixtures to be tested and suitable calibration methods need to be employed.


Two approaches can be combined to enable direct analysis of the naphthas – cooling the spray chamber and introducing oxygen into the nebuliser stream.


Spectroscopy


Overcoming the challenges of direct organic elemental analysis with ICP-MS


Erik Buseth, Environmental and Industrial Market Development Manager, EMEA and India, PerkinElmer


Analysing organic matrices for trace elements is a challenging proposition and frequently involves complex sample preparation steps. This is time consuming and potentially risks introducing contaminants or losing elements of interest. Sample digestion or wet ashing methods result in an aqueous matrix which can then be analysed by conventional ICP-OES or, where superior detection limits are required, by ICP-MS. However, the direct analysis of organic matrices by ICP techniques has historically presented a number of challenges.


Chilling the spray chamber lowers the naphtha vapour pressure, which reduces the solvent vapour loading on the plasma allowing plasma conditions to stabilise. The sample is aspirated through a glass Meinhard® glass spray at a temperature of -20o


nebuliser into a PC3 C. Even at this temperature, significant


differences are noted in the plasma conditions, internal standard recoveries and degree of signal suppression.


To address this issue of inconsistency, the naphthas were diluted. In this case the solvent employed was semiconductor-grade xylene (mixed isomers), also used as the


rinse. As xylene and naphtha are in the same carbon chain group (C8) it is ideally suited as a diluent: the two liquids are readily miscible and have similar boiling- point ranges. Conostan®


organomettalic standards were used as commercial calibration standards are not yet available in a naphtha or xylene matrix. Supplied in


concentrations from 100 to 1000 µg/g in 75 centistoke base mineral oil, mixed and single-element oil analysis standards from Conostan were prepared by dilution with xylene. The use of semiconductor-grade solvent ensures that the naphtha samples are not contaminated with elements of interest. Calibration curves were constructed


using standards at 0.2, 1 and 10 µg/L. Naphtha samples were diluted 1:10 with xylene, which minimised any sample/standards matrix variations, whilst still enabling sub-ppb detection limits.


During analysis, ammonia was used as the reaction gas to remove the argon and carbon-based interferences including 12 overlapping 24


C12 Mg+ , 27 Al+ , 52 Cr+ , and 56 Fe+ C+ , 12 C14 NH+ , 40 Ar12 C+ and 40 Ar16 O+ respectively. The standard sample-


introduction system was adapted to cater for the high organic content and volatility of the samples.


ICP-MS success


No commercial standard reference materials are available for naphtha, as such, each of the three diluted naphthas were spiked with low concentrations of target


elements (0.2 – 1.0 µg/L) to demonstrate the efficacy of the method. Each spiked sample was measured seven times and the standard deviation then multiplied by 3.14 to determine the method detection limits of the naphthas. Table 1 shows the detection limits that were achieved for the three samples.


Table 1: Detection limits for the three naptha solvents.


-LT Peltier-cooled


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