Ask A Chemist

The Effect of Matrices in the Analysis of Mercury by ICP
Bill Driscoll

Mercury analysis is one of the most difficult analyses by ICP and ICP/MS instrumentation.

Mercury standards are sold in nitric acid. However, nitric acid alone is not an ideal matrix for analysis. Diluted mercury standards in nitric acid matrix show a lot of variation when analyzed over a period of time. SPEX CertiPrep conducted a study on the effect of matrices in the analysis of Hg by ICP.

Solution preparation: NIST SRM 3131 was diluted to 100 mg/L in two different nitric acid concentrations, 10% HNO3 and 5% HNO3. These two solutions were further diluted to 2 mg/L on five different days in four different matrices as shown in Table 1

Solution analysis: Initial calibration was done using 2 mg/L Hg in 20% HCl/1% HNO3. Final analysis was done on all the diluted samples at the same date following completion of analysis of a given matrix diluted on different days before taking up the second matrix.

Conclusion: Results are shown in Table 2:  Concentration shown is the average of four replicates in three wavelengths, 253, 194 and 184 nanometers. Consistent analytical results are obtained when mercury is analyzed in 2% HCl + 1% HNO3 or in 20% HCl + 1% HNO3 when the different dilutions are analyzed over a period of time. Addition of gold to nitric acid matrix also produced repeatable results.  F test showed no significant difference in the standard deviations between these matrices while the standard deviation was very significantly higher for the analysis in 2% nitric acid alone. It is concluded that nitric acid alone is not an ideal matrix for the analysis of Mercury.

Table 1:

Table 2:

The process below outlines the difference between a calibration standard and a check standard:

1. Make an educated approximation as to the elements and their concentrations in your unknown sample.

2. You would buy a Certified Reference Material that contains the elements in the sample you'd like to analyze. This is your calibration standard.

3. Dilute the calibration standard to three or four different dilutions and run a curve.

4. Run another known standard (QC check standard) with concentrations within this range and calculate the recovery. Usually the lab defines what a suitable recovery should be depending on the regulations they adhere to.

5. Run your unknown sample.

6. Spike the QC standard at a similar concentration as your step 4. Verify the spike recovery.

Biodiesel consists of esters made from fatty acids that come from a reaction between vegetable oil and an alcohol in the presence of base and a little heat (transesterification).

The possible contaminants are:
1. Unreacted vegetable oil consisting of mono, di, and triglycerides.
2. Methanol or another alcohol, left over from the reaction creating the methyl esters (actual biodiesel) from the vegetable oil.
3. Base, usually sodium or potassium hydroxide, which is the catalyst used to separate the glycerol molecule from the vegetable oil glyceride molecules.
4. Glycerol which usually settles to the bottom of the reaction vessel when biodiesel is made.

SPEX CertiPrep offers a full line of Biodiesel CRMs, visit http://www.spexcertiprep.com/biodiesel for product offerings and descriptions.

Instrument materials (test tubes, autosampler tubes, etc.) and clean lab techniques all can impact your Sodium measurements. Follow these guidelines for minimizing contamination and improving your Sodium measurements:

For test tubes, autosampler tubes, and other vessels:

1. The best material is PTFE, followed by Polypropylene.
2. Cleaning: Soak in 2-5% nitric acid for a few hours.
3. Storing: Fill up with DI water, or 2-5% nitric acid.

ICP Tubing:

1. Using the unused channel of the peristaltic pump, recirculate the blank solution through the pump tubes that are to be cleaned.

Nebulizer:

1. Glass nebulizers contain Sodium. Use plastic or PTFE nebulizers instead.
2. An ultrasonic nebulizer is more effective than a pneumatic nebulizer.
3. PTFE direct injection nebulizers are also very useful.

Torch:

1. Wash torch with DI water or 2% nitric acid (though dilute HF is ideal for cleaning, use of it is prohibited due to legal/regulatory/safety requirements).
2. For sample introduction tubes (central tube of the torch), use alumina tubes.

Keep in mind, clean room environments and high grade chemicals are also extremely useful in reducing contamination.

Acid concentration influences the nebulization process. Concentration of acid in the sample and standard should match perfectly. Amount of acid in the aliquots used for dilution should be taken in to account and further amount should be added if needed to match the standard.

The addition of acid is recommended to prevent the formation of hydroxides. Hydroxides have a tendency to absorb on the surface of storage containers, autosampler tubes and all parts of the sample introduction system. Hence nebulized aerosol droplets will not be a true representation of the standard itself and will lead to lower intensity readings which will cause erroneous results. This is true for the sample as well the standard and one needs to match the amount of acid in both.

The kinetics for formation of hydroxides depends on the element. Quantity of acid should be enough to keep the elements in solution. Any precipitation would disturb the composition of the solution and/or the nebulization.

As a general rule, standards in the ppb range need at minimum 1% acid. Low ppm range standards need 2% acid, and a level of 5% is recommended for concentrations greater than 100 ppm.

If you see some cloudiness or precipitation, it can be inferred that the amount of acid added is not sufficient.

Sometimes, the precipitation may not be visible but still can affect the measurements. Indication for this is an abnormality shown in intensity vs time. Usually, higher concentration of acid will resolve this problem. However, always match the concentration of acid in the sample and the standard.

The density, viscosity and surface tension of the matrix have considerable effect on the droplet size of the aerosol. Only fine droplets are transported to the plasma torch. Matching the acid concentration of the sample and the standard helps in transporting similar quantity of droplets (from both solutions) to the torch, thus improving the accuracy.

The temperature of the optic is not stable; the instrument needs a warm up time before performing the first measurement. For most instruments, the warm up time is usually 15-30 minutes. If it fluctuates even after sufficient warm up time, the temperature control in the instrument is not functioning properly. Call for service from the manufacturer.

The easily ionizable elements have very different reaction on the plasma electron concentration than the internal standards used. These lines are known as “soft lines”. For alkali metals such as those you have mentioned, you should select another internal standard, such as Cesium, or do not use an internal standard. When selecting the internal standard, consideration should be given to the ionization stage (ion or atom) and ionization energy and excitation energy of the analytes of interest. Also, the selected internal standard should not be part of the analytes of interest in the mix.

There are two possibilities for this:

1. There is oxygen in the optical path. Oxygen absorbs the low wavelength photons more than the higher wavelengths, thus decreasing the intensity. Call the manufacturer for service.
2. The optical system has a window, and it may be corroding. The longer wavelength photons can go through but the shorter ones are absorbed. Your option is to change the window(s).

If internal standard is selected correctly, it can compensate for fluctuation in the power outputted from the power source thus increasing the precision of measurements; it will also compensate for the variation in nebulization if it occurs.

The most common ways to resolve a zero-reading reference material are:

1. Check the sample introduction system. The nebulizer should be clean and the tubing connecting the nebulizer, peristaltic pumps and auto-sampler should be tight.
2. Check the spectrum of the element(s) in question. Verify the peak is centered.
3. Check the blank for any contamination.
4. If the CRM is diluted, use the appropriate matrix and adequate acids for dilution.
5. Analyze the solution at different concentrations than the one used (preferably two times higher or lower).

Volatile acids such as Hydrochloric acid can be purified by Isothermal Distillation, also known as Isopiestic Distillation. Place an open beaker containing reagent grade acid and an open beaker containing high purity water within a closed system at room temperature. An empty desiccator can be used to create such a closed system. Acid vapors will be continuously transferred to the high purity water until an equilibrium is established between the two solutions. This should be accomplished in 12 to 24 hours creating high purity acid from the water, at a lower concentration than that of the original reagent grade acid, of course. This method can also be used to purify ammonia as well.

As per our Certificate of Analysis, stock anion standards such as nitrate, nitrite, phosphate, sulfate, and others are guaranteed stable to ±0.5% of the certified concentration for a period of 1 year if materials are kept tightly capped and stored under normal conditions of temperature (50-80°F) and Humidity (30-80%). Refrigeration is not required. If for any reason the standards are refrigerated, bring the solution to room temperature before using.

In general the instruments are accurate to within ±1%. Most of the time, you can further refine the measurements by replications and using an exact calibration standard matching the analyte concentration and matrix of interest.

This is a great question with a very easy solution. While Sb2O3 dissolves easily in tartaric acid and water, the solution is clear at first but a gelatin like substance can form over time. This is a form of mold, and adding a trace amount of nitric acid to the solution can prevent this.

Great question, here are a few conversion factors for our most popular IC Standards:

• 1000 µg/mL Nitrate = 226 µg/mL Nitrogen
• 1000 µg/mL Phosphate = 326 µg/mL Phosphorus
• 1000 µg/mL Nitrate-Nitrogen = 1000 µg/mL Nitrogen
• 1000 µg/mL Phosphate-Phosphorus = 1000 µg/mL Phosphorus
• 1000 µg/mL Nitrite = 305 µg/mL Nitrogen
• 1000 µg/mL Sulfate = 334 µg/mL Sulfur
• 1000 µg/mL Nitrite-Nitrogen = 1000 µg/mL Nitrogen
• 1000 µg/mL Sulfate-Sulfur = 1000 µg/mL Sulfur

To calculate the gravimetric factor of other ions or at other concentrations, the formula is as follows:

(Formula weight of the element / Formula weight of the ion) * Concentration of the Ion

Researchers at the EPA have defined twelve principles of green chemistry. These principles are:

1. Prevent waste
2. Design safer chemicals and products
3. Design less hazardous chemical syntheses
4. Use renewable feedstocks
5. Use catalysts, not stoichiometric reagents
6. Avoid chemical derivatives
7. Maximize atom economy
8. Use safer solvents and reaction conditions
9. Increase energy efficiency
10. Design chemicals and products to degrade after use
11. Analyze in real time to prevent pollution
12. Minimize the potential for accidents

Paul Anastas and John Warner in Green Chemistry: Theory and Practice (Oxford University Press: New York, 1998).

SPEX CertiPrep is constantly revising its policies and procedures to include more green practices each year. We try to limit our production stocks to both ensure quality and reduce waste. Many of our organic products are created for our customers on-demand, which dramatically reduces exposure and waste. SPEX CertiPrep has designed our own acid purification still to purify our own acids.

Our company has active programs to promote reduction, reuse and recycling of resources and materials.

SPEX CertiPrep is committed to increasing green practices and our own environment impact in 2009. Check back with us throughout the year to see updates on our progress!

Melamine is a small man-made, nitrogen-rich molecule which has been in use since it was first synthesized in 1834. Melamine has been combined with formaldehyde to form melamine resin which is used in countertops, whiteboards, flame retardants and home goods.

The scare of the past two years has actually come from melamine and a group of other associated chemicals: ammeline, ammelide and cyanuric acid. The illegal uses of these compounds as counterfeit protein substitutes in food products have caused illness and death in people and animals.

Proteins are organic molecules which contain, among other elements, nitrogen. Testing of many food items for protein involves the calculation of the nitrogen content to determine the amount of protein. Manufacturers from China added melamine and cyanuric acid to boost the nitrogen level of their products and therefore artificially elevate the protein levels of their products.

Unfortunately, melamine and cyanuric acid readily form together a complex network of bonds called melamine cyanurate. This new, very large molecule becomes a physical obstruction in body tissues and organs such as the kidneys. The crystals build up in the systems and shut the organs down. Melamine cyuranate is actually more toxic than either melamine or cyanuric acid alone.

The new awareness of the potential contamination of food stocks has prompted more laboratories to test for melamine, cyanuric acid and their associated compounds. SPEX CertiPrep has several standards for these compounds:

• S-4806: Melamine (500 µg/mL)
• S-4807: Cyanuric Acid (1000 µg/mL)
• S-4808: Ammeline (1000 µg/mL)
• S-4809: Ammelide (100 µg/mL)
• XQ-4042: All four compounds (1000 µg/mL)

Phthalates are esters of 1,2-benzenedicarbolxylic acid. These compounds are used in almost every industry as coatings, binders, lubricants, plasticizers, and many other applications. The most common phthalates can be found in the table below.

The most widely used phthalates in production today are DEHP, DBP, and BBP
Di-(2-ethylhexyl) phthalate (DEHP) is found in the production of polyvinyl chloride (PVC). DEHP is also widely used in the medical device industry. Plastics can contain up to 40% DEHP. Butyl benzyl phthalate is found as a plasticizer in vinyl foams, floor tiles, synthetic leather, and food production line conveyor belts. Dibutyl phthalate (DBP) is found in adhesives, binders and inks.

Phthalates have come under scrutiny in the past decade for their widespread contamination in the environment and their potential health consequences. SPEX CertiPrep has several phthalate standards in stock for your laboratory’s standard needs. Check back with us soon for our latest studies on phthalates in water samples!

There are seven common plastic packaging materials. These food packaging materials each have a different recycling number called a “Plastic Identification Code” (PID). The codes are as follows:

PIC Abbreviation Compound Uses

1. PET Polyethylene Terephthalate Soft drink and water bottles
2. HDPE High Density Polyethylene Milk bottles, retail bags
3. PVC Polyvinyl chloride PVC pipe, plastic films & wraps
4. LDPE Low Density Polyethylene Squeeze bottles, cling wraps, food bags
5. PP Polypropylene Microwave containers
6. PS Polystyrene Foam containers
7. Other Often polycarbonate Refillable bottles, baby bottles, other

Bisphenol A is most commonly found in polycarbonate materials such as baby bottles, sports bottles and reusable drink containers marked with the identification code “seven”.

SPEX CertiPrep has a Bisphenol A (S-509) standard available for your testing needs. Check back with us soon to see our study on Bisphenol A found in beverage containers!

MFSTA is a reagent used for derivatizing compounds in order to detect them on a GC. The chemical name is N-trimethylsilyl-N-methyl trifluoroacetamide. It falls in the category of Silyation reagents which changes the molecule making it more volatile so that in can be analyzed. There are many types of derivitizing reagents. Most reagents fall into the categories of Acylation, Alkylation, Esterification and Silylation.

MFSTA is commonly used for glyceride analysis in biodiesels as well as for melamine in food raw materials.

Yes, vinyl acetate breaks down into acetic acid methyl ester (CAS# 79-20-9) in the presence of methanol and other compounds like ketones. Acetic acid methyl ester is also known as methyl acetate and has the characteristic m/z ions or 43, 59 and 74. SPEX CertiPrep supplies vinyl acetate (S-3800) as a single component standard, but be careful what you blend it with as it will not last long.

Not as stable as you might think. Phenanthrene and Anthracene are isomers; Phenanthrene is stable while Anthracene is not. We have observed hundreds of samples in our past experience where the concentration of Phenanthrene is much higher than that of Anthracene. It ends up that Anthracene breaks down to form 1,4-Anthraquione. We have observed this happening in sealed ampules of calibration standards containing Anthracene. The breakdown reaction takes place over a 4-5 year period when sealed in an amber glass ampoule.

For a more detailed explanation of why this is, you might find this paper of interest.

We have a calibration standard containing Metribuzin and are seeing two peaks for this compound, can you explain this?

Metribuzin breaks down depending on factors such as storage condition, other compounds in the solution, the solution matrix itself and, of course, time.

On a typical GC capillary column, 5% diphenyl, 95% methylpolysiloxane stationary phase for example, the breakdown peak will show up shortly after the Metribuzin. If you have access to a GC/MS you can easily identify Metribuzin, major ion 198 and distinct minor ion of 214. The breakdown product has the same major ion of 198, but contains distinct minor ions of 211, 239 and 254.