Author
Kenneth Ong

PerkinElmer Life and Analytical Sciences 710 Bridgeport Avenue Shelton, CT 06484 USA

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Milk Analysis with ICPMS

The major and trace elements in milk powders can be analyzed using a simple sample preparation method, within one solution. This involves no digestion of the sample but an easy-to-follow and quick preparation of a slurry or suspension of the milk. The ICPMS parameters are optimized to allow extension of liner range for analysis of major elements (in % level) while maintain good sensitivity for analysis of trace elements (in ppb level). This method has been shown to work for whey powders which contain little fat, and for baby milk powders and nutritious drinks containing high fat levels.

Sample Preparation

Approximately 400mg of the sample was weighed accurately into the quartz vessel of the Perkin-Elmer Paar Multiwave system. To each sample was added 4 ml of 65% HNO3 (Merck Suprapur) and 1 ml of 37% HCl (Merck Suprapur). For each run of the program at least one of the vessels contained acid only and was used as a sample blank. The samples were digested using the following program:

Make up to 50mL with DI water.

Dilute 1+1 with DI water and spike with internal standard mix.

Final concentration of internal standards: 10ppb Ga, In, Tm plus 5ppm Au to stabilize the Hg.

All elements can be measured on the milk digest or the milk slurry, however, iodine may be lost during the acid attack and the correct iodine result will be obtained on the slurry. As such for Iodine analysis, the milk powders were analyzed as slurries after preparation with the following method:

0.7g milk powder plus 70mL of DI water, shake and dissolve. The solution is added with 10mL of 3.6% HCl, and spiked with internal standards before make up to 100mL with DI water. Final solution is 0.7% milk powder in a matrix of 0.36%HCl.

Alternatively, Iodine in milk may be extracted via TMAH as outlined by Joaquim et al. (1)

Plasma Conditions

The ICP-MS was optimized for top of the mountain performance for In with the CeO/Ce oxide ratio set to < 3%.

The following plasma conditions were used with a cyclonic spray chamber with High TDS Concentric nebulizer.

Plasma gas:15 L/min
Auxiliary gas: 0.8 L/min
Nebulizer gas: 1.02 L/min
Plasma power: 1400W
Uptake rate: 1 mL/min

For the analysis of milk solutions at 0.5% and 0.7% milk powder the potassium and sodium concentration is very high for ICP-MS analysis, ca. 50-100ppm (mg/L) in solution. To keep these high signals on scale the custom resolution feature of the Elan DRC was utilized. The K and Na were analyzed with a resolution, i.e. peak width measured at 10% peak height, of 0.3-0.4 amu. This attenuates the signals by approximately an order of magnitude allowing much larger than normal signals to be measured easily within the range of the Elan dual detector. (2)

The remaining elements were measured with normal resolution, that is, 0.7 amu peak width and high sensitivity. This feature enables Hg, Cd to be measured in the same solution, at the same dilution (one shot) together with the Na and K at very high concentrations. This feature gives the instrument effectively 10 orders of linear range whereas the detector has 9 orders of linear range available.

The only alternative to this would be to measure the samples twice at two different dilutions.

The samples analyzed as 0.7g powder/100ml of 0.36%HCl, give excellence agreement with Certified value of the major and trace elements.

Elements marked with * were analyzed in DRC mode using NH3 gas flow.

References