Improving Throughput of Environmental Samples by ICPMS Following EPA Method 200.8

The United States Environmental Protection Agency (US EPA) method 200.8 defines the procedure for measuring trace elements in water and soil/sediment samples by ICPMS. The method lists five interferences (isobaric elemental, abundance sensitivity, isobaric polyatomic ion, physical and memory) which can affect the efficiency and accuracy of the analysis. Both the physical interferences and the memory interferences are highly affected by the type of sample introduction system used. The use of the Elemental Scientific SC-FAST AS 200.8 rapid analysis system gives an average 2 fold improvement in the method detection limit (MDL) and reduces the memory interference, resulting in a doubling of the sample throughput.

A total of 33 elements were analyzed using the SC-FAST AS 200.8 method, 27 analytes and 6 internal standards (see Table I) and compared to a standard sample introduction system for washout, method detection limits and sample throughput.

Uptake and Washout
It is important that the nebulizer used also has a low internal dead volume to ensure a rapid rise to steady state signal and return to baseline signal. The time profiles in Figure 1, show that the PFA-ST MicroFlow nebulizer has a much lower dead volume than a standard glass concentric nebulizer, the ST-nebulizer is integral to the improved performance of the SC-FAST AS 200.8 system.

Hundreds of SC-FAST units are already installed and in use:

Blanks and Detection Limits
The improved wash out and reduced matrix loading of the ICP-MS using the SC-FAST results in improved method detection limits (MDL) compared to the standard introduction system (Figure 2). Lower SC-FAST DLs for 14 elements presented in Figure 2 illustrate an average 2 fold improvement over standard sample introduction due to improved signal stability and lower blanks.

Sample Throughput
Typical time profiles for the standard and SC-FAST sample introduction schemes can be seen in Figure 3.

With the SC-FAST, a rinse time of 30 seconds is sufficient to prevent sample carryover of the 5µg/L Hg signal (see Table II) (EPA Method 200.8 recommends a sample wash time of 120 seconds to rinse 5µg/L of Hg from the previous sample before the next sample is analyzed (longer if gold is not added to the rinse solution).

The combined SC-FAST benefits of rapid uptake and rinseout, excellent stability and reduced blanks and DLs improve the performance of EPA Method 200.8 while reducing time for individual sample analysis from 240 s to 110 s (see Table III).

An added benefit of using the SC-FAST to reduce the sample uptake time and remove the fast peristaltic pumping lowers the total amount of sample matrix reaching the torch injector and ICP-MS cones (1). The total sample load on the plasma is reduced by 57% (0.54mL of sample using the SC-FAST compared to 1.25mL with the standard introduction system) resulting in less matrix deposition on the injector and cones between samples, improving long term stability and reducing maintenance.

Conclusion
The reduced sample uptake and rinse times with the SC-FAST mean the total sequence analysis time is more than halved. The improved efficiency in sampling means the costs of analysis are reduced with savings in Argon and other ICP-MS consumables, giving the SC-FAST a rapid return on investment.

References
(1) Direct determination of P, V, Mn, As, Mo, Ba and U in seawater by SF-ICP-MS M. Paul Field, Michèle LaVigne, Kathleen R. Murphy, Gregory M. Ruiz and Robert M. Sherrell, J. Anal. At. Spectrom., 2007

Schematic of SC-FAST AS 200.80

Schematic of SC-FAST AS 200.8


Table I: Elements measured in SC-FAST AS 200.8 Method
6Li 44Ca 59Co 82Se 121Sb 209Bi
9Be 45Sc 60Ni 89Y 137Ba 232Th
23Na 51V 65Cu 98Mo 165Ho 238U
24Mg 52Cr 66Zn 107Ag 202Hg  
27Al 55Mn 71Ga 111Cd 205Tl  
39K 57Fe 75As 115In 208Pb I.S.

Table II: Instrument calibration blank (ICB) ran immediately after 5µg/L Hg calibration standard
  200Hg 202Hg
ICB 1 0.003 0.003
ICB 2 -0.002 0.000
ICB 3 0.001 0.003
ICB 4 0.000 0.001
ICB 5 -0.003 0.002
Std. Dev. 0.0024 0.0013
0.007ppb 0.004ppb
Ir used as I.S. for Hg calibration

Table III: Method Timings (seconds)
Experimental Conditions
Standard SC-FAST
Sample Uptake Delay
30 10
Sample Stabilization
30 0
Sample Reading
20 20
Replicates
3 3
Sample Wash
120 35
Total Analysis Time
240 110

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