Performing EPA method 1631 using the PSA Millennium Merlin 10.035 systems

Posted by: chrisball | Posted on: February 14th, 2012 | 0 Comments

Mercury is naturally present in aquatic systems in very low concentrations. Due to the long range atmospheric transport and deposition of anthropogenic mercury, elevated concentrations of mercury are found even in remote freshwater system although no direct local contamination sources are present. It is long recognised that mercury is one of the most hazardous toxicant to human and the environment. To protect people and the environment from the mercury, governments and regulatory agencies are introducing ever more stringent guidelines. As a result, analysts are challenged to achieve lower detection limits. The use of atomic fluorescence spectrometry for the determination of mercury was first reported by Thompson and Reynolds in 1971. Since then, several authors have described enhancements to the technique that reduced formal instrument detection limits (IDL) for the technique to the 1–10 ng/L range. Knox et al. reported on the use of atomic fluorescence for the determination of mercury with a detection limit of less than 1 ng/L. The USEPA has promulgated Methods 245.7 and 1631 for the determination of low level total mercury in water. The 245.7 method is based on vapor generation atomic fluorescence spectrometry (CV-AFS) without gold amalgamation whereas the 1631 method uses amalgamation to pre-concentrate mercury to provide lower detection limits. Method 245.7 can be performed using the PSA 10.025 or alternatively a PSA 10.035 can be used if the preconcentrator is bypassed.   Here we show how the PSA Millennium Merlin system meets the methodology, operation procedure, analytical sensitivity, and the quality control criteria for this method.

Figure 1           PSA 10.035 Millennium Merlin System

The Millennium Merlin system (Figure 1) is designed for routine use, which provides the full range of facilities required by researchers and method development chemists. The Millennium Merlin system offers standard modules that can be configured to meet current legislation. In addition to the EPA Methods 1631 and 245.7 the CV-AFS also complies to European Hg standards EN13506, EN12338 and EN17852.

Figure 2        Schematic flow diagram of PSA 10.025/10.035 Millennium Merlin. Method 245.7 configuration is when pre-concentration is bypassed

Table 1            EPA Method 1631

The USEPA approved the Method 1631 for the determination of low level total mercury in water which specified a detection limit of 0.2 ng/L. With Millennium Merlin system, even better sensitivity (0.05 ng/L) can be achieved. Apart from the sensitivity, Method 1631 also requires to satisfy a formal quality assurance program which comprises the following requirements, at a minimum:

(a)         A calibration containing a minimum of 5 non-zero points, with 3 system blanks. The lowest calibration level must be lower than 0.5 ng/L. The RSD of the calibration factors less than 15%. A recovery of 75-125% for the lowest point of the calibration.

(b)         Four replicates of initial precision and recovery solution (IPR, 5 ng/L), RSD<21%, recovery of 79-121%

(c)         All bubbler blanks must be <0.25 ng/L with a standard deviation (n-1) <0.10 ng/L. All system blanks and must be <0.5 ng/L with a standard deviation (n-1) <0.1 ng/L. All reagent blanks and method blanks must be <0.20 ng/L and <0.50 ng/L, respectively.

(d)         Matrix spike in duplicate, with a recovery of 71-125% and a relative percent difference (RPD) < 24.

(e)         An ongoing precision and recovery (OPR, 5ng/L) solution must be analysed prior to and after each sample batch, with a recovery of 77-123%.

(f)           A quality control sample (QCS) prepared from a different source of Hg standard must be analysed as an independent check of the system performance.

(g)         A method detection limit (MDL) <0.2 ng/L.

Figure 1        Method setting of Millennium Merlin system Performing EPA Method 1631

Table 2 Sequence of analysis complying EPA Method 1631 with typical data set

EPA Method 1631 and 245.7 allows to measure as many as 20 samples in each sample batch between QC samples. In each batch, a number of QC requirements must be met to demonstrate the initial capability of the laboratory. A typical analytical sequence of 1631 is listed in Table 1 which includes a minimum of 3 system blanks, 1 OPR at the beginning and the end of each batch, 4 IPR, 1 QCS and 2 MS/MSD for every ten samples. The system performance is verified if each QC test in the analysis sequence qualifies the criteria mentioned above in the introduction. In addition, a Method Detection Limit shall be determined to be equal or less than 0.2 ng/L. The results of the measurement shall not be reported if any of the criteria fails.

Each sequence also includes a calibration curve with a minimum of five, non-zero standards with the lowest concentration of 0.5 ng/L (5 ng/L for Method 245.7). A typical calibration curve of Method 245.7 is shown in Figure 4, with the lowest calibration standard of 1ppt, which surpasses the EPA requirement.

Method 1631 is a US EPA specified method for the determination ultra low level of total mercury in water samples, with a detection limit of  0.2 ng/L.

The PSA 10.035 Millennium Merlin 1631 not only meets the all the requirements of Method 1631 for measuring mercury in water but even betters the requirements for accuracy and precision, passing all QC tests and achieving an MDL of 0.15 ng/L.