4 lead determination with internal standard, Reagents and accessories -42, Loading the method – Metrohm 746 VA Trace Analyzer User Manual
Page 127: Intstd.mth -43, Method specifications of the method, Recording the calibration curve -47, Performing the determination -52
4 Introduction to the operation
746 VA Trace Analyzer / 747 VA Stand
4-42
4.4
Lead determination with internal standard
The usual calibration procedure for the majority of the determinations
possible with the 746 VA Trace Analyzer is standard addition
(example, see section 4.2). However, the mass concentration of a
substance can also be calculated using the method of internal stan-
dards. With this calibration procedure, a known quantity of a reference
substance (internal standard) is added to the analysis solution. The
mass concentration of the analyte is calculated from the ratio of the
evaluation quantities of this substance and those of the internal stan-
dard using a proportionality factor determined earlier by measuring
different reference solutions.
Result determination with the aid of the internal standard saves time
compared with standard additions (particularly if several substances
are determined using the same internal standard), but is admissible
only if
• the requirement of a linear relation between analyte and
reference substance holds,
• the matrix of all samples and calibration solutions is identi-
cal or has no influence on the measurement,
• all measurement parameters (capillary, temperature, etc.)
remain unchanged in the measurements, and
• the accuracy of the results obtained is regularly checked
using the standard addition method.
The present example shows you how this procedure is programmed
and performed using cadmium as an internal standard in the determi-
nation of lead in drinking water.
You must first prepare the 746 VA Trace Analyzer and 747 VA Stand as
described in section 4.1 before you can perform this example.
4.4.1
Reagents and accessories
To perform the method
INTSTD.mth
, the following solutions are needed:
1
Acetate buffer 1 mol/L, pH 4.6
2
Lead ion standard solution,
ρρ(Pb
2+
) = 1 g/L
Lead ion standard solution (Metrohm No. 6.2301.100) for preparation of the
following calibration solutions:
•
Calibration solution
Pb-1
ρ(Pb
2+
) = 2.5
µ
g/L
•
Calibration solution
Pb-2
ρ(Pb
2+
) = 5
µ
g/L
•
Calibration solution
Pb-3
ρ(Pb
2+
) = 10
µ
g/L
•
Calibration solution
Pb-4
ρ(Pb
2+
) = 20
µ
g/L
3
Cadmium standard solution,
ρρ(Cd
2+
) = 1 mg/L
4
Conc. hydrochloric acid Suprapur, w(HCl) = 0.3 (30%)