12 | Atomic Spectroscopy
ADVERTISING SUPPLEMENT
THE APPLICATION NOTEBOOK – SEPTEMBER 2007
Matrix Effects in ICP-AES Analysis
Vanaja Sivakumar, Laszlo Ernyei, and Ralph H. Obenauf,
SPEX CertiPrep, Inc.
T
Experimental
Technique: Inductively Coupled Plasma-Optical Emission Spectroscopy (ICPOES) — PerkinElmer Elan DV 3300
Operating Conditions:
RF Generator Power: 1450 W
Plasma Gas flow rate: 15 L/min
Auxiliary gas flow rate: 0.5 L/min
Nebulizer flow rate: 0.65 L/min
Sample uptake: 1 mL/min
Nebulizer: Concentric
Spray chamber: Cyclonic
Injector tube diameter: 1.5 mm
Cd II 214.440
Cr II 267.716
Pb I 217.000
1.080
Cd I 361.051
Cr I 357.869
Pb II 220.353
1.060
Relative intensity
1.040
1.020
1.000
0.980
0.960
0.940
0.920
200
0
400
600
800
1000
1200
Sodium concentration (mg/L)
Figure 1: Effect of sodium concentration.
1.02
Cr I
Cr II
Cd 1
Cd II
Pb I
Pb II
1
0.98
Relative intensity
he sample “matrix” is the bulk composition of the sample such as water, organic compounds, acids, dissolved solids, and salts. Matrix effects can influence the ability of an analytical method to qualitatively identify and quantitatively measure target compounds in environmental and other samples by indirectly
affecting the intensity and resolution of observed signals. To obtain defensible
results, the analyst must account for all matrix effects.
In ICP-OES analysis, the ionic-to-atomic line intensity ratio can be used as
an indicator for determining plasma-related matrix effects. Elements such as
sodium and calcium, which are ubiquitous in nature, have low ionization potentials and as a consequence are some of the most easily ionized elements. In this
study, the influence of sodium as well as the acid concentration on the ionic and
atomic intensity of chromium, cadmium, and lead has been investigated.
0.96
0.94
0.92
0.9
0.88
0.86
0.84
0
10
20
30
40
50
60
Acid concentration (%)
Figure 2: Effect of nitric acid concentration.
Results
Figure 1 represents the relative intensity of ionic lines and atomic lines as well as
the ionic/atomic ratio in the presence of varying concentrations of sodium. The
matrix effect due to varying the concentration of nitric acid and hydrochloric
acid is shown in Figure 2 and Figure 3.
Discussion
Effect of Sodium Concentration
We observed a considerable change in the recovery of cadmium and chromium in
the presence of varying concentrations of sodium. The relative intensity of chromium
and cadmium atom lines was much higher than the respective ionic lines for these
analytes. The effect is significant even at 50 ppm sodium. The lead concentration
is affected, relatively, less by the change in the sodium concentration.
Effect of Acid Concentration
The matrix effect due to changes in acid concentration was very significant for
cadmium and lead in nitric acid; atom line and ion line for both of them showed
lower intensity. The higher the acid concentration, the lower was the intensity for
these two elements; hence, it has poorer detection limits. While the chromium
atom line was somewhat less affected, we observed a decrease in intensity for the
chromium ion line.
We also observed a much smaller, though significant, change in analyte recovery
with increase in hydrochloric acid when compared with nitric acid. Here again, effect of acid on chromium atom line was minimal when compared to chromium ion
Cr I
Cd II
1.05
Cr II
Pb I
Cd 1
Pb II
1.04
1.03
1.02
Relative intensity
Sample Preparation:
SPEX CertiPrep standard CLMS-2 was diluted to 1.0 ppm by 10X dilution with
5% nitric acid and aspirated through channel 1. Various concentrations of sodium
(0, 50, 100, 200, 500, and 1000) were prepared and aspirated through channel 2.
Both solutions were mixed through a “T” tube connector and further mixing
through a coil before entering the nebulizer.
1.01
1
0.99
0.98
0.97
0.96
0.95
0.94
0
10
20
30
40
Acid concentration (%)
50
60
Figure 3: HCl matrix effect.
line; however the acid concentration impacted lead differently. The recovery for lead
ion line was better than the atom line.
Results
Sample matrix has a profound effect on the accuracy of trace elemental analysis by
ICP. Excess acid from acid digestion can be a source of error in the analysis. Because of the different behavior of the atomic and ionic lines, at least two internal
standards are needed to compensate for matrix effects. It is absolutely essential to
match the sample matrix to the standard to ensure accurate and defensible data.
SPEX CertiPrep, Inc.
203 Norcross Avenue, Metuchen, NJ 08840
Tel. (800) LAB-SPEX, Fax (732) 603-9647
[email protected], www.spexcsp.com