Determination of anions in concentrated nitric acid by ion chromatography:
The influence of temperature on column selectivity
Summary
The determination of anions in concentrated nitric acid is of crucial importance in
semiconductor manufacture as even small amounts of ionic impurities significantly
affect the efficiency of the production process. Traditional wet chemical methods
are labor-intensive, yield poor results and suffer from the health and
environmental hazards due to the handling of nitric acid.
Ion chromatography is a very powerful analytical method allowing to reliably
determine ionic species. The analytical challenge treated in the present work
consists in detecting low concentrations (ppm) of chloride and sulfate in the
presence of very high concentrations (percent) of nitric acid or nitrate, respectively.
Due to the high concentrations of nitric acid, the sample can either be neutralized
prior to chromatographic separation or be directly injected after a 1:100 (w/w)
dilution.
The main objective of this work was to develop a direct injection ion
chromatographic method that allows to determine very small chloride and sulfate
concentrations in high-nitrate matrices. Therefore the influence of varying eluent
composition and column temperature was evaluated. Moreover, calibration
linearity and recovery rates for spiked nitric acid samples were determined.
Eluent:
3, 4, 5, 6 and 7 mmol/L Na2CO3
15 mmol/L NaOH
Flow:
0.8 mL/min
L
Loop:
20 μLL
 Metrosep A Supp 16 – 250
0.20
0.15
0.10
0.05
0.30
0.25
0.20
0.15
0.10
0 05
0.05
0.00
0
0.2
0.4
0.6
0.8
1
0
1.2
0.4
15, 20, 25 and 30 mmol/L NaOH
Flow:
0.8 mL/min
Loop:
20 μL
12
15
8
10
4
5
0
2
3
4
5
6
7
8
9
 An eluent consisting of 3…4 mmol/L Na2CO3 and 25 mmol/L NaOH is
ideal.
0.9997
Peak areas in μSmincm , concentrations in ppm
The recovery rate of the spiked chloride and sulfate is a measure for the accuracy
of the method. At first two different diluted nitric acid (1:100, w/w) samples were
analyzed. Afterwards the nitric acid samples were spiked with two different
chloride (0.1 and 0.2 ppm) and sulfate (0.2 and 0.4 ppm) concentrations,
respectively.
Chloride
1
spike Conc.
Column:
Metrosep A Supp 16 – 250
Column temp.:
15, 20, 25, 30, 35, 40,
45, 50, 55 and 60 ˚C
Eluent:
3 mmol/L Na2CO3
Flow:
0.8 mL/min
Loop:
20 μL
[ppm] [ppm]
HNO32
Br–
NO3–
HNO32 + spike 1
SO 42–
60 °C
55 °C
50 °C
45 °C
40 °C
35 °C
NO 2– /SO42–
2
4
6
Decreasing temperature
8
10
[min]
NO3–
Br–
12
14
Peak area  10
P
0.9994
0.15234 0.10 0.31466 0.20 0.63400 0.40 1.64721 1.00 3.41942 2.00
-1
Column temperature
Cl –
Coefficient of
determination
Sulfate
Sample 1
F–
2.4
Spike recovery
Increasing
g the eluent concentration of NaOH improves
p
the separation
p
of the
nitrate and sulfate peak. However, a too high NaOH concentration shortens
retention times and thus affects separation.
35
3
30
25
20
15
10
5
0
2
0.11122 0.05 0.22950 0.10 0.44230 0.20 1.23655 0.50 2.40209 1.00
10
Eluent composition
NO2–
1.6
Chloride
0
1
1.2
Standard 5
Concentration
C
Peak resolution sulfate/nitrate
Peak area  10
P
2, 3, 4, 5, 6 and 7 mmol/L Na2CO3
25
20
Standard 4
Concentration
C
Eluent:
16
Sodium carbonate [mmol/L]
Peak area  10
P
25 ˚C
20
NaOH [mmol/L]
30
Standard 3
Concentration
C
Column temp.:
35
Standard 2
Concentration
C
Metrosep A Supp 16 – 250
Peak area  10
P
Column:
Standard 1
Peak area  10
P
Increasing the Na2CO3 concentration shortens the elution time of all anions.
0.8
Sulfate concentration [ppm]
Chloride concentration [ppm]
 850 Professional
P f i
l IC Anion
A i – MCS
 858 Professional Sample Processor – Pump
y = 0.1721x - 0.004
r2 = 0.9997
0.35
0.00
25 mmol/L NaOH
System setup
0.40
y = 0.2429x - 0.0015
r2 = 0.9994
0.25
Concentration
C
25 ˚C
Peak area [μS·min/cm]
Metrosep A Supp 16 – 250
Column temp.:
Peak area [μS·min/cm]
Column:
Peak resolutiion
sulfate/nitra
ate
I t d ti
Introduction
Calibration
Concentration [m
mmol/L]
Calibration curves were linear with a coefficient of determination of 0.9994 or
better. Spike recovery percentages for chloride and sulfate in a diluted nitric acid
matrix (1:100, w/w) were within 95…102% and 95…106%, respectively. The
direct injection arrangement requires no neutralization of the acid and is highly
suitable for accurately determining low levels (ppm) of chloride and sulfate in the
presence of high quantities of nitrate in the percent range.
Data evaluation
0.30
[μS/cm]
We have developed a direct injection ion chromatography method for the
determination of low levels of anions in a nitrate matrix. Separation on the highcapacity Metrosep A Supp 16 – 250 was achieved by optimizing both eluent
composition and column temperature. An eluent composition of 3 mmol/L Na2CO3
and 25 mmol/L NaOH and a column temperature of 25 ˚C yielded an excellent
chromatographic separation with nitrate eluting as the last anion.
Eluent composition
30 °C
25 °C
20 °C
15 °C
16
 only marginally influences the retention times of fluoride and chloride
 increases retention times of nitrite, bromide and nitrate
 decreases retention times of sulfate
favor the chromatographic
On the one hand lower column temperatures
p
g p
separation of the chloride, sulfate and the nitrate peak. On the other hand nitrite
and sulfate coelute at very low temperature (15 ˚C).
2
3
HNO
+ spike 2
Sulfate
Sample 2
1
Recovery Conc.
Sample 1
1
Recovery spike Conc.
[%]
[ppm]
[%]
[ppm] [ppm]
Sample 2
Recovery Conc.1 Recovery
[%]
[ppm]
[%]
–
0.0519
–
0.0293
–
–
0.1849
–
0.0510
–
0.1
0.1534
101.47
0.1254
96.05
0.2
0.3864
100.77
0.2438
96.39
0.2
0.2552
101.61
0.2212
95.97
0.4
0.6088
105.97
0.4338
95.70
1
mean of four determinations, 21:100 (w/w) diluted
Spike recovery rates for chloride and sulfate ranged between 95...102% and
95...106%, respectively. Chloride concentrations in the two undiluted nitric acid
samples were 5.19 and 2.93 ppm; sulfate concentrations 18.49 and 5.1 ppm,
respectively.
Cl–
Sample 2 + spike 2
Spike conc.
NO3–
SO42–
0.2 ppm chloride
0.4 ppm sulfate
Column:
Metrosep A Supp 16 – 250
Column temp.:
25 ˚C
Eluent:
3 mmol/L Na2CO3
Chloride: 0.2212 ppm
Sulfate: 0.4338 ppm
25 mmol/L NaOH
Flow:
0.8 mL/min
.Loop:
20 μL
 A column temperature of 25 ˚C guarantees an excellent separation of
all anions.
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Metrohm AG, CH-9101 Herisau/Switzerland, [email protected], phone +41 71 353 85 89
Ion
n Chromatograp
phy
N. Seifert, S. Czyborra, A. Rumi