Embed Size (px)
Citation preview
1
Microwave Acid Digestion Guidelines
Recommended Literature
2
Recommended Literature
3
Acids Chemistry
4
Acids
Non-oxidizing• Hydrochloric acid• Hydrofluoric acid• Phosphoric acid• Diluted sulfuric acid• Diluted perchloric acid
Oxidizing• Nitric acid• Hot concentrated
perchloric acid• Concentrated sulfuric
acid• Hydrogen peroxide
Nitric Acid
• Boiling point is 120°C at 65% concentration• Poor oxidizing strength at concentrations less
than 2 M; oxidizing strength increases with concentration and reaction temperature and pressure
• Most common acid for oxidation of organic matrices(CH2)X + HNO3 CO2(g) + NOX(g) + H2O
5
Nitric Acid
• It dissolves most metals forming soluble nitrates, exceptions are Au and Pt (not oxidated) and Al, B, Cr, Ti and Zr (passivated)
• These metals require acid mixtures or diluted nitric acid
• Often mixed with H2O2, HCl and H2SO4
• Available in high purity for trace analysis
Nitric Acid
0
50
100
150
200
250
300
0.00 0.05 0.10 0.15 0.20 0.25Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
6
Hydrochloric Acid
• Boiling point of azeotropic mixture with H2O with 20,4% HCl is 110°C
• Available with 38% concentration• Nonoxidizing• It dissolves salts of weak acids (carbonates,
phosphates) and most metals are soluble with the exception of AgCl, HgCl and TiCl
• Excess of HCl improves the solubility of AgCl, converted into AgCl2-
Hydrochloric Acid
• Strong complexing nature• Widely used for iron-based alloys because of its
ability to hold large amounts of chloro-complex in solution
• Other complexes formed are Ag (I), Au (II), Hg (II), Ga (III), Tl (III), Sn (IV), Fe (II) and Fe (III)
• It does not dissolve oxides of Al, Be, Cr, Ti, Zr, Sn and Sb; sulphates of Ba and Pb, group II fluorides, SiO2, TiO2 and ZrO2
7
Hydrochloric Acid
0
50
100
150
200
250
300
0.00 0.05 0.10 0.15 0.20 0.25Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
Hydrofluoric Acid
Digestion• Boiling point is 108°C at 40% concentration• Nonoxidizing, strong complexing nature• Used in digestion of minerals, ores, soils, rocks
and even botanical samples• Major use is the decomposition of silicates
SiO2 + 6HF H2SiF6 + 2H2O• Often used in combination with HNO3 or HClO4
8
Hydrofluoric Acid
Concentration• Following dissolution, many analyses require
removal of HF to prevent equipment damage or to resolubilize insoluble fluorides
H2SiF6 SiF4 + 2HF• Many analytes such as As, B, Se, Sb, Hg, Cr
may volatilize
Hydrofluoric Acid
Complexation• Alternative approach to remove HF from the
solution, by addition of boric acid• The following reactions take place
H3BO3 + 3HF HBF3(OH) + 2H2OHBF3(OH) + HF HBF4 + H2O
• 10-50 times excess boric acid enhances reaction rate
9
Hydrofluoric Acid
0
50
100
150
200
250
300
0.00 0.05 0.10 0.15 0.20 0.25Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
Sulfuric Acid
• Boiling point is 340°C at 98% concentration, exceeding max working temperature of Teflon vessels
• Careful reaction temperature monitoring is required to prevent vessel damages
• It destroys organics by dehydrating action• Many sulfates are insoluble (Ba, Sr, Pb)
10
Sulfuric Acid
0
50
100
150
200
250
300
350
0.00.00 0.02.30 0.05.00 0.07.30 0.10.00Time (h.mm.ss)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
Perchloric Acid
• Boiling point is 203°C at 72% concentration• Powerful oxidizing acid when used warm• Hot and concentrated decomposes violently
organic matter• Nearly all perchlorates are soluble• HClO4 decomposes at 245°C in microwave
closed vessel with dangerous amounts of by-products and tremendous excess pressure
11
Perchloric Acid
0
50
100
150
200
250
300
350
0 20 40 60 80 100Time (counts)
Tem
pera
ture
(°C
)
0102030405060708090100
Pres
sure
(bar
)
Perchloric Acid• Rule #1: do not use it• Use only very diluted perchloric acid• Mix it with other acids (but never with sulfuric
acid)• Never exceed 200°C• Use it only to perform a two-step digestion• Perchloric acid is normally not required for the
closed vessel microwave digestion of organic samples
12
Hydrogen Peroxide
• Oxidizing agent2H2O2 2H2O + O2
• Added to HNO3 it reduces nitrous vapors and it accelerates the digestion of organic samples by raising the temperature
• Typical mixture ratio is HNO3:H2O2=4:1
Aqua Regia• HCl and HNO3 in 3:1 (v/v) mixture• It produces NOCl (nitrosyl chloride), which
decomposes in NO and Cl2 up on heating3HCl + HNO3 NOCl +Cl2 + H2O
• It dissolves precious metals• It must be freshly prepared and used
immediately, otherwise it evolves chlorine gas overpressurizing and venting the vessel
13
Aqua Regia
0
50
100
150
200
250
300
0.00 0.05 0.10 0.15 0.20 0.25Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
Environmental Samples
14
15
US EPA 3015a
0
40
80
120
160
200
0.00 0.05 0.10 0.15Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
16
US EPA 3015a
0
40
80
120
160
200
0.00 0.05 0.10 0.15 0.20 0.25Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
US EPA 3015a
0
40
80
120
160
200
0.00 0.05 0.10 0.15 0.20 0.25Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
17
18
19
US EPA 3051a
0
40
80
120
160
200
0.00 0.05 0.10 0.15Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
20
US EPA 3051a
0
40
80
120
160
200
0.00 0.05 0.10 0.15Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
21
22
US EPA 3052
0
40
80
120
160
200
0.00 0.05 0.10 0.15 0.20Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
23
Organic Samples
Organic Samples
• Nitric acid is the most common oxidizing agent used to digest organic samples, according to the following reaction
ORG + HNO3 CO2 + H2O + NOX
• Metals are converted into soluble nitrates, available for analysis
24
Temperature
• High fat (cheese, butter, vegetable oil etc.)180°C
• High protein (bovine, serum, albumin)160°C
• High carbohydrates (wheat, sugar etc.)140°C
• Based on sample decomposition with HNO3
0,5 g Olive Oil
0
50
100
150
200
250
0.00 0.05 0.10 0.15 0.20 0.25Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
25
0,5 g Milk Powder
0
50
100
150
200
250
0.00 0.05 0.10 0.15 0.20 0.25Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
0,5 g Noodles
0
50
100
150
200
250
0.00 0.05 0.10 0.15 0.20 0.25Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
26
Pressure
• Temperature is key• Pressure is mean• Microwave heating raises acid
temperature and vapor pressure• Gaseous products (CO2 and NOX) are
formed from sample decomposition
0,1 g Milk Powder
0
50
100
150
200
250
0.00 0.05 0.10 0.15Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
27
0,25 g Milk Powder
0
50
100
150
200
250
0.00 0.05 0.10 0.15Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
0,5 g Milk Powder
0
50
100
150
200
250
0.00 0.05 0.10 0.15Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
28
1,0 g Milk Powder
0
50
100
150
200
250
0.00 0.05 0.10 0.15Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
Milk Powder
Up to 48 bar25 bar~ 100°C220°C1,0 g
25 bar25 bar~ 150°C220°C0,5 g
25 bar25 bar~ 200°C220°C0,25 g
~ 8 bar25 bar220°C220°C0,1 g
Actual pressureSet pressureActual
temperatureSet
temperatureSample weight
29
Limitations of Pressure Control
0
50
100
150
200
250
0.00.00 0.05.00 0.10.00 0.15.00 0.20.000
10
20
30
40
50
Power (watt)/10 Temperature (°C) Pressure (bar)
Limitations of Pressure Control
0
50
100
150
200
250
00:00:00 00:05:00 00:10:00 00:15:00 00:20:000
10
20
30
40
50
Power (watt)/10 Temperature (°C) Pressure (bar)
30
Pressure-based Digestion Quality
• Left– 1,0 gram Leaves• 20 bar
• Right– 0,25 grams Leaves• 20 bar
HNO3
0
50
100
150
200
250
0.00.00 0.05.00 0.10.00 0.15.00 0.20.00Time (hh.mm.ss)
Tem
pera
ture
(°C
)
0
20
40
60
80
Pres
sure
(bar
)
31
Microwave vs. ‘Teflon Bomb’
0,25 g BCR 185 Bovine Liver
0
50
100
150
200
250
0.00.00 0.05.00 0.10.00 0.15.00 0.20.00Time (hh.mm.ss)
Tem
pera
ture
(°C
)
0
20
40
60
80
Pres
sure
(bar
)
32
0,5 g BCR 185 Bovine Liver
0
50
100
150
200
250
0.00.00 0.05.00 0.10.00 0.15.00 0.20.00Time (hh.mm.ss)
Tem
pera
ture
(°C
)
0
20
40
60
80
Pres
sure
(bar
)
0,75 g BCR 185 Bovine Liver
0
50
100
150
200
250
0.00.00 0.05.00 0.10.00 0.15.00 0.20.00Time (hh.mm.ss)
Tem
pera
ture
(°C
)
0
20
40
60
80
Pres
sure
(bar
)
33
1 g BCR 185 Bovine Liver
0
50
100
150
200
250
0.00.00 0.05.00 0.10.00 0.15.00 0.20.00Time (hh.mm.ss)
Tem
pera
ture
(°C
)
0
20
40
60
80
Pres
sure
(bar
)
1,25 g BCR 185 Bovine Liver
0
50
100
150
200
250
0.00.00 0.05.00 0.10.00 0.15.00 0.20.00Time (hh.mm.ss)
Tem
pera
ture
(°C
)
0
20
40
60
80
Pres
sure
(bar
)
34
Effect of Sample Amount
01020304050607080
0.00.00 0.05.00 0.10.00 0.15.00 0.20.00Time (hh.mm.ss)
Pres
sure
(bar
)
Effect of Sample Amount
• Higher pressure conditions• More exothermic reactions• Vessel and microwave system have to be
capable of withstand or handle such over-pressurization
• Vessel design and materials are key factors for safe microwave digestions
35
0,25 g PET
0
50
100
150
200
250
300
0.00 0.10 0.20 0.30 0.40Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
Inserts Technology
36
Inserts Technology
• Sample directly weighed in the quartz insert
• Quartz insert introduced inside the TFM vessel
• HNO3 is added inside the quartz insert• H2O and H2O2 added outside the quartz
insert, in the TFM vessel
0,25 g PET
0
50
100
150
200
250
300
0.00 0.10 0.20 0.30 0.40Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
37
Quartz Inserts
10
9 BCR 186 Pig Kidney
8
7
6 BCR 062 Olive Leaves
5
4
3
0,5 g
BCR 061 Aquatic Plant
2
5 ml H2O and 2 ml H2O2
5 ml HNO3
-Blank1
OutInWeightSampleVessel
Quartz Inserts
*Direct analysis on solid samplewith Milestone DMA-80 gave 1,78 mg/kg
30,0551,824*1,970 ± 0,040BCR 186
30,0040,2840,28 ± 0,02BCR 062
30,0030,2080,23 ± 0,02BCR 061
NSdFound (mg/kg)
Certified (mg/kg)Sample
38
Inorganic Samples
39
Inorganic Samples
• Sample preparation procedures are strongly dependent on the chemical nature of the material to be digested and on the elements to be determined
• The microwave acids choice is often the same as used with “conventional” methods
• Many geological samples may be digested according with the US EPA 3052 method
Inorganic Samples
• Most inorganic samples show non-exothermal reactions and do not produce large amount of gases
• The sample amount does not affect the ratio temperature/pressure as much as with organics
• The digestion efficiency strongly depends on digestion temperature
• The target is to bring the solution to the highest temperature in the shortest time (1st step) and to hold this temperature until the digestion is complete (2nd step)
40
1,0 g Cu Concentrate
0
50
100
150
200
250
0.00 0.05 0.10 0.15 0.20 0.25 0.30Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
2,5 g Cu Concentrate
0
50
100
150
200
250
0.00 0.05 0.10 0.15 0.20 0.25 0.30Time (h.mm)
Tem
pera
ture
(°C
)
0
10
20
30
40
50
Pres
sure
(bar
)
41
0,2 g Ruthenium
0
50
100
150
200
250
300
0.00 0.30 1.00 1.30 2.00 2.30Time (h.mm)
Tem
pera
ture
(°C
)
Our Experience at Your Service
42
