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Analysis of short chain aliphatic carboxylic acids in aqueous environment

by HPLC

Annie Coenen, Jim Dohmen

Jos Hellenbrand, Ynze Mengerink

Ron Peters and Sjoerd van der Wal

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Most industrial aqueous fluids contain

short chain carboxylic acids at ppm levels e.g.

- plant process fluids

- aqueous extracts of organic solvents

- processed drinking water

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Water in “bricks”

O O

OH

[O]

Stale taste correlated with:

Technique

- aldehydes at ppt levels GC-MS

- ketones at ppt levels GC-MS

- carboxylic acids ppb’s LC

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Trace analysis

The most direct method is preferred

Reversed phase HPLC, UV at 200 nm

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HPICE + suppressed conductivity

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minmin

2 4 6 8 10 12 14

0

20

40

60

80

100

120

2 4 6 8 10 12 14

0

20

40

60

80

100

120 API-ES, Neg, Scan, 50

HMB  glutaric malic       glycolic oxalic succinic    malonic lactic 

  

RP-HPLC-ES-MS

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Second choice: pre-column derivatization

Benefits

- detectability - increase in S/N

- higher selectivity

- more retention

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Strategy to derivatize carboxylic acids

R

O

OH

R

O

O

R

O

N

N

R'

R"

R

O

O

R

OO

18Cr6

EDC

BrMMC

15' RT

BHBT

5' 40 o C

2NPH

340/417 nm

357/462 nm

400 nm20' 60 o C

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Derivatization by BHBT

N

N O

S

OO

R

O

ON

N

R2

N

R1 N

N O

S

OO

ON

N

R2

N

R1

R

O

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Derivatization protocol BHBT

Program 10 l 1.5 M EDC in water + 10 l sample + 10 l 20% pyridine in methanol + 10 l 30 mM BHBT in DMSO Mix in reaction vial Inject 0 l at end of previous gradient

Draw 2 l from reaction vial Draw 8 l water React 5’at 40oC inject

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Derivatization of BHBT

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Short chain acids by BHBT

Analysis: d104171b,1,1 Project: dummy01 Instrument: chan381 Method: BHBT

0 min

Acquisition Time: 17 Apr 2001 at 17:10.58

Response(mV)

Time(minutes)

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

C3

C4

C5C6

C7C8 C9

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Process fluid (BHBT-deriv.)

Analysis: d106013a,12,1 Project: dummy01 Instrument: chan381 Method: BHBT

monster 16 10X

Acquisition Time: 02 Jun 2001 at 02:39.59

Response(mV)

Time(minutes)

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

125

2 4 6 8 10 12 14 16 18 20

C7 C8C9

C4

C3

C5

C6

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C1 C2 C3 iC4 C4 C5 C6 C7 iC8

C9

Time(min.)

20

40

60

80

100

0 2 4 6 8 10 12 14 16

Removal of reagent peak: C1- to C9-acid

Using C18 amide column

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Derivatization by NPH

N

N

R

O

ON

N

R2

R1

N

N

ON

N

R2

R1

R

O

O2NO2NEDC-RCOOH

2-NPH

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Derivatization by NPH

Protocol 40 l 40 mM NPH in 50% aq. Ethanol 80 l 250 mM EDC + 1.5% pyridine aq. 400 l sample* vortex 0.5’ React 15’at 60oC Vortex 0.5’ Inject 40 l

* Often samples are pre-diluted to 50% ACN

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NPH: - Robust HPLC system - Little reagent interference

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Comparison of methods

Method Injection R-CO O H LO D LO D RSD

ul R= ppb ng % *

UV200 1000 C6 100 100

HPICE 500 C1 40 20 2

BMMC 20 C6 60 1.2 8

BHBT 2 C6 200 0.4 2

NPH 16 C6 50 1.3 3

*(C7=IS)

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nm360 400 440 480

ABSORBANCE

0

22

4

6

ACID

KETONE

ALDEHYDE

Specific detection of NPH-derivatives

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min2.5 5 7.5 10 12.5 15 17.5 20 22.5

mAU

0

2

4

6

8

10

12

14

16

iC4 C

4

C5

C6

C3-

ald

C3-

keto

ne

C4-

keto

ne

C

4-al

d

min2.5 5 7.5 10 12.5 15 17.5 20 22.5

mAU

-8

-6

-4

-2

0

2

4

iC4

C4

C5

C6

Specific detection of NPH-derivatives

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NPH: - Additional MS detection