AD- A2156 297 ( RR2

* Supercritical Fluid Chromatography for the Analysis of Nitroaromatics, Nitramines and Nitrate Esters

Paul H. Miyares August 1992

a UA E

* _

: GAS *sIC

a 92-27802
For conversion of SI metric units to U.S./Brtlsh customary units

of measurement consult ASTM Standard E380, Metric Practice

Guide, publ/shed by the Amedcan Society for Testing and

Materlols, 1916 Race St., Phiadelohla, Pa. 19103.

Cover: Phase diagram for carbon dioxide.
Special Report 92-21 1

U.S. Army Corps

of Engineers

Cold Regions Research &

Engineering Laboratory

Supercritical Fluid Chromatography

for the Analysis of Nitroaromatics,

Nitramines and Nitrate Esters

Paul H. Miyares August 1992

Accesiofl For

NrIS CRAMI

ToA TO ')

j,,) -------------------------------- ------------------ B Y.. ---------------------.......................

f or

Ava~biiY Co'des o

Dist Speciil

Prepared for

U.S. ARMY WATERWAYS EXPERIMENT STATION

Approved for pubfIc rekle; distribflon Is unllrited.
PREFACE

This report was prepared by Paul H. Miyares, Research Chemist, Geochemical

Sciences Branch, Research Division, U.S. Army Cold Regions Research and Engineering Laboratory. Funding for this research was provided by the U.S. Army

Waterways Experiment Station, Vicksburg, Mississippi (Ann B. Strong, Project

Monitor).

The author gratefully acknowledges Dr. C.L. Grant, Professor Emeritus, University of New Hampshire, and Dr. Thomas F. Jenkins, CRREL, for their technical

reviews of the manuscript. Dr. David Kaplan, U.S. Army Natick Laboratories, is also

acknowledged for supplying standards for 2-Am-DNT and 4-Am-DNT.

The contents of this report are not to be used for advertising or commercial

purposes. Citation of brand names does not constitute an official endorsement or

approval of the use of such commercial products.
Supercritical Fluid Chromatography

for the Analysis of Nitroaromatics,

Nitramines and Nitrate Esters

PAUL H. MIYARES

INTRODUCTION ises to be an improvement, both analytically and

operationally, over accepted HPLC-UV technolSupercritical fluid chromatography (SFC) is a ogy.

rapidly growing branch of chromatographic science offering a unique combination of chromatographic conditions similar to high-perfor- BACKGROUND

mance liquid chromatography (HPLC) with the

high resolution of capillary columns and highly History of SFC

specific detectors similar to gas chromatography "Supercritical" describes the state of a com-

(GC). pound when it is above its critical temperature and

The objective of this study was to examine critical pressure. A supercritical fluid is a subthe feasibility of SFC with a thermionic detector stance in the supercritical state that exhibits both

known as TID-2-H2/air for the determination of gaseous and liquid properties. A fluid in this state

nitroaromatics, nitramines and nitrate esters in is similar to a gas, where the volume expands to fill

environmental samples, using carbon dioxide the volume of the container, but it has densities

(CO2) as the carrier fluid. The specific analytes of and solvating properties that are similar to those of

interest are: a liquid.

"* 2,4,6-trinitrotoluene (TNT); The unique properties of supercritical fluids

"* 2,4-dinitrotoluene (2,4-DNT); were first observed by de la Tour in 1822 upon the

"* 2,6-dinitrotoluene (2,6-DNT); disappearance of the meniscus between gaseous

"* hexahydro-1,3,5-trinitro-1,3,5-nitramine and liquid phases in a closed system at a critical

(RDX); temperature (referenced in Lee and Markides 1987).

"* octahydro-1,3,5,7-tetranitro-1,3,5,7- In 1879, Hannay and Hogarth noted that the soltetrazocine (HMX); vating properties of a supercritical fluid were simi-

"* nitroglycerine (NG); lar to a liquid when they observed similar absorp-

"• pentarythitol tetranitrate (PETN); tion spectra for an inorganic salt in both a

"• methyl-2,4,6-trinitrophenylnitramine supercritical fluid and a liquid (referenced in Lee

(tetryl); and Markides 1987).

"* 2-amino-4,6-dinitrotoluene (2-Am-DNT); Supercritical fluids were introduced to chroma-

"* 4-amino-2,6-dinitrotoluene (4-Am-DNT); tography in 1962, when Klesper et al. published

"• 1,3,5-trinitrobenzene (TNB); and the first study employing supercritical Freons as

"* 1,3-dinitrobenzene (DNB). the mobile phase for the separation of metal porI evaluated the chromatography by testing vari- phyrins. This work was followed by that of

ous separations and comparing them with ac- Giddings et al. (1966) on "dense gas chromatogracepted HPLC separations. I also compared the phy" and others, who explored this new technoldetection capabilities of TID-2-H2/air with those ogy and laid the foundation for modem SFC. In

of the UV detector used with HPLC. The results of 1966 and 1967, Sie et al. published a series of works

this study were used to determine whether an that thoroughly discussed the theory and applicaSFC-TID-2-H 2 /air method for theseanalytes prom- tion of SFC. The advancement of SFC, though, was
hampered by inadequate instrumentation, and it many of these methods are quite sensitive, each

was overshadowed by HPLC for many years (Lee has its advantages and disadvantages. GC methand Markides 1987). ods have demonstrated high sensitivity and selecThe resurgence of SFC in the early 1980s was led tivity, but problems have been encountered with

by growing interest in using supercritical fluids as compounds exhibiting low volatility and thermal

extractants (Lee and Markides 1987). Advance- lability. HPLC methods provide adequate chroments in instrumentation as well as in column matographic resolution and excellent analyte statechnology also contributed. Novotny et al. (1981) bility, but methods of detection, although sensipublished the results of a study using open tubular tive, lack selectivity. Colorimetric and thin-layer

columns, while researchers at Hewlett-Packard chromatographic techniques also lack selectivity

studied the use of packed columns (Gere et al. and are more appropriate for field use.

1982). At the same time, Hewlett-Packard mar- Only a few studies have been published using

keted the first commercially available SFC system SFC to analyze for munitions (West and Lee 1986,

by modifying HPLC equipment. SFC is now an Douse 1988, Griest et al. 1989, Ashraf-Khorassani

increasingly popular analytical technique. There and Taylor 1989a,b, Munder et al. 1991), and only

are several commercially available systems, with three of these deal with any of the analytes of

numerous packed and capillary columns and a interest in this study. In 1988, Douse introduced

host of detectors, including electron capture detec- SFC-TEA into forensic analysis as an alternative to

tion (ECD), flame ionization detection (FID), ther- GC-TEA or HPLC-TEA. He stated that capillary

mal energy analysis (TEA), thermionic ionization GC-TEA is restricted to the determination of therdetection (TID), ultraviolet (UV) and mass spec- mally stable and highly volatile compounds, while

trometry (MS). HPLC-TEA is limited to the determination of nitrate esters and nitramines. To analyze for

SFC for munitions nitroaromatics, a photolytic reactor is required

One of the military's most serious environmen- prior to the TEA pyrolysis unit. Additionally,

tal problems has been soil and water contamina- HPLC-TEA requires a cryogenic trap to prevent

tion from munitions and munitions byproducts. solvent contamination of the detector. These addiContamination has occurred through the disposal tional units make an HPLC-TEA difficult to operof wastewater from manufacturing and load-and- ate and maintain. Douse performed trace determipack facilities, burning of off-specification muni- nations of explosives using a 6.8-m x 0.05-mm-ID

tions and detonation of out-of-date ordnance. Fur- SB Octyl 50 Superbond cross-linked flexible silither contamination may also have occurred at con capillary and TEA detection. He worked with

military impact areas or artillery training facilities a number of munitions of interest, including TNT,

where large numbers of munitions are detonated. RDX, NG, PETN and tetryl (Fig. 1). Previously

Many explosives and their byproducts incorpo- these analytes could not be easily determined

rate three classes of organic compounds: simultaneously by capillary GC due to thermal

nitroaromatics, nitramines and nitrate esters. instability or low volatility or by HPLC due to

Becauseof monitoring requirements atdisposal detector limitations. Unfortunately Douse was

sites, methodology has been developed for the unable to resolve tetryl and RDX. In addition,

determination of these compounds. Several GC HMX had a poor peak shape. This is most likely

techniques have been developed using a variety of due to poor solubility of HMX in C02. Schoenmaker

detectors, such as ECD (Douse 1981, Belkin et al. and Uunk (1987) felt that CO2 has a solvating

1985, Richard and Junk 1986), TEA (Lafleur and power similar to that of methylene chloride and

Mills 1981, Douse 1983), FiD (Jurinski et al. 1975), that itisnotsufficiently polar to eluteHMX (Yarbro

TID (Patterson 1986, West and Lee 1986) and MS and Gere 1987).

(Pereira et al. 1979). Also, a number of HPLC Griestetal.(1989)showedSFC tobecomplemenmethods have been developed using UV (Jenkins tary to GC and HPLC for the analysis of munitions.

et al. 1986, 1989), TEA (Lafleur and Morriseau Using a Deltabond-cyano (5-jim particle size) 25-

1980), electrochemical detection (Maskarinec et al. and 15-cm-long packed column separations and

1984),ECD(Krulletal. 1981)andLC/MS(Voyksner TID detection, Griest et al. successfully resolved

and Yinon 1986). Methods employing direct colo- seven high explosives: TNT, 2,4-DNT, 2,6-DNT,

rimetric analysis (Jenkins and Walsh 1992), as well PETN, NG, tetryl, RDX and HMX. Acceptable

as thin-layer chromatography (Hoffsommer and resolution was achieved for all analytes except

McCullough 1968), have also been used. Although HMX in approximately 20 min (Fig. 2). Although

2
0

a

z

I,-

wz

0F_

I.-

z

0 1

z 0

inject Inject

I I I I I I i I I I I

0 2 4 6 8 0 2 4 6 8 10

Time (min)

Figure 1. Separation of high explosives using an SBOctyl 50 Superbond cross-linked flexible silicon capillary column (6.8 m xO.05 mm ID). (After Douse 1988.)

210 nm

NG

DNT

0 2,6- 0 a 2,4-

-e TNT

PETN

Tetryl

RDX

I I I I

0 5 10 15 20 25

Time (min)

Figure 2. SFC separation of high explosives using a Deltabond-cyano

packed column (2.5 m x 1 mm, with 5-ptm particles). (After Griest et al.

1989.)

3
not shown, a tailing peak for HMX was observed computerized system controller. Data were colafter RDX eluted on a 15-cm column, again due to lected by a Hewlett-Packard model 3393 digital

its low solubility in CO2.The detection limits were integrator.

estimated to be approximately 10 times higher The following Lee Scientific columns were used

than the reporting limits for the analogous HPLC- during the study:

UV method developed by Jenkins et al. (1989).

In 1991, Munder et al. employed on-line SB-Methyl-100 (2.5 m x 0.05 mm ID)

supercritical fluid extraction (SFE) and SFC with SB-Biphenyl-30 (2.5 m x 0.05 mm ID)

triple detection (UV, ECD, FID) for the determina- SB-Cyanopropyl-50 (2.5 m x 0.05 mm ID)

tion of explosives and propellants. They were SB-Cyanopropyl-25 (5.0 m x 0.05 mm ID)

successful in separating or partially separating a SB-Smectic (2.5 m x 0.05 mm ID).

suite of 20 munitions compounds that included

TNT, 2,4--)NT 2,6-DNT, NG, PETN, HMX, RDX The supercritical fluids used as the mobile phase

and tetryl, as well as a number of nitrates, phtha- were SFC-grade CO2, as well as CO2 modified

lates and phenylamines. Although complete reso- with 5 and 10% methanol and 5 and 10% acetone,

lution of all 20 compounds was not achieved, obtained from Scott Specialty Gases. The analytes

those analytes that are of interest in this study were Standard Analytical Reference Materials

were resolved. The detection limits for this method (SARMs) obtained from USATHAMA, except for

were estimated tobe 100 pgforsomeof theanalytes 2-Am-DNT and 4-Am-DNT, which were reagent

for a 1.4-mg soil sample. grade. Methanol, acetonitrile and acetone solvents

All three groups, though, had difficulty work- (all HPLC grade) used to prepare the various

ing with very polar compounds such as HMX, and standards and samples were from Baker,

all felt that the nonpolar characteristics of CO2 Mallinkrodt or Alltech.

were the source of the problem. The solvating Theoptimumseparationforthesuiteofanalytes

power of CO2 may be a limiting factor in the use of of interest was experimentally determined. The

SFC for the determination of the munitions re- separation was performed on an SB-Cyanoprop) 1-

quired by the military. 25 column eluted with 100% CO2 and using the

OtherstudiesincludetwobyAshraf-Khorassani temperature and density conditions outlined in

and Taylor (1989a,b), who used SFC/Fouriertrans- Table 1.

form infrared (FT-IR) spectroscopy in a qualitative Samples were introduced using an LC-type

study to characterize double-base propellants and syringe, overfilling the 0.50-gL sample loop. The

propellant stabilizers. Also, West and Lee (1986) valve was fired and the loop rotated to the in-line

evaluated the use of capillary SFC-TID for the position for 2.5 s. Analytes were detected by the

analysis of polycyclic nitroaromatics. This study TID-2-H 2 /air detector optimized with an H2 flow

described the comparison of three modes of TID. rate of 4.2-4.5 mL/min and a source heating curThey concluded that the TID-2-H2/air mode was rent of 2.7-2.8 A.

sensitive and highly selective to nitro-containing

polycycic aromatics. They found, though, that the

detector limited the chromatography to density SFC COLUMNS

programming at low oven temperatures (40'C)

because of sensitivity toward density change at The first part of this study focused on determinhigh temperatures. They also found that this TID ing whether an adequate separation was feasible

mode has a limited dynamic range. for the analytes of interest. For SFC, as in GC, it is

possible to use either packed or open tubular

(capillary) columns. Packed SFC columns are very

EXPERIMENTAL METHODS similar to LC columns. They have smaller inner

diameter (ID) but similar particle-size (3, 5 or 10

The analyses for this study were performed on gm) packing. These columns have a large number

aLeeScientificSupercriticalFluidChromatograph of theoretical plates per unit distance, which is

model 600 modular system consisting of an SFC advantageous for rapid analysis of simple mixpump, a GC/SFC programmable oven equipped tures but is not advantageous for a complex that

with a high-speed pneumatic injection valve, a contains a large number of analytes. A limitation

time split injector, a thermionicionization detector of packed columns is that they require an eluent

from Detector Engineering Technologies, and a flow rate of 2-20 mL/min (Lee and Markides

4
Table 1. Temperature-density program for optimum

separation using the SB-Cyanopropyl-25 column.

Den ;ity Temperature

program program

Initial settings 0.150 g/mL 100

rC

Injection time 2.5s -

Hold time A 5.0 min 5.0 min

Ramp A 0.1 g/mL.min 35*C/min

Final A 0.30 g/mL 1500C

Hold time B 1.2 min 5.0 mm

Ramp B 0.05 g/mL-rnin 50°C/min

Final B 0.42 g/mL 1750C

Hold time C 5.0 min 19.0 min

Ramp D 0.50 g/mL.rnin -

Final D 0.55 g/mL

Final hold 15.0 min

1987), which prohibits the use of many of the GC- (Fig. 3). The peak shape of HIMX is likely due to the

like detectors unless the column effluent is split. poor solubility of HMX in CO2. The poor peak

For this study I was restricted to capillary columns shapes of the DNT isomers were determined exbecause I chose to use a TID-2-H 2/air (GC-like) perimentally to be a temperature-dependency

detector, which cannot operate under the high- problem. At a set density and temperature, one

flow-rate conditions of the packed columns. isomer would elute with a suitable peak shape and

Capillary SFC columns are similar to GC capil- the other would not. I was unable to determine a

laries but typically have smaller inner diameters density-temperature combination for which both

(0.05 or 0.10 mm ID vs 0.325 mm ID). Theory and isomers would elutewith satisfactory peak shapes.

practice have shown that capillary SFC columns I next turned to an SB-Octyl-50 column (Lee

must be less than 0.10 mm ID to achieve acceptable Scientific), as suggested by Douse (1988). Douse

efficiencies (> 3000 plates per m) (Peaden and Lee found that RDX and tetryl have similar retention

1983, Fields ct ". 1934). A typical SFC capillary (20 times (Fig. 2). I attempted to resolve this problem

m x 0.05 mm) can produce 100,000 theoretical while retaining satisfactory separation of the

plates with a supercritical CO2 mobile phase (Lee analytes of interest by modifying the densityand Markides 1987). Capillary columns typically temperature program. Unfortunately I was unrequire a 1- to 10-mL/min flow rate, which is suf- able to find conditions yielding a separation that

ficiently slow for most GC- and LC-like detectors, was adequate for quantitative work. I went on to

including mass spectrometry. However, the sample test an SB-Smectic colurui (Lee Scientific), which

capacity of these columns is limited. Although no separates compounds on the basis of size and

published data were available, the manufacturer shape, and an SB-Methyl-100 column (Lee Scien-

(Lee Scientific) warns against overloading certain tific), a non-polar phase. The SB-Smectic and SBcolumns with specific classes of compounds. For Methyl-100 showed limited abilities to separate

example, the SB-Methyl-100 is easily overloaded isomers or similar compounds such as TNT and

by strongly polar and hydrogen-bonding materi- TNB (Fig. 4, 5). Also, retention of the smaller

als, and the SB-Cyanopropyl-50 can be easily over- analytes on the SB-Smectic and the less-polar

loaded by aliphatic hydrocarbons. This suggests analytes on the SB-Methyl-100 was so poor that

that the selection of stationary phases is highly peaks were not sufficiently resolved.

dependent on the analytes of interest. Having had only limited success with pubIn my search for a suitable separation, I tested lished capillary separations, I re-examined the

many capillary columns. The first of these was an work of Griest et al. (1988), who adequately reSB-Biphenyl-30 (Lee Scientific). This column was solved a suite of common munitions using a

reported to resolve 2,4-DNT and 2,6-DNT, as well Deltabond-cyano packed column in a20-min analyas TNT and TNB. I encountered a major problem, sis. An SFC study by Ashraf-Khorassani and Tayhowever. HMX and the isomers of DNT did not lor (1989b), as well as HPLC studies by Jenkins and

elute with satisfactory peak shapes for quantitation Walsh (1987), Jenkins et al. t 1988,1989) and Jenkins

5
X

z

~X

COC

St

Cl--

0 4 8 12 16

Time (min)

Figure 3. SFC separation of high explosives using an SB-Biphenyl-30 capillary

column (2.5 m xO.05 mm).

- TNT

- TNB

I . I I * I I * I * I * I

0 2 4 6 0 2 4 6

Time (min)

Figure 4. SFC chromatogram of TNT and TNB using an SBSmectic capillary column (2.5 m x 0.05 mm).

6
"Retention Times An SB-Cyanopropyl-50 (Lee Scientific) 2.5-in

p- NT 5.8 min column showed great promise for e suitable sepaDNB 6.3 ration (Fig. 6). The nitroaromatics and nitrate es2,6 - DNT 6.3 ters eluted between 8 and 11 minutes, with the

2.4 - DNT 6.5

TNB 6.7 RDX and HMX being retained to 16 and 24 min,

TNT 6.7 respectively. With this long retention time, howPETN 4 -Am -DNT 6.8 7.3 ever, the HMX peak was broad and would be hard

Tetryl 7.3 to quantitate, particularly at trace concentrations.

HMX 12.8 I then switched to an SB-Cyanopropyl-25 (Lee

Scientific) 5.0-m column since the reduced polarity should shorten the HMX and RDX retention

times. The combination of reduced polarity, longer

column and slight modifications in the densitytemperature program improved the separation

for the nitroaromatics and nitrate esters over the

SB-Cyanopropyl-50 column and shortened the eluI I tion time for RDX and HMX to 12 and 20 min,

0 4 T8 12 respectively (Fig. 7). The SB-Cyanopropyl-25 colTime (man) umn was able to resolve the majority of the analytes

of interest, although DNB, 2,4-DNT and NG were

Figure 5. SFC separation of high explosives using an not resolved enough to be separately quantitated.

SB-Met hyl-1 00 capillary column (2.5 m xO.05 mm). Also, I was unable to develop a secondary separation adequate for confirmation, which is often

required when analyzing environmental samples.

(1989), also indicated the utility of using the The currently accepted HPLC-UV methods for

cyanopropyl stationary phase to resolve muni- the determination of munitions (Jenkins et al. 1989,

tions compounds. Jenkins (1989) noted that a spe- Miyares and Jenkins 1991) each report a primary

cific interaction exists between the polar and a confirmatory separation. Jenkins et al. emnitroaromatics and nitramines and the -CN por- ployed an LC-18 (25 cm x 4.6 mm, 5 gim) column

tion of the stationary phase that accounts for the for primary analysis and have reported resolution

unique separation and retention order of these of 11 munitions and munitions byproducts, incompounds on an LC-CN phase column. cluding HMX, RDX, TNB, DNB, tetryl, TNT, 2,4-

Z a

a.

I- I

Z z

I I .I II I , I p I I

0 4 8 12 16 20 24 28

Time (min)

Figure 6. SFC separation of high explosives using an SB-Cyanopropyl-50 capillary

column (2.5 m x 0.05 mm).

7
z 00z

0X !C C~O~cf- N crr

z z

I-I

I--

z

0

E

Figure 7. SFC separation of

high explosives using an SBCyanopropyl-25 capillary column (5.0 m xO.05 mm). NG z

II i I I i I i I I I

0 4 8 12 16 20 24

Time (min)

z

O 0

Figure 8. HPLC separation of I- z high explosives using an LC18 packed column (25 cmx 0

4.6 n -,with 5-gm particles, 0 - E " 'i 

50/50tv/v]MeOH/H2Oat 1.5 c c Z Z

mL/min). (After Jenkins and z 0

Walsh 1987, Jenkins et al.

1988, 1989.)

I I I I I

0 4 8 12 16

Time (min)

8
erZ cam

.Z

D 0

CCu Z IIz ozz E Z co x0)

Z,

I- z

0 I- x E, 

0 4 8 12

Time (rnin)

Figure 9. HPLC separation of high I I I I I I I I I

explosives using an LC-CN packed 04 8 12 16

column (25 cm x4.6 mm, with 5-pam Time (min)

particles, 50150 MeOH/H20 [v/v]

eluent at l.5mLlmin). (After Jenkins Figure 10. HPLC separation of high explosives using

and Walsh 1987, Jenkins et al. 1988, an LC-8 packed column (7.5 cm x4.6 mm, with 3-pan

1989.) particles, 70.7127.8/1.5 H20/MeOH/THF [v/v/v] eluent at 2.0 mL/min). (From Miyares and Jenkins 1991 .)

DNT, 2,6-DNT, 2-Am-DNT, 4-Am-DNT and the SFC DETECTORS

th~ree isomers of mono-nitrotoluene (ortho, meta,

para) (Fig. 8). They also reported an adequate The second part of this study concentrated on

confirmation separation employing an LC-CN detection. A wide range of GC-like and HPLC-like

column (25 cm x 4.6 mm, 5 lm=) (Fig. 9). Miyares detectors have been used in conjunction with SFC.

and Jenkins (1991) reported a primary separation These include ECD, FID, TEA, TID, UV, MS and

using an LC-8 column (7.5 cm x 4.6 mm, 3 ý=), FT-II. Many of these have been employed in studwhich resolved HMX, RDX, TNB, DNB, NB, TNT, ies for the analysis of munitions by SFC. For this

2,4-DNT, 2,6-DNT, 2-Am-DNT and 4-Am-DNT study the TID-2-H2/air detector was chosen besufficiently for quantitation (Fig. 10). They also cause of its selectivity toward nitrogen-containing

reported a confirmation separation using an LC-8 compounds. A schematic of the TID-2-H2/air decolumn (3.3 cm x 4.6 mm, 3 gtm) in series with an tector is shown in Figure 12. In this detector a

LC-CN (3.3 cm x 4.6 mm, 3 lain) (Fig. 11). Both highly reactive flame-like gaseous layer surrounds

methods reported by Jenkins et al. and Miyares a hot ceramic surface that contains cesium. Comand Jenkins are carried out under isocratic condi- pounds introduced into the boundary layer are

tions and at room temperature. In contrast the SFC chemically decomposed by active gas-phase chemseparation developed for this study is carried out istry. Electronegative products of the decomposiunder ant intricate combination of temperature tion are selectively ionized at the surface of the

and density ramps (Table 1). Therefore, I see no thermionic source (Patterson 1986). Nitrogen- or

evidence that chromatography for munitions by phosphorous-containing compounds are ionized

SFC has any overwhelming advantages over chru- with especially high efficiency by this process.

matography by HPLC. Patterson (1986) reported that typical GC detec0I9
Co min, with approximately 4 mL/min being opti- co Z

z mum for nitrogen analyses. The optimum current

will vary with the condition of the individual

thermionic source. The current should be high

enough to produce a boundary layer but not so

ca high as to produce a flame. West and Lee (1986)

z used an H2 flow rate of 5.3 mL/min and a current

of 3.3 A. With these settings they reported a low

selectivity (-103 over a hydrocarbon) and a large

z dependency on increasing density of fluid. Such x 0 density changes cause severe baseline drifting,

CM E which greatly hampers the ability to perform any

+ chromatography. X My procedure for optimizing detector response

X: Z was based on the response of TNT. I began by

-- ' setting the current and H2 flow rate in accordance

L~I Thermionicodoaiag

0 4 8 12 16 20

Time (min)

Figure 11. HPLC separation of high explosives using

LC-8 and LC-CN columns in series (both 3.3 cm x4.6

mm, with 3-1m particles, 70.7/27.8/1.5 H20/MeOH/

THF[vivivieluentat 1.5 mL/min). (From Miyaresand I I

Jenkins 1991.)

tion capabilities for this detector are 1-20 pg in- Collector

jected, and the specificity with respect to hydro- C

carbons is 104-105. For SFC-TID the data are far

less favorable. West and Lee (1986) reported a Reactive Boundary

detectable range of 0.4-8 ng injected for a series of

nitrated polycyclic aromatics (S/N = 3) using SFCTID-2-H2/air. They also reported that the linear

range was limited to 10-100 ng injected. Based on (air)

the 500-nL injection that our SFC can deliver, these

figures correspond to 0.8-16 mg/L in solution anmI Sample

4.0-16 mg/g based on a 10.0-mL extraction of 2.0 and Gas 1

g of soil. (H 2)

The sensitivity of the TID-2-H 2/air depends on

the concentration of H2 gas in the boundary layer,

which is controlled by the flow rate of H2 gas and Sample

the temperature of the thermionic source, which in Conduits

turn is controlled by the current applied. The Figure 12. Schematic of the TID-2-H2/air detector.

required flow rate of H2 ranges from 3 to 6 mL/ (After Patterson 1986.)

10
with the manufacturer's recommendations (-2.7 The introduction of samples is very simple. The

A and -4.0 mL/min). I then adjusted the current high-speed pneumatic valve is analogous to the

and flow rate and noted the subsequent effects on Rheodyne valves used in many HPLC systems.

the signal-to-noise ratio. I found that a current One major drawback for capillary SFC, though, is

range of 2.65-2.80 A combined with an H2 flow the limitation on injection size. The injection loop

rate of -3.7-4.2 mL/min resulted in the highest for our system is only 0.50 gL in total volume

achievable signal-to-noise ratio for this system. versus 1.0-2.0j±LforGC and 10-1100l LforHPLC.

Outside of these ranges I observed either reduc- This limits the detection capabilities for the instrutions in signal levels or drastic increases in both ment, and for trace analysis it requires sample

signal and noise levels. At these lower settings I preparation procedures that include a preconcendid not observe the baseline drift with density tration step.

change reported by West and Lee (1986). Analyzing samples under routine conditions

Using the flow and current settings specified using an established method requires comparable

above, some preliminary analyses of munitions steps for both SFC and HPLC once the systems are

standards were carried out. The results showed up and running. Both HPLC and SFC systems can

that some of the analytes could not be detected be equipped with auto-injection and sophisticated

even at concentrations as high as 1.0 g/L.* For the data-handling systems for analyses requiring exanalytes that could be detected, the detection ca- tended (overnight) times. However, operators of

pabilities were consistent with the limits reported HPLC systems may require one to two days of

by West and Lee (1986). These limits were signifi- training on a specific instrument, while SFC operacantly higher than the certified reporting limits tors may require three to five days of instruction.

reported by Jenkins et al. (1989) for the HPLC-UV Developing separations is more difficult in SFC

methods. Because of these results I felt that a than in HPLC. Jenkins (1989) noted that reversedcertified reporting limit test was not warranted. phased (RP)-HPLC separations are based on

solvophobic behavior, where retention times are

strongly correlated to octanol-water partition coSFC OPERATIONS efficients (Kow). McDuffie (1981) showed that this

relationship was so predictable that K0 , values

The third part of this study dealt with the basic could be used to predict capacity factors and thus

operations of an SFC system compared to GC and retention times for an RP-HPLC separation. In SFC

HPLC. Operating an SFC system is very similar to the partitioning of analytes between the mobile

operating a GC system with a few hints of HPLC and stationary phases is part of the mechanism for

tossed in. The Dionex SFC system evaluated in this separation, but the mechanism may also include

study was operated only with capillary columns. specific effects from density, temperature, linear

These columns, as in GC, require precision cutting velocity and volatility of analytes or a combination

of theends with sapphire-tipped cutting tools, and of these effects. Consequently it is a much more

they require precise positioning within the injec- difficult process to understand and interpret, and

tor and detector for optimum performance. They thus a more difficult system for which to predict

are only 195 gm in diameter, compared to 325 gm the proper chromatographic conditions.

for GC. Being smaller and more flexible than GC In SFC one needs to determine the proper fluid,

columns, SFC columns are much harder to install, the proper temperature-density-pressure comHPLC columns can be installed or removed in less bination for that fluid, and the proper stationary

than 1 minute, but SFC columns can take 15 min- phase to achieve adequate separations. In HPLC

utes or longer to install, depending on experience. the strength of the eluent for a given stationary

phase needs to be determined. Although the two

requirements seem to be analogous, determining *Figures 3-7 indicate a significant response for all the temperature-density-pressure combination is

analytes, but here the detector was set at an excessively far more time consuming than determining the

high H2 flow rate and source current to achieve a response to establish retention times and resolutions. proper ratio of two or three solvents in an eluent.

Unfortunately, as discussed earlier, high H2 flow rates The temperature, density and pressure of

and high currents eliminate selectivity by reducing the supercritical fluids are interrelated in such a way

detector to a pseudo-FID, and they also cause excessive that for a given density, there are many temperadrifting in the baseline with changing density. ture and pressure combinations. Also, at low presII
sures the relationship between density and tem- 2-H2/air is significantly less than that for HPLCperature is not linear, where at high pressures they UV. The basic operations and the understanding

appear to have a linear relationship. Due to the of the science of SFC are more difficult than that of

complex interrelationships of these effects, the HPLC.

conditions for separation are often some combina- In conclusion, SFC-TID-2-H2 /air is selective,

tion of temperature varying at constant density, and there exists the potential capability of analyzdensity varying at constant temperature and tem- ing the three class of compounds of interest to the

perature varying with varying density. In con- military, but the limited detection capabilities, the

trast, developing an isocratic or gradient elution column limitations and the difficulty of basic use

HPLC separation requires a systematic varying of and understanding of the instrumentation far outthe percentage of each component of solvents in weigh its potential capabilities.

the eluent until the desired resolution is achieved.

For both techniques, choosing a column will be

specific to the analytes of interest and may require LITERATURE CITED

testing of more than one column before the separation is adequate. Ashraf-Khorassani, M. and L.T. Taylor (1989a)

Qualitative supercritical fluid chromatography/

Fourier transform infrared spectroscopy study of

SFC WITH MODIFIED FLUIDS methylene chloride and supercritical carbon dioxide extracts of double-base propellant. Analytical

During this study I examined the effects of Chemistry, 61: 145-148.

using solvent-modified CO2 as the carrier fluid. Ashraf-Khorassani, M. and L.T. Taylor (1989b)

The solvents tested were methanol and acetone, Analysis of propellant stabilizer components via

each at 5 and 10% concentration, by volume. Both packed and capillary supercritical fluid chromasolvents showed potential for a slight to moderate tography/Fourier transform infrared spectrompositive effect on the chromatography. Retention etry. Journal of High Resolution Chromatography, 12:

times decreased slightly with resolution being 40-44.

maintained. The greatest effects were seen with Belkin, F., R.W. Bishop and M.V. Sheely (1985)

acetone over methanol-modified fluid. Unfortu- Analysis of explosives in water by capillary gas

nately the use of the modifiers had an adverse chromatography. Journal of Chromatographic Scieffect on the detector, causing baseline drifting ence, 24: 532-534.

and inconsistencies in analyte responses. de la Tour, C. (1822) Ann. Chim. Phys, 21:127 (cited

in Lee and Markides 1987).

Douse, J.M.F. (1981) Trace analysis of explosives

CONCLUSION at the low picogram level by silica capillary column gas-liquid chromatography with electronThe use of SFC with GC-like selective detectors capture detection. Journal of Chromatography, 208:

has been shown by many to be an excellent tech- 83-88.

nique to fill in the analytical gaps between GC, Douse, J.M.F. (1983) Trace analysis of explosives

where thermal lability and volatility are the limit- at the low picogram level by silica capillary coling factors, and HPLC, where available detectors umn gas chromatography with thermal energy

limit selectivity. Some papers have shown that analysis detection. Journal of Chromatography, 256:

SFC is an adequate technique for some specific 359-362.

munitions applications. For routine environmen- Douse, J.M.F. (1988) Trace analysis of explosives

tal monitoring and site characterization required by capillary supercritical fluid chromatography

by the military, though, SFC does not offer advan- with thermal energy analysis detection. Journal of

tages over the accepted HPLC-UV methods. Al- Chromatography, 445: 244-250.

though separations are possible for some combi- Fields, S.M., R.C. Kong, J.C. Fjeldsted, P.A.

nationsof munitions compounds, separations were Peadon and M.L. Lee (1984) Effect of column

not adequate for quantitation of all the analytes of diameter on efficiency in capillary supercritical

interest in this study. Also, the possibility of a fluid chromatography. Journal of High Resolution

confirmatory separation is meager because of the Chromatography and Chromatographic Communicalimited number of stationary phases available for tions, 7: 312-318.

capillary columns. The sensitivity of the SFC-TID- Gere, D.R., R. Board and D. McManigill (1982)

12
Supercritical fluid chromatography with small (HPLC-ECD). Analytical Letters, 14:1363-1376.

particle diameter packedcolumns. Analytical Chem- Lafleur, A.L. and B.D. Morriseau (1980) Identifiistry, 54: 736. cation of explosives at trace levels by high perGiddings, J.C. (1966) Some aspects of pressure- formance liquid chromatography with nitrosylinduced equilibrium shifts in chromatography. specific detector. Analytical Chemistry, 52. 1313-

Separation Science, 1(1): 73-80. 1318.

Griest, W.H., C. Guzman and M. Dekker (1989) Lafleur, A.L. and K.M. Mills (1981) Trace level

Packed-column supercritical fluid chromato- determination of selected nitroaromatic comgraphicseparationof highly explosive compounds. pounds by gas chromatography with pyrolysis/

Journal of Chromatography, 467: 423-429. chemiluminescent detection. Analytical Chemistry,

Hannay J.B., and J. Hogarth (1879) Proceedings of 53:1202-1205.

the Royal Society (London), 29:324 (cited in Lee and Lee, M.L. and K.E. Markides (1987) ChromatogMarkides 1987). raphy with supercritical fluids. Science, 235:1342-

Hoffsommer, J.C. and J.F. McCullough (1968) 1347.

Quantitative analysis of polynitroaromatic com- Maskarinec, M.P., D.L. Manning, R.W. Harvey,

pounds in complex mixtures by combination thin- W.H. Griest and B.A. Tomkins (1984) Determinalayer chromatography and visible spectrometry. tion of munitions components in water by resin

Jomrnal of Chromatography, 39: 508-514. adsorption and high-performance liquid chromaJenkins, T.F. (1989) Development of an analytical tography electrochemical detection. Journal of Chromethod for the determination of extractable matography, 302: 51-63.

nitroaromatics and nitramines in soils. Ph.D. Dis- McDuffie, B. (1981) Estimation of octanol/water

sertation, University of New Hampshire. partition coefficients for organic pollutants using

Jenkins, T.F. and M.E. Walsh (1987) Develop- reversed-phase HPLC. Chemosphere, 10: 73-83.

ment of an analytical method for explosive resi- Miyares, P.H. and T.F. Jenkins (1991) Improved

dues in soil. USA Cold Regions Research and salting-out extraction/preconcentration method

Engineering Laboratory, CRREL Report 87-7. for determination of nitroaromatics and nitramines

Jenkins, T.F. and M.E. Walsh (1992) Develop- in water. USA Cold Regions Research and Engiment of field screening methods for TNT, 2,4-DNT neering Laboratory, Special Report 91-18.

and RDX in soil. Talanta, 39: 419-428. Munder, A., R.G. Christensen and S.A. Wise

Jenkins, T.F., D.C. Leggett, C.L. Grant and C.F. (1991)Microanalysis of explosives and propellents

Bauer (1986) Reversed-phase high-performance by on-line supercritical fluid extraction/chromaliquid chromatographic determination of nitro- tography with triple detection. Journal of

organics in munitions wastewater. Analytical Chem- Microcolumn Separations, 3:127-140.

istry, 58: 170-175. Myers, M.N. and J.C. Giddings (1965) High colJenkins, T.F., P.H. Miyares and M.E. Walsh (1988) umn efficiency in gas-liquid chromatography at

An improved RP-HPLC method for determining inlet pressuires of 2500 p.s.i. Analytical Chemistry,

nitroaromatics and nitramines in water. USA Cold 37(12): 1453.

Regions Research and Engineering Laboratory, Novotny, M., S.R. Springston, P.A. Peaden, J.C.

Special Report 88-23. Fjeldsted and M.L. Lee (1981) Capillary

Jenkins, T.F., M.E. Walsh, P.W. Schumacher, P.H. supercritical fluid chromatography. Analytical

Miyares, C.F. Bauer and C.L. Grant (1989) Liquid Chemistry, 53(3): 407A.

chromatographic method for the determination of Patterson, P.L. (1986) Recent advances in thermiextractable nitroaromatic and nitramine residues onic ionization detection for gas chromatography.

in soil. Journal of the AOAC, 72. 890-899. Journal of Chromatographic Science, 24: 41-52.

jurinski, N.B., G.E. Podolak and T.L. Hess (1975) Peaden, P.A. and M.L. Lee (1983) Theoretical treatComparison of analytical methods for trace quan- ment of resolving power in open tubular column

tities of 2,4,6-trinitrotoluene. American Industrial supercritical fluid chromatography. Journal ofChroHygiene Association Journal, 497-502. matography, 259:1-16.

Klesper, E., A.H. Corwin and D.A. Turner (1962) Pereira, W.E., D.C. Short, D.B. Manigold and

High pressure gas chromatography above critical P.K. Roscio (1979) Isolation and characterization

temperatures. Journal of Organic Chemistry, 700. of TNT and its metabolities in groundwater by gas

Krull, L.S., E.A. Davis, C. Santasania, S. Kruas, A. chromatography-mass spectrometry-computer

Basch and Y. Bamberger (1981) Trace analysis of techniques. Bulletin of Environmental Contaminaexplosives by HPLC-electron capture detection tion and Toxicology, 21: 554-562.

13
Richard, J.J. and G.A. Junk (1986) Determinatioa effect of pressure on partition coefficients in gasof munitions in water using macroreticular resins, liquid chromatography with carbon dioxide as a

Analytical Chemistry, 58: 723-725. carrier gas. Separation Science, 1, 459.

Schoenmakers, P.J. and L.G.NI. Uunk (1987) Voyksner, R.D. and J. Yinon (1986) Trace analysis

Supercritical fluid chromatography-Recent and of explosives by thermospray high-performance

future developments. European Chromatography liquid chromatography-mass spectrometry. JourNews, 1(3): 14-22. nal of Chromatography, 354: 393-405.

SFC Applications (1989) Symposium/Workshop on West, W.R. and M.L. Lee (1986) Evaluation of a

Supercritical Fluid Chromatography, Snow Bird, Utah, thermionic detector for capillary supercritical fluid

June 13-15, 1989. chromatography of nitrated polycycic aromatic

Sie, S.T. and G.W.A. Rijnder (1967) Chromatog- compounds. Journal of High Resolution Chromatography with supercritical fluids. Analytica Chimica raphy and Chromatography Communications, 9:161-

Acta, 38: 31. 167.

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(1966) High-pressure gas chromatography and out of supercritical fluid chromatography. Chrochromatography with supercritical fluids. 1) The matography, 49-55.

14
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4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Supercritical Fluid Chromatography for the Analysis of

Nitroaromatics, Nitramines and Nitrate Esters

6. AUTHORS

Paul H. Miyares

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

REPORT NUMBER

U.S. Army Cold Regions Research and Engineering Laboratory Special Report 92-21

72 Lyme Road

Hanover, N.H. 03755-1290

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AGENCY REPORT NUMBER

U.S. Army

Waterways Experiment Station

Vicksburg, Mississippi 39180-0631

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Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161.

13. ABSTRACT (Maximum 200 words)

A supercritical fluid chromatographic (SFC) system equipped with a capillary column and a thermionic ionization detector

(TID) was evaluated as a potential analytical tool for the simultaneous determination of nitroaromatics, nitramines and nitrate

esters. Examination of several stationary phases and modified fluids was carried out while determining optimal conditions for

a suitable separation. The results indicate that a cyanopropyl stationary phase is best suited for these analytes, but the available

percentages of cyanopropyl in the phase (i.e. 25% and 50%) do not give total resolution. The performance and usability of the

TID was evaluated. Detection limits are estimated to be several times greater than those for standard HPLC and GC methods.

Also, the usability and performance of the SFC were compared with HPLC and GC. SFC-TID can be used for the simultaneous

determination of nitroaromatic, nitramines and nitrate esters, but current column and detection capability limitations greatly

reduce its potential.

14. SUBJECT TERMS 15. NUMBER OF PAGES

Explosives Supercritical Fluid Chromatography 20

Chromatography 16. PRICE CODE

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