A review of direct liquid introduction interfacing for LC/MS Part I: Instrumental aspects

A Review of Direct Liquid Introduction Interfacing for LC/MS Part I: Instrumental Aspects

W. M. A. Niessen*

Laboratory for Analytical Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands

I Key Words

Liquid chromatography/mass spectrometry Direct liquid introduction Diaphragm Liquid jet formation Desolvation

Summary

Considerable progress has been made in the coupling of liquid chromatography and mass spectrometry over the past ten years. Three interfaces tend to dominate the LC/MS market: transport systems, direct liquid introduction, and the thermospray interface. In this paper the developments in direct liquid introduction interfacing for LC/MS wil l be reviewed. The paper wil l be published in two parts. Mass spectrometry and applications will be discussed in the second part. This first part of the review concentrates on the various instrumental aspects of direct liquid introduction, such as the design of vacuum systems, the interface probes and the desolvation chambers.

Introduction

The coupling of Liquid Chromatography and Mass Spectro- metry (LC/MS) has attracted considerable interest in recent years. The subject has been extensively reviewed (see e.g. [1-3]), LC/MS interfaces can be classified in two major groups: interfaces for indirect and for direct introduction of the column effluent. In an indirect introduction interface some kind of a mechanical device is used to transfer the column effluent to the MS vacuum. The transport system [4] is a typical example of an indirect introduction type of interface.

In a direct introduction system the column effluent flows directly into the mass spectrometric vacuum system through some kind of a tube. The direct introduction seems to be the most simple way of coupling LC and MS. The first experiments in this direction were carried out by Tal'roze and coworkers [5, 6] in the early seventies. The flow of liquid through a capillary tube into the mass spectrometric

Chromatographia Vol. 21, No. 5, May 1986 Review Paper

vacuum system was only restricted by viscous forces. The same approach was used by the group of McLafferty [7] in their early studies. However, it can be shown theoretically [8 -10 ] that in all practical cases the solvent wil l evaporate inside the capillary. In such an interface (a capillary inlet) the liquid is vaporized before entering the vacuum system. This type of interface will not be discussed in this review [8, 10].

One of the most important interfaces is the Direct Liquid Introduction (DLI) interface. In a DLI interface (part of) the column effluent is nebulized by the disintegration into small droplets of a liquid jet formed at a small diaphragm. After desolvation of the droplets in a desolvation chamber the solutes can be analyzed using chemical ionization with the reversed phase solvents as the reagent gas. Significant progress in DLI interface development has been achieved since the introduction of the present form in the late seventies. In the following discussions the various aspects of the DLI interface wil l be discussed. In the first part of this paper the instrumental aspects of the DLI interface will be discussed. The chemical ionization pro- cesses and the applications of DLI-LC/MS wil l be reviewed in the second part.

Two other important types of direct introduction interfaces wil l not be discussed in this review, as they both differ from DLI in the nebulization process: the pneumatic nebulizer [11] and the thermospray interface [12]. How- ever, some of the topics discussed here do apply to these types of interfaces as well.

Design of The Vacuum System

Although some LC/MS experiments have been accom- plished utilizing unmodified MS vacuum systems, most interface designers agree on the necessity of enlarging the pumping capacity of a standard MS vacuum system. The design of vacuum systems for LC/MS has been discussed by Arpino et al. [13].

In order to avoid loss of injected solute one wants to introduce as much of the LC column effluent as possible. However, the maximum liquid flow-rate FI that can be introduced into a MS vacuum system is limited by the pumping capacity of that system. Upon evaporation the

277

0009-5893/86/5 0277-11 $ 03.00/0 �9 1986 Friedr. Vieweg & Sohn Verlagsgesellschaft mbH

l iqu id f low-rate w i l l result in gas f low-rate Fg, which can be calculated (at a pressure o f 1Pa and a temperature of

273K) using the equat ion:

FIPVm (1) F g - M

in which Vm is the molar gas volume (2240Pa m3/mole) , p is the l iquid density (kg/m 3) and M is the molecular

mass (kg/mole) of the l iquid. The gas f low-rate is thus

given in Pam3/s. In order to maintain a vacuum system

at a pressure P, whi le in t roducing a gas f low-rate Fg, the

system must have an effective pumping speed Serf (in m3/s) given by:

Fg Self = -~- (2)

The effective pumping speed of a pump depends on both

the max imum pumping speed of that pump and the con-

ductances of pipelines and openings.

In modern d i f ferent ia l ly pumped MS vacuum systems (Fig. l a) typical operat ing pressures are between 10 - 4 and 10-3pa in the analyzer region and between 10 - 3

and 5" 10 -2pa in the ion source housing. In LC/MS large

effective pumping speeds are required. Three aspects of the vacuum system design have to be discussed: the pumping eff iciency at the ion source housing, the ways to enlarge

the pumping capacity of the vacuum system, and the area of the opening between the ion source housing and the

analyzer region.

Ion Source Region

The max imum l iquid f low-rates that can be introduced

into the ion source housing evacuated by an oil d i f fusion pump can be calculated, using the equations for the con- ductances U of openings and tubes given by Arp ino et al.

[13]. In Table I some figures are given for water and ace-

ton i t r i le at three d i f ferent pumping speeds. The max imum

l iquid f low-rates are about 0.1/11/s in practical cases, which

is qui te low compared to a f low-rate of about 20/~l/s f rom

a conventional LC column. However, the relat ively small

gain achieved by the use of larger pumps does not just i fy

the larger expenses and di f f icul t ies of the instal lat ion of

(a) I I B ,N-- LI, I

11 DP

cb) I P I

OP

I IF CP

AN 1 I I F

(c) IN--"

I DP

AN 1 DP

Fig. 1

IN-- L_ll I DP

'~ 1 I ANI I F DP

Schematic diagrams of mass spectrometric vacuum systems (ex. cluding the pump on the direct insertion inlet), with AN = analyzer region; IS = ion source housing; IN = inlet; DP= diffusion pump; B = baffle; RP = rotation pump; CP = eryopump.

a) standard differentially pumped MS vacuum system. b) vacuum system as used in transport systems and MAGIC. c} vacuum system as used in conventional DLI equipment. d} vacuum system as used in thermospray and "hot-DLl' equip-

ment.

larger di f fusion pumps. Other ways of enlarging the pump.

ing capacity at the ion source housing are more successful.

Three d i f ferent approaches to that effect have been proposed. These approaches can be distinguished w i th respect

to the posit ion of the addi t ional vacuum pump(s) relative

to the ion source.

Table I. Maximum flow-rates of gas, Fg, and liquid, FI, that can be introduced into the ion source housing (preSsure 10 -2 Pa) with vacuum systems, differing in effective pumping capacity, Self, and conductance, U, of the pipelines: 1) Oil diffusion pump (ODP) with Smax = 0.5m3/s connected to the source housing by a 15cm X 15cm i.d.

tube. 2) ODP with Smax = 0.7m3/s connected to the source housing by a 15cm X 15cm i.d. tube. 3) ODP with Smax = 1.3m3/s connected to the source housing by a 15cm X 30cm i.d. tube. 4) Cryopump with 300cm 2 active surface area.

Reprinted from [341 with permission. Copyright 1979 Elsevier Scientific Publishers

water

1 2 3

U (m3/s) 1.7 1.7 8.5 - Sef f (m3/s) 0.38 0.49 1.13 4.97 Fg (ml/min STP) 2.4 3.1 7.0 30.8

~ Fl(#l/min) 1.9 2.5 5.6 24.8

4 1

acetonitrile

2 3 4

1.1 1.1 5.6 - 0.34 0.43 1.06 3.29 2.1 2.7 6.6 20.4 5.0 6.2 15.3 47.6

278 Chromatographia Vol. 21, No. 5, May 1986 Rev;ew Paper

In the vacuum system in use with transport systems [4], but also with the electrospray interface [14] and the rnonodisperse aerosol generating interface (MAGIC) [15] most of the LC solvent is pumped away at the inlet side of the ion source (Fig. lb ) . This approach seems to be the most versatile one, especially from the MS point of view, as it keeps most of the mobile phase contaminants out of the high vacuum region of the MS. It makes LC and MS virtually independent. Specificially, it opens the possibility to obtain both CI and El spectra. In practice the maximum liquid flow-rate that can be handled in MAGIC is between 2 and 10/zl/s. The system is compatible with the flow-rate of microbore columns and, when using a moderate split, with that of conventional columns as well.

In the vacuum system in use with commercially available DLI interfaces the additional pumping speed is obtained by means of a liquid nitrogen trap (cryopump) in the ion source housing (Fig. lc). In Fig. 2 the maximum pumping speeds of a cryopump and a diffusion pump are compared as a function of the pressure. Cryopumps can achieve very high pumping speeds, that can be approximately calculated with the equation:

1

( RT ~2 Smax = A \ ~ - - ~ ] (3)

in which A is the active surface area of the pump, R is the gas constant (8.3J/Kmole) and T is the temperature [13]. This equation applies for the pressure range of 1 0 - 4 < P < 10-2pa. In Table I some figures are shown for a 300cm 2 active surface area cryopump. The maximum liquid flow- rate that can be introduced in this system is between 0.5 and 2/~l/s, which makes the system compatible with the flow-rates of microbore columns, while for conventional columns a split is necessary (split ratios larger than 10).

In the vacuum system used with thermospray interfaces an additional mechanical pump is installed at the outlet side of the ion source (Fig. 1 d). In order to avoid frequent cleaning of the pump oil a liquid nitrogen trap is enclosed between the pump and the ion source [12]. Flow-rates of 15-30/.d/s can be introduced in this system. It is ful ly compatible with the high liquid flow-rates of conventional LC columns. This approach has recently also been employed in a DLI system [10], using liquid flow-rates of between 2 and lO/~l/s. Higher flow-rates result in incomplete solvent evaporation.

Analyzer Region

The oil diffusion pump at the analyzer region has to remove the gas, which passes the baffle between the two vacuum regions. This gas flow depends on the area of the openings in the baffle (ion beam entrance slit and leaks) and the pressure drop across them. Arpino et al. [13] showed that the area of the openings can be as larger as 1 cm 2. This does not pose problems on the baffle design and on the size of the ion beam entrace slit.

,

4

Smax L

f

r 11~6 1=0"5

0.5_

0

o]• ~o "3 1'0-2 1'0-1

t ,

L 1'(~6 1=0-5 ' t'0-4 1'0-3 1!0 -2 1!61

Fig. 2 Comparison of the maximum pumping speeds of a 300cm 2 active surface area cryopump (CP) and a O.5m3/s oil diffusion pump (DP). (1 Tort = 130 Pa) Reproduced from [ 13] with permission. Copyright 1979 Elsevier Scientific Publishers.

Cryopump Cleaning

In most of the present DLI equipment, for example in the commercially available DLI interfaces, a liquid nitrogen cooled surface (a cryopump) is installed to trap the solvent vapours in the high vacuum ion source housing. A cryopump will consume a few liters of liquid nitrogen per day.

Daily cleaning of such a cryopump is necessary. It is not advised to regenerate the cryopump by allowing it to warm to room temperature overnight, as the evaporating solvent wil l cause a temporarily high pressure in the system. Be- sides, the solvents vapours will in part accumulate in the pump oils. A cryopump, which can easily be removed for cleaning purpose without turning down the complete vacuum system is a better solution to the problem of cryopump regeneration [13].

Design of the Interface

Types of Restrictions

An essential part of the DLI interface is the restriction in the tube connecting the LC and the MS. Several types of restrictions have been proposed.

In principle a small bore capillary tube is a restriction. However, both theoretically [8 -10] and experimentally [10] it was shown that the solvent wil l evaporate inside the capillary in most practical applications. Frequent plugging of the tube is observed. This plugging can be the result of clogging by the precipitation of non volatile materials from the mobile phase, or freezing of the liquid

Chromatographia Vol. 21, No. 5, May 1986 Review Paper 279

near the end of the tube. The freezing is a result of too low temperatures of the tube or of liquid flow-rates near the evaporation rate [10].

By drawing out the tip of the capillary tube in a flame an additional flow restriction is formed. In their early experiments Arpino et al. [16] used a O.076mm i.d. glass capillary with a tip narrowed in this way. Plugging of these restrictive tips still occurs regularly.

The principle of contemporary DLI interfaces is based on the proposal by Melera [18] to replace the narrowed tip, as used by Arpino et al. [16], by a small diaphragm. This results in the formation of a liquid jet, which breaks down into small droplets. The process of disintegrating liquid jets is known for quite many years. It was described as early as 1833.

The disintegration of the liquid jet is the result of surface instabilities on the jet. The surface instabilities can have a natural origin, but can also be artif icially produced. Disintegration with the use of artificial surface instabilities; produced by vibrations of piezo-electric crystals, have been utilized for the generation of monodisperse aerosols [ 19 - 20]. Experiments with such a device for LC/MS applications have been described by Willoughby [21]. However, no measurable differences were observed in the droplet size distribution of sprays produced by applying either natural or artificial surface instabilities to the liquid jet. Ultrasonic vibrations in DLI-type LC/MS interfaces have also been employed by Christensen and coworkers [22].

Disintegrating liquid jets were also used by Tijssen et al. [17[, who drew a 2Mm i.d. conical t ip at the end of 10-50 //m i.d. open tubular columns (see Fig. 5a).

In a recent paper Arpino and Beaugrand [10] tried to replace the diaphragms by short capillary tubes of equivalent internal diameter (see below).

Flow conditions for liquid jet formation

A certain minimum linear liquid velocity, Ujet, min, is necessary to form the liquid jet. This velocity can be calculated using:

1

in which 3' is the surface tension of the solvent, and dje t iS the inner diameter of the diaphragm [17, 19, 20]. The pressure drop over the diaphragm, APd, necessary to form a liquid jet with a flow-rate Fie t can be calculated with an equation given by Arpino et al. [9, 10]:

( Fj2t P / z~P d = 1.27 ~ e t ] (5)

for a liquid jet formed in vacuum.

In Fig. 3 the minimum liquid flow-rate to form a liquid jet is given as a function of the diaphragm diameter for reversed phase solvents. A poor agreement exists between the theoretically calculated minimum jet flow-rates and the experimentally determined ones, especially with diaphragms smaller than 10/1m i.d. [10, 23]. This is probably due to the fact that a 4/~m i.d. pinhole in a 50pro thick

200~

150,

(nL/s)

100

50

Fig. 3 Minimum flow-rate to form a liquid jet, Fje t, as a function of the diameter of the diaphragm, die t. Average line for reversed phase solvents.

plate is not a real pinhole, but in fact acts as a tube. In Fig. 4 some experimental pressure drops over a diaphragm are compared with theoretically expected values [23].

In the commercially available DLI interfaces, and in most laboratory-built probes as well, diaphragms of 2 - ~ m i.d. are used. Liquid flow-rates considerably higher than the minimum values are used in most experiments. Nickel or stainless steel diaphragms are employed in most cases, although a glass diaphragm has also been described [15].

As the small diaphragms still happen to become plugged, research has been directed towards other restrictions, giving better reliability and lower costs in routine applications [10]. Higher liquid flow-rates can be introduced when changing the vacuum system to the geometry drawn in Fig. l d. In that case larger diaphragm diameters can be used. For diaphragms later than 10#m and liquid temperature of upto 100~ the minimum jet flow-rates are well above the rate of evaporation in the tube. Therefore, it is also possible to form stable liquid jets at larger diaphragm openings.

Replacement of the diaphragms with short narrow bore capillary tubes, which are cheaper than the precision pinholes used as diaphragm, has also been tried. The same liquid flow-rates are necessary to form a liquid jet at a capillary tube as at a diaphragm. However, the pressure drop over a capillary tube is considerably higher than that over a diaphragm. As the pressure drop over a capillary tube is proportional to the solvent viscosity, which has a strong temperature dependence, the liquid temperature can be used to overcome the problems with this higher pressure drop. This approach is called 'hot-DLl'. Experi- ments with both lO/zm i.d. diaphragms and capillary tubes have been performed in order to test the possibilities of this approach [10].

280 Chromatographia Vol. 21, No. 5, May 1986 Review Paper

0.5

0.3

(NPa }

0.1"

�9 o ~

2

F (~t/s)

0./,

0.2,

o 0:2 F o.6

Fig. 4 Comparison of experimental and calculated values of the pressure drop. Pi, over a diaphragm as a function of flowrate, F. Solvent: methanol. a) diaphragm with a stated diameter of 12.5 -+ 2#m, with 1. theory for a 12.5p, m i.d. diaphragm, and 2. theory for a 14#m i.d. diaphragm. b) diaphragm with a stated diameter of 4 + 2#m, with 1. theory for a 4/~m i.d. diaphragm, and 2. theory for a 6p, m i.d. diaphragm.

For the case of liquid jet formation on a conical tr ip Tijssen et al. [17] derived a complicated equation to de- scribe the pressure drop. The pressure drop over a conical tip is considerably larger than that over a diaphragm. From the flow-rate and pressure requirements to form a liquid jet with a conical t ip Tijssen et al. [17] concluded, that this approach restricts the optimal performance of the open tubular columns, which was actually the topic of their research.

Interface Probe Designs

Several DLI probes are reported in the literature. In Fig. 5 the schematic diagrams of some of these DLI interface probes are shown.

In Fig. 5a the conical tip, used by Tijssen et al. [17] is shown. The use of a conical t ip of 40#m i.d. on a 220/~m i.d. packed microcapillary column was recently described by Alborn and Stenhagen [24].

The probe tip of the DLI interface built by Arpino and coworkers [9[ is drawn in Fig. 5b. This probe tip design is essentially equivalent to the tips of commercially available DLI probes from Nermag and Hewlett-Packard (see also Fig. 6a).

Two DLI probes, designed for interfacing microbore columns and MS are shown in Fig. 5c and 5d. In the first probe the column effluent traverses a narrow bore capillary between column and diaphragm [25--26], while in the latter the microbore column is enciosed in the probe [27], thus avoiding external peak broadening. Minimizing external peak broadening also plays a major role in the DLI probe shown in Fig. 5e, designed for use with open tubular columns. An additional liquid flow is used to form the liquid jet [28].

The monodisperse aerosol generator, designed by Willough- by and Browner [15], is drawn in Fig. 5f. A flow of nitrogen is used to help in traversing the desolvation chamber.

The probe used in the experiments on liquid jet formation on short narrowbore capillaries [10] is shown in Fig. 5g.

DLI type interface probes were also used in experiments in the coupling of supercritical fluid chromatography and mass spectrometry (SFC/MS). In these probes 1-2/~m i.d. diaphragms were used. One of the probes described is shown in Fig. 5h. The pressurized fluid expands momen- tari ly after leaving the diaphragm. Both El and Cl spectra can be obtained [29-30] .

Stability of the Liquid Jet

After connecting the column to the interface probe the liquid jet can be started. The liquid jet has to be started in atmospheric pressure. When a stable liquid jet is established the DLI probe is introduced into the high vacuum through a direct insertion inlet. A stable liquid jet is straight, coaxial with the probe and about 2--4cm long (with 3-5/~m i.d. diaphragms). Starting the liquid jet under reduced pressure will fail, due to freezing of the droplets flowing out of the diaphragm before the necessary jet velocity is established. The liquid jet must be stable and straight under atmospheric pressure; reduced pressure will not alter its appearance with respect to this. A straight liquid jet is a prerequisite for stable and reliable results in DLI-LC/MS. When the liquid jet deviates from coaxial direction, considerable changes in the spectrum, especially in the fragmentation pattern, can be observed [9-10, 30-33].

During the work with the DLI interface probe sudden changes in the direction of the liquid jet can occur, while it may also be straight and coaxial for a long period. In most cases the sudden changes in the direction of the liquid jet can be detected by monitoring the ion source pressure. Only minor fluctuations have to be found. The changes in the direction of the liquid jet are probably due to the deposition of packing material or other solid particles from the LC mobile phase on the sharp edges of the diaphragm [32]. Directional changes mostly have no effects on the pressure drop over the diaphragm.

Clogging of the diagphragm is a frequently encountered problem. A number of precautions has to be taken. Special care is taken in filtering the LC mobile phases prior to use. The use of 0.5/~m porosity endfrits on the column is recommended. In some applications line-filters are inserted between the column and the diaphragm, causing a considerable external peak broadening. Efficient cooling of the

Chromatographia Vol. 21, No. 5, May 1986 Rev;ew Paper 281

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Fig. 5

Schematic diagrams of various interface probe tip designs. a) Conical tip on a glass open capillary column of Tijssen et al. [17|. Reproduced from [17] with permission. Copyright 1981 Elsevier Scientific Publishers. b) Tip of the interface probe of Arpino et el. [9], with 1. liquid inlet; 2. liquid outlet; 3, diaphragm; 8. temperature regulating water-inlet. Reproduced from [9] with permission. Copyright 1981 Elsevier Scientific Publishers.

DLI probe is also reported to prevent clogging of the diaphragm [9, 32-35].

In order to prevent clogging of the diaphragm and con- tamination of the ion source in the analysis of complex biological mixtures a flow-switching system was proposed. The parts of the chromatogram not of interest are directed to waste, while the interesting parts are directed to the MS ion source [34, 36--37].

Although these precautions surely diminish the problems of clogged diaphragms, they can not completely solve them. Deposition of dissolved column material on the diaphragm was proved by Mauchamp and Krien [32] with the use of Scanning Electron Microscopes. Considerable dissolution of column material in daily LC practice was also observed in studies on the coupling of HPLC and Inductively Coupled Argon Plasma Atomic Emission Spectroscopy (ICAP-AES). Especially when restarting the mobile phase high silica concentrations can be detected in the column effluent [33, 37]. It is advised to discard the first few column volumes of mobile phase before connecting the column to the interface probe. It appears necessary to renew the diaphragm regularly, about every few days.

Desoivat ion

Desolvation of Droplets

Almost immediately after the formation of the liquid jet it disintegrates into a mist of droplets, which appear to have a narrow size distribution [15, 21, 34]. Theory pre- dicts, that the droplets have a diameter of about twice the diaphragm diameter [19]. The droplet diameters measured by Willoughby [21 ] are in agreement with this.

The next step in the DLI process is the desolvation of the droplets. On their travel from the probe tip towards the ion source through the so-called desolvation chamber the droplets evaporate. The rate of evaporation will depend on both heat and mass transfer. The evaporation of the

Fig. 5 continued

c) Micro-LC interface probe of Henion and Wachs [25], with A. Liquid inlet; B. water cooling inlet; H. diaphragm; I. removable endcap. Reproduced from [25| with permission. Copyright 1981 American Chemical Society. d) Micro-LC interface probe of Krien et al. [27]. with 4. diaphragm; 5. microbore LC column; 7. 0.5#m porosity fitter. Re- produced from [27] with permission. Copyright 1982 Elsevier Scientific Publishers. e) DLI-probe for open tubular columns [28], with 1. open tubular column; 2. Make-up liquid; 3. diaphragm; 5. cooling water inlet. f) Monodisperse aerosol generator of Willoughby and Browner [15]. Reproduced from [15] with permission. Copyright 1984 American Chemical Society. g) Interface probe with capillary tubing in stead of a diaphragm of Arpino and Beaugrand [10]. Reproduced from [10] with permission. Copyright 1985 Elsevier Scientific Publishers. h) Interface probe for Supercritical Fluid Chromatography/Mass Spectrometry of Smith et al. [30]. Reproduced from [30] with permission. Copyright 1984 American Chemical Society.

solvent requires a considerable amount of energy. In the absence of heat transfer, the droplets will cool very rapidly upon evaporation, It has been calculated that in an entirely adiabatic process a droplet of acetonitrile at room temperature will freeze (at - 92~ after the evaporation of only 23% of the liquid [9]. Thus, heat transfer from the surrounding gas is of prime importance in the desolvation process, for which only a few milliseconds are available. In most DLI interfaces the desolvation chamber is heated in order to promote the heat exchange. However, the temperature of the droplets wil l not rise significantly in the evaporation process. This description of the desolvation process clarifies the fact, that thermolabile and even non- volatile molecules can be transmitted to the ion source [9, 10, 18, 34, 35].

The importance of the heat exchange in the desolvation process has been shown in several experiments [9, 10, 21, 23]. For example, Arpino and Beaugrand [10] described experiments with a 12cm long liquid jet, produced with a flow-rate of 100/~l/min from a 10/am i.d. capillary tube or diaphragm. In the low pressure desolvation chamber the heat exchange was insufficient to deliver the energy necessary for the evaporation process. The liquid jet froze rapidly, and more easily when using a diaphragm in stead of a capillary. These problems with heat transfer prevents the DLI interface from being used at flow-rates over 10/~l/s.

Freezing of liquid jets has been described by several authors [9-11, 23]. The frozen material can have several appear- ances: a solid bar of frozen liquid building up from the diaphragm, a solid mass protruding from the interface, a cloud of solid particles, and even a bar growing from the testchamber wall towards the probe tip.

Willoughby and Browner [15, 21 ] pointed out that the heat exchange between the droplets and the surrounding gas is more efficient at higher pressures. Therefore, they decided to accomplish the desolvation process in MAGIC at atmospheric pressure rather than at a pressure of about 100Pa, which is used in conventional desotvation chambers.

Desolvation Chamber Design

Several designs of desolvation chambers have been proposed (Fig. 6). The purpose of inserting a desolvation chamber is to provide a heated zone between the probe tip and the ion source in order to promote the desolvation of the droplets.

Most desolvation chambers simply consist of more or less straight tubes, heated by either separate heaters or heat transfer from the ion source (Fig. 6a). However some ingenious devices have also been described.

Dedieu and coworkers [35, 39] proposed a desolvation chamber with a convergent-divergent internal geometry (Fig. 6b), in order to concentrate and accelerate the droplets in their flight. In this way the desolvation of the droplets is improved and contacts with the walls of the desolvation chamber are avoided. A similar device was recently constructed by Esmans et al. [40].

Arpino et al. [41] attempted to use the high positive charges on the liquid droplets produced in the nebulization process to assist in the ionization of solutes. These ex-

Chromatographia Vol. 21, No. 5, May 1986 Review Paper 283

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Fig. 6

Schematic diagrams of the various desolvation chamber designs. a) Standard desolvation chamber as depicted by Melera {18], also showing the DLI interface probe tip. Reproduced from [181 with permission. b) Convergent-divergent internal geometry of the desolvation chamber of Dedieu et al. [35, 39], with 1. DLI probe;4, pre-evaporation zone; 5. heated divergent zone; 6. heater; 8. ion source block. Reproduced from [39] with permission. Copyright 1982 American Chemical Society. c) Conical internal geometry of the desolvation chamber of Arpino et al. [41], with 1. DLI probe; 2. Heaters; 4. Desolvation chamber;

6. Ion source block. Reproduced from [41] with permission. Copy- right 1983 Elsevier Scientific Publishers. d) Solvent stripping extended desolvation chamber of Sugnaux et al. [43{, with 1. DLI probe; 3. gap between desolvation chamber and ion source block; 7. heater. Reproduced from [43] with permission. Copyright 1983 Elsevier Scientific Publishers. e) Vespel desolvation chamber of Voyksner et al. [44]. Reproduced from [44] with permission. Copyright 1984 American Chemical Society. f) Desolvation chamber and aerosol beam separator in use with MAGIC [15]. Reproduced from [15] with permission. Copyright 1984 American Chemical Society.

284 Chromatographia Vol. 21, No. 5, May 1986 Review Paper

periments are based on the observation, that ions pre- formed in solution can be extracted from highly charged droplets into the gas-phase [42]. These experiments wil l be discussed in more detail later on. The desolvation chamber used in these experiments has a conical internal geometry, wi th the apex located on the electron beam trajectory inside the ion source (Fig. 6c). The same desolvation chamber was used in the experiments with larger restriction diameters [10].

A special extended desolvation chamber (Fig. 6d) was designed by Sugnaux and coworkers [43] to give easier control of the desolvation process and the ion source pressure. The pressure in a particular ion source can be regulated by two parameters: the flow-rate of the liquid introduced, and the distance between the probe t ip and the desolvation chamber. Accurate control of the flow- rate in a split-type DLI interface is dif f icult. Therefore, most workers use the probe position in optimization procedures [43-45] . However, Sugnaux et el. [43] showed that regulating the pressure in this way is di f f icul t with the standard desolvation chamber, and much easier with the extended desolvation chamber (see Fig. 7a). Even better results were obtained with the so-called 'solvent- stripping' extended desolvation chamber, in which part of the solvent vapours is pumped away through an ad- justable slit between desolvation chamber and ion source. The functioning of the desolvation chambers is depicted schematically in Fig. 7b.

A non-heated Vespel desolvation chamber (Fig. 6e) was described by Voyksner et el. [44] for the analysis of thermolabile pesticides. The use of a non-heated desolvation chamber is somewhat surprising in the light of the discussions above, but the results appear to be satisfactory, especially in the analysis of thermolabile compounds. A similar device was recently described by Shalaby [31], in this way avoiding the necessity of cooling the DLI probe.

The desolvation chamber in MAGIC differs significantly from the others [15]. It was designed for the desolvation of droplets at atmospheric pressure, and the subsequent transfer of solutes through a series of vacuum chambers towards the ion source. The fl ight of the droplets is sup- ported by the use of a carrier gas stream. A schematic diagram of the desolvation chamber and the two-stage aerosol-beam separator is shown in Fig. 6f.

Liquid Chromatography

Column types

Up to about 1977 analytical LC columns with 3-4 .6mm i.d. were used in almost every LC lab. Starting with the work of Ishii, and Scott and Kucera several new column types were introduced wi th smaller internal diameters [46]. Al l these types of columns have been coupled with MS uti l izing various interfaces. In DLI interfacing some of these column types can have advantages in LC/MS coupling in comparison with conventional packed columns.

In the DLI interface a solvent splitter is used when working with conventional columns. When using a split, i.e. ad-

~reSIurlr

[~o.] A

i ! ~ ! i i i i J L | i 1 5 10

f r o n t r162 r [ m m ] O~=tance

a, STANDARD H P

i

"r d ~-y ,

5 ~'n

b. EXTENDED

bY

v

m

C. SOLVENT STRIPPING

; ,~ ' d " ; w t m

Fig. 7 a) Plots of ion source pressure versus distance of the DLI probe tip and desolvation chamber (DSC) for: 1. the Hewlett-Packard OSC, 2. the extended DSC, and 3. the solvent-stripping extended DSC. Solvent: 65:35 methanol water. Temperature of the OSC and ion source 200~ (1 Torr = 130 Pa) b) Schematic diagram showing the functioning of the three dif- ferent desolvation chambers. The arrows indicate the direction of vapour flow, Reproduced from 143] with permission. Copyright 1983 Elsevier Scientific Publishers.

mitt ing a fixed flow-rate to the ion source, the MS acts as a concentration sensitive detector, rather than as a mass sensitive detector. As small bore columns deliver peaks in a smaller volume, a better signal for the same amount injected is obtained. The mass detection l imit decreases proportional to the inverse of the column diameter.

This has first been demonstrated by Henion et el. [48]. Using a conventional column and a 99 : 1 split-ratio Henion reported good signal to noise ratios for 500pg injections of methylparathion [49]. Similar signal to noise ratios were obtained for 30pg injections of phenothiazine derivatives on a 0.5mm i.d. PFTE column and no split [48]. Similar results were later reported by others.

Although the mass detection l imit is improved when decreasing the column diameter there is no gain in the con-

Chromatographia Vot. 21, No. 5, May 1986 Review Paper 285

centration detection limit. A given analytical determina- tion with small bore columns can always be carried out equally well on a standard column using a split, provided there is enough sample available to increase the injected mass and volume in proportion. In that case there is no fundamental advantage for the small bore columns, and it becomes a matter of convenience, cost and elegance in the system itself or in the sample pretreatment, which column type is to be preferred.

This is valid as long as a split is needed. When the column diameter is decreased so far that the flowrate becomes even smaller than the maximum input to the MS, the MS is a mass sensitive detector. No further improvements can be expected. Mass detection limits are constant on de- crease of the column diameter, while the concentration detection l imit will deteriorate. This is due to the fact, that smaller injection volumes are allowed under these conditions. This is the situation with most of the packed microcapillary columns and even more so with open tubular columns. The latter ones (5-25/~m i.d.) are developed as they can achieve high resolving power in a much shorter time.

Small bore packed columns offer the additional advantage over conventional columns that very long columns with high plate numbers can be made and operated easily.

The use of small bore columns in LC/MS, especially with DLI interfaces, is strongly recommended in many research and review papers. Microbore columns with good chromato- graphic performance are now commercially available from several manufacturers, as is the modified LC equipments. However, it turns out that relatively few applications are done with such columns.

For practical applications the situation can be summarized as follows. Columns with about 1 mm inner diameter offer some minor advantages in general, but are a must when the sample amount is limited. The work on even smaller bore packed columns and open tubulars is still in a research stage.

LC Mobile Phase

In our discussion on chemical ionization with DLI interfaces emphasis wil l be laid on the use of acetonitrile-water and methanol-water mixtures as LC mobile phases. These mobile phases are used in the majority of the DLI experiments. Because acetonitrile tends to form less high mass solvent cluster ions, acetonitrile-water mixtures are used most. However, DLI interfaces are not restricted to those solvents.

The use of some other organic modifiers was described: chloroacetonitrile [50], proprionitrile [51] and ethanol [52].

Several other solvent additives, such as buffers and ion pairing reagents, have also been used: ammonium formiate [34, 36], ammonium acetate [10, 18, 53], acetic acid [22], triethylamine [54, 55], citric acid, disodium-hydrogen- phosphate, sodium n-octylsulfonate and Na2EDTA [56], trifluoroacetic acid and triethylammoniumacetate [55].

Several normal phase applications have been described. The solvent systems used in these applications are hexane

[27, 32], dichloromethane [32], hexane-isopropanol [32, 57], hexane-dichloromethane [58] and dichloromethane- methanol-acetic acid [31].

Gradient elution has also been performed in several applications (e.g. see [18, 22, 34, 45, 53]).

External Peakbroadening

In most DLI experiments the column end is connected to the DLI interface probe and the column effluent flows through a narrow capillary to the diaphragm. A tube of about 40cm long will certainly result in some external peak broadening. For the standard DLI probe manu- factured by Hewlett-Packard a internal volume of 20- 40/,zl was reported [40, 53]. Recently a low dead volume probe was introduced by the same manufacturer, having an internal volume of about 10pl [40]. This constitutes a considerable improvement, although a volume of 10#1 is still quite high in microbore experiments.

External broadening of the interface probe was also discussed by Melera [18], using incorrect equations, and by Kenyon et al. [34]. In the latter paper the external peak broadening in the LC/MS interface is calculated by com- paring peak parameters of the LC/MS with those of the UV, which is placed in series. This method is used by several authors. However, wide-bore outlet capillaries are present in most UV detectors, contributing significantly the peak broadening. Such methods give false information on the performance of the LC/MS system, and may at the same time also lead to significant de- terioration of that performance.

Most workers visually compare the peak profiles of UV and LC/MS experiments and conclude that there is almost no external peak broadening in the interface. This approach, which is incorrect from a chromatographers point of view, has recently been discussed elegantly by Karger and Vouros [47].

From the various figures reported on external peak broadening it can be concluded that for use with conventional columns the use of a DLI interface will give some deteriora- tion of the column performance. Short connecting tubes and a narrow bore tube in the interface probe are recommended for those applications in which the efficiency of the separation is important. For 1ram i.d. microbore columns commercially available DLI probes are not very suitable with respect to external peak broadening. An interface probe, as drawn in Fig. 5d, is a better solution, provided the capillary between injection valve and column is very short and narrow bore. When the efficiency in the separation is important in solving an analytical problem with DLI-LC/MS it is recommended to modify the outlet tube of the UV detector or to refrain from the use of an on line UV detector in LC/MS applications.

The external peak broadening of the DLI interface designed for use with open tubular columns was also studied. The interface design allows the kinetically optimal use of open tubular columns of 8-25/1m i.d. and about 5m long without significant contribution due to the interface [28, 59].

286 Chromatographia Vol. 21, No. 5. May 1986 Review Paper

Discussions on some trends in modern LC, and especially in LC column technology, all po int to high demands on

instrument developments. The increasing column effi-

ciencies wi l l require higher scan speeds, especially in sector instruments, and higher data acquisition frequencies. Detection l imits in LC/MS appear to be another l imit ing factor. The effects of decreasing the column diameter on the detection l imits were already discussed, as were the problems encountered in microcapil lary and open tubular columns. These topics were discussed in more detail by Guiochon and Arpino [60] and by Karger and Vouros [47].

Acknowledgement Prof. Dr. H. Poppe and Prof. Dr. N. M. M. Nibbering are greatly thanked for reading and discussing the manuscript

and for giving many helpful suggestions.

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Received: Oct. 14, 1985 Accepted: Oct. 30, 1985 B

Chromatographia Vol. 21, No. 5. May 1986 Review Paper 287

Documents

A review of direct liquid introduction interfacing for LC/MS Part I: Instrumental aspects