2373
Anal, Chem. 1985, 57,2373-2378
RECEIVED for review April 1,1985. Accepted June 10,1985. Although the research described in this article has been funded in part by the United States Environmental Protection Agency under assistance agreement number CR-809397 to Southern Illinois University, it has not been subjected to the Agency’s
required peer and administrative review and therefore does not necessarily reflect the view of the Agency and no official endorsement should be inferred. Partial support by the National Science Foundation under Grant CHE-8215371 is also acknowledged.
Thermospray Liquid ChromatographIMass Spectrometer Interface with Direct Electrical Heating of the Capillary Marvin L. Vestal* and Gordon J. Fergusson
Department of Chemistry, University of Houston-University Park, Houston, Texas 77004
A new thermospray vaporizer using direct electrical heating of a stainless steel capillary tube has been developed. This new vaporlzer Is powered by a feedback controlled triac power supply and provides both improved performance and better stability than previous thermospray interfaces. The theoretical background for the improved thermospray system Is presented together with results of recent experimental studies on the effects on performance of various operating parameters.
Our efforts to develop a practical LC/MS interface have concentrated on vaporizing the LC effluent by heating the end of a capillary tube connecting the LC to the MS (1-5). This work has evolved from the original use of a 50-W COz laser (I) through oxy-hydrogen torches (2,3) to the use of simple electrical cartridge heaters imbedded in a copper block brazed to the end of the capillary (4,5). In our most recent work this “indirect” electrical heating has been replaced by “direct” electrical heating in which the power required to vaporize the liquid is supplied by passing an electrical current through the capillary tube itself. A new control system has been developed for supplying the power to the vaporizer. This system is controlled by feedback from a thermocouple attached to the capillary so that the fraction of liquid vaporized can be held nearly constant even though the flow rate may vary. This new controller also provides the compensation necessary for controlling the vaporizer when the composition of the LC eluent changes, for example, during gradient elution. Our early attempts to develop an LC/MS interface applicable to nonvolatile samples in aqueous effluents at conventional LC flow rates (ca. 1 mL/min) were based on the premise that very rapid heating over a short length of the capillary was required to vaporize the liquid without pyrolyzing the sample, but recent work has shown that this premise was false. This history of our work on capillary vaporizers leading to the development of a practical “thermospray” system is summarized in Table I. As can be seen from the table, we have moved progressively to longer heated lengths and lower surface temperatures. At each step we have found both improved performance under optimum conditions and better stability. As a result the directly heated vaporizer can be operated routinely at or near its optimum, even though the solvent flow rate or composition may change. EXPERIMENTAL SECTION A schematic diagram of the thermospray LC/MS interface using a directly heated capillary is shown in Figure 1. The ion
Table I. Summary of the Characteristics of Different Thermospray Vaporizers method C02 laser
oxy-hydrogen flame indirect electrical direct electrical
heated energy total surface length, cm flux, W/cm2 power, W temp, O C 0.03 0.3
3 x 104 5 x 103
3 30
25 50
>loo0 -1000
700
100
250
70
100
200
source and quadrupole mass spectrometer are essentially the same as that described previously (4). The vaporizer is mounted in a probe, 6.3 mm o.d., which passes through the vacuum wall via a standard tube fitting with a Teflon ferrule. This allows the position of the vaporizer to be adjusted relative to the ion exit aperture while the instrument is operating. Temperatures of the vaporizer capillary are monitored by iron-constantan thermocouples brazed to the capillary tube near the exit into the mass spectrometer ion source and in the region near the entrance of liquid into the heated portion as described in more detail below. The source block temperature and the temperature of the vapor in the ion source are monitored by similar thermocouples located as shown in Figure 1. A thoriated irridium filament is available as an alternative source of ionization by producing an electron beam transverse to the figure at the point indicated, but this filamentwas not heated for any of the experiments described in this paper. RESULTS AND DISCUSSIONS Recent research has provided a much more detailed understanding of the processes occurring when a flowing liquid is vaporized as it is forced under pressure through a heated capillary tube. The temperature profiles produced by direct electrical heating of the capillary are illustrated in Figure 2. In this experiment 53 W was dissipated in the capillary with water flowing through at rates in the range from 0.7 to 4.0 mL/min as indicated by the parameter in Figure 2. The experimental result at 0.7 mL/min is compared with a calculated profile in Figure 3 where the processes assumed to occur in different regions of the heated capillary are represented schematically. At the inlet end the flowing water is heated until vaporization begins. From this point and downstream the temperature remains nearly constant until vaporization is complete since the heat flux is used to provide the latent heat of vaporization. The slight decrease in temperature along the vaporization region results from the fact that the pressure is decreasing toward the exit end. At the point corresponding to complete vaporization the temperature again rises rapidly since only the heat capacity of the vapor
0003-2700/85/0357-2373$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
2374
W
SOURCE MOUNTING
E
TO TRAP
a
MECHANICAL PUMP
BLOCK TEMPERATURE
SOURCE BUX;K
Flgure 1. Thermospray LC/MS interface using direct electrical heating of capillary. I
3501
4
'
1
'
1
I
10 MS
t
53 WATTS
OPTOCOUPLER
-.Aq LlOUID
IN
71 STAINLESS (IC
+
MOVEABLE TC
CAPILLARY llJrnrnOOx 015mm1Dl
6
-
FIXED TC
Figure 4. Block diagram of the triac controlled ac supply used for direct heating of capillary vaporizers.
where AHvis the total specific enthalpy (J/g) to convert liquid at the entrance temperature, To,to vapor at exit temperature, T,, and CLis the specific heat capacity of the liquid. The total power coupled into the liquid in the region between the entrance to the vaporizer and the location of the thermocouple monitoring T1 is given by
r !
= CL(T1- To)F (2) where Tl is the temperature at the control point. If the tube W1
is mounted and insulated so that essentially all of the power dissipated in the tube is coupled into the flowing fluid, then uniform heating of the tube implies that
--
-lO..lO IN
-~ -+
ami
I
I
1
VAPORIZPTION ONSET BUBBLES
IN LIQUID
I
0 .T
VAPORIZATION DROPLETS COMPLETE
VAPOR
Flgure 3. Comparison between calculated (line)and measured (points) temperature profile at 0.7 mL/min together with a schematic representation of the model used in the calculation.
is available to absorb the input heat. In these initial studies a manually controlled dc power supply was employed. More recently a triac controlled ac supply has been developed. A block diagram of this controller is shown in Figure 4. The power output to the capillary is controlled by feedback so as to maintain the temperature, TI, constant as indicated by a thermocouple attached to the capillary near the inlet end where no vaporization occurs. The total power which must be coupled into the flowing liquid to vaporize a fraction, f , of a given mass flow F (g/s) is
W = F A H V + F(l - f j C ~ ( T2 To)
(1)
w,/w = L1/L
(3)
where L is the total heated length and L1 is the length of the heated portion up to the point monitored by T,. Combining eq 1 through 3 and solving for T1 gives
= To + (L,/L)(Tz- To)
Ti(f)=
To + (L,/L)VAHV/CL+ (1 - f ) ( T z- To)]
0
f
