Thermocouples only introduce additional error due to cold junctions.
The PT100 is platinum resistance thermometer (RTD), so don't require for a cold junction compensator.
[Don't be embarassed. It happened to me too after hours of moderating job.]
How achieve high accuracy temperature measurements is not then an easy matter, however some suggestion may be given here.
1. Four wires PT100 should be selected or at least 3. This will compensate (sensing) for leads resistance.
2. Choose good sensor. Rosemount or Minco o maybe Heraus. In PT100 the worst error is long time stability, due to PT100 contructions factors.
3. Select a class A PT100. That's means 0.1 + 0.002|t| (°C). Class B is 0.3 + 0.002|t|.
4. When you have purchased PT100, send it to NIST if you live in USA or EAL if you live in EUROPE and ask for certification with not less of 6 temperature points, one of them passing from 0°C (you will need it, because you require R0, that is R at 0°C). From the certificate, calculate the coefficients A, B and C. R0 is reported at 0°C.
5. If you cannot calculate the coefficient for your RTD, you may keep the standard coefficients, that is A=3.90830e-3, B= -5.7750e-7, C=-4.2735e-12 and R0=100 ohms. In that case you will have and error of +/-0.5°C, provided that you have purchased a class A PT100.
6. Linearize RTD using VanDusen equations. Hence : Rt=R0+(1+At+Bt^2) from 0 to 850°C, or Rt=R0+[1+At+Bt^2+Ct^3(t-100)] from -200 to 0°C. I guess you will use a micro to do this job. Analogue methods are difficult to implement if you need high accuracy.
7. Excitate the PT100 with a costant current generator. The value should be less of 1mA to prevent the auto power error (the PT100 dissipate a power so it will generate heat due to joule effect). A current value of 0.5mA or better 0.2mA should be nice. Obviously a good ADC with high resolution is a must. Select a delta sigma one. ADI or Linear Technology would be ideal.
8. You may use a DC current to excitate RTD. Some ones suggest to use an AC current to null the effects of parassite cold junctions that would exist from Platinum and Copper junction (cable and connectors) and prevent the autopower due to Peltier coefficients of above junctions. I don't think so. The Peltier auto power maybe calculated from the FEM of the parassite junctions multiplied by excitation current, let say 1mA, so given that the FEM is not more of 15uV for platinum, the Peltier auto power is 15uV x 1mA=15nW (joule effect due the Peltier juction). This value is unmeasureable compared to auto power of PT100 excitated at 1mA, for instance at 0°C, that is 100*1mA^2= 0.1mW. To reduce the effects of parassites junctions you can revert excitation current at low frequency, 5 or 10 time per seconds and then take averages of absolute values. However this add an additional complessity to your system. From my experience a DC method will suffice to achieve 0.1°C or bit better, provided that you have considered what above said.
Followings note maybe useful.